Produced by Louise Hope




[Transcriber's Note:

DO NOT TRY THIS AT HOME.

This e-text comes in three different forms: unicode (UTF-8), Latin-1 and
ascii-7. Use the one that works best on your text reader.

  --If "oe" displays as a single character, and apostrophes and quotation
  marks are "curly" or angled, you have the utf-8 version (best). If any
  part of this paragraph displays as garbage, try changing your text
  reader's "character set" or "file encoding". If that doesn't work,
  proceed to:
  --In the Latin-1 version, "oe" is two letters, but the word "aeriform"
  is usually written with dieresis (dots) over the "e", and "ae" is a
  single letter. Apostrophes and quotation marks will be straight
  ("typewriter" form). Again, if you see any garbage in this paragraph
  and can't get it to display properly, use:
  --The ASCII-7 or rock-bottom version. All necessary text will still be
  there; it just won't be as pretty.

The full caption of each Plate is given after its first mention in the
text--generally a few pages before the Plate's physical appearance, as
specified in the caption.

Many terms used in this book are different from today's standard
terminology. Note in particular:

  oxy-muriatic acid = the element chlorine
  phosphat of lime = calcium diphosphate _or_ the element calcium
  glucium = the element beryllium

  muriatic acid = hydrochloric acid
  muriat of lime = calcium chloride
  oxymuriate of potash = potassium chlorate
  carbonic acid = carbon dioxide

Further details and more examples are at the end of the e-text.

Each Volume had its own table of contents. They have been merged for
this e-text, but the Vol. II title page was retained. Some Conversations
were renumbered between the 4th and 5th edition, resulting in the
apparent disappearance of Conversations XI and XII.

Typographical errors are listed at the end of the text.]


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                 CONVERSATIONS
                       ON
                   CHEMISTRY;

                    In Which
          The Elements Of That Science
                      Are
             _Familiarly Explained_
                      And
          Illustrated By Experiments.


                IN TWO VOLUMES.

    _The Fifth Edition, revised, corrected,_
          _and considerably enlarged._

                    VOL. I.
               ON SIMPLE BODIES.


  _London:_
  Printed For Longman, Hurst, Rees, Orme, and Brown,
  Paternoster-Row.
  1817.




Printed by A. Strahan, Printers-Street, London.




ADVERTISEMENT.


_The Author, in this fifth edition, has endeavoured to give an account
of the principal discoveries which have been made within the last four
years in Chemical Science, and of the various important applications,
such as the gas-lights, and the miner's-lamp, to which they have given
rise. But in regard to doctrines or principles, the work has undergone
no material alteration._

_London_, _July_, 1817.




PREFACE.


In venturing to offer to the public, and more particularly to the female
sex, an Introduction to Chemistry, the author, herself a woman,
conceives that some explanation may be required; and she feels it the
more necessary to apologise for the present undertaking, as her
knowledge of the subject is but recent, and as she can have no real
claims to the title of chemist.

On attending for the first time experimental lectures, the author found
it almost impossible to derive any clear or satisfactory information
from the rapid demonstrations which are usually, and perhaps
necessarily, crowded into popular courses of this kind. But frequent
opportunities having afterwards occurred of conversing with a friend on
the subject of chemistry, and of repeating a variety of experiments, she
became better acquainted with the principles of that science, and began
to feel highly interested in its pursuit. It was then that she
perceived, in attending the excellent lectures delivered at the Royal
Institution, by the present Professor of Chemistry, the great advantage
which her previous knowledge of the subject, slight as it was, gave her
over others who had not enjoyed the same means of private instruction.
Every fact or experiment attracted her attention, and served to explain
some theory to which she was not a total stranger; and she had the
gratification to find that the numerous and elegant illustrations, for
which that school is so much distinguished, seldom failed to produce on
her mind the effect for which they were intended.

Hence it was natural to infer, that familiar conversation was, in
studies of this kind, a most useful auxiliary source of information; and
more especially to the female sex, whose education is seldom calculated
to prepare their minds for abstract ideas, or scientific language.

As, however, there are but few women who have access to this mode of
instruction; and as the author was not acquainted with any book that
could prove a substitute for it, she thought that it might be useful for
beginners, as well as satisfactory to herself, to trace the steps by
which she had acquired her little stock of chemical knowledge, and to
record, in the form of dialogue, those ideas which she had first derived
from conversation.

But to do this with sufficient method, and to fix upon a mode of
arrangement, was an object of some difficulty. After much hesitation,
and a degree of embarrassment, which, probably, the most competent
chemical writers have often felt in common with the most superficial,
a mode of division was adopted, which, though the most natural, does not
always admit of being strictly pursued--it is that of treating first of
the simplest bodies, and then gradually rising to the most intricate
compounds.

It is not the author's intention to enter into a minute vindication of
this plan. But whatever may be its advantages or inconveniences, the
method adopted in this work is such, that a young pupil, who should
occasionally recur to it, with a view to procure information on
particular subjects, might often find it obscure or unintelligible; for
its various parts are so connected with each other as to form an
uninterrupted chain of facts and reasonings, which will appear
sufficiently clear and consistent to those only who may have patience to
go through the whole work, or have previously devoted some attention to
the subject.

It will, no doubt, be observed, that in the course of these
Conversations, remarks are often introduced, which appear much too acute
for the young pupils, by whom they are supposed to be made. Of this
fault the author is fully aware. But, in order to avoid it, it would
have been necessary either to omit a variety of useful illustrations, or
to submit to such minute explanations and frequent repetitions, as would
have rendered the work tedious, and therefore less suited to its
intended purpose.

In writing these pages, the author was more than once checked in her
progress by the apprehension that such an attempt might be considered by
some, either as unsuited to the ordinary pursuits of her sex, or
ill-justified by her own recent and imperfect knowledge of the subject.
But, on the one hand, she felt encouraged by the establishment of those
public institutions, open to both sexes, for the dissemination of
philosophical knowledge, which clearly prove that the general opinion no
longer excludes women from an acquaintance with the elements of science;
and, on the other, she flattered herself that whilst the impressions
made upon her mind, by the wonders of Nature, studied in this new point
of view, were still fresh and strong, she might perhaps succeed the
better in communicating to others the sentiments she herself
experienced.

The reader will soon perceive, in perusing this work, that he is often
supposed to have previously acquired some slight knowledge of natural
philosophy, a circumstance, indeed, which appears very desirable. The
author's original intention was to commence this work by a small tract,
explaining, on a plan analogous to this, the most essential rudiments of
that science. This idea she has since abandoned; but the manuscript was
ready, and might, perhaps, have been printed at some future period, had
not an elementary work of a similar description, under the tide of
"Scientific Dialogues," been pointed out to her, which, on a rapid
perusal, she thought very ingenious, and well calculated to answer its
intended object.




  Contents Of
  _The First Volume_.

  ON SIMPLE BODIES.


CONVERSATION I.
                                                                  Page

  ON THE GENERAL PRINCIPLES OF CHEMISTRY.                            1

Connexion between Chemistry and Natural Philosophy. --Improved State
of modern Chemistry. --Its use in the Arts. --The general Objects of
Chemistry. --Definition of Elementary Bodies. --Definition of
Decomposition. --Integrant and Constituent Particles. --Distinction
between Simple and Compound Bodies. --Classification of Simple
Bodies. --Of Chemical Affinity, or Attraction of Composition.
--Examples of Composition and Decomposition.


CONVERSATION II.

  ON LIGHT AND HEAT.                                                26

Light and Heat capable of being separated. --Dr. Herschel's
Experiments. --Phosphorescence. --Of Caloric. --Its two
Modifications. --Free Caloric. --Of the three different States of
Bodies, solid, fluid, and aeriform. --Dilatation of solid Bodies.
--Pyrometer. --Dilatation of Fluids. --Thermometer. --Dilatation of
Elastic Fluids. --Air Thermometer. --Equal Diffusion of Caloric.
--Cold a Negative Quality. --Professor Prevost's Theory of the
Radiation of Heat. --Professor Pictet's Experiments on the Reflexion
of Heat. --Mr. Leslie's Experiments on the Radiation of Heat.


CONVERSATION III.

  CONTINUATION OF THE SUBJECT.                                      70

Of the different Power of Bodies to conduct Heat. --Attempt to
account for this Power. --Count Rumford's Theory of the
non-conducting Power of Fluids. --Phenomena of Boiling. --Of
Solution in general. --Solvent Power of Water. --Difference between
Solution and Mixture. --Solvent Power of Caloric. --Of Clouds, Rain,
Dr. Wells' theory of Dew, Evaporation, &c. --Influence of
Atmospherical Pressure on Evaporation. --Ignition.


CONVERSATION IV.

  ON COMBINED CALORIC, COMPREHENDING SPECIFIC HEAT
  AND LATENT HEAT.                                                 122

Of Specific Heat. --Of the different Capacities of Bodies for Heat.
--Specific Heat not perceptible by the Senses. --How to be
ascertained. --Of Latent Heat. --Distinction between Latent and
Specific Heat. --Phenomena attending the Melting of Ice and the
Formation of Vapour. --Phenomena attending the Formation of Ice, and
the Condensation of Elastic Fluids. --Instances of Condensation, and
consequent Disengagement of Heat, produced by Mixtures, by the
Slaking of Lime. --General Remarks on Latent Heat. --Explanation of
the Phenomena of Ether boiling, and Water freezing, at the same
Temperature. --Of the Production of Cold by Evaporation.
--Calorimeter. --Meteorological Remarks.


CONVERSATION V.

  ON THE CHEMICAL AGENCIES OF ELECTRICITY.                         160

Of Positive and Negative Electricity. --Galvani's Discoveries.
--Voltaic Battery. --Electrical Machine. --Theory of Voltaic
Excitement.


CONVERSATION VI.

  ON OXYGEN AND NITROGEN.                                          181

The Atmosphere composed of Oxygen and Nitrogen in the State of Gas.
--Definition of Gas. --Distinction between Gas and Vapour. --Oxygen
essential to Combustion and Respiration. --Decomposition of the
Atmosphere by Combustion. --Nitrogen Gas obtained by this Process.
--Of Oxygenation in general. --Of the Oxydation of Metals. --Oxygen
Gas obtained from Oxyd of Manganese. --Description of a Water-Bath
for collecting and preserving Gases. --Combustion of Iron Wire in
Oxygen Gas. --Fixed and volatile Products of Combustion. --Patent
Lamps. --Decomposition of the Atmosphere by Respiration.
--Recomposition of the Atmosphere.


CONVERSATION VII.

  ON HYDROGEN.                                                     214

Of Hydrogen. --Of the Formation of Water by the Combustion of
Hydrogen. --Of the Decomposition of Water. --Detonation of Hydrogen
Gas. --Description of Lavoisier's Apparatus for the formation of
Water. --Hydrogen Gas essential to the Production of Flame.
--Musical Tones produced by the Combustion of Hydrogen Gas within a
Glass Tube. --Combustion of Candles explained. --Gas lights.
--Detonation of Hydrogen Gas in Soap Bubbles. --Air Balloons.
--Meteorological Phenomena ascribed to Hydrogen Gas. --Miner's Lamp.

  [Transcriber's Note:
  The final two pages of the Table of Contents for Volume I were
  missing; everything after "Decomposition of Water" was supplied
  from earlier and later editions, compared against the body text.
  The section marked "Diamond" (Conv. IX) was called "Diamond is
  Carbon(e) in a state of perfect purity" in the 4th edn., "Diamond"
  alone in later editions.]


CONVERSATION VIII.

  ON SULPHUR AND PHOSPHORUS.                                       256

Natural History of Sulphur. --Sublimation. --Alembic. --Combustion
of Sulphur in Atmospheric Air. --Of Acidification in general.
--Nomenclature of the Acids. --Combustion of Sulphur in Oxygen Gas.
--Sulphuric Acid. --Sulphurous Acid. --Decomposition of Sulphur.
--Sulphurated Hydrogen Gas. --Harrogate, or Hydro-sulphurated
Waters. --Phosphorus. --History of its Discovery. --Its Combustion
in Oxygen Gas. --Phosphoric Acid. --Phosphorus Acid. --Eudiometer.
--Combination of Phosphorus with Sulphur. --Phosphorated Hydrogen
Gas. --Nomenclature of Binary Compounds. --Phosphoret of Lime
burning under Water.


CONVERSATION IX.

  ON CARBON.                                                       282

Method of obtaining pure Charcoal. --Method of making common
Charcoal. --Pure Carbon not to be obtained by Art. --Diamond.
--Properties of Carbon. --Combustion of Carbon. --Production of
Carbonic Acid Gas. --Carbon susceptible of only one Degree of
Acidification. --Gaseous Oxyd of Carbon. --Of Seltzer Water and
other Mineral Waters. --Effervescence. --Decomposition of Water by
Carbon. --Of Fixed and Essential Oils. --Of the Combustion of Lamps
and Candles. --Vegetable Acids. --Of the Power of Carbon to revive
Metals.


CONVERSATION X.

  ON METALS.                                                       314

Natural History of Metals. --Of Roasting, Smelting, &c. --Oxydation
of metals by the Atmosphere. --Change of Colours produced by
different degrees of Oxydation. --Combustion of Metals. --Perfect
Metals burnt by Electricity only. --Some Metals revived by Carbon
and other Combustibles. --Perfect Metals revived by Heat alone. --Of
the Oxydation of certain Metals by the Decomposition of Water. Power
of Acids to promote this Effect. --Oxydation of Metals by Acids.
--Metallic Neutral Salts. --Previous oxydation of the Metal
requisite. --Crystallisation. --Solution distinguished from
Dissolution. --Five metals susceptible of acidification. --Meteoric
Stones. --Alloys, Soldering, Plating, &c. --Of Arsenic, and of the
caustic Effects of Oxygen. --Of Verdigris, Sympathetic Ink, &c. --Of
the new Metals discovered by Sir H. Davy.


  Contents Of
  _The Second Volume_.

  ON COMPOUND BODIES.


CONVERSATION XIII.
                                                                  Page

  ON THE ATTRACTION OF COMPOSITION.                                  1

Of the laws which regulate the Phenomena of the Attraction of
Composition. --1. It takes place only between Bodies of a different
Nature. --2. Between the most minute Particles only. --3. Between 2,
3, 4, or more Bodies. --Of Compound or Neutral Salts. --4. Produces
a Change of Temperature. --5. The Properties which characterise
Bodies in their separate State, destroyed by Combination. --6. The
Force of Attraction estimated by that which is required by the
Separation of the Constituents. --7. Bodies have amongst themselves
different Degrees of Attraction. --Of simple elective and double
elective Attractions. --Of quiescent and divellent Forces. --Law of
definite Proportions. --Decomposition of Salts by Voltaic
Electricity.


CONVERSATION XIV.

  ON ALKALIES.                                                      19

Of the Composition and general Properties of the Alkalies. --Of
Potash. --Manner of preparing it. --Pearlash. --Soap. --Carbonat of
Potash. --Chemical Nomenclature. --Solution of Potash. --Of Glass.
--Of Nitrat of Potash or Saltpetre. --Effect of Alkalies on
Vegetable Colours. --Of Soda. --Of Ammonia or Volatile Alkali.
--Muriat of Ammonia. --Ammoniacal Gas. --Composition of Ammonia.
--Hartshorn and Sal Volatile. --Combustion of Ammoniacal Gas.


CONVERSATION XV.

  ON EARTHS.                                                        44

Composition of the Earths. --Of their Incombustibility. --Form the
Basis of all Minerals. --Their Alkaline Properties. --Silex; its
Properties and Uses in the Arts. --Alumine; its Uses in Pottery, &c.
--Alkaline Earths. --Barytes. --Lime; its extensive chemical
Properties and Uses in the Arts. --Magnesia. --Strontian.


CONVERSATION XVI.

  ON ACIDS.                                                         69

Nomenclature of the Acids. --Of the Classification of Acids. --1st
Class --Acids of simple and known Radicals, or Mineral Acids.
--2d Class --Acids of double Radicals, or Vegetable Acids.
--3d Class --Acids of triple Radicals or Animal Acids. --Of the
Decomposition of Acids of the 1st Class by Combustible bodies.


CONVERSATION XVII.

  OF THE SULPHURIC AND PHOSPHORIC ACIDS: OR, THE COMBINATIONS OF
  OXYGEN WITH SULPHUR AND WITH PHOSPHORUS; AND OF THE SULPHATS
  AND PHOSPHATS.                                                    80

Of the Sulphuric Acid. --Combustion of Animal or Vegetable Bodies by
this Acid. --Method of preparing it. -- The Sulphurous Acid obtained
in the Form of Gas. --May be obtained from Sulphuric Acid. --May be
reduced to Sulphur. --Is absorbable by Water. --Destroys Vegetable
Colours. --Oxyd of Sulphur. --Of Salts in general. --Sulphats.
--Sulphat of Potash, or Sal Polychrest. --Cold produced by the
melting of Salts. --Sulphat of Soda, or Glauber's Salt. --Heat
evolved during the Formation of Salts. --Crystallisation of Salts.
--Water of Crystallisation. --Efflorescence and Deliquescence of
Salts. --Sulphat of Lime, Gypsum or Plaister of Paris. --Sulphat of
Magnesia. --Sulphat of Alumine, or Alum. --Sulphat of Iron. --Of
Ink. --Of the Phosphoric and Phosphorous Acids. --Phosphorus
obtained from Bones. --Phosphat of Lime.


CONVERSATION XVIII.

  OF THE NITRIC AND CARBONIC ACIDS: OR THE COMBINATION OF
  OXYGEN WITH NITROGEN AND WITH CARBON; AND OF THE NITRATS AND
  CARBONATS.                                                       100

Nitrogen susceptible of various Degrees of Acidification. --Of the
Nitric Acid. --Its Nature and Composition discovered by
Mr. Cavendish. --Obtained from Nitrat of Potash. --Aqua Fortis.
--Nitric Acid may be converted into Nitrous Acid. --Nitric Oxyd Gas.
--Its Conversion into Nitrous Acid Gas. --Used as an Eudiometrical
Test. --Gaseous Oxyd of Nitrogen, or exhilarating Gas, obtained from
Nitrat of Ammonia. --Its singular Effects on being respired.
--Nitrats. --Of Nitrat of Potash, Nitre or Saltpetre. --Of
Gunpowder. --Causes of Detonation. --Decomposition of Nitre.
--Deflagration. --Nitrat of Ammonia. --Nitrat of Silver. --Of the
Carbonic Acid. --Formed by the Combustion of Carbon. --Constitutes a
component Part of the Atmosphere. --Exhaled in some Caverns.
--Grotto del Cane. --Great Weight of this Gas. --Produced from
calcareous Stones by Sulphuric Acid. --Deleterious Effects of this
Gas when respired. --Sources which keep up a Supply of this Gas in
the Atmosphere. --Its Effects on Vegetation. --Of the Carbonats of
Lime; Marble, Chalk, Shells, Spars, and calcareous Stones.


CONVERSATION XIX.

  ON THE BORACIC, FLUORIC, MURIATIC, AND OXYGENATED MURIATIC ACIDS;
  AND ON MURIATS.                                                  131

On the Boracic Acid. --Its Decomposition by Sir H. Davy. --Its Basis
Boracium. --Its Recomposition. --Its Uses in the Arts. --Borax or
Borat of Soda. --Of the Fluoric Acid. --Obtained from Fluor;
corrodes Siliceous Earth; its supposed Composition. --Fluorine; its
supposed Basis. --Of the Muriatic Acid. --Obtained from Muriats.
--Its gaseous Form. --Is absorbable by Water. --Its Decomposition.
--Is susceptible of a stronger Degree of Oxygenation. --Oxygenated
Muriatic Acid. --Its gaseous Form and other Properties. --Combustion
of Bodies in this Gas. --It dissolves Gold. --Composition of Aqua
Regia. --Oxygenated Muriatic Acid destroys all Colours. --Sir H.
Davy's Theory of the Nature of Muriatic and Oxymuriatic Acid.
--Chlorine. --Used for Bleaching and for Fumigations. --Its
offensive Smell, &c. --Muriats. --Muriat of Soda, or common Salt.
--Muriat of Ammonia. --Oxygenated Muriat of Potash. --Detonates with
Sulphur, Phosphorus, &c. --Experiment of burning Phosphorus under
Water by means of this Salt and of Sulphuric Acid.


CONVERSATION XX.

  ON THE NATURE AND COMPOSITION OF VEGETABLES.                     162

Of organised Bodies. --Of the Functions of Vegetables. --Of the
Elements of Vegetables. --Of the Materials of Vegetables. --Analysis
of Vegetables. --Of Sap. --Mucilage, or Gum. --Sugar. --Manna, and
Honey. --Gluten. --Vegetable Oils. --Fixed Oils, Linseed, Nut, and
Olive Oils. --Volatile Oils, forming Essences and Perfumes.
--Camphor. --Resins and Varnishes. --Pitch, Tar, Copal, Mastic, &c.
--Gum Resins. --Myrrh, Assafoetida, &c. --Caoutchouc, or Gum Elastic.
--Extractive colouring Matter; its Use in the Arts of Dyeing and
Painting. --Tannin; its Use in the Art of preparing Leather. --Woody
Fibre. --Vegetable Acids. --The Alkalies and Salts contained in
Vegetables.


CONVERSATION XXI.

  ON THE DECOMPOSITION OF VEGETABLES.                              202

Of Fermentation in general. --Of the Saccharine Fermentation, the
Product of which is Sugar. --Of the Vinous Fermentation, the Product
of which is Wine. --Alcohol, or Spirit of Wine. --Analysis of Wine
by Distillation. --Of Brandy, Rum, Arrack, Gin, &c. --Tartrit of
Potash, or Cream of Tartar. --Liqueurs. --Chemical Properties of
Alcohol. --Its Combustion. --Of Ether. --Of the Acetous
Fermentation, the Product of which is Vinegar. --Fermentation of
Bread. --Of the Putrid Fermentation, which reduces Vegetables to
their Elements. --Spontaneous Succession of these Fermentations.
--Of Vegetables said to be petrified. --Of Bitumens: Naphtha,
Asphaltum, Jet, Coal, Succin, or Yellow Amber. --Of Fossil Wood,
Peat, and Turf.


CONVERSATION XXII.

  HISTORY OF VEGETATION.                                           243

Connexion between the Vegetable and Animal Kingdoms. --Of Manures.
--Of Agriculture. --Inexhaustible Sources of Materials for the
Purposes of Agriculture. --Of sowing Seed. --Germination of the
Seed. --Function of the Leaves of Plants. --Effects of Light and Air
on Vegetation. --Effects of Water on Vegetation. --Effects of
Vegetation on the Atmosphere. --Formation of Vegetable Materials by
the Organs of Plants. --Vegetable Heat. --Of the Organs of Plants.
--Of the Bark, consisting of Epidermis, Parenchyma, and Cortical
Layers. --Of Alburnum, or Wood. --Leaves, Flowers, and Seeds.
--Effects of the Season on Vegetation. --Vegetation of Evergreens in
Winter.


CONVERSATION XXIII.

  ON THE COMPOSITION OF ANIMALS.                                   276

Elements of Animals. --Of the principal Materials of Animals, viz.
--Gelatine, Albumen, Fibrine, Mucus. --Of Animal Acids. --Of Animal
Colours, Prussian Blue, Carmine, and Ivory Black.


CONVERSATION XXIV.

  ON THE ANIMAL ECONOMY.                                           297

Of the principal Animal Organs. --Of Bones, Teeth, Horns, Ligaments,
and Cartilage. --Of the Muscles, constituting the Organs of Motion.
--Of the Vascular System, for the Conveyance of Fluids. --Of the
Glands, for the Secretion of Fluids. --Of the Nerves, constituting
the Organs of Sensation. --Of the Cellular Substance which connects
the several Organs. --Of the Skin.


CONVERSATION XXV.

  ON ANIMALISATION, NUTRITION, AND RESPIRATION.                    314

Digestion. --Solvent Power of the Gastric Juice. --Formation of a
Chyle. --Its Assimilation, or Conversion into Blood. --Of
Respiration. --Mechanical Process of Respiration. --Chemical Process
of Respiration. --Of the Circulation of the Blood. --Of the
Functions of the Arteries, the Veins, and the Heart. --Of the Lungs.
--Effects of Respiration on the Blood.


CONVERSATION XXVI.

  ON ANIMAL HEAT; AND OF VARIOUS ANIMAL PRODUCTS.                  336

Of the Analogy of Combustion and Respiration. --Animal Heat evolved
in the Lungs. --Animal Heat evolved in the Circulation. --Heat
produced by Fever. --Perspiration. --Heat produced by Exercise.
--Equal Temperature of Animals at all Seasons. --Power of the Animal
Body to resist the Effects of Heat. --Cold produced by Perspiration.
--Respiration of Fish and of Birds. --Effects of Respiration on
Muscular Strength. --Of several Animal Products, viz. Milk, Butter,
and Cheese; Spermaceti; Ambergris; Wax; Lac; Silk; Musk; Civet;
Castor. --Of the putrid Fermentation. --Conclusion.




CONVERSATIONS

ON

CHEMISTRY.




CONVERSATION I.

ON THE GENERAL PRINCIPLES OF CHEMISTRY.


MRS. B.

As you have now acquired some elementary notions of NATURAL PHILOSOPHY,
I am going to propose to you another branch of science, to which I am
particularly anxious that you should devote a share of your attention.
This is CHEMISTRY, which is so closely connected with Natural
Philosophy, that the study of the one must be incomplete without some
knowledge of the other; for, it is obvious that we can derive but a very
imperfect idea of bodies from the study of the general laws by which
they are governed, if we remain totally ignorant of their intimate
nature.

CAROLINE.

To confess the truth, Mrs. B., I am not disposed to form a very
favourable idea of chemistry, nor do I expect to derive much
entertainment from it. I prefer the sciences which exhibit nature on a
grand scale, to those that are confined to the minutiae of petty details.
Can the studies which we have lately pursued, the general properties of
matter, or the revolutions of the heavenly bodies, be compared to the
mixing up of a few insignificant drugs? I grant, however, there may be
entertaining experiments in chemistry, and should not dislike to try
some of them: the distilling, for instance, of lavender, or rose
water . . . . . .

MRS. B.

I rather imagine, my dear Caroline, that your want of taste for
chemistry proceeds from the very limited idea you entertain of its
object. You confine the chemist's laboratory to the narrow precincts of
the apothecary's and perfumer's shops, whilst it is subservient to an
immense variety of other useful purposes. Besides, my dear, chemistry is
by no means confined to works of art. Nature also has her laboratory,
which is the universe, and there she is incessantly employed in chemical
operations. You are surprised, Caroline, but I assure you that the most
wonderful and the most interesting phenomena of nature are almost all of
them produced by chemical powers. What Bergman, in the introduction to
his history of chemistry, has said of this science, will give you a more
just and enlarged idea of it. The knowledge of nature may be divided, he
observes, into three periods. The first was that in which the attention
of men was occupied in learning the external forms and characters of
objects, and this is called _Natural History_. In the second, they
considered the effects of bodies acting on each other by their
mechanical power, as their weight and motion, and this constitutes the
science of _Natural Philosophy_. The third period is that in which the
properties and mutual action of the elementary parts of bodies was
investigated. This last is the science of CHEMISTRY, and I have no doubt
you will soon agree with me in thinking it the most interesting.

You may easily conceive, therefore, that without entering into the
minute details of practical chemistry, a woman may obtain such a
knowledge of the science as will not only throw an interest on the
common occurrences of life, but will enlarge the sphere of her ideas,
and render the contemplation of nature a source of delightful
instruction.

CAROLINE.

If this is the case, I have certainly been much mistaken in the notion I
had formed of chemistry. I own that I thought it was chiefly confined to
the knowledge and preparation of medicines.

MRS. B.

That is only a branch of chemistry which is called Pharmacy; and, though
the study of it is certainly of great importance to the world at large,
it belongs exclusively to professional men, and is therefore the last
that I should advise you to pursue.

EMILY.

But, did not the chemists formerly employ themselves in search of the
philosopher's stone, or the secret of making gold?

MRS. B.

These were a particular set of misguided philosophers, who dignified
themselves with the name of Alchemists, to distinguish their pursuits
from those of the common chemists, whose studies were confined to the
knowledge of medicines.

But, since that period, chemistry has undergone so complete a
revolution, that, from an obscure and mysterious art, it is now become a
regular and beautiful science, to which art is entirely subservient. It
is true, however, that we are indebted to the alchemists for many very
useful discoveries, which sprung from their fruitless attempts to make
gold, and which, undoubtedly, have proved of infinitely greater
advantage to mankind than all their chimerical pursuits.

The modern chemists, instead of directing their ambition to the vain
attempt of producing any of the original substances in nature, rather
aim at analysing and imitating her operations, and have sometimes
succeeded in forming combinations, or effecting decompositions, no
instances of which occur in the chemistry of Nature. They have little
reason to regret their inability to make gold, whilst, by their
innumerable inventions and discoveries, they have so greatly stimulated
industry and facilitated labour, as prodigiously to increase the
luxuries as well as the necessaries of life.

EMILY.

But, I do not understand by what means chemistry can facilitate labour;
is not that rather the province of the mechanic?

MRS. B.

There are many ways by which labour may be rendered more easy,
independently of mechanics; but even the machine, the most wonderful in
its effects, the Steam-engine, cannot be understood without the
assistance of chemistry. In agriculture, a chemical knowledge of the
nature of soils, and of vegetation, is highly useful; and, in those arts
which relate to the comforts and conveniences of life, it would be
endless to enumerate the advantages which result from the study of this
science.

CAROLINE.

But, pray, tell us more precisely in what manner the discoveries of
chemists have proved so beneficial to society?

MRS. B.

That would be an injudicious anticipation; for you would not comprehend
the nature of such discoveries and useful applications, as well as you
will do hereafter. Without a due regard to method, we cannot expect to
make any progress in chemistry. I wish to direct your observations
chiefly to the chemical operations of Nature; but those of Art are
certainly of too high importance to pass unnoticed. We shall therefore
allow them also some share of our attention.

EMILY.

Well, then, let us now set to work regularly. I am very anxious to
begin.

MRS. B.

The object of chemistry is to obtain a knowledge of the intimate nature
of bodies, and of their mutual action on each other. You find therefore,
Caroline, that this is no narrow or confined science, which comprehends
every thing material within our sphere.

CAROLINE.

On the contrary, it must be inexhaustible; and I am a loss to conceive
how any proficiency can be made in a science whose objects are so
numerous.

MRS. B.

If every individual substance were formed of different materials, the
study of chemistry would, indeed, be endless; but you must observe that
the various bodies in nature are composed of certain elementary
principles, which are not very numerous.

CAROLINE.

Yes; I know that all bodies are composed of fire, air, earth, and water;
I learnt that many years ago.

MRS. B.

But you must now endeavour to forget it. I have already informed you
what a great change chemistry has undergone since it has become a
regular science. Within these thirty years especially, it has
experienced an entire revolution, and it is now proved, that neither
fire, air, earth, nor water, can be called elementary bodies. For an
elementary body is one that has never been decomposed, that is to say,
separated into other substances; and fire, air, earth, and water, are
all of them susceptible of decomposition.

EMILY.

I thought that decomposing a body was dividing it into its minutest
parts. And if so, I do not understand why an elementary substance is not
capable of being decomposed, as well as any other.

MRS. B.

You have misconceived the idea of _decomposition_; it is very different
from mere _division_. The latter simply reduces a body into parts, but
the former separates it into the various ingredients, or materials, of
which it is composed. If we were to take a loaf of bread, and separate
the several ingredients of which it is made, the flour, the yeast, the
salt, and the water, it would be very different from cutting or
crumbling the loaf into pieces.

EMILY.

I understand you now very well. To decompose a body is to separate from
each other the various elementary substances of which it consists.

CAROLINE.

But flour, water, and other materials of bread, according to our
definition, are not elementary substances?

MRS. B.

No, my dear; I mentioned bread rather as a familiar comparison, to
illustrate the idea, than as an example.

The elementary substances of which a body is composed are called the
_constituent_ parts of that body; in decomposing it, therefore, we
separate its constituent parts. If, on the contrary, we divide a body by
chopping it to pieces, or even by grinding or pounding it to the finest
powder, each of these small particles will still consist of a portion of
the several constituent parts of the whole body: these are called the
_integrant_ parts; do you understand the difference?

EMILY.

Yes, I think, perfectly. We _decompose_ a body into its _constituent_
parts; and _divide_ it into its _integrant_ parts.

MRS. B.

Exactly so. If therefore a body consists of only one kind of substance,
though it may be divided into its integrant parts, it is not possible to
decompose it. Such bodies are therefore called _simple_ or _elementary_,
as they are the elements of which all other bodies are composed.
_Compound bodies_ are such as consist of more than one of these
elementary principles.

CAROLINE.

But do not fire, air, earth, and water, consist, each of them, but of
one kind of substance?

MRS. B.

No, my dear; they are every one of them susceptible of being separated
into various simple bodies. Instead of four, chemists now reckon upwards
of forty elementary substances. The existence of most of these is
established by the clearest experiments; but, in regard to a few of
them, particularly the most subtle agents of nature, _heat_, _light_,
and _electricity_, there is yet much uncertainty, and I can only give
you the opinion which seems most probably deduced from the latest
discoveries. After I have given you a list of the elementary bodies,
classed according to their properties, we shall proceed to examine each
of them separately, and then consider them in their combinations with
each other.

Excepting the more general agents of nature, heat, light, and
electricity, it would seem that the simple form of bodies is that of a
metal.

CAROLINE.

You astonish me! I thought the metals were only one class of minerals,
and that there were besides, earths, stones, rocks, acids, alkalies,
vapours, fluids, and the whole of the animal and vegetable kingdoms.

MRS. B.

You have made a tolerably good enumeration, though I fear not arranged
in the most scientific order. All these bodies, however, it is now
strongly believed, may be ultimately resolved into metallic substances.
Your surprise at this circumstance is not singular, as the decomposition
of some of them, which has been but lately accomplished, has excited the
wonder of the whole philosophical world.

But to return to the list of simple bodies--these being usually found in
combination with oxygen, I shall class them according to their
properties when so combined. This will, I think, facilitate their future
investigation.

EMILY.

Pray what is oxygen?

MRS. B.

A simple body; at least one that is supposed to be so, as it has never
been decomposed. It is always found united with the negative
electricity. It will be one of the first of the elementary bodies whose
properties I shall explain to you, and, as you will soon perceive, it is
one of the most important in nature; but it would be irrelevant to enter
upon this subject at present. We must now confine our attention to the
enumeration and classification of the simple bodies in general. They may
be arranged as follows:

CLASS I.

_Comprehending the imponderable agents, viz._

  HEAT or CALORIC,
  LIGHT,
  ELECTRICITY.

CLASS II.

_Comprehending agents capable of uniting with inflammable bodies, and in
most instances of effecting their combustion._

  OXYGEN,
  CHLORINE,
  IODINE.*

    [Footnote *: It has been questioned by some eminent chemists,
    whether these two last agents should not be classed among the
    inflammable bodies, as they are capable of combining with oxygen,
    as well as with inflammable bodies. But they seem to be more
    distinctly characterised by their property of supporting
    combustion than by any other quality.]

CLASS III.

_Comprehending bodies capable of uniting with oxygen, and, forming with
it various compounds. This class may be divided as follows:_

DIVISION 1.

  HYDROGEN, _forming_ water.

DIVISION 2.

_Bodies forming acids._

  NITROGEN,    _forming_ nitric acid.
  SULPHUR,     _forming_ sulphuric acid.
  PHOSPHORUS,  _forming_ phosphoric acid.
  CARBON,      _forming_ carbonic acid.
  BORACIUM,    _forming_ boracic acid.
  FLUORIUM,    _forming_ fluoric acid.
  MURIATIUM,   _forming_ muriatic acid.

DIVISION 3.

_Metallic bodies forming alkalies._

  POTASSIUM,  _forming_ potash.
  SODIUM,     _forming_ soda.
  AMMONIUM,   _forming_ ammonia.

DIVISION 4.

_Metallic bodies forming earths._

  CALCIUM, _or metal forming_ lime.
  MAGNIUM,    _forming_ magnesia.
  BARIUM,     _forming_ barytes.
  STRONTIUM,  _forming_ strontites.
  SILICIUM,   _forming_ silex.
  ALUMIUM,    _forming_ alumine.
  YTTRIUM,    _forming_ yttria.
  GLUCIUM,    _forming_ glucina.
  ZIRCONIUM,  _forming_ zirconi.*

    [Footnote *: Of all these earths, three or four only have as yet
    been distinctly decomposed.]

DIVISION 5.

_Metals, either naturally metallic, or yielding their oxygen to carbon
or to heat alone._

_Subdivision 1._

_Malleable Metals._

  GOLD,
  PLATINA,
  PALLADIUM,
  SILVER*
  MERCURY**
  TIN,
  COPPER,
  IRON,
  LEAD,
  NICKEL,
  ZINC.

    [Footnote *: These first four metals have commonly been
    distinguished by the appellation of perfect or noble metals, on
    account of their possessing the characteristic properties of
    ductility, malleability, inalterability, and great specific
    gravity, in an eminent degree.]

    [Footnote **: Mercury, in its liquid state, cannot, of course,
    be called a malleable metal. But when frozen, it possesses a
    considerable degree of malleability.]

_Subdiv. 2._

_Brittle Metals._

  ARSENIC,
  BISMUTH,
  ANTIMONY,
  MANGANESE,
  TELLURIUM,
  COBALT,
  TUNGSTEN,
  MOLYBDENUM,
  TITANIUM,
  CHROME,
  URANIUM,
  COLUMBIUM _or_ TANTALIUM,
  IRIDIUM,
  OSMIUM,
  RHODIUM.*

    [Footnote *: These last four or five metallic bodies are placed
    under this class for the sake of arrangement, though some of their
    properties have not been yet fully investigated.]

CAROLINE.

Oh, what a formidable list! You will have much to do to explain it,
Mrs. B.; for I assure you it is perfectly unintelligible to me, and I
think rather perplexes than assists me.

MRS. B.

Do not let that alarm you, my dear; I hope that hereafter this
classification will appear quite clear, and, so far from perplexing you,
will assist you in arranging your ideas. It would be in vain to attempt
forming a division that would appear perfectly clear to a beginner: for
you may easily conceive that a chemical division being necessarily
founded on properties with which you are almost wholly unacquainted, it
is impossible that you should at once be able to understand its meaning
or appreciate its utility.

But, before we proceed further, it will be necessary to give you some
idea of chemical attraction, a power on which the whole science depends.

_Chemical Attraction_, or the _Attraction of Composition_, consists in
the peculiar tendency which bodies of a different nature have to unite
with each other. It is by this force that all the compositions, and
decompositions, are effected.

EMILY.

What is the difference between chemical attraction, and the attraction
of cohesion, or of aggregation, which you often mentioned to us, in
former conversations?

MRS. B.

The attraction of cohesion exists only between particles of the _same_
nature, whether simple or compound; thus it unites the particles of a
piece of metal which is a simple substance, and likewise the particles
of a loaf of bread which is a compound. The attraction of composition,
on the contrary, unites and maintains, in a state of combination,
particles of a _dissimilar_ nature; it is this power that forms each of
the compound particles of which bread consists; and it is by the
attraction of cohesion that all these particles are connected into a
single mass.

EMILY.

The attraction of cohesion, then, is the power which unites the
integrant particles of a body: the attraction of composition that which
combines the constituent particles. Is it not so?

MRS. B.

Precisely: and observe that the attraction of cohesion unites particles
of a similar nature, without changing their original properties; the
result of such an union, therefore, is a body of the same kind as the
particles of which it is formed; whilst the attraction of composition,
by combining particles of a dissimilar nature, produces compound bodies,
quite different from any of their constituents. If, for instance, I pour
on the piece of copper, contained in this glass, some of this liquid
(which is called nitric acid), for which it has a strong attraction,
every particle of the copper will combine with a particle of acid, and
together they will form a new body, totally different from either the
copper or the acid.

Do you observe the internal commotion that already begins to take place?
It is produced by the combination of these two substances; and yet the
acid has in this case to overcome not only the resistance which the
strong cohesion of the particles of copper opposes to their combination
with it, but also to overcome the weight of the copper, which makes it
sink to the bottom of the glass, and prevents the acid from having such
free access to it as it would if the metal were suspended in the liquid.

EMILY.

The acid seems, however, to overcome both these obstacles without
difficulty, and appears to be very rapidly dissolving the copper.

MRS. B.

By this means it reduces the copper into more minute parts than could
possibly be done by any mechanical power. But as the acid can act only
on the surface of the metal, it will be some time before the union of
these two bodies will be completed.

You may, however, already see how totally different this compound is
from either of its ingredients. It is neither colourless, like the acid,
nor hard, heavy, and yellow like the copper. If you tasted it, you would
no longer perceive the sourness of the acid. It has at present the
appearance of a blue liquid; but when the union is completed, and the
water with which the acid is diluted is evaporated, the compound will
assume the form of regular crystals, of a fine blue colour, and
perfectly transparent*. Of these I can shew you a specimen, as I have
prepared some for that purpose.

    [Footnote *: These crystals are more easily obtained from a
    mixture of sulphuric with a little nitric acid.]

CAROLINE.

How very beautiful they are, in colour, form, and transparency!

EMILY.

Nothing can be more striking than this example of chemical attraction.

MRS. B.

The term _attraction_ has been lately introduced into chemistry as a
substitute for the word _affinity_, to which some chemists have
objected, because it originated in the vague notion that chemical
combinations depended upon a certain resemblance, or relationship,
between particles that are disposed to unite; and this idea is not only
imperfect, but erroneous, as it is generally particles of the most
dissimilar nature, that have the greatest tendency to combine.

CAROLINE.

Besides, there seems to be no advantage in using a variety of terms to
express the same meaning; on the contrary it creates confusion; and as
we are well acquainted with the term Attraction in natural philosophy,
we had better adopt it in chemistry likewise.

MRS. B.

If you have a clear idea of the meaning, I shall leave you at liberty to
express it in the terms you prefer. For myself, I confess that I think
the word Attraction best suited to the general law that unites the
integrant particles of bodies; and Affinity better adapted to that which
combines the constituent particles, as it may convey an idea of the
preference which some bodies have for others, which the term _attraction
of composition_ does not so well express.

EMILY.

So I think; for though that preference may not result from any
relationship, or similitude, between the particles (as you say was once
supposed), yet, as it really exists, it ought to be expressed.

MRS. B.

Well, let it be agreed that you may use the terms _affinity_, _chemical
attraction_ and _attraction of composition_, indifferently, provided you
recollect that they have all the same meaning.

EMILY.

I do not conceive how bodies can be decomposed by chemical attraction.
That this power should be the means of composing them, is very obvious;
but that it should, at the same time, produce exactly the contrary
effect, appears to me very singular.

MRS. B.

To decompose a body is, you know, to separate its constituent parts,
which, as we have just observed, cannot be done by mechanical means.

EMILY.

No: because mechanical means separate only the integrant particles; they
act merely against the attraction of cohesion, and only divide a
compound into smaller parts.

MRS. B.

The decomposition of a body is performed by chemical powers. If you
present to a body composed of two principles, a third, which has a
greater affinity for one of them than the two first have for each other,
it will be decomposed, that is, its two principles will be separated by
means of the third body. Let us call two ingredients, of which the body
is composed, A and B. If we present to it another ingredient C, which
has a greater affinity for B than that which unites A and B, it
necessarily follows that B will quit A to combine with C. The new
ingredient, therefore, has effected a decomposition of the original body
A B; A has been left alone, and a new compound, B C, has been formed.

EMILY.

We might, I think, use the comparison of two friends, who were very
happy in each other's society, till a third disunited them by the
preference which one of them gave to the new-comer.

MRS. B.

Very well. I shall now show you how this takes place in chemistry.

Let us suppose that we wish to decompose the compound we have just
formed by the combination of the two ingredients, copper and nitric
acid; we may do this by presenting to it a piece of iron, for which the
acid has a stronger attraction than for copper; the acid will,
consequently, quit the copper to combine with the iron, and the copper
will be what the chemists call _precipitated_, that is to say, it will
be thrown down in its separate state, and reappear in its simple form.

In order to produce this effect, I shall dip the blade of this knife
into the fluid, and, when I take it out, you will observe, that, instead
of being wetted with a bluish liquid, like that contained in the glass,
it will be covered with a thin coat of copper.

CAROLINE.

So it is really! but then is it not the copper, instead of the acid,
that has combined with the iron blade?

MRS. B.

No; you are deceived by appearances: it is the acid which combines with
the iron, and, in so doing, deposits or precipitates the copper on the
surface of the blade.

EMILY.

But, cannot three or more substances combine together, without any of
them being precipitated?

MRS. B.

That is sometimes the case; but, in general, the stronger affinity
destroys the weaker; and it seldom happens that the attraction of
several substances for each other is so equally balanced as to produce
such complicated compounds.

CAROLINE.

But, pray, Mrs. B., what is the cause of the chemical attraction of
bodies for each other? It appears to me more extraordinary or unnatural,
if I may use the expression, than the attraction of cohesion, which
unites particles of a similar nature.

MRS. B.

Chemical attraction may, like that of cohesion or gravitation, be one of
the powers inherent in matter which, in our present state of knowledge,
admits of no other satisfactory explanation than an immediate reference
to a divine cause. Sir H. Davy, however, whose important discoveries
have opened such improved views in chemistry, has suggested an
hypothesis which may throw great light upon that science. He supposes
that there are two kinds of electricity, with one or other of which all
bodies are united. These we distinguish by the names of _positive_ and
_negative_ electricity; those bodies are disposed to combine, which
possess opposite electricities, as they are brought together by the
attraction which these electricities have for each other. But, whether
this hypothesis be altogether founded on truth or not, it is impossible
to question the great influence of electricity in chemical combinations.

EMILY.

So, that we must suppose that the two electricities always attract each
other, and thus compel the bodies in which they exist to combine?

CAROLINE.

And may not this be also the cause of the attraction of cohesion?

MRS. B.

No, for in particles of the same nature the same electricities must
prevail, and it is only the different or opposite electric fluids that
attract each other.

CAROLINE.

These electricities seem to me to be a kind of chemical spirit, which
animates the particles of bodies, and draws them together.

EMILY.

If it is known, then, with which of the electricities bodies are united,
it can be inferred which will, and which will not, combine together?

MRS. B.

Certainly. --I should not omit to mention, that some doubts have been
entertained whether electricity be really a material agent, or whether
it might not be a power inherent in bodies, similar to, or, perhaps
identical with, attraction.

EMILY.

But what then would be the electric spark which is visible, and must
therefore be really material?

MRS. B.

What we call the electric spark, may, Sir H. Davy says, be merely the
heat and light, or fire produced by the chemical combinations with which
these phenomena are always connected. We will not, however, enter more
fully on this important subject at present, but reserve the principal
facts which relate to it to a future conversation.

Before we part, however, I must recommend you to fix in your memory the
names of the simple bodies, against our next interview.




CONVERSATION II.

ON LIGHT AND HEAT OR CALORIC.


CAROLINE.

We have learned by heart the names of all the simple bodies which you
have enumerated, and we are now ready to enter on the examination of
each of them successively. You will begin, I suppose, with LIGHT?

MRS. B.

Respecting the nature of light we have little more than conjectures. It
is considered by most philosophers as a real substance, immediately
emanating from the sun, and from all luminous bodies, from which it is
projected in right lines with prodigious velocity. Light, however, being
imponderable, it cannot be confined and examined by itself; and
therefore it is to the effects it produces on other bodies, rather than
to its immediate nature, that we must direct our attention.

The connection between light and heat is very obvious; indeed, it is
such, that it is extremely difficult to examine the one independently of
the other.

EMILY.

But, is it possible to separate light from heat; I thought they were
only different degrees of the same thing, fire?

MRS. B.

I told you that fire was not now considered as a simple element. Whether
light and heat be altogether different agents, or not, I cannot pretend
to decide; but, in many cases, light may be separated from heat. The
first discovery of this was made by a celebrated Swedish chemist,
Scheele. Another very striking illustration of the separation of heat
and light was long after pointed out by Dr. Herschell. This philosopher
discovered that these two agents were emitted in the rays of the sun,
and that heat was less refrangible than light; for, in separating the
different coloured rays of light by a prism (as we did some time ago),
he found that the greatest heat was beyond the spectrum, at a little
distance from the red rays, which, you may recollect, are the least
refrangible.

EMILY.

I should like to try that experiment.

MRS. B.

It is by no means an easy one: the heat of a ray of light, refracted by
a prism, is so small, that it requires a very delicate thermometer to
distinguish the difference of the degree of heat within and without the
spectrum. For in this experiment the heat is not totally separated from
the light, each coloured ray retaining a certain portion of it, though
the greatest part is not sufficiently refracted to fall within the
spectrum.

EMILY.

I suppose, then, that those coloured rays which are the least
refrangible, retain the greatest quantity of heat?

MRS. B.

They do so.

EMILY.

Though I no longer doubt that light and heat can be separated, Dr.
Herschell's experiment does not appear to me to afford sufficient proof
that they are essentially different; for light, which you call a simple
body, may likewise be divided into the various coloured rays.

MRS. B.

No doubt there must be some difference in the various coloured rays.
Even their chemical powers are different. The blue rays, for instance,
have the greatest effect in separating oxygen from bodies, as was found
by Scheele; and there exist also, as Dr. Wollaston has shown, rays more
refrangible than the blue, which produce the same chemical effect, and,
what is very remarkable, are invisible.

EMILY.

Do you think it possible that heat may be merely a modification of
light?

MRS. B.

That is a supposition which, in the present state of natural philosophy,
can neither be positively affirmed nor denied. Let us, therefore,
instead of discussing theoretical points, be contented with examining
what is known respecting the chemical effects of light.

Light is capable of entering into a kind of transitory union with
certain substances, and this is what has been called phosphorescence.
Bodies that are possessed of this property, after being exposed to the
sun's rays, appear luminous in the dark. The shells of fish, the bones
of land animals, marble, limestone, and a variety of combinations of
earths, are more or less powerfully phosphorescent.

CAROLINE.

I remember being much surprised last summer with the phosphorescent
appearance of some pieces of rotten wood, which had just been dug out of
the ground; they shone so bright that I at first supposed them to be
glow-worms.

EMILY.

And is not the light of a glow-worm of a phosphorescent nature?

MRS. B.

It is a very remarkable instance of phosphorescence in living animals;
this property, however, is not exclusively possessed by the glow-worm.
The insect called the lanthorn-fly, which is peculiar to warm climates,
emits light as it flies, producing in the dark a remarkably sparkling
appearance. But it is more common to see animal matter in a dead state
possessed of a phosphorescent quality; sea fish is often eminently so.

EMILY.

I have heard that the sea has sometimes had the appearance of being
illuminated, and that the light is supposed to proceed from the spawn of
fishes floating on its surface.

MRS. B.

This light is probably owing to that or some other animal matter. Sea
water has been observed to become luminous from the substance of a fresh
herring having been immersed in it; and certain insects, of the Medusa
kind, are known to produce similar effects.

But the strongest phosphorescence is produced by chemical compositions
prepared for the purpose, the most common of which consists of oyster
shells and sulphur, and is known by the name of Canton's Phosphorus.

EMILY.

I am rather surprised, Mrs. B., that you should have said so much of the
light emitted by phosphorescent bodies without taking any notice of that
which is produced by burning bodies.

MRS. B.

The light emitted by the latter is so intimately connected with the
chemical history of combustion, that I must defer all explanation of it
till we come to the examination of that process, which is one of the
most interesting in chemical science.

Light is an agent capable of producing various chemical changes. It is
essential to the welfare both of the animal and vegetable kingdoms; for
men and plants grow pale and sickly if deprived of its salutary
influence. It is likewise remarkable for its property of destroying
colour, which renders it of great consequence in the process of
bleaching.

EMILY.

Is it not singular that light, which in studying optics we were taught
to consider as the source and origin of colours, should have also the
power of destroying them?

CAROLINE.

It is a fact, however, that we every day experience; you know how it
fades the colours of linens and silks.

EMILY.

Certainly. And I recollect that endive is made to grow white instead of
green, by being covered up so as to exclude the light. But by what means
does light produce these effects?

MRS. B.

This I cannot attempt to explain to you until you have obtained a
further knowledge of chemistry. As the chemical properties of light can
be accounted for only in their reference to compound bodies, it would be
useless to detain you any longer on this subject; we may therefore pass
on to the examination of heat, or caloric, with which we are somewhat
better acquainted.

HEAT and LIGHT may be always distinguished by the different sensations
they produce, _Light_ affects the sense of sight; _Caloric_ that of
feeling; the one produces _Vision_, the other the sensation of _Heat_.

Caloric is found to exist in a variety of forms or modifications, and I
think it will be best to consider it under the two following heads, viz.

1. FREE OR RADIANT CALORIC.

2. COMBINED CALORIC.

The first, FREE or RADIANT CALORIC, is also called HEAT OF TEMPERATURE;
it comprehends all heat which is perceptible to the senses, and affects
the thermometer.

EMILY.

You mean such as the heat of the sun, of fire, of candles, of stoves; in
short, of every thing that burns?

MRS. B.

And likewise of things that do not burn, as, for instance, the warmth of
the body; in a word, all heat that is _sensible_, whatever may be its
degree, or the source from which it is derived.

CAROLINE.

What then are the other modifications of caloric? It must be a strange
kind of heat that cannot be perceived by our senses.

MRS. B.

None of the modifications of caloric should properly be called _heat_;
for heat, strictly speaking, is the sensation produced by caloric, on
animated bodies; this word, therefore, in the accurate language of
science, should be confined to express the sensation. But custom has
adapted it likewise to inanimate matter, and we say _the heat of an
oven_, _the heat of the sun_, without any reference to the sensation
which they are capable of exciting.

It was in order to avoid the confusion which arose from thus confounding
the cause and effect, that modern chemists adopted the new word
_caloric_, to denote the principle which produces heat; yet they do not
always, in compliance with their own language, limit the word _heat_ to
the expression of the sensation, since they still frequently employ it
in reference to the other modifications of caloric which are quite
independent of sensation.

CAROLINE.

But you have not yet explained to us what these other modifications of
caloric are.

MRS. B.

Because you are not acquainted with the properties of free caloric, and
you know that we have agreed to proceed with regularity.

One of the most remarkable properties of free caloric is its power of
_dilating_ bodies. This fluid is so extremely subtle, that it enters and
pervades all bodies whatever, forces itself between their particles, and
not only separates them, but frequently drives them asunder to a
considerable distance from each other. It is thus that caloric dilates
or expands a body so as to make it occupy a greater space than it did
before.

EMILY.

The effect it has on bodies, therefore, is directly contrary to that of
the attraction of cohesion; the one draws the particles together, the
other drives them asunder.

MRS. B.

Precisely. There is a continual struggle between the attraction of
aggregation, and the expansive power of caloric; and from the action of
these two opposite forces, result all the various forms of matter, or
degrees of consistence, from the solid, to the liquid and aeriform
state. And accordingly we find that most bodies are capable of passing
from one of these forms to the other, merely in consequence of their
receiving different quantities of caloric.

CAROLINE.

That is very curious; but I think I understand the reason of it. If a
great quantity of caloric is added to a solid body, it introduces itself
between the particles in such a manner as to overcome, in a considerable
degree, the attraction of cohesion; and the body, from a solid, is then
converted into a fluid.

MRS. B.

This is the case whenever a body is fused or melted; but if you add
caloric to a liquid, can you tell me what is the consequence?

CAROLINE.

The caloric forces itself in greater abundance between the particles of
the fluid, and drives them to such a distance from each other, that
their attraction of aggregation is wholly destroyed: the liquid is then
transformed into vapour.

MRS. B.

Very well; and this is precisely the case with boiling water, when it is
converted into steam or vapour, and with all bodies that assume an
aeriform state.

EMILY.

I do not well understand the word aeriform?

MRS. B.

Any elastic fluid whatever, whether it be merely vapour or permanent
air, is called aeriform.

But each of these various states, solid, liquid, and aeriform, admit of
many different degrees of density, or consistence, still arising
(chiefly at least) from the different quantities of caloric the bodies
contain. Solids are of various degrees of density, from that of gold, to
that of a thin jelly. Liquids, from the consistence of melted glue, or
melted metals, to that of ether, which is the lightest of all liquids.
The different elastic fluids (with which you are not yet acquainted) are
susceptible of no less variety in their degrees of density.

EMILY.

But does not every individual body also admit of different degrees of
consistence, without changing its state?

MRS. B.

Undoubtedly; and this I can immediately show you by a very simple
experiment. This piece of iron now exactly fits the frame, or ring, made
to receive it; but if heated red hot, it will no longer do so, for its
dimensions will be so much increased by the caloric that has penetrated
into it, that it will be much too large for the frame.

The iron is now red hot; by applying it to the frame, we shall see how
much it is dilated.

EMILY.

Considerably so indeed! I knew that heat had this effect on bodies, but
I did not imagine that it could be made so conspicuous.

MRS. B.

By means of this instrument (called a Pyrometer) we may estimate, in the
most exact manner, the various dilatations of any solid body by heat.
The body we are now going to submit to trial is this small iron bar;
I fix it to this apparatus, (PLATE I. Fig. 1.) and then heat it by
lighting the three lamps beneath it: when the bar expands, it increases
in length as well as thickness; and, as one end communicates with this
wheel-work, whilst the other end is fixed and immoveable, no sooner does
it begin to dilate than it presses against the wheel-work, and sets in
motion the index, which points out the degrees of dilatation on the
dial-plate.

  [Illustration: Plate I. Vol. I. p. 38.

  Fig. 1. Pyrometer.
  A.A Bar of Metal.
  1.2.3 Lamps burning.
  B.B Wheel work.
  C Index.

  Fig. 2
  A.A Glass tubes with bulbs.
  B.B Glasses of water in which they are immersed.]

EMILY.

This is, indeed, a very curious instrument; but I do not understand the
use of the wheels: would it not be more simple, and answer the purpose
equally well, if the bar, in dilating, pressed against the index, and
put it in motion without the intervention of the wheels?

MRS. B.

The use of the wheels is merely to multiply the motion, and therefore
render the effect of the caloric more obvious; for if the index moved no
more than the bar increased in length, its motion would scarcely be
perceptible; but by means of the wheels it moves in a much greater
proportion, which therefore renders the variations far more conspicuous.

By submitting different bodies to the test of the pyrometer, it is found
that they are far from dilating in the same proportion. Different metals
expand in different degrees, and other kinds of solid bodies vary still
more in this respect. But this different susceptibility of dilatation is
still more remarkable in fluids than in solid bodies, as I shall show
you. I have here two glass tubes, terminated at one end by large bulbs.
We shall fill the bulbs, the one with spirit of wine, the other with
water. I have coloured both liquids, in order that the effect may be
more conspicuous. The spirit of wine, you see, dilates by the warmth of
my hand as I hold the bulb.

EMILY.

It certainly does, for I see it is rising into the tube. But water, it
seems, is not so easily affected by heat; for scarcely any change is
produced on it by the warmth of the hand.

MRS. B.

True; we shall now plunge the bulbs into hot water, (PLATE I. Fig. 2.)
and you will see both liquids rise in the tubes; but the spirit of wine
will ascend highest.

CAROLINE.

How rapidly it expands! Now it has nearly reached the top of the tube,
though the water has hardly begun to rise.

EMILY.

The water now begins to dilate. Are not these glass tubes, with liquids
rising within them, very like thermometers?

MRS. B.

A thermometer is constructed exactly on the same principle, and these
tubes require only a scale to answer the purpose of thermometers: but
they would be rather awkward in their dimensions. The tubes and bulbs of
thermometers, though of various sizes, are in general much smaller than
these; the tube too is hermetically closed, and the air excluded from
it. The fluid most generally used in thermometers is mercury, commonly
called quicksilver, the dilatations and contractions of which correspond
more exactly to the additions, and subtractions, of caloric, than those
of any other fluid.

CAROLINE.

Yet I have often seen coloured spirit of wine used in thermometers.

MRS. B.

The expansions and contractions of that liquid are not quite so uniform
as those of mercury; but in cases in which it is not requisite to
ascertain the temperature with great precision, spirit of wine will
answer the purpose equally well, and indeed in some respects better, as
the expansion of the latter is greater, and therefore more conspicuous.
This fluid is used likewise in situations and experiments in which
mercury would be frozen; for mercury becomes a solid body, like a piece
of lead or any other metal, at a certain degree of cold: but no degree
of cold has ever been known to freeze spirit of wine.

A thermometer, therefore, consists of a tube with a bulb, such as you
see here, containing a fluid whose degrees of dilatation and contraction
are indicated by a scale to which the tube is fixed. The degree which
indicates the boiling point, simply means that, when the fluid is
sufficiently dilated to rise to this point, the heat is such that water
exposed to the same temperature will boil. When, on the other hand, the
fluid is so much condensed as to sink to the freezing point, we know
that water will freeze at that temperature. The extreme points of the
scales are not the same in all thermometers, nor are the degrees always
divided in the same manner. In different countries philosophers have
chosen to adopt different scales and divisions. The two thermometers
most used are those of Fahrenheit, and of Reaumur; the first is
generally preferred by the English, the latter by the French.

EMILY.

The variety of scale must be very inconvenient, and I should think
liable to occasion confusion, when French and English experiments are
compared.

MRS. B.

The inconvenience is but very trifling, because the different gradations
of the scales do not affect the principle upon which thermometers are
constructed. When we know, for instance, that Fahrenheit's scale is
divided into 212 degrees, in which 32 deg. corresponds with the freezing
point, and 212 deg. with the point of boiling water: and that Reaumur's is
divided only into 80 degrees, in which 0 deg. denotes the freezing point,
and 80 deg. that of boiling water, it is easy to compare the two scales
together, and reduce the one into the other. But, for greater
convenience, thermometers are sometimes constructed with both these
scales, one on either side of the tube; so that the correspondence of
the different degrees of the two scales is thus instantly seen. Here
is one of these scales, (PLATE II. Fig. 1.) by which you can at
once perceive that each degree of Reaumur's corresponds to 2-1/4 of
Fahrenheit's division. But I believe the French have, of late, given
the preference to what they call the centigrade scale, in which the
space between the freezing and the boiling point is divided into 100
degrees.

  [Illustration: Plate II. Vol. I. p. 42.

  Fig. 1. Thermometer.
  Fahrenheit's Scale.
  Reaumur's Scale.
  Boiling point of Water
  Freezing point of Water

  Fig. 2. Differential Thermometer.]

CAROLINE.

That seems to me the most reasonable division, and I cannot guess
why the freezing point is called 32 deg., or what advantage is derived
from it.

MRS. B.

There really is no advantage in it; and it originated in a mistaken
opinion of the instrument-maker, Fahrenheit, who first constructed these
thermometers. He mixed snow and salt together, and produced by that
means a degree of cold which he concluded was the greatest possible, and
therefore made his scale begin from that point. Between that and boiling
water he made 212 degrees, and the freezing point was found to be at
32 deg.

EMILY.

Are spirit of wine, and mercury, the only liquids used in the
construction of thermometers?

MRS. B.

I believe they are the only liquids now in use, though some others, such
as linseed oil, would make tolerable thermometers: but for experiments
in which a very quick and delicate test of the changes of temperature is
required, air is the fluid sometimes employed. The bulb of air
thermometers is filled with common air only, and its expansion and
contraction are indicated by a small drop of any coloured liquor, which
is suspended within the tube, and moves up and down, according as the
air within the bulb and tube expands or contracts. But in general, air
thermometers, however sensible to changes of temperature, are by no
means accurate in their indications.

I can, however, show you an air thermometer of a very peculiar
construction, which is remarkably well adapted for some chemical
experiments, as it is equally delicate and accurate in its indications.

CAROLINE.

It looks like a double thermometer reversed, the tube being bent, and
having a large bulb at each of its extremities. (PLATE II. Fig. 2.)

EMILY.

Why do you call it an air thermometer; the tube contains a coloured
liquid?

MRS. B.

But observe that the bulbs are filled with air, the liquid being
confined to a portion of the tube, and answering only the purpose of
showing, by its motion in the tube, the comparative dilatation or
contraction of the air within the bulbs, which afford an indication of
their relative temperature. Thus if you heat the bulb A, by the warmth
of your hand, the fluid will rise towards the bulb B, and the contrary
will happen if you reverse the experiment.

But if, on the contrary, both tubes are of the same temperature, as is
the case now, the coloured liquid, suffering an equal pressure on each
side, no change of level takes place.

CAROLINE.

This instrument appears, indeed, uncommonly delicate. The fluid is set
in motion by the mere approach of my hand.

MRS. B.

You must observe, however, that this thermometer cannot indicate the
temperature of any particular body, or of the medium in which it is
immersed; it serves only to point out the _difference_ of temperature
between the two bulbs, when placed under different circumstances. For
this reason it has been called _differential_ thermometer. You will see
by-and-bye to what particular purposes this instrument applies.

EMILY.

But do common thermometers indicate the exact quantity of caloric
contained either in the atmosphere, or in any body with which they are
in contact?

MRS. B.

No: first, because there are other modifications of caloric which do not
affect the thermometer; and, secondly, because the temperature of a
body, as indicated by the thermometer, is only relative. When, for
instance, the thermometer remains stationary at the freezing point, we
know that the atmosphere (or medium in which it is placed, whatever it
may be) is as cold as freezing water; and when it stands at the boiling
point, we know that this medium is as hot as boiling water; but we do
not know the positive quantity of heat contained either in freezing or
boiling water, any more than we know the real extremes of heat and cold;
and consequently we cannot determine that of the body in which the
thermometer is placed.

CAROLINE.

I do not quite understand this explanation.

MRS. B.

Let us compare a thermometer to a well, in which the water rises to
different heights, according as it is more or less supplied by the
spring which feeds it: if the depth of the well is unfathomable, it must
be impossible to know the absolute quantity of water it contains; yet we
can with the greatest accuracy measure the number of feet the water has
risen or fallen in the well at any time, and consequently know the
precise quantity of its increase or diminution, without having the least
knowledge of the whole quantity of water it contains.

CAROLINE.

Now I comprehend it very well; nothing appears to me to explain a thing
so clearly as a comparison.

EMILY.

But will thermometers bear any degree of heat?

MRS. B.

No; for if the temperature were much above the highest degree marked on
the scale of the thermometer, the mercury would burst the tube in an
attempt to ascend. And at any rate, no thermometer can be applied to
temperatures higher than the boiling point of the liquid used in its
construction, for the steam, on the liquid beginning to boil, would
burst the tube. In furnaces, or whenever any very high temperature is to
be measured, a pyrometer, invented by Wedgwood, is used for that
purpose. It is made of a certain composition of baked clay, which has
the peculiar property of contracting by heat, so that the degree of
contraction of this substance indicates the temperature to which it has
been exposed.

EMILY.

But is it possible for a body to contract by heat? I thought that heat
dilated all bodies whatever.

MRS. B.

This is not an exception to the rule. You must recollect that the bulk
of the clay is not compared, whilst hot, with that which it has when
cold; but it is from the change which the clay has undergone by _having
been_ heated that the indications of this instrument are derived. This
change consists in a beginning fusion which tends to unite the particles
of clay more closely, thus rendering it less pervious or spongy.

Clay is to be considered as a spongy body, having many interstices or
pores, from its having contained water when soft. These interstices are
by heat lessened, and would by extreme heat be entirely obliterated.

CAROLINE.

And how do you ascertain the degrees of contraction of Wedgwood's
pyrometer?

MRS. B.

The dimensions of a piece of clay are measured by a scale graduated on
the side of a tapered groove, formed in a brass ruler; the more the clay
is contracted by the heat, the further it will descend into the narrow
part of the tube.

Before we quit the subject of expansion, I must observe to you that, as
liquids expand more readily than solids, so elastic fluids, whether air
or vapour, are the most expansible of all bodies.

It may appear extraordinary that all elastic fluids whatever, undergo
the same degree of expansion from equal augmentations of temperature.

EMILY.

I suppose, then, that all elastic fluids are of the same density?

MRS. B.

Very far from it; they vary in density, more than either liquids or
solids. The uniformity of their expansibility, which at first may appear
singular, is, however, readily accounted for. For if the different
susceptibilities of expansion of bodies arise from their various degrees
of attraction of cohesion, no such difference can be expected in elastic
fluids, since in these the attraction of cohesion does not exist, their
particles being on the contrary possessed of an elastic or repulsive
power; they will therefore all be equally expanded by equal degrees of
caloric.

EMILY.

True; as there is no power opposed to the expansive force of caloric in
elastic bodies, its effect must be the same in all of them.

MRS. B.

Let us now proceed to examine the other properties of free caloric.

Free caloric always tends to diffuse itself equally, that is to say,
when two bodies are of different temperatures, the warmer gradually
parts with its heat to the colder, till they are both brought to the
same temperature. Thus, when a thermometer is applied to a hot body, it
receives caloric; when to a cold one, it communicates part of its own
caloric, and this communication continues until the thermometer and the
body arrive at the same temperature.

EMILY.

Cold, then, is nothing but a negative quality, simply implying the
absence of heat.

MRS. B.

Not the total absence, but a diminution of heat; for we know of no body
in which some caloric may not be discovered.

CAROLINE.

But when I lay my hand on this marble table I feel it _positively_ cold,
and cannot conceive that there is any caloric in it.

MRS. B.

The cold you experience consists in the loss of caloric that your hand
sustains in an attempt to bring its temperature to an equilibrium with
the marble. If you lay a piece of ice upon it, you will find that the
contrary effect will take place; the ice will be melted by the heat
which it abstracts from the marble.

CAROLINE.

Is it not in this case the air of the room, which being warmer than the
marble, melts the ice?

MRS. B.

The air certainly acts on the surface which is exposed to it, but the
table melts that part with which it is in contact.

CAROLINE.

But why does caloric tend to an equilibrium? It cannot be on the same
principle as other fluids, since it has no weight?

MRS. B.

Very true, Caroline, that is an excellent objection. You might also,
with some propriety, object to the term _equilibrium_ being applied to a
body that is without weight; but I know of no expression that would
explain my meaning so well. You must consider it, however, in a
figurative rather than a literal sense; its strict meaning is an _equal
diffusion_. We cannot, indeed, well say by what power it diffuses itself
equally, though it is not surprising that it should go from the parts
which have the most to those which have the least. This subject is best
explained by a theory suggested by Professor Prevost of Geneva, which is
now, I believe, generally adopted.

According to this theory, caloric is composed of particles perfectly
separate from each other, every one of which moves with a rapid velocity
in a certain direction. These directions vary as much as imagination can
conceive, the result of which is, that there are rays or lines of these
particles moving with immense velocity in every possible direction.
Caloric is thus universally diffused, so that when any portion of space
happens to be in the neighbourhood of another, which contains more
caloric, the colder portion receives a quantity of calorific rays from
the latter, sufficient to restore an equilibrium of temperature. This
radiation does not only take place in free space, but extends also to
bodies of every kind. Thus you may suppose all bodies whatever
constantly radiating caloric: those that are of the same temperature
give out and absorb equal quantities, so that no variation of
temperature is produced in them; but when one body contains more free
caloric than another, the exchange is always in favour of the colder
body, until an equilibrium is effected; this you found to be the case
when the marble table cooled your hand, and again when it melted the
ice.

CAROLINE.

This reciprocal radiation surprises me extremely; I thought, from what
you first said, that the hotter bodies alone emitted rays of caloric
which were absorbed by the colder; for it seems unnatural that a hot
body should receive any caloric from a cold one, even though it should
return a greater quantity.

MRS. B.

It may at first appear so, but it is no more extraordinary than that a
candle should send forth rays of light to the sun, which, you know, must
necessarily happen.

CAROLINE.

Well, Mrs. B--, I believe that I must give up the point. But I wish I
could _see_ these rays of caloric; I should then have greater faith in
them.

MRS. B.

Will you give no credit to any sense but that of sight? You may feel the
rays of caloric which you receive from any body of a temperature higher
than your own; the loss of the caloric you part with in return, it is
true, is not perceptible; for as you gain more than you lose, instead of
suffering a diminution, you are really making an acquisition of caloric.
It is, therefore, only when you are parting with it to a body of a lower
temperature, that you are sensible of the sensation of cold, because you
then sustain an absolute loss of caloric.

EMILY.

And in this case we cannot be sensible of the small quantity of heat we
receive in exchange from the colder body, because it serves only to
diminish the loss.

MRS. B.

Very well, indeed, Emily. Professor Pictet, of Geneva, has made some
very interesting experiments, which prove not only that caloric radiates
from all bodies whatever, but that these rays may be reflected,
according to the laws of optics, in the same manner as light. I shall
repeat these experiments before you, having procured mirrors fit for the
purpose; and it will afford us an opportunity of using the differential
thermometer, which is particularly well adapted for these experiments.
--I place an iron bullet, (PLATE III. Fig. 1.) about two inches in
diameter, and heated to a degree not sufficient to render it luminous,
in the focus of this large metallic concave mirror. The rays of heat
which fall on this mirror are reflected, agreeably to the property of
concave mirrors, in a parallel direction, so as to fall on a similar
mirror, which, you see, is placed opposite to the first, at the distance
of about ten feet; thence the rays converge to the focus of the second
mirror, in which I place one of the bulbs of this thermometer. Now,
observe in what manner it is affected by the caloric which is reflected
on it from the heated bullet. --The air is dilated in the bulb which we
placed in the focus of the mirror, and the liquor rises considerably in
the opposite leg.

  [Illustration: Plate III. Vol. I. p. 54
  Mr. Pictet's Apparatus for the Reflection of Heat.

  Fig. 1.
  A.A. & B.B Concave mirrors fixed on stands.
  C Heated Bullet placed in the focus of the mirror A.
  D Thermometer, with its bulb placed in the focus of the mirror B.
  1.2.3.4 Rays of Caloric radiating from the bullet & falling on the
    mirror A.
  5.6.7.8 The same rays reflected from the mirror A to the mirror B.
  9.10.11.12 The same rays reflected by the mirror B to the
    Thermometer.]

EMILY.

But would not the same effect take place, if the rays of caloric from
the heated bullet fell directly on the thermometer, without the
assistance of the mirrors?

MRS. B.

The effect would in that case be so trifling, at the distance at which
the bullet and the thermometer are from each other, that it would be
almost imperceptible. The mirrors, you know, greatly increase the
effect, by collecting a large quantity of rays into a focus; place your
hand in the focus of the mirror, and you will find it much hotter there
than when you remove it nearer to the bullet.

EMILY.

That is very true; it appears extremely singular to feel the heat
diminish in approaching the body from which it proceeds.

CAROLINE.

And the mirror which produces so much heat, by converging the rays, is
itself quite cold.

MRS. B.

The same number of rays that are dispersed over the surface of the
mirror are collected by it into the focus; but, if you consider how
large a surface the mirror presents to the rays, and, consequently, how
much they are diffused in comparison to what they are at the focus,
which is little more than a point, I think you can no longer wonder that
the focus should be so much hotter than the mirror.

The principal use of the mirrors in this experiment is, to prove that
the calorific emanation is reflected in the same manner as light.

CAROLINE.

And the result, I think, is very conclusive.

MRS. B.

The experiment may be repeated with a wax taper instead of the bullet,
with a view of separating the light from the caloric. For this purpose a
transparent plate of glass must be interposed between the mirrors; for
light, you know, passes with great facility through glass, whilst the
transmission of caloric is almost wholly impeded by it. We shall find,
however, in this experiment, that some few of the calorific rays pass
through the glass together with the light, as the thermometer rises a
little; but, as soon as the glass is removed, and a free passage left to
the caloric, it will rise considerably higher.

EMILY.

This experiment, as well as that of Dr. Herschell's, proves that light
and heat may be separated; for in the latter experiment the separation
was not perfect, any more than in that of Mr. Pictet.

CAROLINE.

I should like to repeat this experiment, with the difference of
substituting a cold body instead of the hot one, to see whether cold
would not be reflected as well as heat.

MRS. B.

That experiment was proposed to Mr. Pictet by an incredulous philosopher
like yourself, and he immediately tried it by substituting a piece of
ice in the place of the heated bullet.

CAROLINE.

Well, Mrs. B., and what was the result?

MRS. B.

That we shall see; I have procured some ice for the purpose.

EMILY.

The thermometer falls considerably!

CAROLINE.

And does not that prove that cold is not merely a _negative_ quality,
implying simply an inferior degree of heat? The cold must be _positive_,
since it is capable of reflection.

MRS. B.

So it at first appeared to Mr. Pictet; but upon a little consideration
he found that it afforded only an additional proof of the reflection of
heat: this I shall endeavour to explain to you.

According to Mr. Prevost's theory, we suppose that all bodies whatever
radiate caloric; the thermometer used in these experiments therefore
emits calorific rays in the same manner as any other substance. When its
temperature is in equilibrium with that of the surrounding bodies, it
receives as much caloric as it parts with, and no change of temperature
is produced. But when we introduce a body of a lower temperature, such
as a piece of ice, which parts with less caloric than it receives, the
consequence is, that its temperature is raised, whilst that of the
surrounding bodies is proportionally lowered.

EMILY.

If, for instance, I was to bring a large piece of ice into this room,
the ice would in time be melted, by absorbing caloric from the general
radiation which is going on throughout the room; and as it would
contribute very little caloric in return for what is absorbed, the room
would necessarily be cooled by it.

MRS. B.

Just so; and as in consequence of the mirrors, a more considerable
exchange of rays takes place between the ice and the thermometer, than
between these and any of the surrounding bodies, the temperature of the
thermometer must be more lowered than that of any other adjacent object.

CAROLINE.

I confess I do not perfectly understand your explanation.

MRS. B.

This experiment is exactly similar to that made with the heated bullet:
for, if we consider the thermometer as the hot body (which it certainly
is in comparison to the ice), you may then easily understand that it is
by the loss of the calorific rays which the thermometer sends to the
ice, and not by any cold rays received from it, that the fall of the
mercury is occasioned: for the ice, far from emitting rays of cold,
sends forth rays of caloric, which diminish the loss sustained by the
thermometer.

Let us say, for instance, that the radiation of the thermometer towards
the ice is equal to 20, and that of the ice towards the thermometer to
10: the exchange in favour of the ice is as 20 is to 10, or the
thermometer absolutely loses 10, whilst the ice gains 10.

CAROLINE.

But if the ice actually sends rays of caloric to the thermometer, must
not the latter fall still lower when the ice is removed?

MRS. B.

No; for the space that the ice occupied, admits rays from all the
surrounding bodies to pass through it; and those being of the same
temperature as the thermometer, will not affect it, because as much heat
now returns to the thermometer as radiates from it.

CAROLINE.

I must confess that you have explained this in so satisfactory a manner,
that I cannot help being convinced now that cold has no real claim to
the rank of a positive being.

MRS. B.

Before I conclude the subject of radiation I must observe to you that
different bodies, (or rather surfaces,) possess the power of radiating
caloric in very different degrees.

Some very curious experiments have been made by Mr. Leslie on this
subject, and it was for this purpose that he invented the differential
thermometer; with its assistance he ascertained that black surfaces
radiate most, glass next, and polished surfaces the least of all.

EMILY.

Supposing these surfaces, of course, to be all of the same temperature.

MRS. B.

Undoubtedly. I will now show you the very simple and ingenious
apparatus, by means of which he made these experiments. This cubical tin
vessel or canister, has each of its sides externally covered with
different materials; the one is simply blackened; the next is covered
with white paper; the third with a pane of glass, and in the fourth the
polished tin surface remains uncovered. We shall fill this vessel with
hot water, so that there can be no doubt but that all its sides will be
of the same temperature. Now let us place it in the focus of one of the
mirrors, making each of its sides front it in succession. We shall begin
with the black surface.

CAROLINE.

It makes the thermometer which is in the focus of the other mirror rise
considerably. Let us turn the paper surface towards the mirror. The
thermometer falls a little, therefore of course this side cannot emit or
radiate so much caloric as the blackened side.

EMILY.

This is very surprising; for the sides are exactly of the same size, and
must be of the same temperature. But let us try the glass surface.

MRS. B.

The thermometer continues falling, and with the plain surface it falls
still lower; these two surfaces therefore radiate less and less.

CAROLINE.

I think I have found out the reason of this.

MRS. B.

I should be very happy to hear it, for it has not yet (to my knowledge)
been accounted for.

CAROLINE.

The water within the vessel gradually cools, and the thermometer in
consequence gradually falls.

MRS. B.

It is true that the water cools, but certainly in much less proportion
than the thermometer descends, as you will perceive if you now change
the tin surface for the black one.

CAROLINE.

I was mistaken certainly, for the thermometer rises again now that the
black surface fronts the mirror.

MRS. B.

And yet the water in the vessel is still cooling, Caroline.

EMILY.

I am surprised that the tin surface should radiate the least caloric,
for a metallic vessel filled with hot water, a silver teapot, for
instance, feels much hotter to the hand than one of black earthen ware.

MRS. B.

That is owing to the different power which various bodies possess for
_conducting_ caloric, a property which we shall presently examine. Thus,
although a metallic vessel feels warmer to the hand, a vessel of this
kind is known to preserve the heat of the liquid within, better than one
of any other materials; it is for this reason that silver teapots make
better tea than those of earthen ware.

EMILY.

According to these experiments, light-coloured dresses, in cold weather,
should keep us warmer than black clothes, since the latter radiate so
much more than the former.

MRS. B.

And that is actually the case.

EMILY.

This property, of different surfaces to radiate in different degrees,
appears to me to be at variance with the equilibrium of caloric; since
it would imply that those bodies which radiate most, must ultimately
become coldest.

Suppose that we were to vary this experiment, by using two metallic
vessels full of boiling water, the one blackened, the other not; would
not the black one cool the first?

CAROLINE.

True; but when they were both brought down to the temperature of the
room, the interchange of caloric between the canisters and the other
bodies of the room being then equal, their temperatures would remain the
same.

EMILY.

I do not see why that should be the case; for if different surfaces of
the same temperature radiate in different degrees when heated, why
should they not continue to do so when cooled down to the temperature of
the room?

MRS. B.

You have started a difficulty, Emily, which certainly requires
explanation. It is found by experiment that the power of absorption
corresponds with and is proportional to that of radiation; so that under
equal temperatures, bodies compensate for the greater loss they sustain
in consequence of their greater radiation by their greater absorption;
so that if you were to make your experiment in an atmosphere heated like
the canisters, to the temperature of boiling water, though it is true
that the canisters would radiate in different degrees, no change of
temperature would be produced in them, because they would each absorb
caloric in proportion to their respective radiation.

EMILY.

But would not the canisters of boiling water also absorb caloric in
different degrees in a room of the common temperature?

MRS. B.

Undoubtedly they would. But the various bodies in the room would not, at
a lower temperature, furnish either of the canisters with a sufficiency
of caloric to compensate for the loss they undergo; for, suppose the
black canister to absorb 400 rays of caloric, whilst the metallic one
absorbed only 200; yet if the former radiate 800, whilst the latter
radiates only 400, the black canister will be the first cooled down to
the temperature of the room. But from the moment the equilibrium of
temperature has taken place, the black canister, both receiving and
giving out 400 rays, and the metallic one 200, no change of temperature
will take place.

EMILY.

I now understand it extremely well. But what becomes of the surplus of
calorific rays, which good radiators emit and bad radiators refuse to
receive; they must wander about in search of a resting-place?

MRS. B.

They really do so; for they are rejected and sent back, or, in other
words, _reflected_ by the bodies which are bad radiators of caloric; and
they are thus transmitted to other bodies which happen to lie in their
way, by which they are either absorbed or again reflected, according as
the property of reflection, or that of absorption, predominates in these
bodies.

CAROLINE.

I do not well understand the difference between radiating and reflecting
caloric, for the caloric that is reflected from a body proceeds from it
in straight lines, and may surely be said to radiate from it?

MRS. B.

It is true that there at first appears to be a great analogy between
_radiation_ and _reflection_, as they equally convey the idea of the
transmission of caloric.

But if you consider a little, you will perceive that when a body
_radiates_ caloric, the heat which it emits not only proceeds from, but
has its origin in the body itself. Whilst when a body _reflects_
caloric, it parts with none of its own caloric, but only reflects that
which it receives from other bodies.

EMILY.

Of this difference we have very striking examples before us, in the tin
vessel of water, and the concave mirrors; the first radiates its own
heat, the latter reflect the heat which they receive from other bodies.

CAROLINE.

Now, that I understand the difference, it no longer surprises me that
bodies which radiate, or part with their own caloric freely, should not
have the power of transmitting with equal facility that which they
receive from other bodies.

EMILY.

Yet no body can be said to possess caloric of its own, if all caloric is
originally derived from the sun.

MRS. B.

When I speak of a body radiating its own caloric, I mean that which it
has absorbed and incorporated either immediately from the sun's rays, or
through the medium of any other substance.

CAROLINE.

It seems natural enough that the power of absorption should be in
opposition to that of reflection, for the more caloric a body receives,
the less it will reject.

EMILY.

And equally so that the power of radiation should correspond with that
of absorption. It is, in fact, cause and effect; for a body cannot
radiate heat without having previously absorbed it; just as a spring
that is well fed flows abundantly.

MRS. B.

Fluids are in general very bad radiators of caloric; and air neither
radiates nor absorbs caloric in any sensible degree.

We have not yet concluded our observations on free caloric. But I shall
defer, till our next meeting, what I have further to say on this
subject. I believe it will afford us ample conversation for another
interview.




CONVERSATION III.

CONTINUATION OF THE SUBJECT.


MRS. B.

In our last conversation, we began to examine the tendency of caloric to
restore an equilibrium of temperature. This property, when once well
understood, affords the explanation of a great variety of facts which
appeared formerly unaccountable. You must observe, in the first place,
that the effect of this tendency is gradually to bring all bodies that
are in contact to the same temperature. Thus, the fire which burns in
the grate, communicates its heat from one object to another, till every
part of the room has an equal proportion of it.

EMILY.

And yet this book is not so cold as the table on which it lies, though
both are at an equal distance from the fire, and actually in contact
with each other, so that, according to your theory, they should be
exactly of the same temperature.

CAROLINE.

And the hearth, which is much nearer the fire than the carpet, is
certainly the colder of the two.

MRS. B.

If you ascertain the temperature of these several bodies by a
thermometer (which is a much more accurate test than your feeling), you
will find that it is exactly the same.

CAROLINE.

But if they are of the same temperature, why should the one feel colder
than the other?

MRS. B.

The hearth and the table feel colder than the carpet or the book,
because the latter are not such good _conductors of heat_ as the former.
Caloric finds a more easy passage through marble and wood, than through
leather and worsted; the two former will therefore absorb heat more
rapidly from your hand, and consequently give it a stronger sensation of
cold than the two latter, although they are all of them really of the
same temperature.

CAROLINE.

So, then, the sensation I feel on touching a cold body, is in proportion
to the rapidity with which my hand yields its heat to that body?

MRS. B.

Precisely; and, if you lay your hand successively on every object in the
room, you will discover which are good, and which are bad conductors of
heat, by the different degrees of cold you feel. But, in order to
ascertain this point, it is necessary that the several substances should
be of the same temperature, which will not be the case with those that
are very near the fire, or those that are exposed to a current of cold
air from a window or door.

EMILY.

But what is the reason that some bodies are better conductors of heat
than others?

MRS. B.

This is a point not well ascertained. It has been conjectured that a
certain union or adherence takes place between the caloric and the
particles of the body through which it passes. If this adherence be
strong, the body detains the heat, and parts with it slowly and
reluctantly; if slight, it propagates it freely and rapidly. The
conducting power of a body is therefore, inversely, as its tendency to
unite with caloric.

EMILY.

That is to say, that the best conductors are those that have the least
affinity for caloric.

MRS. B.

Yes; but the term affinity is objectionable in this case, because, as
that word is used to express a chemical attraction (which can be
destroyed only by decomposition), it cannot be applicable to the slight
and transient union that takes place between free caloric and the bodies
through which it passes; an union which is so weak, that it constantly
yields to the tendency which caloric has to an equilibrium. Now you
clearly understand, that the passage of caloric, through bodies that are
good conductors, is much more rapid than through those that are bad
conductors, and that the former both give and receive it more quickly,
and therefore, in a given time, more abundantly, than bad conductors,
which makes them feel either hotter or colder, though they may be, in
fact, both of the same temperature.

CAROLINE.

Yes, I understand it now; the table, and the book lying upon it, being
really of the same temperature, would each receive, in the same space of
time, the same quantity of heat from my hand, were their conducting
powers equal; but as the table is the best conductor of the two, it will
absorb the heat from my hand more rapidly, and consequently produce a
stronger sensation of cold than the book.

MRS. B.

Very well, my dear; and observe, likewise, that if you were to heat the
table and the book an equal number of degrees above the temperature of
your body, the table, which before felt the colder, would now feel the
hotter of the two; for, as in the first case it took the heat most
rapidly from your hand, so it will now impart heat most rapidly to it.
Thus the marble table, which seems to us colder than the mahogany one,
will prove the hotter of the two to the ice; for, if it takes heat more
rapidly from our hands, which are warmer, it will give out heat more
rapidly to the ice, which is colder. Do you understand the reason of
these apparently opposite effects?

EMILY.

Perfectly. A body which is a good conductor of caloric, affords it a
free passage; so that it penetrates through that body more rapidly than
through one which is a bad conductor; and consequently, if it is colder
than your hand, you lose more caloric, and if it is hotter, you gain
more than with a bad conductor of the same temperature.

MRS. B.

But you must observe that this is the case only when the conductors are
either hotter or colder than your hand; for, if you heat different
conductors to the temperature of your body, they will all feel equally
warm, since the exchange of caloric between bodies of the same
temperature is equal. Now, can you tell me why flannel clothing, which
is a very bad conductor of heat, prevents our feeling cold?

CAROLINE.

It prevents the cold from penetrating . . . . . . . .

MRS. B.

But you forget that cold is only a negative quality.

CAROLINE.

True; it only prevents the heat of our bodies from escaping so rapidly
as it would otherwise do.

MRS. B.

Now you have explained it right; the flannel rather keeps in the heat,
than keeps out the cold. Were the atmosphere of a higher temperature
than our bodies, it would be equally efficacious in keeping their
temperature at the same degree, as it would prevent the free access of
the external heat, by the difficulty with which it conducts it.

EMILY.

This, I think, is very clear. Heat, whether external or internal, cannot
easily penetrate flannel; therefore in cold weather it keeps us warm;
and if the weather was hotter than our bodies, it would keep us cool.

MRS. B.

The most dense bodies are, generally speaking, the best conductors of
heat; probably because the denser the body the greater are the number of
points or particles that come in contact with caloric. At the common
temperature of the atmosphere a piece of metal will feel much colder
than a piece of wood, and the latter than a piece of woollen cloth; this
again will feel colder than flannel; and down, which is one of the
lightest, is at the same time one of the warmest bodies.

CAROLINE.

This is, I suppose, the reason that the plumage of birds preserves them
so effectually from the influence of cold in winter?

MRS. B.

Yes; but though feathers in general are an excellent preservative
against cold, down is a kind of plumage peculiar to aquatic birds, and
covers their chest, which is the part most exposed to the water; for
though the surface of the water is not of a lower temperature than the
atmosphere, yet, as it is a better conductor of heat, it feels much
colder, consequently the chest of the bird requires a warmer covering
than any other part of its body. Besides, the breasts of aquatic birds
are exposed to cold not only from the temperature of the water, but also
from the velocity with which the breast of the bird strikes against it;
and likewise from the rapid evaporation occasioned in that part by the
air against which it strikes, after it has been moistened by dipping
from time to time into the water.

If you hold a finger of one hand motionless in a glass of water, and at
the same time move a finger of the other hand swiftly through water of
the same temperature, a different sensation will be soon perceived in
the different fingers.

Most animal substances, especially those which Providence has assigned
as a covering for animals, such as fur, wool, hair, skin, &c. are bad
conductors of heat, and are, on that account, such excellent
preservatives against the inclemency of winter, that our warmest apparel
is made of these materials.

EMILY.

Wood is, I dare say, not so good a conductor as metal, and it is for
that reason, no doubt, that silver teapots have always wooden handles.

MRS. B.

Yes; and it is the facility with which metals conduct caloric that made
you suppose that a silver pot radiated more caloric than an earthen one.
The silver pot is in fact hotter to the hand when in contact with it;
but it is because its conducting power more than counterbalances its
deficiency in regard to radiation.

We have observed that the most dense bodies are in general the best
conductors; and metals, you know, are of that class. Porous bodies, such
as the earths and wood, are worse conductors, chiefly, I believe, on
account of their pores being filled with air; for air is a remarkably
bad conductor.

CAROLINE.

It is a very fortunate circumstance that air should be a bad conductor,
as it tends to preserve the heat of the body when exposed to cold
weather.

MRS. B.

It is one of the many benevolent dispensations of Providence, in order
to soften the inclemency of the seasons, and to render almost all
climates habitable to man.

In fluids of different densities, the power of conducting heat varies no
less remarkably; if you dip your hand into this vessel full of mercury,
you will scarcely conceive that its temperature is not lower than that
of the atmosphere.

CAROLINE.

Indeed I know not how to believe it, it feels so extremely cold. --But
we may easily ascertain its true temperature by the thermometer. --It is
really not colder than the air;--the apparent difference then is
produced merely by the difference of the conducting power in mercury and
in air.

MRS. B.

Yes; hence you may judge how little the sense of feeling is to be relied
on as a test of the temperature of bodies, and how necessary a
thermometer is for that purpose.

It has indeed been doubted whether fluids have the power of conducting
caloric in the same manner as solid bodies. Count Rumford, a very few
years since, attempted to prove, by a variety of experiments, that
fluids, when at rest, were not at all endowed with this property.

CAROLINE.

How is that possible, since they are capable of imparting cold or heat
to us; for if they did not conduct heat, they would neither take it
from, nor give it to us?

MRS. B.

Count Rumford did not mean to say that fluids would not communicate
their heat to solid bodies; but only that heat does not pervade fluids,
that is to say, is not transmitted from one particle of a fluid to
another, in the same manner as in solid bodies.

EMILY.

But when you heat a vessel of water over the fire, if the particles of
water do not communicate heat to each other, how does the water become
hot throughout?

MRS. B.

By constant agitation. Water, as you have seen, expands by heat in the
same manner as solid bodies; the heated particles of water, therefore,
at the bottom of the vessel, become specifically lighter than the rest
of the liquid, and consequently ascend to the surface, where, parting
with some of their heat to the colder atmosphere, they are condensed,
and give way to a fresh succession of heated particles ascending from
the bottom, which having thrown off their heat at the surface, are in
their turn displaced. Thus every particle is successively heated at the
bottom, and cooled at the surface of the liquid; but as the fire
communicates heat more rapidly than the atmosphere cools the succession
of surfaces, the whole of the liquid in time becomes heated.

CAROLINE.

This accounts most ingeniously for the propagation of heat upwards. But
suppose you were to heat the upper surface of a liquid, the particles
being specifically lighter than those below, could not descend: how
therefore would the heat be communicated downwards?

MRS. B.

If there were no agitation to force the heated surface downwards, Count
Rumford assures us that the heat would not descend. In proof of this he
succeeded in making the upper surface of a vessel of water boil and
evaporate, while a cake of ice remained frozen at the bottom.

CAROLINE.

That is very extraordinary indeed!

MRS. B.

It appears so, because we are not accustomed to heat liquids by their
upper surface; but you will understand this theory better if I show you
the internal motion that takes place in liquids when they experience a
change of temperature. The motion of the liquid itself is indeed
invisible from the extreme minuteness of its particles; but if you mix
with it any coloured dust, or powder, of nearly the same specific
gravity as the liquid, you may judge of the internal motion of the
latter by that of the coloured dust it contains. --Do you see the small
pieces of amber moving about in the liquid contained in this phial?

CAROLINE.

Yes, perfectly.

MRS. B.

We shall now immerse the phial in a glass of hot water, and the motion
of the liquid will be shown, by that which it communicates to the amber.

EMILY.

I see two currents, the one rising along the sides of the phial, the
other descending in the centre: but I do not understand the reason of
this.

MRS. B.

The hot water communicates its caloric, through the medium of the phial,
to the particles of the fluid nearest to the glass; these dilate and
ascend laterally to the surface, where, in parting with their heat, they
are condensed, and in descending, form the central current.

CAROLINE.

This is indeed a very clear and satisfactory experiment; but how much
slower the currents now move than they did at first?

MRS. B.

It is because the circulation of particles has nearly produced an
equilibrium of temperature between the liquid in the glass and that in
the phial.

CAROLINE.

But these communicate laterally, and I thought that heat in liquids
could be propagated only upwards.

MRS. B.

You do not take notice that the heat is imparted from one liquid to the
other, through the medium of the phial itself, the external surface of
which receives the heat from the water in the glass, whilst its internal
surface transmits it to the liquid it contains. Now take the phial out
of the hot water, and observe the effect of its cooling.

EMILY.

The currents are reversed; the external current now descends, and the
internal one rises. --I guess the reason of this change:-- the phial
being in contact with cold air instead of hot water, the external
particles are cooled instead of being heated; they therefore descend and
force up the central particles, which, being warmer, are consequently
lighter.

MRS. B.

It is just so. Count Rumford hence infers that no alteration of
temperature can take place in a fluid, without an internal motion of its
particles, and as this motion is produced only by the comparative levity
of the heated particles, heat cannot be propagated downwards.

But though I believe that Count Rumford's theory as to heat being
incapable of pervading fluids is not strictly correct, yet there is, no
doubt, much truth in his observation, that the communication is
materially promoted by a motion of the parts; and this accounts for the
cold that is found to prevail at the bottom of the lakes in Switzerland,
which are fed by rivers issuing from the snowy Alps. The water of these
rivers being colder, and therefore more dense than that of the lakes,
subsides to the bottom, where it cannot be affected by the warmer
temperature of the surface; the motion of the waves may communicate this
temperature to some little depth, but it can descend no further than the
agitation extends.

EMILY.

But when the atmosphere is colder than the lake, the colder surface of
the water will descend, for the very reason that the warmer will not.

MRS. B.

Certainly: and it is on this account that neither a lake, nor any body
of water whatever, can be frozen until every particle of the water has
risen to the surface to give off its caloric to the colder atmosphere;
therefore the deeper a body of water is, the longer will be the time it
requires to be frozen.

EMILY.

But if the temperature of the whole body of water be brought down to the
freezing point, why is only the surface frozen?

MRS. B.

The temperature of the whole body is lowered, but not to the freezing
point. The diminution of heat, as you know, produces a contraction in
the bulk of fluids, as well as of solids. This effect, however, does not
take place in water below the temperature of 40 degrees, which is 8
degrees above the freezing point. At that temperature, therefore, the
internal motion, occasioned by the increased specific gravity of the
condensed particles, ceases; for when the water at the surface no longer
condenses, it will no longer descend, and leave a fresh surface exposed
to the atmosphere: this surface alone, therefore, will be further
exposed to its severity, and will soon be brought down to the freezing
point, when it becomes ice, which being a bad conductor of heat,
preserves the water beneath a long time from being affected by the
external cold.

CAROLINE.

And the sea does not freeze, I suppose, because its depth is so great,
that a frost never lasts long enough to bring down the temperature of
such a great body of water to 40 degrees?

MRS. B.

That is one reason why the sea, as a large mass of water, does not
freeze. But, independently of this, salt water does not freeze till it
is cooled much below 32 degrees, and with respect to the law of
condensation, salt water is an exception, as it condenses even many
degrees below the freezing point. When the caloric of fresh water,
therefore, is imprisoned by the ice on its surface, the ocean still
continues throwing off heat into the atmosphere, which is a most signal
dispensation of Providence to moderate the intensity of the cold in
winter.

CAROLINE.

This theory of the non-conducting power of liquids, does not, I suppose,
hold good with respect to air, otherwise the atmosphere would not be
heated by the rays of the sun passing through it?

MRS. B.

Nor is it heated in that way. The pure atmosphere is a perfectly
transparent medium, which neither radiates, absorbs, nor conducts
caloric, but transmits the rays of the sun to us without in any way
diminishing their intensity. The air is therefore not more heated, by
the sun's rays passing through it, than diamond, glass, water, or any
other transparent medium.

CAROLINE.

That is very extraordinary! Are glass windows not heated then by the sun
shining on them?

MRS. B.

No; not if the glass be perfectly transparent. A most convincing proof
that glass transmits the rays of the sun without being heated by them is
afforded by the burning lens, which by converging the rays to a focus
will set combustible bodies on fire, without its own temperature being
raised.

EMILY.

Yet, Mrs. B., if I hold a piece of glass near the fire it is almost
immediately warmed by it; the glass therefore must retain some of the
caloric radiated by the fire? Is it that the solar rays alone pass
freely through glass without paying tribute? It seems unaccountable that
the radiation of a common fire should have power to do what the sun's
rays cannot accomplish.

MRS. B.

It is not because the rays from the fire have more power, but rather
because they have less, that they heat glass and other transparent
bodies. It is true, however, that as you approach the source of heat the
rays being nearer each other, the heat is more condensed, and can
produce effects of which the solar rays, from the great distance of
their source, are incapable. Thus we should find it impossible to roast
a joint of meat by the sun's rays, though it is so easily done by
culinary heat. Yet caloric emanated from burning bodies, which is
commonly called _culinary heat_, has neither the intensity nor the
velocity of solar rays. All caloric, we have said, is supposed to
proceed originally from the sun; but after having been incorporated with
terrestrial bodies, and again given out by them, though its nature is
not essentially altered, it retains neither the intensity nor the
velocity with which it first emanated from that luminary; it has
therefore not the power of passing through transparent mediums, such as
glass and water, without being partially retained by those bodies.

EMILY.

I recollect that in the experiment on the reflection of heat, the glass
skreen which you interposed between the burning taper and the mirror,
arrested the rays of caloric, and suffered only those of light to pass
through it.

CAROLINE.

Glass windows, then, though they cannot be heated by the sun shining on
them, may be heated internally by a fire in the room? But, Mrs. B.,
since the atmosphere is not warmed by the solar rays passing through it,
how does it obtain heat; for all the fires that are burning on the
surface of the earth would contribute very little towards warming it?

EMILY.

The radiation of heat is not confined to burning bodies: for all bodies,
you know, have that property; therefore, not only every thing upon the
surface of the earth, but the earth itself, must radiate heat; and this
terrestrial caloric, not having, I suppose, sufficient power to traverse
the atmosphere, communicates heat to it.

MRS. B.

Your inference is extremely well drawn, Emily; but the foundation on
which it rests is not sound; for the fact is, that terrestrial or
culinary heat, though it cannot pass through the denser transparent
mediums, such as glass or water, without loss, traverses the atmosphere
completely: so that all the heat which the earth radiates, unless it
meet with clouds or any foreign body to intercept its passage, passes
into the distant regions of the universe.

CAROLINE.

What a pity that so much heat should be wasted!

MRS. B.

Before you are tempted to object to any law of nature, reflect whether
it may not prove to be one of the numberless dispensations of Providence
for our good. If all the heat which the earth has received from the sun,
since the creation had been accumulated in it, its temperature by this
time would, no doubt, have been more elevated than any human being could
have borne.

CAROLINE.

I spoke indeed very inconsiderately. But, Mrs. B., though the earth, at
such a high temperature, might have scorched our feet, we should always
have had a cool refreshing air to breathe, since the radiation of the
earth does not heat the atmosphere.

EMILY.

The cool air would have afforded but very insufficient refreshment,
whilst our bodies were exposed to the burning radiation of the earth.

MRS. B.

Nor should we have breathed a cool air; for though it is true that heat
is not communicated to the atmosphere by radiation, yet the air is
warmed by contact with heated bodies, in the same manner as solids or
liquids. The stratum of air which is immediately in contact with the
earth is heated by it; it becomes specifically lighter and rises, making
way for another stratum of air which is in its turn heated and carried
upwards; and thus each successive stratum of air is warmed by coming in
contact with the earth. You may perceive this effect in a sultry day, if
you attentively observe the strata of air near the surface of the earth;
they appear in constant agitation, for though it is true the air is
itself invisible, yet the sun shining on the vapours floating in it,
render them visible, like the amber dust in the water. The temperature
of the surface of the earth is therefore the source from whence the
atmosphere derives its heat, though it is communicated neither by
radiation, nor transmitted from one particle of it to another by the
conducting power; but every particle of air must come in contact with
the earth in order to receive heat from it.

EMILY.

Wind then by agitating the air should contribute to cool the earth and
warm the atmosphere, by bringing a more rapid succession of fresh strata
of air in contact with the earth, and yet in general wind feels cooler
than still air?

MRS. B.

Because the agitation of the air carries off heat from the surface of
our bodies more rapidly than still air, by occasioning a greater number
of points of contact in a given time.

EMILY.

Since it is from the earth and not the sun that the atmosphere receives
its heat, I no longer wonder that elevated regions should be colder than
plains and valleys; it was always a subject of astonishment to me, that
in ascending a mountain and approaching the sun, the air became colder
instead of being more heated.

MRS. B.

At the distance of about a hundred million of miles, which we are from
the sun, the approach of a few thousand feet makes no sensible
difference, whilst it produces a very considerable effect with regard to
the warming the atmosphere at the surface of the earth.

CAROLINE.

Yet as the warm air rises from the earth and the cold air descends to
it, I should have supposed that heat would have accumulated in the upper
regions of the atmosphere, and that we should have felt the air warmer
as we ascended?

MRS. B.

The atmosphere, you know, diminishes in density, and consequently in
weight, as it is more distant from the earth; the warm air, therefore,
rises only till it meets with a stratum of air of its own density; and
it will not ascend into the upper regions of the atmosphere until all
the parts beneath have been previously heated. The length of summer even
in warm climates does not heat the air sufficiently to melt the snow
which has accumulated during the winter on very high mountains, although
they are almost constantly exposed to the heat of the sun's rays, being
too much elevated to be often enveloped in clouds.

EMILY.

These explanations are very satisfactory; but allow me to ask you one
more question respecting the increased levity of heated liquids. You
said that when water was heated over the fire, the particles at the
bottom of the vessel ascended as soon as heated, in consequence of their
specific levity: why does not the same effect continue when the water
boils, and is converted into steam? and why does the steam rise from the
surface, instead of the bottom of the liquid?

MRS. B.

The steam or vapour does ascend from the bottom, though it seems to
arise from the surface of the liquid. We shall boil some water in this
Florence flask, (PLATE IV. Fig. 1.) in order that you may be well
acquainted with the process of ebullition;--you will then see, through
the glass, that the vapour rises in bubbles from the bottom. We shall
make it boil by means of a lamp, which is more convenient for this
purpose than the chimney fire.

  [Illustration: Plate IV. Vol. I. p. 84.

  Fig. 1. Pneumatic Pump.
  Ether evaporated & water frozen in the air pump.
  A Phial of Ether.
  B Glass vessel containing water.
  C.C Thermometers one in the Ether, the other in the water.

  Fig. 2. Boiling water in a flask over a Patent lamp.]

EMILY.

I see some small bubbles ascend, and a great many appear all over the
inside of the flask; does the water begin to boil already?

MRS. B.

No; what you now see are bubbles of air, which were either dissolved in
the water, or attached to the inner surface of the flask, and which,
being rarefied by the heat, ascend in the water.

EMILY.

But the heat which rarefies the air inclosed in the water must rarefy
the water at the same time; therefore, if it could remain stationary in
the water when both were cold, I do not understand why it should not
when both are equally heated?

MRS. B.

Air being much less dense than water, is more easily rarefied; the
former, therefore, expands to a great extent, whilst the latter
continues to occupy nearly the same space; for water dilates
comparatively but very little without changing its state and becoming
vapour. Now that the water in the flask begins to boil, observe what
large bubbles rise from the bottom of it.

EMILY.

I see them perfectly; but I wonder that they have sufficient power to
force themselves through the water.

CAROLINE.

They _must_ rise, you know, from their specific levity.

MRS. B.

You are right, Caroline; but vapour has not in all liquids (when brought
to the degree of vaporization) the power of overcoming the pressure of
the less heated surface. Metals, for instance, mercury excepted,
evaporate only from the surface; therefore no vapour will ascend from
them till the degree of heat which is necessary to form it has reached
the surface; that is to say, till the whole of the liquid is brought to
a state of ebullition.

EMILY.

I have observed that steam, immediately issuing from the spout of a
teakettle, is less visible than at a further distance from it; yet it
must be more dense when it first evaporates, than when it begins to
diffuse itself in the air.

MRS. B.

When the steam is first formed, it is so perfectly dissolved by caloric,
as to be invisible. In order however to understand this, it will be
necessary for me to enter into some explanation respecting the nature of
SOLUTION. Solution takes place whenever a body is melted in a fluid. In
this operation the body is reduced to such a minute state of division by
the fluid, as to become invisible in it, and to partake of its fluidity;
but in common solutions this happens without any decomposition, the body
being only divided into its integrant particles by the fluid in which it
is melted.

CAROLINE.

It is then a mode of destroying the attraction of aggregation.

MRS. B.

Undoubtedly. --The two principal solvent fluids are _water_, and
_caloric_. You may have observed that if you melt salt in water, it
totally disappears, and the water remains clear, and transparent as
before; yet though the union of these two bodies appears so perfect, it
is not produced by any chemical combination; both the salt and the water
remain unchanged; and if you were to separate them by evaporating the
latter, you would find the salt in the same state as before.

EMILY.

I suppose that water is a solvent for solid bodies, and caloric for
liquids?

MRS. B.

Liquids of course can only be converted into vapour by caloric. But the
solvent power of this agent is not at all confined to that class of
bodies; a great variety of solid substances are dissolved by heat: thus
metals, which are insoluble in water, can be dissolved by intense heat,
being first fused or converted into a liquid, and then rarefied into an
invisible vapour. Many other bodies, such as salt, gums, &c. yield to
either of these solvents.

CAROLINE.

And that, no doubt, is the reason why hot water will melt them so much
better than cold water?

MRS. B.

It is so. Caloric may, indeed, be considered as having, in every
instance, some share in the solution of a body by water, since water,
however low its temperature may be, always contains more or less
caloric.

EMILY.

Then, perhaps, water owes its solvent power merely to the caloric
contained in it?

MRS. B.

That, probably, would be carrying the speculation too far; I should
rather think that water and caloric unite their efforts to dissolve a
body, and that the difficulty or facility of effecting this, depend both
on the degree of attraction of aggregation to be overcome, and on the
arrangement of the particles which are more or less disposed to be
divided and penetrated by the solvent.

EMILY.

But have not all liquids the same solvent power as water?

MRS. B.

The solvent power of other liquids varies according to their nature, and
that of the substances submitted to their action. Most of these
solvents, indeed, differ essentially from water, as they do not merely
separate the integrant particles of the bodies which they dissolve, but
attack their constituent principles by the power of chemical attraction,
thus producing a true decomposition. These more complicated operations
we must consider in another place, and confine our attention at present
to the solutions by water and caloric.

CAROLINE.

But there are a variety of substances which, when dissolved in water,
make it thick and muddy, and destroy its transparency.

MRS. B.

In this case it is not a solution, but simply a mixture. I shall show
you the difference between a solution and a mixture, by putting some
common salt into one glass of water, and some powder of chalk into
another; both these substances are white, but their effect on the water
will be very different.

CAROLINE.

Very different indeed! The salt entirely disappears and leaves the water
transparent, whilst the chalk changes it into an opaque liquid like
milk.

EMILY.

And would lumps of chalk and salt produce similar effects on water?

MRS. B.

Yes, but not so rapidly; salt is, indeed, soon melted though in a lump;
but chalk, which does not mix so readily with water, would require a
much greater length of time; I therefore preferred showing you the
experiment with both substances reduced to powder, which does not in any
respect alter their nature, but facilitates the operation merely by
presenting a greater quantity of surface to the water.

I must not forget to mention a very curious circumstance respecting
solutions, which is, that a fluid is not nearly so much increased in
bulk by holding a body in solution, as it would by mere mixture with the
body.

CAROLINE.

That seems impossible; for two bodies cannot exist together in the same
space.

MRS. B.

Two bodies may, by condensation, occupy less space when in union than
when separate, and this I can show you by an easy experiment.

This phial, which contains some salt, I shall fill with water, pouring
it in quickly, so as not to dissolve much of the salt; and when it is
quite full I cork it. --If I now shake the phial till the salt is
dissolved, you will observe that it is no longer full.

CAROLINE.

I shall try to add a little more salt. --But now, you see, Mrs. B., the
water runs over.

MRS. B.

Yes; but observe that the last quantity of salt you put in remains solid
at the bottom, and displaces the water; for it has already melted all
the salt it is capable of holding in solution. This is called the point
of _saturation_; and the water in this case is said to be _saturated_
with salt.

EMILY.

I think I now understand the solution of a solid body by water
perfectly: but I have not so clear an idea of the solution of a liquid
by caloric.

MRS. B.

It is probably of a similar nature; but as caloric is an invisible
fluid, its action as a solvent is not so obvious as that of water.
Caloric, we may conceive, dissolves water, and converts it into vapour
by the same process as water dissolves salt; that is to say, the
particles of water are so minutely divided by the caloric as to become
invisible. Thus, you are now enabled to understand why the vapour of
boiling water, when it first issues from the spout of a kettle, is
invisible; it is so, because it is then completely dissolved by caloric.
But the air with which it comes in contact, being much colder than the
vapour, the latter yields to it a quantity of its caloric. The particles
of vapour being thus in a great measure deprived of their solvent,
gradually collect, and become visible in the form of steam, which is
water in a state of imperfect solution; and if you were further to
deprive it of its caloric, it would return to its original liquid state.

CAROLINE.

That I understand very well. If you hold a cold plate over a tea-urn,
the steam issuing from it will be immediately converted into drops of
water by parting with its caloric to the plate; but in what state is the
steam, when it becomes invisible by being diffused in the air?

MRS. B.

It is not merely diffused, but is again dissolved by the air.

EMILY.

The air, then, has a solvent power, like water and caloric?

MRS. B.

This was formerly believed to be the case. But it appears from more
recent enquiries that the solvent power of the atmosphere depends solely
upon the caloric contained in it. Sometimes the watery vapour diffused
in the atmosphere is but imperfectly dissolved, as is the case in the
formation of clouds and fogs; but if it gets into a region sufficiently
warm, it becomes perfectly invisible.

EMILY.

Can any water dissolve in the atmosphere without its being previously
converted into vapour by boiling?

MRS. B.

Unquestionably; and this constitutes the difference between
_vaporization_ and _evaporation_. Water, when heated to the boiling
point, can no longer exist in the form of water, and must necessarily be
converted into vapour or steam, whatever may be the state and
temperature of the surrounding medium; this is called vaporization. But
the atmosphere, by means of the caloric it contains, can take up a
certain portion of water at any temperature, and hold it in a state of
solution. This is simply evaporation. Thus the atmosphere is continually
carrying off moisture from the surface of the earth, until it is
saturated with it.

CAROLINE.

That is the case, no doubt, when we feel the atmosphere damp.

MRS. B.

On the contrary, when the moisture is well dissolved it occasions no
humidity: it is only when in a state of imperfect solution and floating
in the atmosphere, in the form of watery vapour, that it produces
dampness. This happens more frequently in winter than in summer; for the
lower the temperature of the atmosphere, the less water it can dissolve;
and in reality it never contains so much moisture as in a dry hot
summer's day.

CAROLINE.

You astonish me! But why, then, is the air so dry in frosty weather,
when its temperature is at the lowest?

EMILY.

This, I conjecture, proceeds not so much from the moisture being
dissolved, as from its being frozen; is not that the case?

MRS. B.

It is; and the freezing of the watery vapour which the atmospheric heat
could not dissolve, produces what is called a hoar frost; for the
particles descend in freezing, and attach themselves to whatever they
meet with on the surface of the earth.

The tendency of free caloric to an equilibrium, together with its
solvent power, are likewise connected with the phenomena of rain, of
dew, &c. When moist air of a certain temperature happens to pass through
a colder region of the atmosphere, it parts with a portion of its heat
to the surrounding air; the quantity of caloric, therefore, which served
to keep the water in a state of vapour, being diminished, the watery
particles approach each other, and form themselves into drops of water,
which being heavier than the atmosphere, descend to the earth. There are
also other circumstances, and particularly the variation in the weight
of the atmosphere, which may contribute to the formation of rain. This,
however, is an intricate subject, into which we cannot more fully enter
at present.

EMILY.

In what manner do you account for the formation of dew?

MRS. B.

Dew is a deposition of watery particles or minute drops from the
atmosphere, precipitated by the coolness of the evening.

CAROLINE.

This precipitation is owing, I suppose, to the cooling of the
atmosphere, which prevents its retaining so great a quantity of watery
vapour in solution as during the heat of the day.

MRS. B.

Such was, from time immemorial, the generally received opinion
respecting the cause of dew; but it has been very recently proved by a
course of ingenious experiments of Dr. Wells, that the deposition of dew
is produced by the cooling of the surface of the earth, which he has
shown to take place previously to the cooling of the atmosphere; for on
examining the temperature of a plot of grass just before the dew-fall,
he found that it was considerably colder than the air a few feet above
it, from which the dew was shortly after precipitated.

EMILY.

But why should the earth cool in the evening sooner than the atmosphere?

MRS. B.

Because it parts with its heat more readily than the air; the earth is
an excellent radiator of caloric, whilst the atmosphere does not possess
that property, at least in any sensible degree. Towards evening,
therefore, when the solar heat declines, and when after sunset it
entirely ceases, the earth rapidly cools by radiating heat towards the
skies; whilst the air has no means of parting with its heat but by
coming into contact with the cooled surface of the earth, to which it
communicates its caloric. Its solvent power being thus reduced, it is
unable to retain so large a portion of watery vapour, and deposits those
pearly drops which we call dew.

EMILY.

If this be the cause of dew, we need not be apprehensive of receiving
any injury from it; for it can be deposited only on surfaces that are
colder than the atmosphere, which is never the case with our bodies.

MRS. B.

Very true; yet I would not advise you for this reason to be too
confident of escaping all the ill effects which may arise from exposure
to the dew; for it may be deposited on your clothes, and chill you
afterwards by its evaporation from them. Besides, whenever the dew is
copious, there is a chill in the atmosphere which it is not always safe
to encounter.

CAROLINE.

Wind, then, must promote the deposition of dew, by bringing a more rapid
succession of particles of air in contact with the earth, just as it
promotes the cooling of the earth and warming of the atmosphere during
the heat of the day?

MRS. B.

Yes; provided the wind be unattended with clouds, for these
accumulations of moisture not only prevent the free radiation of the
earth towards the upper regions, but themselves radiate towards the
earth; under these circumstances much less dew is formed than on fine
clear nights, when the radiation of the earth passes without obstacle
through the atmosphere to the distant regions of space, whence it
receives no caloric in exchange. The dew continues to be deposited
during the night, and is generally most abundant towards morning, when
the contrast between the temperature of the earth and that of the air is
greatest. After sunrise the equilibrium of temperature between these two
bodies is gradually restored by the solar rays passing freely through
the atmosphere to the earth; and later in the morning the temperature of
the earth gains the ascendency, and gives out caloric to the air by
contact, in the same manner as it receives it from the air during the
night. --Can you tell me, now, why a bottle of wine taken fresh from the
cellar (in summer particularly), will soon be covered with dew; and even
the glasses into which the wine is poured will be moistened with a
similar vapour?

EMILY.

The bottle being colder than the surrounding air, must absorb caloric
from it; the moisture therefore which that air contained becomes
visible, and forms the dew which is deposited on the bottle.

MRS. B.

Very well, Emily. Now, Caroline, can you inform me why, in a warm room,
or close carriage, the contrary effect takes place; that is to say, that
the inside of the windows is covered with vapour?

CAROLINE.

I have heard that it proceeds from the breath of those within the room
or the carriage; and I suppose it is occasioned by the windows which,
being colder than the breath, deprive it of part of its caloric, and by
this means convert it into watery vapour.

MRS. B.

You have both explained it extremely well. Bodies attract dew in
proportion as they are good radiators of caloric, as it is this quality
which reduces their temperature below that of the atmosphere; hence we
find that little or no dew is deposited on rocks, sand, water; while
grass and living vegetables, to which it is so highly beneficial,
attract it in abundance--another remarkable instance of the wise and
bountiful dispensations of Providence.

EMILY.

And we may again observe it in the abundance of dew in summer, and in
hot climates, when its cooling effects are so much required; but I do
not understand what natural cause increases the dew in hot weather?

MRS. B.

The more caloric the earth receives during the day, the more it will
radiate afterwards, and consequently the more rapidly its temperature
will be reduced in the evening, in comparison to that of the atmosphere.
In the West-Indies especially, where the intense heat of the day is
strongly contrasted with the coolness of the evening, the dew is
prodigiously abundant. During a drought, the dew is less plentiful, as
the earth is not sufficiently supplied with moisture to be able to
saturate the atmosphere.

CAROLINE.

I have often observed, Mrs. B., that when I walk out in frosty weather,
with a veil over my face, my breath freezes upon it. Pray what is the
reason of that?

MRS. B.

It is because the cold air immediately seizes on the caloric of your
breath, and, by robbing it of its solvent, reduces it to a denser fluid,
which is the watery vapour that settles on your veil, and there it
continues parting with its caloric till it is brought down to the
temperature of the atmosphere, and assumes the form of ice.

You may, perhaps, have observed that the breath of animals, or rather
the moisture contained in it, is visible in damp weather, or during a
frost. In the former case, the atmosphere being over-saturated with
moisture, can dissolve no more. In the latter, the cold condenses it
into visible vapour; and for the same reason, the steam arising from
water that is warmer than the atmosphere, becomes visible. Have you
never taken notice of the vapour rising from your hands after having
dipped them into warm water?

CAROLINE.

Frequently, especially in frosty weather.

MRS. B.

We have already observed that pressure is an obstacle to evaporation:
there are liquids that contain so great a quantity of caloric, and whose
particles consequently adhere so slightly together, that they may be
rapidly converted into vapour without any elevation of temperature,
merely by taking off the weight of the atmosphere. In such liquids, you
perceive, it is the pressure of the atmosphere alone that connects their
particles, and keeps them in a liquid state.

CAROLINE.

I do not well understand why the particles of such fluids should be
disunited and converted into vapour, without any elevation of
temperature, in spite of the attraction of cohesion.

MRS. B.

It is because the degree of heat at which we usually observe these
fluids is sufficient to overcome their attraction of cohesion. Ether is
of this description; it will boil and be converted into vapour, at the
common temperature of the air, if the pressure of the atmosphere be
taken off.

EMILY.

I thought that ether would evaporate without either the pressure of the
atmosphere being taken away, or heat applied; and that it was for that
reason so necessary to keep it carefully corked up?

MRS. B.

It is true it will evaporate, but without ebullition; what I am now
speaking of is the vaporization of ether, or its conversion into vapour
by boiling. I am going to show you how suddenly the ether in this phial
will be converted into vapour, by means of the air-pump. --Observe with
what rapidity the bubbles ascend, as I take off the pressure of the
atmosphere.

CAROLINE.

It positively boils: how singular to see a liquid boil without heat!

MRS. B.

Now I shall place the phial of ether in this glass, which it nearly
fits, so as to leave only a small space, which I fill with water; and in
this state I put it again under the receiver. (PLATE IV. Fig. 1.)* You
will observe, as I exhaust the air from it, that whilst the ether boils,
the water freezes.

    [Footnote *: Two pieces of thin glass tubes, sealed at one end,
    might answer this purpose better. The experiment, however, as here
    described, is difficult, and requires a very nice apparatus. But
    if, instead of phials or tubes, two watch-glasses be used, water
    may be frozen almost instantly in the same manner. The two glasses
    are placed over one another, with a few drops of water interposed
    between them, and the uppermost glass is filled with ether. After
    working the pump for a minute or two, the glasses are found to
    adhere strongly together, and a thin layer of ice is seen between
    them.]

CAROLINE.

It is indeed wonderful to see water freeze in contact with a boiling
fluid!

EMILY.

I am at a loss to conceive how the ether can pass to the state of vapour
without an addition of caloric. Does it not contain more caloric in a
state of vapour, than in a state of liquidity?

MRS. B.

It certainly does; for though it is the pressure of the atmosphere which
condenses it into a liquid, it is by forcing out the caloric that
belongs to it when in an aeriform state.

EMILY.

You have, therefore, two difficulties to explain, Mrs. B. --First, from
whence the ether obtains the caloric necessary to convert it into vapour
when it is relieved from the pressure of the atmosphere; and, secondly,
what is the reason that the water, in which the bottle of ether stands,
is frozen?

CAROLINE.

Now, I think, I can answer both these questions. The ether obtains the
addition of caloric required, from the water in the glass; and the loss
of caloric, which the latter sustains, is the occasion of its freezing.

MRS. B.

You are perfectly right; and if you look at the thermometer which I have
placed in the water, whilst I am working the pump, you will see that
every time bubbles of vapour are produced, the mercury descends; which
proves that the heat of the water diminishes in proportion as the ether
boils.

EMILY.

This I understand now very well; but if the water freezes in consequence
of yielding its caloric to the ether, the equilibrium of heat must, in
this case, be totally destroyed. Yet you have told us, that the exchange
of caloric between two bodies of equal temperature, was always equal;
how, then, is it that the water, which was originally of the same
temperature as the ether, gives out caloric to it, till the water is
frozen, and the ether made to boil?

MRS. B.

I suspected that you would make these objections; and, in order to
remove them, I enclosed two thermometers in the air-pump; one which
stands in the glass of water, the other in the phial of ether; and you
may see that the equilibrium of temperature is not destroyed; for as the
thermometer descends in the water, that in the ether sinks in the same
manner; so that both thermometers indicate the same temperature, though
one of them is in a boiling, the other in a freezing liquid.

EMILY.

The ether, then, becomes colder as it boils? This is so contrary to
common experience, that I confess it astonishes me exceedingly.

CAROLINE.

It is, indeed, a most extraordinary circumstance. But pray, how do you
account for it?

MRS. B.

I cannot satisfy your curiosity at present; for before we can attempt to
explain this apparent paradox, it is necessary to become acquainted with
the subject of LATENT HEAT: and that, I think, we must defer till our
next interview.

CAROLINE.

I believe, Mrs. B., that you are glad to put off the explanation; for it
must be a very difficult point to account for.

MRS. B.

I hope, however, that I shall do it to your complete satisfaction.

EMILY.

But before we part, give me leave to ask you one question. Would not
water, as well as ether, boil with less heat, if deprived of the
pressure of the atmosphere?

MRS. B.

Undoubtedly. You must always recollect that there are two forces to
overcome, in order to make a liquid boil or evaporate; the attraction of
aggregation, and the weight of the atmosphere. On the summit of a high
mountain (as Mr. De Saussure ascertained on Mount Blanc) much less heat
is required to make water boil, than in the plain, where the weight of
the atmosphere is greater.* Indeed if the weight of the atmosphere be
entirely removed by means of a good air-pump, and if water be placed in
the exhausted receiver, it will evaporate so fast, however cold it
maybe, as to give it the appearance of boiling from the surface. But
without the assistance of the air-pump, I can show you a very pretty
experiment, which proves the effect of the pressure of the atmosphere in
this respect.

Observe, that this Florence flask is about half full of water, and the
upper half of invisible vapour, the water being in the act of boiling.
--I take it from the lamp, and cork it carefully--the water, you see,
immediately ceases boiling. --I shall now dip the flask into a bason of
cold water.**

    [Footnote *: On the top of Mount Blanc, water boiled when heated
    only to 187 degrees, instead of 212 degrees.]

    [Footnote **: The same effect may be produced by wrapping a cold
    wet linen cloth round the upper part of the flask. In order to
    show how much the water cools whilst it is boiling, a thermometer,
    graduated on the tube itself, may be introduced into the bottle
    through the cork.]

CAROLINE.

But look, Mrs. B., the hot water begins to boil again, although the cold
water must rob it more and more of its caloric! What can be the reason
of that?

MRS. B.

Let us examine its temperature. You see the thermometer immersed in it
remains stationary at 180 degrees, which is about 30 degrees below the
boiling point. When I took the flask from the lamp, I observed to you
that the upper part of it was filled with vapour; this being compelled
to yield its caloric to the cold water, was again condensed into water--
What, then, filled the upper part of the flask?

EMILY.

Nothing; for it was too well corked for the air to gain admittance, and
therefore the upper part of the flask must be a vacuum.

MRS. B.

The water below, therefore, no longer sustains the pressure of the
atmosphere, and will consequently boil at a much lower temperature.
Thus, you see, though it had lost many degrees of heat, it began boiling
again the instant the vacuum was formed above it. The boiling has now
ceased, the temperature of the water being still farther reduced; if it
had been ether, instead of water, it would have continued boiling much
longer, for ether boils, under the usual atmospheric pressure, at a
temperature as low as 100 degrees; and in a vacuum it boils at almost
any temperature; but water being a more dense fluid, requires a more
considerable quantity of caloric to make it evaporate quickly, even when
the pressure of the atmosphere is removed.

EMILY.

What proportion of vapour can the atmosphere contain in a state of
solution?

MRS. B.

I do not know whether it has been exactly ascertained by experiment; but
at any rate this proportion must vary, both according to the temperature
and the weight of the atmosphere; for the lower the temperature, and the
greater the pressure, the smaller must be the proportion of vapour that
the atmosphere can contain.

To conclude the subject of free caloric, I should mention _Ignition_, by
which is meant that emission of light which is produced in bodies at a
very high temperature, and which is the effect of accumulated caloric.

EMILY.

You mean, I suppose, that light which is produced by a burning body?

MRS. B.

No: ignition is quite independent of combustion. Clay, chalk, and indeed
all incombustible substances, may be made red hot. When a body burns,
the light emitted is the effect of a chemical change which takes place,
whilst ignition is the effect of caloric alone, and no other change than
that of temperature is produced in the ignited body.

All solid bodies, and most liquids, are susceptible of ignition, or, in
other words, of being heated so as to become luminous; and it is
remarkable that this takes place pretty nearly at the same temperature
in all bodies, that is, at about 800 degrees of Fahrenheit's scale.

EMILY.

But how can liquids attain so high a temperature, without being
converted into vapour?

MRS. B.

By means of confinement and pressure. Water confined in a strong iron
vessel (called Papin's digester) can have its temperature raised to
upwards of 400 degrees. Sir James Hall has made some very curious
experiments on the effects of heat assisted by pressure; by means of
strong gun-barrels, he succeeded in melting a variety of substances
which were considered as infusible: and it is not unlikely that, by
similar methods, water itself might be heated to redness.

EMILY.

I am surprised at that: for I thought that the force of steam was such
as to destroy almost all mechanical resistance.

MRS. B.

The expansive force of steam is prodigious; but in order to subject
water to such high temperatures, it is prevented by confinement from
being converted into steam, and the expansion of heated water is
comparatively trifling. --But we have dwelt so long on the subject of
free caloric, that we must reserve the other modifications of that agent
to our next meeting, when we shall endeavour to proceed more rapidly.




CONVERSATION IV.

ON COMBINED CALORIC, COMPREHENDING SPECIFIC AND LATENT HEAT.


MRS. B.

We are now to examine the other modifications of caloric.

CAROLINE.

I am very curious to know of what nature they can be; for I have no
notion of any kind of heat that is not perceptible to the senses.

MRS. B.

In order to enable you to understand them, it will be necessary to enter
into some previous explanations.

It has been discovered by modern chemists, that bodies of a different
nature, heated to the same temperature, do not contain the same quantity
of caloric.

CAROLINE.

How could that be ascertained? Have you not told us that it is
impossible to discover the absolute quantity of caloric which bodies
contain?

MRS. B.

True; but at the same time I said that we were enabled to form a
judgment of the proportions which bodies bore to each other in this
respect. Thus it is found that, in order to raise the temperature of
different bodies the same number of degrees, different quantities of
caloric are required for each of them. If, for instance, you place a
pound of lead, a pound of chalk, and a pound of milk, in a hot oven,
they will be gradually heated to the temperature of the oven; but the
lead will attain it first, the chalk next, and the milk last.

CAROLINE.

That is a natural consequence of their different bulks; the lead being
the smallest body, will be heated soonest, and the milk, which is the
largest, will require the longest time.

MRS. B.

That explanation will not do, for if the lead be the least in bulk, it
offers also the least surface to the caloric, the quantity of heat
therefore which can enter into it in the same space of time is
proportionally smaller.

EMILY.

Why, then, do not the three bodies attain the temperature of the oven at
the same time?

MRS. B.

It is supposed to be on account of the different capacity of these
bodies for caloric.

CAROLINE.

What do you mean by the capacity of a body for caloric?

MRS. B.

I mean a certain disposition of bodies to require more or less caloric
for raising their temperature to any degree of heat. Perhaps the fact
may be thus explained:

Let us put as many marbles into this glass as it will contain, and pour
some sand over them--observe how the sand penetrates and lodges between
them. We shall now fill another glass with pebbles of various forms--you
see that they arrange themselves in a more compact manner than the
marbles, which, being globular, can touch each other by a single point
only. The pebbles, therefore, will not admit so much sand between them;
and consequently one of these glasses will necessarily contain more sand
than the other, though both of them be equally full.

CAROLINE.

This I understand perfectly. The marbles and the pebbles represent two
bodies of different kinds, and the sand the caloric contained in them;
it appears very plain, from this comparison, that one body may admit of
more caloric between its particles than another.

MRS. B.

You can no longer be surprised, therefore, that bodies of a different
capacity for caloric should require different proportions of that fluid
to raise their temperatures equally.

EMILY.

But I do not conceive why the body that contains the most caloric should
not be of the highest temperature; that is to say, feel hot in
proportion to the quantity of caloric it contains?

MRS. B.

The caloric that is employed in filling the capacity of a body, is not
free caloric; but is imprisoned as it were in the body, and is therefore
imperceptible: for we can feel only the caloric which the body parts
with, and not that which it retains.

CAROLINE.

It appears to me very extraordinary that heat should be confined in a
body in such a manner as to be imperceptible.

MRS. B.

If you lay your hand on a hot body, you feel only the caloric which
leaves it, and enters your hand; for it is impossible that you should be
sensible of that which remains in the body. The thermometer, in the same
manner, is affected only by the free caloric which a body transmits to
it, and not at all by that which it does not part with.

CAROLINE.

I begin to understand it: but I confess that the idea of insensible heat
is so new and strange to me, that it requires some time to render it
familiar.

MRS. B.

Call it insensible caloric, and the difficulty will appear much less
formidable. It is indeed a sort of contradiction to call it heat, when
it is so situated as to be incapable of producing that sensation. Yet
this modification of caloric is commonly called SPECIFIC HEAT.

CAROLINE.

But it certainly would have been more correct to have called it
_specific caloric_.

EMILY.

I do not understand how the term _specific_ applies to this modification
of caloric?

MRS. B.

It expresses the relative quantity of caloric which different _species_
of bodies of the same weight and temperature are capable of containing.
This modification is also frequently called _heat of capacity_, a term
perhaps preferable, as it explains better its own meaning.

You now understand, I suppose, why the milk and chalk required a longer
portion of time than the lead to raise their temperature to that of the
oven?

EMILY.

Yes: the milk and chalk having a greater capacity for caloric than the
lead, a greater proportion of that fluid became insensible in those
bodies: and the more slowly, therefore, their temperature was raised.

CAROLINE.

But might not this difference proceed from the different conducting
powers of heat in these three bodies, since that which is the best
conductor must necessarily attain the temperature of the oven first?

MRS. B.

Very well observed, Caroline. This objection would be insurmountable, if
we could not, by reversing the experiment, prove that the milk, the
chalk, and the lead, actually absorbed different quantities of caloric,
and we know that if the different time they took in heating, proceeded
merely from their different conducting powers, they would each have
acquired an equal quantity of caloric.

CAROLINE.

Certainly. But how can you reverse this experiment?

MRS. B.

It may be done by cooling the several bodies to the same degree in an
apparatus adapted to receive and measure the caloric which they give
out. Thus, if you plunge them into three equal quantities of water, each
at the same temperature, you will be able to judge of the relative
quantity of caloric which the three bodies contained, by that, which, in
cooling, they communicated to their respective portions of water: for
the same quantity of caloric which they each absorbed to raise their
temperature, will abandon them in lowering it; and on examining the
three vessels of water, you will find the one in which you immersed the
lead to be the least heated; that which held the chalk will be the next;
and that which contained the milk will be heated the most of all. The
celebrated Lavoisier has invented a machine to estimate, upon this
principle, the specific heat of bodies in a more perfect manner; but I
cannot explain it to you, till you are acquainted with the next
modification of caloric.

EMILY.

The more dense a body is, I suppose, the less is its capacity for
caloric?

MRS. B.

This is not always the case with bodies of different nature; iron, for
instance, contains more specific heat than tin, though it is more dense.
This seems to show that specific heat does hot merely depend upon the
interstices between the particles; but, probably, also upon some
peculiar constitution of the bodies which we do not comprehend.

EMILY.

But, Mrs. B., it would appear to me more proper to compare bodies by
_measure_, rather than by _weight_, in order to estimate their specific
heat. Why, for instance, should we not compare _pints_ of milk, of
chalk, and of lead, rather than _pounds_ of those substances; for equal
weights may be composed of very different quantities?

MRS. B.

You are mistaken, my dear; equal weight must contain equal quantities of
matter; and when we wish to know what is the relative quantity of
caloric, which substances of various kinds are capable of containing
under the same temperature, we must compare equal weights, and not equal
bulks of those substances. Bodies of the same weight may undoubtedly be
of very different dimensions; but that does not change their real
quantity of matter. A pound of feathers does not contain one atom more
than a pound of lead.

CAROLINE.

I have another difficulty to propose. It appears to me, that if the
temperature of the three bodies in the oven did not rise equally, they
would never reach the same degree; the lead would always keep its
advantage over the chalk and milk, and would perhaps be boiling before
the others had attained the temperature of the oven. I think you might
as well say that, in the course of time, you and I should be of the same
age?

MRS. B.

Your comparison is not correct, Caroline. As soon as the lead reached
the temperature of the oven, it would remain stationary; for it would
then give out as much heat as it would receive. You should recollect
that the exchange of radiating heat, between two bodies of equal
temperature, is equal: it would be impossible, therefore, for the lead
to accumulate heat after having attained the temperature of the oven;
and that of the chalk and milk therefore would ultimately arrive at the
same standard. Now I fear that this will not hold good with respect to
our ages, and that, as long as I live, I shall never cease to keep my
advantage over you.

EMILY.

I think that I have found a comparison for specific heat, which is very
applicable. Suppose that two men of equal weight and bulk, but who
required different quantities of food to satisfy their appetites, sit
down to dinner, both equally hungry; the one would consume a much
greater quantity of provisions than the other, in order to be equally
satisfied.

MRS. B.

Yes, that is very fair; for the quantity of food necessary to satisfy
their respective appetites, varies in the same manner as the quantity of
caloric requisite to raise equally the temperature of different bodies.

EMILY.

The thermometer, then, affords no indication of the specific heat of
bodies?

MRS. B.

None at all: no more than satiety is a test of the quantity of food
eaten. The thermometer, as I have repeatedly said, can be affected only
by free caloric, which alone raises the temperature of bodies.

But there is another mode of proving the existence of specific heat,
which affords a very satisfactory illustration of that modification.
This, however, I did not enlarge upon before, as I thought it might
appear to you rather complicated. --If you mix two fluids of different
temperatures, let us say the one at 50 degrees, and the other at 100
degrees, of what temperature do you suppose the mixture will be?

CAROLINE.

It will be no doubt the medium between the two, that is to say, 75
degrees.

MRS. B.

That will be the case if the two bodies happen to have the same capacity
for caloric; but if not, a different result will be obtained. Thus, for
instance, if you mix together a pound of mercury, heated at 50 degrees,
and a pound of water heated at 100 degrees, the temperature of the
mixture, instead of being 75 degrees, will be 80 degrees; so that the
water will have lost only 12 degrees, whilst the mercury will have
gained 38 degrees; from which you will conclude that the capacity of
mercury for heat is less than that of water.

CAROLINE.

I wonder that mercury should have so little specific heat. Did we not
see it was a much better conductor of heat than water?

MRS. B.

And it is precisely on that account that its specific heat is less. For
since the conductive power of bodies depends, as we have observed
before, on their readiness to receive heat and part with it, it is
natural to expect that those bodies which are the worst conductors
should absorb the most caloric before they are disposed to part with it
to other bodies. But let us now proceed to LATENT HEAT.

CAROLINE.

And pray what kind of heat is that?

MRS. B.

It is another modification of combined caloric, which is so analogous to
specific heat, that most chemists make no distinction between them; but
Mr. Pictet, in his Essay on Fire, has so clearly discriminated them,
that I am induced to adopt his view of the subject. We therefore call
_latent heat_ that portion of insensible caloric which is employed in
changing the state of bodies; that is to say, in converting solids into
liquids, or liquids; into vapour. When a body changes its state from
solid to liquid, or from liquid to vapour, its expansion occasions a
sudden and considerable increase of capacity for heat, in consequence of
which it immediately absorbs a quantity of caloric, which becomes fixed
in the body which it has transformed; and, as it is perfectly concealed
from our senses, it has obtained the name of _latent_ heat.

CAROLINE.

I think it would be much more correct to call this modification latent
caloric instead of latent heat, since it does not excite the sensation
of heat.

MRS. B.

This modification of heat was discovered and named by Dr. Black long
before the French chemists introduced the term caloric, and we must not
presume to alter it, as it is still used by much better chemists than
ourselves. And, besides, you are not to suppose that the nature of heat
is altered by being variously modified: for if latent heat and specific
heat do not excite the same sensations as free caloric, it is owing to
their being in a state of confinement, which prevents them from acting
upon our organs; and consequently, as soon as they are extricated from
the body in which they are imprisoned, they return to their state of
free caloric.

EMILY.

But I do not yet clearly see in what respect latent heat differs from
specific heat; for they are both of them imprisoned and concealed in
bodies.

MRS. B.

Specific heat is that which is employed in filling the capacity of a
body for caloric, in the state in which this body actually exists; while
latent heat is that which is employed only in effecting a change of
state, that is, in converting bodies from a solid to a liquid, or from a
liquid to an aeriform state. But I think that, in a general point of
view, both these modifications might be comprehended under the name of
_heat of capacity_, as in both cases the caloric is equally engaged in
filling the capacities of bodies.

I shall now show you an experiment, which I hope will give you a clear
idea of what is understood by latent heat.

The snow which you see in this phial has been cooled by certain chemical
means (which I cannot well explain to you at present), to 5 or 6 degrees
below the freezing point, as you will find indicated by the thermometer
which is placed in it. We shall expose it to the heat of a lamp, and you
will see the thermometer gradually rise, till it reaches the freezing
point----

EMILY.

But there it stops, Mrs. B., and yet the lamp burns just as well as
before. Why is not its heat communicated to the thermometer?

CAROLINE.

And the snow begins to melt, therefore it must be rising above the
freezing point?

MRS. B.

The heat no longer affects the thermometer, because it is wholly
employed in converting the ice into water. As the ice melts, the caloric
becomes _latent_ in the new-formed liquid, and therefore cannot raise
its temperature; and the thermometer will consequently remain
stationary, till the whole of the ice be melted.

CAROLINE.

Now it is all melted, and the thermometer begins to rise again.

MRS. B.

Because the conversion of the ice into water being completed, the
caloric no longer becomes latent; and therefore the heat which the water
now receives raises its temperature, as you find the thermometer
indicates.

EMILY.

But I do not think that the thermometer rises so quickly in the water as
it did in the ice, previous to its beginning to melt, though the lamp
burns equally well?

MRS. B.

That is owing to the different specific heat of ice and water. The
capacity of water for caloric being greater than that of ice, more heat
is required to raise its temperature, and therefore the thermometer
rises slower in the water than in the ice.

EMILY.

True; you said that a solid body always increased its capacity for heat
by becoming fluid; and this is an instance of it.

MRS. B.

Yes, and the latent heat is that which is absorbed in consequence of the
greater capacity which the water has for heat, in comparison to ice.

I must now tell you a curious calculation founded on that consideration.
I have before observed to you that though the thermometer shows us the
comparative warmth of bodies, and enables us to determine the same point
at different times and places, it gives us no idea of the absolute
quantity of heat in any body. We cannot tell how low it ought to fall by
the privation of all heat, but an attempt has been made to infer it in
the following manner. It has been found by experiment, that the capacity
of water for heat, when compared with that of ice, is as 10 to 9, so
that, at the same temperature, ice contains one tenth of caloric less
than water. By experiment also it is observed, that in order to melt
ice, there must be added to it as much heat, as would, if it did not
melt it, raise its temperature 140 degrees. This quantity of heat is
therefore absorbed when the ice, by being converted into water, is made
to contain one-ninth more caloric than it did before. Therefore 140
degrees is a ninth part of the heat contained in ice at 30 degrees; and
the point of zero, or the absolute privation of heat, must consequently
be 1260 degrees below 32 degrees.

This mode of investigating so curious a question is ingenious, but its
correctness is not yet established by similar calculations for other
bodies. The points of absolute cold, indicated by this method in various
bodies, are very remote from each other; it is however possible, that
this may arise from some imperfection in the experiments.

CAROLINE.

It is indeed very ingenious--but we must now attend to our present
experiment. The water begins to boil, and the thermometer is again
stationary.

MRS. B.

Well, Caroline, it is your turn to explain the phenomenon.

CAROLINE.

It is wonderfully curious! The caloric is now busy in changing the water
into steam, in which it hides itself, and becomes insensible. This is
another example of latent heat, producing a change of form. At first it
converted a solid body into a liquid, and now it turns the liquid into
vapour!

MRS. B.

You see, my dear, how easily you have become acquainted with these
modifications of insensible heat, which at first appeared so
unintelligible. If, now, we were to reverse these changes, and condense
the vapour into water, and the water into ice, the latent heat would
re-appear entirely, in the form of free caloric.

EMILY.

Pray do let us see the effect of latent heat returning to its free
state.

MRS. B.

For the purpose of showing this, we need simply conduct the vapour
through this tube into this vessel of cold water, where it will part
with its latent heat and return to its liquid form.

EMILY.

How rapidly the steam heats the water!

MRS. B.

That is because it does not merely impart its free caloric to the water,
but likewise its latent heat. This method of heating liquids, has been
turned to advantage, in several economical establishments. The
steam-kitchens, which are getting into such general use, are upon the
same principle. The steam is conveyed through a pipe in a similar
manner, into the several vessels which contain the provisions to be
dressed, where it communicates to them its latent caloric, and returns
to the state of water. Count Rumford makes great use of this principle
in many of his fire-places: his grand maxim is to avoid all unnecessary
waste of caloric, for which purpose he confines the heat in such a
manner, that not a particle of it shall unnecessarily escape; and while
he economises the free caloric, he takes care also to turn the latent
heat to advantage. It is thus that he is enabled to produce a degree of
heat superior to that which is obtained in common fire-places, though he
employs less fuel.

EMILY.

When the advantages of such contrivances are so clear and plain,
I cannot understand why they are not universally used.

MRS. B.

A long time is always required before innovations, however useful, can
be reconciled with the prejudices of the vulgar.

EMILY.

What a pity it is that there should be a prejudice against new
inventions; how much more rapidly the world would improve, if such
useful discoveries were immediately and universally adopted!

MRS. B.

I believe, my dear, that there are as many novelties attempted to be
introduced, the adoption of which would be prejudicial to society, as
there are of those which would be beneficial to it. The well-informed,
though by no means exempt from error, have an unquestionable advantage
over the illiterate, in judging what is likely or not to prove
serviceable; and therefore we find the former more ready to adopt such
discoveries as promise to be really advantageous, than the latter, who
having no other test of the value of a novelty but time and experience,
at first oppose its introduction. The well-informed, however, are
frequently disappointed in their most sanguine expectations, and the
prejudices of the vulgar, though they often retard the progress of
knowledge, yet sometimes, it must be admitted, prevent the propagation
of error. --But we are deviating from our subject.

We have converted steam into water, and are now to change water into
ice, in order to render the latent heat sensible, as it escapes from the
water on its becoming solid. For this purpose we must produce a degree
of cold that will make water freeze.

CAROLINE.

That must be very difficult to accomplish in this warm room.

MRS. B.

Not so much as you think. There are certain chemical mixtures which
produce a rapid change from the solid to the fluid state, or the
reverse, in the substances combined, in consequence of which change
latent heat is either extricated or absorbed.

EMILY.

I do not quite understand you.

MRS. B.

This snow and salt, which you see me mix together, are melting rapidly;
heat, therefore, must be absorbed by the mixture, and cold produced.

CAROLINE.

It feels even colder than ice, and yet the snow is melted. This is very
extraordinary.

MRS. B.

The cause of the intense cold of the mixture is to be attributed to the
change from a solid to a fluid state. The union of the snow and salt
produces a new arrangement of their particles, in consequence of which
they become liquid; and the quantity of caloric, required to effect this
change, is seized upon by the mixture wherever it can be obtained. This
eagerness of the mixture for caloric, during its liquefaction, is such,
that it converts part of its own free caloric into latent heat, and it
is thus that its temperature is lowered.

EMILY.

Whatever you put in this mixture, therefore, would freeze?

MRS. B.

Yes; at least any fluid that is susceptible of freezing at that
temperature. I have prepared this mixture of salt and snow for the
purpose of freezing the water from which you are desirous of seeing the
latent heat escape. I have put a thermometer in the glass of water that
is to be frozen, in order that you may see how it cools.

CAROLINE.

The thermometer descends, but the heat which the water is now losing, is
its _free_, not its _latent_ heat.

MRS. B.

Certainly; it does not part with its latent heat till it changes its
state and is converted into ice.

EMILY.

But here is a very extraordinary circumstance! The thermometer is fallen
below the freezing point, and yet the water is not frozen.

MRS. B.

That is always the case previous to the freezing of water when it is in
a state of rest. Now it begins to congeal, and you may observe that the
thermometer again rises to the freezing point.

CAROLINE.

It appears to me very strange that the thermometer should rise the very
moment that the water freezes; for it seems to imply that the water was
colder before it froze than when in the act of freezing.

MRS. B.

It is so; and after our long dissertation on this circumstance, I did
not think it would appear so surprising to you. Reflect a little, and I
think you will discover the reason of it.

CAROLINE.

It must be, no doubt, the extrications of latent heat, at the instant
the water freezes, that raises the temperature.

MRS. B.

Certainly; and if you now examine the thermometer, you will find that
its rise was but temporary, and lasted only during the disengagement of
the latent heat--now that all the water is frozen it falls again, and
will continue to fall till the ice and mixture are of an equal
temperature.

EMILY.

And can you show us any experiments in which liquids, by being mixed,
become solid, and disengage latent heat?

MRS. B.

I could show you several; but you are not yet sufficiently advanced to
understand them well. I shall, however, try one, which will afford you a
striking instance of the fact. The fluid which you see in this phial
consists of a quantity of a certain salt called _muriat of lime_,
dissolved in water. Now, if I pour into it a few drops of this other
fluid, called _sulphuric acid_, the whole, or very nearly the whole,
will be instantaneously converted into a solid mass.

EMILY.

How white it turns! I feel the latent heat escaping, for the bottle is
warm, and the fluid is changed to a solid white substance like chalk!

CAROLINE.

This is, indeed, the most curious experiment we have seen yet. But pray
what is that white vapour that ascends from the mixture?

MRS. B.

You are not yet enough of a chemist to understand that. --But take care,
Caroline, do not approach too near it, for it has a very pungent smell.

I shall show you another instance similar to that of the water, which
you observed to become warmer as it froze. I have in this phial a
solution of a salt called sulphat of soda or Glauber's salt, made very
strong, and corked up when it was hot, and kept without agitation till
it became cold, as you may feel the phial is. Now when I take out the
cork and let the air fall upon it, (for being closed when boiling, there
was a vacuum in the upper part) observe that the salt will suddenly
crystallize. . . .

CAROLINE.

Surprising! how beautifully the needles of salt have shot through the
whole phial!

MRS. B.

Yes, it is very striking--but pray do not forget the object of the
experiment. Feel how warm the phial has become by the conversion of part
of the liquid into a solid.

EMILY.

Quite warm I declare! this is a most curious experiment of the
disengagement of latent heat.

MRS. B.

The slakeing of lime is another remarkable instance of the extrication
of latent heat. Have you never observed how quick-lime smokes when water
is poured upon it, and how much heat it produces?

CAROLINE.

Yes; but I do not understand what change of state takes place in the
lime that occasions its giving out latent heat; for the quick-lime,
which is solid, is (if I recollect right) reduced to powder, by this
operation, and is, therefore, rather expanded than condensed.

MRS. B.

It is from the water, not the lime, that the latent heat is set free.
The water incorporates with, and becomes solid in the lime; in
consequence of which, the heat, which kept it in a liquid state, is
disengaged, and escapes in a sensible form.

CAROLINE.

I always thought that the heat originated in the lime. It seems very
strange that water, and cold water too, should contain so much heat.

EMILY.

After this extrication of caloric, the water must exist in a state of
ice in the lime, since it parts with the heat which kept it liquid.

MRS. B.

It cannot properly be called ice, since ice implies a degree of cold, at
least equal to the freezing point. Yet as water, in combining with lime,
gives out more heat than in freezing, it must be in a state of still
greater solidity in the lime, than it is in the form of ice; and you may
have observed that it does not moisten or liquefy the lime in the
smallest degree.

EMILY.

But, Mrs. B., the smoke that rises is white; if it was only pure caloric
which escaped, we might feel, but could not see it.

MRS. B.

This white vapour is formed by some of the particles of lime, in a state
of fine dust, which are carried off by the caloric.

EMILY.

In all changes of state, then, a body either absorbs or disengages
latent heat?

MRS. B.

You cannot exactly say _absorbs latent heat_, as the heat becomes latent
only on being confined in the body; but you may say, generally, that
bodies, in passing from a solid to a liquid form, or from the liquid
state to that of vapour, absorb heat; and that when the reverse takes
place, heat is disengaged.*

    [Footnote *: This rule, if not universal, admits of very few
    exceptions.]

EMILY.

We can now, I think, account for the ether boiling, and the water
freezing in vacuo, at the same temperature.**

    [Footnote **: See page 102.]

MRS. B.

Let me hear how you explain it.

EMILY.

The latent heat, which the water gave out in freezing, was immediately
absorbed by the ether, during its conversion into vapour; and therefore,
from a latent state in one liquid, it passed into a latent state in the
other.

MRS. B.

But this only partly accounts for the result of the experiment; it
remains to be explained why the temperature of the ether, while in a
state of ebullition, is brought down to the freezing temperature of the
water. --It is because the ether, during its evaporation, reduces its
own temperature, in the same proportion as that of the water, by
converting its free caloric into latent heat: so that, though one liquid
boils, and the other freezes, their temperatures remain in a state of
equilibrium.

EMILY.

But why does not water, as well as ether, reduce its own temperature by
evaporating?

MRS. B.

The fact is that it does, though much less rapidly than ether. Thus, for
instance, you may often have observed, in the heat of summer, how much
any particular spot may be cooled by watering, though the water used for
that purpose be as warm as the air itself. Indeed so much cold may be
produced by the mere evaporation of water, that the inhabitants of
India, by availing themselves of the most favourable circumstances for
this process which their warm climate can afford, namely, the cool of
the night, and situations most exposed to the night breeze, succeed in
causing water to freeze, though the temperature of the air be as high as
60 degrees. The water is put into shallow earthen trays, so as to expose
an extensive surface to the process of evaporation, and in the morning,
the water is found covered with a thin cake of ice, which is collected
in sufficient quantity to be used for purposes of luxury.

CAROLINE.

How delicious it must be to drink liquids so cold in those tropical
climates! But, Mrs. B., could we not try that experiment?

MRS. B.

If we were in the country, I have no doubt but that we should be able to
freeze water, by the same means, and under similar circumstances. But we
can do it immediately, upon a small scale, in this very room, in which
the thermometer stands at 70 degrees. For this purpose we need only
place some water in a little cup under the receiver of the air-pump
(PLATE V. fig. 1.), and exhaust the air from it. What will be the
consequence, Caroline?

  [Illustration: Plate V. Vol. I. page 138.

  Fig. 1.
  The air-pump & receiver for Mr. Leslie's experiment.
  C a saucer with sulphuric Acid.
  B a glass or earthen cup containing Water.
  D a stand for the cup with its legs made of Glass.
  A a Thermometer.

  Fig. 2. Dr. Wollaston's Cryophorus.

  Fig. 5. Dr. Marcet's mode of using the Cryophorus.

  Fig. 3. & 4. the different parts of Fig. 5. seen separate.]

CAROLINE.

Of course the water will evaporate more quickly, since there will no
longer be any atmospheric pressure on its surface: but will this be
sufficient to make the water freeze?

MRS. B.

Probably not, because the vapour will not be carried off fast enough;
but this will be accomplished without difficulty if we introduce into
the receiver (fig. 1.), in a saucer, or other large shallow vessel, some
strong sulphuric acid, a substance which has a great attraction for
water, whether in the form of vapour, or in the liquid state. This
attraction is such that the acid will instantly absorb the moisture as
it rises from the water, so as to make room for the formation of fresh
vapour; this will of course hasten the process, and the cold produced
from the rapid evaporation of the water, will, in a few minutes, be
sufficient to freeze its surface.* We shall now exhaust the air from the
receiver.

    [Footnote *: This experiment was first devised by Mr. Leslie, and
    has since been modified in a variety of forms.]

EMILY.

Thousands of small bubbles already rise through the water from the
internal surface of the cup; what is the reason of this?

MRS. B.

These are bubbles of air which were partly attached to the vessel, and
partly diffused in the water itself; and they expand and rise in
consequence of the atmospheric pressure being removed.

CAROLINE.

See, Mrs. B.; the thermometer in the cup is sinking fast; it has already
descended to 40 degrees!

EMILY.

The water seems now and then violently agitated on the surface, as if it
was boiling; and yet the thermometer is descending fast!

MRS. B.

You may call it _boiling_, if you please, for this appearance is, as
well as boiling, owing to the rapid formation of vapour; but here, as
you have just observed, it takes place from the surface, for it is only
when heat is applied to the bottom of the vessel that the vapour is
formed there. --Now crystals of ice are actually shooting all over the
surface of the water.

CAROLINE.

How beautiful it is! The surface is now entirely frozen--but the
thermometer remains at 32 degrees.

MRS. B.

And so it will, conformably with our doctrine of latent heat, until the
whole of the water is frozen; but it will then again begin to descend
lower and lower, in consequence of the evaporation which goes on from
the surface of the ice.

EMILY.

This is a most interesting experiment; but it would be still more
striking if no sulphuric acid were required.

MRS. B.

I will show you a freezing instrument, contrived by Dr. Wollaston, upon
the same principle as Mr. Leslie's experiment, by which water may be
frozen by its own evaporation alone, without the assistance of sulphuric
acid.

This tube, which, as you see (PLATE V. fig. 2.), is terminated at each
extremity by a bulb, one of which is half full of water, is internally
perfectly exhausted of air; the consequence of this is, that the water
in the bulb is always much disposed to evaporate. This evaporation,
however, does not proceed sufficiently fast to freeze the water; but if
the empty ball be cooled by some artificial means, so as to condense
quickly the vapour which rises from the water, the process may be thus
so much promoted as to cause the water to freeze in the other ball. Dr.
Wollaston has called this instrument _Cryophorus_.

CAROLINE.

So that cold seems to perform here the same part which the sulphuric
acid acted in Mr. Leslie's experiment?

MRS. B.

Exactly so; but let us try the experiment.

EMILY.

How will you cool the instrument? You have neither ice nor snow.

MRS. B.

True: but we have other means of effecting this.* You recollect what an
intense cold can be produced by the evaporation of ether in an exhausted
receiver. We shall inclose the bulb in this little bag of fine flannel
(fig. 3.), then soke it in ether, and introduce it into the receiver of
the air-pump. (Fig. 5.) For this purpose we shall find it more
convenient to use a cryophorus of this shape (fig. 4.), as its elongated
bulb passes easily through a brass plate which closes the top of the
receiver. If we now exhaust the receiver quickly, you will see, in less
than a minute, the water freeze in the other bulb, out of the receiver.

    [Footnote *: This mode of making the experiment was proposed, and
    the particulars detailed, by Dr. Marcet, in the 34th vol. of
    Nicholson's Journal, page 119.]

EMILY.

The bulb already looks quite dim, and small drops of water are
condensing on its surface.

CAROLINE.

And now crystals of ice shoot all over the water. This is, indeed,
a very curious experiment!

MRS. B.

You will see, some other day, that, by a similar method, even
quicksilver may be frozen. --But we cannot at present indulge in any
further digression.

Having advanced so far on the subject of heat, I may now give you an
account of the calorimeter, an instrument invented by Lavoisier, upon
the principles just explained, for the purpose of estimating the
specific heat of bodies. It consists of a vessel, the inner surface of
which is lined with ice, so as to form a sort of hollow globe of ice, in
the midst of which the body, whose specific heat is to be ascertained,
is placed. The ice absorbs caloric from this body, till it has brought
it down to the freezing point; this caloric converts into water a
certain portion of the ice which runs out through an aperture at the
bottom of the machine; and the quantity of ice changed to water is a
test of the quantity of caloric which the body has given out in
descending from a certain temperature to the freezing point.

CAROLINE.

In this apparatus, I suppose, the milk, chalk, and lead, would melt
different quantities of ice, in proportion to their different capacities
for caloric?

MRS. B.

Certainly: and thence we are able to ascertain, with precision, their
respective capacities for heat. But the calorimeter affords us no more
idea of the absolute quantity of heat contained in a body, than the
thermometer; for though by means of it we extricate both the free and
combined caloric, yet we extricate them only to a certain degree, which
is the freezing point; and we know not how much they contain of either
below that point.

EMILY.

According to the theory of latent heat, it appears to me that the
weather should be warm when it freezes, and cold in a thaw: for latent
heat is liberated from every substance that it freezes, and such a large
supply of heat must warm the atmosphere; whilst, during a thaw, that
very quantity of free heat must be taken from the atmosphere, and return
to a latent state in the bodies which it thaws.

MRS. B.

Your observation is very natural; but consider that in a frost the
atmosphere is so much colder than the earth, that all the caloric which
it takes from the freezing bodies is insufficient to raise its
temperature above the freezing point; otherwise the frost must cease.
But if the quantity of latent heat extricated does not destroy the
frost, it serves to moderate the suddenness of the change of temperature
of the atmosphere, at the commencement both of frost, and of a thaw. In
the first instance, its extrication diminishes the severity of the cold;
and, in the latter, its absorption moderates the warmth occasioned by a
thaw: it even sometimes produces a discernible chill, at the breaking up
of a frost.

CAROLINE.

But what are the general causes that produce those sudden changes in the
weather, especially from hot to cold, which we often experience?

MRS. B.

This question would lead us into meteorological discussions, to which I
am by no means competent. One circumstance, however, we can easily
understand. When the air has passed over cold countries, it will
probably arrive here at a temperature much below our own, and then it
must absorb heat from every object it meets with, which will produce a
general fall of temperature.

CAROLINE.

But pray, now that we know so much of the effects of heat, will you
inform us whether it is really a distinct body, or, as I have heard,
a peculiar kind of motion produced in bodies?

MRS. B.

As I before told you, there is yet much uncertainty as to the nature of
these subtle agents. But I am inclined to consider heat not as mere
motion, but as a separate substance. Late experiments too appear to make
it a compound body, consisting of the two electricities, and in our next
conversation I shall inform you of the principal facts on which that
opinion is founded.




CONVERSATION V.

ON THE CHEMICAL AGENCIES OF ELECTRICITY.


MRS. B.

Before we proceed further it will be necessary to give you some account
of certain properties of electricity, which have of late years been
discovered to have an essential connection with the phenomena of
chemistry.

CAROLINE.

It is ELECTRICITY, if I recollect right, which comes next in our list of
simple substances?

MRS. B.

I have placed electricity in that list, rather from the necessity of
classing it somewhere, than from any conviction that it has a right to
that situation, for we are as yet so ignorant of its intimate nature,
that we are unable to determine, not only whether it is simple or
compound, but whether it is in fact a material agent; or, as Sir H. Davy
has hinted, whether it may not be merely a property inherent in matter.
As, however, it is necessary to adopt some hypothesis for the
explanation of the discoveries which this agent has enabled us to make,
I have chosen the opinion, at present most prevalent, which supposes the
existence of two kinds of electricity, distinguished by the names of
_positive_ and _negative_ electricity.

CAROLINE.

Well, I must confess, I do not feel nearly so interested in a science in
which so much uncertainty prevails, as in those which rest upon
established principles; I never was fond of electricity, because,
however beautiful and curious the phenomena it exhibits may be, the
theories, by which they were explained, appeared to me so various, so
obscure and inadequate, that I always remained dissatisfied. I was in
hopes that the new discoveries in electricity had thrown so great a
light on the subject, that every thing respecting it would now have been
clearly explained.

MRS. B.

That is a point which we are yet far from having attained. But, in spite
of the imperfection of our theories, you will be amply repaid by the
importance and novelty of the subject. The number of new facts which
have already been ascertained, and the immense prospect of discovery
which has lately been opened to us, will, I hope, ultimately lead to a
perfect elucidation of this branch of natural science; but at present
you must be contented with studying the effects, and in some degree
explaining the phenomena, without aspiring to a precise knowledge of the
remote cause of electricity.

You have already obtained some notions of electricity: in our present
conversation, therefore, I shall confine myself to that part of the
science which is of late discovery, and is more particularly connected
with chemistry.

It was a trifling and accidental circumstance which first gave rise to
this new branch of physical science. Galvani, a professor of natural
philosophy at Bologna, being engaged (about twenty years ago) in some
experiments on muscular irritability, observed, that when a piece of
metal was laid on the nerve of a frog, recently dead, whilst the limb
supplied by that nerve rested upon some other metal, the limb suddenly
moved, on a communication being made between the two pieces of metal.

EMILY.

How is this communication made?

MRS. B.

Either by bringing the two metals into contact, or by connecting them by
means of a metallic conductor. But without subjecting a frog to any
cruel experiments, I can easily make you sensible of this kind of
electric action. Here is a piece of zinc, (one of the metals I mentioned
in the list of elementary bodies)--put it _under_ your tongue, and this
piece of silver _upon_ your tongue, and let both the metals project a
little beyond the tip of the tongue--very well--now make the projecting
parts of the metals touch each other, and you will instantly perceive a
peculiar sensation.

EMILY.

Indeed I did, a singular taste, and I think a degree of heat: but I can
hardly describe it.

MRS. B.

The action of these two pieces of metal on the tongue is, I believe,
precisely similar to that made on the nerve of a frog. I shall not
detain you by a detailed account of the theory by which Galvani
attempted to account for this fact, as his explanation was soon
overturned by subsequent experiments, which proved that _Galvanism_ (the
name this new power had obtained) was nothing more than electricity.
Galvani supposed that the virtue of this new agent resided in the nerves
of the frog, but Volta, who prosecuted this subject with much greater
success, shewed that the phenomena did not depend on the organs of the
frog, but upon the electrical agency of the metals, which is excited by
the moisture of the animal, the organs of the frog being only a delicate
test of the presence of electric influence.

CAROLINE.

I suppose, then, the saliva of the mouth answers the same purpose as the
moisture of the frog, in exciting the electricity of the pieces of
silver and zinc with which Emily tried the experiment on her tongue.

MRS. B.

Precisely. It does not appear, however, necessary that the fluid used
for this purpose should be of an animal nature. Water, and acids very
much diluted by water, are found to be the most effectual in promoting
the developement of electricity in metals; and, accordingly, the
original apparatus which Volta first constructed for this purpose,
consisted of a pile or succession of plates of zinc and copper, each
pair of which was connected by pieces of cloth or paper impregnated with
water; and this instrument, from its original inconvenient structure and
limited strength, has gradually arrived at its present state of power
and improvement, such as is exhibited in the Voltaic battery. In this
apparatus, a specimen of which you see before you (PLATE VI. fig. 1.),
the plates of zinc and copper are soldered together in pairs, each pair
being placed at regular distances in wooden troughs and the interstices
being filled with fluid.

  [Illustration: Plate VI. p. 151.

  Fig. 1. Voltaic Battery.
  Fig. 2.
  Fig. 4.
  Fig. 1. 2. & 4. Voltaic Batteries

  Fig. 3. Electrical Machine.
  A the Cylinder.
  B the Conductor.
  R the Rubber.
  C the Chain.]

CAROLINE.

Though you will not allow us to enquire into the precise cause of
electricity, may we not ask in what manner the fluid acts on the metals
so as to produce it?

MRS. B.

The action of the fluid on the metals, whether water or acid be used, is
entirely of a chemical nature. But whether electricity is excited by
this chemical action, or whether it is produced by the contact of the
two metals, is a point upon which philosophers do not yet perfectly
agree.

EMILY.

But can the mere contact of two metals, without any intervening fluid,
produce electricity?

MRS. B.

Yes, if they are afterwards separated. It is an established fact, that
when two metals are put in contact, and afterwards separated, that which
has the strongest attraction for oxygen exhibits signs of positive, the
other of negative electricity.

CAROLINE.

It seems then but reasonable to infer that the power of the Voltaic
battery should arise from the contact of the plates of zinc and copper.

MRS. B.

It is upon this principle that Volta and Sir H. Davy explain the
phenomena of the pile; but notwithstanding these two great authorities,
many philosophers entertain doubts on the truth of this theory. The
principal difficulty which occurs in explaining the phenomena of the
Voltaic battery on this principle, is, that two such plates show no
signs of different states of electricity whilst in contact, but only on
being separated after contact. Now in the Voltaic battery, those plates
that are in contact always continue so, being soldered together: and
they cannot therefore receive a succession of charges. Besides, if we
consider the mere disturbance of the balance of electricity by the
contact of the plates, as the sole cause of the production of Voltaic
electricity, it remains to be explained how this disturbed balance
becomes an inexhaustible source of electrical energy, capable of pouring
forth a constant and copious supply of electrical fluid, though without
any means of replenishing itself from other sources. This subject, it
must be owned, is involved in too much obscurity to enable us to speak
very decidedly in favour of any theory. But, in order to avoid
perplexing you with different explanations, I shall confine myself to
one which appears to me to be least encumbered with difficulties, and
most likely to accord with truth.*

This theory supposes the electricity to be excited by the chemical
action of the acid on the zinc; but you are yet such novices in
chemistry, that I think it will be necessary to give you some previous
explanation of the nature of this action.

All metals have a strong attraction for oxygen, and this element is
found in great abundance both in water and in acids. The action of the
diluted acid on the zinc consists therefore in its oxygen combining with
it, and dissolving its surface.

    [Footnote *: This mode of explaining the phenomena of the Voltaic
    pile is called the _chemical theory_ of electricity, because it
    ascribes the cause of these phenomena to certain chemical changes
    which take place during their appearance. In the preceding edition
    of this work, the same theory was presented in a more elaborate,
    but less easy form than it is in this. The mode of viewing the
    subject which is here sketched was long since suggested by Dr.
    Bostock, of whose theory, however, this is by no means to be
    considered as a complete statement.]

CAROLINE.

In the same manner I suppose as we saw an acid dissolve copper?

MRS. B.

Yes; but in the Voltaic battery the diluted acid is not strong enough to
produce so complete an effect; it acts only on the surface of the zinc,
to which it yields its oxygen, forming upon it a film or crust, which is
a compound of the oxygen and the metal.

EMILY.

Since there is so strong a chemical attraction between oxygen and
metals, I suppose they are naturally in different states of electricity?

MRS. B.

Yes; it appears that all metals are united with the positive, and that
oxygen is the grand source of the negative electricity.

CAROLINE.

Does not then the acid act on the plates of copper, as well as on those
of zinc?

MRS. B.

No; for though copper has an affinity for oxygen, it is less strong than
that of zinc; and therefore the energy of the acid is only exerted upon
the zinc.

It will be best, I believe, in order to render the action of the Voltaic
battery more intelligible, to confine our attention at first to the
effect produced on two plates only. (PLATE VI. fig. 2.)

If a plate of zinc be placed opposite to one of copper, or any other
metal less attractive of oxygen, and the space between them (suppose of
half an inch in thickness), be filled with an acid or any fluid capable
of oxydating the zinc, the oxydated surface will have its capacity for
electricity diminished, so that a quantity of electricity will be
evolved from that surface. This electricity will be received by the
contiguous fluid, by which it will be transmitted to the opposite
metallic surface, the copper, which is not oxydated, and is therefore
disposed to receive it; so that the copper plate will thus become
positive, whilst the zinc plate will be in the negative state.

This evolution of electrical fluid however will be very limited; for as
these two plates admit of but very little accumulation of electricity,
and are supposed to have no communication with other bodies, the action
of the acid, and further developement of electricity, will be
immediately stopped.

EMILY.

This action, I suppose, can no more continue to go on, than that of a
common electrical machine, which is not allowed to communicate with
other bodies?

MRS. B.

Precisely; the common electrical machine, when excited by the friction
of the rubber, gives out both the positive and negative electricities.--
(PLATE VI. Fig. 3.) The positive, by the rotation of the glass cylinder,
is conveyed into the conductor, whilst the negative goes into the
rubber. But unless there is a communication made between the rubber and
the ground, but a very inconsiderable quantity of electricity can be
excited; for the rubber, like the plates of the battery, has too small a
capacity to admit of an accumulation of electricity. Unless therefore
the electricity can pass out of the rubber, it will not continue to go
into it, and consequently no additional accumulation will take place.
Now as one kind of electricity cannot be given out without the other,
the developement of the positive electricity is stopped as well as that
of the negative, and the conductor therefore cannot receive a succession
of charges.

CAROLINE.

But does not the conductor, as well as the rubber, require a
communication with the earth, in order to get rid of its electricity?

MRS. B.

No; for it is susceptible of receiving and containing a considerable
quantity of electricity, as it is much larger than the rubber, and
therefore has a greater capacity; and this continued accumulation of
electricity in the conductor is what is called a charge.

EMILY.

But when an electrical machine is furnished with two conductors to
receive the two electricities, I suppose no communication with the earth
is required?

MRS. B.

Certainly not, until the two are fully charged; for the two conductors
will receive equal quantities of electricity.

CAROLINE.

I thought the use of the chain had been to convey the electricity _from_
the ground into the machine?

MRS. B.

That was the idea of Dr. Franklin, who supposed that there was but one
kind of electricity, and who, by the terms positive and negative (which
he first introduced), meant only different quantities of the same kind
of electricity. The chain was in that case supposed to convey
electricity _from_ the ground through the rubber into the conductor. But
as we have adopted the hypothesis of two electricities, we must consider
the chain as a vehicle to conduct the negative electricity into the
earth.

EMILY.

And are both kinds of electricity produced whenever electricity is
excited?

MRS. B.

Yes, invariably. If you rub a tube of glass with a woollen cloth, the
glass becomes positive, and the cloth negative. If, on the contrary, you
excite a stick of sealing-wax by the same means, it is the rubber which
becomes positive, and the wax negative.

But with regard to the Voltaic battery, in order that the acid may act
freely on the zinc, and the two electricities be given out without
interruption, some method must be devised, by which the plates may part
with their electricities as fast as they receive them. --Can you think
of any means by which this might be effected?

EMILY.

Would not two chains or wires, suspended from either plate to the
ground, conduct the electricities into the earth, and thus answer the
purpose?

MRS. B.

It would answer the purpose of carrying off the electricity, I admit;
but recollect, that though it is necessary to find a vent for the
electricity, yet we must not lose it, since it is the power which we are
endeavouring to obtain. Instead, therefore, of conducting it into the
ground, let us make the wires, from either plate, meet: the two
electricities will thus be brought together, and will combine and
neutralize each other; and as long as this communication continues, the
two plates having a vent for their respective electricities, the action
of the acid will go on freely and uninterruptedly.

EMILY.

That is very clear, so far as two plates only are concerned; but I
cannot say I understand how the energy of the succession of plates, or
rather pairs of plates, of which the Galvanic trough is composed, is
propagated and accumulated throughout a battery?

MRS. B.

In order to shew you how the intensity of the electricity is increased
by increasing the number of plates, we will examine the action of four
plates; if you understand these, you will readily comprehend that of any
number whatever. In this figure (PLATE VI. Fig. 4.), you will observe
that the two central plates are united; they are soldered together, (as
we observed in describing the Voltaic trough,) so as to form but one
plate which offers two different surfaces, the one of copper, the other
of zinc.

Now you recollect that, in explaining the action of two plates, we
supposed that a quantity of electricity was evolved from the surface of
the first zinc plate, in consequence of the action of the acid, and was
conveyed by the interposed fluid to the copper plate, No. 2, which thus
became positive. This copper plate communicates its electricity to the
contiguous zinc plate, No. 3, in which, consequently, some accumulation
of electricity takes place. When, therefore, the fluid in the next cell
acts upon the zinc plate, electricity is extricated from it in larger
quantity, and in a more concentrated form, than before. This
concentrated electricity is again conveyed by the fluid to the next pair
of plates, No. 4 and 5, when it is farther increased by the action of
the fluid in the third cell, and so on, to any number of plates of which
the battery may consist; so that the electrical energy will continue to
accumulate in proportion to the number of double plates, the first zinc
plate of the series being the most negative, and the last copper plate
the most positive.

CAROLINE.

But does the battery become more and more strongly charged, merely by
being allowed to stand undisturbed?

MRS. B.

No, for the action will soon stop, as was explained before, unless a
vent be given to the accumulated electricities. This is easily done,
however, by establishing a communication by means of the wires
(Fig. 1.), between the two ends of the battery: these being brought into
contact, the two electricities meet and neutralize each other, producing
the shock and other effects of electricity; and the action goes on with
renewed energy, being no longer obstructed by the accumulation of the
two electricities which impeded its progress.

EMILY.

Is it the union of the two electricities which produces the electric
spark?

MRS. B.

Yes; and it is, I believe, this circumstance which gave rise to Sir H.
Davy's opinion that caloric may be a compound of the two electricities.

CAROLINE.

Yet surely caloric is very different from the electrical spark?

MRS. B.

The difference may consist probably only in intensity: for the heat of
the electric spark is considerably more intense, though confined to a
very minute spot, than any heat we can produce by other means.

EMILY.

Is it quite certain that the electricity of the Voltaic battery is
precisely of the same nature as that of the common electrical machine?

MRS. B.

Undoubtedly; the shock given to the human body, the spark, the
circumstance of the same substances which are conductors of the one
being also conductors of the other, and of those bodies, such as glass
and sealing-wax, which are non-conductors of the one, being also
non-conductors of the other, are striking proofs of it. Besides, Sir H.
Davy has shewn in his Lectures, that a Leyden jar, and a common electric
battery, can be charged with electricity obtained from a Voltaic
battery, the effect produced being perfectly similar to that obtained by
a common machine.

Dr. Wollaston has likewise proved that similar chemical decompositions
are effected by the electric machine and by the Voltaic battery; and has
made other experiments which render it highly probable, that the origin
of both electricities is essentially the same, as they show that the
rubber of the common electrical machine, like the zinc in the Voltaic
battery, produces the two electricities by combining with oxygen.

CAROLINE.

But I do not see whence the rubber obtains oxygen, for there is neither
acid nor water used in the common machine, and I always understood that
the electricity was excited by the friction.

MRS. B.

It appears that by friction the rubber obtains oxygen from the
atmosphere, which is partly composed of that element. The oxygen
combines with the amalgam of the rubber, which is of a metallic nature,
much in the same way as the oxygen of the acid combines with the zinc in
the Voltaic battery, and it is thus that the two electricities are
disengaged.

CAROLINE.

But, if the electricities of both machines are similar, why not use the
common machine for chemical decompositions?

MRS. B.

Though its effects are similar to those of the Voltaic battery, they are
incomparably weaker. Indeed Dr. Wollaston, in using it for chemical
decompositions, was obliged to act upon the most minute quantities of
matter, and though the result was satisfactory in proving the similarity
of its effects to those of the Voltaic battery, these effects were too
small in extent to be in any considerable degree applicable to chemical
decomposition.

CAROLINE.

How terrible, then, the shock must be from a Voltaic battery, since it
is so much more powerful than an electrical machine!

MRS. B.

It is not nearly so formidable as you think; at least it is by no means
proportional to the chemical effect. The great superiority of the
Voltaic battery consists in the large _quantity_ of electricity that
passes; but in regard to the _rapidity_ or _intensity_ of the charge, it
is greatly surpassed by the common electrical machine. It would seem
that the shock or sensation depends chiefly upon the intensity; whilst,
on the contrary, for chemical purposes, it is quantity which is
required. In the Voltaic battery, the electricity, though copious, is so
weak as not to be able to force its way through the fluid which
separates the plates, whilst that of a common machine will pass through
any space of water.

CAROLINE.

Would not it be possible to increase the intensity of the Voltaic
battery till it should equal that of the common machine?

MRS. B.

It can actually be increased till it imitates a weak electrical machine,
so as to produce a visible spark when accumulated in a Leyden jar. But
it can never be raised sufficiently to pass through any considerable
extent of air, because of the ready communication through the fluids
employed.

By increasing the number of plates of a battery, you increase its
_intensity_, whilst, by enlarging the dimensions of the plates, you
augment its _quantity_; and, as the superiority of the battery over the
common machine consists entirely in the quantity of electricity
produced, it was at first supposed that it was the size, rather than the
number of plates that was essential to the augmentation of power. It
was, however, found upon trial, that the quantity of electricity
produced by the Voltaic battery, even when of a very moderate size, was
sufficiently copious, and that the chief advantage in this apparatus was
obtained by increasing the intensity, which, however, still falls very
short of that of the common machine.

I should not omit to mention, that a very splendid, and, at the same
time, most powerful battery, was, a few years ago, constructed under the
direction of Sir H. Davy, which he repeatedly exhibited in his course of
electro-chemical lectures. It consists of two thousand double plates of
zinc and copper, of six square inches in dimensions, arranged in troughs
of Wedgwood-ware, each of which contains twenty of these plates. The
troughs are furnished with a contrivance for lifting the plates out of
them in a very convenient and expeditious manner.*

    [Footnote *: A model of this mode of construction is exhibited in
    PLATE XIII. Fig. 1.]

CAROLINE.

Well, now that we understand the nature of the action of the Voltaic
battery, I long to hear an account of the discoveries to which it has
given rise.

MRS. B.

You must restrain your impatience, my dear, for I cannot with any
propriety introduce the subject of these discoveries till we come to
them in the regular course of our studies. But, as almost every
substance in nature has already been exposed to the influence of the
Voltaic battery, we shall very soon have occasion to notice its effects.




CONVERSATION VI.

ON OXYGEN AND NITROGEN.


MRS. B.

To-day we shall examine the chemical properties of the ATMOSPHERE.

CAROLINE.

I thought that we were first to learn the nature of OXYGEN, which come
next in our table of simple bodies?

MRS. B.

And so you shall; the atmosphere being composed of two principles,
OXYGEN and NITROGEN, we shall proceed to analyse it, and consider its
component parts separately.

EMILY.

I always thought that the atmosphere had been a very complicated fluid,
composed of all the variety of exhalations from the earth.

MRS. B.

Such substances may be considered rather as heterogeneous and
accidental, than as forming any of its component parts; and the
proportion they bear to the whole mass is quite inconsiderable.

ATMOSPHERICAL AIR is composed of two gasses, known by the names of
OXYGEN GAS and NITROGEN or AZOTIC GAS.

EMILY.

Pray what is a gas?

MRS. B.

The name of gas is given to any fluid capable of existing constantly in
an aeriform state, under the pressure and at the temperature of the
atmosphere.

CAROLINE.

Is not water, or any other substance, when evaporated by heat, called
gas?

MRS. B.

No, my dear; vapour is, indeed, an elastic fluid, and bears a strong
resemblance to a gas; there are, however, several points in which they
essentially differ, and by which you may always distinguish them. Steam,
or vapour, owes its elasticity merely to a high temperature, which is
equal to that of boiling water. And it differs from boiling water only
by being united with more caloric, which, as we before explained, is in
a latent state. When steam is cooled, it instantly returns to the form
of water; but air, or gas, has never yet been rendered liquid or solid
by any degree of cold.

EMILY.

But does not gas, as well as vapour, owe its elasticity to caloric?

MRS. B.

It was the prevailing opinion; and the difference of gas or vapour was
thought to depend on the different manner in which caloric was united
with the basis of these two kinds of elastic fluids. In vapour, it was
considered as in a latent state; in gas, it was said to be chemically
combined. But the late researches of Sir H. Davy have given rise to a
new theory respecting gasses; and there is now reason to believe that
these bodies owe their permanently elastic state, not solely to caloric,
but likewise to the prevalence of either the one or the other of the two
electricities.

EMILY.

When you speak, then, of the simple bodies oxygen and nitrogen, you mean
to express those substances which are the basis of the two gasses?

MRS. B.

Yes, in strict propriety, for they can properly be called gasses only
when brought to an aeriform state.

CAROLINE.

In what proportions are they combined in the atmosphere?

MRS. B.

The oxygen gas constitutes a little more than one-fifth, and the
nitrogen gas a little less than four-fifths. When separated, they are
found to possess qualities totally different from each other. For oxygen
gas is essential both to respiration and combustion, while neither of
these processes can be performed in nitrogen gas.

CAROLINE.

But if nitrogen gas is unfit for respiration, how does it happen that
the large proportion of it which enters into the composition of the
atmosphere is not a great impediment to breathing?

MRS. B.

We should breathe more freely than our lungs could bear, if we respired
oxygen gas alone. The nitrogen is no impediment to respiration, and
probably, on the contrary, answers some useful purpose, though we do not
know in what manner it acts in that process.

EMILY.

And by what means can the two gasses, which compose the atmospheric air,
be separated?

MRS. B.

There are many ways of analysing the atmosphere: the two gasses may be
separated first by combustion.

EMILY.

You surprise me! how is it possible that combustion should separate
them?

MRS. B.

I should previously remind you that oxygen is supposed to be the only
simple body naturally combined with negative electricity. In all the
other elements the positive electricity prevails, and they have
consequently, all of them, an attraction for oxygen.*

    [Footnote *: If chlorine or oxymuriatic gas be a simple body,
    according to Sir H. Davy's view of the subject, it must be
    considered as an exception to this statement; but this subject
    cannot be discussed till the properties and nature of chlorine
    come under examination.]

CAROLINE.

Oxygen the only negatively electrified body! that surprises me
extremely; how then are the combinations of the other bodies performed,
if, according to your explanation of chemical attraction, bodies are
supposed only to combine in virtue of their opposite states of
electricity?

MRS. B.

Observe that I said, that oxygen was the only _simple_ body, naturally
negative. Compound bodies, in which oxygen prevails over the other
component parts, are also negative, but their negative energy is greater
or less in proportion as the oxygen predominates. Those compounds into
which oxygen enters in less proportion than the other constituents, are
positive, but their positive energy is diminished in proportion to the
quantity of oxygen which enters into their composition.

All bodies, therefore, that are not already combined with oxygen, will
attract it, and, under certain circumstances, will absorb it from the
atmosphere, in which case the nitrogen gas will remain alone, and may
thus be obtained in its separate state.

CAROLINE.

I do not understand how a gas can be absorbed?

MRS. B.

It is only the oxygen, or basis of the gas, which is absorbed; and the
two electricities escaping, that is to say, the negative from the
oxygen, the positive from the burning body, unite and produce caloric.

EMILY.

And what becomes of this caloric?

MRS. B.

We shall make this piece of dry wood attract oxygen from the atmosphere,
and you will see what becomes of the caloric.

CAROLINE.

You are joking, Mrs. B--; you do not mean to decompose the atmosphere
with a piece of dry stick?

MRS. B.

Not the whole body of the atmosphere, certainly; but if we can make this
piece of wood attract any quantity of oxygen from it, a proportional
quantity of atmospherical air will be decomposed.

CAROLINE.

If wood has so strong an attraction for oxygen, why does it not
decompose the atmosphere spontaneously?

MRS. B.

It is found by experience, that an elevation of temperature is required
for the commencement of the union of the oxygen and the wood.

This elevation of temperature was formerly thought to be necessary, in
order to diminish the cohesive attraction of the wood, and enable the
oxygen to penetrate and combine with it more readily. But since the
introduction of the new theory of chemical combination, another cause
has been assigned, and it is now supposed that the high temperature, by
exalting the electrical energies of bodies, and consequently their force
of attraction, facilitates their combination.

EMILY.

If it is true, that caloric is composed of the two electricities, an
elevation of temperature must necessarily augment the electric energies
of bodies.

MRS. B.

I doubt whether that would be a necessary consequence; for, admitting
this composition of caloric, it is only by its being decomposed that
electricity can be produced. Sir H. Davy, however, in his numerous
experiments, has found it to be an almost invariable rule that the
electrical energies of bodies are increased by elevation of temperature.

What means then shall we employ to raise the temperature of the wood, so
as to enable it to attract oxygen from the atmosphere?

CAROLINE.

Holding it near the fire, I should think, would answer the purpose.

MRS. B.

It may, provided you hold it sufficiently close to the fire; for a very
considerable elevation of temperature is required.

CAROLINE.

It has actually taken fire, and yet I did not let it touch the coals,
but I held it so very close that I suppose it caught fire merely from
the intensity of the heat.

MRS. B.

Or you might say, in other words, that the caloric which the wood
imbibed, so much elevated its temperature, and exalted its electric
energy, as to enable it to attract oxygen very rapidly from the
atmosphere.

EMILY.

Does the wood absorb oxygen while it is burning?

MRS. B.

Yes, and the heat and light are produced by the union of the two
electricities which are set at liberty, in consequence of the oxygen
combining with the wood.

CAROLINE.

You astonish me! the heat of a burning body proceeds then as much from
the atmosphere as from the body itself?

MRS. B.

It was supposed that the caloric, given out during combustion, proceeded
entirely, or nearly so, from the decomposition of the oxygen gas; but,
according to Sir H. Davy's new view of the subject, both the oxygen gas,
and the combustible body, concur in supplying the heat and light, by the
union of their opposite electricities.

EMILY.

I have not yet met with any thing in chemistry that has surprised or
delighted me so much as this explanation of combustion. I was at first
wondering what connection there could be between the affinity of a body
for oxygen and its combustibility; but I think I understand it now
perfectly.

MRS. B.

Combustion then, you see, is nothing more than the rapid combination of
a body with oxygen, attended by the disengagement of light and heat.

EMILY.

But are there no combustible bodies whose attraction for oxygen is so
strong, that they will combine with it, without the application of heat?

CAROLINE.

That cannot be; otherwise we should see bodies burning spontaneously.

MRS. B.

But there are some instances of this kind, such as phosphorus,
potassium, and some compound bodies, which I shall hereafter make you
acquainted with. These bodies, however, are prepared by art, for in
general, all the combustions that could occur spontaneously, at the
temperature of the atmosphere, have already taken place; therefore new
combustions cannot happen without the temperature of the body being
raised. Some bodies, however, will burn at a much lower temperature than
others.

CAROLINE.

But the common way of burning a body is not merely to approach it to one
already on fire, but rather to put the one in actual contact with the
other, as when I burn this piece of paper by holding it in the flame of
the fire.

MRS. B.

The closer it is in contact with the source of caloric, the sooner will
its temperature be raised to the degree necessary for it to burn. If you
hold it near the fire, the same effect will be produced; but more time
will be required, as you found to be the case with the piece of stick.

EMILY.

But why is it not necessary to continue applying caloric throughout the
process of combustion, in order to keep up the electric energy of the
wood, which is required to enable it to combine with the oxygen?

MRS. B.

The caloric which is gradually produced by the two electricities during
combustion, keeps up the temperature of the burning body; so that when
once combustion has begun, no further application of caloric is
required.

CAROLINE.

Since I have learnt this wonderful theory of combustion, I cannot take
my eyes from the fire; and I can scarcely conceive that the heat and
light, which I always supposed to proceed entirely from the coals, are
really produced as much by the atmosphere.

EMILY.

When you blow the fire, you increase the combustion, I suppose, by
supplying the coals with a greater quantity of oxygen gas?

MRS. B.

Certainly; but of course no blowing will produce combustion, unless the
temperature of the coals be first raised. A single spark, however, is
sometimes sufficient to produce that effect; for, as I said before, when
once combustion has commenced, the caloric disengaged is sufficient to
elevate the temperature of the rest of the body, provided that there be
a free access of oxygen. It however sometimes happens that if a fire be
ill made, it will be extinguished before all the fuel is consumed, from
the very circumstance of the combustion being so slow that the caloric
disengaged is insufficient to keep up the temperature of the fuel. You
must recollect that there are three things required in order to produce
combustion; a combustible body, oxygen, and a temperature at which the
one will combine with the other.

EMILY.

You said that combustion was one method of decomposing the atmosphere,
and obtaining the nitrogen gas in its simple state; but how do you
secure this gas, and prevent it from mixing with the rest of the
atmosphere?

MRS. B.

It is necessary for this purpose to burn the body within a close vessel,
which is easily done. --We shall introduce a small lighted taper (PLATE
VII. Fig. 1.) under this glass receiver, which stands in a bason over
water, to prevent all communication with the external air.

  [Illustration: Plate VII. Vol. I. p. 181.

  Fig. 1. Combustion of a taper under a receiver.

  Fig. 2. A Retort on a stand.

  Fig. 3. Preparation of oxygen gas.
  A Furnace.
  B Earthen Retort in the furnace.
  C Water bath.
  D Receiver.
  E.E Tube conveying the gas from the Retort through the water into
    the Receiver.
  F.F.F Shelf perforated on which the Receiver stands.

  Fig. 4. Combustion of iron wire in oxygen gas.]

CAROLINE.

How dim the light burns already! --It is now extinguished.

MRS. B.

Can you tell us why it is extinguished?

CAROLINE.

Let me consider. --The receiver was full of atmospherical air; the
taper, in burning within it, must have combined with the oxygen
contained in that air, and the caloric that was disengaged produced the
light of the taper. But when the whole of the oxygen was absorbed, the
whole of its electricity was disengaged; consequently no more caloric
could be produced, the taper ceased to burn, and the flame was
extinguished.

MRS. B.

Your explanation is perfectly correct.

EMILY.

The two constituents of the oxygen gas being thus disposed of, what
remains under the receiver must be pure nitrogen gas?

MRS. B.

There are some circumstances which prevent the nitrogen gas, thus
obtained, from being perfectly pure; but we may easily try whether the
oxygen has disappeared, by putting another lighted taper under it. --You
see how instantaneously the flame is extinguished, for want of oxygen to
supply the negative electricity required for the formation of caloric;
and were you to put an animal under the receiver, it would immediately
be suffocated. But that is an experiment which I do not think your
curiosity will tempt you to try.

EMILY.

Certainly not. --But look, Mrs. B., the receiver is full of a thick
white smoke. Is that nitrogen gas?

MRS. B.

No, my dear; nitrogen gas is perfectly transparent and invisible, like
common air. This cloudiness proceeds from a variety of exhalations,
which arise from the burning taper, and the nature of which you cannot
yet understand.

CAROLINE.

The water within the receiver has now risen a little above its level in
the bason. What is the reason of this?

MRS. B.

With a moment's reflection, I dare say, you would have explained it
yourself. The water rises in consequence of the oxygen gas within it
having been destroyed, or rather decomposed, by the combustion of the
taper.

CAROLINE.

Then why did not the water rise immediately when the oxygen gas was
destroyed?

MRS. B.

Because the heat of the taper, whilst burning, produced a dilatation of
the air in the vessel, which at first counteracted this effect.

Another means of decomposing the atmosphere is the _oxygenation_ of
certain metals. This process is very analogous to combustion; it is,
indeed, only a more general term to express the combination of a body
with oxygen.

CAROLINE.

In what respect, then, does it differ from combustion?

MRS. B.

The combination of oxygen in combustion is always accompanied by a
disengagement of light and heat; whilst this circumstance is not a
necessary consequence of simple oxygenation.

CAROLINE.

But how can a body absorb oxygen without the combination of the two
electricities which produce caloric?

MRS. B.

Oxygen does not always present itself in a gaseous state; it is a
constituent part of a vast number of bodies, both solid and liquid, in
which it exists in a much denser state than in the atmosphere; and from
these bodies it may be obtained without much disengagement of caloric.
It may likewise, in some cases, be absorbed from the atmosphere without
any sensible production of light and heat; for, if the process be slow,
the caloric is disengaged in such small quantities, and so gradually,
that it is not capable of producing either light or heat. In this case
the absorption of oxygen is called _oxygenation_ or _oxydation_, instead
of _combustion_, as the production of sensible light and heat is
essential to the latter.

EMILY.

I wonder that metals can unite with oxygen; for, as they are so dense,
their attraction of aggregation must be very great; and I should have
thought that oxygen could never have penetrated such bodies.

MRS. B.

Their strong attraction for oxygen counterbalances this obstacle. Most
metals, however, require to be made red-hot before they are capable of
attracting oxygen in any considerable quantity. By this combination they
lose most of their metallic properties, and fall into a kind of powder,
formerly called _calx_, but now much more properly termed an _oxyd_;
thus we have _oxyd of lead_, _oxyd of iron_, &c.

EMILY.

And in the Voltaic battery, it is, I suppose, an oxyd of zinc, that is
formed by the union of the oxygen with that metal?

MRS. B.

Yes, it is.

CAROLINE.

The word oxyd, then, simply means a metal combined with oxygen?

MRS. B.

Yes; but the term is not confined to metals, though chiefly applied to
them. Any body whatever, that has combined with a certain quantity of
oxygen, either by means of oxydation or combustion, is called an _oxyd_,
and is said to be _oxydated_ or _oxygenated_.

EMILY.

Metals, when converted into oxyds, become, I suppose, negative?

MRS. B.

Not in general; because in most oxyds the positive energy of the metal
more than counterbalances the native energy of the oxygen with which it
combines.

This black powder is an oxyd of manganese, a metal which has so strong
an affinity for oxygen, that it attracts that substance from the
atmosphere at any known temperature: it is therefore never found in its
metallic form, but always in that of an oxyd, in which state, you see,
it has very little of the appearance of a metal. It is now heavier than
it was before oxydation, in consequence of the additional weight of the
oxygen with which it has combined.

CAROLINE.

I am very glad to hear that; for I confess I could not help having some
doubts whether oxygen was really a substance, as it is not to be
obtained in a simple and palpable state; but its weight is, I think,
a decisive proof of its being a real body.

MRS. B.

It is easy to estimate its weight, by separating it from the manganese,
and finding how much the latter has lost.

EMILY.

But if you can take the oxygen from the metal, shall we not then have it
in its palpable simple state?

MRS. B.

No; for I can only separate the oxygen from the manganese, by presenting
to it some other body, for which it has a greater affinity than for the
manganese. Caloric affording the two electricities is decomposed, and
one of them uniting with the oxygen, restores it to the aeriform state.

EMILY.

But you said just now, that manganese would attract oxygen from the
atmosphere in which it is combined with the negative electricity; how,
therefore, can the oxygen have a superior affinity for that electricity,
since it abandons it to combine with the manganese?

MRS. B.

I give you credit for this objection, Emily; and the only answer I can
make to it is, that the mutual affinities of metals for oxygen, and of
oxygen for electricity, vary at different temperatures; a certain degree
of heat will, therefore, dispose a metal to combine with oxygen, whilst,
on the contrary, the former will be compelled to part with the latter,
when the temperature is further increased. I have put some oxyd of
manganese into a retort, which is an earthen vessel with a bent neck,
such as you see here. (PLATE VII. Fig. 2.) --The retort containing the
manganese you cannot see, as I have enclosed it in this furnace, where
it is now red-hot. But, in order to make you sensible of the escape of
the gas, which is itself invisible, I have connected the neck of the
retort with this bent tube, the extremity of which is immersed in this
vessel of water. (PLATE VII. Fig. 3.) --Do you see the bubbles of air
rise through the water?

CAROLINE.

Perfectly. This, then, is pure oxygen gas; what a pity it should be
lost! Could you not preserve it?

MRS. B.

We shall collect it in this receiver. --For this purpose, you observe,
I first fill it with water, in order to exclude the atmospherical air;
and then place it over the bubbles that issue from the retort, so as to
make them rise through the water to the upper part of the receiver.

EMILY.

The bubbles of oxygen gas rise, I suppose, from their specific levity?

MRS. B.

Yes; for though oxygen forms rather a heavy gas, it is light compared to
water. You see how it gradually displaces the water from the receiver.
It is now full of gas, and I may leave it inverted in water on this
shelf, where I can keep the gas as long as I choose, for future
experiments. This apparatus (which is indispensable in all experiments
in which gases are concerned) is called a water-bath.

CAROLINE.

It is a very clever contrivance, indeed; equally simple and useful. How
convenient the shelf is for the receiver to rest upon under water, and
the holes in it for the gas to pass into the receiver! I long to make
some experiments with this apparatus.

MRS. B.

I shall try your skill that way, when you have a little more experience.
I am now going to show you an experiment, which proves, in a very
striking manner, how essential oxygen is to combustion. You will see
that iron itself will burn in this gas, in the most rapid and brilliant
manner.

CAROLINE.

Really! I did not know that it was possible to burn iron.

EMILY.

Iron is a simple body, and you know, Caroline, that all simple bodies
are naturally positive, and therefore must have an affinity for oxygen.

MRS. B.

Iron will, however, not burn in atmospherical air without a very great
elevation of temperature; but it is eminently combustible in pure oxygen
gas; and what will surprise you still more, it can be set on fire
without any considerable rise of temperature. You see this spiral iron
wire--I fasten it at one end to this cork, which is made to fit an
opening at the top of the glass-receiver. (PLATE VII. Fig. 4.)

EMILY.

I see the opening in the receiver; but it is carefully closed by a
ground glass-stopper.

MRS. B.

That is in order to prevent the gas from escaping; but I shall take out
the stopper, and put in the cork, to which the wire hangs. --Now I mean
to burn this wire in the oxygen gas, but I must fix a small piece of
lighted tinder to the extremity of it, in order to give the first
impulse to combustion; for, however powerful oxygen is in promoting
combustion, you must recollect that it cannot take place without some
elevation of temperature. I shall now introduce the wire into the
receiver, by quickly changing the stoppers.

CAROLINE.

Is there no danger of the gas escaping while you change the stoppers?

MRS. B.

Oxygen gas is a little heavier than atmospherical air, therefore it will
not mix with it very rapidly; and, if I do not leave the opening
uncovered, we shall not lose any----

CAROLINE.

Oh, what a brilliant and beautiful flame!

EMILY.

It is as white and dazzling as the sun! --Now a piece of the melted wire
drops to the bottom: I fear it is extinguished; but no, it burns again
as bright as ever.

MRS. B.

It will burn till the wire is entirely consumed, provided the oxygen is
not first expended: for you know it can burn only while there is oxygen
to combine with it.

CAROLINE.

I never saw a more beautiful light. My eyes can hardly bear it! How
astonishing to think that all this caloric was contained in the small
quantity of gas and iron that was enclosed in the receiver; and that,
without producing any sensible heat!

CAROLINE.

How wonderfully quick combustion goes on in pure oxygen gas! But pray,
are these drops of burnt iron as heavy as the wire was before?

MRS. B.

They are even heavier; for the iron, in burning, has acquired exactly
the weight of the oxygen which has disappeared, and is now combined with
it. It has become an oxyd of iron.

CAROLINE.

I do not know what you mean by saying that the oxygen has _disappeared_,
Mrs. B., for it was always invisible.

MRS. B.

True, my dear; the expression was incorrect. But though you could not
see the oxygen gas, I believe you had no doubt of its presence, as the
effect it produced on the wire was sufficiently evident.

CAROLINE.

Yes, indeed; yet you know it was the caloric, and not the oxygen gas
itself, that dazzled us so much.

MRS. B.

You are not quite correct in your turn, in saying the caloric dazzled
you; for caloric is invisible; it affects only the sense of feeling; it
was the light which dazzled you.

CAROLINE.

True; but light and caloric are such constant companions, that it is
difficult to separate them, even in idea.

MRS. B.

The easier it is to confound them, the more careful you should be in
making the distinction.

CAROLINE.

But why has the water now risen, and filled part of the receiver?

MRS. B.

Indeed, Caroline, I did not suppose you would have asked such a
question! I dare say, Emily, you can answer it.

EMILY.

Let me reflect . . . . . . The oxygen has combined with the wire; the
caloric has escaped; consequently nothing can remain in the receiver,
and the water will rise to fill the vacuum.

CAROLINE.

I wonder that I did not think of that. I wish that we had weighed the
wire and the oxygen gas before combustion; we might then have found
whether the weight of the oxyd was equal to that of both.

MRS. B.

You might try the experiment if you particularly wished it; but I can
assure you, that, if accurately performed, it never fails to show that
the additional weight of the oxyd is precisely equal to that of the
oxygen absorbed, whether the process has been a real combustion, or a
simple oxygenation.

CAROLINE.

But this cannot be the case with combustions in general; for when any
substance is burnt in the common air, so far from increasing in weight,
it is evidently diminished, and sometimes entirely consumed.

MRS. B.

But what do you mean by the expression _consumed_? You cannot suppose
that the smallest particle of any substance in nature can be actually
destroyed. A compound body is decomposed by combustion; some of its
constituent parts fly off in a gaseous form, while others remain in a
concrete state; the former are called the _volatile_, the latter the
_fixed products_ of combustion. But if we collect the whole of them, we
shall always find that they exceed the weight of the combustible body,
by that of the oxygen which has combined with them during combustion.

EMILY.

In the combustion of a coal fire, then, I suppose that the ashes are
what would be called the fixed product, and the smoke the volatile
product?

MRS. B.

Yet when the fire burns best, and the quantity of volatile products
should be the greatest, there is no smoke; how can you account for that?

EMILY.

Indeed I cannot; therefore I suppose that I was not right in my
conjecture.

MRS. B.

Not quite: ashes, as you supposed, are a fixed product of combustion;
but smoke, properly speaking, is not one of the volatile products, as it
consists of some minute undecomposed particles of the coals that are
carried off by the heated air without being burnt, and are either
deposited in the form of soot, or dispersed by the wind. Smoke,
therefore, ultimately, becomes one of the _fixed_ products of
combustion. And you may easily conceive that the stronger the fire is,
the less smoke is produced, because the fewer particles escape
combustion. On this principle depends the invention of Argand's Patent
Lamps; a current of air is made to pass through the cylindrical wick of
the lamp, by which means it is so plentifully supplied with oxygen, that
scarcely a particle of oil escapes combustion, nor is there any smoke
produced.

EMILY.

But what then are the volatile products of combustion?

MRS. B.

Various new compounds, with which you are not yet acquainted, and which
being converted by caloric either into vapour or gas, are invisible; but
they can be collected, and we shall examine them at some future period.

CAROLINE.

There are then other gases, besides the oxygen and nitrogen gases.

MRS. B.

Yes, several: any substance that can assume and maintain the form of an
elastic fluid at the temperature of the atmosphere, is called a gas. We
shall examine the several gases in their respective places; but we must
now confine our attention to those that compose the atmosphere.

I shall show you another method of decomposing the atmosphere, which is
very simple. In breathing, we retain a portion of the oxygen, and expire
the nitrogen gas; so that if we breathe in a closed vessel, for a
certain length of time, the air within it will be deprived of its oxygen
gas. Which of you will make the experiment?

CAROLINE.

I should be very glad to try it.

MRS. B.

Very well; breathe several times through this glass tube into the
receiver with which it is connected, until you feel that your breath is
exhausted.

CAROLINE.

I am quite out of breath already!

MRS. B.

Now let us try the gas with a lighted taper.

EMILY.

It is very pure nitrogen gas, for the taper is immediately extinguished.

MRS. B.

That is not a proof of its being pure, but only of the absence of
oxygen, as it is that principle alone which can produce combustion,
every other gas being absolutely incapable of it.

EMILY.

In the methods which you have shown us, for decomposing the atmosphere,
the oxygen always abandons the nitrogen; but is there no way of taking
the nitrogen from the oxygen, so as to obtain the latter pure from the
atmosphere?

MRS. B.

You must observe, that whenever oxygen is taken from the atmosphere, it
is by decomposing the oxygen gas; we cannot do the same with the
nitrogen gas, because nitrogen has a stronger affinity for caloric than
for any other known principle: it appears impossible therefore to
separate it from the atmosphere by the power of affinities. But if we
cannot obtain the oxygen gas, by this means, in its separate state, we
have no difficulty (as you have seen) to procure it in its gaseous form,
by taking it from those substances that have absorbed it from the
atmosphere, as we did with the oxyd of manganese.

EMILY.

Can atmospherical air be recomposed, by mixing due proportions of oxygen
and nitrogen gases?

MRS. B.

Yes: if about one part of oxygen gas be mixed with about four parts of
nitrogen gas, atmospherical air is produced.*

    [Footnote *: The proportion of oxygen in the atmosphere varies
    from 21 to 22 per cent.]

EMILY.

The air, then, must be an oxyd of nitrogen?

MRS. B.

No, my dear; for there must be a chemical combination between oxygen and
nitrogen in order to produce an oxyd; whilst in the atmosphere these two
substances are separately combined with caloric, forming two distinct
gases, which are simply mixed in the formation of the atmosphere.

I shall say nothing more of oxygen and nitrogen at present, as we shall
continually have occasion to refer to them in our future conversations.
They are both very abundant in nature; nitrogen is the most plentiful in
the atmosphere, and exists also in all animal substances; oxygen forms a
constituent part, both of the animal and vegetable kingdoms, from which
it may be obtained by a variety of chemical means. But it is now time to
conclude our lesson. I am afraid you have learnt more to-day than you
will be able to remember.

CAROLINE.

I assure you that I have been too much interested in it, ever to forget
it. In regard to nitrogen there seems to be but little to remember; it
makes a very insignificant figure in comparison to oxygen, although it
composes a much larger portion of the atmosphere.

MRS. B.

Perhaps this insignificance you complain of may arise from the compound
nature of nitrogen, for though I have hitherto considered it as a simple
body, because it is not known in any natural process to be decomposed,
yet from some experiments of Sir H. Davy, there appears to be reason for
suspecting that nitrogen is a compound body, as we shall see afterwards.
But even in its simple state, it will not appear so insignificant when
you are better acquainted with it; for though it seems to perform but a
passive part in the atmosphere, and has no very striking properties,
when considered in its separate state, yet you will see by-and-bye what
a very important agent it becomes, when combined with other bodies. But
no more of this at present; we must reserve it for its proper place.




CONVERSATION VII.

ON HYDROGEN.


CAROLINE.

The next simple bodies we come to are CHLORINE and IODINE. Pray what
kinds of substances are these; are they also invisible?

MRS. B.

No; for chlorine, in the state of gas, has a distinct greenish colour,
and is therefore visible; and iodine, in the same state, has a beautiful
claret-red colour. The knowledge of these two bodies, however, and the
explanation of their properties, imply various considerations, which you
would not yet be able to understand; we shall therefore defer their
examination to some future conversation, and we shall pass on to the
next simple substance, HYDROGEN, which we cannot, any more than oxygen,
obtain in a visible or palpable form. We are acquainted with it only in
its gaseous state, as we are with oxygen and nitrogen.

CAROLINE.

But in its gaseous state it cannot be called a simple substance, since
it is combined with heat and electricity?

MRS. B.

True, my dear; but as we do not know in nature of any substance which is
not more or less combined with caloric and electricity, we are apt to
say that a substance is in its pure state when combined with those
agents only.

Hydrogen was formerly called _inflammable air_, as it is extremely
combustible, and burns with a great flame. Since the invention of the
new nomenclature, it has obtained the name of hydrogen, which is derived
from two Greek words, the meaning of which is, _to produce water_.

EMILY.

And how does hydrogen produce water?

MRS. B.

By its combustion. Water is composed of eighty-five parts, by weight, of
oxygen, combined with fifteen parts of hydrogen; or of two parts, by
bulk of hydrogen gas, to one part of oxygen gas.

CAROLINE.

Really! is it possible that water should be a combination of two gases,
and that one of these should be inflammable air! Hydrogen must be a most
extraordinary gas that will produce both fire and water.

EMILY.

But I thought you said that combustion could take place in no gas but
oxygen?

MRS. B.

Do you recollect what the process of combustion consists in?

EMILY.

In the combination of a body with oxygen, with disengagement of light
and heat.

MRS. B.

Therefore when I say that hydrogen is combustible, I mean that it has an
affinity for oxygen; but, like all other combustible substances, it
cannot burn unless supplied with oxygen, and also heated to a proper
temperature.

CAROLINE.

The simply mixing fifteen parts of hydrogen, with eighty-five parts of
oxygen gas, will not, therefore, produce water?

MRS. B.

No; water being a much denser fluid than gases, in order to reduce these
gases to a liquid, it is necessary to diminish the quantity of caloric
or electricity which maintains them in an elastic form.

EMILY.

That I should think might be done by combining the oxygen and hydrogen
together; for in combining they would give out their respective
electricities in the form of caloric, and by this means would be
condensed.

CAROLINE.

But you forget, Emily, that in order to make the oxygen and hydrogen
combine, you must begin by elevating their temperature, which increases,
instead of diminishing, their electric energies.

MRS. B.

Emily is, however, right; for though it is necessary to raise their
temperature, in order to make them combine, as that combination affords
them the means of parting with their electricities, it is eventually the
cause of the diminution of electric energy.

CAROLINE.

You love to deal in paradoxes to-day, Mrs. B. --Fire, then, produces
water?

MRS. B.

The combustion of hydrogen gas certainly does; but you do not seem to
have remembered the theory of combustion so well as you thought you
would. Can you tell me what happens in the combustion of hydrogen gas?

CAROLINE.

The hydrogen combines with the oxygen, and their opposite electricities
are disengaged in the form of caloric. --Yes, I think I understand it
now--by the loss of this caloric, the gases are condensed into a liquid.

EMILY.

Water, then, I suppose, when it evaporates and incorporates with the
atmosphere, is decomposed and converted into hydrogen and oxygen gases?

MRS. B.

No, my dear--there you are quite mistaken: the decomposition of water is
totally different from its evaporation; for in the latter case (as you
should recollect) water is only in a state of very minute division; and
is merely suspended in the atmosphere, without any chemical combination,
and without any separation of its constituent parts. As long as these
remain combined, they form WATER, whether in a state of liquidity, or in
that of an elastic fluid, as vapour, or under the solid form of ice.

In our experiments on latent heat, you may recollect that we caused
water successively to pass through these three forms, merely by an
increase or diminution of caloric, without employing any power of
attraction, or effecting any decomposition.

CAROLINE.

But are there no means of decomposing water?

MRS. B.

Yes, several: charcoal, and metals, when heated red hot, will attract
the oxygen from water, in the same manner as they will from the
atmosphere.

CAROLINE.

Hydrogen, I see, is like nitrogen, a poor dependant friend of oxygen,
which is continually forsaken for greater favourites.

MRS. B.

The connection, or friendship, as you choose to call it, is much more
intimate between oxygen and hydrogen, in the state of water, than
between oxygen and nitrogen, in the atmosphere; for, in the first case,
there is a chemical union and condensation of the two substances; in the
latter, they are simply mixed together in their gaseous state. You will
find, however, that, in some cases, nitrogen is quite as intimately
connected with oxygen, as hydrogen is. --But this is foreign to our
present subject.

EMILY.

Water, then, is an oxyd, though the atmospherical air is not?

MRS. B.

It is not commonly called an oxyd, though, according to our definition,
it may, no doubt, be referred to that class of bodies.

CAROLINE.

I should like extremely to see water decomposed.

MRS. B.

I can gratify your curiosity by a much more easy process than the
oxydation of charcoal or metals: the decomposition of water by these
latter means takes up a great deal of time, and is attended with much
trouble; for it is necessary that the charcoal or metal should be made
red hot in a furnace, that the water should pass over them in a state of
vapour, that the gas formed should be collected over the water-bath, &c.
In short, it is a very complicated affair. But the same effect may be
produced with the greatest facility, by the action of the Voltaic
battery, which this will give me an opportunity of exhibiting.

CAROLINE.

I am very glad of that, for I longed to see the power of this apparatus
in decomposing bodies.

MRS. B.

For this purpose I fill this piece of glass-tube (PLATE VIII. fig. 1.)
with water, and cork it up at both ends; through one of the corks I
introduce that wire of the battery which conveys the positive
electricity; and the wire which conveys the negative electricity is made
to pass through the other cork, so that the two wires approach each
other sufficiently near to give out their respective electricities.

  [Illustration: Plate VIII. Vol. I. p. 206

  Fig. 1. Apparatus for the decomposition of water by the Voltaic
    Battery.

  Fig. 2. Apparatus for decomposing water by Voltaic Electricity
    & obtaining the gasses separate.

  Fig. 3. Apparatus for preparing & collecting hydrogen gas.

  Fig. 4. Receiver full of hydrogen gas inverted over water.

  Fig. 5 Slow combustion of hydrogen gas.

  Fig. 6. Apparatus for illustrating the formation of water by the
    combustion of hydrogen gas.

  Fig. 7. Apparatus for producing harmonic sounds by the combustion
    of hydrogen gas.]

CAROLINE.

It does not appear to me that you approach the wires so near as you did
when you made the battery act by itself.

MRS. B.

Water being a better conductor of electricity than air, the two wires
will act on each other at a greater distance in the former than in the
latter.

EMILY.

Now the electrical effect appears: I see small bubbles of air emitted
from each wire.

MRS. B.

Each wire decomposes the water, the positive by combining with its
oxygen which is negative, the negative by combining with its hydrogen
which is positive.

CAROLINE.

That is wonderfully curious! But what are the small bubbles of air?

MRS. B.

Those that appear to proceed from the positive wire, are the result of
the decomposition of the water by that wire. That is to say, the
positive electricity having combined with some of the oxygen of the
water, the particles of hydrogen which were combined with that portion
of oxygen are set at liberty, and appear in the form of small bubbles of
gas or air.

EMILY.

And I suppose the negative fluid having in the same manner combined with
some of the hydrogen of the water, the particles of oxygen that were
combined with it, are set free, and emitted in a gaseous form.

MRS. B.

Precisely so. But I should not forget to observe, that the wires used in
this experiment are made of platina, a metal which is not capable of
combining with oxygen; for otherwise the wire would combine with the
oxygen, and the hydrogen alone would be disengaged.

CAROLINE.

But could not water be decomposed without the electric circle being
completed? If, for instance, you immersed only the positive wire in the
water, would it not combine with the oxygen, and the hydrogen gas be
given out?

MRS. B.

No; for as you may recollect, the battery cannot act unless the circle
be completed; since the positive wire will not give out its electricity,
unless attracted by that of the negative wire.

CAROLINE.

I understand it now. --But look, Mrs. B., the decomposition of the water
which has now been going on for some time, does not sensibly diminish
its quantity--what is the reason of that?

MRS. B.

Because the quantity decomposed is so extremely small. If you compare
the density of water with that of the gases into which it is resolved,
you must be aware that a single drop of water is sufficient to produce
thousands of such small bubbles as those you now perceive.

CAROLINE.

But in this experiment, we obtain the oxygen and hydrogen gases mixed
together. Is there any means of procuring the two gases separately?

MRS. B.

They can be collected separately with great ease, by modifying a little
the experiment. Thus if instead of one tube, we employ two, as you see
here, (c, d, PLATE VIII. fig. 2.) both tubes being closed at one end,
and open at the other; and if after filling these tubes with water, we
place them standing in a glass of water (e), with their open end
downwards, you will see that the moment we connect the wires (a, b)
which proceed upwards from the interior of each tube, the one with one
end of the battery, and the other with the other end, the water in the
tubes will be decomposed; hydrogen will be given out round the wire in
the tube connected with the positive end of the battery, and oxygen in
the other; and these gases will be evolved, exactly in the proportions
which I have before mentioned, namely, two measures of hydrogen for one
of oxygen. We shall now begin the experiment, but it will be some time
before any sensible quantity of the gases can be collected.

EMILY.

The decomposition of water in this way, slow as it is, is certainly very
striking; but I confess that I should be still more gratified, if you
could shew it us on a larger scale, and by a quicker process. I am sorry
that the decomposition of water by charcoal or metals is attended with
so much inconvenience.

MRS. B.

Water may be decomposed by means of metals without any difficulty; but
for this purpose the intervention of an acid is required. Thus, if we
add some sulphuric acid (a substance with the nature of which you are
not yet acquainted) to the water which the metal is to decompose, the
acid disposes the metal to combine with the oxygen of the water so
readily and abundantly, that no heat is required to hasten the process.
Of this I am going to shew you an instance. I put into this bottle the
water that is to be decomposed, as also the metal that is to effect that
decomposition by combining with the oxygen, and the acid which is to
facilitate the combination of the metal and the oxygen. You will see
with what violence these will act on each other.

CAROLINE.

But what metal is it that you employ for this purpose?

MRS. B.

It is iron; and it is used in the state of filings, as these present a
greater surface to the acid than a solid piece of metal. For as it is
the surface of the metal which is acted upon by the acid, and is
disposed to receive the oxygen produced by the decomposition of the
water, it necessarily follows that the greater is the surface, the more
considerable is the effect. The bubbles which are now rising are
hydrogen gas----

CAROLINE.

How disagreeably it smells!

MRS. B.

It is indeed unpleasant, though, I believe, not particularly hurtful. We
shall not, however, suffer any more to escape, as it will be wanted for
experiments. I shall, therefore, collect it in a glass-receiver, by
making it pass through this bent tube, which will conduct it into the
water-bath. (PLATE VIII. fig. 3.)

EMILY.

How very rapidly the gas escapes! it is perfectly transparent, and
without any colour whatever. --Now the receiver is full----

MRS. B.

We shall, therefore, remove it, and substitute another in its place. But
you must observe, that when the receiver is full, it is necessary to
keep it inverted with the mouth under water, otherwise the gas would
escape. And in order that it may not be in the way, I introduce within
the bath, under the water, a saucer, into which I slide the receiver, so
that it can be taken out of the bath and conveyed any where, the water
in the saucer being equally effectual in preventing its escape as that
in the bath. (PLATE VIII. fig. 4.)

EMILY.

I am quite surprised to see what a large quantity of hydrogen gas can be
produced by such a small quantity of water, especially as oxygen is the
principal constituent of water.

MRS. B.

In weight it is; but not in volume. For though the proportion, by
weight, is nearly six parts of oxygen to one of hydrogen, yet the
proportion of the volume of the gases, is about one part of oxygen to
two of hydrogen; so much heavier is the former than the latter.

CAROLINE.

But why is the vessel in which the water is decomposed so hot? As the
water changes from a liquid to a gaseous form, cold should be produced
instead of heat.

MRS. B.

No; for if one of the constituents of water is converted into a gas, the
other becomes solid in combining with the metal.

EMILY.

In this case, then, neither heat nor cold should be produced?

MRS. B.

True: but observe that the sensible heat which is disengaged in this
operation, is not owing to the decomposition of the water, but to an
extrication of heat produced by the mixture of water and sulphuric acid.
I will mix some water and sulphuric acid together in this glass, that
you may feel the surprising quantity of heat that is disengaged by their
union--now take hold of the glass----

CAROLINE.

Indeed I cannot; it feels as hot as boiling water. I should have
imagined there would have been heat enough disengaged to have rendered
the liquid solid.

MRS. B.

As, however, it does not produce that effect, we cannot refer this heat
to the modification called latent heat. We may, however, I think,
consider it as heat of capacity, as the liquid is condensed by its loss;
and if you were to repeat the experiment, in a graduated tube, you would
find that the two liquids, when mixed, occupy considerably less space
than they did separately. --But we will reserve this to another
opportunity, and attend at present to the hydrogen gas which we have
been producing.

If I now set the hydrogen gas, which is contained in this receiver, at
liberty all at once, and kindle it as soon as it comes in contact with
the atmosphere, by presenting it to a candle, it will so suddenly and
rapidly decompose the oxygen gas, by combining with its basis, that an
explosion, or a _detonation_ (as chemists commonly call it), will be
produced. For this purpose, I need only take up the receiver, and
quickly present its open mouth to the candle---- so . . . .

CAROLINE.

It produced only a sort of hissing noise, with a vivid flash of light.
I had expected a much greater report.

MRS. B.

And so it would have been, had the gases been closely confined at the
moment they were made to explode. If, for instance, we were to put in
this bottle a mixture of hydrogen gas and atmospheric air; and if, after
corking the bottle, we should kindle the mixture by a very small
orifice, from the sudden dilatation of the gases at the moment of their
combination, the bottle must either fly to pieces, or the cork be blown
out with considerable violence.

CAROLINE.

But in the experiment which we have just seen, if you did not kindle the
hydrogen gas, would it not equally combine with the oxygen?

MRS. B.

Certainly not; for, as I have just explained to you, it is necessary
that the oxygen and hydrogen gases be burnt together, in order to
combine chemically and produce water.

CAROLINE.

That is true; but I thought this was a different combination, for I see
no water produced.

MRS. B.

The water resulting from this detonation was so small in quantity, and
in such a state of minute division, as to be invisible. But water
certainly was produced; for oxygen is incapable of combining with
hydrogen in any other proportions than those that form water; therefore
water must always be the result of their combination.

If, instead of bringing the hydrogen gas into sudden contact with the
atmosphere (as we did just now) so as to make the whole of it explode
the moment it is kindled, we allow but a very small surface of gas to
burn in contact with the atmosphere, the combustion goes on quietly and
gradually at the point of contact, without any detonation, because the
surfaces brought together are too small for the immediate union of
gases. The experiment is a very easy one. This phial, with a narrow
neck, (PLATE VIII. fig. 5.) is full of hydrogen gas, and is carefully
corked. If I take out the cork without moving the phial, and quickly
approach the candle to the orifice, you will see how different the
result will be----

EMILY.

How prettily it burns, with a blue flame! The flame is gradually sinking
within the phial--now it has entirely disappeared. But does not this
combustion likewise produce water?

MRS. B.

Undoubtedly. In order to make the formation of the water sensible to
you, I shall procure a fresh supply of hydrogen gas, by putting into
this bottle (PLATE VIII. fig. 6.) iron filings, water, and sulphuric
acid, materials similar to those which we have just used for the same
purpose. I shall then cork up the bottle, leaving only a small orifice
in the cork, with a piece of glass-tube fixed to it, through which the
gas will issue in a continued rapid stream.

CAROLINE.

I hear already the hissing of the gas through the tube, and I can feel a
strong current against my hand.

MRS. B.

This current I am going to kindle with the candle--see how vividly it
burns----

EMILY.

It burns like a candle with a long flame. But why does this combustion
last so much longer than in the former experiment?

MRS. B.

The combustion goes on uninterruptedly as long as the new gas continues
to be produced. Now if I invert this receiver over the flame, you will
soon perceive its internal surface covered with a very fine dew, which
is pure water----

CAROLINE.

Yes, indeed; the glass is now quite dim with moisture! How glad I am
that we can see the water produced by this combustion.

EMILY.

It is exactly what I was anxious to see; for I confess I was a little
incredulous.

MRS. B.

If I had not held the glass-bell over the flame, the water would have
escaped in the state of vapour, as it did in the former experiment. We
have here, of course, obtained but a very small quantity of water; but
the difficulty of procuring a proper apparatus, with sufficient
quantities of gases, prevents my showing it you on a larger scale.

The composition of water was discovered about the same period, both by
Mr. Cavendish, in this country, and by the celebrated French chemist
Lavoisier. The latter invented a very perfect and ingenious apparatus to
perform, with great accuracy, and upon a large scale, the formation of
water by the combination of oxygen and hydrogen gases. Two tubes,
conveying due proportions, the one of oxygen, the other of hydrogen gas,
are inserted at opposite sides of a large globe of glass, previously
exhausted of air; the two streams of gas are kindled within the globe,
by the electrical spark, at the point where they come in contact; they
burn together, that is to say, the hydrogen combines with the oxygen,
the caloric is set at liberty, and a quantity of water is produced
exactly equal, in weight, to that of the two gases introduced into the
globe.

CAROLINE.

And what was the greatest quantity of water ever formed in this
apparatus?

MRS. B.

Several ounces; indeed, very nearly a pound, if I recollect right; but
the operation lasted many days.

EMILY.

This experiment must have convinced all the world of the truth of the
discovery. Pray, if improper proportions of the gases were mixed and set
fire to, what would be the result?

MRS. B.

Water would equally be formed, but there would be a residue of either
one or other of the gases, because, as I have already told you, hydrogen
and oxygen will combine only in the proportions requisite for the
formation of water.

EMILY.

Look, Mrs. B., our experiment with the Voltaic battery (PLATE VIII. fig.
2.) has made great progress; a quantity of gas has been formed in each
tube, but in one of them there is twice as much gas as in the other.

MRS. B.

Yes; because, as I said before, water is composed of two volumes of
hydrogen to one of oxygen--and if we should now mix these gases together
and set fire to them by an electrical spark, both gases would entirely
disappear, and a small quantity of water would be formed.

There is another curious effect produced by the combustion of hydrogen
gas, which I shall show you, though I must acquaint you first, that I
cannot well explain the cause of it. For this purpose, I must put some
materials into our apparatus, in order to obtain a stream of hydrogen
gas, just as we have done before. The process is already going on, and
the gas is rushing through the tube--I shall now kindle it with the
taper----

EMILY.

It burns exactly as it did before---- What is the curious effect which
you were mentioning?

MRS. B.

Instead of the receiver, by means of which we have just seen the drops
of water form, we shall invert over the flame this piece of tube, which
is about two feet in length, and one inch in diameter (PLATE VIII.
fig. 7.); but you must observe that it is open at both ends.

EMILY.

What a strange noise it makes! something like the AEolian harp, but not
so sweet.

CAROLINE.

It is very singular, indeed; but I think rather too powerful to be
pleasing. And is not this sound accounted for?

MRS. B.

That the percussion of glass, by a rapid stream of gas, should produce a
sound, is not extraordinary: but the sound here is so peculiar, that no
other gas has a similar effect. Perhaps it is owing to a brisk vibratory
motion of the glass, occasioned by the successive formation and
condensation of small drops of water on the sides of the glass tube, and
the air rushing in to replace the vacuum formed.*

    [Footnote *: This ingenious explanation was first suggested by
    Dr. Delarive. --See Journals of the Royal Institution, vol. i.
    p. 259.]

CAROLINE.

How very much this flame resembles the burning of a candle.

MRS. B.

The burning of a candle is produced by much the same means. A great deal
of hydrogen is contained in candles, whether of tallow or wax. This
hydrogen being converted into gas by the heat of the candle, combines
with the oxygen of the atmosphere, and flame and water result from this
combination. So that, in fact, the flame of a candle is owing to the
combustion of hydrogen gas. An elevation of temperature, such as is
produced by a lighted match or taper, is required to give the first
impulse to the combustion; but afterwards it goes on of itself, because
the candle finds a supply of caloric in the successive quantities of
heat which results from the union of the two electricities given out by
the gases during their combustion. But there are other circumstances
connected with the combustion of candles and lamps, which I cannot
explain to you till you are acquainted with _carbon_, which is one of
their constituent parts. In general, however, whenever you see flame,
you may infer that it is owing to the formation and burning of hydrogen
gas*; for flame is the peculiar mode of burning hydrogen gas, which,
with only one or two apparent exceptions, does not belong to any other
combustible.

    [Footnote *: Or rather, _hydro-carbonat_, a gas composed of
    hydrogen and carbon, which will be noticed under the head
    _Carbon_.]

EMILY.

You astonish me! I understood that flame was the caloric produced by the
union of the two electricities, in all combustions whatever?

MRS. B.

Your error proceeded from your vague and incorrect idea of flame; you
have confounded it with light and caloric in general. Flame always
implies caloric, since it is produced by the combustion of hydrogen gas;
but all caloric does not imply flame. Many bodies burn with intense heat
without producing flame. Coals, for instance, burn with flame until all
the hydrogen which they contain is evaporated; but when they afterwards
become red hot, much more caloric is disengaged than when they produce
flame.

CAROLINE.

But the iron wire, which you burnt in oxygen gas, appeared to me to emit
flame; yet, as it was a simple metal, it could contain no hydrogen?

MRS. B.

It produced a sparkling dazzling blaze of light, but no real flame.

EMILY.

And what is the cause of the regular shape of the flame of a candle?

MRS. B.

The regular stream of hydrogen gas which exhales from its combustible
matter.

CAROLINE.

But the hydrogen gas must, from its great levity, ascend into the upper
regions of the atmosphere; why therefore does not the flame continue to
accompany it?

MRS. B.

The combustion of the hydrogen gas is completed at the point where the
flame terminates; it then ceases to be hydrogen gas, as it is converted
by its combination with oxygen into watery vapour; but in a state of
such minute division as to be invisible.

CAROLINE.

I do not understand what is the use of the wick of a candle, since the
hydrogen gas burns so well without it?

MRS. B.

The combustible matter of the candle must be decomposed in order to
emit the hydrogen gas, and the wick is instrumental in effecting this
decomposition. Its combustion first melts the combustible matter,
and . . . .

CAROLINE.

But in lamps the combustible matter is already fluid, and yet they also
require wicks?

MRS. B.

I am going to add that, afterwards, the burning wick (by the power of
capillary attraction) gradually draws up the fluid to the point where
combustion takes place; for you must have observed that the wick does
not burn quite to the bottom.

CAROLINE.

Yes; but I do not understand why it does not.

MRS. B.

Because the air has not so free an access to that part of the wick which
is immediately in contact with the candle, as to the part just above, so
that the heat there is not sufficient to produce its decomposition; the
combustion therefore begins a little above this point.

CAROLINE.

But, Mrs. B., in those beautiful lights, called _gas-lights_, which are
now seen in many streets, and will, I hope, be soon adopted every where,
I can perceive no wick at all. How are these lights managed?

MRS. B.

I am glad you have put me in mind of saying a few words on this very
useful and interesting improvement. In this mode of lighting, the gas is
conveyed to the extremity of a tube, where it is kindled, and burns as
long as the supply continues. There is, therefore, no occasion for a
wick, or any other fuel whatever.

EMILY.

But how is all this gas procured in such large quantities?

MRS. B.

It is obtained from coal, by distillation. --Coal, when exposed to heat
in a close vessel, is decomposed; and hydrogen, which is one of its
constituents, rises in the state of gas, combined with another of its
component parts, carbon, forming a compound gas, called _Hydrocarbonat_,
the nature of which we shall again have an opportunity of noticing when
we treat of carbon. This gas, like hydrogen, is perfectly transparent,
invisible, and highly inflammable; and in burning it emits that vivid
light which you have so often observed.

CAROLINE.

And does the process for procuring it require nothing but heating the
coals, and conveying the gas through tubes?

MRS. B.

Nothing else; except that the gas must be made to pass, immediately at
its formation, through two or three large vessels of water, in which it
deposits some other ingredients, and especially water, tar, and oil,
which also arise from the distillation of coals. The gas-light
apparatus, therefore, consists simply in a large iron vessel, in which
the coals are exposed to the heat of a furnace,--some reservoirs of
water, in which the gas deposits its impurities,--and tubes that convey
it to the desired spot, being propelled with uniform velocity through
the tubes by means of a certain degree of pressure which is made upon
the reservoir.

EMILY.

What an admirable contrivance! Do you not think, Mrs. B., that it will
soon get into universal use?

MRS. B.

Most probably, as to the lighting of streets, offices, and public
places, as it far surpasses any former invention for that purpose; but
as to the interior of private houses, this mode of lighting has not yet
been sufficiently tried to know whether it will be found generally
desirable, either in regard to economy or convenience. It may, however,
be considered as one of the happiest applications of chemistry to the
comforts of life; and there is every reason to suppose that it will
answer the full extent of public, expectation.

I have another experiment to show you with hydrogen gas, which I think
will entertain you. Have you ever blown bubbles with soap and water?

EMILY.

Yes, often, when I was a child; and I used to make them float in the air
by blowing them upwards.

MRS. B.

We shall fill some such bubbles with hydrogen gas, instead of
atmospheric air, and you will see with what ease and rapidity they will
ascend, without the assistance of blowing, from the lightness of the
gas. --Will you mix some soap and water whilst I fill this bladder with
the gas contained in the receiver which stands on the shelf in the
water-bath?

CAROLINE.

What is the use of the brass-stopper and turn-cock at the top of the
receiver?

MRS. B.

It is to afford a passage to the gas when required. There is, you see,
a similar stop-cock fastened to this bladder, which is made to fit that
on the receiver. I screw them one on the other, and now turn the two
cocks, to open a communication between the receiver and the bladder;
then, by sliding the receiver off the shelf, and gently sinking it into
the bath, the water rises in the receiver and forces the gas into the
bladder. (PLATE IX. fig. 1.)

  [Illustration: Plate IX. Vol. I. p. 228

  Fig. 1. Apparatus for transferring gases from a Receiver into a
    bladder.
  Fig. 2. Apparatus for blowing Soap bubbles.]

CAROLINE.

Yes, I see the bladder swell as the water rises in the receiver.

MRS. B.

I think that we have already a sufficient quantity in the bladder for
our purpose; we must be careful to stop both the cocks before we
separate the bladder from the receiver, lest the gas should escape.
--Now I must fix a pipe to the stopper of the bladder, and by dipping
its mouth into the soap and water, take up a few drops--then I again
turn the cock, and squeeze the bladder in order to force the gas into
the soap and water at the mouth of the pipe. (PLATE IX. fig. 2.)

EMILY.

There is a bubble--but it bursts before it leaves the mouth of the pipe.

MRS. B.

We must have patience and try again; it is not so easy to blow bubbles
by means of a bladder, as simply with the breath.

CAROLINE.

Perhaps there is not soap enough in the water; I should have had warm
water, it would have dissolved the soap better.

EMILY.

Does not some of the gas escape between the bladder and the pipe?

MRS. B.

No, they are perfectly air tight; we shall succeed presently, I dare
say.

CAROLINE.

Now a bubble ascends; it moves with the rapidity of a balloon. How
beautifully it refracts the light!

EMILY.

It has burst against the ceiling--you succeed now wonderfully; but why
do they all ascend and burst against the ceiling?

MRS. B.

Hydrogen gas is so much lighter than atmospherical air, that it ascends
rapidly with its very light envelope, which is burst by the force with
which it strikes the ceiling.

Air-balloons are filled with this gas, and if they carried no other
weight than their covering, would ascend as rapidly as these bubbles.

CAROLINE.

Yet their covering must be much heavier than that of these bubbles?

MRS. B.

Not in proportion to the quantity of gas they contain. I do not know
whether you have ever been present at the filling of a large balloon.
The apparatus for that purpose is very simple. It consists of a number
of vessels, either jars or barrels, in which the materials for the
formation of the gas are mixed, each of these being furnished with a
tube, and communicating with a long flexible pipe, which conveys the gas
into the balloon.

EMILY.

But the fire-balloons which were first invented, and have been since
abandoned, on account of their being so dangerous, were constructed,
I suppose, on a different principle.

MRS. B.

They were filled simply with atmospherical air, considerably rarefied by
heat; and the necessity of having a fire underneath the balloon, in
order to preserve the rarefaction of the air within it, was the
circumstance productive of so much danger.

If you are not yet tired of experiments, I have another to show you. It
consists in filling soap-bubbles with a mixture of hydrogen and oxygen
gases, in the proportions that form water; and afterwards setting fire
to them.

EMILY.

They will detonate, I suppose?

MRS. B.

Yes, they will. As you have seen the method of transferring the gas from
the receiver into the bladder, it is not necessary to repeat it. I have
therefore provided a bladder which contains a due proportion of oxygen
and hydrogen gases, and we have only to blow bubbles with it.

CAROLINE.

Here is a fine large bubble rising--shall I set fire to it with the
candle?

MRS. B.

If you please . . . .

CAROLINE.

Heavens, what an explosion! --It was like the report of a gun: I confess
it frightened me much. I never should have imagined it could be so loud.

EMILY.

And the flash was as vivid as lightning.

MRS. B.

The combination of the two gases takes place during that instant of time
that you see the flash, and hear the detonation.

EMILY.

This has a strong resemblance to thunder and lightning.

MRS. B.

These phenomena, however, are generally of an electrical nature. Yet
various meteorological effects may be attributed to accidental
detonations of hydrogen gas in the atmosphere; for nature abounds with
hydrogen: it constitutes a very considerable portion of the whole mass
of water belonging to our globe, and from that source almost every other
body obtains it. It enters into the composition of all animal
substances, and of a great number of minerals; but it is most abundant
in vegetables. From this immense variety of bodies, it is often
spontaneously disengaged; its great levity makes it rise into the
superior regions of the atmosphere; and when, either by an electrical
spark, or any casual elevation of temperature, it takes fire, it may
produce such meteors or luminous appearances as are occasionally seen in
the atmosphere. Of this kind are probably those broad flashes which we
often see on a summer-evening, without hearing any detonation.

EMILY.

Every flash, I suppose, must produce a quantity of water?

CAROLINE.

And this water, naturally, descends in the form of rain?

MRS. B.

That probably is often the case, though it is not a necessary
consequence; for the water may be dissolved by the atmosphere, as it
descends towards the lower regions, and remain there in the form of
clouds.

The application of electrical attraction to chemical phenomena is likely
to lead to many very interesting discoveries in meteorology; for
electricity evidently acts a most important part in the atmosphere. This
subject however, is, as yet, not sufficiently developed for me to
venture enlarging upon it. The phenomena of the atmosphere are far from
being well understood; and even with the little that is known, I am but
imperfectly acquainted.


But before we take leave of hydrogen, I must not omit to mention to you
a most interesting discovery of Sir H. Davy, which is connected with
this subject.

CAROLINE.

You allude, I suppose, to the new miner's lamp, which has of late been
so much talked of? I have long been desirous of knowing what that
discovery was, and what purpose it was intended to answer.

MRS. B.

It often happens in coal-mines, that quantities of the gas, called by
chemists _hydro-carbonat_, or by the miners _fire-damp_, (the same from
which the gas-lights are obtained,) ooze out from fissures in the beds
of coal, and fill the cavities in which the men are at work; and this
gas being inflammable, the consequence is, that when the men approach
those places with a lighted candle, the gas takes fire, and explosions
happen which destroy the men and horses employed in that part of the
colliery, sometimes in great numbers.

EMILY.

What tremendous accidents these must be! But whence does that gas
originate?

MRS. B.

Being the chief product of the combustion of coal, no wonder that
inflammable gas should occasionally appear in situations in which this
mineral abounds, since there can be no doubt that processes of
combustion are frequently taking place at a great depth under the
surface of the earth; and therefore those accumulations of gas may arise
either from combustions actually going on, or from former combustions,
the gas having perhaps been confined there for ages.

CAROLINE.

And how does Sir H. Davy's lamp prevent those dreadful explosions?

MRS. B.

By a contrivance equally simple and ingenious; and one which does no
less credit to the philosophical views from which it was deduced, than
to the philanthropic motives from which the enquiry sprung. The
principle of the lamp is shortly this: It was ascertained, two or three
years ago, both by Mr. Tennant and by Sir Humphry himself, that the
combustion of inflammable gas could not be propagated through small
tubes; so that if a jet of an inflammable gaseous mixture, issuing from
a bladder or any other vessel, through a small tube, be set fire to, it
burns at the orifice of the tube, but the flame never penetrates into
the vessel. It is upon this fact that Sir Humphry's safety-lamp is
founded.

EMILY.

But why does not the flame ever penetrate through the tube into the
vessel from which the gas issues, so as to explode at once the whole of
the gas?

MRS. B.

Because, no doubt, the inflamed gas is so much cooled in its passage
through a small tube as to cease to burn before the combustion reaches
the reservoir.

CAROLINE.

And how can this principle be applied to the construction of a lamp?

MRS. B.

Nothing easier. You need only suppose a lamp enclosed all round in glass
or horn, but having a number of small open tubes at the bottom, and
others at the top, to let the air in and out. Now, if such a lamp or
lanthorn be carried into an atmosphere capable of exploding, an
explosion or combustion of the gas will take place within the lamp; and
although the vent afforded by the tubes will save the lamp from
bursting, yet, from the principle just explained, the combustion will
not be propagated to the external air through the tubes, so that no
farther consequence will ensue.

EMILY.

And is that all the mystery of that valuable lamp?

MRS. B.

No; in the early part of the enquiry a lamp of this kind was actually
proposed; but it was but a rude sketch compared to its present state of
improvement. Sir H. Davy, after a succession of trials, by which he
brought his lamp nearer and nearer to perfection, at last conceived the
happy idea that if the lamp were surrounded with a wire-work or
wire-gauze, of a close texture, instead of glass or horn, the tubular
contrivance I have just described would be entirely superseded, since
each of the interstices of the gauze would act as a tube in preventing
the propagation of explosions; so that this pervious metallic covering
would answer the various purposes of transparency, of permeability to
air, and of protection against explosion. This idea, Sir Humphry
immediately submitted to the test of experiment, and the result has
answered his most sanguine expectations, both in his laboratory and in
the collieries, where it has already been extensively tried. And he has
now the happiness of thinking that his invention will probably be the
means of saving every year a number of lives, which would have been lost
in digging out of the bowels of the earth one of the most valuable
necessaries of life. Here is one of these lamps, every part of which you
will at once comprehend. (See PLATE X. fig. 1.)

  [Illustration: Plate X.

  Fig. 1.
  A. the cistern containing the Oil
  B. the rim or screw by which the gauze cage is fixed to the cistern.
  C. apperture for supplying Oil.
  E. a wire for trimming the wick.
  D. F. the wire gauze cylinder.
  G. a double top.

  Fig. 2.
  A. the reservoir of condensed air.
  B. the condensing Syringe.
  C. the bladder for Oxygen.
  D. the moveable jet.]

CAROLINE.

How very simple and ingenious! But I do not yet well see why an
explosion taking place within the lamp should not communicate to the
external air around it, through the interstices of the wire?

MRS. B.

This has been and is still a subject of wonder, even to philosophers;
and the only mode they have of explaining it is, that flame or ignition
cannot pass through a fine wire-work, because the metallic wire cools
the flame sufficiently to extinguish it in passing through the gauze.
This property of the wire-gauze is quite similar to that of the tubes
which I mentioned on introducing the subject; for you may consider each
interstice of the gauze as an extremely short tube of a very small
diameter.

EMILY.

But I should expect the wire would often become red-hot, by the burning
of the gas within the lamp?

MRS. B.

And this is actually the case, for the top of the lamp is very apt to
become red-hot. But, fortunately, inflammable gaseous mixtures cannot be
exploded by red-hot wire, the intervention of actual flame being
required for that purpose; so that the wire does not set fire to the
explosive gas around it.

EMILY.

I can understand that; but if the wire be red-hot, how can it cool the
flame within, and prevent its passing through the gauze?

MRS. B.

The gauze, though red-hot, is not so hot as the flame by which it has
been heated; and as metallic wire is a good conductor, the heat does not
much accumulate in it, as it passes off quickly to the other parts of
the lamp, as well as to any contiguous bodies.

CAROLINE.

This is indeed a most interesting discovery, and one which shows at once
the immense utility with which science may be practically applied to
some of the most important purposes.




CONVERSATION VIII.

ON SULPHUR AND PHOSPHORUS.


MRS. B.

SULPHUR is the next substance that comes under our consideration. It
differs in one essential point from the preceding, as it exists in a
solid form at the temperature of the atmosphere.

CAROLINE.

I am glad that we have at last a solid body to examine; one that we can
see and touch. Pray, is it not with sulphur that the points of matches
are covered, to make them easily kindle?

MRS. B.

Yes, it is; and you therefore already know that sulphur is a very
combustible substance. It is seldom discovered in nature in a pure
unmixed state; so great is its affinity for other substances, that it is
almost constantly found combined with some of them. It is most commonly
united with metals, under various forms, and is separated from them by a
very simple process. It exists likewise in many mineral waters, and some
vegetables yield it in various proportions, especially those of the
cruciform tribe. It is also found in animal matter; in short, it may be
discovered in greater or less quantity, in the mineral, vegetable, and
animal kingdoms.

EMILY.

I have heard of _flowers of sulphur_, are they the produce of any plant?

MRS. B.

By no means: they consist of nothing more than common sulphur, reduced
to a very fine powder by a process called _sublimation_. --You see some
of it in this phial; it is exactly the same substance as this lump of
sulphur, only its colour is a paler yellow, owing to its state of very
minute division.

EMILY.

Pray what is sublimation?

MRS. B.

It is the evaporation, or, more properly speaking, the volatilisation of
solid substances, which, in cooling, condense again in a concrete form.
The process, in this instance, must be performed in a closed vessel,
both to prevent combustion, which would take place if the access of air
were not carefully precluded, and likewise in order to collect the
substance after the operation. As it is rather a slow process, we shall
not try the experiment now; but you will understand it perfectly if I
show you the apparatus used for the purpose. (PLATE XI. fig. 1.) Some
lumps of sulphur are put into a receiver of this kind, which is called a
_cucurbit_. Its shape, you see, somewhat resembles that of a pear, and
is open at the top, so as to adapt itself exactly to a kind of conical
receiver of this sort, called the head. The cucurbit, thus covered with
its head, is placed over a sand-bath; this is nothing more than a vessel
full of sand, which is kept heated by a furnace, such as you see here,
so as to preserve the apparatus in a moderate and uniform temperature.
The sulphur then soon begins to melt, and immediately after this,
a thick white smoke rises, which is gradually deposited within the head,
or upper part of the apparatus, where it condenses against the sides,
somewhat in the form of a vegetation, whence it has obtained the name of
flowers of sulphur. This apparatus, which is called an _alembic_, is
highly useful in all kinds of distillations, as you will see when we
come to treat of those operations. Alembics are not commonly made of
glass, like this, which is applicable only to distillations upon a very
small scale. Those used in manufactures are generally made of copper,
and are, of course, considerably larger. The principal construction,
however, is always the same, although their shape admits of some
variation.

  [Illustration: Plate XI. Vol. I. p. 237.

  Fig. 1. Sublimation of Sulphur.
  A Alembic.
  B Sand-bath.
  C Furnace.

  Fig. 2. Eudiometer.

  Fig. 3. Decomposition of water by Carbon.
  A Retort containing water.
  B Lamp to heat the water.
  C.C Porcelain tube containing Carbone.
  D Furnace through which the tube passes.
  E Receiver for the gas produced.
  F Water bath.]

CAROLINE.

What is the use of that neck, or tube, which bends down from the upper
piece of the apparatus?

MRS. B.

It is of no use in sublimations; but in distillations (the general
object of which is to evaporate, by heat, in closed vessels, the
volatile parts of a compound body, and to condense them again into a
liquid,) it serves to carry off the condensed fluid, which otherwise
would fall back into the cucurbit. But this is rather foreign to our
present subject. Let us return to the sulphur. You now perfectly
understand, I suppose, what is meant by sublimation?

EMILY.

I believe I do. Sublimation appears to consist in destroying, by means
of heat, the attraction of aggregation of the particles of a solid body,
which are thus volatilised; and as soon as they lose the caloric which
produced that effect, they are deposited in the form of a fine powder.

CAROLINE.

It seems to me to be somewhat similar to the transformation of water
into vapour, which returns to its liquid state when deprived of caloric.

EMILY.

There is this difference, however, that the sulphur does not return to
its former state, since, instead of lumps, it changes to a fine powder.

MRS. B.

Chemically speaking, it is exactly the same substance, whether in the
form of lump or powder. For if this powder be melted again by heat, it
will, in cooling, be restored to the same solid state in which it was
before its sublimation.

CAROLINE.

But if there be no real change, produced by the sublimation of the
sulphur, what is the use of that operation?

MRS. B.

It divides the sulphur into very minute parts, and thus disposes it to
enter more readily into combination with other bodies. It is used also
as a means of purification.

CAROLINE.

Sublimation appears to me like the beginning of combustion, for the
completion of which one circumstance only is wanting, the absorption of
oxygen.

MRS. B.

But that circumstance is every thing. No essential alteration is
produced in sulphur by sublimation; whilst in combustion it combines
with the oxygen, and forms a new compound totally different in every
respect from sulphur in its pure state. --We shall now _burn_ some
sulphur, and you will see how very different the result will be. For
this purpose I put a small quantity of flowers of sulphur into this cup,
and place it in a dish, into which I have poured a little water: I now
set fire to the sulphur with the point of this hot wire; for its
combustion will not begin unless its temperature be considerably raised.
--You see that it burns with a faint blueish flame; and as I invert over
it this receiver, white fumes arise from the sulphur, and fill the
vessel. --You will soon perceive that the water is rising within the
receiver, a little above its level in the plate. --Well, Emily, can you
account for this?

EMILY.

I suppose that the sulphur has absorbed the oxygen from the
atmospherical air within the receiver, and that we shall find some
oxygenated sulphur in the cup. As for the white smoke, I am quite at a
loss to guess what it may be.

MRS. B.

Your first conjecture is very right: but you are mistaken in the last;
for nothing will be left in the cup. The white vapour is the oxygenated
sulphur, which assumes the form of an elastic fluid of a pungent and
offensive smell, and is a powerful acid. Here you see a chemical
combination of oxygen and sulphur, producing a true gas, which would
continue such under the pressure and at the temperature of the
atmosphere, if it did not unite with the water in the plate, to which it
imparts its acid taste, and all its acid properties. --You see, now,
with what curious effects the combustion of sulphur is attended.

CAROLINE.

This is something quite new; and I confess that I do not perfectly
understand why the sulphur turns acid.

MRS. B.

It is because it unites with oxygen, which is the acidifying principle.
And, indeed, the word _oxygen_ is derived from two Greek words
signifying _to produce an acid_.

CAROLINE.

Why, then, is not water, which contains such a quantity of oxygen, acid?

MRS. B.

Because hydrogen, which is the other constituent of water, is not
susceptible of acidification. --I believe it will be necessary, before
we proceed further, to say a few words of the general nature of acids,
though it is rather a deviation from our plan of examining the simple
bodies separately, before we consider them in a state of combination.

Acids may be considered as a peculiar class of _burnt_ bodies, which
during their combustion, or combination with oxygen, have acquired very
characteristic properties. They are chiefly discernible by their sour
taste, and by turning red most of the blue vegetable colours. These two
properties are common to the whole class of acids; but each of them is
distinguished by other peculiar qualities. Every acid consists of some
particular substance, (which constitutes its basis, and is different in
each,) and of oxygen, which is common to them all.

EMILY.

But I do not clearly see the difference between acids and oxyds.

MRS. B.

Acids were, in fact, oxyds, which, by the addition of a sufficient
quantity of oxygen, have been converted into acids. For acidification,
you must observe, always implies previous oxydation, as a body must have
combined with the quantity of oxygen requisite to constitute it an oxyd,
before it can combine with the greater quantity that is necessary to
render it an acid.

CAROLINE.

Are all oxyds capable of being converted into acids?

MRS. B.

Very far from it; it is only certain substances which will enter into
that peculiar kind of union with oxygen that produces acids, and the
number of these is proportionally very small; but all burnt bodies may
be considered as belonging either to the class of oxyds, or to that of
acids. At a future period, we shall enter more at large into this
subject. At present, I have but one circumstance further to point out to
your observation respecting acids: it is, that most of them are
susceptible of two degrees of acidification, according to the different
quantities of oxygen with which their basis combines.

EMILY.

And how are these two degrees of acidification distinguished?

MRS. B.

By the peculiar properties which result from them. The acid we have just
made is the first or weakest degree of acidification, and is called
_sulphureous acid_; if it were fully saturated with oxygen, it would be
called _sulphuric acid_. You must therefore remember, that in this, as
in all acids, the first degree of acidification is expressed by the
termination in _ous_; the stronger, by the termination in _ic_.

CAROLINE.

And how is the sulphuric acid made?

MRS. B.

By burning sulphur in pure oxygen gas, and thus rendering its combustion
much more complete. I have provided some oxygen gas for this purpose; it
is in that bottle, but we must first decant the gas into the glass
receiver which stands on the shelf in the bath, and is full of water.

CAROLINE.

Pray, let me try to do it, Mrs. B.

MRS. B.

It requires some little dexterity--hold the bottle completely under
water, and do not turn the mouth upwards, till it is immediately under
the aperture in the shelf, through which the gas is to pass into the
receiver, and then turn it up gradually. --Very well, you have only let
a few bubbles escape, and that must be expected at a first trial. --Now
I shall put this piece of sulphur into the receiver, through the opening
at the top, and introduce along with it a small piece of lighted tinder
to set fire to it. --This requires being done very quickly, lest the
atmospherical air should get in, and mix with the pure oxygen gas.

EMILY.

How beautifully it burns!

CAROLINE.

But it is already buried in the thick vapour. This, I suppose, is
sulphuric acid?

EMILY.

Are these acids always in a gaseous state?

MRS. B.

Sulphureous acid, as we have already observed, is a permanent gas, and
can be obtained in a liquid form only by condensing it in water. In its
pure state, the sulphureous acid is invisible, and it now appears in the
form of a white smoke, from its combining with the moisture. But the
vapour of sulphuric acid, which you have just seen to rise during the
combustion, is not a gas, but only a vapour, which condenses into liquid
sulphuric acid, by losing its caloric. But it appears from Sir H. Davy's
experiments, that this formation and condensation of sulphuric acid
requires the presence of water, for which purpose the vapour is received
into cold water, which may afterwards be separated from the acid by
evaporation.

Sulphur has hitherto been considered as a simple substance; but Sir H.
Davy has suspected that it contains a small portion of hydrogen, and
perhaps also of oxygen.

On submitting sulphur to the action of the Voltaic battery, he observed
that the negative wire gave out hydrogen; and the existence of hydrogen
in sulphur was rendered still more probable by his observing that a
small quantity of water was produced during the combustion of sulphur.

EMILY.

And pray of what nature is sulphur when perfectly pure?

MRS. B.

Sulphur has probably never been obtained perfectly free from
combination, so that its radical may possibly possess properties very
different from those of common sulphur. It has been suspected to be of a
metallic nature; but this is mere conjecture.

Before we quit the subject of sulphur, I must tell you that it is
susceptible of combining with a great variety of substances, and
especially with hydrogen, with which you are already acquainted.
Hydrogen gas can dissolve a small portion of it.

EMILY.

What! can a gas dissolve a solid substance?

MRS. B.

Yes; a solid substance may be so minutely divided by heat, as to become
soluble in a gas: and there are several instances of it. But you must
observe, that, in this case, a chemical union or combination of the
sulphur with the hydrogen gas is produced. In order to effect this, the
sulphur must be strongly heated in contact with the gas; the heat
reduces the sulphur to such a state of extreme division, and diffuses it
so thoroughly through the gas, that they combine and incorporate
together. And as a proof that there must be a chemical union between the
sulphur and the gas, it is sufficient to remark that they are not
separated when the sulphur loses the caloric by which it was
volatilized. Besides, it is evident, from the peculiar fetid smell of
this gas, that it is a new compound totally different from either of its
constituents; it is called _sulphuretted hydrogen gas_, and is contained
in great abundance in sulphureous mineral waters.

CAROLINE.

Are not the Harrogate waters of this nature?

MRS. B.

Yes; they are naturally impregnated with sulphuretted hydrogen gas, and
there are many other springs of the same kind, which shows that this gas
must often be formed in the bowels of the earth by spontaneous processes
of nature.

CAROLINE.

And could not such waters be made artificially by impregnating common
water with this gas?

MRS. B.

Yes; they can be so well imitated, as perfectly to resemble the
Harrogate waters.

Sulphur combines likewise with phosphorus, and with the alkalies, and
alkaline earths, substances with which you are yet unacquainted. We
cannot, therefore, enter into these combinations at present. In our next
lesson we shall treat of phosphorus.

EMILY.

May we not begin that subject to-day; this lesson has been so short?

MRS. B.

I have no objection, if you are not tired. What do you say, Caroline?

CAROLINE.

I am as desirous as Emily of prolonging the lesson to-day, especially as
we are to enter on a new subject; for I confess that sulphur has not
appeared to me so interesting as the other simple bodies.

MRS. B.

Perhaps you may find phosphorus more entertaining. You must not,
however, be discouraged when you meet with some parts of a study less
amusing than others; it would answer no good purpose to select the most
pleasing parts, since, if we did not proceed with some method, in order
to acquire a general idea of the whole, we could scarcely expect to take
interest in any particular subjects.


PHOSPHORUS.

PHOSPHORUS is considered as a simple body; though, like sulphur, it has
been suspected of containing hydrogen. It was not known by the earlier
chemists. It was first discovered by Brandt, a chemist of Hamburgh,
whilst employed in researches after the philosopher's stone; but the
method of obtaining it remained a secret till it was a second time
discovered both by Kunckel and Boyle, in the year 1680. You see a
specimen of phosphorus in this phial; it is generally moulded into small
sticks of a yellowish colour, as you find it here.

CAROLINE.

I do not understand in what the discovery consisted; there may be a
secret method of making an artificial composition, but how can you talk
of _making_ a substance which naturally exists?

MRS. B.

A body may exist in nature so closely combined with other substances, as
to elude the observation of chemists, or render it extremely difficult
to obtain it in its separate state. This is the case with phosphorus,
which is always so intimately combined with other substances, that its
existence remained unnoticed till Brandt discovered the means of
obtaining it free from other combinations. It is found in all animal
substances, and is now chiefly extracted from bones, by a chemical
process. It exists also in some plants, that bear a strong analogy to
animal matter in their chemical composition.

EMILY.

But is it never found in its pure separate state?

MRS. B.

Never, and this is the reason that it has remained so long undiscovered.

Phosphorus is eminently combustible; it melts and takes fire at the
temperature of one hundred degrees, and absorbs in its combustion nearly
once and a half its own weight of oxygen.

CAROLINE.

What! will a pound of phosphorus consume a pound and half of oxygen?

MRS. B.

So it appears from accurate experiments. I can show you with what
violence it combines with oxygen, by burning some of it in that gas. We
must manage the experiment in the same manner as we did the combustion
of sulphur. You see I am obliged to cut this little bit of phosphorus
under water, otherwise there would be danger of its taking fire by the
heat of my fingers. I now put into the receiver, and kindle it by means
of a hot wire.

EMILY.

What a blaze! I can hardly look at it. I never saw any thing so
brilliant. Does it not hurt your eyes, Caroline?

CAROLINE.

Yes; but still I cannot help looking at it. A prodigious quantity of
oxygen must indeed be absorbed, when so much light and caloric are
disengaged!

MRS. B.

In the combustion of a pound of phosphorus, a sufficient quantity of
caloric is set free to melt upwards of a hundred pounds of ice; this has
been computed by direct experiments with the calorimeter.

EMILY.

And is the result of this combustion, like that of sulphur, an acid?

MRS. B.

Yes; phosphoric acid. And had we duly proportioned the phosphorus and
the oxygen, they would have been completely converted into phosphoric
acid, weighing together, in this new state, exactly the sum of their
weights separately. The water would have ascended into the receiver, on
account of the vacuum formed, and would have filled it entirely. In this
case, as in the combustion of sulphur, the acid vapour formed is
absorbed and condensed in the water of the receiver. But when this
combustion is performed without any water or moisture being present, the
acid then appears in the form of concrete whitish flakes, which are,
however, extremely ready to melt upon the least admission of moisture.

EMILY.

Does phosphorus, in burning in atmospherical air, produce, like sulphur,
a weaker sort of the same acid?

MRS. B.

No: for it burns in atmospherical air, nearly at the same temperature as
in pure oxygen gas; and it is in both cases so strongly disposed to
combine with the oxygen, that the combustion is perfect, and the product
similar; only in atmospherical air, being less rapidly supplied with
oxygen, the process is performed in a slower manner.

CAROLINE.

But is there no method of acidifying phosphorus in a slighter manner, so
as to form _phosphorus_ acid?

MRS. B.

Yes, there is. When simply exposed to the atmosphere, phosphorus
undergoes a kind of slow combustion at any temperature above zero.

EMILY.

But is not the process in this case rather an oxydation than a
combustion? For if the oxygen is too slowly absorbed for a sensible
quantity of light and heat to be disengaged, it is not a true
combustion.

MRS. B.

The case is not as you suppose: a faint light is emitted which is very
discernible in the dark; but the heat evolved is not sufficiently strong
to be sensible: a whitish vapour arises from this combustion, which,
uniting with water, condenses into liquid phosphorus acid.

CAROLINE.

Is it not very singular that phosphorus should burn at so low a
temperature in atmospherical air, whilst it does not burn in pure oxygen
without the application of heat?

MRS. B.

So it at first appears. But this circumstance seems to be owing to the
nitrogen gas of the atmosphere. This gas dissolves small particles of
phosphorus, which being thus minutely divided and diffused in the
atmospherical air, combines with the oxygen, and undergoes this slow
combustion. But the same effect does not take place in oxygen gas,
because it is not capable of dissolving phosphorus; it is therefore
necessary, in this case, that heat should be applied to effect that
division of particles, which, in the former instance, is produced by the
nitrogen.

EMILY.

I have seen letters written with phosphorus, which are invisible by
day-light, but may be read in the dark by their own light. They look as
if they were written with fire; yet they do not seem to burn.

MRS. B.

But they do really burn; for it is by their slow combustion that the
light is emitted; and phosphorus acid is the result of this combustion.

Phosphorus is sometimes used as a test to estimate the purity of
atmospherical air. For this purpose, it is burnt in a graduated tube,
called an _Eudiometer_ (PLATE XI. fig. 2.), and from the quantity of air
which the phosphorus absorbs, the proportion of oxygen in the air
examined is deduced; for the phosphorus will absorb all the oxygen, and
the nitrogen alone will remain.

EMILY.

And the more oxygen is contained in the atmosphere, the purer,
I suppose, it is esteemed?

MRS. B.

Certainly. Phosphorus, when melted, combines with a great variety of
substances. With sulphur it forms a compound so extremely combustible,
that it immediately takes fire on coming in contact with the air. It is
with this composition that phosphoric matches are prepared, which kindle
as soon as they are taken out of their case and are exposed to the air.

EMILY.

I have a box of these curious matches; but I have observed, that in very
cold weather, they will not take fire without being previously rubbed.

MRS. B.

By rubbing them you raise their temperature; for, you know, friction is
one of the means of extricating heat.

EMILY.

Will phosphorus combine with hydrogen gas, as sulphur does?

MRS. B.

Yes; and the compound gas which results from this combination has a
smell still more fetid than the sulphuretted hydrogen; it resembles that
of garlic.

The _phosphoretted hydrogen gas_ has this remarkable peculiarity, that
it takes fire spontaneously in the atmosphere, at any temperature. It is
thus, probably, that are produced those transient flames, or flashes of
light, called by the vulgar _Will-of-the Whisp_, or more properly
_Ignes-fatui_, which are often seen in church-yards, and places where
the putrefactions of animal matter exhale phosphorus and hydrogen gas.

CAROLINE.

Country people, who are so much frightened by those appearances, would
soon be reconciled to them, if they knew from what a simple cause they
proceed.

MRS. B.

There are other combinations of phosphorus that have also very singular
properties, particularly that which results from its union with lime.

EMILY.

Is there any name to distinguish the combination of two substances, like
phosphorus and lime, neither of which are oxygen, and which cannot
therefore produce either an oxyd or an acid?

MRS. B.

The names of such combinations are composed from those of their
ingredients, merely by a slight change in their termination. Thus the
combination of sulphur with lime is called a _sulphuret_, and that of
phosphorus, a _phosphuret of lime_. This latter compound, I was going to
say, has the singular property of decomposing water, merely by being
thrown into it. It effects this by absorbing the oxygen of water, in
consequence of which bubbles of hydrogen gas ascend, holding in solution
a small quantity of phosphorus.

EMILY.

These bubbles then are _phosphoretted hydrogen gas_?

MRS. B.

Yes; and they produce the singular appearance of a flash of fire issuing
from water, as the bubbles kindle and detonate on the surface of the
water, at the instant that they come in contact with the atmosphere.

CAROLINE.

Is not this effect nearly similar to that produced by the combination of
phosphorus and sulphur, or, more properly speaking, the _phosphuret of
sulphur_?

MRS. B.

Yes; but the phenomenon appears more extraordinary in this case, from
the presence of water, and from the gaseous form of the combustible
compound. Besides, the experiment surprises by its great simplicity. You
only throw a piece of phosphoret of lime into a glass of water, and
bubbles of fire will immediately issue from it.

CAROLINE.

Cannot we try the experiment?

MRS. B.

Very easily: but we must do it in the open air; for the smell of the
phosphorated hydrogen gas is so extremely fetid, that it would be
intolerable in the house. But before we leave the room, we may produce,
by another process, some bubbles of the same gas, which are much less
offensive.

There is in this little glass retort a solution of potash in water;
I add to it a small piece of phosphorus. We must now heat the retort
over the lamp, after having engaged its neck under water--you see it
begins to boil; in a few minutes bubbles will appear, which take fire
and detonate as they issue from the water.

CAROLINE.

There is one--and another. How curious it is! --But I do not understand
how this is produced.

MRS. B.

It is the consequence of a display of affinities too complicated,
I fear, to be made perfectly intelligible to you at present.

In a few words, the reciprocal action of the potash, phosphorus,
caloric, and water are such, that some of the water is decomposed, and
the hydrogen gas thereby formed carries off some minute particles of
phosphorus, with which it forms phosphoretted hydrogen gas, a compound
which spontaneously takes fire at almost any temperature.

EMILY.

What is that circular ring of smoke which slowly rises from each bubble
after its detonation?

MRS. B.

It consists of water and phosphoric acid in vapour, which are produced
by the combustion of hydrogen and phosphorus.




CONVERSATION IX.

ON CARBON.


CAROLINE.

To-day, Mrs. B., I believe we are to learn the nature and properties of
CARBON. This substance is quite new to me; I never heard it mentioned
before.

MRS. B.

Not so new as you imagine; for carbon is nothing more than charcoal in a
state of purity, that is to say, unmixed with any foreign ingredients.

CAROLINE.

But charcoal is made by art, Mrs. B., and a body consisting of one
simple substance cannot be fabricated?

MRS. B.

You again confound the idea, of making a simple body, with that of
separating it from a compound. The chemical processes by which a simple
body is obtained in a state of purity, consist in _unmaking_ the
compound in which it is contained, in order to separate from it the
simple substance in question. The method by which charcoal is usually
obtained, is, indeed, commonly called _making_ it; but, upon
examination, you will find this process to consist simply in separating
it from other substances with which it is found combined in nature.

Carbon forms a considerable part of the solid matter of all organised
bodies; but it is most abundant in the vegetable creation, and it is
chiefly obtained from wood. When the oil and water (which are other
constituents of vegetable matter) are evaporated, the black, porous,
brittle substance that remains, is charcoal.

CAROLINE.

But if heat be applied to the wood in order to evaporate the oil and
water, will not the temperature of the charcoal be raised so as to make
it burn; and if it combines with oxygen, can we any longer call it pure?

MRS. B.

I was going to say, that, in this operation, the air must be excluded.

CAROLINE.

How then can the vapour of the oil and water fly off?

MRS. B.

In order to produce charcoal in its purest state (which is, even then,
but a less imperfect sort of carbon), the operation should be performed
in an earthen retort. Heat being applied to the body of the retort, the
evaporable part of the wood will escape through its neck, into which no
air can penetrate as long as the heated vapour continues to fill it. And
if it be wished to collect these volatile products of the wood, this can
easily be done by introducing the neck of the retort into the water-bath
apparatus, with which you are acquainted. But the preparation of common
charcoal, such as is used in kitchens and manufactures, is performed on
a much larger scale, and by an easier and less expensive process.

EMILY.

I have seen the process of making common charcoal. The wood is ranged on
the ground in a pile of a pyramidical form, with a fire underneath; the
whole is then covered with clay, a few holes only being left for the
circulation of air.

MRS. B.

These holes are closed as soon as the wood is fairly lighted, so that
the combustion is checked, or at least continues but in a very imperfect
manner; but the heat produced by it is sufficient to force out and
volatilize, through the earthy cover, most part of the oily and watery
principles of the wood, although it cannot reduce it to ashes.

EMILY.

Is pure carbon as black as charcoal?

MRS. B.

The purest charcoal we can prepare is so; but chemists have never yet
been able to separate it entirely from hydrogen. Sir H. Davy says, that
the most perfect carbon that is prepared by art contains about five per
cent. of hydrogen; he is of opinion, that if we could obtain it quite
free from foreign ingredients, it would be metallic, in common with
other simple substances.

But there is a form in which charcoal appears, that I dare say will
surprise you. --This ring, which I wear on my finger, owes its
brilliancy to a small piece of carbon.

CAROLINE.

Surely, you are jesting, Mrs. B.?

EMILY.

I thought your ring was diamond?

MRS. B.

It is so. But diamond is nothing more than carbon in a crystallized
state.

EMILY.

That is astonishing! Is it possible to see two things apparently more
different than diamond and charcoal?

CAROLINE.

It is, indeed, curious to think that we adorn ourselves with jewels of
charcoal!

MRS. B.

There are many other substances, consisting chiefly of carbon, that are
remarkably white. Cotton, for instance, is almost wholly carbon.

CAROLINE.

That, I own, I could never have imagined! --But pray, Mrs. B., since it
is known of what substance diamond and cotton are composed, why should
they not be manufactured, or imitated, by some chemical process, which
would render them much cheaper, and more plentiful than the present mode
of obtaining them?

MRS. B.

You might as well, my dear, propose that we should make flowers and
fruit, nay, perhaps even animals, by a chemical process; for it is known
of what these bodies consist, since every thing which we are acquainted
with in nature is formed from the various simple substances that we have
enumerated. But you must not suppose that a knowledge of the component
parts of a body will in every case enable us to imitate it. It is much
less difficult to decompose bodies, and discover of what materials they
are made, than it is to recompose them. The first of these processes is
called _analysis_, the last _synthesis_. When we are able to ascertain
the nature of a substance by both these methods, so that the result of
one confirms that of the other, we obtain the most complete knowledge of
it that we are capable of acquiring. This is the case with water, with
the atmosphere, with most of the oxyds, acids, and neutral salts, and
with many other compounds. But the more complicated combinations of
nature, even in the mineral kingdom, are in general beyond our reach,
and any attempt to imitate organised bodies must ever prove fruitless;
their formation is a secret that rests in the bosom of the Creator. You
see, therefore, how vain it would be to attempt to make cotton by
chemical means. But, surely, we have no reason to regret our inability
in this instance, when nature has so clearly pointed out a method of
obtaining it in perfection and abundance.

CAROLINE.

I did not imagine that the principle of life could be imitated by the
aid of chemistry; but it did not appear to me ridiculous to suppose that
chemists might attain a perfect imitation of inanimate nature.

MRS. B.

They have succeeded in this point in a variety of instances; but, as you
justly observe, the principle of life, or even the minute and intimate
organisation of the vegetable kingdom, are secrets that have almost
entirely eluded the researches of philosophers; nor do I imagine that
human art will ever be capable of investigating them with complete
success.

EMILY.

But diamond, since it consists of one simple unorganised substance,
might be, one would think, perfectly imitable by art?

MRS. B.

It is sometimes as much beyond our power to obtain a simple body in a
state of perfect purity, as it is to imitate a complicated combination;
for the operations by which nature separates bodies are frequently as
inimitable as those which she uses for their combination. This is the
case with carbon; all the efforts of chemists to separate it entirely
from other substances have been fruitless, and in the purest state in
which it can be obtained by art, it still retains a portion of hydrogen,
and probably of some other foreign ingredients. We are ignorant of the
means which nature employs to crystallize it. It may probably be the
work of ages, to purify, arrange, and unite the particles of carbon in
the form of diamond. Here is some charcoal in the purest state we can
procure it: you see that it is a very black, brittle, light, porous
substance, entirely destitute of either taste or smell. Heat, without
air, produces no alteration in it, as it is not volatile; but, on the
contrary, it invariably remains at the bottom of the vessel after all
the other parts of the vegetable are evaporated.

EMILY.

Yet carbon is, no doubt, combustible, since you say that charcoal would
absorb oxygen if air were admitted during its preparation?

CAROLINE.

Unquestionably. Besides, you know, Emily, how much it is used in
cooking. But pray what is the reason that charcoal burns without smoke,
whilst a wood fire smokes so much?

MRS. B.

Because, in the conversion of wood into charcoal, the volatile particles
of the former have been evaporated.

CAROLINE.

Yet I have frequently seen charcoal burn with flame; therefore it must,
in that case, contain some hydrogen.

MRS. B.

Very true; but you must recollect that charcoal, especially that which
is used for common purposes, is not perfectly pure. It generally retains
some remains of the various other component parts of vegetables, and
hydrogen particularly, which accounts for the flame in question.

CAROLINE.

But what becomes of the carbon itself during its combustion?

MRS. B.

It gradually combines with the oxygen of the atmosphere, in the same way
as sulphur and phosphorus, and, like those substances, it is converted
into a peculiar acid, which flies off in a gaseous form. There is this
difference, however, that the acid is not, in this instance, as in the
two cases just mentioned, a mere condensable vapour, but a permanent
elastic fluid, which always remains in the state of gas, under any
pressure and at any temperature. The nature of this acid was first
ascertained by Dr. Black, of Edinburgh; and, before the introduction of
the new nomenclature, it was called _fixed air_. It is now distinguished
by the more appropriate name of _carbonic acid gas_.

EMILY.

Carbon, then, can be volatilized by burning, though, by heat alone, no
such effect is produced?

MRS. B.

Yes; but then it is no longer simple carbon, but an acid of which carbon
forms the basis. In this state, carbon retains no more appearance of
solidity or corporeal form, than the basis of any other gas. And you
may, I think, from this instance, derive a more clear idea of the basis
of the oxygen, hydrogen, and nitrogen gases, the existence of which, as
real bodies, you seemed to doubt, because they were not to be obtained
simply in a solid form.

EMILY.

That is true; we may conceive the basis of the oxygen, and of the other
gases, to be solid, heavy substances, like carbon; but so much expanded
by caloric as to become invisible.

CAROLINE.

But does not the carbonic acid gas partake of the blackness of charcoal?

MRS. B.

Not in the least. Blackness, you know, does not appear to be essential
to carbon, and it is pure carbon, and not charcoal, that we must
consider as the basis of carbonic acid. We shall make some carbonic
acid, and, in order to hasten the process, we shall burn the carbon in
oxygen gas.

EMILY.

But do you mean then to burn diamond?

MRS. B.

Charcoal will answer the purpose still better, being softer and more
easy to inflame; besides the experiments on diamond are rather
expensive.

CAROLINE.

But is it possible to burn diamond?

MRS. B.

Yes, it is; and in order to effect this combustion, nothing more is
required than to apply a sufficient degree of heat by means of the
blow-pipe, and of a stream of oxygen gas. Indeed it is by burning
diamond that its chemical nature has been ascertained. It has long been
known as a combustible substance, but it is within these few years only
that the product of its combustion has been proved to be pure carbonic
acid. This remarkable discovery is due to Mr. Tennant.

Now let us try to make some carbonic acid. --Will you, Emily, decant
some oxygen gas from this large jar into the receiver in which we are to
burn the carbon; and I shall introduce this small piece of charcoal,
with a little lighted tinder, which will be necessary to give the first
impulse to the combustion.

EMILY.

I cannot conceive how so small a piece of tinder, and that but just
lighted, can raise the temperature of the carbon sufficiently to set
fire to it; for it can produce scarcely any sensible heat, and it hardly
touches the carbon.

MRS. B.

The tinder thus kindled has only heat enough to begin its own
combustion, which, however, soon becomes so rapid in the oxygen gas, as
to raise the temperature of the charcoal sufficiently for this to burn
likewise, as you see is now the case.

EMILY.

I am surprised that the combustion of carbon is not more brilliant; it
does not give out near so much light or caloric as phosphorus, or
sulphur. Yet since it combines with so much oxygen, why is not a
proportional quantity of light and heat disengaged from the
decomposition of the oxygen gas, and the union of its electricity with
that of the charcoal?

MRS. B.

It is not surprising that less light and heat should be liberated in
this than in almost any other combustion, since the oxygen, instead of
entering into a solid or liquid combination, as it does in the
phosphoric and sulphuric acids, is employed in forming another elastic
fluid; it therefore parts with less of its caloric.

EMILY.

True; and, on second consideration, it appears, on the contrary,
surprising that the oxygen should, in its combination with carbon,
retain a sufficient portion of caloric to maintain both substances in a
gaseous state.

CAROLINE.

We may then judge of the degree of solidity in which oxygen is combined
in a burnt body, by the quantity of caloric liberated during its
combustion?

MRS. B.

Yes; provided that you take into the account the quantity of oxygen
absorbed by the combustible body, and observe the proportion which the
caloric bears to it.

CAROLINE.

But why should the water, after the combustion of carbon, rise in the
receiver, since the gas within it retains an aeriform state?

MRS. B.

Because the carbonic acid gas is gradually absorbed by the water; and
this effect would be promoted by shaking the receiver.

EMILY.

The charcoal is now extinguished, though it is not nearly consumed; it
has such an extraordinary avidity for oxygen, I suppose, that the
receiver did not contain enough to satisfy the whole.

MRS. B.

That is certainly the case; for if the combustion were performed in the
exact proportions of 28 parts of carbon to 72 of oxygen, both these
ingredients would disappear, and 100 parts of carbonic acid would be
produced.

CAROLINE.

Carbonic acid must be a very strong acid, since it contains so great a
proportion of oxygen?

MRS. B.

That is a very natural inference; yet it is erroneous. For the carbonic
is the weakest of all the acids. The strength of an acid seems to depend
upon the nature of its basis, and its mode of combination, as well as
upon the proportion of the acidifying principle. The same quantity of
oxygen that will convert some bodies into strong acids, will only be
sufficient simply to oxydate others.

CAROLINE.

Since this acid is so weak, I think chemists should have called it the
_carbonous_, instead of the _carbonic_ acid.

EMILY.

But, I suppose, the carbonous acid is still weaker, and is formed by
burning carbon in atmospherical air.

MRS. B.

It has been lately discovered, that carbon may be converted into a gas,
by uniting with a smaller proportion of oxygen; but as this gas does not
possess any acid properties, it is no more than an oxyd; it is called
_gaseous oxyd of carbon_.

CAROLINE.

Pray is not carbonic acid a very wholesome gas to breathe, as it
contains so much oxygen?

MRS. B.

On the contrary, it is extremely pernicious. Oxygen, when in a state of
combination with other substances, loses, in almost every instance, its
respirable properties, and the salubrious effects which it has on the
animal economy when in its unconfined state. Carbonic acid is not only
unfit for respiration, but extremely deleterious if taken into the
lungs.

EMILY.

You know, Caroline, how very unwholesome the fumes of burning charcoal
are reckoned.

CAROLINE.

Yes; but, to confess the truth, I did not consider that a charcoal fire
produced carbonic acid gas. --Can this gas be condensed into a liquid?

MRS. B.

No: for, as I told you before, it is a permanent elastic fluid. But
water can absorb a certain quantity of this gas, and can even be
impregnated with it, in a very strong degree, by the assistance of
agitation and pressure, as I am going to show you. I shall decant some
carbonic acid gas into this bottle, which I fill first with water, in
order to exclude the atmospherical air; the gas is then introduced
through the water, which you see it displaces, for it will not mix with
it in any quantity, unless strongly agitated, or allowed to stand over
it for some time. The bottle is now about half full of carbonic acid
gas, and the other half is still occupied by the water. By corking the
bottle, and then violently shaking it, in this way, I can mix the gas
and water together. --Now will you taste it?

EMILY.

It has a distinct acid taste.

CAROLINE.

Yes, it is sensibly sour, and appears full of little bubbles.

MRS. B.

It possesses likewise all the other properties of acids, but, of course,
in a less degree than the pure carbonic acid gas, as it is so much
diluted by water.

This is a kind of artificial Seltzer water. By analysing that which is
produced by nature, it was found to contain scarcely any thing more than
common water impregnated with a certain proportion of carbonic acid gas.
We are, therefore, able to imitate it, by mixing those proportions of
water and carbonic acid. Here, my dear, is an instance, in which, by a
chemical process, we can exactly copy the operations of nature; for the
artificial Seltzer waters can be made in every respect similar to those
of nature; in one point, indeed, the former have an advantage, since
they may be prepared stronger, or weaker, as occasion requires.

CAROLINE.

I thought I had tasted such water before. But what renders it so brisk
and sparkling?

MRS. B.

This sparkling, or effervescence, as it is called, is always occasioned
by the action of an elastic fluid escaping from a liquid; in the
artifical Seltzer water, it is produced by the carbonic acid, which
being lighter than the water in which it was strongly condensed, flies
off with great rapidity the instant the bottle is uncorked; this makes
it necessary to drink it immediately. The bubbling that took place in
this bottle was but trifling, as the water was but very slightly
impregnated with carbonic acid. It requires a particular apparatus to
prepare the gaseous artificial mineral waters.

EMILY.

If, then, a bottle of Seltzer water remains for any length of time
uncorked, I suppose it returns to the state of common water?

MRS. B.

The whole of the carbonic acid gas, or very nearly so, will soon
disappear; but there is likewise in Seltzer water a very small quantity
of soda, and of a few other saline or earthy ingredients, which will
remain in the water, though it should be kept uncorked for any length of
time.

CAROLINE.

I have often heard of people drinking soda-water. Pray what sort of
water is that?

MRS. B.

It is a kind of artificial Seltzer water, holding in solution, besides
the gaseous acid, a particular saline substance, called soda, which
imparts to the water certain medicinal qualities.

CAROLINE.

But how can these waters be so wholesome, since carbonic acid is so
pernicious?

MRS. B.

A gas, we may conceive, though very prejudicial to breathe, may be
beneficial to the stomach. --But it would be of no use to attempt
explaining this more fully at present.

CAROLINE.

Are waters never impregnated with other gases?

MRS. B.

Yes; there are several kinds of gaseous waters. I forgot to tell you
that waters have, for some years past, been prepared, impregnated both
with oxygen and hydrogen gases. These are not an imitation of nature,
but are altogether obtained by artificial means. They have been lately
used medicinally, particularly on the continent, where, I understand,
they have acquired some reputation.

EMILY.

If I recollect right, Mrs. B., you told us that carbon was capable of
decomposing water; the affinity between oxygen and carbon must,
therefore, be greater than between oxygen and hydrogen?

MRS. B.

Yes; but this is not the case unless their temperature be raised to a
certain degree. It is only when carbon is red-hot, that it is capable of
separating the oxygen from the hydrogen. Thus, if a small quantity of
water be thrown on a red-hot fire, it will increase rather than
extinguish the combustion; for the coals or wood (both of which contain
a quantity of carbon) decompose the water, and thus supply the fire both
with oxygen and hydrogen gases. If, on the contrary, a large mass of
water be thrown over the fire, the diminution of heat thus produced is
such, that the combustible matter loses the power of decomposing the
water, and the fire is extinguished.

EMILY.

I have heard that fire-engines sometimes do more harm than good, and
that they actually increase the fire when they cannot throw water enough
to extinguish it. It must be owing, no doubt, to the decomposition of
the water by the carbon during the conflagration.

MRS. B.

Certainly. --The apparatus which you see here (PLATE XI. fig. 3.), may
be used to exemplify what we have just said. It consists in a kind of
open furnace, through which a porcelain tube, containing charcoal,
passes. To one end of the tube is adapted a glass retort with water in
it; and the other end communicates with a receiver placed on the
water-bath. A lamp being applied to the retort, and the water made to
boil, the vapour is gradually conveyed through the red-hot charcoal, by
which it is decomposed; and the hydrogen gas which results from this
decomposition is collected in the receiver. But the hydrogen thus
obtained is far from being pure; it retains in solution a minute portion
of carbon, and contains also a quantity of carbonic acid. This renders
it heavier than pure hydrogen gas, and gives it some peculiar
properties; it is distinguished by the name of _carbonated hydrogen
gas_.

CAROLINE.

And whence does it obtain the carbonic acid that is mixed with it?

EMILY.

I believe I can answer that question, Caroline. --From the union of the
oxygen (proceeding from the decomposed water) with the carbon, which,
you know, makes carbonic acid.

CAROLINE.

True; I should have recollected that. --The product of the decomposition
of water by red-hot charcoal, therefore, is carbonated hydrogen gas, and
carbonic acid gas.

MRS. B.

You are perfectly right now.

Carbon is frequently found combined with hydrogen in a state of
solidity, especially in coals, which owe their combustible nature to
these two principles.

EMILY.

Is it the hydrogen, then, that produces the flame of coals?

MRS. B.

It is so; and when all the hydrogen is consumed, the carbon continues to
burn without flame. But again, as I mentioned when speaking of the
gas-lights, the hydrogen gas produced by the burning of coals is not
pure; for, during the combustion, particles of carbon are successively
volatilized with the hydrogen, with which they form what is called a
_hydro-carbonat_, which is the principal product of this combustion.

Carbon is a very bad conductor of heat; for this reason, it is employed
(in conjunction with other ingredients) for coating furnaces and other
chemical apparatus.

EMILY.

Pray what is the use of coating furnaces?

MRS. B.

In most cases, in which a furnace is used, it is necessary to produce
and preserve a great degree of heat, for which purpose every possible
means are used to prevent the heat from escaping by communicating with
other bodies, and this object is attained by coating over the inside of
the furnace with a kind of plaster, composed of materials that are bad
conductors of heat.

Carbon, combined with a small quantity of iron, forms a compound called
plumbago, or black-lead, of which pencils are made. This substance,
agreeably to the nomenclature, is _a carburet of iron_.

EMILY.

Why, then, is it called black-lead?

MRS. B.

It is an ancient name given to it by ignorant people, from its shining
metallic appearance; but it is certainly a most improper name for it, as
there is not a particle of lead in the composition. There is only one
mine of this mineral, which is in Cumberland. It is supposed to approach
as nearly to pure carbon as the best prepared charcoal does, as it
contains only five parts of iron, unadulterated by any other foreign
ingredients. There is another carburet of iron, in which the iron,
though united only to an extremely small proportion of carbon, acquires
very remarkable properties; this is steel.

CAROLINE.

Really; and yet steel is much harder than iron?

MRS. B.

But carbon is not ductile like iron, and therefore may render the steel
more brittle, and prevent its bending so easily. Whether it is that the
carbon, by introducing itself into the pores of the iron, and, by
filling them, makes the metal both harder and heavier; or whether this
change depends upon some chemical cause, I cannot pretend to decide. But
there is a subsequent operation, by which the hardness of steel is very
much increased, which simply consists in heating the steel till it is
red-hot, and then plunging it into cold water.

Carbon, besides the combination just mentioned, enters into the
composition of a vast number of natural productions, such, for instance,
as all the various kinds of oils, which result from the combination of
carbon, hydrogen, and caloric, in various proportions.

EMILY.

I thought that carbon, hydrogen, and caloric, formed carbonated hydrogen
gas?

MRS. B.

That is the case when a small portion of carbonic acid gas is held in
solution by hydrogen gas. Different proportions of the same principles,
together with the circumstances of their union, produce very different
combinations; of this you will see innumerable examples. Besides, we are
not now talking of gases, but of carbon and hydrogen, combined only with
a quantity of caloric sufficient to bring them to the consistency of oil
or fat.

CAROLINE.

But oil and fat are not of the same consistence?

MRS. B.

Fat is only congealed oil; or oil, melted fat. The one requires a little
more heat to maintain it in a fluid state than the other. Have you never
observed the fat of meat turned to oil by the caloric it has imbibed
from the fire?

EMILY.

Yet oils in general, as salad-oil, and lamp-oil, do not turn to fat when
cold?

MRS. B.

Not at the common temperature of the atmosphere, because they retain too
much caloric to congeal at that temperature; but if exposed to a
sufficient degree of cold, their latent heat is extricated, and they
become solid fat substances. Have you never seen salad oil frozen in
winter?

EMILY.

Yes; but it appears to me in that state very different from animal fat.

MRS. B.

The essential constituent parts of either vegetable or animal oils are
the same, carbon and hydrogen; their variety arises from the different
proportions of these substances, and from other accessory ingredients
that may be mixed with them. The oil of a whale, and the oil of roses,
are, in their essential constituent parts, the same; but the one is
impregnated with the offensive particles of animal matter, the other
with the delicate perfume of a flower.

The difference of _fixed oils_, and _volatile_ or _essential oils_,
consists also in the various proportions of carbon and hydrogen. Fixed
oils are those which will not evaporate without being decomposed; this
is the case with all common oils, which contain a greater proportion of
carbon than the essential oils. The essential oils (which comprehend the
whole class of essences and perfumes) are lighter; they contain more
equal proportions of carbon and hydrogen, and are volatilized or
evaporated without being decomposed.

EMILY.

When you say that one kind of oil will evaporate, and the other be
decomposed, you mean, I suppose, by the application of heat?

MRS. B.

Not necessarily; for there are oils that will evaporate slowly at the
common temperature of the atmosphere; but for a more rapid
volatilization, or for their decomposition, the assistance of heat is
required.

CAROLINE.

I shall now remember, I think, that fat and oil are really the same
substances, both consisting of carbon and hydrogen; that in fixed oils
the carbon preponderates, and heat produces a decomposition; while, in
essential oils, the proportion of hydrogen is greater, and heat produces
a volatilization only.

EMILY.

I suppose the reason why oil burns so well in lamps is because its two
constituents are so combustible?

MRS. B.

Certainly; the combustion of oil is just the same as that of a candle;
if tallow, it is only oil in a concrete state; if wax, or spermaceti,
its chief chemical ingredients are still hydrogen and carbon.

EMILY.

I wonder, then, there should be so great a difference between tallow and
wax?

MRS. B.

I must again repeat, that the same substances, in different proportions,
produce results that have sometimes scarcely any resemblance to each
other. But this is rather a general remark that I wish to impress upon
your minds, than one which is applicable to the present case; for tallow
and wax are far from being very dissimilar; the chief difference
consists in the wax being a purer compound of carbon and hydrogen than
the tallow, which retains more of the gross particles of animal matter.
The combustion of a candle, and that of a lamp, both produce water and
carbonic acid gas. Can you tell me how these are formed?

EMILY.

Let me reflect . . . . Both the candle and lamp burn by means of fixed
oil--this is decomposed as the combustion goes on; and the constituent
parts of the oil being thus separated, the carbon unites to a portion of
oxygen from the atmosphere to form carbonic acid gas, whilst the
hydrogen combines with another portion of oxygen, and forms with it
water. --The products, therefore, of the combustion of oils are water
and carbonic acid gas.

CAROLINE.

But we see neither water nor carbonic acid produced by the combustion of
a candle.

MRS. B.

The carbonic acid gas, you know, is invisible, and the water being in a
state of vapour, is so likewise. Emily is perfectly correct in her
explanation, and I am very much pleased with it.

All the vegetable acids consist of various proportions of carbon and
hydrogen, acidified by oxygen. Gums, sugar, and starch, are likewise
composed of these ingredients; but, as the oxygen which they contain is
not sufficient to convert them into acids, they are classed with the
oxyds, and called vegetable oxyds.

CAROLINE.

I am very much delighted with all these new ideas; but, at the same
time, I cannot help being apprehensive that I may forget many of them.

MRS. B.

I would advise you to take notes, or, what would answer better still, to
write down, after every lesson, as much of it as you can recollect. And,
in order to give you a little assistance, I shall lend you the heads or
index, which I occasionally consult for the sake of preserving some
method and arrangement in these conversations. Unless you follow some
such plan, you cannot expect to retain nearly all that you learn, how
great soever be the impression it may make on you at first.

EMILY.

I will certainly follow your advice. --Hitherto I have found that I
recollected pretty well what you have taught us; but the history of
carbon is a more extensive subject than any of the simple bodies we have
yet examined.

MRS. B.

I have little more to say on carbon at present; but hereafter you will
see that it performs a considerable part in most chemical operations.

CAROLINE.

That is, I suppose, owing to its entering into the composition of so
great a variety of substances?

MRS. B.

Certainly; it is the basis, you have seen, of all vegetable matter; and
you will find that it is very essential to the process of animalization.
But in the mineral kingdom also, particularly in its form of carbonic
acid, we shall often discover it combined with a great variety of
substances.

In chemical operations, carbon is particularly useful, from its very
great attraction for oxygen, as it will absorb this substance from many
oxygenated or burnt bodies, and thus deoxygenate, or _unburn_ them, and
restore them to their original combustible state.

CAROLINE.

I do not understand how a body can be _unburnt_, and restored to its
original state. This piece of tinder, for instance, that has been burnt,
if by any means the oxygen were extracted from it, would not be restored
to its former state of linen; for its texture is destroyed by burning,
and that must be the case with all organized or manufactured substances,
as you observed in a former conversation.

MRS. B.

A compound body is decomposed by combustion in a way which generally
precludes the possibility of restoring it to its former state; the
oxygen, for instance, does not become fixed in the tinder, but it
combines with its volatile parts, and flies off in the shape of gas, or
watery vapour. You see, therefore, how vain it would be to attempt the
recomposition of such bodies. But, with regard to simple bodies, or at
least bodies whose component parts are not disturbed by the process of
oxygenation or deoxygenation, it is often possible to restore them,
after combustion, to their original state. --The metals, for instance,
undergo no other alteration by combustion than a combination with
oxygen; therefore, when the oxygen is taken from them, they return to
their pure metallic state. But I shall say nothing further of this at
present, as the metals will furnish ample subject for another morning;
and they are the class of simple bodies that come next under
consideration.




CONVERSATION X.

ON METALS.


MRS. B.

The METALS, which we are now to examine, are bodies of a very different
nature from those which we have hitherto considered. They do not, like
the bases of gases, elude the immediate observation of our senses; for
they are the most brilliant, the most ponderous, and the most palpable
substances in nature.

CAROLINE.

I doubt, however, whether the metals will appear to us so interesting,
and give us so much entertainment as those mysterious elements which
conceal themselves from our view. Besides, they cannot afford so much
novelty; they are bodies with which we are already so well acquainted.

MRS. B.

You are not aware, my dear, of the interesting discoveries which were a
few years ago made by Sir H. Davy respecting this class of bodies. By
the aid of the Voltaic battery, he has obtained from a variety of
substances, metals before unknown, the properties of which are equally
new and curious. We shall begin, however, by noticing those metals with
which you profess to be so well acquainted. But the acquaintance, you
will soon perceive, is but very superficial; and I trust that you will
find both novelty and entertainment in considering the metals in a
chemical point of view. To treat of this subject fully, would require a
whole course of lectures; for metals form of themselves a most important
branch of practical chemistry. We must, therefore, confine ourselves to
a general view of them. These bodies are seldom found naturally in their
metallic form: they are generally more or less oxygenated or combined
with sulphur, earths, or acids, and are often blended with each other.
They are found buried in the bowels of the earth in most parts of the
world, but chiefly in mountainous districts, where the surface of the
globe has suffered from the earthquakes, volcanos, and other convulsions
of nature. They are spread in strata or beds, called veins, and these
veins are composed of a certain quantity of metal, combined with various
earthy substances, with which they form minerals of different nature and
appearance, which are called _ores_.

CAROLINE.

I now feel quite at home, for my father has a lead-mine in Yorkshire,
and I have heard a great deal about veins of ore, and of the _roasting_
and _smelting_ of the lead; but, I confess, that I do not understand in
what these operations consist.

MRS. B.

Roasting is the process by which the volatile parts of the ore are
evaporated; smelting, that by which the pure metal is afterwards
separated from the earthy remains of the ore. This is done by throwing
the whole into a furnace, and mixing with it certain substances that
will combine with the earthy parts and other foreign ingredients of the
ore; the metal being the heaviest, falls to the bottom, and runs out by
proper openings in its pure metallic state.

EMILY.

You told us in a preceding lesson that metals had a great affinity for
oxygen. Do they not, therefore, combine with oxygen, when strongly
heated in the furnace, and run out in the state of oxyds?

MRS. B.

No; for the scoriae, or oxyd, which soon forms on the surface of the
fused metal, when it is oxydable, prevents the air from having any
further influence on the mass; so that neither combustion nor
oxygenation can take place.

CAROLINE.

Are all the metals equally combustible?

MRS. B.

No; their attraction for oxygen varies extremely. There are some that
will combine with it only at a very high temperature, or by the
assistance of acids; whilst there are others that oxydate spontaneously
and with great rapidity, even at the lowest temperature; such is in
particular manganese, which scarcely ever exists in the metallic state,
as it immediately absorbs oxygen on being exposed to the air, and
crumbles to an oxyd in the course of a few hours.

EMILY.

Is not that the oxyd from which you extracted the oxygen gas?

MRS. B.

It is: so that, you see, this metal attracts oxygen at a low
temperature, and parts with it when strongly heated.

EMILY.

Is there any other metal that oxydates at the temperature of the
atmosphere?

MRS. B.

They all do, more or less, excepting gold, silver, and platina.

Copper, lead, and iron, oxydate slowly in the air, and cover themselves
with a sort of rust, a process which depends on the gradual conversion
of the surface into an oxyd. This rusty surface preserves the interior
metal from oxydation, as it prevents the air from coming in contact with
it. Strictly speaking, however, the word rust applies only to the oxyd,
which forms on the surface of iron, when exposed to air and moisture,
which oxyd appears to be united with a small portion of carbonic acid.

EMILY.

When metals oxydate from the atmosphere without an elevation of
temperature, some light and heat, I suppose, must be disengaged, though
not in sufficient quantities to be sensible.

MRS. B.

Undoubtedly; and, indeed, it is not surprising that in this case the
light and heat should not be sensible, when you consider how extremely
slow, and, indeed, how imperfectly, most metals oxydate by mere exposure
to the atmosphere. For the quantity of oxygen with which metals are
capable of combining, generally depends upon their temperature; and the
absorption stops at various points of oxydation, according to the degree
to which their temperature is raised.

EMILY.

That seems very natural; for the greater the quantity of caloric
introduced into a metal, the more will its positive electricity be
exalted, and consequently the stronger will be its affinity for oxygen.

MRS. B.

Certainly. When the metal oxygenates with sufficient rapidity for light
and heat to become sensible, combustion actually takes place. But this
happens only at very high temperatures, and the product is nevertheless
an oxyd; for though, as I have just said, metals will combine with
different proportions of oxygen, yet with the exception of only five of
them, they are not susceptible of acidification.

Metals change colour during the different degrees of oxydation which
they undergo. Lead, when heated in contact with the atmosphere, first
becomes grey; if its temperature be then raised, it turns yellow, and a
still stronger heat changes it to red. Iron becomes successively a
green, brown, and white oxyd. Copper changes from brown to blue, and
lastly green.

EMILY.

Pray, is the white lead with which houses are painted prepared by
oxydating lead?

MRS. B.

Not merely by oxydating, but by being also united with carbonic acid. It
is a carbonat of lead. The mere oxyd of lead is called red lead.
Litharge is another oxyd of lead, containing less oxygen. Almost all the
metallic oxyds are used as paints. The various sorts of ochres consist
chiefly of iron more or less oxydated. And it is a remarkable
circumstance, that if you burn metals rapidly, the light or flame they
emit during combustion partakes of the colours which the oxyd
successively assumes.

CAROLINE.

How is that accounted for, Mrs. B.? For light, you know, does not
proceed from the burning body, but from the decomposition of the oxygen
gas?

MRS. B.

The correspondence of the colour of the light with that of the oxyd
which emits it, is, in all probability, owing to some particles of the
metal which are volatilised and carried off by the caloric.

CAROLINE.

It is then a sort of metallic gas.

EMILY.

Why is it reckoned so unwholesome to breathe the air of a place in which
metals are melting?

MRS. B.

Perhaps the notion is too generally entertained. But it is true with
respect to lead, and some other noxious metals, because, unless care be
taken, the particles of the oxyd which are volatilised by the heat are
inhaled in with the breath, and may produce dangerous effects.

I must show you some instances of the combustion of metals; it would
require the heat of a furnace to make them burn in the common air, but
if we supply them with a stream of oxygen gas, we may easily
accomplish it.

CAROLINE.

But it will still, I suppose, be necessary in some degree to raise their
temperature?

MRS. B.

This, as you shall see, is very easily done, particularly if the
experiment be tried upon a small scale. --I begin by lighting this piece
of charcoal with the candle, and then increase the rapidity of its
combustion by blowing upon it with a blow-pipe. (PLATE XII. fig. 1.)

  [Illustration: Plate XII.
  Apparatus for the combustion of metals by means of oxygen gas.

  Fig. 1.
  Igniting charcoal with a taper & blow-pipe.

  Fig. 2.
  Combustion of metals by means of a blow-pipe conveying a stream of
    oxygen gas from a gas holder.]

EMILY.

That I do not understand; for it is not every kind of air, but merely
oxygen gas, that produces combustion. Now you said that in breathing we
inspired, but did not expire oxygen gas. Why, therefore, should the air
which you breathe through the blow-pipe promote the combustion of the
charcoal?

MRS. B.

Because the air, which has but once passed through the lungs, is yet but
little altered, a small portion only of its oxygen being destroyed; so
that a great deal more is gained by increasing the rapidity of the
current, by means of the blow-pipe, than is lost in consequence of the
air passing once through the lungs, as you shall see--

EMILY.

Yes, indeed, it makes the charcoal burn much brighter.

MRS. B.

Whilst it is red-hot, I shall drop some iron filings on it, and supply
them with a current of oxygen gas, by means of this apparatus, (PLATE
XII. fig 2.) which consists simply of a closed tin cylindrical vessel,
full of oxygen gas, with two apertures and stop-cocks, by one of which a
stream of water is thrown into the vessel through a long funnel, whilst
by the other the gas is forced out through a blow-pipe adapted to it, as
the water gains admittance. --Now that I pour water into the funnel, you
may hear the gas issuing from the blow-pipe--I bring the charcoal close
to the current, and drop the filings upon it--

CAROLINE.

They emit much the same vivid light as the combustion of the iron wire
in oxygen gas.

MRS. B.

The process is, in fact, the same; there is only some difference in the
mode of conducting it. Let us burn some tin in the same manner--you see
that it is equally combustible. --Let us now try some copper--

CAROLINE.

This burns with a greenish flame; it is, I suppose, owing to the colour
of the oxyd?

EMILY.

Pray, shall we not also burn some gold?

MRS. B.

That is not in our power, at least in this way. Gold, silver, and
platina, are incapable of being oxydated by the greatest heat that we
can produce by the common method. It is from this circumstance, that
they have been called perfect metals. Even these, however, have an
affinity for oxygen; but their oxydation or combustion can be performed
only by means of acids or by electricity. The spark given out by the
Voltaic battery produces at the point of contact a greater degree of
heat than any other process; and it is at this very high temperature
only that the affinity of these metals for oxygen will enable them to
act on each other.

I am sorry that I cannot show you the combustion of the perfect metals
by this process, but it requires a considerable Voltaic battery. You
will see these experiments performed in the most perfect manner, when
you attend the chemical lectures of the Royal Institution. But in the
mean time I can, without difficulty, show you an ingenious apparatus
lately contrived for the purpose of producing intense heats, the power
of which nearly equals that of the largest Voltaic batteries. It simply
consists, you see, in a strong box, made of iron or copper, (PLATE X.
fig. 2.) to which may be adapted this air-syringe or condensing-pump,
and a stop-cock terminating in a small orifice similar to that of a
blow-pipe. By working the condensing syringe, up and down in this
manner, a quantity of air is accumulated in the vessel, which may be
increased to almost any extent; so that if we now turn the stop-cock,
the condensed air will rush out, forming a jet of considerable force;
and if we place the flame of a lamp in the current, you will see how
violently the flame is driven in that direction.

CAROLINE.

It seems to be exactly the same effect as that of a blow-pipe worked by
the mouth, only much stronger.

EMILY.

Yes; and this new instrument has this additional advantage, that it does
not fatigue the mouth and lungs like the common blow-pipe, and requires
no art in blowing.

MRS. B.

Unquestionably; but yet this blow-pipe would be of very limited utility,
if its energy and power could not be greatly increased by some other
contrivance. Can you imagine any mode of producing such an effect?

EMILY.

Could not the reservoir be charged with pure oxygen, instead of common
air, as in the case of the gas-holder?

MRS. B.

Undoubtedly; and this is precisely the contrivance I allude to. The
vessel need only be supplied with air from a bladder full of oxygen,
instead of the air of the room, and this, you see, may be easily done by
screwing the bladder on the upper part of the syringe, so that in
working the syringe the oxygen gas is forced from the bladder into the
condensing vessel.

CAROLINE.

With the aid of this small apparatus, therefore, we could obtain the
same effects as those we have just produced with the gas-holder, by
means of a column of water forcing the gas out of it?

MRS. B.

Yes; and much more conveniently so. But there is a mode of using this
apparatus by which more powerful effects still may be obtained. It
consists in condensing in the reservoir, not oxygen alone, but a mixture
of oxygen and hydrogen in the exact proportion in which they unite to
produce water; and then kindling the jet formed by the mixed gases. The
heat disengaged by this combustion, without the help of any lamp, is
probably the most intense known; and various effects are said to have
been obtained from it which exceed all expectation.

CAROLINE.

But why should we not try this experiment?

MRS. B.

Because it is not exempt from danger; the combustion (notwithstanding
various contrivances which have been resorted to with a view to prevent
accident) being apt to penetrate into the inside of the vessel, and to
produce a dangerous and violent explosion. --We shall, therefore, now
proceed in our subject.

CAROLINE.

I think you said the oxyds of metals could be restored to their metallic
state?

MRS. B.

Yes; this is called _reviving_ a metal. Metals are in general capable of
being revived by charcoal, when heated red hot, charcoal having a
greater attraction for oxygen than the metals. You need only, therefore,
decompose, or unburn the oxyd, by depriving it of its oxygen, and the
metal will be restored to its pure state.

EMILY.

But will the carbon, by this operation, be burnt, and be converted into
carbonic acid?

MRS. B.

Certainly. There are other combustible substances to which metals at a
high temperature will part with their oxygen. They will also yield it to
each other, according to their several degrees of attraction for it; and
if the oxygen goes into a more dense state in the metal which it enters,
than it existed in that which it quits, a proportional disengagement of
caloric will take place.

CAROLINE.

And cannot the oxyds of gold, silver, and platina, which are formed by
means of acids or of the electric fluid, be restored to their metallic
state?

MRS. B.

Yes, they may; and the intervention of a combustible body is not
required; heat alone will take the oxygen from them, convert it into a
gas, and revive the metal.

EMILY.

You said that rust was an oxyd of iron; how is it, then, that water, or
merely dampness, produces it, which, you know, it very frequently does
on steel grates, or any iron instruments?

MRS. B.

In that case the metal decomposes the water, or dampness (which is
nothing but water in a state of vapour), and obtains the oxygen from it.

CAROLINE.

I thought that it was necessary to bring metals to a very high
temperature to enable them to decompose water.

MRS. B.

It is so, if it is required that the process should be performed
rapidly, and if any considerable quantity is to be decomposed. Rust, you
knew, is sometimes months in forming, and then it is only the surface of
the metal that is oxydated.

EMILY.

Metals, then, that do not rust, are incapable of spontaneous oxydation,
either by air or water?

MRS. B.

Yes; and this is the case with the perfect metals, which, on that
account, preserve their metallic lustre so well.

EMILY.

Are all metals capable of decomposing water, provided their temperature
be sufficiently raised?

MRS. B.

No; a certain degree of attraction is requisite, besides the assistance
of heat. Water, you recollect, is composed of oxygen and hydrogen; and,
unless the affinity of the metal for oxygen be stronger than that of
hydrogen, it is in vain that we raise its temperature, for it cannot
take the oxygen from the hydrogen. Iron, zinc, tin, and antimony, have a
stronger affinity for oxygen than hydrogen has, therefore these four
metals are capable of decomposing water. But hydrogen having an
advantage over all the other metals with respect to its affinity for
oxygen, it not only withholds its oxygen from them, but is even capable,
under certain circumstances, of taking the oxygen from the oxyds of
these metals.

EMILY.

I confess that I do not quite understand why hydrogen can take oxygen
from those metals that do not decompose water.

CAROLINE.

Now I think I do perfectly. Lead, for instance, will not decompose
water, because it has not so strong an attraction for oxygen as hydrogen
has. Well, then, suppose the lead to be in a state of oxyd; hydrogen
will take the oxygen from the lead, and unite with it to form water,
because hydrogen has a stronger attraction for oxygen, than oxygen has
for lead; and it is the same with all the other metals which do not
decompose water.

EMILY.

I understand your explanation, Caroline, very well; and I imagine that
it is because lead cannot decompose water that it is so much employed
for pipes for conveying that fluid.

MRS. B.

Certainly; lead is, on that account, particularly appropriate to such
purposes; whilst, on the contrary, this metal, if it was oxydable by
water, would impart to it very noxious qualities, as all oxyds of lead
are more or less pernicious.

But, with regard to the oxydation of metals, the most powerful mode of
effecting it is by means of acids. These, you know, contain a much
greater proportion of oxygen than either air or water; and will, most of
them, easily yield it to metals. Thus, you recollect, the zinc plates of
the Voltaic battery are oxydated by the acid and water, much more
effectually than by water alone.

CAROLINE.

And I have often observed that if I drop vinegar, lemon, or any acid on
the blade of a knife, or on a pair of scissars, it will immediately
produce a spot of rust.

EMILY.

Metals have, then, three ways of obtaining oxygen; from the atmosphere,
from water, and from acids.

MRS. B.

The two first you have already witnessed, and I shall now show you how
metals take the oxygen from an acid. This bottle contains nitric acid;
I shall pour some of it over this piece of copper-leaf . . . . . . .

CAROLINE.

Oh, what a disagreeable smell!

EMILY.

And what is it that produces the effervescency and that thick yellow
vapour?

MRS. B.

It is the acid, which being abandoned by the greatest part of its
oxygen, is converted into a weaker acid, which escapes in the form of
gas.

CAROLINE.

And whence proceeds this heat?

MRS. B.

Indeed, Caroline, I think you might now be able to answer that question
yourself.

CAROLINE.

Perhaps it is that the oxygen enters into the metal in a more solid
state than it existed in the acid, in consequence of which caloric is
disengaged.

MRS. B.

If the combination of the oxygen and the metal results from the union of
their opposite electricities, of course caloric must be given out.

EMILY.

The effervescence is over; therefore I suppose that the metal is now
oxydated.

MRS. B.

Yes. But there is another important connection between metals and acids,
with which I must now make you acquainted. Metals, when in the state of
oxyds, are capable of being dissolved by acids. In this operation they
enter into a chemical combination with the acid, and form an entirely
new compound.

CAROLINE.

But what difference is there between the _oxydation_ and the
_dissolution_ of the metal by an acid?

MRS. B.

In the first case, the metal merely combines with a portion of oxygen
taken from the acid, which is thus partly deoxygenated, as in the
instance you have just seen; in the second case, the metal, after being
previously oxydated, is actually dissolved in the acid, and enters into
a chemical combination with it, without producing any further
decomposition or effervescence. --This complete combination of an oxyd
and an acid forms a peculiar and important class of compound salts.

EMILY.

The difference between an oxyd and a compound salt, therefore, is very
obvious: the one consists of a metal and oxygen; the other of an oxyd
and an acid.

MRS. B.

Very well: and you will be careful to remember that the metals are
incapable of entering into this combination with acids, unless they are
previously oxydated; therefore, whenever you bring a metal in contact
with an acid, it will be first oxydated and afterwards dissolved,
provided that there be a sufficient quantity of acid for both
operations.

There are some metals, however, whose solution is more easily
accomplished, by diluting the acid in water; and the metal will, in this
case, be oxydated, not by the acid, but by the water, which it will
decompose. But in proportion as the oxygen of the water oxydates the
surface of the metal, the acid combines with it, washes it off, and
leaves a fresh surface for the oxygen to act upon: then other coats of
oxyd are successively formed, and rapidly dissolved by the acid, which
continues combining with the new-formed surfaces of oxyd till the whole
of the metal is dissolved. During this process the hydrogen gas of the
water is disengaged, and flies off with effervescence.

EMILY.

Was not this the manner in which the sulphuric acid assisted the iron
filings in decomposing water?

MRS. B.

Exactly; and it is thus that several metals, which are incapable alone
of decomposing water, are enabled to do it by the assistance of an acid,
which, by continually washing off the covering of oxyd, as it is formed,
prepares a fresh surface of metal to act upon the water.

CAROLINE.

The acid here seems to act a part not very different from that of a
scrubbing-brush. --But pray would not this be a good method of cleaning
metallic utensils?

MRS. B.

Yes; on some occasions a weak acid, as vinegar, is used for cleaning
copper. Iron plates, too, are freed from the rust on their surface by
diluted muriatic acid, previous to their being covered with tin. You
must remember, however, that in this mode of cleaning metals the acid
should be quickly afterwards wiped off, otherwise it would produce fresh
oxyd.

CAROLINE.

Let us watch the dissolution of the copper in the nitric acid; for I am
very impatient to see the salt that is to result from it. The mixture is
now of a beautiful blue colour; but there is no appearance of the
formation of a salt; it seems to be a tedious operation.

MRS. B.

The crystallisation of the salt requires some length of time to be
completed; if, however, you are so impatient, I can easily show you a
metallic salt already formed.

CAROLINE.

But that would not satisfy my curiosity half so well as one of our own
manufacturing.

MRS. B.

It is one of our own preparing that I mean to show you. When we
decomposed water a few days since, by the oxydation of iron filings
through the assistance of sulphuric acid, in what did the process
consist?

CAROLINE.

In proportion as the water yielded its oxygen to the iron, the acid
combined with the new-formed oxyd, and the hydrogen escaped alone.

MRS. B.

Very well; the result, therefore, was a compound salt, formed by the
combination of sulphuric acid with oxyd of iron. It still remains in the
vessel in which the experiment was performed. Fetch it, and we shall
examine it.

EMILY.

What a variety of processes the decomposition of water, by a metal and
an acid, implies; 1st, the decomposition of the water; 2dly, the
oxydation of the metal; and 3dly, the formation of a compound salt.

CAROLINE.

Here it is, Mrs. B. --What beautiful green crystals! But we do not
perceive any crystals in the solution of copper in nitrous acid?

MRS. B.

Because the salt is now suspended in the water which the nitrous acid
contains, and will remain so till it is deposited in consequence of rest
and cooling.

EMILY.

I am surprised that a body so opake as iron can be converted into such
transparent crystals.

MRS. B.

It is the union with the acid that produces the transparency; for if the
pure metal were melted, and afterwards permitted to cool and
crystallise, it would be found just as opake as before.

EMILY.

I do not understand the exact meaning of _crystallisation_?

MRS. B.

You recollect that when a solid body is dissolved either by water or
caloric it is not decomposed; but that its integrant parts are only
suspended in the solvent. When the solution is made in water, the
integrant particles of the body will, on the water being evaporated,
again unite into a solid mass by the force of their mutual attraction.
But when the body is dissolved by caloric alone, nothing more is
necessary, in order to make its particles reunite, than to reduce its
temperature. And, in general, if the solvent, whether water or caloric,
be slowly separated by evaporation or by cooling, and care taken that
the particles be not agitated during their reunion, they will arrange
themselves in regular masses, each individual substance assuming a
peculiar form or arrangement; and this is what is called
crystallisation.

EMILY.

Crystallisation, therefore, is simply the reunion of the particles of a
solid body that has been dissolved in a fluid.

MRS. B.

That is a very good definition of it. But I must not forget to observe,
that _heat_ and _water_ may unite their solvent powers; and, in this
case, crystallisation may be hastened by cooling, as well as by
evaporating the liquid.

CAROLINE.

But if the body dissolved is of a volatile nature, will it not evaporate
with the fluid?

MRS. B.

A crystallised body held in solution only by water is scarcely ever so
volatile as the fluid itself, and care must be taken to manage the heat
so that it may be sufficient to evaporate the water only.

I should not omit also to mention that bodies, in crystallising from
their watery solution, always retain a small portion of water, which
remains confined in the crystal in a solid form, and does not reappear
unless the body loses its crystalline state. This is called the _water
of crystallisation_. But you must observe, that whilst a body may be
separated from its solution in water or caloric simply by cooling or by
evaporation, an acid can be taken from a metal with which it is combined
only by stronger affinities, which produce a decomposition.

EMILY.

Are the perfect metals susceptible of being dissolved and converted into
compound salts by acids?

MRS. B.

Gold is acted upon by only one acid, the _oxygenated muriatic_, a very
remarkable acid, which, when in its most concentrated state, dissolves
gold or any other metal, by burning them rapidly.

Gold can, it is true, be dissolved likewise by a mixture of two acids,
commonly called _aqua regia_; but this mixed solvent derives that
property from containing the peculiar acid which I have just mentioned.
Platina is also acted upon by this acid only; silver is dissolved by
nitric acid.

CAROLINE.

I think you said that some of the metals might be so strongly oxydated
as to become acid?

MRS. B.

There are five metals, arsenic, molybdena, chrome, tungsten, and
columbium, which are susceptible of combining with a sufficient quantity
of oxygen to be converted into acids.

CAROLINE.

Acids are connected with metals in such a variety of ways, that I am
afraid of some confusion in remembering them. --In the first place,
acids will yield their oxygen to metals. Secondly, they will combine
with them in their state of oxyds, to form compound salts; and lastly,
several of the metals are themselves susceptible of acidification.

MRS. B.

Very well; but though metals have so great an affinity for acids, it is
not with that class of bodies alone that they will combine. They are
most of them, in their simple state, capable of uniting with sulphur,
with phosphorus, with carbon, and with each other; these combinations,
according to the nomenclature which was explained to you on a former
occasion, are called _sulphurets_, _phosphorets_, _carburets_, &c.

The metallic phosphorets offer nothing very remarkable. The sulphurets
form the peculiar kind of mineral called _pyrites_, from which certain
kinds of mineral waters, as those of Harrogate, derive their chief
chemical properties. In this combination, the sulphur, together with the
iron, have so strong an attraction for oxygen, that they obtain it both
from the air and from water, and by condensing it in a solid form,
produce the heat which raises the temperature of the water in such a
remarkable degree.

EMILY.

But if pyrites obtain oxygen from water, that water must suffer a
decomposition, and hydrogen gas be evolved.

MRS. B.

That is actually the case in the hot springs alluded to, which give out
an extremely fetid gas, composed of hydrogen impregnated with sulphur.

CAROLINE.

If I recollect right, steel and plumbago, which you mentioned in the
last lesson, are both carburets of iron?

MRS. B.

Yes; and they are the only carburets of much consequence.

A curious combination of metals has lately very much attracted the
attention of the scientific world: I mean the meteoric stones that fall
from the atmosphere. They consist principally of native or pure iron,
which is never found in that state in the bowels of the earth; and
contain also a small quantity of nickel and chrome, a combination
likewise new in the mineral kingdom.

These circumstances have led many scientific persons to believe that
those substances have fallen from the moon, or some other planet, while
others are of opinion either that they are formed in the atmosphere, or
are projected into it by some unknown volcano on the surface of our
globe.

CAROLINE.

I have heard much of these stones, but I believe many people are of
opinion that they are formed on the surface of the earth, and laugh at
their pretended celestial origin.

MRS. B.

The fact of their falling is so well ascertained, that I think no person
who has at all investigated the subject, can now entertain any doubt of
it. Specimens of these stones have been discovered in all parts of the
world, and to each of them some tradition or story of its fall has been
found connected. And as the analysis of all those specimens affords
precisely the same results, there is strong reason to conjecture that
they all proceed from the same source. It is to Mr. Howard that
philosophers are indebted for having first analysed these stones, and
directed their attention to this interesting subject.

CAROLINE.

But pray, Mrs. B., how can solid masses of iron and nickel be formed
from the atmosphere, which consists of the two airs, nitrogen and
oxygen?

MRS. B.

I really do not see how they could, and think it much more probable that
they fall from the moon. --But we must not suffer this digression to
take up too much of our time.

The combinations of metals with each other are called alloys; thus brass
is an alloy of copper and zinc; bronze, of copper and tin, &c.

EMILY.

And is not pewter also a combination of metal?

MRS. B.

It is. The pewter made in this country is mostly composed of tin, with a
very small proportion of zinc and lead.

CAROLINE.

Block-tin is a kind of pewter, I believe?

MRS. B.

Properly speaking, block-tin means tin in blocks, or square massive
ingots; but in the sense in which it is used by ignorant workmen, it is
iron plated with tin, which renders it more durable, as tin will not so
easily rust. Tin alone, however, would be too soft a metal to be worked
for common use, and all tin-vessels and utensils are in fact made of
plates of iron, thinly coated with tin, which prevents the iron from
rusting.

CAROLINE.

Say rather _oxydating_, Mrs. B. --Rust is a word that should be exploded
in chemistry.

MRS. B.

Take care, however, not to introduce the word oxydate, instead of rust,
in general conversation; for you would probably not be understood, and
you might be suspected of affectation.

Metals differ very much in their affinity for each other; some will not
unite at all, others readily combine together, and on this property of
metals the art of _soldering_ depends.

EMILY.

What is soldering?

MRS. B.

It is joining two pieces of metal together, by a more fusible metal
interposed between them. Thus tin is a solder for lead; brass, gold, or
silver, are solder for iron, &c.

CAROLINE.

And is not _plating_ metals something of the same nature?

MRS. B.

In the operation of plating, two metals are united, one being covered
with the other, but without the intervention of a third; iron or copper
may thus be covered with gold or silver.

EMILY.

Mercury appears to me of a very different nature from the other metals.

MRS. B.

One of its greatest peculiarities is, that it retains a fluid state at
the temperature of the atmosphere. All metals are fusible at different
degrees of heat, and they have likewise each the property of freezing or
becoming solid at a certain fixed temperature. Mercury congeals only at
seventy-two degrees below the freezing point.

EMILY.

That is to say, that in order to freeze, it requires a temperature of
seventy-two degrees colder than that at which water freezes.

MRS. B.

Exactly so.

CAROLINE.

But is the temperature of the atmosphere ever so low as that?

MRS. B.

Yes, often in Siberia; but happily never in this part of the globe.
Here, however, mercury may be congealed by artificial cold; I mean such
intense cold as can be produced by some chemical mixtures, or by the
rapid evaporation of ether under the air-pump.*

    [Footnote *: By a process analogous to that described, page 155.
    of this volume.]

CAROLINE.

And can mercury be made to boil and evaporate?

MRS. B.

Yes, like any other liquid; only it requires a much greater degree of
heat. At the temperature of six hundred degrees, it begins to boil and
evaporate like water.

Mercury combines with gold, silver, tin, and with several other metals;
and, if mixed with any of them in a sufficient proportion, it penetrates
the solid metal, softens it, loses its own fluidity, and forms an
_amalgam_, which is the name given to the combination of any metal with
mercury, forming a substance more or less solid, according as the
mercury or the other metal predominates.

EMILY.

In the list of metals there are some whose names I have never before
heard mentioned.

MRS. B.

Besides those which Sir H. Davy has obtained, there are several that
have been recently discovered, whose properties are yet but little
known, as for instance, titanium, which was discovered by the Rev. Mr.
Gregor, in the tin-mines of Cornwall; columbium or tantalium, which has
lately been discovered by Mr. Hatchett; and osmium, iridium, palladium,
and rhodium, all of which Dr. Wollaston and Mr. Tennant found mixed in
minute quantities with crude platina, and the distinct existence of
which they proved by curious and delicate experiments.

CAROLINE.

Arsenic has been mentioned amongst the metals. I had no notion that it
belonged to that class of bodies, for I had never seen it but as a
powder, and never thought of it but as a most deadly poison.

MRS. B.

In its pure metallic state, I believe, it is not so poisonous; but it
has such a great affinity for oxygen, that it absorbs it from the
atmosphere at its natural temperature: you have seen it, therefore, only
in its state of oxyd, when, from its combination with oxygen, it has
acquired its very poisonous properties.

CAROLINE.

Is it possible that oxygen can impart poisonous qualities? That valuable
substance which produces light and fire, and which all bodies in nature
are so eager to obtain?

MRS. B.

Most of the metallic oxyds are poisonous, and derive this property from
their union with oxygen. The white lead, so much used in paint, owes its
pernicious effects to oxygen. In general, oxygen, in a concrete state,
appears to be particularly destructive in its effects on flesh or any
animal matter; and those oxyds are most caustic that have an acrid
burning taste, which proceeds from the metal having but a slight
affinity for oxygen, and therefore easily yielding it to the flesh,
which it corrodes and destroys.

EMILY.

What is the meaning of the word _caustic_, which you have just used?

MRS. B.

It expresses that property which some bodies possess, of disorganizing
and destroying animal matter, by operating a kind of combustion, or at
least a chemical decomposition. You must often have heard of caustic
used to burn warts, or other animal excrescences; most of these bodies
owe their destructive power to the oxygen with which they are combined.
The common caustic, called _lunar caustic_, is a compound formed by the
union of nitric acid and silver; and it is supposed to owe its caustic
qualities to the oxygen contained in the nitric acid.

CAROLINE.

But, pray, are not acids still more caustic than oxyds, as they contain
a greater proportion of oxygen?

MRS. B.

Some of the acids are; but the caustic property of a body depends not
only upon the quantity of oxygen which it contains, but also upon its
slight affinity for that principle, and the consequent facility with
which it yields it.

EMILY.

Is not this destructive property of oxygen accounted for?

MRS. B.

It proceeds probably from the strong attraction of oxygen for hydrogen;
for if the one rapidly absorb the other from the animal fibre,
a disorganisation of the substance must ensue.

EMILY.

Caustics are, then, very properly said to burn the flesh, since the
combination of oxygen and hydrogen is an actual combustion.

CAROLINE.

Now, I think, this effect would be more properly termed an oxydation, as
there is no disengagement of light and heat.

MRS. B.

But there really is a sensation of heat produced by the action of
caustics.

EMILY.

If oxygen is so caustic, why does not that which is contained in the
atmosphere burn us?

MRS. B.

Because it is in a gaseous state, and has a greater attraction for its
electricity than for the hydrogen of our bodies. Besides, should the air
be slightly caustic, we are in a great measure sheltered from its
effects by the skin; you know how much a wound, however trifling, smarts
on being exposed to it.

CAROLINE.

It is a curious idea, however, that we should live in a slow fire. But,
if the air was caustic, would it not have an acrid taste?

MRS. B.

It possibly may have such a taste; though in so slight a degree, that
custom has rendered it insensible.

CAROLINE.

And why is not water caustic? When I dip my hand into water, though
cold, it ought to burn me from the caustic nature of its oxygen.

MRS. B.

Your hand does not decompose the water; the oxygen in that state is much
better supplied with hydrogen than it would be by animal matter, and if
its causticity depend on its affinity for that principle, it will be
very far from quitting its state of water to act upon your hand. You
must not forget that oxyds are caustic in proportion as the oxygen
adheres slightly to them.

EMILY.

Since the oxyd of arsenic is poisonous, its acid, I suppose, is fully as
much so?

MRS. B.

Yes; it is one of the strongest poisons in nature.

EMILY.

There is a poison called _verdigris_, which forms on brass and copper
when not kept very clean; and this, I have heard, is an objection to
these metals being made into kitchen utensils. Is this poison likewise
occasioned by oxygen?

MRS. B.

It is produced by the intervention of oxygen; for verdigris is a
compound salt formed by the union of vinegar and copper; it is of a
beautiful green colour, and much used in painting.

EMILY.

But, I believe, verdigris is often formed on copper when no vinegar has
been in contact with it.

MRS. B.

Not real verdigris, but compound salts, somewhat resembling it, may be
produced by the action of any acid on copper.

The solution of copper in nitric acid, if evaporated, affords a salt
which produces an effect on tin that will surprise you, and I have
prepared some from the solution we made before, that I might show it to
you. I shall first sprinkle some water on this piece of tin-foil, and
then some of the salt. --Now observe that I fold it up suddenly, and
press it into one lump.

CAROLINE.

What a prodigious vapour issues from it--and sparks of fire I declare!

MRS. B.

I thought it would surprise you. The effect, however, I dare say you
could account for, since it is merely the consequence of the oxygen of
the salt rapidly entering into a closer combination with the tin.

There is also a beautiful green salt too curious to be omitted; it is
produced by the combination of cobalt with muriatic acid, which has the
singular property of forming what is called _sympathetic ink_.
Characters written with this solution are invisible when cold, but when
a gentle heat is applied, they assume a fine bluish green colour.

CAROLINE.

I think one might draw very curious landscapes with the assistance of
this ink; I would first make a water-colour drawing of a winter-scene,
in which the trees should be leafless, and the grass scarcely green:
I would then trace all the verdure with the invisible ink, and whenever
I chose to create spring, I should hold it before the fire, and its
warmth would cover the landscape with a rich verdure.

MRS. B.

That will be a very amusing experiment, and I advise you by all means to
try it.

  [Transcriber's Note:
  Several cobalt compounds, including the cobalt chloride described
  here, are still in use as invisible ("sympathetic") inks. They are
  safe if used appropriately.]

Before we part, I must introduce to your acquaintance the curious metals
which Sir H. Davy has recently discovered. The history of these
extraordinary bodies is yet so much in its infancy, that I shall confine
myself to a very short account of them; it is more important to point
out to you the vast, and apparently inexhaustible, field of research
which has been thrown open to our view by Sir H. Davy's memorable
discoveries, than to enter into a minute account of particular bodies or
experiments.

CAROLINE.

But I have heard that these discoveries, however splendid and
extraordinary, are not very likely to prove of any great benefit to the
world, as they are rather objects of curiosity than of use.

MRS. B.

Such may be the illiberal conclusions of the ignorant and narrow-minded;
but those who can duly estimate the advantages of enlarging the sphere
of science, must be convinced that the acquisition of every new fact,
however unconnected it may at first appear with practical utility, must
ultimately prove beneficial to mankind. But these remarks are scarcely
applicable to the present subject; for some of the new metals have
already proved eminently useful as chemical agents, and are likely soon
to be employed in the arts. For the enumeration of these metals, I must
refer you to our list of simple bodies; they are derived from the
alkalies, the earths, and three of the acids, all of which had been
hitherto considered as undecompoundable or simple bodies.

When Sir H. Davy first turned his attention to the effects of the
Voltaic battery, he tried its power on a variety of compound bodies, and
gradually brought to light a number of new and interesting facts, which
led the way to more important discoveries. It would be highly
interesting to trace his steps in this new department of science, but it
would lead us too far from our principal object. A general view of his
most remarkable discoveries is all that I can aim at, or that you could,
at present, understand.

The facility with which compound bodies yielded to the Voltaic
electricity, induced him to make trial of its effects on substances
hitherto considered as simple, but which he suspected of being compound,
and his researches were soon crowned with the most complete success.

The body which he first submitted to the Voltaic battery, and which had
never yet been decomposed, was one of the fixed alkalies, called potash.
This substance gave out an elastic fluid at the positive wire, which was
ascertained to be oxygen, and at the negative wire, small globules of a
very high metallic lustre, very similar in appearance to mercury; thus
proving that potash, which had hitherto been considered as a simple
incombustible body, was in fact a metallic oxyd; and that its
incombustibility proceeded from its being already combined with oxygen.

EMILY.

I suppose the wires used in this experiment were of platina, as they
were when you decomposed water; for if of iron, the oxygen would have
combined with the wire, instead of appearing in the form of gas.

MRS. B.

Certainly: the metal, however, would equally have been disengaged. Sir
H. Davy has distinguished this new substance by the name of POTASSIUM,
which is derived from that of the alkali, from which it is procured.
I have some small pieces of it in this phial, but you have already seen
it, as it is the metal which we burnt in contact with sulphur.

EMILY.

What is the liquid in which you keep it?

MRS. B.

It is naptha, a bituminous liquid, with which I shall hereafter make you
acquainted. It is almost the only fluid in which potassium can be
preserved, as it contains no oxygen, and this metal has so powerful an
attraction for oxygen, that it will not only absorb it from the air, but
likewise from water, or any body whatever that contains it.

EMILY.

This, then, is one of the bodies that oxydates spontaneously without the
application of heat?

MRS. B.

Yes; and it has this remarkable peculiarity that it attracts oxygen much
more rapidly from water than from air; so that when thrown into water,
however cold, it actually bursts into flame. I shall now throw a small
piece, about the size of a pin's head, on this drop of water.

CAROLINE.

It instantaneously exploded, producing a little flash of light! this is,
indeed, a most curious substance!

MRS. B.

By its combustion it is reconverted into potash; and as potash is now
decidedly a compound body, I shall not enter into any of its properties
till we have completed our review of the simple bodies; but we may here
make a few observations on its basis, potassium. If this substance is
left in contact with air, it rapidly returns to the state of potash,
with a disengagement of heat, but without any flash of light.

EMILY.

But is it not very singular that it should burn better in water than in
air?

CAROLINE.

I do not think so: for if the attraction of potassium for oxygen is so
strong that it finds no more difficulty in separating it from the
hydrogen in water, than in absorbing it from the air, it will no doubt
be more amply and rapidly supplied by water than by air.

MRS. B.

That cannot, however, be precisely the reason, for when potassium is
introduced under water, without contact of air, the combustion is not so
rapid, and indeed, in that case, there is no luminous appearance; but a
violent action takes place, much heat is excited, the potash is
regenerated, and hydrogen gas is evolved.

Potassium is so eminently combustible, that instead of requiring, like
other metals, an elevation of temperature, it will burn rapidly in
contact with water, even below the freezing point. This you may witness
by throwing a piece on this lump of ice.

CAROLINE.

It again exploded with flame, and has made a deep hole in the ice.

MRS. B.

This hole contains a solution of potash; for the alkali being extremely
soluble, disappears in the water at the instant it is produced. Its
presence, however, may be easily ascertained, alkalies having the
property of changing paper, stained with turmeric, to a red colour; if
you dip one end of this slip of paper into the hole in the ice you will
see it change colour, and the same, if you wet it with the drop of water
in which the first piece of potassium was burnt.

CAROLINE.

It has indeed changed the paper from yellow to red.

MRS. B.

This metal will burn likewise in carbonic acid gas, a gas that had
always been supposed incapable of supporting combustion, as we were
unacquainted with any substance that had a greater attraction for oxygen
than carbon. Potassium, however, readily decomposes this gas, by
absorbing its oxygen, as I shall show you. This retort is filled with
carbonic acid gas. --I will put a small piece of potassium in it; but
for this combustion a slight elevation of temperature is required, for
which purpose I shall hold the retort over the lamp.

CAROLINE.

Now it has taken fire, and burns with violence! It has burst the retort.

MRS. B.

Here is the piece of regenerated potash; can you tell me why it is
become so black?

EMILY.

No doubt it is blackened by the carbon, which, when its oxygen entered
into combination with the potassium, was deposited on its surface.

MRS. B.

You are right. This metal is perfectly fluid at the temperature of one
hundred degrees; at fifty degrees it is solid, but soft and malleable;
at thirty-two degrees it is hard and brittle, and its fracture exhibits
an appearance of confused crystallization. It is scarcely more than half
as heavy as water; its specific gravity being about six when water is
reckoned at ten; so that this metal is actually lighter than any known
fluid, even than ether.

Potassium combines with sulphur and phosphorus, forming sulphurets and
phosphurets; it likewise forms alloys with several metals, and
amalgamates with mercury.

EMILY.

But can a sufficient quantity of potassium be obtained, by means of the
Voltaic battery, to admit of all its properties and relations to other
bodies being satisfactorily ascertained?

MRS. B.

Not easily; but I must not neglect to inform you that a method of
obtaining this metal in considerable quantities has since been
discovered. Two eminent French chemists, Thenard and Gay Lussac,
stimulated by the triumph which Sir H. Davy had obtained, attempted to
separate potassium from its combination with oxygen, by common chemical
means, and without the aid of electricity. They caused red hot potash in
a state of fusion to filter through iron turnings in an iron tube,
heated to whiteness. Their experiment was crowned with the most complete
success; more potassium was obtained by this single operation, that
could have been collected in many weeks by the most diligent use of the
Voltaic battery.

EMILY.

In this experiment, I suppose, the oxygen quitted its combination with
the potassium to unite with the iron turnings?

MRS. B.

Exactly so; and the potassium was thus obtained in its simple state.
From that time it has become a most convenient and powerful instrument
of deoxygenation in chemical experiments. This important improvement,
engrafted on Sir H. Davy's previous discoveries, served but to add to
his glory, since the facts which he had established, when possessed of
only a few atoms of this curious substance, and the accuracy of his
analytical statements, were all confirmed when an opportunity occurred
of repeating his experiments upon this substance, which can now be
obtained in unlimited quantities.

CAROLINE.

What a satisfaction Sir H. Davy must have felt, when by an effort of
genius he succeeded in bringing to light and actually giving existence,
to these curious bodies, which without him might perhaps have ever
remained concealed from our view!

MRS. B.

The next substance which Sir H. Davy submitted to the influence of the
Voltaic battery was _Soda_, the other fixed alkali, which yielded to the
same powers of decomposition; from this alkali too, a metallic substance
was obtained, very analogous in its properties to that which had been
discovered in potash; Sir H. Davy has called it SODIUM. It is rather
heavier than potassium, though considerably lighter than water; it is
not so easily fusible as potassium.

Encouraged by these extraordinary results, Sir H. Davy next performed a
series of beautiful experiments on _Ammonia_, or the volatile alkali,
which, from analogy, he was led to suspect might also contain oxygen.
This he soon ascertained to be the fact, but he has not yet succeeded in
obtaining the basis of ammonia in a separate state; it is from analogy,
and from the power which the volatile alkali has, in its gaseous form,
to oxydate iron, and also from the amalgams which can be obtained from
ammonia by various processes, that the proofs of that alkali being also
a metallic oxyd are deduced.

Thus, then, the three alkalies, two of which had always been considered
as simple bodies, have now lost all claim to that title, and I have
accordingly classed the alkalies amongst the compounds, whose properties
we shall treat of in a future conversation.

EMILY.

What are the other newly discovered metals which you have alluded to in
your list of simple bodies?

MRS. B.

They are the metals of the earths which became next the object of Sir H.
Davy's researches; these bodies had never yet been decomposed, though
they were strongly suspected not only of being compounds, but of being
metallic oxyds. From the circumstance of their incombustibility it was
conjectured, with some plausibility, that they might possibly be bodies
that had been already burnt.

CAROLINE.

And metals, when oxydated, become, to all appearance, a kind of earthy
substance.

MRS. B.

They have, besides, several features of resemblance with metallic oxyds;
Sir H. Davy had therefore great reason to be sanguine in his
expectations of decomposing them, and he was not disappointed. He could
not, however, succeed in obtaining the basis of the earths in a pure
separate state; but metallic alloys were formed with other metals, which
sufficiently proved the existence of the metallic basis of the earths.

The last class of new metallic bodies which Sir H. Davy discovered was
obtained from the three undecompounded acids, the boracic, the fluoric,
and the muriatic acids; but as you are entirely unacquainted with these
bodies, I shall reserve the account of their decomposition till we come
to treat of their properties as acids.

Thus in the course of two years, by the unparalleled exertions of a
single individual, chemical science has assumed a new aspect. Bodies
have been brought to light which the human eye never before beheld, and
which might have remained eternally concealed under their impenetrable
disguise.

It is impossible at the present period to appreciate to their full
extent the consequences which science or the arts may derive from these
discoveries; we may, however, anticipate the most important results.

In chemical analysis we are now in possession of more energetic agents
of decomposition than were ever before known.

In geology new views are opened, which will probably operate a
revolution in that obscure and difficult science. It is already proved
that all the earths, and, in fact, the solid surface of this globe, are
metallic bodies mineralized by oxygen, and as our planet has been
calculated to be considerably more dense upon the whole than on the
surface, it is reasonable to suppose that the interior part is composed
of a metallic mass, the surface of which only has been mineralized by
the atmosphere.

The eruptions of volcanos, those stupendous problems of nature, admit
now of an easy explanation. For if the bowels of the earth are the grand
recess of these newly discovered inflammable bodies, whenever water
penetrates into them, combustions and explosions must take place; and it
is remarkable that the lava which is thrown out, is the very kind of
substance which might be expected to result from these combustions.

I must now take my leave of you; we have had a very long conversation
to-day, and I hope you will be able to recollect what you have learnt.
At our next interview we shall enter on a new subject.


END OF THE FIRST VOLUME.


  Printed by A. Strahan,
  Printers-Street, London.

       *       *       *       *       *
           *       *       *       *

                 CONVERSATIONS
                       ON
                   CHEMISTRY;

                    In Which
          The Elements Of That Science
                      Are
             _Familiarly Explained_
                      And
          Illustrated By Experiments.


                IN TWO VOLUMES.

    _The Fifth Edition, revised, corrected,_
          _and considerably enlarged._

                    VOL. II.
              ON COMPOUND BODIES.


  _London:_
  Printed For Longman, Hurst, Rees, Orme, and Brown,
  Paternoster-Row.
  1817.




CONVERSATION XIII.

ON THE ATTRACTION OF COMPOSITION.


MRS. B.

Having completed our examination of the simple or elementary bodies, we
are now to proceed to those of a compound nature; but before we enter on
this extensive subject, it will be necessary to make you acquainted with
the principal laws by which chemical combinations are governed.

You recollect, I hope, what we formerly said of the nature of the
attraction of composition, or chemical attraction, or affinity, as it is
also called?

EMILY.

Yes, I think perfectly; it is the attraction that subsists between
bodies of a different nature, which occasions them to combine and form a
compound, when they come in contact, and, according to Sir H. Davy's
opinion, this effect is produced by the attraction of the opposite
electricities, which prevail in bodies of different kinds.

MRS. B.

Very well; your definition comprehends the first law of chemical
attraction, which is, that _it takes place only between bodies of a
different nature_; as, for instance, between an acid and an alkali;
between oxygen and a metal, &c.

CAROLINE.

That we understand of course; for the attraction between particles of a
similar nature is that of aggregation, or cohesion, which is independent
of any chemical power.

MRS. B.

The 2d law of chemical attraction is, that _it takes place only between
the most minute particles of bodies_; therefore, the more you divide the
particles of the bodies to be combined, the more readily they act upon
each other.

CAROLINE.

That is again a circumstance which we might have supposed, for the finer
the particles of the two substances are, the more easily and perfectly
they will come in contact with each other, which must greatly facilitate
their union. It was for this purpose, you said, that you used iron
filings, in preference to wires or pieces of iron, for the decomposition
of water.

MRS. B.

It was once supposed that no mechanical power could divide bodies into
particles sufficiently minute for them to act on each other; and that,
in order to produce the extreme division requisite for a chemical
action, one, if not both of the bodies, should be in a fluid state.
There are, however, a few instances in which two solid bodies, very
finely pulverized, exert a chemical action on one another; but such
exceptions to the general rule are very rare indeed.

EMILY.

In all the combinations that we have hitherto seen, one of the
constituents has, I believe, been either liquid or aeriform. In
combustions, for instance, the oxygen is taken from the atmosphere, in
which it existed in the state of gas; and whenever we have seen acids
combine with metals or with alkalies, they were either in a liquid or an
aeriform state.

MRS. B.

The 3d law of chemical attraction is, that _it can take place between
two, three, four, or even a greater number of bodies_.

CAROLINE.

Oxyds and acids are bodies composed of two constituents; but I recollect
no instance of the combination of a greater number of principles.

MRS. B.

The compound salts, formed by the union of the metals with acids, are
composed of three principles. And there are salts formed by the
combination of the alkalies with the earths which are of a similar
description.

CAROLINE.

Are they of the same kind as the metallic salts?

MRS. B.

Yes; they are very analogous in their nature, although different in many
of their properties.

A methodical nomenclature, similar to that of the acids, has been
adopted for the compound salts. Each individual salt derives its name
from its constituent parts, so that every name implies a knowledge of
the composition of the salt.

The three alkalies, the alkaline earths, and the metals, are called
_salifiable bases_ or _radicals_; and the acids, _salifying principles_.
The name of each salt is composed both of that of the acid and the
salifiable base; and it terminates in _at_ or _it_, according to the
degree of the oxygenation of the acid. Thus, for instance, all those
salts which are formed by the combination of the sulphuric acid with any
of the salifiable bases are called _sulphats_, and the name of the
radical is added for the specific distinction of the salt; if it be
potash, it will compose a _sulphat of potash_; if ammonia, _sulphat of
ammonia_, &c.

EMILY.

The crystals which we obtained from the combination of iron and
sulphuric acid were therefore _sulphat of iron_?

MRS. B.

Precisely; and those which we prepared by dissolving copper in nitric
acid, _nitrat of copper_, and so on. --But this is not all; if the salt
be formed by that class of acids which ends in _ous_, (which you know
indicates a less degree of oxygenation,) the termination of the name of
the salt will be in _it_, as _sulphit of potash_, _sulphit of
ammonia_, &c.

EMILY.

There must be an immense number of compound salts, since there is so
great a variety of salifiable radicals, as well as of salifying
principles.

MRS. B.

Their real number cannot be ascertained, since it increases every day.
But we must not proceed further in the investigation of the compound
salts, until we have completed the examination of the nature of the
ingredients of which they are composed.

The 4th law of chemical attraction is, that _a change of temperature
always takes place at the moment of combination_. This arises from the
extrication of the two electricities in the form of caloric, which takes
place when bodies unite; and also sometimes in part from a change of
capacity of the bodies for heat, which always takes place when the
combination is attended with an increase of density, but more especially
when the compound passes from the liquid to the solid form. I shall now
show you a striking instance of a change of temperature from chemical
union, merely by pouring some nitrous acid on this small quantity of oil
of turpentine--the oil will instantly combine with the oxygen of the
acid, and produce a considerable change of temperature.

CAROLINE.

What a blaze! The temperature of the oil and the acid must be greatly
raised, indeed, to produce such a violent combustion.

MRS. B.

There is, however, a peculiarity in this combustion, which is, that the
oxygen, instead of being derived from the atmosphere alone, is
principally supplied by the acid itself.

EMILY.

And are not all combustions instances of the change of temperature
produced by the chemical combination of two bodies?

MRS. B.

Undoubtedly; when oxygen loses its gaseous form, in order to combine
with a solid body, it becomes condensed, and the caloric evolved
produces the elevation of temperature. The specific gravity of bodies is
at the same time altered by chemical combination; for in consequence of
a change of capacity for heat, a change of density must be produced.

CAROLINE.

That was the case with the sulphuric acid and water, which, by being
mixed together, gave out a great deal of heat, and increased in density.

MRS. B.

The 5th law of chemical attraction is, that _the properties which
characterise bodies, when separate, are altered or destroyed by their
combination_.

CAROLINE.

Certainly; what, for instance, can be so different from water as the
hydrogen and oxygen gases?

EMILY.

Or what more unlike sulphat of iron than iron or sulphuric acid?

MRS. B.

Every chemical combination is an illustration of this rule. But let us
proceed--

The 6th law is, that _the force of chemical affinity between the
constituents of a body is estimated by that which is required for their
separation_. This force is not always proportional to the facility with
which bodies unite; for manganese, for instance, which, you know, is so
much disposed to unite with oxygen that it is never found in a metallic
state, yields it more easily than any other metal.

EMILY.

But, Mrs. B., you speak of estimating the force of attraction between
bodies, by the force required to separate them; how can you measure
these forces?

MRS. B.

They cannot be precisely measured, but they are comparatively
ascertained by experiment, and can be represented by numbers which
express the relative degrees of attraction.

The 7th law is, that _bodies have amongst themselves different degrees
of attraction_. Upon this law, (which you may have discovered yourselves
long since,) the whole science of chemistry depends; for it is by means
of the various degrees of affinity which bodies have for each other,
that all the chemical compositions and decompositions are effected.
Every chemical fact or experiment is an instance of the same kind; and
whenever the decomposition of a body is performed by the addition of any
single new substance, it is said to be effected by _simple elective
attractions_. But it often happens that no simple substance will
decompose a body, and that, in order to effect this, you must offer to
the compound a body which is itself composed of two, or sometimes three
principles, which would not, each separately, perform the decomposition.
In this case there are two new compounds formed in consequence of a
reciprocal decomposition and recomposition. All instances of this kind
are called _double elective attractions_.

CAROLINE.

I confess I do not understand this clearly.

MRS. B.

You will easily comprehend it by the assistance of this diagram, in
which the reciprocal forces of attraction are represented by numbers:

                   _Original Compound_
                      Sulphat of Soda.

                      Soda    8 Sulphuric Acid

                              |
                              | _Quies-_
                              |
                              | _cent_
                              |
  _Result_                                        _Result_
  Nitrat        7 _Divellent Attractions_ 6} 13   Sulphat
  of Soda                                         of Lime
                              |
                              |
                              | _Attrac-_
                              |
                              | _tions_
                              |

                 Nitric Acid  4  Lime
                             --
                             12

                   _Original Compound_
                      Nitrat of Lime.

We here suppose that we are to decompose sulphat of soda; that is, to
separate the acid from the alkali; if, for this purpose, we add some
lime, in order to make it combine with the acid, we shall fail in our
attempt, because the soda and the sulphuric acid attract each other by a
force which is superior, and (by way of supposition) is represented by
the number 8; while the lime tends to unite with this acid by an
affinity equal only to the number 6. It is plain, therefore, that the
sulphat of soda will not be decomposed, since a force equal to 8 cannot
be overcome by a force equal only to 6.

CAROLINE.

So far, this appears very clear.

MRS. B.

If, on the other hand, we endeavour to decompose this salt by nitric
acid, which tends to combine with soda, we shall be equally
unsuccessful, as nitric acid tends to unite with the alkali by a force
equal only to 7.

In neither of these cases of simple elective attraction, therefore, can
we accomplish our purpose. But let us previously combine together the
lime and nitric acid, so as to form a nitrat of lime, a compound salt,
the constituents of which are united by a power equal to 4. If then we
present this compound to the sulphat of soda, a decomposition will
ensue, because the sum of the forces which tend to preserve the two
salts in their actual state is not equal to that of the forces which
tend to decompose them, and to form new combinations. The nitric acid,
therefore, will combine with the soda, and the sulphuric acid with the
lime.

CAROLINE.

I understand you now very well. This double effect takes place because
the numbers 8 and 4, which represent the degrees of attraction of the
constituents of the two original salts, make a sum less than the numbers
7 and 6, which represent the degrees of attraction of the two new
compounds that will in consequence be formed.

MRS. B.

Precisely so.

CAROLINE.

But what is the meaning of _quiescent_ and _divellent_ forces, which are
written in the diagram?

MRS. B.

Quiescent forces are those which tend to preserve compounds in a state
of rest, or such as they actually are: divellent forces, those which
tend to destroy that state of combination, and to form new compounds.

These are the principal circumstances relative to the doctrine of
chemical attractions, which have been laid down as rules by modern
chemists; a few others might be mentioned respecting the same theory,
but of less importance, and such as would take us too far from our plan.
I should, however, not omit to mention that Mr. Berthollet, a celebrated
French chemist, has questioned the uniform operation of elective
attraction, and has advanced the opinion, that, in chemical
combinations, the changes which take place depend not only upon the
affinities, but also, in some degree, on the respective quantities of
the substances concerned, on the heat applied during the process, and
some other circumstances.

CAROLINE.

In that case, I suppose, there would hardly be two compounds exactly
similar, though composed of the same materials?

MRS. B.

On the contrary, it is found that a remarkable uniformity prevails, as
to proportions, between the ingredients of bodies of similar
composition. Thus water, as you may recollect to have seen in a former
conversation, is composed of two volumes of hydrogen gas to one of
oxygen, and this is always found to be precisely the proportion of its
constituents, from whatever source the water be derived. The same
uniformity prevails with regard to the various salts; the acid and
alkali, in each kind of salt, being always found to combine in the same
proportions. Sometimes, it is true, the same acid, and the same alkali,
are capable of making two distinct kinds of salts; but in all these
cases it is found that one of the salts contains just twice, or in some
instances, thrice as much acid, or alkali, as the other.

EMILY.

If the proportions in which bodies combine are so constant and so well
defined, how can Mr. Berthollet's remark be reconciled with this uniform
system of combination?

MRS. B.

Great as that philosopher's authority is in chemistry, it is now
generally supposed that his doubts on this subject were in a great
degree groundless, and that the exceptions he has observed in the laws
of definite proportions, have been only apparent, and may be accounted
for consistently with those laws.

CAROLINE.

Pray, Mrs. B., can you decompose a salt by means of electricity, in the
same way as we decompose water?

MRS. B.

Undoubtedly; and I am glad this question occurred to you, because it
gives me an opportunity of showing you some very interesting experiments
on the subject.

If we dissolve a quantity, however small, of any salt in a glass of
water, and if we plunge into it the extremities of the wires which
proceed from the two ends of the Voltaic battery, the salt will be
gradually decomposed, the acid being attracted by the positive, and the
alkali by the negative wire.

EMILY.

But how can you render that decomposition perceptible?

MRS. B.

By placing in contact with the extremities of each wire, in the
solution, pieces of paper stained with certain vegetable colours, which
are altered by the contact of an acid or an alkali. Thus this blue
vegetable preparation called litmus becomes red when touched by an acid;
and the juice of violets becomes green by the contact of an alkali.

But the experiment can be made in a much more distinct manner, by
receiving the extremities of the wires into two different vessels, so
that the alkali shall appear in one vessel and the acid in the other.

CAROLINE.

But then the Voltaic circle will not be completed; how can any effect be
produced?

MRS. B.

You are right; I ought to have added that the two vessels must be
connected together by some interposed substance capable of conducting
electricity. A piece of moistened cotton-wick answers this purpose very
well. You see that the cotton (PLATE XIII. fig. 2. c.) has one end
immersed in one glass and the other end in the other, so as to establish
a communication between any fluids contained in them. We shall now put
into each of the glasses a little glauber salt, or sulphat of soda,
(which consists of an acid and an alkali,) and then we shall fill the
glasses with water, which will dissolve the salt. Let us now connect the
glasses by means of the wires (e, d,) with the two ends of the battery,
thus . . . .

  [Illustration: Plate XIII. Vol. II. page 16.

  Fig. 1. Voltaic Battery of improved construction with the Plates
    out of the Cells.

  Fig. 2. 3 & 4. Instances of Chemical decomposition by the Voltaic
    Battery.]

CAROLINE.

The wires are already giving out small bubbles; is this owing to the
decomposition of the salt?

MRS. B.

No; these are bubbles produced by the decomposition of the water, as you
saw in a former experiment. In order to render the separation of the
acid from the alkali visible, I pour into the glass (a), which is
connected with the positive wire, a few drops of a solution of litmus,
which the least quantity of acid turns red; and into the other
glass (b), which is connected with the negative wire, I pour a few drops
of the juice of violets . . . .

EMILY.

The blue solution is already turning red all round the wire.

CAROLINE.

And the violet solution is beginning to turn green. This is indeed very
singular!

MRS. B.

You will be still more astonished when we vary the experiment in this
manner:-- These three glasses (fig. 3. f, g, h,) are, as in the former
instance, connected together by wetted cotton, but the middle one alone
contains a saline solution, the two others containing only distilled
water, coloured as before by vegetable infusions. Yet, on making the
connection with the battery, the alkali will appear in the negative
glass (h), and the acid in the positive glass (f), though neither of
them contained any saline matter.

EMILY.

So that the acid and alkali must be conveyed right and left from the
central glass, into the other glasses, by means of the connecting
moistened cotton?

MRS. B.

Exactly so; and you may render the experiment still more striking, by
putting into the central glass (k, fig. 3.) an alkaline solution, the
glauber salt being placed into the negative glass (l), and the positive
glass (i) containing only water. The acid will be attracted by the
positive wire (m), and will actually appear in the vessel (i), after
passing through the alkaline solution (k), without combining with it,
although, you know, acids and alkalies are so much disposed to combine.
--But this conversation has already much exceeded our usual limits, and
we cannot enlarge more upon this interesting subject at present.




CONVERSATION XIV.

ON ALKALIES.


MRS. B.

Having now given you some idea of the laws by which chemical attractions
are governed, we may proceed to the examination of bodies which are
formed in consequence of these attractions.

The first class of compounds that present themselves to our notice, in
our gradual ascent to the most complicated combinations, are bodies
composed of only two principles. The sulphurets, phosphurets, carburets,
&c. are of this description; but the most numerous and important of
these compounds are the combinations of oxygen with the various simple
substances with which it has a tendency to unite. Of these you have
already acquired some knowledge, but it will be necessary to enter into
further particulars respecting the nature and properties of those most
deserving our notice. Of this class are the ALKALIES and the EARTHS,
which we shall successively examine.

We shall first take a view of the alkalies, of which there are three,
viz. POTASH, SODA, and AMMONIA. The two first are called _fixed
alkalies_, because they exist in a solid form at the temperature of the
atmosphere, and require a great heat to be volatilised. They consist, as
you already know, of metallic bases combined with oxygen. In potash, the
proportions are about eighty-six parts of potassium to fourteen of
oxygen; and in soda, seventy-seven parts of sodium to twenty-three of
oxygen. The third alkali, ammonia, has been distinguished by the name of
_volatile alkali_, because its natural form is that of gas. Its
composition is of a more complicated nature, of which we shall speak
hereafter.

Some of the earths bear so strong a resemblance in their properties to
the alkalies, that it is difficult to know under which head to place
them. The celebrated French chemist, Fourcroy, has classed two of them
(barytes and strontites) with the alkalies; but as lime and magnesia
have almost an equal title to that rank, I think it better not to
separate them, and therefore have adopted the common method of classing
them with the earths, and of distinguishing them by the name of
_alkaline earths_.

The general properties of alkalies are, an acrid burning taste,
a pungent smell, and a caustic action on the skin and flesh.

CAROLINE.

I wonder they should be caustic, Mrs. B., since they contain so little
oxygen.

MRS. B.

Whatever substance has an affinity for any one of the constituents of
animal matter, sufficiently powerful to decompose it, is entitled to the
appellation of caustic. The alkalies, in their pure state, have a very
strong attraction for water, for hydrogen, and for carbon, which, you
know, are the constituent principles of oil, and it is chiefly by
absorbing these substances from animal matter that they effect its
decomposition; for, when diluted with a sufficient quantity of water, or
combined with any oily substance, they lose their causticity.

But, to return to the general properties of alkalies--they change, as we
have already seen, the colour of syrup of violets, and other blue
vegetable infusions, to green; and have, in general, a very great
tendency to unite with acids, although the respective qualities of these
two classes of bodies form a remarkable contrast.

We shall examine the result of the combination of acids and alkalies
more particularly hereafter. It will be sufficient at present to inform
you, that whenever acids are brought in contact with alkalies, or
alkaline earths, they unite with a remarkable eagerness, and form
compounds perfectly different from either of their constituents; these
bodies are called _neutral_ or _compound salts_.

The dry white powder which you see in this phial is pure caustic POTASH;
it is very difficult to preserve it in this state, as it attracts, with
extreme avidity, the moisture from the atmosphere, and if the air were
not perfectly excluded, it would, in a very short time, be actually
melted.

EMILY.

It is then, I suppose, always found in a liquid state?

MRS. B.

No; it exists in nature in a great variety of forms and combinations,
but is never found in its pure separate state; it is combined with
carbonic acid, with which it exists in every part of the vegetable
kingdom, and is most commonly obtained from the ashes of vegetables,
which are the residue that remains after all the other parts have been
volatilised by combustion.

CAROLINE.

But you once said, that after all the volatile parts of a vegetable were
evaporated, the substance that remained was charcoal?

MRS. B.

I am surprised that you should still confound the processes of
volatilisation and combustion. In order to procure charcoal, we
evaporate such parts as can be reduced to vapour by the operation of
heat alone; but when we _burn_ the vegetable, we burn the carbon also,
and convert it into carbonic acid gas.

CAROLINE.

That is true; I hope I shall make no more mistakes in my favourite
theory of combustion.

MRS. B.

Potash derives its name from the _pots_ in which the vegetables, from
which it was obtained, used formerly to be burnt; the alkali remained
mixed with the ashes at the bottom, and was thence called potash.

EMILY.

The ashes of a wood-fire, then, are potash, since they are vegetable
ashes?

MRS. B.

They always contain more or less potash, but are very far from
consisting of that substance alone, as they are a mixture of various
earths and salts which remain after the combustion of vegetables, and
from which it is not easy to separate the alkali in its pure form. The
process by which potash is obtained, even in the imperfect state in
which it is used in the arts, is much more complicated than simple
combustion. It was once deemed impossible to separate it entirely from
all foreign substances, and it is only in chemical laboratories that it
is to be met with in the state of purity in which you find it in this
phial. Wood-ashes are, however, valuable for the alkali which they
contain, and are used for some purposes without any further preparation.
Purified in a certain degree, they make what is commonly called
_pearlash_, which is of great efficacy in taking out grease, in washing
linen, &c.; for potash combines readily with oil or fat, with which it
forms a compound well known to you under the name of _soap_.

CAROLINE.

Really! Then I should think it would be better to wash all linen with
pearlash than with soap, as, in the latter case, the alkali being
already combined with oil, must be less efficacious in extracting
grease.

MRS. B.

Its effect would be too powerful on fine linen, and would injure its
texture; pearlash is therefore only used for that which is of a strong
coarse kind. For the same reason you cannot wash your hands with plain
potash; but, when mixed with oil in the form of soap, it is soft as well
as cleansing, and is therefore much better adapted to the purpose.

Caustic potash, as we already observed, acts on the skin, and animal
fibre, in virtue of its attraction for water and oil, and converts all
animal matter into a kind of saponaceous jelly.

EMILY.

Are vegetables the only source from which potash can be derived?

MRS. B.

No: for though far most abundant in vegetables, it is by no means
confined to that class of bodies, being found also on the surface of the
earth, mixed with various minerals, especially with earths and stones,
whence it is supposed to be conveyed into vegetables by the roots of the
plant. It is also met with, though in very small quantities, in some
animal substances. The most common state of potash is that of
_carbonat_; I suppose you understand what that is?

EMILY.

I believe so; though I do not recollect that you ever mentioned the word
before. If I am not mistaken, it must be a compound salt, formed by the
union of carbonic acid with potash.

MRS. B.

Very true; you see how admirably the nomenclature of modern chemistry is
adapted to assist the memory; when you hear the name of a compound, you
necessarily learn what are its constituent parts; and when you are
acquainted with these constituents, you can immediately name the
compound which they form.

CAROLINE.

Pray, how were bodies arranged and distinguished before this
nomenclature was introduced?

MRS. B.

Chemistry was then a much more difficult study; for every substance had
an arbitrary name, which it derived either from the person who
discovered it, as _Glauber's salts_ for instance; or from some other
circumstance relative to it, though quite unconnected with its real
nature, as potash.

These names have been retained for some of the simple bodies; for as
this class is not numerous, and therefore can easily be remembered, it
has not been thought necessary to change them.

EMILY.

Yet I think it would have rendered the new nomenclature more complete to
have methodised the names of the elementary, as well as of the compound
bodies, though it could not have been done in the same manner. But the
names of the simple substances might have indicated their nature, or, at
least, some of their principal properties; and if, like the acids and
compound salts, all the simple bodies had a similar termination, they
would have been immediately known as such. So complete and regular a
nomenclature would, I think, have given a clearer and more comprehensive
view of chemistry than the present, which is a medley of the old and new
terms.

MRS. B.

But you are not aware of the difficulty of introducing into science an
entire set of new terms; it obliges all the teachers and professors to
go to school again, and if some of the old names, that are least
exceptionable, were not left as an introduction to the new ones, few
people would have had industry and perseverance enough to submit to the
study of a completely new language; and the inferior classes of artists,
who can only act from habit and routine, would, at least for a time,
have felt material inconvenience from a total change of their habitual
terms. From these considerations, Lavoisier and his colleagues, who
invented the new nomenclature, thought it most prudent to leave a few
links of the old chain, in order to connect it with the new one.
Besides, you may easily conceive the inconvenience which might arise
from giving a regular nomenclature to substances, the simple nature of
which is always uncertain; for the new names might, perhaps, have proved
to have been founded in error. And, indeed, cautious as the inventors of
the modern chemical language have been, it has already been found
necessary to modify it in many respects. In those few cases, however, in
which new terms have been adopted to designate simple bodies, these
names have been so contrived as to indicate one of the chief properties
of the body in question; this is the case with oxygen, which, as I
explained to you, signifies generator of acids; and hydrogen generator
of water. If all the elementary bodies had a similar termination, as you
propose, it would be necessary to change the name of any that might
hereafter be found of a compound nature, which would be very
inconvenient in this age of discovery.

But to return to the alkalies. --We shall now try to melt some of this
caustic potash in a little water, as a circumstance occurs during its
solution very worthy of observation. --Do you feel the heat that is
produced?

CAROLINE.

Yes, I do; but is not this directly contrary to our theory of latent
heat, according to which heat is disengaged when fluids become solid,
and cold produced when solids are melted?

MRS. B.

The latter is really the case in all solutions; and if the solution of
caustic alkalies seems to make an exception to the rule, it does not,
I believe, form any solid objection to the theory. The matter may be
explained thus: When water first comes in contact with the potash, it
produces an effect similar to the slaking of lime, that is, the water is
solidified in combining with the potash, and thus loses its latent heat;
this is the heat that you now feel, and which is, therefore, produced
not by the melting of the solid, but by the solidification of the fluid.
But when there is more water than the potash can absorb and solidify,
the latter then yields to the solvent power of the water; and if we do
not perceive the cold produced by its melting, it is because it is
counterbalanced by the heat previously disengaged.*

A very remarkable property of potash is the formation of glass by its
fusion with siliceous earth. You are not yet acquainted with this last
substance, further than its being in the list of simple bodies. It is
sufficient, for the present, that you should know that sand and flint
are chiefly composed of it; alone, it is infusible, but mixed with
potash, it melts when exposed to the heat of a furnace, combines with
the alkali, and runs into glass.

    [Footnote *: This defence of the general theory, however
    plausible, is liable to some obvious objections. The phenomenon
    might perhaps be better accounted for by supposing that a solution
    of alkali in water has less capacity for heat than either water or
    alkali in their separate state.]

CAROLINE.

Who would ever have supposed that the same substance which converts
transparent oil into such an opake body as soap, should transform that
opake substance, sand, into transparent glass!

MRS. B.

The transparency, or opacity of bodies, does not, I conceive, depend so
much upon their intimate nature, as upon the arrangement of their
particles: we cannot have a more striking instance of this, than is
afforded by the different states of carbon, which, though it commonly
appears in the form of a black opake body, sometimes assumes the most
dazzling transparent form in nature, that of diamond, which, you
recollect, is carbon, and which, in all probability, derives its
beautiful transparency from the peculiar arrangement of its particles
during their crystallisation.

EMILY.

I never should have supposed that the formation of glass was so simple a
process as you describe it.

MRS. B.

It is by no means an easy operation to make perfect glass; for if the
sand, or flint, from which the siliceous earth is obtained, be mixed
with any metallic particles, or other substance, which cannot be
vitrified, the glass will be discoloured, or defaced, by opake specks.

CAROLINE.

That, I suppose, is the reason why objects so often appear irregular and
shapeless through a common glass-window.

MRS. B.

This species of imperfection proceeds, I believe, from another cause. It
is extremely difficult to prevent the lower part of the vessels, in
which the materials of glass are fused, from containing a more dense
vitreous matter than the upper, on account of the heavier ingredients
falling to the bottom. When this happens, it occasions the appearance of
veins or waves in the glass, from the difference of density in its
several parts, which produces an irregular refraction of the rays of
light that pass through it.

Another species of imperfection sometimes arises from the fusion not
being continued for a length of time sufficient to combine the two
ingredients completely, or from the due proportion of potash and silex
(which are as two to one) not being carefully observed; the glass, in
those cases, will be liable to alteration from the action of the air, of
salts, and especially of acids, which will effect its decomposition by
combining with the potash, and forming compound salts.

EMILY.

What an extremely useful substance potash is!

MRS. B.

Besides the great importance of potash in the manufactures of glass and
soap, it is of very considerable utility in many of the other arts, and
in its combinations with several acids, particularly the nitric, with
which it forms saltpetre.

CAROLINE.

Then saltpetre must be a _nitrat of potash_? But we are not yet
acquainted with the nitric acid?

MRS. B.

We shall therefore defer entering into the particulars of these
combinations till we come to a general review of the compound salts. In
order to avoid confusion, it will be better at present to confine
ourselves to the alkalies.

EMILY.

Cannot you show us the change of colour which you said the alkalies
produced on blue vegetable infusions?

MRS. B.

Yes; very easily. I shall dip a piece of white paper into this syrup of
violets, which, you see, is of a deep blue, and dyes the paper of the
same colour. --As soon as it is dry, we shall dip it into a solution of
potash, which, though itself colourless, will turn the paper green--

CAROLINE.

So it has, indeed! And do the other alkalies produce a similar effect?

MRS. B.

Exactly the same. --We may now proceed to SODA, which, however
important, will detain us but a very short time; as in all its general
properties it very strongly resembles potash; indeed, so great is their
similitude, that they have been long confounded, and they can now
scarcely be distinguished, except by the difference of the salts which
they form with acids.

The great source of this alkali is the sea, where, combined with a
peculiar acid, it forms the salt with which the waters of the ocean are
so strongly impregnated.

EMILY.

Is not that the common table salt?

MRS. B.

The very same; but again we must postpone entering into the particulars
of this interesting combination, till we treat of the neutral salts.
Soda may be obtained from common salt; but the easiest and most usual
method of procuring it is by the combustion of marine plants, an
operation perfectly analogous to that by which potash is obtained from
vegetables.

EMILY.

From what does soda derive its name?

MRS. B.

From a plant called by us _soda_, and by the Arabs _kali_, which affords
it in great abundance. Kali has, indeed, given its name to the alkalies
in general.

CAROLINE.

Does soda form glass and soap in the same manner as potash?

MRS. B.

Yes, it does; it is of equal importance in the arts, and is even
preferred to potash for some purposes; but you will not be able to
distinguish their properties till we examine the compound salts which
they form with acids; we must therefore leave soda for the present, and
proceed to AMMONIA, or the VOLATILE ALKALI.

EMILY.

I long to hear something of this alkali; is it not of the same nature as
hartshorn?

MRS. B.

Yes, it is, as you will see by-and-bye. This alkali is seldom found in
nature in its pure state; it is most commonly extracted from a compound
salt, called _sal ammoniac_, which was formerly imported from _Ammonia_,
a region of Libya, from which both these salts and the alkali derive
their names. The crystals contained in this bottle are specimens of this
salt, which consists of a combination of ammonia and muriatic acid.

CAROLINE.

Then it should be called _muriat of ammonia_; for though I am ignorant
what muriatic acid is, yet I know that its combination with ammonia
cannot but be so called; and I am surprised to see sal ammoniac
inscribed on the label.

MRS. B.

That is the name by which it has been so long known, that the modern
chemists have not yet succeeded in banishing it altogether; and it is
still sold under that name by druggists, though by scientific chemists
it is more properly called muriat of ammonia.

CAROLINE.

Both the popular and the common name should be inscribed on labels--this
would soon introduce the new nomenclature.

EMILY.

By what means can the ammonia be separated from the muriatic acid?

MRS. B.

By chemical attractions; but this operation is too complicated for you
to understand, till you are better acquainted with the agency of
affinities.

EMILY.

And when extracted from the salt, what kind of substance is ammonia?

MRS. B.

Its natural form, at the temperature of the atmosphere, when free from
combination, is that of gas; and in this state it is called _ammoniacal
gas_. But it mixes very readily with water, and can be thus obtained in
a liquid form.

CAROLINE.

You said that ammonia was more complicated in its composition than the
other alkalies; pray of what principles does it consist?

MRS. B.

It was discovered a few years since, by Berthollet, a celebrated French
chemist, that it consisted of about one part of hydrogen to four parts
of nitrogen. Having heated ammoniacal gas under a receiver, by causing
the electrical spark to pass repeatedly through it, he found that it
increased considerably in bulk, lost all its alkaline properties, and
was actually converted into hydrogen and nitrogen gases; and from the
latest and most accurate experiments, the proportions appear to be, one
volume of nitrogen gas to three of hydrogen gas.

CAROLINE.

Ammonia, therefore, has not, like the two other alkalies, a metallic
basis?

MRS. B.

It is believed it has, though it is extremely difficult to reconcile
that idea with what I have just stated of its chemical nature. But the
fact is, that although this supposed metallic basis of ammonia has never
been obtained distinct and separate, yet both Professor Berzelius, of
Stockholm, and Sir H. Davy, have succeeded in forming a combination of
mercury with the basis of ammonia, which has so much the appearance of
an amalgam, that it strongly corroborates the idea of ammonia having a
metallic basis.* But these theoretical points are full of difficulties
and doubts, and it would be useless to dwell any longer upon them.

Let us therefore return to the properties of volatile alkali. Ammoniacal
gas is considerably lighter than oxygen gas, and only about half the
weight of atmospherical air. It possesses most of the properties of the
fixed alkalies; but cannot be of so much use in the arts on account of
its volatile nature. It is, therefore, never employed in the manufacture
of glass, but it forms soap with oils equally as well as potash and
soda; it resembles them likewise in its strong attraction for water; for
which reason it can be collected in a receiver over mercury only.

    [Footnote *: This amalgam is easily obtained, by placing a globule
    of mercury upon a piece of muriat, or carbonat of ammonia, and
    electrifying this globule by the Voltaic battery. The globule
    instantly begins to expand to three or four times its former size,
    and becomes much less fluid, though without losing its metallic
    lustre, a change which is ascribed to the metallic basis of
    ammonia uniting with the mercury. This is an extremely curious
    experiment.]

CAROLINE.

I do not understand this?

MRS. B.

Do you recollect the method which we used to collect gases in a
glass-receiver over water?

CAROLINE.

Perfectly.

MRS. B.

Ammoniacal gas has so strong a tendency to unite with water, that,
instead of passing through that fluid, it would be instantaneously
absorbed by it. We can therefore neither use water for that purpose, nor
any other liquid of which water is a component part; so that, in order
to collect this gas, we are obliged to have recourse to mercury,
(a liquid which has no action upon it,) and a mercurial bath is used
instead of a water bath, such as we employed on former occasions. Water
impregnated with this gas is nothing more than the fluid which you
mentioned at the beginning of the conversation--hartshorn; it is the
ammoniacal gas escaping from the water which gives it so powerful a
smell.

EMILY.

But there is no appearance of effervescence in hartshorn.

MRS. B.

Because the particles of gas that rise from the water are too subtle and
minute for their effect to be visible.

Water diminishes in density, by being impregnated with ammoniacal gas;
and this augmentation of bulk increases its capacity for caloric.

EMILY.

In making hartshorn, then, or impregnating water with ammonia, heat must
be absorbed, and cold produced?

MRS. B.

That effect would take place if it was not counteracted by another
circumstance; the gas is liquefied by incorporating with the water, and
gives out its latent heat. The condensation of the gas more than
counterbalances the expansion of the water; therefore, upon the whole,
heat is produced. --But if you dissolve ammoniacal gas with ice or snow,
cold is produced. --Can you account for that?

EMILY.

The gas, in being condensed into a liquid, must give out heat; and, on
the other hand, the snow or ice, in being rarefied into a liquid, must
absorb heat; so that, between the opposite effects, I should have
supposed the original temperature would have been preserved.

MRS. B.

But you have forgotten to take into the account the rarefaction of the
water (or melted ice) by the impregnation of the gas; and this is the
cause of the cold which is ultimately produced.

CAROLINE.

Is the _sal volatile_ (the smell of which so strongly resembles
hartshorn) likewise a preparation of ammonia?

MRS. B.

It is carbonat of ammonia dissolved in water; and which, in its concrete
state, is commonly called salts of hartshorn. Ammonia is caustic, like
the fixed alkalies, as you may judge by the pungent effects of
hartshorn, which cannot be taken internally, nor applied to delicate
external parts, without being plentifully diluted with water. --Oil and
acids are very excellent antidotes for alkaline poisons; can you guess
why?

CAROLINE.

Perhaps, because the oil combines with the alkali, and forms soap, and
thus destroys its caustic properties; and the acid converts it into a
compound salt, which, I suppose, is not so pernicious as caustic alkali.

MRS. B.

Precisely so.

Ammoniacal gas, if it be mixed with atmospherical air, and a burning
taper repeatedly plunged into it, will burn with a large flame of a
peculiar yellow colour.

EMILY.

But pray tell me, can ammonia be procured from this Lybian salt only?

MRS. B.

So far from it, that it is contained in, and may be extracted from, all
animal substances whatever. Hydrogen and nitrogen are two of the chief
constituents of animal matter; it is therefore not surprising that they
should occasionally meet and combine in those proportions that compose
ammonia. But this alkali is more frequently generated by the spontaneous
decomposition of animal substances; the hydrogen and nitrogen gases that
arise from putrefied bodies combine, and form the volatile alkali.

Muriat of ammonia, instead of being exclusively brought from Lybia, as
it originally was, is now chiefly prepared in Europe, by chemical
processes. Ammonia, although principally extracted from this salt, can
also be produced by a great variety of other substances. The horns of
cattle, especially those of deer, yield it in abundance, and it is from
this circumstance that a solution of ammonia in water has been called
hartshorn. It may likewise be procured from wool, flesh, and bones; in a
word, any animal substance whatever yields it by decomposition.

We shall now lay aside the alkalies, however important the subject may
be, till we treat of their combination with acids. The next time we meet
we shall examine the earths.




CONVERSATION XV.

ON EARTHS.


MRS. B.

The EARTHS, which we are to-day to examine, are nine in number:

  SILEX,
  ALUMINE,
  BARYTES,
  LIME,
  MAGNESIA,
  STRONTITES,
  YTTRIA,
  GLUCINA,
  ZIRCONIA.

The last three are of late discovery; their properties are but
imperfectly known; and, as they have not yet been applied to use, it
will be unnecessary to enter into any particulars respecting them; we
shall confine our remarks, therefore, to the first five. They are
composed, as you have already learnt, of a metallic basis combined with
oxygen; and, from this circumstance, are incombustible.

CAROLINE.

Yet I have seen turf burnt in the country, and it makes an excellent
fire; the earth becomes red hot, and produces a very great quantity of
heat.

MRS. B.

It is not the earth that burns, my dear, but the roots, grass, and other
remnants of vegetables that are intermixed with it. The caloric, which
is produced by the combustion of these substances, makes the earth red
hot, and this being a bad conductor of heat, retains its caloric a long
time; but were you to examine it when cooled, you would find that it had
not absorbed one particle of oxygen, nor suffered any alteration from
the fire. Earth is, however, from the circumstance just mentioned, an
excellent radiator of heat, and owes its utility, when mixed with fuel,
solely to that property. It is in this point of view that Count Rumford
has recommended balls of incombustible substances to be arranged in
fire-places, and mixed with the coals, by which means the caloric
disengaged by the combustion of the latter is more perfectly reflected
into the room, and an expense of fuel is saved.

EMILY.

I expected that the list of earths would be much more considerable. When
I think of the great variety of soils, I am astonished that there is not
a greater number of earths to form them.

MRS. B.

You might, indeed, almost confine that number to four; for barytes,
strontites, and the others of late discovery, act but so small a part in
this great theatre, that they cannot be reckoned as essential to the
general formation of the globe. And you must not confine your idea of
earths to the formation of soil; for rock, marble, chalk, slate, sand,
flint, and all kinds of stones, from the precious jewels to the
commonest pebbles; in a word, all the immense variety of mineral
products, may be referred to some of these earths, either in a simple
state, or combined the one with the other, or blended with other
ingredients.

CAROLINE.

Precious stones composed of earth! That seems very difficult to
conceive.

EMILY.

Is it more extraordinary than that the most precious of all jewels,
diamond, should be composed of carbon? But diamond forms an exception,
Mrs. B.; for, though a stone, it is not composed of earth.

MRS. B.

I did not specify the exception, as I knew you were so well acquainted
with it. Besides, I would call a diamond a mineral rather than a stone,
as the latter term always implies the presence of some earth.

CAROLINE.

I cannot conceive how such coarse materials can be converted into such
beautiful productions.

MRS. B.

We are very far from understanding all the secret resources of nature;
but I do not think the spontaneous formation of the crystals, which we
call precious stones, one of the most difficult phenomena to comprehend.

By the slow and regular work of ages, perhaps of hundreds of ages, these
earths may be gradually dissolved by water, and as gradually deposited
by their solvent in the undisturbed process of crystallisation. The
regular arrangement of their particles, during their reunion in a solid
mass, gives them that brilliancy, transparency, and beauty, for which
they are so much admired; and renders them in appearance so totally
different from their rude and primitive ingredients.

CAROLINE.

But how does it happen that they are spontaneously dissolved, and
afterwards crystallised?

MRS. B.

The scarcity of many kinds of crystals, as rubies, emeralds, topazes,
&c. shows that their formation is not an operation very easily carried
on in nature. But cannot you imagine that when water, holding in
solution some particles of earth, filters through the crevices of hills
or mountains, and at length dribbles into some cavern, each successive
drop may be slowly evaporated, leaving behind it the particle of earth
which it held in solution? You know that crystallisation is more regular
and perfect, in proportion as the evaporation of the solvent is slow and
uniform; nature, therefore, who knows no limit of time, has, in all
works of this kind, an infinite advantage over any artist who attempts
to imitate such productions.

EMILY.

I can now conceive that the arrangement of the particles of earth,
during crystallisation, may be such as to occasion transparency, by
admitting a free passage to the rays of light; but I cannot understand
why crystallised earths should assume such beautiful colours as most of
them do. Sapphire, for instance, is of a celestial blue; ruby, a deep
red; topaz, a brilliant yellow?

MRS. B.

Nothing is more simple than to suppose that the arrangement of their
particles is such, as to transmit some of the coloured rays of light,
and to reflect others, in which case the stone must appear of the colour
of the rays which it reflects. But besides, it frequently happens that
the colour of a stone is owing to a mixture of some metallic matter.

CAROLINE.

Pray, are the different kinds of precious stones each composed of one
individual earth, or are they formed of a combination of several earths?

MRS. B.

A great variety of materials enters into the composition of most of
them; not only several earths, but sometimes salts and metals. The
earths, however, in their simple state, frequently form very beautiful
crystals; and, indeed, it is in that state only that they can be
obtained perfectly pure.

EMILY.

Is not the Derbyshire spar produced by the crystallisation of earths, in
the way you have just explained? I have been in some of the
subterraneous caverns where it is found, which are similar to those you
have described.

MRS. B.

Yes; but this spar is a very imperfect specimen of crystallisation; it
consists of a variety of ingredients confusedly blended together, as you
may judge by its opacity, and by the various colours and appearances
which it exhibits.

But, in examining the earths in their most perfect and agreeable form,
we must not lose sight of that state in which they are commonly found,
and which, if less pleasing to the eye, is far more interesting by its
utility.

All the earths are more or less endowed with alkaline properties; but
there are four, barytes, magnesia, lime, and strontites, which are
called _alkaline earths_, because they possess those qualities in so
great a degree, as to entitle them, in most respects, to the rank of
alkalies. They combine and form compound salts with acids, in the same
way as alkalies; they are, like them, susceptible of a considerable
degree of causticity, and are acted upon in a similar manner by chemical
tests. --The remaining earths, silex and alumine, with one or two others
of late discovery, are in some degree more earthy, that is to say, they
possess more completely the properties common to all the earths, which
are, insipidity, dryness, unalterableness in the fire, infusibility, &c.

CAROLINE.

Yet, did you not tell us that silex, or siliceous earth, when mixed with
an alkali, was fusible, and run into glass?

MRS. B.

Yes, my dear; but the characteristic properties of earths, which I have
mentioned, are to be considered as belonging to them in a state of
purity only; a state in which they are very seldom to be met with in
nature. --Besides these general properties, each earth has its own
specific characters, by which it is distinguished from any other
substance. --Let us therefore review them separately.


SILEX, or SILICA, abounds in flint, sand, sandstone, agate, jasper, &c.;
it forms the basis of many precious stones, and particularly of those
which strike fire with steel. It is rough to the touch, scratches and
wears away metals; it is acted upon by no acid but the fluoric, and is
not soluble in water by any known process; but nature certainly
dissolves it by means with which we are unacquainted, and thus produces
a variety of siliceous crystals, and amongst these _rock crystal_, which
is the purest specimen of this earth. Silex appears to have been
intended by Providence to form the solid basis of the globe, to serve as
a foundation for the original mountains, and give them that hardness and
durability which has enabled them to resist the various revolutions
which the surface of the earth has successively undergone. From these
mountains siliceous rocks have, during the course of ages, been
gradually detached by torrents of water, and brought down in fragments;
these, in the violence and rapidity of their descent, are sometimes
crumbled to sand, and in this state form the beds of rivers and of the
sea, chiefly composed of siliceous materials. Sometimes the fragments
are broken without being pulverised by their fall, and assume the form
of pebbles, which gradually become rounded and polished.

EMILY.

Pray what is the true colour of silex, which forms such a variety of
different coloured substances? Sand is brown, flint is nearly black, and
precious stones are of all colours.

MRS. B.

Pure silex, such as is found only in the chemist's laboratory, is
perfectly white, and the various colours which it assumes, in the
different substances you have just mentioned, proceed from the different
ingredients with which it is mixed in them.

CAROLINE.

I wonder that silex is not more valuable, since it forms the basis of so
many precious stones.

MRS. B.

You must not forget that the value we set upon precious stones depends
in a great measure upon the scarcity with which nature affords them;
for, were those productions either common or perfectly imitable by art,
they would no longer, notwithstanding their beauty, be so highly
esteemed. But the real value of siliceous earth, in many of the most
useful arts, is very extensive. Mixed with clay, it forms the basis of
all the various kinds of earthen ware, from the most common utensils to
the most refined ornaments.

EMILY.

And we must recollect its importance in the formation of glass with
potash.

MRS. B.

Nor should we omit to mention, likewise, many other important uses of
silex, such as being the chief ingredient of some of the most durable
cements, of mortar, &c.

I said before, that siliceous earth combined with no acid but the
fluoric; it is for this reason that glass is liable to be attacked by
that acid only, which, from its strong affinity for silex, forces that
substance from its combination with the potash, and thus destroys the
glass.

We will now hasten to proceed to the other earths, for I am rather
apprehensive of your growing weary of this part of our subject.

CAROLINE.

The history of the earths is not quite so entertaining as that of the
simple substances.

MRS. B.

Perhaps not; but it is absolutely indispensable that you should know
something of them; for they form the basis of so many interesting and
important compounds, that their total omission would throw great
obscurity on our general outline of chemical science. We shall, however,
review them in as cursory a manner as the subject can admit of.


ALUMINE derives its name from a compound salt called _alum_, of which it
forms the basis.

CAROLINE.

But it ought to be just the contrary, Mrs. B.; the simple body should
give, instead of taking, its name from the compound.

MRS. B.

That is true; but as the compound salt was known long before its basis
was discovered, it was very natural that when the earth was at length
separated from the acid, it should derive its name from the compound
from which it was obtained. However, to remove your scruples, we will
call the salt according to the new nomenclature, _sulphat of alumine_.
From this combination, alumine may be obtained in its pure state; it is
then soft to the touch, makes a paste with water, and hardens in the
fire. In nature, it is found chiefly in clay, which contains a
considerable proportion of this earth; it is very abundant in fuller's
earth, slate, and a variety of other mineral productions. There is
indeed scarcely any mineral substance more useful to mankind than
alumine. In the state of clay, it forms large strata of the earth, gives
consistency to the soil of valleys, and of all low and damp spots, such
as swamps and marshes. The beds of lakes, ponds, and springs, are almost
entirely of clay; instead of allowing of the filtration of water, as
sand does, it forms an impenetrable bottom, and by this means water is
accumulated in the caverns of the earth, producing those reservoirs
whence springs issue, and spout out at the surface.

EMILY.

I always thought that these subterraneous reservoirs of water were
bedded by some hard stone, or rock, which the water could not penetrate.

MRS. B.

That is not the case; for in the course of time water would penetrate,
or wear away silex, or any other kind of stone, while it is effectually
stopped by clay, or alumine.

The solid compact soils, such as are fit for corn, owe their consistence
in a great measure to alumine; this earth is therefore used to improve
sandy or chalky soils, which do not retain a sufficient quantity of
water for the purpose of vegetation.

Alumine is the most essential ingredient in all potteries. It enters
into the composition of brick, as well as that of the finest porcelain;
the addition of silex and water hardens it, renders it susceptible of a
degree of vitrification, and makes it perfectly fit for its various
purposes.

CAROLINE.

I can scarcely conceive that brick and china should be made of the same
materials.

MRS. B.

Brick consists almost entirely of baked clay; but a certain proportion
of silex is essential to the formation of earthen or stone ware. In
common potteries sand is used for that purpose; a more pure silex is,
I believe, necessary for the composition of porcelain, as well as a
finer kind of clay; and these materials are, no doubt, more carefully
prepared, and curiously wrought, in the one case than in the other.
Porcelain owes its beautiful semitransparency to a commencement of
vitrification.

EMILY.

But the commonest earthen-ware, though not transparent, is covered with
a kind of glazing.

MRS. B.

That precaution is equally necessary for use as for beauty, as the ware
would be liable to be spoiled and corroded by a variety of substances,
if not covered with a coating of this kind. In porcelain it consists of
enamel, which is a fine white opake glass, formed of metallic oxyds,
sand, salts, and such other materials as are susceptible of
vitrification. The glazing of common earthen-ware is made chiefly of
oxyd of lead, or sometimes merely of salt, which, when thinly spread
over earthen vessels, will, at a certain heat, run into opake glass.

CAROLINE.

And of what nature are the colours which are used for painting
porcelain?

MRS. B.

They are all composed of metallic oxyds, so that these colours, instead
of receiving injury from the application of fire, are strengthened and
developed by its action, which causes them to undergo different degrees
of oxydation.

Alumine and silex are not only often combined by art, but they have in
nature a very strong tendency to unite, and are found combined, in
different proportions, in various gems and other minerals. Indeed, many
of the precious stones, such as ruby, oriental sapphire, amethyst, &c.
consist chiefly of alumine.


We may now proceed to the alkaline earths, I shall say but a few words
on BARYTES, as it is hardly ever used, except in chemical laboratories.
It is remarkable for its great weight, and its strong alkaline
properties, such as destroying animal substances, turning green some
blue vegetable colours, and showing a powerful attraction for acids;
this last property it possesses to such a degree, particularly with
regard to the sulphuric acid, that it will always detect its presence in
any substance or combination whatever, by immediately uniting with it,
and forming a sulphat of barytes. This renders it a very valuable
chemical test. It is found pretty abundantly in nature in the state of
carbonat, from which the pure earth can be easily separated.


The next earth we have to consider is LIME. This is a substance of too
great and general importance to be passed over so slightly as the last.

Lime is strongly alkaline. In nature it is not met with in its simple
state, as its affinity for water and carbonic acid is so great, that it
is always found combined with these substances, with which it forms the
common lime-stone; but it is separated in the kiln from these
ingredients, which are volatilised whenever a sufficient degree of heat
is applied.

EMILY.

Pure lime, then, is nothing but lime-stone, which has been deprived, in
the kiln, of its water and carbonic acid?

MRS. B.

Precisely: in this state it is called _quick-lime_, and it is so
caustic, that it is capable of decomposing the dead bodies of animals
very rapidly, without their undergoing the process of putrefaction.
--I have here some quick lime, which is kept carefully corked up in a
bottle to prevent the access of air; for were it at all exposed to the
atmosphere, it would absorb both moisture and carbonic acid gas from it,
and be soon slaked. Here is also some lime-stone--we shall pour a little
water on each, and observe the effects that result from it.

CAROLINE.

How the quick-lime hisses! It is become excessively hot! --It swells,
and now it bursts and crumbles to powder, while the water appears to
produce no kind of alteration on the lime-stone.

MRS. B.

Because the lime-stone is already saturated with water, whilst the
quick-lime, which has been deprived of it in the kiln, combines with it
with very great avidity, and produces this prodigious disengagement of
heat, the cause of which I formerly explained to you; do you
recollect it?

EMILY.

Yes; you said that the heat did not proceed from the lime, but from the
water which was _solidified_, and thus parted with its heat of
liquidity.

MRS. B.

Very well. If we continue to add successive quantities of water to the
lime after being slaked and crumbled as you see, it will then gradually
be diffused in the water, till it will at length be dissolved in it, and
entirely disappear; but for this purpose it requires no less than 700
times its weight of water. This solution is called _lime-water_.

CAROLINE.

How very small, then, is the proportion of lime dissolved!

MRS. B.

Barytes is still of more difficult solution; it dissolves only in 900
times its weight of water: but it is much more soluble in the state of
crystals. The liquid contained in this bottle is lime-water; it is often
used as a medicine, chiefly, I believe, for the purpose of combining
with, and neutralising, the superabundant acid which it meets with in
the stomach.

EMILY.

I am surprised that it is so perfectly clear; it does not at all partake
of the whiteness of the lime.

MRS. B.

Have you forgotten that, in solutions, the solid body is so minutely
subdivided by the fluid as to become invisible, and therefore will not
in the least degree impair the transparency of the solvent?

I said that the attraction of lime for carbonic acid was so strong, that
it would absorb it from the atmosphere. We may see this effect by
exposing a glass of lime-water to the air; the lime will then separate
from the water, combine with the carbonic acid, and re-appear on the
surface in the form of a white film, which is carbonat of lime, commonly
called _chalk_.

CAROLINE.

Chalk is, then, a compound salt! I never should have supposed that those
immense beds of chalk, that we see in many parts of the country, were a
salt. --Now, the white film begins to appear on the surface of the
water; but it is far from resembling hard solid chalk.

MRS. B.

That is owing to its state of extreme division; in a little time it will
collect into a more compact mass, and subside at the bottom of the
glass.

If you breathe into lime-water, the carbonic acid, which is mixed with
the air that you expire, will produce the same effect. It is an
experiment very easily made; --I shall pour some lime-water into this
glass tube, and, by breathing repeatedly into it, you will soon perceive
a precipitation of chalk--

EMILY.

I see already a small white cloud formed.

MRS. B.

It is composed of minute particles of chalk; at present it floats in the
water, but it will soon subside.

Carbonat of lime, or chalk, you see, is insoluble in water, since the
lime which was dissolved re-appears when converted into chalk; but you
must take notice of a very singular circumstance, which is, that chalk
is soluble in water impregnated with carbonic acid.

CAROLINE.

It is very curious, indeed, that carbonic acid gas should render lime
soluble in one instance, and insoluble in the other!

MRS. B.

I have here a bottle of Seltzer water, which, you know, is strongly
impregnated with carbonic acid:-- let us pour a little of it into a
glass of lime-water. You see that it immediately forms a precipitation
of carbonat of lime?

EMILY.

Yes, a white cloud appears.

MRS. B.

I shall now pour an additional quantity of the Seltzer water into the
lime-water--

EMILY.

How singular! The cloud is re-dissolved, and the liquid is again
transparent.

MRS. B.

All the mystery depends upon this circumstance, that carbonat of lime is
soluble in carbonic acid, whilst it is insoluble in water; the first
quantity of carbonic acid, therefore, which I introduce into the
lime-water, was employed in forming the carbonat of lime, which remained
visible, until an additional quantity of carbonic acid dissolved it.
Thus, you see, when the lime and carbonic acid are in proper proportions
to form chalk, the white cloud appears, but when the acid predominates,
the chalk is no sooner formed than it is dissolved.

CAROLINE.

That is now the case; but let us try whether a further addition of
lime-water will again precipitate the chalk.

EMILY.

It does, indeed! The cloud re-appears, because, I suppose, there is now
no more of the carbonic acid than is necessary to form chalk; and, in
order to dissolve the chalk, a superabundance of acid is required.

MRS. B.

We have, I think, carried this experiment far enough; every repetition
would but exhibit the same appearances.

Lime combines with most of the acids, to which the carbonic (as being
the weakest) readily yields it; but these combinations we shall have an
opportunity of noticing more particularly hereafter. It unites with
phosphorus, and with sulphur, in their simple state; in short, of all
the earths, lime is that which nature employs most frequently, and most
abundantly, in its innumerable combinations. It is the basis of all
calcareous earths and stones; we find it likewise in the animal and the
vegetable creations.

EMILY.

And in the arts is not lime of very great utility?

MRS. B.

Scarcely any substance more so; you know that it is a most essential
requisite in building, as it constitutes the basis of all cements, such
as mortar, stucco, plaister, &c.

Lime is also of infinite importance in agriculture; it lightens and
warms soils that are too cold, and compact, in consequence of too great
a proportion of clay. --But it would be endless to enumerate the various
purposes for which it is employed; and you know enough of it to form
some idea of its importance; we shall, therefore, now proceed to the
third alkaline earth, MAGNESIA.

CAROLINE.

I am already pretty well acquainted with that earth; it is a medicine.

MRS. B.

It is in the state of carbonat that magnesia is usually employed
medicinally; it then differs but little in appearance from its simple
form, which is that of a very fine light white powder. It dissolves in
2000 times its weight of water, but forms with acids extremely soluble
salts. It has not so great an attraction for acids as lime, and
consequently yields them to the latter. It is found in a great variety
of mineral combinations, such as slate, mica, amianthus, and more
particularly in a certain lime stone, which has lately been discovered
by Mr. Tennant to contain it in very great quantities. It does not
attract and solidify water, like lime: but when mixed with water and
exposed to the atmosphere, it slowly absorbs carbonic acid from the
latter, and thus loses its causticity. Its chief use in medicine is,
like that of lime, derived from its readiness to combine with, and
neutralise, the acid which it meets with in the stomach.

EMILY.

Yet, you said that it was taken in the state of carbonat, in which case
it has already combined with an acid?

MRS. B.

Yes; but the carbonic is the last of all the acids in the order of
affinities; it will therefore yield the magnesia to any of the others.
It is, however, frequently taken in its caustic state as a remedy for
flatulence. Combined with sulphuric acid, magnesia forms another and
more powerful medicine, commonly called _Epsom salt_.

CAROLINE.

And properly, _sulphat of magnesia_, I suppose? Pray why was it ever
called Epsom salt?

MRS. B.

Because there is a spring in the neighbourhood of Epsom which contains
this salt in great abundance.

The last alkaline earth which we have to mention is STRONTIAN, or
STRONTITES, discovered by Dr. Hope a few years ago. It so strongly
resembles barytes in its properties, and is so sparingly found in
nature, and of so little use in the arts, that it will not be necessary
to enter into any particulars respecting it. One of the remarkable
characteristic properties of strontites is, that its salts, when
dissolved in spirit of wine, tinge the flame of a deep red, or blood
colour.




CONVERSATION XVI.

ON ACIDS.


MRS. B.

We may now proceed to the acids. Of the metallic oxyds, you have already
acquired some general notions. This subject, though highly interesting
in its details, is not of sufficient importance to our concise view of
chemistry, to be particularly treated of; but it is absolutely necessary
that you should be better acquainted with the acids, and likewise with
their combinations with the alkalies, which form the triple compounds
called NEUTRAL SALTS.

The class of acids is characterised by very distinct properties. They
all change blue vegetable infusions to a red colour: they are all more
or less sour to the taste; and have a general tendency to combine with
the earths, alkalies, and metallic oxyds.

You have, I believe, a clear idea of the nomenclature by which the base
(or radical) of the acid, and the various degrees of acidification, are
expressed?

EMILY.

Yes, I think so; the acid is distinguished by the name of its base, and
its degree of oxydation, that is, the quantity of oxygen it contains, by
the termination of that name in _ous_ or _ic_; thus sulphure_ous_ acid
is that formed by the smallest proportion of oxygen combined with
sulphur; sulphur_ic_ acid that which results from the combination of
sulphur with the greatest quantity of oxygen.

MRS. B.

A still greater latitude may, in many cases, be allowed to the
proportions of oxygen than can be combined with acidifiable radicals;
for several of these radicals are susceptible of uniting with a quantity
of oxygen so small as to be insufficient to give them the properties of
acids; in these cases, therefore, they are converted into oxyds. Such is
sulphur, which by exposure to the atmosphere with a degree of heat
inadequate to produce inflammation, absorbs a small proportion of
oxygen, which colours it red or brown. This, therefore, is the first
degree of oxygenation of sulphur; the 2d converts it into sulphur_ous_
acid; the 3d into the sulphur_ic_ acid; and 4thly, if it was found
capable of combining with a still larger proportion of oxygen, it would
then be termed _super-oxygenated sulphuric acid_.

EMILY.

Are these various degrees of oxygenation common to all the acids?

MRS. B.

No; they vary much in this respect: some are susceptible of only one
degree of oxygenation; others, of two, or three; there are but very few
that will admit of more.

CAROLINE.

The modern nomenclature must be of immense advantage in pointing out so
easily the nature of the acids, and their various degrees of
oxygenation.

MRS. B.

Till lately many of the acids had not been decomposed; but analogy
afforded so strong a proof of their compound nature, that I never could
reconcile myself to classing them with the simple bodies, though this
division has been adopted by several chemical writers. At present there
are only the muriatic and the fluoric acids, which have not had their
bases distinctly separated.

CAROLINE.

We have heard of a great variety of acids; pray how many are there in
all?

MRS. B.

I believe there are reckoned at present thirty-four, and their number is
constantly increasing, as the science improves; but the most important,
and those to which we shall almost entirely confine our attention, are
but few. I shall, however, give you a general view of the whole; and
then we shall more particularly examine those that are the most
essential.

This class of bodies was formerly divided into mineral, vegetable, and
animal acids, according to the substances from which they were commonly
obtained.

CAROLINE.

That, I should think, must have been an excellent arrangement; why was
it altered?

MRS. B.

Because in many cases it produced confusion. In which class, for
instance, would you place carbonic acid?

CAROLINE.

Now I see the difficulty. I should be at a loss where to place it, as
you have told us that it exists in the animal, vegetable, and mineral
kingdoms.

EMILY.

There would be the same objection with respect to phosphoric acid,
which, though obtained chiefly from bones, can also, you said, be found
in small quantities in stones, and likewise in some plants.

MRS. B.

You see, therefore, the propriety of changing this mode of
classification. These objections do not exist in the present
nomenclature; for the composition and nature of each individual acid is
in some degree pointed out, instead of the class of bodies from which it
is extracted; and, with regard to the more general division of acids,
they are classed under these three heads:

First, Acids of known or supposed simple bases, which are formed by the
union of these bases with oxygen. They are the following:

The _Sulphuric_
    _Carbonic_
    _Nitric_
    _Phosphoric_
    _Arsenical_  Acids, of known and simple bases.
    _Tungstenic_
    _Molybdenic_
    _Boracic_
    _Fluoric_
    _Muriatic_

This class comprehends the most anciently known and most important
acids. The sulphuric, nitric, and muriatic were formerly, and are still
frequently, called _mineral acids_.

2dly, Acids that have double or binary radicals, and which consequently
consist of triple combinations. These are the vegetable acids, whose
common radical is a compound of hydrogen and carbon.

CAROLINE.

But if the basis of all the vegetable acids be the same, it should form
but one acid; it may indeed combine with different proportions of
oxygen, but the nature of the acid must be the same.

MRS. B.

The only difference that exists in the basis of vegetable acids, is the
various proportions of hydrogen and carbon from which they are severally
composed. But this is enough to produce a number of acids apparently
very dissimilar. That they do not, however, differ essentially, is
proved by their susceptibility of being converted into each other, by
the addition or subtraction of a portion of hydrogen or of carbon. The
names of these acids are,

The _Acetic_
    _Oxalic_
    _Tartarous_
    _Citric_
    _Malic_     Acids, of double bases, being of vegetable origin.
    _Gallic_
    _Mucous_
    _Benzoic_
    _Succinic_
    _Camphoric_
    _Suberic_

The 3d class of acids consists of those which have triple radicals, and
are therefore of a still more compound nature. This class comprehends
the animal acids, which are,

The _Lactic_
    _Prussic_
    _Formic_  Acids, of triple bases, or animal acids.
    _Bombic_
    _Sebacic_
    _Zoonic_
    _Lithic_

I have given you this summary account or enumeration of the acids, as
you may find it more satisfactory to have at once an outline or a
general notion of the extent of the subject; but we shall now confine
ourselves to the first class, which requires our more immediate
attention; and defer the few remarks which we shall have to make on the
others, till we treat of the chemistry of the animal and vegetable
kingdoms.

The acids of simple and known radicals are all capable of being
decomposed by combustible bodies, to which they yield their oxygen. If,
for instance, I pour a drop of sulphuric acid on this piece of iron, it
will produce a spot of rust, you know what that is?

CAROLINE.

Yes; it is an oxyd, formed by the oxygen of the acid combining with the
iron.

MRS. B.

In this case you see the sulphur deposits the oxygen by which it was
acidified on the metal. And again, if we pour some acid on a compound
combustible substance, (we shall try it on this piece of wood,) it will
combine with one or more of the constituents of that substance, and
occasion a decomposition.

EMILY.

It has changed the colour of the wood to black. How is that?

MRS. B.

The oxygen deposited by the acid has burnt it; you know that wood in
burning becomes black before it is reduced to ashes. Whether it derives
the oxygen which burns it from the atmosphere, or from any other source,
the chemical effect on the wood is the same. In the case of real
combustion, wood becomes black, because it is reduced to the state of
charcoal by the evaporation of its other constituents. But can you tell
me the reason why wood turns black when burnt by the application of an
acid?

CAROLINE.

First, tell me what are the ingredients of wood?

MRS. B.

Hydrogen and carbon are the chief constituents of wood, as of all other
vegetable substances.

CAROLINE.

Well, then, I suppose that the oxygen of the acid combines with the
hydrogen of the wood, to form water; and that the carbon of the wood,
remaining alone, appears of its usual black colour.

MRS. B.

Very well indeed, my dear; that is certainly the most plausible
explanation.

EMILY.

Would not this be a good method of making charcoal?

MRS. B.

It would be an extremely expensive, and, I believe, very imperfect
method; for the action of the acid on the wood, and the heat produced by
it, are far from sufficient to deprive the wood of all its evaporable
parts.

CAROLINE.

What is the reason that vinegar, lemon, and the acid of fruits, do not
produce this effect on wood?

MRS. B.

They are vegetable acids, whose bases are composed of hydrogen and
carbon; the oxygen, therefore, will not be disposed to quit this
radical, where it is already united with hydrogen. The strongest of
these may, perhaps, yield a little of their oxygen to the wood, and
produce a stain upon it; but the carbon will not be sufficiently
uncovered to assume its black colour. Indeed, the several mineral acids
themselves possess this power of charring wood in very different
degrees.

EMILY.

Cannot vegetable acids be decomposed, by any combustibles?

MRS. B.

No; because their radical is composed of two substances which have a
greater attraction for oxygen than any known body.

CAROLINE.

And are those strong acids, which burn and decompose wood, capable of
producing similar effects on the skin and flesh of animals?

MRS. B.

Yes; all the mineral acids, and one of them more especially, possess
powerful caustic qualities. They actually corrode and destroy the skin
and flesh; but they do not produce upon these exactly the same
alteration they do on wood, probably because there is a great proportion
of nitrogen and other substances in animal matter, which prevents the
separation of carbon from being so conspicuous.




CONVERSATION XVII.

OF THE SULPHURIC AND PHOSPHORIC ACIDS; OR THE COMBINATIONS OF OXYGEN
WITH SULPHUR AND PHOSPHORUS; AND OF THE SULPHATS AND PHOSPHATS.


MRS. B.

In addition to the general survey which we have taken of acids, I think
you will find it interesting to examine individually a few of the most
important of them, and likewise some of their principal combinations
with the alkalies, alkaline earths, and metals. The first of the acids,
in point of importance, is the SULPHURIC, formerly called _oil of
vitriol_.

CAROLINE.

I have known it a long time by that name, but had no idea that it was
the same fluid as sulphuric acid. What resemblance or connection can
there be between oil of vitriol and this acid?

MRS. B.

Vitriol is the common name for sulphat of iron, a salt which is formed
by the combination of sulphuric acid and iron; the sulphuric acid was
formerly obtained by distillation from this salt, and it very naturally
received its name from the substance which afforded it.

CAROLINE.

But it is still usually called oil of vitriol?

MRS. B.

Yes; a sufficient length of time has not yet elapsed, since the
invention of the new nomenclature, for it to be generally disseminated;
but, as it is adopted by all scientific chemists, there is every reason
to suppose that it will gradually become universal. When I received this
bottle from the chemists, _oil of vitriol_ was inscribed on the label;
but, as I knew you were very punctilious in regard to the nomenclature,
I changed it, and substituted the words _sulphuric acid_.

EMILY.

This acid has neither colour nor smell, but it appears much thicker than
water.

MRS. B.

It is nearly twice as heavy as water, and has, you see, an oily
consistence.

CAROLINE.

And it is probably from this circumstance that it has been called an
oil, for it can have no real claim to that name, as it does not contain
either hydrogen or carbon, which are the essential constituents of oil.

MRS. B.

Certainly; and therefore it would be the more absurd to retain a name
which owed its origin to such a mistaken analogy.

Sulphuric acid, in its purest state, would probably be a concrete
substance, but its attraction for water is such, that it is impossible
to obtain that acid perfectly free from it; it is, therefore, always
seen in a liquid form, such as you here find it. One of the most
striking properties of sulphuric acid is that of evolving a considerable
quantity of heat when mixed with water; this I have already shown you.

EMILY.

Yes, I recollect it; but what was the degree of heat produced by that
mixture?

MRS. B.

The thermometer may be raised by it to 300 degrees, which is
considerably above the temperature of boiling water.

CAROLINE.

Then water might be made to boil in that mixture?

MRS. B.

Nothing more easy, provided that you employ sufficient quantities of
acid and of water, and in the due proportions. The greatest heat is
produced by a mixture of one part of water to four of the acid: we shall
make a mixture of these proportions, and immerse in it this thin glass
tube, which is full of water.

CAROLINE.

The vessel feels extremely hot, but the water does not boil yet.

MRS. B.

You must allow some time for the heat to penetrate the tube, and raise
the temperature of the water to the boiling point--

CAROLINE.

Now it boils--and with increasing violence.

MRS. B.

But it will not continue boiling long; for the mixture gives out heat
only while the particles of the water and the acid are mutually
penetrating each other: as soon as the new arrangement of those
particles is effected, the mixture will gradually cool, and the water
return to its former temperature.

You have seen the manner in which sulphuric acid decomposes all
combustible substances, whether animal, vegetable, or mineral, and burns
them by means of its oxygen?

CAROLINE.

I have very unintentionally repeated the experiment on my gown, by
letting a drop of the acid fall upon it, and it has made a stain, which,
I suppose, will never wash out.

MRS. B.

No, certainly; for before you can put it into water, the spot will
become a hole, as the acid has literally burnt the muslin.

CAROLINE.

So it has, indeed! Well, I will fasten the stopper, and put the bottle
away, for it is a dangerous substance. --Oh, now I have done worse
still, for I have spilt some on my hand!

MRS. B.

It is then burned, as well as your gown, for you know that oxygen
destroys animal as well as vegetable matters; and, as far as the
decomposition of the skin of your finger is effected, there is no
remedy; but by washing it immediately in water, you will dilute the
acid, and prevent any further injury.

CAROLINE.

It feels extremely hot, I assure you.

MRS. B.

You have now learned, by experience, how cautiously this acid must be
used. You will soon become acquainted with another acid, the nitric,
which, though it produces less heat on the skin, destroys it still
quicker, and makes upon it an indelible stain. You should never handle
any substances of this kind, without previously dipping your fingers in
water, which will weaken their caustic effects. But, since you will not
repeat the experiment, I must put in the stopper, for the acid attracts
the moisture from the atmosphere, which would destroy its strength and
purity.

EMILY.

Pray, how can sulphuric acid be extracted from sulphat of iron by
distillation?

MRS. B.

The process of distillation, you know, consists in separating substances
from one another by means of their different degrees of volatility, and
by the introduction of a new chemical agent, caloric. Thus, if sulphat
of iron be exposed in a retort to a proper degree of heat, it will be
decomposed, and the sulphuric acid will be volatilised.

EMILY.

But now that the process of forming acids by the combustion of their
radicals is known, why should not this method be used for making
sulphuric acid?

MRS. B.

This is actually done in most manufactures; but the usual method of
preparing sulphuric acid does not consist in burning the sulphur in
oxygen gas (as we formerly did by the way of experiment), but in heating
it together with another substance, nitre, which yields oxygen in
sufficient abundance to render the combustion in common air rapid and
complete.

CAROLINE.

This substance, then, answers the same purpose as oxygen gas?

MRS. B.

Exactly. In manufactures the combustion is performed in a leaden
chamber, with water at the bottom, to receive the vapour and assist its
condensation. The combustion is, however, never so perfect but that a
quantity of _sulphureous_ acid is formed at the same time; for you
recollect that the sulphureous acid, according to the chemical
nomenclature, differs from the sulphuric only by containing less oxygen.

From its own powerful properties, and from the various combinations into
which it enters, sulphuric acid is of great importance in many of the
arts.

It is used also in medicine in a state of great dilution; for were it
taken internally, in a concentrated state, it would prove a most
dangerous poison.

CAROLINE.

I am sure it would burn the throat and stomach.

MRS. B.

Can you think of any thing that would prove an antidote to this poison?

CAROLINE.

A large draught of water to dilute it.

MRS. B.

That would certainly weaken the caustic power of the acid, but it would
increase the heat to an intolerable degree. Do you recollect nothing
that would destroy its deleterious properties more effectually?

EMILY.

An alkali might, by combining with it; but, then, a pure alkali is
itself a poison, on account of its causticity.

MRS. B.

There is no necessity that the alkali should be caustic. Soap, in which
it is combined with oil; or magnesia, either in the state of carbonat,
or mixed with water, would prove the best antidotes.

EMILY.

In those cases then, I suppose, the potash and the magnesia would quit
their combinations to form salts with the sulphuric acid?

MRS. B.

Precisely.

We may now make a few observations on the sulphure_ous_ acid, which we
have found to be the product of sulphur slowly and imperfectly burnt.
This acid is distinguished by its pungent smell, and its gaseous form.

CAROLINE.

Its aeriform state is, I suppose, owing to the smaller proportion of
oxygen, which renders it lighter than sulphur_ic_ acid?

MRS. B.

Probably; for by adding oxygen to the weaker acid, it may be converted
into the stronger kind. But this change of state may also be connected
with a change of affinity with regard to caloric.

EMILY.

And may sulphureous acid be obtained from sulphuric acid by a diminution
of oxygen?

MRS. B.

Yes; it can be done by bringing any combustible substance in contact
with the acid. This decomposition is most easily performed by some of
the metals; these absorb a portion of the oxygen from the sulphuric
acid, which is thus converted into the sulphureous, and flies off in its
gaseous form.

CAROLINE.

And cannot the sulphureous acid itself be decomposed and reduced to
sulphur?

MRS. B.

Yes; if this gas be heated in contact with charcoal, the oxygen of the
gas will combine with it, and the pure sulphur is regenerated.

Sulphureous acid is readily absorbed by water; and in this liquid state
it is found particularly useful in bleaching linen and woollen cloths,
and is much used in manufactures for those purposes. I can show you its
effect in destroying colours, by taking out vegetable stains--I think I
see a spot on your gown, Emily, on which we may try the experiment.

EMILY.

It is the stain of mulberries; but I shall be almost afraid of exposing
my gown to the experiment, after seeing the effect which the sulphuric
acid produced on that of Caroline--

MRS. B.

There is no such danger from the sulphureous; but the experiment must be
made with great caution, for, during the formation of sulphureous acid
by combustion, there is always some sulphuric produced.

CAROLINE.

But where is your sulphureous acid?

MRS. B.

We may easily prepare some ourselves, simply by burning a match; we must
first wet the stain with water, and now hold it in this way, at a little
distance, over the lighted match: the vapour that arises from it is
sulphureous acid, and the stain, you see, gradually disappears.

EMILY.

I have frequently taken out stains by this means, without understanding
the nature of the process. But why is it necessary to wet the stain
before it is exposed to the acid fumes?

MRS. B.

The moisture attracts and absorbs the sulphureous acid; and it serves
likewise to dilute any particles of sulphuric acid which might injure
the linen.

Sulphur is susceptible of a third combination with oxygen, in which the
proportion of the latter is too small to render the sulphur acid. It
acquires this slight oxygenation by mere exposure to the atmosphere,
without any elevation of temperature: in this case, the sulphur does not
change its natural form, but is only discoloured, being changed to red
or brown; and in this state it is an oxyd of sulphur.

Before we take leave of the sulphuric acid, we shall say a few words of
its principal combinations. It unites with all the alkalies, alkaline
earths and metals, to form compound salts.

CAROLINE.

Pray, give me leave to interrupt you for a moment: you have never
mentioned any other salts than the compound or neutral salts; is there
no other kind?

MRS. B.

The term _salt_ has been used, from time immemorial, as a kind of
general name for any substance that has savour, odour, is soluble in
water, and crystallisable, whether it be of an acid, an alkaline, or
compound nature; but the compound salts alone retain that appellation in
modern chemistry.

The most important of the salts, formed by the combinations of the
sulphuric acid, are, first, _sulphat of potash_, formerly called _sal
polychrest_: this is a very bitter salt, much used in medicine; it is
found in the ashes of most vegetables, but it may be prepared
artificially by the immediate combination of sulphuric acid and potash.
This salt is easily soluble in boiling water. Solubility is, indeed,
a property common to all salts; and they always produce cold in melting.

EMILY.

That must be owing to the caloric which they absorb in passing from a
solid to a fluid form.

MRS. B.

That is, certainly, the most probable explanation.

_Sulphat of soda_, commonly called Glauber's salt, is another medicinal
salt, which is still more bitter than the preceding. We must prepare
some of these compounds, that you may observe the phenomena which take
place during their formation. We need only pour some sulphuric acid over
the soda which I have put into this glass.

CAROLINE.

What an amazing heat is disengaged! --I thought you said that cold was
produced by the melting of salts?

MRS. B.

But you must observe that we are now _making_, not _melting_ a salt.
Heat is disengaged during the formation of compound salts, and a faint
light is also emitted, which may sometimes be perceived in the dark.

EMILY.

And is this heat and light produced by the union of the opposite
electricities of the alkali and the acid?

MRS. B.

No doubt it is, if that theory be true.

CAROLINE.

The union of an acid and an alkali is then an actual combustion?

MRS. B.

Not precisely, though there is certainly much analogy in these
processes.

CAROLINE.

Will this sulphat of soda become solid?

MRS. B.

We have not, I suppose, mixed the acid and the alkali in the exact
proportions that are required for the formation of the salt, otherwise
the mixture would have been almost immediately changed to a solid mass;
but, in order to obtain it in crystals, as you see it in this bottle, it
would be necessary first to dilute it with water, and afterwards to
evaporate the water, during which operation the salt would gradually
crystallise.

CAROLINE.

But of what use is the addition of water, if it is afterwards to be
evaporated?

MRS. B.

When suspended in water, the acid and the alkali are more at liberty to
act on each other, their union is more complete, and the salt assumes
the regular form of crystals during the slow evaporation of its solvent.

Sulphat of soda liquefies by heat, and effloresces in the air.

EMILY.

Pray what is the meaning of the word _effloresces_? I do not recollect
your having mentioned it before.

MRS. B.

A salt is said to effloresce when it loses its water of crystallisation
on being exposed to the atmosphere, and is thus gradually converted into
a dry powder: you may observe that these crystals of sulphat of soda are
far from possessing the transparency which belongs to their crystalline
state; they are covered with a white powder, occasioned by their having
been exposed to the atmosphere, which has deprived their surface of its
lustre, by absorbing its water of crystallisation. Salts are, in
general, either _efflorescent_ or _deliquescent_: this latter property
is precisely the reverse of the former; that is to say, deliquescent
salts absorb water from the atmosphere, and are moistened and gradually
melted by it. Muriat of lime is an instance of great deliquescence.

EMILY.

But are there no salts that have the same degree of attraction for water
as the atmosphere, and that will consequently not be affected by it?

MRS. B.

Yes; there are many such salts, as, for instance, common salt, sulphat
of magnesia, and a variety of others.

_Sulphat of lime_ is very frequently met with in nature, and constitutes
the well-known substance called _gypsum_, or _plaster of Paris_.

_Sulphat of magnesia_, commonly called _Epsom salt_, is another very
bitter medicine, which is obtained from sea-water and from several
springs, or may be prepared by the direct combination of its
ingredients.

We have formerly mentioned _sulphat of alumine_ as constituting the
common _alum_; it is found in nature chiefly in the neighbourhood of
volcanos, and is particularly useful in the arts, from its strong
astringent qualities. It is chiefly employed by dyers and
calico-printers, to fix colours; and is used also in the manufacture of
some kinds of leather.

Sulphuric acid combines also with the metals.

CAROLINE.

One of these combinations, _sulphat of iron_, we are already well
acquainted with.

MRS. B.

That is the most important metallic salt formed by sulphuric acid, and
the only one that we shall here notice. It is of great use in the arts;
and, in medicine, it affords a very valuable tonic: it is of this salt
that most of those preparations called _steel medicines_ are composed.

CAROLINE.

But does any carbon enter into these compositions to form steel?

MRS. B.

Not an atom: they are, therefore, very improperly called steel: but it
is the vulgar appellation, and medical men themselves often comply with
the general custom.

Sulphat of iron may be prepared, as you have seen, by dissolving iron in
sulphuric acid; but it is generally obtained from the natural production
called _Pyrites_, which being a sulphuret of iron, requires only
exposure to the atmosphere to be oxydated, in order to form the salt;
this, therefore, is much the most easy way of procuring it on a large
scale.

EMILY.

I am surprised to find that both acids and compound salts are generally
obtained from their various combinations, rather than from the immediate
union of their ingredients.

MRS. B.

Were the simple bodies always at hand, their combinations would
naturally be the most convenient method of forming compounds; but you
must consider that, in most instances, there is great difficulty and
expense in obtaining the simple ingredients from their combinations; it
is, therefore, often more expedient to procure compounds from the
decomposition of other compounds. But, to return to the sulphat of iron.
--There is a certain vegetable acid called _Gallic acid_, which has the
remarkable property of precipitating this salt black--I shall pour a few
drops of the gallic acid into this solution of sulphat of iron--

CAROLINE.

It is become as black as ink!

MRS. B.

And it is ink in reality. Common writing ink is a precipitate of sulphat
of iron by gallic acid; the black colour is owing to the formation of
gallat of iron, which being insoluble, remains suspended in the fluid.

This acid has also the property of altering the colour of iron in its
metallic state. You may frequently see its effect on the blade of a
knife, that has been used to cut certain kinds of fruits.

CAROLINE.

True; and that is, perhaps, the reason that a silver knife is preferred
to cut fruits; the gallic acid, I suppose, does not act upon silver.
--Is this acid found in all fruits?

MRS. B.

It is contained, more or less, in the rind of most fruits and roots,
especially the radish, which, if scraped with a steel or iron knife, has
its bright red colour changed to a deep purple, the knife being at the
same time blackened. But the vegetable substance in which the gallic
acid most abounds is _nutgall_, a kind of excrescence that grows on
oaks, and from which the acid is commonly obtained for its various
purposes.


MRS. B.

We now come to the PHOSPHORIC and PHOSPHOROUS ACIDS. In treating of
phosphorus, you have seen how these acids may be obtained from it by
combustion?

EMILY.

Yes; but I should be much surprised if it was the usual method of
obtaining them, since it is so very difficult to procure phosphorus in
its pure state.

MRS. B.

You are right, my dear; the phosphoric acid, for general purposes, is
extracted from bones, in which it is contained in the state of phosphat
of lime; from this salt the phosphoric acid is separated by means of the
sulphuric, which combines with the lime. In its pure state, phosphoric
acid is either liquid or solid, according to its degree of
concentration.

Among the salts formed by this acid, _phosphat of lime_ is the only one
that affords much interest; and this, we have already observed,
constitutes the basis of all bones. It is also found in very small
quantities in some vegetables.




CONVERSATION XVIII.

OF THE NITRIC AND CARBONIC ACIDS: OR THE COMBINATIONS OF OXYGEN WITH
NITROGEN AND CARBON; AND OF THE NITRATS AND CARBONATS.


MRS. B.

I am almost afraid of introducing the subject of the NITRIC ACID, as I
am sure that I shall be blamed by Caroline for not having made her
acquainted with it before.

CAROLINE.

Why so, Mrs. B.?

MRS. B.

Because you have long known its radical, which is nitrogen or azote; and
in treating of that element, I did not even hint that it was the basis
of an acid.

CAROLINE.

And what could be your reason for not mentioning this acid sooner?

MRS. B.

I do not know whether you will think the reason sufficiently good to
acquit me; but the omission, I assure you, did not proceed from
negligence. You may recollect that nitrogen was one of the first simple
bodies which we examined; you were then ignorant of the theory of
combustion, which I believe was, for the first time, mentioned in that
lesson; and therefore it would have been in vain, at that time, to have
attempted to explain the nature and formation of acids.

CAROLINE.

I wonder, however, that it never occurred to us to enquire whether
nitrogen could be acidified; for, as we knew it was classed among the
combustible bodies, it was natural to suppose that it might produce an
acid.

MRS. B.

That is not a necessary consequence; for it might combine with oxygen
only in the degree requisite to form an oxyd. But you will find that
nitrogen is susceptible of various degrees of oxygenation, some of which
convert it merely into an oxyd, and others give it all the acid
properties.

The acids, resulting from the combination of oxygen and nitrogen, are
called the NITROUS and NITRIC acids. We will begin with the NITRIC, in
which nitrogen is in the highest state of oxygenation. This acid
naturally exists in the form of gas; but is so very soluble in water,
and has so great an affinity for it, that one grain of water will absorb
and condense ten grains of acid gas, and form the limpid fluid which you
see in this bottle.

CAROLINE.

What a strong offensive smell it has!

MRS. B.

This acid contains a greater abundance of oxygen than any other, but it
retains it with very little force.

EMILY.

Then it must be a powerful caustic, both from the facility with which it
parts with its oxygen, and the quantity which it affords?

MRS. B.

Very well, Emily; both cause and effect are exactly such as you
describe: nitric acid burns and destroys all kinds of organised matter.
It even sets fire to some of the most combustible substances. --We shall
pour a little of it over this piece of dry warm charcoal--you see it
inflames it immediately; it would do the same with oil of turpentine,
phosphorus, and several other very combustible bodies. This shows you
how easily this acid is decomposed by combustible bodies, since these
effects must depend upon the absorption of its oxygen.

Nitric acid has been used in the arts from time immemorial, but it is
only within these twenty-five years that its chemical nature has been
ascertained. The celebrated Mr. Cavendish discovered that it consisted
of about 10 parts of nitrogen and 25 of oxygen.* These principles, in
their gaseous state, combine at a high temperature; and this may be
effected by repeatedly passing the electrical spark through a mixture of
the two gases.

    [Footnote *: The proportion stated by Sir H. Davy, in his Chemical
    Researches, is as 1 to 2.389.]

EMILY.

The nitrogen and oxygen gases, of which the atmosphere is composed, do
not combine, I suppose, because their temperature is not sufficiently
elevated?

CAROLINE.

But in a thunder-storm, when the lightning repeatedly passes through
them, may it not produce nitric acid? We should be in a strange
situation, if a violent storm should at once convert the atmosphere into
nitric acid.

MRS. B.

There is no danger of it, my dear; the lightning can affect but a very
small portion of the atmosphere, and though it were occasionally to
produce a little nitric acid, yet this never could happen to such an
extent as to be perceivable.

EMILY.

But how could the nitric acid be known, and used, before the method of
combining its constituents was discovered?

MRS. B.

Before that period the nitric acid was obtained, and it is indeed still
extracted, for the common purposes of art, from the compound salt which
it forms with potash, commonly called _nitre_.

CAROLINE.

Why is it so called? Pray, Mrs. B., let these old unmeaning names be
entirely given up, by us at least; and let us call this salt _nitrat of
potash_.

MRS. B.

With all my heart; but it is necessary that I should, at least, mention
the old names, and more especially those which are yet in common use;
otherwise, when you meet with them, you would not be able to understand
their meaning.

EMILY.

And how is the acid obtained from this salt?

MRS. B.

By the intervention of sulphuric acid, which combines with the potash,
and sets the nitric acid at liberty. This I can easily show you, by
mixing some nitrat of potash and sulphuric acid in this retort, and
heating it over a lamp; the nitric acid will come over in the form of
vapour, which we shall collect in a glass bell. This acid, diluted in
water, is commonly called _aqua fortis_, if Caroline will allow me to
mention that name.

CAROLINE.

I have often heard that aqua fortis will dissolve almost all metals; it
is no doubt because it yields its oxygen so easily.

MRS. B.

Yes; and from this powerful solvent property, it derived the name of
aqua fortis, or strong water. Do you not recollect that we oxydated, and
afterwards dissolved, some copper in this acid?

EMILY.

If I remember right, the nitrat of copper was the first instance you
gave us of a compound salt.

CAROLINE.

Can the nitric acid be completely decomposed and converted into nitrogen
and oxygen?

EMILY.

That cannot be the case, Caroline; since the acid can be decomposed only
by the combination of its constituents with other bodies.

MRS. B.

True; but caloric is sufficient for this purpose. By making the acid
pass through a red hot porcelain tube, it is decomposed; the nitrogen
and oxygen regain the caloric which they had lost in combining, and are
thus both restored to their gaseous state.

The nitric acid may also be partly decomposed, and is by this means
converted into NITROUS ACID.

CAROLINE.

This conversion must be easily effected, as the oxygen is so slightly
combined with the nitrogen.

MRS. B.

The partial decomposition of nitric acid is readily effected by most
metals; but it is sufficient to expose the nitric acid to a very strong
light to make it give out oxygen gas, and thus be converted into nitrous
acid. Of this acid there are various degrees, according to the
proportions of oxygen which it contains; the strongest, and that into
which the nitric is first converted, is of a yellow colour, as you see
in this bottle.

CAROLINE.

How it fumes when the stopper is taken out!

MRS. B.

The acid exists naturally in a gaseous state, and is here so strongly
concentrated in water, that it is constantly escaping.

Here is another bottle of nitrous acid, which, you see, is of an orange
red; this acid is weaker, the nitrogen being combined with a smaller
quantity of oxygen; and with a still less proportion of oxygen it is an
olive-green colour, as it appears in this third bottle. In short, the
weaker the acid, the deeper is its colour.

Nitrous acid acts still more powerfully on some inflammable substances
than the nitric.

EMILY.

I am surprised at that, as it contains less oxygen.

MRS. B.

But, on the other hand, it parts with its oxygen much more readily: you
may recollect that we once inflamed oil with this acid.

The next combinations of nitrogen and oxygen form only oxyds of
nitrogen, the first of which is commonly called _nitrous air_; or more
properly _nitric oxyd gas_. This may be obtained from nitric acid, by
exposing the latter to the action of metals, as in dissolving them it
does not yield the whole of its oxygen, but retains a portion of this
principle sufficient to convert it into this peculiar gas, a specimen of
which I have prepared, and preserved within this inverted glass bell.

EMILY.

It is a perfectly invisible elastic fluid.

MRS. B.

Yes; and it may be kept any length of time in this manner over water, as
it is not, like the nitric and nitrous acids, absorbable by it. It is
rather heavier than atmospherical air, and is incapable of supporting
either combustion or respiration. I am going to incline the glass gently
on one side, so as to let some of the gas escape--

EMILY.

How very curious! --It produces orange fumes like the nitrous acid! that
is the more extraordinary, as the gas within the glass is perfectly
invisible.

MRS. B.

It would give me much pleasure if you could make out the reason of this
curious change without requiring any further explanation.

CAROLINE.

It seems, by the colour and smell, as if it were converted into nitrous
acid gas: yet that cannot be, unless it combines with more oxygen; and
how can it obtain oxygen the very instant it escapes from the glass?

EMILY.

From the atmosphere, no doubt. Is it not so, Mrs. B.?

MRS. B.

You have guessed it; as soon as it comes in contact with the atmosphere,
it absorbs from it the additional quantity of oxygen necessary to
convert it into nitrous acid gas. And, if I now remove the bottle
entirely from the water, so as to bring at once the whole of the gas
into contact with the atmosphere, this conversion will appear still more
striking--

EMILY.

Look, Caroline, the whole capacity of the bottle is instantly tinged of
an orange colour!

MRS. B.

Thus, you see, it is the most easy process imaginable to convert
_nitrous oxyd gas_ into _nitrous acid gas_. The property of attracting
oxygen from the atmosphere, without any elevation of temperature, has
occasioned this gaseous oxyd being used as a test for ascertaining the
degree of purity of the atmosphere. I am going to show you how it is
applied to this purpose. --You see this graduated glass tube, which is
closed at one end, (PLATE X. Fig. 2.) --I first fill it with water, and
then introduce a certain measure of nitrous gas, which, not being
absorbable by water, passes through it, and occupies the upper part of
the tube. I must now add rather above two-thirds of oxygen gas, which
will just be sufficient to convert the nitrous oxyd gas into nitrous
acid gas.

CAROLINE.

So it has! --I saw it turn of an orange colour; but it immediately
afterwards disappeared entirely, and the water, you see, has risen, and
almost filled the tube.

MRS. B.

That is because the acid gas is absorbable by water, and in proportion
as the gas impregnates the water, the latter rises in the tube. When the
oxygen gas is very pure, and the required proportion of nitrous oxyd gas
very exact, the whole is absorbed by the water; but if any other gas be
mixed with the oxygen, instead of combining with the nitrous oxygen, it
will remain and occupy the upper part of the tube; or, if the gases be
not in the due proportion, there will be a residue of that which
predominates. --Before we leave this subject, I must not forget to
remark that nitrous acid may be formed by dissolving nitrous oxyd gas in
nitric acid. This solution may be effected simply by making bubbles of
nitrous oxyd gas pass through nitric acid.

EMILY.

That is to say, that nitrogen at its highest degree of oxygenation,
being mixed with nitrogen at its lowest degree of oxygenation, will
produce a kind of intermediate substance, which is nitrous acid.

MRS. B.

You have stated the fact with great precision. --There are various other
methods of preparing nitrous oxyd, and of obtaining it from compound
bodies; but it is not necessary to enter into these particulars. It
remains for me only to mention another curious modification of
oxygenated nitrogen, which has been distinguished by the name of
_gaseous oxyd of nitrogen_. It is but lately that this gas has been
accurately examined, and its properties have been investigated chiefly
by Sir H. Davy. It has obtained also the name of _exhilarating_ gas,
from the very singular property which that gentleman has discovered in
it, of elevating the animal spirits, when inhaled into the lungs, to a
degree sometimes resembling delirium or intoxication.

CAROLINE.

Is it respirable, then?

MRS. B.

It can scarcely be called respirable, as it would not support life for
any length of time; but it may be breathed for a few moments without any
other effects, than the singular exhilaration of spirits I have just
mentioned. It affects different people, however, in a very different
manner. Some become violent, even outrageous: others experience a
languor, attended with faintness; but most agree in opinion, that the
sensations it excites are extremely pleasant.

CAROLINE.

I think I should like to try it--how do you breathe it?

MRS. B.

By collecting the gas in a bladder, to which a short tube with a
stop-cock is adapted; this is applied to the mouth with one hand, whilst
the nostrils are kept closed with the other, that the common air may
have no access. You then alternately inspire, and expire the gas, till
you perceive its effects. But I cannot consent to your making the
experiment; for the nerves are sometimes unpleasantly affected by it,
and I would not run any risk of that kind.

EMILY.

I should like, at least, to see somebody breathe it; but pray by what
means is this curious gas obtained?

MRS. B.

It is procured from _nitrat of ammonia_, an artificial salt which yields
this gas on the application of a gentle heat. I have put some of the
salt into a retort, and by the aid of a lamp the gas will be
extricated.--

CAROLINE.

Bubbles of air begin to escape through the neck of the retort into the
water apparatus; will you not collect them?

MRS. B.

The gas that first comes over need not be preserved, as it consists of
little more than the common air that was in the retort; besides, there
is always in this experiment a quantity of watery vapour which must come
away before the nitrous oxyd appears.

EMILY.

Watery vapour! Whence does that proceed? There is no water in nitrat of
ammonia?

MRS. B.

You must recollect that there is in every salt a quantity of water of
crystallisation, which may be evaporated by heat alone. But, besides
this, water is actually generated in this experiment, as you will see
presently. First tell me, what are the constituent parts of nitrat of
ammonia?

EMILY.

Ammonia, and nitric acid: this salt, therefore, contains three different
elements, nitrogen and hydrogen, which produce the ammonia; and oxygen,
which, with nitrogen, forms the acid.

MRS. B.

Well then, in this process the ammonia is decomposed; the hydrogen quits
the nitrogen to combine with some of the oxygen of the nitric acid, and
forms with it the watery vapour which is now coming over. When that is
effected, what will you expect to find?

EMILY.

Nitrous acid instead of nitric acid, and nitrogen instead of ammonia.

MRS. B.

Exactly so; and the nitrous acid and nitrogen combine, and form the
gaseous oxyd of nitrogen, in which the proportion of oxygen is 37 parts
to 63 of nitrogen.

You may have observed, that for a little while no bubbles of air have
come over, and we have perceived only a stream of vapour condensing as
it issued into the water. --Now bubbles of air again make their
appearance, and I imagine that by this time all the watery vapour is
come away, and that we may begin to collect the gas. We may try whether
it is pure, by filling a phial with it, and plunging a taper into
it--yes, it will do now, for the taper burns brighter than in the common
air, and with a greenish flame.

CAROLINE.

But how is that? I thought no gas would support combustion but oxygen or
chlorine.

MRS. B.

Or any gas that contains oxygen, and is ready to yield it, which is the
case with this in a considerable degree; it is not, therefore,
surprising that it should accelerate the combustion of the taper.

You see that the gas is now produced in great abundance; we shall
collect a large quantity of it, and I dare say that we shall find some
of the family who will be curious to make the experiment of respiring
it. Whilst this process is going on, we may take a general survey of the
most important combinations of the nitric and nitrous acids with the
alkalies.

The first of these is _nitrat of potash_, commonly called _nitre_ or
_saltpetre_.

CAROLINE.

Is not that the salt with which gunpowder is made?

MRS. B.

Yes. Gunpowder is a mixture of five parts of nitre to one of sulphur,
and one of charcoal. --Nitre from its great proportion of oxygen, and
from the facility with which it yields it, is the basis of most
detonating compositions.

EMILY.

But what is the cause of the violent detonation of gunpowder when set
fire to?

MRS. B.

Detonation may proceed from two causes; the sudden formation or
destruction of an elastic fluid. In the first case, when either a solid
or liquid is instantaneously converted into an elastic fluid, the
prodigious and sudden expansion of the body strikes the air with great
violence, and this concussion produces the sound called detonation.

CAROLINE.

That I comprehend very well; but how can a similar effect be produced by
the destruction of a gas?

MRS. B.

A gas can be destroyed only by condensing it to a liquid or solid state;
when this takes place suddenly, the gas, in assuming a new and more
compact form, produces a vacuum, into which the surrounding air rushes
with great impetuosity; and it is by that rapid and violent motion that
the sound is produced. In all detonations, therefore, gases are either
suddenly formed, or destroyed. In that of gunpowder, can you tell me
which of these two circumstances takes place?

EMILY.

As gunpowder is a solid, it must, of course, produce the gases in its
detonation; but how, I cannot tell.

MRS. B.

The constituents of gunpowder, when heated to a certain degree, enter
into a number of new combinations, and are instantaneously converted
into a variety of gases, the sudden expansion of which gives rise to the
detonation.

CAROLINE.

And in what instance does the destruction or condensation of gases
produce detonation?

MRS. B.

I can give you one with which you are well acquainted; the sudden
combination of the oxygen and hydrogen gases.

CAROLINE.

True; I recollect perfectly that hydrogen detonates with oxygen when the
two gases are converted into water.

MRS. B.

But let us return to the nitrat of potash. --This salt is decomposed
when exposed to heat, and mixed with any combustible body, such as
carbon, sulphur, or metals, these substances oxydating rapidly at the
expense of the nitrat. I must show you an instance of this. --I expose
to the fire some of the salt in a small iron ladle, and, when it is
sufficiently heated, add to it some powdered charcoal; this will attract
the oxygen from the salt, and be converted into carbonic acid.--

EMILY.

But what occasions that crackling noise, and those vivid flashes that
accompany it?

MRS. B.

The rapidity with which the carbonic acid gas is formed occasions a
succession of small detonations, which, together with the emission of
flame, is called _deflagration_.

_Nitrat of ammonia_ we have already noticed, on account of the gaseous
oxyd of nitrogen which is obtained from it.

_Nitrat of silver_ is the lunar caustic, so remarkable for its property
of destroying animal fibre, for which purpose it is often used by
surgeons. --We have said so much on a former occasion, on the mode in
which caustics act on animal matter, that I shall not detain you any
longer on this subject.


We now come to the CARBONIC ACID, which we have already had many
opportunities of noticing. You recollect that this acid may be formed by
the combustion of carbon, whether in its imperfect state of charcoal, or
in its purest form of diamond. And it is not necessary, for this
purpose, to burn the carbon in oxygen gas, as we did in the preceding
lecture; for you need only light a piece of charcoal and suspend it
under a receiver on the water bath. The charcoal will soon be
extinguished, and the air in the receiver will be found mixed with
carbonic acid. The process, however, is much more expeditious if the
combustion be performed in pure oxygen gas.

CAROLINE.

But how can you separate the carbonic acid, obtained in this manner,
from the air with which it is mixed?

MRS. B.

The readiest mode is to introduce under the receiver a quantity of
caustic lime, or caustic alkali, which soon attracts the whole of the
carbonic acid to form a carbonat. --The alkali is found increased in
weight, and the volume of the air is diminished by a quantity equal to
that of the carbonic acid which was mixed with it.

EMILY.

Pray is there no method of obtaining pure carbon from carbonic acid?

MRS. B.

For a long time it was supposed that carbonic acid was not
decompoundable; but Mr. Tennant discovered, a few years ago, that this
acid may be decomposed by burning phosphorus in a closed vessel with
carbonat of soda or carbonat of lime: the phosphorus absorbs the oxygen
from the carbonat, whilst the carbon is separated in the form of a black
powder. This decomposition, however, is not effected simply by the
attraction of the phosphorus for oxygen, since it is weaker than that of
charcoal; but the attraction of the alkali of lime for the phosphoric
acid, unites its power at the same time.

CAROLINE.

Cannot we make that experiment?

MRS. B.

Not easily; it requires being performed with extreme nicety, in order to
obtain any sensible quantity of carbon, and the experiment is much too
delicate for me to attempt it. But there can be no doubt of the accuracy
of Mr. Tennant's results; and all chemists now agree, that one hundred
parts of carbonic acid gas consists of about twenty-eight parts of
carbon to seventy-two of oxygen gas. But if you recollect, we decomposed
carbonic acid gas the other day by burning potassium in it.

CAROLINE.

True, so we did; and found the carbon precipitated on the regenerated
potash.

MRS. B.

Carbonic acid gas is found very abundantly in nature; it is supposed to
form about one thousandth part of the atmosphere, and is constantly
produced by the respiration of animals; it exists in a great variety of
combinations, and is exhaled from many natural decompositions. It is
contained in a state of great purity in certain caves, such as the
_Grotto del Cane_, near Naples.

EMILY.

I recollect having read an account of that grotto, and of the cruel
experiments made on the poor dogs, to gratify the curiosity of
strangers. But I understood that the vapour exhaled by this cave was
called _fixed air_.

MRS. B.

That is the name by which carbonic acid was known before its chemical
composition was discovered. --This gas is more destructive of life than
any other; and if the poor animals that are submitted to its effects are
not plunged into cold water as soon as they become senseless, they do
not recover. It extinguishes flame instantaneously. I have collected
some in this glass, which I will pour over the candle.

CAROLINE.

This is extremely singular--it seems to extinguish it as it were by
enchantment, as the gas is invisible. I never should have imagined that
gas could have been poured like a liquid.

MRS. B.

It can be done with carbonic acid only, as no other gas is sufficiently
heavy to be susceptible of being poured out in the atmospherical air
without mixing with it.

EMILY.

Pray by what means did you obtain this gas?

MRS. B.

I procured it from marble. Carbonic acid gas has so strong an attraction
for all the alkalies and alkaline earths, that these are always found in
nature in the state of carbonats. Combined with lime, this acid forms
chalk, which may be considered as the basis of all kinds of marbles, and
calcareous stones. From these substances carbonic acid is easily
separated, as it adheres so slightly to its combinations, that the
carbonats are all decomposable by any of the other acids. I can easily
show you how I obtained this gas; I poured some diluted sulphuric acid
over pulverised marble in this bottle (the same which we used the other
day to prepare hydrogen gas), and the gas escaped through the tube
connected with it; the operation still continues, as you may easily
perceive--

EMILY.

Yes, it does; there is a great fermentation in the glass vessel. What
singular commotion is excited by the sulphuric acid taking possession of
the lime, and driving out the carbonic acid!

CAROLINE.

But did the carbonic acid exist in a gaseous state in the marble?

MRS. B.

Certainly not; the acid, when in a state of combination, is capable of
existing in a solid form.

CAROLINE.

Whence, then, does it obtain the caloric necessary to convert it into
gas?

MRS. B.

It may be supplied in this case from the mixture of sulphuric acid and
water, which produces an evolution of heat, even greater than is
required for the purpose; since, as you may perceive by touching the
glass vessel, a considerable quantity of the caloric disengaged becomes
sensible. But a supply of caloric may be obtained also from a diminution
of capacity for heat, occasioned by the new combination which takes
place; and, indeed, this must be the case when other acids are employed
for the disengagement of carbonic acid gas, which do not, like the
sulphuric, produce heat on being mixed with water. Carbonic acid may
likewise be disengaged from its combinations by heat alone, which
restores it to its gaseous state.

CAROLINE.

It appears to me very extraordinary that the same gas, which is produced
by the burning of wood and coals, should exist also in such bodies as
marble, and chalk, which are incombustible substances.

MRS. B.

I will not answer that objection, Caroline, because I think I can put
you in a way of doing it yourself. Is carbonic acid combustible?

CAROLINE.

Why, no--because it is a body that has been already burnt; it is carbon
only, and not the acid, that is combustible.

MRS. B.

Well, and what inference do you draw from this?

CAROLINE.

That carbonic acid cannot render the bodies with which it is united
combustible; but that simple carbon does, and that it is in this
elementary state that it exists in wood, coals, and a great variety of
other combustible bodies. --Indeed, Mrs. B., you are very ungenerous;
you are not satisfied with convincing me that my objections are
frivolous, but you oblige me to prove them so myself.

MRS. B.

You must confess, however, that I make ample amends for the detection of
error, when I enable you to discover the truth. You, understand, now,
I hope, that carbonic acid is equally produced by the decomposition of
chalk, or by the combustion of charcoal. These processes are certainly
of a very different nature; in the first case the acid is already
formed, and requires nothing more than heat to restore it to its gaseous
state; whilst, in the latter, the acid is actually made by the process
of combustion.

CAROLINE.

I understand it now perfectly. But I have just been thinking of another
difficulty, which, I hope, you will excuse my not being able to remove
myself. How does the immense quantity of calcareous earth, which is
spread all over the globe, obtain the carbonic acid with which it is
combined?

MRS. B.

The question is, indeed, not very easy to answer; but I conceive that
the general carbonisation of calcareous matter may have been the effect
of a general combustion, occasioned by some revolution of our globe, and
producing an immense supply of carbonic acid, with which the calcareous
matter became impregnated; or that this may have been effected by a
gradual absorption of carbonic acid from the atmosphere. --But this
would lead us to discussions which we cannot indulge in, without
deviating too much from our subject.

EMILY.

How does it happen that we do not perceive the pernicious effects of the
carbonic acid which is floating in the atmosphere?

MRS. B.

Because of the state of very great dilution in which it exists there.
But can you tell me, Emily, what are the sources which keep the
atmosphere constantly supplied with this acid?

EMILY.

I suppose the combustion of wood, coals, and other substances, that
contain carbon.

MRS. B.

And also the breath of animals.

CAROLINE.

The breath of animals! I thought you said that this gas was not at all
respirable, but on the contrary, extremely poisonous.

MRS. B.

So it is; but although animals cannot breathe in carbonic acid gas, yet,
in the process of respiration, they have the power of forming this gas
in their lungs; so that the air which we _expire_, or reject from the
lungs, always contains a certain proportion of carbonic acid, which is
much greater than that which is commonly found in the atmosphere.

CAROLINE.

But what is it that renders carbonic acid such a deadly poison?

MRS. B.

The manner in which this gas destroys life, seems to be merely by
preventing the access of respirable air; for carbonic acid gas, unless
very much diluted with common air, does not penetrate into the lungs, as
the windpipe actually contracts and refuses it admittance. --But we must
dismiss this subject at present, as we shall have an opportunity of
treating of respiration much more fully, when we come to the chemical
functions of animals.

EMILY.

Is carbonic acid as destructive to the life of vegetables as it is to
that of animals?

MRS. B.

If a vegetable be completely immersed in it, I believe it generally
proves fatal to it; but mixed in certain proportions with atmospherical
air, it is, on the contrary, very favourable to vegetation.

You remember, I suppose, our mentioning the mineral waters, both natural
and artificial, which contain carbonic acid gas?

CAROLINE.

You mean the Seltzer water?

MRS. B.

That is one of those which are the most used; there are, however,
a variety of others into which carbonic acid enters as an ingredient:
all these waters are usually distinguished by the name of _acidulous_ or
_gaseous mineral waters_.

The class of salts called _carbonats_ is the most numerous in nature; we
must pass over them in a very cursory manner, as the subject is far too
extensive for us to enter on it in detail. The state of carbonat is the
natural state of a vast number of minerals, and particularly of the
alkalies and alkaline earths, as they have so great an attraction for
the carbonic acid, that they are almost always found combined with it;
and you may recollect that it is only by separating them from this acid,
that they acquire that causticity and those striking qualities which I
have formerly described. All marbles, chalks, shells, calcareous spars,
and lime-stones of every description, are neutral salts, in which
_lime_, their common basis, has lost all its characteristic properties.

EMILY.

But if all these various substances are formed by the union of lime with
carbonic acid, whence arises their diversity of form and appearance?

MRS. B.

Both from the different proportions of their component parts, and from a
variety of foreign ingredients which may be occasionally blended with
them: the veins and colours of marbles, for instance, proceed from a
mixture of metallic substances; silex and alumine also frequently enter
into these combinations. The various carbonats, therefore, that I have
enumerated, cannot be considered as pure unadulterated neutral salts,
although they certainly belong to that class of bodies.




CONVERSATION XIX.

ON THE BORACIC, FLUORIC, MURIATIC, AND OXYGENATED MURIATIC ACIDS; AND ON
MURIATS. --ON IODINE AND IODIC ACID.


MRS. B.

We now come to the three remaining acids with simple bases, the compound
nature of which, though long suspected, has been but recently proved.
The chief of these is the muriatic; but I shall first describe the two
others, as their bases have been obtained more distinctly than that of
the muriatic acid.

You may recollect I mentioned the BORACIC ACID. This is found very
sparingly in some parts of Europe, but for the use of manufactures we
have always received it from the remote country of Thibet, where it is
found in some lakes, combined with soda. It is easily separated from the
soda by sulphuric acid, and appears in the form of shining scales, as
you see here.

CAROLINE.

I am glad to meet with an acid which we need not be afraid to touch; for
I perceive, from your keeping it in a piece of paper, that it is more
innocent than our late acquaintance, the sulphuric and nitric acids.

MRS. B.

Certainly; but being more inert, you will not find its properties so
interesting. However, its decomposition, and the brilliant spectacle it
affords when its basis again unites with oxygen, atones for its want of
other striking qualities.

Sir H. Davy succeeded in decomposing the boracic acid, (which had till
then been considered as undecompoundable,) by various methods. On
exposing this acid to the Voltaic battery, the positive wire gave out
oxygen, and on the negative wire was deposited a black substance, in
appearance resembling charcoal. This was the basis of the acid, which
Sir H. Davy has called _Boracium_, or _Boron_.

The same substance was obtained in more considerable quantities, by
exposing the acid to a great heat in an iron gun-barrel.

A third method of decomposing the boracic acid consisted in burning
potassium in contact with it in vacuo. The potassium attracts the oxygen
from the acid, and leaves its basis in a separate state.

The recomposition of this acid I shall show you, by burning some of its
basis, which you see here, in a retort full of oxygen gas. The heat of a
candle is all that is required for this combustion.--

EMILY.

The light is astonishingly brilliant, and what beautiful sparks it
throws out!

MRS. B.

The result of this combustion is the boracic acid, the nature of which,
you see, is proved both by analytic and synthetic means. Its basis has
not, it is true, a metallic appearance; but it makes very hard alloys
with other metals.

EMILY.

But pray, Mrs. B., for what purpose is the boracic acid used in
manufactures?

MRS. B.

Its principal use is in conjunction with soda, that is, in the state of
_borat of soda_, which in the arts is commonly called borax. This salt
has a peculiar power of dissolving metallic oxyds, and of promoting the
fusion of substances capable of being melted; it is accordingly employed
in various metallic arts; it is used, for example, to remove the oxyd
from the surface of metals, and is often employed in the assaying of
metallic ores.


Let us now proceed to the FLUORIC ACID. This acid is obtained from a
substance which is found frequently in mines, and particularly in those
of Derbyshire, called _fluor_, a name which it acquired from the
circumstance of its being used to render the ores of metals more fluid
when heated.

CAROLINE.

Pray is not this the Derbyshire spar, of which so many ornaments are
made?

MRS. B.

The same; but though it has long been employed for a variety of
purposes, its nature was unknown until Scheele, the great Swedish
chemist, discovered that it consisted of lime united with a peculiar
acid, which obtained the name of _fluoric acid_. It is easily separated
from the lime by the sulphuric acid, and unless condensed in water,
ascends in the form of gas. A very peculiar property of this acid is its
union with siliceous earths, which I have already mentioned. If the
distillation of this acid is performed in glass vessels, they are
corroded, and the siliceous part of the glass comes over, united with
the gas; if water is then admitted, part of the silex is deposited, as
you may observe in this jar.

CAROLINE.

I see white flakes forming on the surface of the water; is that silex?

MRS. B.

Yes it is. This power of corroding glass has been used for engraving, or
rather etching, upon it. The glass is first covered with a coat of wax,
through which the figures to be engraved are to be scratched with a pin;
then pouring the fluoric acid over the wax, it corrodes the glass where
the scratches have been made.

CAROLINE.

I should like to have a bottle of this acid, to make engravings.

MRS. B.

But you could not have it in a _glass_ bottle, for in that case the acid
would be saturated with silex, and incapable of executing an engraving;
the same thing would happen were the acid kept in vessels of porcelain
or earthen-ware; this acid must therefore be both prepared and preserved
in vessels of silver.

If it be distilled from fluor spar and vitriolic acid, in silver or
leaden vessels, the receiver being kept very cold during the
distillation, it assumes the form of a dense fluid, and in that state is
the most intensely corrosive substance known. This seems to be the acid
combined with a little water. It may be called _hydro-fluoric acid_; and
Sir H. Davy has been led, from some late experiments on the subject, to
consider _pure_ fluoric acid as a compound of a certain unknown
principle, which he calls _fluorine_, with hydrogen.

Sir H. Davy has also attempted to decompose the fluoric acid by burning
potassium in contact with it; but he has not yet been able by this or
any other method, to obtain its basis in a distinct separate state.


We shall conclude our account of the acids with that of the MURIATIC
ACID, which is perhaps the most curious and interesting of all of them.
It is found in nature combined with soda, lime, and magnesia. _Muriat of
soda_ is the common sea-salt, and from this substance the acid is
usually disengaged by means of the sulphuric acid. The natural state of
the muriatic acid is that of an invisible permanent gas, at the common
temperature of the atmosphere; but it has a remarkably strong attraction
for water, and assumes the form of a whitish cloud whenever it meets any
moisture to combine with. This acid is remarkable for its peculiar and
very pungent smell, and possesses, in a powerful degree, most of the
acid properties. Here is a bottle containing muriatic acid in a liquid
state.

CAROLINE.

And how is it liquefied?

MRS. B.

By impregnating water with it; its strong attraction for water makes it
very easy to obtain it in a liquid form. Now, if I open the phial, you
may observe a kind of vapour rising from it, which is muriatic acid gas,
of itself invisible, but made apparent by combining with the moisture of
the atmosphere.

EMILY.

Have you not any of the pure muriatic acid gas?

MRS. B.

This jar is full of that acid in its gaseous state--it is inverted over
mercury instead of water, because, being absorbable by water, this gas
cannot be confined by it. --I shall now raise the jar a little on one
side, and suffer some of the gas to escape. --You see that it
immediately becomes visible in the form of a cloud.

EMILY.

It must be, no doubt, from its uniting with the moisture of the
atmosphere, that it is converted into this dewy vapour.

MRS. B.

Certainly; and for the same reason, that is to say, its extreme
eagerness to unite with water, this gas will cause snow to melt as
rapidly as an intense fire.

This acid proved much more refractory when Sir H. Davy attempted to
decompose it than the other two undecompounded acids. It is singular
that potassium will burn in muriatic acid, and be converted into potash,
without decomposing the acid, and the result of this combustion is a
_muriat of potash_; for the potash, as soon as it is regenerated,
combines with the muriatic acid.

CAROLINE.

But how can the potash be regenerated if the muriatic acid does not
oxydate the potassium?

MRS. B.

The potassium, in this process, obtains oxygen from the moisture with
which the muriatic acid is always combined, and accordingly hydrogen,
resulting from the decomposition of the moisture, is invariably evolved.

EMILY.

But why not make these experiments with dry muriatic acid?

MRS. B.

Dry acids cannot be acted on by the Voltaic battery, because acids are
non-conductors of electricity, unless moistened. In the course of a
number of experiments which Sir H. Davy made upon acids in a state of
dryness, he observed that the presence of water appeared always
necessary to develop the acid properties, so that acids are not even
capable of reddening vegetable blues if they have been carefully
deprived of moisture. This remarkable circumstance led him to suspect,
that water, instead of oxygen, may be the acidifying principle; but this
he threw out rather as a conjecture than as an established point.

Sir H. Davy obtained very curious results from burning potassium in a
mixture of phosphorus and muriatic acid, and also of sulphur and
muriatic acid; the latter detonates with great violence. All his
experiments, however, failed in presenting to his view the basis of the
muriatic acid, of which he was in search; and he was at last induced to
form an opinion respecting the nature of this acid, which I shall
presently explain.

EMILY.

Is this acid susceptible of different degrees of oxygenation?

MRS. B.

Yes, for though we cannot deoxygenate this acid, yet we may add oxygen
to it.

CAROLINE.

Why, then, is not the least degree of oxygenation of the acid called the
_muriatous_, and the higher degree the _muriatic_ acid?

MRS. B.

Because, instead of becoming, like other acids, more dense, and more
acid by an addition of oxygen, it is rendered on the contrary more
volatile, more pungent, but less acid, and less absorbable by water.
These circumstances, therefore, seem to indicate the propriety of making
an exception to the nomenclature. The highest degree of oxygenation of
this acid has been distinguished by the additional epithet of
_oxygenated_, or, for the sake of brevity, _oxy_, so that it is called
the _oxygenated_, or _oxy-muriatic acid_. This likewise exists in a
gaseous form, at the temperature of the atmosphere; it is also
susceptible of being absorbed by water, and can be congealed, or
solidified, by a certain degree of cold.

EMILY.

And how do you obtain the oxy-muriatic acid?

MRS. B.

In various ways; but it may be most conveniently obtained by distilling
liquid muriatic acid over oxyd of manganese, which supplies the acid
with the additional oxygen. One part of the acid being put into a
retort, with two parts of the oxyd of manganese, and the heat of a lamp
applied, the gas is soon disengaged, and may be received over water, as
it is but sparingly absorbed by it. --I have collected some in this
jar--

CAROLINE.

It is not invisible, like the generality of gases; for it is of a
yellowish colour.

MRS. B.

The muriatic acid extinguishes flame, whilst, on the contrary, the
oxy-muriatic makes the flame larger, and gives it a dark red colour. Can
you account for this difference in the two acids?

EMILY.

Yes, I think so; the muriatic acid will not supply the flame with the
oxygen necessary for its support; but when this acid is further
oxygenated, it will part with its additional quantity of oxygen, and in
this way support combustion.

MRS. B.

That is exactly the case; indeed the oxygen added to the muriatic acid,
adheres so slightly to it, that it is separated by mere exposure to the
sun's rays. This acid is decomposed also by combustible bodies, many of
which it burns, and actually inflames, without any previous increase of
temperature.

CAROLINE.

That is extraordinary, indeed! I hope you mean to indulge us with some
of these experiments?

MRS. B.

I have prepared several glass jars of oxy-muriatic acid gas for that
purpose. In the first we shall introduce some Dutch gold leaf. --Do you
observe that it takes fire?

EMILY.

Yes, indeed it does--how wonderful it is! It became immediately red hot,
but was soon smothered in a thick vapour.

CAROLINE.

What a disagreeable smell!

MRS. B.

We shall try the same experiment with phosphorus in another jar of this
acid. --You had better keep your handkerchief to your nose when I open
it--now let us drop into it this little piece of phosphorus--

CAROLINE.

It burns really; and almost as brilliantly as in oxygen gas! But, what
is most extraordinary, these combustions take place without the metal or
phosphorus being previously lighted, or even in the least heated.

MRS. B.

All these curious effects are owing to the very great facility with
which this acid yields oxygen to such bodies as are strongly disposed to
combine with it. It appears extraordinary indeed to see bodies, and
metals in particular, melted down and inflamed, by a gas without any
increase of temperature, either of the gas, or of the combustible. The
phenomenon, however, is, you see, well accounted for.

EMILY.

Why did you burn a piece of Dutch gold leaf rather than a piece of any
other metal?

MRS. B.

Because, in the first place, it is a composition of metals (consisting
chiefly of copper) which burns readily; and I use a thin metallic leaf
in preference to a lump of metal, because it offers to the action of the
gas but a small quantity of matter under a large surface. Filings, or
shavings, would answer the purpose nearly as well; but a lump of metal,
though the surface would oxydate with great rapidity, would not take
fire. Pure gold is not inflamed by oxy-muriatic acid gas, but it is
rapidly oxydated, and dissolved by it; indeed, this acid is the only one
that will dissolve gold.

EMILY.

This, I suppose, is what is commonly called _aqua regia_, which you know
is the only thing that will act upon gold.

MRS. B.

That is not exactly the case either; for aqua regia is composed of a
mixture of muriatic acid and nitric acid. --But, in fact, the result of
this mixture is the formation of oxy-muriatic acid, as the muriatic acid
oxygenates itself at the expence of the nitric; this mixture, therefore,
though it bears the name of _nitro-muriatic acid_, acts on gold merely
in virtue of the oxy-muriatic acid which it contains.

Sulphur, volatile oils, and many other substances, will burn in the same
manner in oxy-muriatic acid gas; but I have not prepared a sufficient
quantity of it, to show you the combustion of all these bodies.

CAROLINE.

There are several jars of the gas yet remaining.

MRS. B.

We must reserve these for future experiments. The oxy-muriatic acid does
not, like other acids, redden the blue vegetable colours; but it totally
destroys any colour, and turns all vegetables perfectly white. Let us
collect some vegetable substances to put into this glass, which is full
of gas.

EMILY.

Here is a sprig of myrtle--

CAROLINE.

And here some coloured paper--

MRS. B.

We shall also put in this piece of scarlet riband, and a rose--

EMILY.

Their colours begin to fade immediately! But how does the gas produce
this effect?

MRS. B.

The oxygen combines with the colouring matter of these substances, and
destroys it; that is to say, destroys the property which these colours
had of reflecting only one kind of rays, and renders them capable of
reflecting them all, which, you know, will make them appear white. Old
prints may be cleaned by this acid, for the paper will be whitened
without injury to the impression, as printer's ink is made of materials
(oil and lamp black) which are not acted upon by acids.

This property of the oxy-muriatic acid has lately been employed in
manufactures in a variety of bleaching processes; but for these purposes
the gas must be dissolved in water, as the acid is thus rendered much
milder and less powerful in its effects; for, in a gaseous state, it
would destroy the texture, as well as the colour of the substance
submitted to its action.

CAROLINE.

Look at the things which we put into the gas; they have now entirely
lost their colour!

MRS. B.

The effect of the acid is almost completed; and, if we were to examine
the quantity that remains, we should find it to consist chiefly of
muriatic acid.

The oxy-muriatic acid has been used to purify the air in fever hospitals
and prisons, as it burns and destroys putrid effluvia of every kind. The
infection of the small-pox is likewise destroyed by this gas, and matter
that has been submitted to its influence will no longer generate that
disorder.

CAROLINE.

Indeed, I think the remedy must be nearly as bad as the disease; the
oxy-muriatic acid has such a dreadfully suffocating smell.

MRS. B.

It is certainly extremely offensive; but by keeping the mouth shut, and
wetting the nostrils with liquid ammonia, in order to neutralize the
vapour as it reaches the nose, its prejudicial effects may be in some
degree prevented. At any rate, however, this mode of disinfection can
hardly be used in places that are inhabited. And as the vapour of nitric
acid, which is scarcely less efficacious for this purpose, is not at all
prejudicial, it is usually preferred on such occasions.

CAROLINE.

You have not told us yet what is Sir H. Davy's new opinion respecting
the nature of muriatic acid, to which you alluded a few minutes ago?

MRS. B.

True; I avoided noticing it then, because you could not have understood
it without some previous knowledge of the oxy-muriatic acid, which I
have but just introduced to your acquaintance.

Sir H. Davy's idea is that muriatic acid, instead of being a compound,
consisting of an unknown basis and oxygen, is formed by the union of
oxy-muriatic gas with hydrogen.

EMILY.

Have you not told us just now that oxy-muriatic gas was itself a
compound of muriatic acid and oxygen?

MRS. B.

Yes; but according to Sir H. Davy's hypothesis, oxy-muriatic gas is
considered as a simple body, which contains no oxygen--as a substance of
its own kind, which has a great analogy to oxygen in most of its
properties, though in others it differs entirely from it. --According to
this view of the subject, the name of _oxy-muriatic acid_ can no longer
be proper, and therefore Sir H. Davy has adopted that of _chlorine_, or
_chlorine gas_, a name which is simply expressive of its greenish
colour; and in compliance with that philosopher's theory, we have placed
chlorine in our table among the simple bodies.

CAROLINE.

But what was Sir H. Davy's reason for adopting an opinion so contrary to
that which had hitherto prevailed?

MRS. B.

There are many circumstances which are favourable to the new doctrine;
but the clearest and simplest fact in its support is, that if hydrogen
gas and oxy-muriatic gas be mixed together, both these gases disappear,
and muriatic acid gas is formed.

EMILY.

That seems to be a complete proof; is it not considered as perfectly
conclusive?

MRS. B.

Not so decisive as it appears at first sight; because it is argued by
those who still incline to the old doctrine, that muriatic acid gas,
however dry it may be, always contains a certain quantity of water,
which is supposed essential to its formation. So that, in the experiment
just mentioned, this water is supplied by the union of the hydrogen gas
with the oxygen of the oxy-muriatic acid; and therefore the mixture
resolves itself into the base of muriatic acid and water, that is,
muriatic acid gas.

CAROLINE.

I think the old theory must be the true one; for otherwise how could you
explain the formation of oxy-muriatic gas, from a mixture of muriatic
acid and oxyd of manganese?

MRS. B.

Very easily; you need only suppose that in this process the muriatic
acid is decomposed; its hydrogen unites with the oxygen of the manganese
to form water, and the chlorine appears in its separate state.

EMILY.

But how can you explain the various combustions which take place in
oxy-muriatic gas, if you consider it as containing no oxygen?

MRS. B.

We need only suppose that combustion is the result of intense chemical
action; so that chlorine, like oxygen, in combining with bodies, forms
compounds which have less capacity for caloric than their constituent
principles, and, therefore, caloric is evolved at the moment of their
combination.

EMILY.

If, then, we may explain every thing by either theory, to which of the
two shall we give the preference?

MRS. B.

It will, perhaps, be better to wait for more positive proofs, if such
can be obtained, before we decide positively upon the subject. The new
doctrine has certainly gained ground very rapidly, and may be considered
as nearly established; but several competent judges still refuse their
assent to it, and until that theory is very generally adopted, it may be
as well for us still occasionally to use the language to which chemists
have long been accustomed. --But let us proceed to the examination of
salts formed by muriatic acid.

Among the compound salts formed by muriatic acid, the _muriat of soda_,
or common salt, is the most interesting.* The uses and properties of
this salt are too well known to require much comment. Besides the
pleasant flavour it imparts to the food, it is very wholesome, when not
used to excess, as it assists the process of digestion.

Sea-water is the great source from which muriat of soda is extracted by
evaporation. But it is also found in large solid masses in the bowels of
the earth, in England, and in many other parts of the world.

    [Footnote *: According to Sir H. Davy's views of the nature of the
    muriatic and oxy-muriatic acids, dry muriat of soda is a compound
    of sodium and chlorine, for it may be formed by the direct
    combination of oxy-muriatic gas and sodium. In his opinion,
    therefore, what we commonly call muriat of soda contains neither
    soda nor muriatic acid.]

EMILY.

I thought that salts, when solid, were always in the state of crystals;
but the common table-salt is in the form of a coarse white powder.

MRS. B.

Crystallisation depends, as you may recollect, on the slow and regular
reunion of particles dissolved in a fluid; common sea-salt is only in a
state of imperfect crystallisation, because the process by which it is
prepared is not favourable to the formation of regular crystals. But if
you dissolve it, and afterwards evaporate the water slowly, you will
obtain a regular crystallisation.

_Muriat of ammonia_ is another combination of this acid, which we have
already mentioned as the principal source from which ammonia is derived.

I can at once show you the formation of this salt by the immediate
combination of muriatic acid with ammonia. --These two glass jars
contain, the one muriatic acid gas, the other ammoniacal gas, both of
which are perfectly invisible--now, if I mix them together, you see they
immediately form an opake white cloud, like smoke. --If a thermometer
was placed in the jar in which these gases are mixed, you would perceive
that some heat is at the same time produced.

EMILY.

The effects of chemical combinations are, indeed, wonderful! --How
extraordinary it is that two invisible bodies should become visible by
their union!

MRS. B.

This strikes you with astonishment, because it is a phenomenon which
nature seldom exhibits to our view; but the most common of her
operations are as wonderful, and it is their frequency only that
prevents our regarding them with equal admiration. What would be more
surprising, for instance, than combustion, were it not rendered so
familiar by custom?

EMILY.

That is true. --But pray, Mrs. B., is this white cloud the salt that
produces ammonia? How different it is from the solid muriat of ammonia
which you once showed us!

MRS. B.

It is the same substance which first appears in the state of vapour, but
will soon be condensed by cooling against the sides of the jar, in the
form of very minute crystals.

We may now proceed to the _oxy-muriats_. In this class of salts the
_oxy-muriat of potash_ is the most worthy of our attention, for its
striking properties. The acid, in this state of combination, contains a
still greater proportion of oxygen than when alone.

CAROLINE.

But how can the oxy-muriatic acid acquire an increase of oxygen by
combining with potash?

MRS. B.

It does not really acquire an additional quantity of oxygen, but it
loses some of the muriatic acid, which produces the same effect, as the
acid which remains is proportionably super-oxygenated.*

If this salt be mixed, and merely rubbed together with sulphur,
phosphorus, charcoal, or indeed any other combustible, it explodes
strongly.

    [Footnote *: According to Sir H. Davy's new views, just explained,
    oxy-muriat of potash is a compound of chlorine with oxyd of
    potassium.]

CAROLINE.

Like gun-powder, I suppose, it is suddenly converted into elastic
fluids?

MRS. B.

Yes; but with this remarkable difference, that no increase of
temperature, any further than is produced by gentle friction, is
required in this instance. Can you tell me what gases are generated by
the detonation of this salt with charcoal?

EMILY.

Let me consider . . . . . The oxy-muriatic acid parts with its excess of
oxygen to the charcoal, by which means it is converted into muriatic
acid gas; whilst the charcoal, being burnt by the oxygen, is changed to
carbonic acid gas. --What becomes of the potash I cannot tell.

MRS. B.

That is a fixed product which remains in the vessel.

CAROLINE.

But since the potash does not enter into the new combinations, I do not
understand of what use it is in this operation. Would not the
oxy-muriatic acid and the charcoal produce the same effect without it?

MRS. B.

No; because there would not be that very great concentration of oxygen
which the combination with the potash produces, as I have just
explained.

I mean to show you this experiment, but I would advise you not to repeat
it alone; for if care be not taken to mix only very small quantities at
a time, the detonation will be extremely violent, and may be attended
with dangerous effects. You see I mix an exceedingly small quantity of
the salt with a little powdered charcoal, in this Wedgwood mortar, and
rub them together with the pestle--

CAROLINE.

Heavens! How can such a loud explosion be produced by so small a
quantity of matter?

MRS. B.

You must consider that an extremely small quantity of solid substance
may produce a very great volume of gases; and it is the sudden evolution
of these which occasions the sound.

EMILY.

Would not oxy-muriat of potash make stronger gunpowder than nitrat of
potash?

MRS. B.

Yes; but the preparation, as well as the use of this salt, is attended
with so much danger, that it is never employed for that purpose.

CAROLINE.

There is no cause to regret it, I think; for the common gunpowder is
quite sufficiently destructive.

MRS. B.

I can show you a very curious experiment with this salt; but it must
again be on condition that you will never attempt to repeat it by
yourselves. I throw a small piece of phosphorus into this glass of
water; then a little oxy-muriat of potash; and, lastly, I pour in (by
means of this funnel, so as to bring it in contact with the two other
ingredients at the bottom of the glass) a small quantity of sulphuric
acid--

CAROLINE.

This is, indeed, a beautiful experiment! The phosphorus takes fire and
burns from the bottom of the water.

EMILY.

How wonderful it is to see flame bursting out under water, and rising
through it! Pray, how is this accounted for?

MRS. B.

Cannot you find it out, Caroline?

EMILY.

Stop--I think I can explain it. Is it not because the sulphuric acid
decomposes the salt by combining with the potash, so as to liberate the
oxy-muriatic acid gas by which the phosphoric is set on fire?

MRS. B.

Very well, Emily; and with a little more reflection you would have
discovered another concurring circumstance, which is, that an increase
of temperature is produced by the mixture of the sulphuric acid and
water, which assists in promoting the combustion of the phosphorus.


I must, before we part, introduce to your acquaintance the
newly-discovered substance IODINE, which you may recollect we placed
next to oxygen and chlorine in our table of simple bodies.

CAROLINE.

Is this also a body capable of maintaining combustion like oxygen and
chlorine?

MRS. B.

It is; and although it does not so generally disengage light and heat
from inflammable bodies, as oxygen and chlorine do, yet it is capable of
combining with most of them; and sometimes, as in the instance of
potassium and phosphorus, the combination is attended with an actual
appearance of light and heat.

CAROLINE.

But what sort of a substance is iodine: what is its form, and colour?

MRS. B.

It is a very singular body, in many respects. At the ordinary
temperature of the atmosphere, it commonly appears in the form of
blueish black crystalline scales, such as you see in this tube.

CAROLINE.

They shine like black lead, and some of the scales have the shape of
lozenges.

MRS. B.

That is actually the form which the crystals of iodine often assume. But
if we heat them gently, by holding the tube over the flame of a candle,
see what a change takes place in them.

CAROLINE.

How curious! They seem to melt, and the tube immediately fills with a
beautiful violet vapour. But look, Mrs. B., the same scales are now
appearing at the other end of the tube.

MRS. B.

This is in fact a sublimation of iodine, from one part of the tube to
another; but with this remarkable peculiarity, that, while in the
gaseous state, iodine assumes that bright violet colour, which, as you
may already perceive, it loses as the tube cools, and the substance
resumes its usual solid form. --It is from the violet colour of the gas
that iodine has obtained its name.

CAROLINE.

But how is this curious substance obtained?

MRS. B.

It is found in the ley of ashes of sea-weeds, after the soda has been
separated by crystallisation; and it is disengaged by means of sulphuric
acid, which expels it from the alkaline ley in the form of a violet gas,
which may be collected and condensed in the way you have just seen.
--This interesting discovery was made in the year 1812, by M. Courtois,
a manufacturer of saltpetre at Paris.

CAROLINE.

And pray, Mrs. B., what is the proof of iodine being a simple body?

MRS. B.

It is considered as a simple body, both because it is not capable of
being resolved into other ingredients; and because it is itself capable
of combining with other bodies, in a manner analogous to oxygen and
chlorine. The most curious of these combinations is that which it forms
with hydrogen gas, the result of which is a peculiar gaseous acid.

CAROLINE.

Just as chlorine and hydrogen gas form muriatic acid? In this respect
chlorine and iodine seem to bear a strong analogy to each other.

MRS. B.

That is indeed the case; so that if the theory of the constitution of
either of these two bodies be true, it must be true also in regard to
the other; if erroneous in the one, the theory must fall in both.

But it is now time to conclude; we have examined such of the acids and
salts as I conceived would appear to you most interesting. --I shall not
enter into any particulars respecting the metallic acids, as they offer
nothing sufficiently striking for our present purpose.




CONVERSATION XX.

ON THE NATURE AND COMPOSITION OF VEGETABLES.


MRS. B.

We have hitherto treated only of the simplest combinations of elements,
such as alkalies, earths, acids, compound salts, stones, &c.; all of
which belong to the mineral kingdom. It is time now to turn our
attention to a more complicated class of compounds, that of ORGANISED
BODIES, which will furnish us with a new source of instruction and
amusement.

EMILY.

By organised bodies, I suppose, you mean the vegetable and animal
creation? I have, however, but a very vague idea of the word
_organisation_, and I have often wished to know more precisely what it
means.

MRS. B.

Organised bodies are such as are endowed by nature with various parts,
peculiarly constructed and adapted to perform certain functions
connected with life. Thus you may observe, that mineral compounds are
formed by the simple effect of mechanical or chemical attraction, and
may appear to some to be in a great measure the productions of chance;
whilst organised bodies bear the most striking and impressive marks of
design, and are eminently distinguished by that unknown principle,
called _life_, from which the various organs derive the power of
exercising their respective functions.

CAROLINE.

But in what manner does life enable these organs to perform their
several functions?

MRS. B.

That is a mystery which, I fear, is enveloped in such profound darkness
that there is very little hope of our ever being able to unfold it. We
must content ourselves with examining the effects of this principle; as
for the cause, we have been able only to give it a name, without
attaching any other meaning to it than the vague and unsatisfactory idea
of au unknown agent.

CAROLINE.

And yet I think I can form a very clear idea of life.

MRS. B.

Pray let me hear how you would define it?

CAROLINE.

It is perhaps more easy to conceive than to express--let me consider--
Is not life the power which enables both the animal and the vegetable
creation to perform the various functions which nature has assigned to
them?

MRS. B.

I have nothing to object to your definition; but you will allow me to
observe, that you have only mentioned the effects which the unknown
cause produces, without giving us any notion of the cause itself.

EMILY.

Yes, Caroline, you have told us what life _does_, but you have not told
us what it _is_.

MRS. B.

We may study its operations, but we should puzzle ourselves to no
purpose by attempting to form an idea of its real nature.

We shall begin with examining its effects in the vegetable world, which
constitutes the simplest class of organised bodies; these we shall find
distinguished from the mineral creation, not only by their more
complicated nature, but by the power which they possess within
themselves, of forming new chemical arrangements of their constituent
parts, by means of appropriate organs. Thus, though all vegetables are
ultimately composed of hydrogen, carbon, and oxygen, (with a few other
occasional ingredients,) they separate and combine these principles by
their various organs, in a thousand ways, and form, with them, different
kinds of juices and solid parts, which exist ready made in vegetables,
and may, therefore, be considered as their immediate materials.

These are:

  _Sap_,
  _Mucilage_,
  _Sugar_,
  _Fecula_,
  _Gluten_,
  _Fixed Oil_,
  _Volatile Oil_,
  _Camphor_,
  _Resins_,
  _Gum Resins_,
  _Balsams_,
  _Caoutchouc_,
  _Extractive colouring Matter_,
  _Tannin_,
  _Woody Fibre_,
  _Vegetable Acids_, _&c._

CAROLINE.

What a long list of names! I did not suppose that a vegetable was
composed of half so many ingredients.

MRS. B.

You must not imagine that every one of these materials is formed in each
individual plant. I only mean to say, that they are all derived
exclusively from the vegetable kingdom.

EMILY.

But does each particular part of the plant, such as the root, the bark,
the stem, the seeds, the leaves, consist of one of these ingredients
only, or of several of them combined together?

MRS. B.

I believe there is no part of a plant which can be said to consist
solely of any one particular ingredient; a certain number of vegetable
materials must always be combined for the formation of any particular
part, (of a seed for instance,) and these combinations are carried on by
sets of vessels, or minute organs, which select from other parts, and
bring together, the several principles required for the development and
growth of those particular parts which they are intended to form and to
maintain.

EMILY.

And are not these combinations always regulated by the laws of chemical
attraction?

MRS. B.

No doubt; the organs of plants cannot force principles to combine that
have no attraction for each other; nor can they compel superior
attractions to yield to those of inferior power; they probably act
rather mechanically, by bringing into contact such principles, and in
such proportions, as will, by their chemical combination, form the
various vegetable products.

CAROLINE.

We may then consider each of these organs as a curiously constructed
apparatus, adapted for the performance of a variety of chemical
processes.

MRS. B.

Exactly so. As long as the plant lives and thrives, the carbon,
hydrogen, and oxygen, (the chief constituents of its immediate
materials,) are so balanced and connected together, that they are not
susceptible of entering into other combinations; but no sooner does
death take place, than this state of equilibrium is destroyed, and new
combinations produced.

EMILY.

But why should death destroy it; for these principles must remain in the
same proportions, and consequently, I should suppose, in the same order
of attractions?

MRS. B.

You must remember, that in the vegetable, as well as in the animal
kingdom, it is by the principle of _life_ that the organs are enabled to
act; when deprived of that agent or stimulus, their power ceases, and an
order of attractions succeeds similar to that which would take place in
mineral or unorganised matter.

EMILY.

It is this new order of attractions, I suppose, that destroys the
organisation of the plant after death; for if the same combinations
still continued to prevail, the plant would always remain in the state
in which it died?

MRS. B.

And that, you know, is never the case; plants may be partially preserved
for some time after death, by drying; but in the natural course of
events they all return to the state of simple elements; a wise and
admirable dispensation of Providence, by which dead plants are rendered
fit to enrich the soil, and become subservient to the nourishment of
living vegetables.

CAROLINE.

But we are talking of the dissolution of plants, before we have examined
them in their living state.

MRS. B.

That is true, my dear. But I wished to give you a general idea of the
nature of vegetation, before we entered into particulars. Besides, it is
not so irrelevant as you suppose to talk of vegetables in their dead
state, since we cannot analyse them without destroying life; and it is
only by hastening to submit them to examination, immediately after they
have ceased to live, that we can anticipate their natural decomposition.
There are two kinds of analysis of which vegetables are susceptible;
first, that which separates them into their immediate materials, such as
sap, resin, mucilage, &c.; secondly, that which decomposes them into
their primitive elements, as carbon, hydrogen, and oxygen.

EMILY.

Is there not a third kind of analysis of plants, which consists in
separating their various parts, as the stem, the leaves, and the several
organs of the flower?

MRS. B.

That, my dear, is rather the department of the botanist; we shall
consider these different parts of plants only, as the organs by which
the various secretions or separations are performed; but we must first
examine the nature of these secretions.

The _sap_ is the principal material of vegetables, since it contains the
ingredients that nourish every part of the plant. The basis of this
juice, which the roots suck up from the soil, is water; this holds in
solution the various other ingredients required by the several parts of
the plant, which are gradually secreted from the sap by the different
organs appropriated to that purpose, as it passes them in circulating
through the plant.


_Mucus_, or _mucilage_, is a vegetable substance, which, like all the
others, is secreted from the sap; when in excess, it exudes from trees
in the form of gum.

CAROLINE.

Is that the gum so frequently used instead of paste or glue?

MRS. B.

It is; almost all fruit-trees yield some sort of gum, but that most
commonly used in the arts is obtained from a species of acacia-tree in
Arabia, and is called _gum arabic_; it forms the chief nourishment of
the natives of those parts, who obtain it in great quantities from
incisions which they make in the trees.

CAROLINE.

I did not know that gum was eatable.

MRS. B.

There is an account of a whole ship's company being saved from starving
by feeding on the cargo, which was gum senegal. I should not, however,
imagine, that it would be either a pleasant or a particularly eligible
diet to those who have not, from their birth, been accustomed to it. It
is, however, frequently taken medicinally, and considered as very
nourishing. Several kinds of vegetable acids may be obtained, by
particular processes, from gum or mucilage, the principal of which is
called the _mucous acid_.


_Sugar_ is not found in its simple state in plants, but is always mixed
with gum, sap, or other ingredients; this saccharine matter is to be met
with in every vegetable, but abounds most in roots, fruits, and
particularly in the sugar-cane.

EMILY.

If all vegetables contain sugar, why is it extracted exclusively from
the sugar-cane?

MRS. B.

Because it is both most abundant in that plant, and most easily obtained
from it. Besides, the sugars produced by other vegetables differ a
little in their nature.

During the late troubles in the West-Indies, when Europe was but
imperfectly supplied with sugar, several attempts were made to extract
it from other vegetables, and very good sugar was obtained from parsnips
and from carrots; but the process was too expensive to carry this
enterprize to any extent.

CAROLINE.

I should think that sugar might be more easily obtained from sweet
fruits, such as figs, dates, &c.

MRS. B.

Probably; but it would be still more expensive, from the high price of
those fruits.

EMILY.

Pray, in what manner is sugar obtained from the sugar-cane?

MRS. B.

The juice of this plant is first expressed by passing it between two
cylinders of iron. It is then boiled with lime-water, which makes a
thick scum rise to the surface. The clarified liquor is let off below
and evaporated to a very small quantity, after which it is suffered to
crystallise by standing in a vessel, the bottom of which is perforated
with holes, that are imperfectly stopped, in order that the syrup may
drain off. The sugar obtained by this process is a coarse brown powder,
commonly called raw or moist sugar; it undergoes another operation to be
refined and converted into loaf sugar. For this purpose it is dissolved
in water, and afterwards purified by an animal fluid called albumen.
White of eggs chiefly consist of this fluid, which is also one of the
constituent parts of blood; and consequently eggs, or bullocks' blood,
are commonly used for this purpose.

The albuminous fluid being diffused through the syrup, combines with all
the solid impurities contained in it, and rises with them to the
surface, where it forms a thick scum; the clear liquor is then again
evaporated to a proper consistence, and poured into moulds, in which, by
a confused crystallisation, it forms loaf-sugar. But an additional
process is required to whiten it; to this effect the mould is inverted,
and its open base is covered with clay, through which water is made to
pass; the water slowly trickling through the sugar, combines with and
carries off the colouring matter.

CAROLINE.

I am very glad to hear that the blood that is used to purify sugar does
not remain in it; it would be a disgusting idea. I have heard of some
improvements by the late Mr. Howard, in the process of refining sugar.
Pray what are they?

MRS. B.

It would be much too long to give you an account of the process in
detail. But the principal improvement relates to the mode of evaporating
the syrup, in order to bring it to the consistency of sugar. Instead of
boiling the syrup in a large copper, over a strong fire, Mr. Howard
carries off the water by means of a large air-pump, in a way similar to
that used in Mr. Leslie's experiment for freezing water by evaporation;
that is, the syrup being exposed to a vacuum, the water evaporates
quickly, with no greater heat than that of a little steam, which is
introduced round the boiler. The air-pump is of course of large
dimensions, and is worked by a steam engine. A great saving is thus
obtained, and a striking instance afforded of the power of science in
suggesting useful economical improvements.

EMILY.

And pray how is sugar-candy and barley-sugar prepared?

MRS. B.

Candied sugar is nothing more than the regular crystals, obtained by
slow evaporation from a solution of sugar. Barley-sugar is sugar melted
by heat, and afterwards cooled in moulds of a spiral form.

Sugar may be decomposed by a red heat, and, like all other vegetable
substances, resolved into carbonic acid and hydrogen. The formation and
the decomposition of sugar afford many very interesting particulars,
which we shall fully examine, after having gone through the other
materials of vegetables. We shall find that there is reason to suppose
that sugar is not, like the other materials, secreted from the sap by
appropriate organs; but that it is formed by a peculiar process with
which you are not yet acquainted.

CAROLINE.

Pray, is not honey of the same nature as sugar?

MRS. B.

Honey is a mixture of saccharine matter and gum.

EMILY.

I thought that honey was in some measure an animal substance, as it is
prepared by the bees.

MRS. B.

It is rather collected by them from flowers, and conveyed to their
store-houses, the hives. It is the wax only that undergoes a real
alteration in the body of the bee, and is thence converted into an
animal substance.

Manna is another kind of sugar, which is united with a nauseous
extractive matter, to which it owes its peculiar taste and colour. It
exudes like gum from various trees in hot climates, some of which have
their leaves glazed by it.

The next of the vegetable materials is _fecula_; this is the general
name given to the farinaceous substance contained in all seeds, and in
some roots, as the potatoe, parsnip, &c. It is intended by nature for
the first aliment of the young vegetable; but that of one particular
grain is become a favourite and most common food of a large part of
mankind.

EMILY.

You allude, I suppose, to bread, which is made of wheat-flower?

MRS. B.

Yes. The fecula of wheat contains also another vegetable substance which
seems peculiar to that seed, or at least has not as yet been obtained
from any other. This is _gluten_, which is of a sticky, ropy, elastic
nature; and it is supposed to be owing to the viscous qualities of this
substance, that wheat-flour forms a much better paste than any other.

EMILY.

Gluten, by your description, must be very like gum?

MRS. B.

In their sticky nature they certainly have some resemblance; but gluten
is essentially different from gum in other points, and especially in its
being insoluble in water, whilst gum, you know, is extremely soluble.

The _oils_ contained in vegetables all consist of hydrogen and carbon in
various proportions. They are of two kinds, _fixed_ and _volatile_, both
of which we formerly mentioned. Do you remember in what the difference
between fixed and volatile oil consists?

EMILY.

If I recollect rightly, the former are decomposed by heat, whilst the
latter are merely volatilised by it.

MRS. B.

Very well. Fixed oil is contained only in the seeds of plants, excepting
in the olive, in which it is produced in, and expressed from, the fruit.
We have already observed that seeds contain also fecula; these two
substances, united with a little mucilage, form the white substance
contained in the seeds or kernels of plants, and is destined for the
nourishment of the young plant, to which the seed gives birth. The milk
of almonds, which is expressed from the seed of that name, is composed
of these three substances.

EMILY.

Pray, of what nature is the linseed oil which is used in painting?

MRS. B.

It is a fixed oil, obtained from the seed of flax. Nut oil, which is
frequently used for the same purpose, is expressed from walnuts.

Olive oil is that which is best adapted to culinary purposes.

CAROLINE.

And what are the oils used for burning?

MRS. B.

Animal oils most commonly; but the preference given to them is owing to
their being less expensive; for vegetable oils burn equally well, and
are more pleasant, as their smell is not offensive.

EMILY.

Since oil is so good a combustible, what is the reason that lamps so
frequently require trimming?

MRS. B.

This sometimes proceeds from the construction of the lamp, which may not
be sufficiently favourable to a perfect combustion; but there is
certainly a defect in the nature of oil itself, which renders it
necessary for the best-constructed lamps to be occasionally trimmed.
This defect arises from a portion of mucilage which it is extremely
difficult to separate from the oil, and which being a bad combustible,
gathers round the wick, and thus impedes its combustion, and
consequently dims the light.

CAROLINE.

But will not oils burn without a wick?

MRS. B.

Not unless their temperature be elevated to five or six hundred degrees;
the wick answers this purpose, as I think I once before explained to
you. The oil rises between the fibres of the cotton by capillary
attraction, and the heat of the burning wick volatilises it, and brings
it successively to the temperature at which it is combustible.

EMILY.

I suppose the explanation which you have given with regard to the
necessity of trimming lamps, applies also to candles, which so often
require snuffing?

MRS. B.

I believe it does; at least, in some degree. But besides the
circumstance just explained, the common sorts of oils are not very
highly combustible, so that the heat produced by a candle, which is a
coarse kind of animal oil, being insufficient to volatilise them
completely, a quantity of soot is gradually deposited on the wick, which
dims the light, and retards the combustion.

CAROLINE.

Wax candles then contain no incombustible matter, since they do not
require snuffing?

MRS. B.

Wax is a much better combustible than tallow, but still not perfectly
so, since it likewise contains some particles that are unfit for
burning; but when these gather round the wick, (which in a wax light is
comparatively small,) they weigh it down on one side, and fall off
together with the burnt part of the wick.

CAROLINE.

As oils are such good combustibles, I wonder that they should require so
great an elevation of temperature before they begin to burn?

MRS. B.

Though fixed oils will not enter into actual combustion below the
temperature of about four hundred degrees, yet they will slowly absorb
oxygen at the common temperature of the atmosphere. Hence arises a
variety of changes in oils which modify their properties and uses in the
arts.

If oil simply absorbs, and combines with oxygen, it thickens and changes
to a kind of wax. This change is observed to take place on the external
parts of certain vegetables, even during their life. But it happens in
many instances that the oil does not retain all the oxygen which it
attracts, but that part of it combines with, or burns, the hydrogen of
the oil, thus forming a quantity of water, which gradually goes off by
evaporation. In this case the alteration of the oil consists not only in
the addition of a certain quantity of oxygen, but in the diminution of
the hydrogen. These oils are distinguished by the name of _drying oils_.
Linseed, poppy, and nut-oils, are of this description.

EMILY.

I am well acquainted with drying oils, as I continually use them in
painting. But I do not understand why the acquisition of oxygen on one
hand, and a loss of hydrogen on the other, should render them drying?

MRS. B.

This, I conceive, may arise from two reasons; either from the oxygen
which is added being less favourable to the state of fluidity than the
hydrogen, which is subtracted; or from this additional quantity of
oxygen giving rise to new combinations, in consequence of which the most
fluid parts of the oil are liberated and volatilised.

For the purpose of painting, the drying quality of oil is further
increased by adding a quantity of oxyd of lead to it, by which means it
is more rapidly oxygenated.

The rancidity of oil is likewise owing to their oxygenation. In this
case a new order of attraction takes place, from which a peculiar acid
is formed, called the _sebacic acid_.

CAROLINE.

Since the nature and composition of oil is so well known, pray could not
oil be actually _made_, by combining its principles?

MRS. B.

That is by no means a necessary consequence; for there are innumerable
varieties of compound bodies which we can decompose, although we are
unable to reunite their ingredients. This, however, is not the case with
oil, as it has very lately been discovered, that it is possible to form
oil, by a peculiar process, from the action of oxygenated muriatic acid
gas on hydro-carbonate.

We now pass to the _volatile_ or _essential oils_. These form the basis
of all the vegetable perfumes, and are contained, more or less, in every
part of the plant excepting the seed; they are, at least, never found in
that part of the seed which contains the embrio plant.

EMILY.

The smell of flowers, then, proceeds from volatile oil?

MRS. B.

Certainly; but this oil is often most abundant in the rind of fruits, as
in oranges, lemons, &c. from which it may be extracted by the slightest
pressure; it is found also in the leaves of plants, and even in the
wood.

CAROLINE.

Is it not very plentiful in the leaves of mint, and of thyme, and all
the sweet-smelling herbs?

MRS. B.

Yes, remarkably so; and in geranium leaves also, which have a much more
powerful odour than the flowers.

The perfume of sandal fans is an instance of its existence in wood. In
short, all vegetable odours or perfumes are produced by the evaporation
of particles of these volatile oils.

EMILY.

They are, I suppose, very light, and of very thin consistence, since
they are so volatile?

MRS. B.

They vary very much in this respect, some of them being as thick as
butter, whilst others are as fluid as water. In order to be prepared for
perfumes, or essences, these oils are first properly purified, and then
either distilled with spirit of wine, as in the case with lavender
water, or simply mixed with a large proportion of water, as is often
done with regard to peppermint. Frequently, also, these odoriferous
waters are prepared merely by soaking the plants in water, and
distilling. The water then comes over impregnated with the volatile oil.

CAROLINE.

Such waters are frequently used to take spots of grease out of cloth, or
silk; how do they produce that effect?

MRS. B.

By combining with the substance that forms these stains; for volatile
oils, and likewise the spirit in which they are distilled, will dissolve
wax, tallow, spermaceti, and resins; if, therefore, the spot proceeds
from any of these substances, it will remove it. Insects of every kind
have a great aversion to perfumes, so that volatile oils are employed
with success in museums for the preservation of stuffed birds and other
species of animals.

CAROLINE.

Pray does not the powerful smell of camphor proceed from a volatile oil?

MRS. B.

_Camphor_ seems to be a substance of its own kind, remarkable by many
peculiarities. But if not exactly of the same nature as volatile oil, it
is at least very analogous to it. It is obtained chiefly from the
camphor-tree, a species of laurel which grows in China, and in the
Indian isles, from the stem and roots of which it is extracted. Small
quantities have also been distilled from thyme, sage, and other aromatic
plants; and it is deposited in pretty large quantities by some volatile
oils after long standing. It is extremely volatile and inflammable. It
is insoluble in water, but is soluble in oils, in which state, as well
as in its solid form, it is frequently applied to medicinal purposes.
Amongst the particular properties of camphor, there is one too singular
to be passed over in silence. If you take a small piece of camphor, and
place it on the surface of a bason of pure water, it will immediately
begin to move round and round with great rapidity; but if you pour into
the bason a single drop of any odoriferous fluid, it will instantly put
a stop to this motion. You can at any time try this very simple
experiment; but you must not expect that I shall be able to account for
this phenomenon, as nothing satisfactory has yet been advanced for its
explanation.

CAROLINE.

It is very singular indeed; and I will certainly try the experiment.
Pray what are _resins_, which you just now mentioned?

MRS. B.

They are volatile oils, that have been acted on, and peculiarly
modified, by oxygen.

CAROLINE.

They are, therefore, oxygenated volatile oils?

MRS. B.

Not exactly; for the process does not appear to consist so much in the
oxygenation of the oil, as in the combustion of a portion of its
hydrogen, and a small portion of its carbon. For when resins are
artificially made by the combination of volatile oils with oxygen, the
vessel in which the process is performed is bedewed with water, and the
air included within is loaded with carbonic acid.

EMILY.

This process must be, in some respects, similar to that for preparing
drying oils?

MRS. B.

Yes; and it is by this operation that both of them acquire a greater
degree of consistence. Pitch, tar, and turpentine, are the most common
resins; they exude from the pine and fir trees. Copal, mastic, and
frankincense, are also of this class of vegetable substances.

EMILY.

Is it of these resins that the mastic and copal varnishes, so much used
in painting, are made?

MRS. B.

Yes. Dissolved either in oil, or in alcohol, resins form varnishes. From
these solutions they may be precipitated by water, in which they are
insoluble. This I can easily show you. --If you will pour some water
into this glass of mastic varnish, it will combine with the alcohol in
which the resin is dissolved, and the latter will be precipitated in the
form of a white cloud--

EMILY.

It is so. And yet how is it that pictures or drawings, varnished with
this solution, may safely be washed with water?

MRS. B.

As the varnish dries, the alcohol evaporates, and the dry varnish or
resin which remains, not being soluble in water, will not be acted on
by it.

There is a class of compound resins called _gum-resins_, which are
precisely what their name denotes, that is to say, resins combined with
mucilage. Myrrh and assafoetida are of this description.

CAROLINE.

Is it possible that a substance of so disagreeable a smell as assafoetida
can be formed from a volatile oil?

MRS. B.

The odour of volatile oils is by no means always grateful. Onions and
garlic derive their smell from volatile oils, as well as roses and
lavender.

There is still another form under which volatile oils present
themselves, which is that of _balsams_. These consist of resinous juices
combined with a peculiar acid, called the benzoic acid. Balsams appear
to have been originally volatile oils, the oxygenation of which has
converted one part into a resin, and the other part into an acid, which,
combined together, form a balsam; such are the balsams of Peru, Tolu,
&c.


We shall now take leave of the oils and their various modifications, and
proceed to the next vegetable substance, which is _caoutchouc_. This is
a white milky glutinous fluid, which acquires consistence, and blackens
in drying, in which state it forms the substance with which you are so
well acquainted, under the name of gum-elastic.

CAROLINE.

I am surprised to hear that gum-elastic was ever white, or ever fluid!
And from what vegetable is it procured?

MRS. B.

It is obtained from two or three different species of trees, in the
East-Indies, and South-America, by making incisions in the stem. The
juice is collected as it trickles from these incisions, and moulds of
clay, in the form of little bottles of gum-elastic, are dipped into it.
A layer of this juice adheres to the clay and dries on it; and several
layers are successively added by repeating this till the bottle is of
sufficient thickness. It is then beaten to break down the clay, which is
easily shaken out. The natives of the countries where this substance is
produced sometimes make shoes and boots of it by a similar process, and
they are said to be extremely pleasant and serviceable, both from their
elasticity, and their being water-proof.


The substance which comes next in our enumeration of the immediate
ingredients of vegetables, is _extractive matter_. This is a term,
which, in a general sense, may be applied to any substance extracted
from vegetables; but it is more particularly understood to relate to the
extractive _colouring matter_ of plants. A great variety of colours are
prepared from the vegetable kingdom, both for the purposes of painting
and of dying; all the colours called _lakes_ are of this description;
but they are less durable than mineral colours, for, by long exposure to
the atmosphere, they either darken or turn yellow.

EMILY.

I know that in painting, the lakes are reckoned far less durable colours
than the ochres; but what is the reason of it?

MRS. B.

The change which takes place in vegetable colours is owing chiefly to
the oxygen of the atmosphere slowly burning their hydrogen, and leaving,
in some measure, the blackness of the carbon exposed. Such change cannot
take place in ochre, which is altogether a mineral substance.

Vegetable colours have a stronger affinity for animal than for vegetable
substances, and this is supposed to be owing to a small quantity of
nitrogen which they contain. Thus, silk and worsted will take a much
finer vegetable dye than linen and cotton.

CAROLINE.

Dying, then, is quite a chemical process?

MRS. B.

Undoubtedly. The condition required to form a good dye is, that the
colouring matter should be precipitated, or fixed, on the substance to
be dyed, and should form a compound not soluble in the liquids to which
it will probably be exposed. Thus, for instance, printed or dyed linens
or cottons must be able to resist the action of soap and water, to which
they must necessarily be subject in washing; and woollens and silks
should withstand the action of grease and acids, to which they may
accidentally be exposed.

CAROLINE.

But if linen and cotton have not a sufficient affinity for colouring
matter, how are they made to resist the action of washing, which they
always do when they are well printed?

MRS. B.

When the substance to be dyed has either no affinity for the colouring
matter, or not sufficient power to retain it, the combination is
effected, or strengthened, by the intervention of a third substance,
called a _mordant_, or basis. The mordant must have a strong affinity
both for the colouring matter and the substance to be dyed, by which
means it causes them to combine and adhere together.

CAROLINE.

And what are the substances that perform the office of thus reconciling
the two adverse parties?

MRS. B.

The most common mordant is sulphat of alumine, or alum. Oxyds of tin and
iron, in the state of compound salts, are likewise used for that
purpose.

_Tannin_ is another vegetable ingredient of great importance in the
arts. It is obtained chiefly from the bark of trees; but it is found
also in nut-galls, and in some other vegetables.

EMILY.

Is that the substance commonly called _tan_, which is used in
hot-houses?

MRS. B.

Tan is the prepared bark in which the peculiar substance, tannin, is
contained. But the use of tan in hot-houses is of much less importance
than in the operation of _tanning_, by which skin is converted into
leather.

EMILY.

Pray, how is this operation performed?

MRS. B.

Various methods are employed for this purpose, which all consist in
exposing skin to the action of tannin, or of substances containing this
principle, in sufficient quantities, and disposed to yield it to the
skin. The most usual way is to infuse coarsely powdered oak bark in
water, and to keep the skin immersed in this infusion for a certain
length of time. During this process, which is slow and gradual, the skin
is found to have increased in weight, and to have acquired a
considerable tenacity and impermeability to water. This effect may be
much accelerated by using strong saturations of the tanning principle
(which can be extracted from bark), instead of employing the bark
itself. But this quick mode of preparation does not appear to make
equally good leather.

Tannin is contained in a great variety of astringent vegetable
substances, as galls, the rose-tree, and wine; but it is nowhere so
plentiful as in bark. All these substances yield it to water, from which
it may be precipitated by a solution of isinglass, or glue, with which
it strongly unites and forms an insoluble compound. Hence its valuable
property of combining with skin (which consists chiefly of glue), and of
enabling it to resist the action of water.

EMILY.

Might we not see that effect by pouring a little melted isinglass into a
glass of wine, which you say contains tannin?

MRS. B.

Yes. I have prepared a solution of isinglass for that very purpose. --Do
you observe the thick muddy precipitate? --That is the tannin combined
with the isinglass.

CAROLINE.

This precipitate must then be of the same nature as leather?

MRS. B.

It is composed of the same ingredients; but the organisation and texture
of the skin being wanting, it has neither the consistence nor the
tenacity of leather.

CAROLINE.

One might suppose that men who drink large quantities of red wine stand
a chance of having the coats of their stomachs converted into leather,
since tannin has so strong an affinity for skin.

MRS. B.

It is not impossible but that the coats of their stomachs may be, in
some measure, tanned, or hardened by the constant use of this liquor;
but you must remember that where a number of other chemical agents are
concerned, and, above all, where life exists, no certain chemical
inference can be drawn.

I must not dismiss this subject, without mentioning a recent discovery
of Mr. Hatchett, which relates to it. This gentleman found that a
substance very similar to tannin, possessing all its leading properties,
and actually capable of tanning leather, may be produced by exposing
carbon, or any substance containing carbonaceous matter, whether
vegetable, animal, or mineral, to the action of nitric acid.

CAROLINE.

And is not this discovery very likely to be of use to manufactures?

MRS. B.

That is very doubtful, because tannin, thus artificially prepared, must
probably always be more expensive than that which is obtained from bark.
But the fact is extremely curious, as it affords one of those very rare
instances of chemistry being able to imitate the proximate principles of
organised bodies.


The last of the vegetable materials is _woody fibre_; it is the hardest
part of plants. The chief source from which this substance is derived is
wood, but it is also contained, more or less, in every solid part of
that plant. It forms a kind of skeleton of the part to which it belongs,
and retains its shape after all the other materials have disappeared. It
consists chiefly of carbon, united with a small proportion of salts, and
the other constituents common to all vegetables.

EMILY.

It is of woody fibre, then, that the common charcoal is made?

MRS. B.

Yes. Charcoal, as you may recollect, is obtained from wood, by the
separation of all its evaporable parts.

Before we take leave of the vegetable materials, it will be proper, at
least, to enumerate the several vegetable acids which we either have
had, or may have occasion to mention. I believe I formerly told you that
their basis, or radical, was uniformly composed of hydrogen and carbon,
and that their difference consisted only in the various proportions of
oxygen which they contained.


The following are the names of the vegetable acids:

The _Mucous Acid_, obtained from gum or mucilage;
    _Suberic_   -  -  - from cork;
    _Camphoric_ -  -  - from camphor;
    _Benzoic_   -  -  - from balsams;
    _Gallic_    -  -  - from galls, bark, &c.
    _Malic_     -  -  - from ripe fruits;
    _Citric_    -  -  - from lemon juice;
    _Oxalic_    -  -  - from sorrel;
    _Succinic_  -  -  - from amber;
    _Tartarous_ -  -  - from tartrit of potash:
    _Acetic_    -  -  - from vinegar.

They are all decomposable by heat, soluble in water, and turn vegetable
blue colours red. The _succinic_, the _tartarous_, and the _acetous
acids_, are the products of the decomposition of vegetables; we shall,
therefore, reserve their examination for a future period.

The _oxalic acid_, distilled from sorrel, is the highest term of
vegetable acidification; for, if more oxygen be added to it, it loses
its vegetable nature, and is resolved into carbonic acid and water;
therefore, though all the other acids may be converted into the oxalic
by an addition of oxygen, the oxalic itself is not susceptible of a
further degree of oxygenation; nor can it be made, by any chemical
processes, to return to a state of lower acidification.


To conclude this subject, I have only to add a few words on the _gallic
acid_. . . . .

CAROLINE.

Is not this the same acid before mentioned, which forms ink, by
precipitating sulphat of iron from its solution?

MRS. B.

Yes. Though it is usually extracted from galls, on account of its being
most abundant in that vegetable substance, it may also be obtained from
a great variety of plants. It constitutes what is called the _astringent
principle_ of vegetables; it is generally combined with tannin, and you
will find that an infusion of tea, coffee, bark, red-wine, or any
vegetable substance that contains the astringent principle, will make a
black precipitate with a solution of sulphat of iron.

CAROLINE.

But pray what are galls?

MRS. B.

They are excrescences which grow on the bark of young oaks, and are
occasioned by an insect which wounds the bark of trees, and lays its
eggs in the aperture. The lacerated vessels of the tree then discharge
their contents, and form an excrescence, which affords a defensive
covering for these eggs. The insect, when come to life, first feeds on
this excrescence, and some time afterward eats its way out, as it
appears from a hole which is formed in all gall-nuts that no longer
contain an insect. It is in hot climates only that strongly astringent
gall-nuts are found; those which are used for the purpose of making ink
are brought from Aleppo.

EMILY.

But are not the oak-apples, which grow on the leaves of the oak in this
country, of a similar nature?

MRS. B.

Yes; only they are an inferior species of galls, containing less of the
astringent principle, and therefore less applicable to useful purposes.

CAROLINE.

Are the vegetable acids never found but in their pure uncombined state?

MRS. B.

By no means; on the contrary, they are frequently met with in the state
of compound salts; these, however, are in general not fully saturated
with the salifiable bases, so that the acid predominates; and, in this
state, they are called _acidulous_ salts. Of this kind is the salt
called cream of tartar.

CAROLINE.

Is not the salt of lemon, commonly used to take out ink-spots and
stains, of this nature?

MRS. B.

No; that salt consists of the oxalic acid, combined with a little
potash. It is found in that state in sorrel.

CAROLINE.

And pray how does it take out ink-spots?

MRS. B.

By uniting with the iron, and rendering it soluble in water.


Besides the vegetable materials which we have enumerated, a variety of
other substances, common to the three kingdoms, are found in vegetables,
such as potash, which was formerly supposed to belong exclusively to
plants, and was, in consequence, called the vegetable alkali.

Sulphur, phosphorus, earths, and a variety of metallic oxyds, are also
found in vegetables, but only in small quantities. And we meet sometimes
with neutral salts, formed by the combination of these ingredients.




CONVERSATION XXI.

ON THE DECOMPOSITION OF VEGETABLES.


CAROLINE.

The account which you have given us, Mrs. B., of the materials of
vegetables, is, doubtless, very instructive; but it does not completely
satisfy my curiosity. I wish to know how plants obtain the principles
from which their various materials are formed; by what means these are
converted into vegetable matter, and how they are connected with the
life of the plant?

MRS. B.

This implies nothing less than a complete history of the chemistry and
physiology of vegetation, subjects on which we have yet but very
imperfect notions. Still I hope that I shall be able, in some measure,
to satisfy your curiosity. But, in order to render the subject more
intelligible, I must first make you acquainted with the various changes
which vegetables undergo, when the vital power no longer enables them to
resist the common laws of chemical attraction.

The composition of vegetables being more complicated than that of
minerals, the former more readily undergo chemical changes than the
latter: for the greater the variety of attractions, the more easily is
the equilibrium destroyed, and a new order of combinations introduced.

EMILY.

I am surprised that vegetables should be so easily susceptible of
decomposition; for the preservation of the vegetable kingdom is
certainly far more important than that of minerals.

MRS. B.

You must consider, on the other hand, how much more easily the former is
renewed than the latter. The decomposition of the vegetable takes place
only after the death of the plant, which, in the common course of
nature, happens when it has yielded fruit and seeds to propagate its
species. If, instead of thus finishing its career, each plant was to
retain its form and vegetable state, it would become an useless burden
to the earth and its inhabitants. When vegetables, therefore, cease to
be productive, they cease to live, and nature then begins her process of
decomposition, in order to resolve them into their chemical
constituents, hydrogen, carbon, and oxygen; those simple and primitive
ingredients, which she keeps in store for all her combinations.

EMILY.

But since no system of combination can be destroyed, except by the
establishment of another order of attractions, how can the decomposition
of vegetables reduce them to their simple elements?

MRS. B.

It is a very long process, during which a variety of new combinations
are successively established and successively destroyed: but, in each of
these changes, the ingredients of vegetable matter tend to unite in a
more simple order of compounds, till they are at length brought to their
elementary state, or, at least, to their most simple order of
combinations. Thus you will find that vegetables are in the end almost
entirely reduced to water and carbonic acid; the hydrogen and carbon
dividing the oxygen between them, so as to form with it these two
substances. But the variety of intermediate combinations that take place
during the several stages of the decomposition of vegetables, present us
with a new set of compounds, well worthy of our examination.

CAROLINE.

How is it possible that vegetables, while putrefying, should produce any
thing worthy of observation?

MRS. B.

They are susceptible of undergoing certain changes before they arrive at
the state of putrefaction, which is the final term of decomposition; and
of these changes we avail ourselves for particular and important
purposes. But, in order to make you understand this subject, which is of
considerable importance, I must explain it more in detail.

The decomposition of vegetables is always attended by a violent internal
motion, produced by the disunion of one order of particles, and the
combination of another. This is called FERMENTATION. There are several
periods at which this process stops, so that a state of rest appears to
be restored, and the new order of compounds fairly established. But,
unless means be used to secure these new combinations in their actual
state, their duration will be but transient, and a new fermentation will
take place, by which the compound last formed will be destroyed; and
another, and less complex order, will succeed.

EMILY.

The fermentations, then, appear to be only the successive steps by which
a vegetable descends to its final dissolution.

MRS. B.

Precisely so. Your definition is perfectly correct.

CAROLINE.

And how many fermentations, or new arrangements, does a vegetable
undergo before it is reduced to its simple ingredients?

MRS. B.

Chemists do not exactly agree in this point; but there are, I think,
four distinct fermentations, or periods, at which the decomposition of
vegetable matter stops and changes its course. But every kind of
vegetable matter is not equally susceptible of undergoing all these
fermentations.

There are likewise several circumstances required to produce
fermentation. Water and a certain degree of heat are both essential to
this process, in order to separate the particles, and thus weaken their
force of cohesion, that the new chemical affinities may be brought into
action.

CAROLINE.

In frozen climates, then, how can the spontaneous decomposition of
vegetables take place?

MRS. B.

It certainly cannot; and, accordingly, we find scarcely any vestiges of
vegetation where a constant frost prevails.

CAROLINE.

One would imagine that, on the contrary, such spots would be covered
with vegetables; for, since they cannot be decomposed, their number must
always increase.

MRS. B.

But, my dear, heat and water are quite as essential to the formation of
vegetables, as they are to their decomposition. Besides, it is from the
dead vegetables, reduced to their elementary principles, that the rising
generation is supplied with sustenance. No young plant, therefore, can
grow unless its predecessors contribute both to its formation and
support; and these not only furnish the seed from which the new plant
springs, but likewise the food by which it is nourished.

CAROLINE.

Under the torrid zone, therefore, where water is never frozen, and the
heat is very great, both the processes of vegetation and of fermentation
must, I suppose, be extremely rapid?

MRS. B.

Not so much as you imagine: for in such climates great part of the water
which it requires for these processes is in an aeriform state, which is
scarcely more conducive either to the growth or formation of vegetables
than that of ice. In those latitudes, therefore, it is only in low damp
situations, sheltered by woods from the sun's rays, that the smaller
tribes of vegetables can grow and thrive during the dry season, as dead
vegetables seldom retain water enough to produce fermentation, but are,
on the contrary, soon dried up by the heat of the sun, which enables
them to resist that process; so that it is not till the fall of the
autumnal rains (which are very violent in such climates), that
spontaneous fermentation can take place.

The several fermentations derive their names from their principal
products. The first is called the _saccharine fermentation_, because its
product is _sugar_.

CAROLINE.

But sugar, you have told us, is found in all vegetables; it cannot,
therefore, be the product of their decomposition.

MRS. B.

It is true that this fermentation is not confined to the decomposition
of vegetables, as it continually takes place during their life; and,
indeed, this circumstance has, till lately, prevented it from being
considered as one of the fermentations. But the process appears so
analogous to the other fermentations, and the formation of sugar,
whether in living or dead vegetable matter is so evidently a new
compound, proceeding from the destruction of the previous order of
combinations, and essential to the subsequent fermentations, that it is
now, I believe, generally esteemed the first step, or necessary
preliminary, to decomposition, if not an actual commencement of that
process.

CAROLINE.

I recollect your hinting to us that sugar was supposed not to be
secreted from the sap, in the same manner as mucilage, fecula, oil, and
the other ingredients of vegetables.

MRS. B.

It is rather from these materials, than from the sap itself, that sugar
is formed; and it is developed at particular periods, as you may observe
in fruits, which become sweet in ripening, sometimes even after they
have been gathered. Life, therefore, is not essential to the formation
of sugar, whilst on the contrary, mucilage, fecula, and the other
vegetable materials that are secreted from the sap by appropriate
organs, whose powers immediately depend on the vital principle, cannot
be produced but during the existence of that principle.

EMILY.

The ripening of fruits is, then, their first step to destruction, as
well as their last towards perfection?

MRS. B.

Exactly. --A process analogous to the saccharine fermentation takes
place also during the cooking of certain vegetables. This is the case
with parsnips, carrots, potatoes, &c. in which sweetness is developed by
heat and moisture; and we know that if we carried the process a little
farther, a more complete decomposition would ensue. The same process
takes place also in seeds previous to their sprouting.

CAROLINE.

How do you reconcile this to your theory, Mrs. B.? Can you suppose that
a decomposition is the necessary precursor of life?

MRS. B.

That is indeed the case. The materials of the seed must be decomposed,
and the seed disorganized, before a plant can sprout from it. Seeds,
besides the embrio plant, contain (as we have already observed) fecula,
oil, and a little mucilage. These substances are destined for the
nourishment of the future plant; but they undergo some change before
they can be fit for this function. The seeds, when buried in the earth,
with a certain degree of moisture and of temperature, absorb water,
which dilates them, separates their particles, and introduces a new
order of attractions, of which sugar is the product. The substance of
the seed is thus softened, sweetened, and converted into a sort of white
milky pulp, fit for the nourishment of the embrio plant.

The saccharine fermentation of seeds is artificially produced, for the
purpose of making _malt_, by the following process:-- A quantity of
barley is first soaked in water for two or three days: the water being
afterwards drained off, the grain heats spontaneously, swells, bursts,
sweetens, shows a disposition to germinate, and actually sprouts to the
length of an inch, when the process is stopped by putting it into a
kiln, where it is well dried at a gentle heat. In this state it is crisp
and friable, and constitutes the substance called _malt_, which is the
principal ingredient of beer.

EMILY.

But I hope you will tell us how malt is made into beer?

MRS. B.

Certainly; but I must first explain to you the nature of the second
fermentation, which is essential to that operation. This is called the
_vinous fermentation_, because its product is _wine_.

EMILY.

How very different the decomposition of vegetables is from what I had
imagined! The products of their disorganisation appear almost superior
to those which they yield during their state of life and perfection.

MRS. B.

And do you not, at the same time, admire the beautiful economy of
Nature, which, whether she creates, or whether she destroys, directs all
her operations to some useful and benevolent purpose? --It appears that
the saccharine fermentation is extremely favourable, if not absolutely
essential, as a previous step, to the vinous fermentation; so that if
sugar be not developed during the life of the plant, the saccharine
fermentation must be artificially produced before the vinous
fermentation can take place. This is the case with barley, which does
not yield any sugar until it is made into malt; and it is in that state
only that it is susceptible of undergoing the vinous fermentation by
which it is converted into beer.

CAROLINE.

But if the product of the vinous fermentation is always wine, beer
cannot have undergone that process, for beer is certainly not wine.

MRS. B.

Chemically speaking, beer may be considered as the wine of grain. For it
is the product of the fermentation of malt, just as wine is that of the
fermentation of grapes, or other fruits.

The consequence of the vinous fermentation is the decomposition of the
saccharine matter, and the formation of a spirituous liquor from the
constituents of the sugar. But, in order to promote this fermentation,
not only water and a certain degree of heat are necessary, but also some
other vegetable ingredients, besides the sugar, as fecula, mucilage,
acids, salts, extractive matter, &c. all of which seem to contribute to
this process; and give to the liquor its peculiar taste.

EMILY.

It is, perhaps, for this reason that wine is not obtained from the
fermentation of pure sugar; but that fruits are chosen for that purpose,
as they contain not only sugar, but likewise the other vegetable
ingredients which promote the vinous fermentation, and give the peculiar
flavour.

MRS. B.

Certainly. And you must observe also, that the relative quantity of
sugar is not the only circumstance to be considered in the choice of
vegetable juices for the formation of wine; otherwise the sugar-cane
would be best adapted for that purpose. It is rather the manner and
proportion in which the sugar is mixed with other vegetable ingredients
that influences the production and qualities of wine. And it is found
that the juice of the grape not only yields the most considerable
proportion of wine, but that it likewise affords it of the most grateful
flavour.

EMILY.

I have seen a vintage in Switzerland, and I do not recollect that heat
was applied, or water added, to produce the fermentation of the grapes.

MRS. B.

The common temperature of the atmosphere in the cellars in which the
juice of the grape is fermented is sufficiently warm for this purpose;
and as the juice contains an ample supply of water, there is no occasion
for any addition of it. But when fermentation is produced in dry malt,
a quantity of water must necessarily be added.

EMILY.

But what are precisely the changes that happen during the vinous
fermentation?

MRS. B.

The sugar is decomposed, and its constituents are recombined into two
new substances; the one a peculiar liquid substance, called _alcohol_ or
_spirit of wine_, which remains in the fluid; the other, carbonic acid
gas, which escapes during the fermentation. Wine, therefore, as I before
observed, in a general point of view, may be considered as a liquid of
which alcohol constitutes the essential part. And the varieties of
strength and flavour of the different kinds of wine are to be attributed
to the different qualities of the fruits from which they are obtained,
independently of the sugar.

CAROLINE.

I am astonished to hear that so powerful a liquid as spirit of wine
should be obtained from so mild a substance as sugar.

MRS. B.

Can you tell me in what the principal difference consists between
alcohol and sugar?

CAROLINE.

Let me reflect . . . . . Sugar consists of carbon, hydrogen, and oxygen.
If carbonic acid be subtracted from it, during the formation of alcohol,
the latter will contain less carbon and oxygen than sugar does;
therefore hydrogen must be the prevailing principle of alcohol.

MRS. B.

It is exactly so. And this very large proportion of hydrogen accounts
for the lightness and combustible property of alcohol, and of spirits in
general, all of which consist of alcohol variously modified.

EMILY.

And can sugar be recomposed from the combination of alcohol and carbonic
acid?

MRS. B.

Chemists have never been able to succeed in effecting this; but from
analogy, I should suppose such a recomposition possible. Let us now
observe more particularly the phenomena that take place during the
vinous fermentation. At the commencement of this process, heat is
evolved, and the liquor swells considerably from the formation of the
carbonic acid, which is disengaged in such prodigious quantities as
would be fatal to any person who should unawares inspire it; an accident
which has sometimes happened. If the fermentation be stopped by putting
the liquor into barrels, before the whole of the carbonic acid is
evolved, the wine is brisk, like Champagne, from the carbonic acid
imprisoned in it, and it tastes sweet, like cyder, from the sugar not
being completely decomposed.

EMILY.

But I do not understand why heat should be evolved during this
operation. For, as there is a considerable formation of gas, in which a
proportionable quantity of heat must become insensible, I should have
imagined that cold, rather than heat, would have been produced.

MRS. B.

It appears so on first consideration; but you must recollect that
fermentation is a complicated chemical process; and that, during the
decompositions and recompositions attending it, a quantity of chemical
heat may be disengaged, sufficient both to develope the gas, and to
effect an increase of temperature. When the fermentation is completed,
the liquid cools and subsides, the effervescence ceases, and the thick,
sweet, sticky juice of the fruit is converted into a clear, transparent,
spirituous liquor, called wine.

EMILY.

How much I regret not having been acquainted with the nature of the
vinous fermentation, when I had an opportunity of seeing the process!

MRS. B.

You have an easy method of satisfying yourself in that respect by
observing the process of brewing, which, in every essential
circumstance, is similar to that of making wine, and is really a very
curious chemical operation.

Although we cannot actually make wine at this moment, it will be easy to
show you the mode of analyzing it. This is done by distillation. When
wine of any kind is submitted to this operation, it is found to contain
brandy, water, tartar, extractive colouring matter, and some vegetable
acids. I have put a little port wine into this alembic of glass (PLATE
XIV. Fig. 1.), and on placing the lamp under it, you will soon see the
spirit and water successively come over--

  [Illustration: Plate XIV. Vol. II. p. 213.

  Fig. 1.
  A Alembic.
  B Lamp.
  C Wine glass.

  Fig. 2. Alcohol blowpipe.
  D the Lamp.
  E the vessel in which the Alcohol is boiling.
  F a safety valve.
  G the inflamed jet or steam of alcohol directed towards a glass
    tube H.]

EMILY.

But you do not mention alcohol amongst the _products_ of the
distillation of wine; and yet that is its most essential ingredient?

MRS. B.

The alcohol is contained in the brandy which is now coming over, and
dropping from the still. Brandy is nothing more than a mixture of
alcohol and water; and in order to obtain the alcohol pure, we must
again distil it from brandy.

CAROLINE.

I have just taken a drop on my finger; it tastes like strong brandy, but
it is without colour, whilst brandy is of a deep yellow.

MRS. B.

It is not so naturally; in its pure state brandy is colourless, and it
obtains the yellow tint you observe, by extracting the colouring matter
from the new oaken casks in which it is kept. But if it does not acquire
the usual tinge in this way, it is the custom to colour the brandy used
in this country artificially, with a little burnt sugar, in order to
give it the appearance of having been long kept.

CAROLINE.

And is rum also distilled from wine?

MRS. B.

By no means; it is distilled from the sugar-cane, a plant which contains
so great a quantity of sugar, that it yields more alcohol than almost
any other vegetable. After the juice of the cane has been pressed out
for making sugar, what still remains in the bruised cane is extracted by
water, and this watery solution of sugar is fermented, and produces rum.

The spirituous liquor called _arack_ is in a similar manner distilled
from the product of the vinous fermentation of rice.

EMILY.

But rice has no sweetness; does it contain any sugar?

MRS. B.

Like barley and most other seeds, it is insipid until it has undergone
the saccharine fermentation; and this, you must recollect, is always a
previous step to the vinous fermentation in those vegetables in which
sugar is not already formed. Brandy may in the same manner be obtained
from malt.

CAROLINE.

You mean from beer, I suppose; for the malt must have previously
undergone the vinous fermentation.

MRS. B.

Beer is not precisely the product of the vinous fermentation of malt.
For hops are a necessary ingredient for the formation of that liquor;
whilst brandy is distilled from pure fermented malt. But brandy might,
no doubt, be distilled from beer as well as from any other liquor that
has undergone the vinous fermentation; for since the basis of brandy is
alcohol, it may be obtained from any liquid that contains that
spirituous substance.

EMILY.

And pray, from what vegetable is the favourite spirit of the lower
orders of people, gin, extracted?

MRS. B.

The spirit (which is the same in all fermented liquors) may be obtained
from any kind of grain; but the peculiar flavour which distinguishes gin
is that of juniper berries, which are distilled together with the
grain--

I think the brandy contained in the wine which we are distilling must,
by this time, be all come over. Yes--taste the liquid that is now
dropping from the alembic--

CAROLINE.

It is perfectly insipid, like water.

MRS. B.

It is water, which, as I was telling you, is the second product of wine,
and comes over after all the spirit, which is the lightest part, is
distilled. --The tartar and extractive colouring matter we shall find in
a solid form at the bottom of the alembic.

EMILY.

They look very like the lees of wine.

MRS. B.

And in many respects they are of a similar nature; for lees of wine
consist chiefly of tartrit of potash; a salt which exists in the juice
of the grape, and in many other vegetables, and is developed only by the
vinous fermentation. During this operation it is precipitated, and
deposits itself on the internal surface of the cask in which the wine is
contained. It is much used in medicine, and in various arts,
particularly dying, under the name of _cream of tartar_, and it is from
this salt that the tartarous acid is obtained.

CAROLINE.

But the medicinal cream of tartar is in appearance quite different from
these dark-coloured dregs; it is perfectly colourless.

MRS. B.

Because it consists of the pure salts only, in its crystallised form;
whilst in the instance before us it is mixed with the deep-coloured
extractive matter, and other foreign ingredients.

EMILY.

Pray cannot we now obtain pure alcohol from the brandy which we have
distilled?

MRS. B.

We might; but the process would be tedious: for in order to obtain
alcohol perfectly free from water, it is necessary to distil, or, as the
distillers call it, _rectify_ it several times. You must therefore allow
me to produce a bottle of alcohol that has been thus purified. This is a
very important ingredient, which has many striking properties, besides
its forming the basis of all spirituous liquors.

EMILY.

It is alcohol, I suppose, that produces intoxication?

MRS. B.

Certainly; but the stimulus and momentary energy it gives to the system,
and the intoxication it occasions when taken in excess, are
circumstances not yet accounted for.

CAROLINE.

I thought that it produced these effects by increasing the rapidity of
the circulation of the blood; for drinking wine or spirits, I have
heard, always quickens the pulse.

MRS. B.

No doubt; the spirit, by stimulating the nerves, increases the action of
the muscles; and the heart, which is one of the strongest muscular
organs, beats with augmented vigour, and propels the blood with
accelerated quickness. After such a strong excitation the frame
naturally suffers a proportional degree of depression, so that a state
of debility and languor is the invariable consequence of intoxication.
But though these circumstances are well ascertained, they are far from
explaining why alcohol should produce such effects.

EMILY.

Liqueurs are the only kind of spirits which I think pleasant. Pray of
what do they consist?

MRS. B.

They are composed of alcohol, sweetened with syrup, and flavoured with
volatile oil.

The different kinds of odoriferous spirituous waters are likewise
solutions of volatile oil in alcohol, as lavender water, eau de
Cologne, &c.

The chemical properties of alcohol are important and numerous. It is one
of the most powerful chemical agents, and is particularly useful in
dissolving a variety of substances, which are soluble neither by water
nor heat.

EMILY.

We have seen it dissolve copal and mastic to form varnishes; and these
resins are certainly not soluble in water, since water precipitates them
from their solution in alcohol.

MRS. B.

I am happy to find that you recollect these circumstances so well. The
same experiment affords also an instance of another property of
alcohol,--its tendency to unite with water; for the resin is
precipitated in consequence of losing the alcohol, which abandons it
from its preference for water. It is attended also, as you may
recollect, with the same peculiar circumstance of a disengagement of
heat and consequent diminution of bulk, which we have supposed to be
produced by a mechanical penetration of particles by which latent heat
is forced out.

Alcohol unites thus readily not only with resins and with water, but
with oils and balsams; these compounds form the extensive class of
elixirs, tinctures, quintessences, &c.

EMILY.

I suppose that alcohol must be highly combustible, since it contains so
large a proportion of hydrogen?

MRS. B.

Extremely so; and it will burn at a very moderate temperature.

CAROLINE.

I have often seen both brandy and spirit of wine burnt; they produce a
great deal of flame, but not a proportional quantity of heat, and no
smoke whatever.

MRS. B.

The last circumstance arises from their combustion being complete; and
the disproportion between the flame and heat shows you that these are by
no means synonymous.

The great quantity of flame proceeds from the combustion of the hydrogen
to which, you know, that manner of burning is peculiar. --Have you not
remarked also that brandy and alcohol will burn without a wick? --They
take fire at so low a temperature, that this assistance is not required
to concentrate the heat and volatilise the fluid.

CAROLINE.

I have sometimes seen brandy burnt by merely heating it in a spoon.

MRS. B.

The rapidity of the combustion of alcohol may, however, be prodigiously
increased by first volatilising it. An ingenious instrument has been
constructed on this principle to answer the purpose of a blow-pipe,
which may be used for melting glass, or other chemical purposes. It
consists of a small metallic vessel (PLATE XIV. Fig. 2.), of a spherical
shape, which contains the alcohol, and is heated by the lamp beneath it;
as soon as the alcohol is volatilised, it passes through the spout of
the vessel, and issues just above the wick of the lamp, which
immediately sets fire to the stream of vapour, as I shall show you--

EMILY.

With what amazing violence it burns! The flame of alcohol, in the state
of vapour, is, I fancy, much hotter than when the spirit is merely burnt
in a spoon?

MRS. B.

Yes; because in this way the combustion goes on much quicker, and, of
course, the heat is proportionally increased. --Observe its effect on
this small glass tube, the middle of which I present to the extremity of
the flame, where the heat is greatest.

CAROLINE.

The glass, in that spot, is become red hot, and bends from its own
weight.

MRS. B.

I have now drawn it asunder, and am going to blow a ball at one of the
heated ends; but I must previously close it up, and flatten it with this
little metallic instrument, otherwise the breath would pass through the
tube without dilating any part of it. --Now, Caroline, will you blow
strongly into the tube whilst the closed end is red hot.

EMILY.

You blowed too hard; for the ball suddenly dilated to a great size, and
then burst in pieces.

MRS. B.

You will be more expert another time; but I must caution you, should you
ever use this blow-pipe, to be very careful that the combustion of the
alcohol does not go on with too great violence, for I have seen the
flame sometimes dart out with such force as to reach the opposite wall
of the room, and set the paint on fire. There is, however, no danger of
the vessel bursting, as it is provided with a safety tube, which affords
an additional vent for the vapour of alcohol when required.

The products of the combustion of alcohol consist in a great proportion
of water, and a small quantity of carbonic acid. There is no smoke or
fixed remains whatever. --How do you account for that, Emily?

EMILY.

I suppose that the oxygen which the alcohol absorbs in burning, converts
its hydrogen into water and its carbon into carbonic acid gas, and thus
it is completely consumed.

MRS. B.

Very well. --_Ether_, the lightest of all fluids, and with which you are
well acquainted, is obtained from alcohol, of which it forms the
lightest and most volatile part.

EMILY.

Ether, then, is to alcohol, what alcohol is to brandy?

MRS. B.

No: there is an essential difference. In order to obtain alcohol from
brandy, you need only deprive the latter of its water; but for the
formation of ether, the alcohol must be decomposed, and one of its
constituents partly subtracted. I leave you to guess which of them
it is--

EMILY.

It cannot be hydrogen, as ether is more volatile than alcohol, and
hydrogen is the lightest of all its ingredients: nor do I suppose that
it can be oxygen, as alcohol contains so small a proportion of that
principle; it is, therefore, most probably, carbon, a diminution of
which would not fail to render the new compound more volatile.

MRS. B.

You are perfectly right. The formation of ether consists simply in
subtracting from the alcohol a certain proportion of carbon; this is
effected by the action of the sulphuric, nitric, or muriatic acids, on
alcohol. The acid and carbon remain at the bottom of the vessel, whilst
the decarbonised alcohol flies off in the form of a condensable vapour,
which is ether.

Ether is the most inflammable of all fluids, and burns at so slow a
temperature that the heat evolved during its combustion is more than is
required for its support, so that a quantity of ether is volatilised,
which takes fire, and gradually increases the violence of the
combustion.


Sir Humphry Davy has lately discovered a very singular fact respecting
the vapour of ether. If a few drops of ether be poured into a
wine-glass, and a fine platina wire, heated almost to redness, be held
suspended in the glass, close to the surface of the ether, the wire soon
becomes intensely red-hot, and remains so for any length of time. We may
easily try the experiment. . . . .

CAROLINE.

How very curious! The wire is almost white hot, and a pungent smell
rises from the glass. Pray how is this accounted for?

MRS. B.

This is owing to a very peculiar property of the vapour of ether, and
indeed of many other combustible gaseous bodies. At a certain
temperature lower than that of ignition, these vapours undergo a slow
and imperfect combustion, which does not give rise, in any sensible
degree, to the phenomena of light and flame, and yet extricates a
quantity of caloric sufficient to react upon the wire and make it
red-hot, and the wire in its turn keeps up the effect as long as the
emission of vapour continues.

CAROLINE.

But why should not an iron or silver wire produce the same effect?

MRS. B.

Because either iron or silver, being much better conductors of heat than
platina, the heat is carried off too fast by those metals to allow the
accumulation of caloric necessary to produce the effect in question.


Ether is so light that it evaporates at the common temperature of the
atmosphere; it is therefore necessary to keep it confined by a well
ground glass stopper. No degree of cold known has ever frozen it.

CAROLINE.

Is it not often taken medicinally?

MRS. B.

Yes; it is one of the most effectual antispasmodic medicines, and the
quickness of its effects, as such, probably depends on its being
instantly converted into vapour by the heat of the stomach, through the
intervention of which it acts on the nervous system. But the frequent
use of ether, like that of spirituous liquors, becomes prejudicial, and,
if taken to excess, it produces effects similar to those of
intoxication.

We may now take our leave of the vinous fermentation, of which, I hope,
you have acquired a clear idea; as well as of the several products that
are derived from it.

CAROLINE.

Though this process appears, at first sight, so much complicated, it
may, I think, be summed up in a few words, as it consists in the
conversion of sugar and fermentable bodies into alcohol and carbonic
acid, which give rise both to the formation of wine, and of all kinds of
spirituous liquors.

MRS. B.

We shall now proceed to the _acetous fermentation_, which is thus
called, because it converts wine into vinegar, by the formation of the
acetous acid, which is the basis or radical of vinegar.

CAROLINE.

But is not the acidifying principle of the acetous acid the same as that
of all other acids, oxygen?

MRS. B.

Certainly; and on that account the contact of air is essential to this
fermentation, as it affords the necessary supply of oxygen. Vinegar, in
order to obtain pure acetous acid from it, must be distilled and
rectified by certain processes.

EMILY.

But pray, Mrs. B., is not the acetous acid frequently formed without
this fermentation taking place? Is it not, for instance, contained in
acid fruits, and in every substance that becomes sour?

MRS. B.

No, not in fruits; you confound it with the citric, the malic, the
oxalic, and other vegetable acids, to which living vegetables owe their
acidity. But whenever a vegetable substance turns sour, after it has
ceased to live, the acetous acid is developed by means of the acetous
fermentation, in which the substance advances a step towards its final
decomposition.

Amongst the various instances of acetous fermentation, that of bread is
usually classed.

CAROLINE.

But the fermentation of bread is produced by yeast; how does that
effect it?

MRS. B.

It is found by experience that any substance that has already undergone
a fermentation, will readily excite it in one that is susceptible of
that process. If, for instance, you mix a little vinegar with wine, that
is intended to be acidified, it will absorb oxygen more rapidly, and the
process be completed much sooner, than if left to ferment spontaneously.
Thus yeast, which is a product of the fermentation of beer, is used to
excite and accelerate the fermentation of malt, which is to be converted
into beer, as well as that of paste which is to be made into bread.

CAROLINE.

But if bread undergoes the acetous fermentation, why is it not sour?

MRS. B.

It acquires a certain savour which corrects the heavy insipidity of
flour, and may be reckoned a first degree of acidification; or if the
process were carried further, the bread would become decidedly acid.

There are, however, some chemists who do not consider the fermentation
of bread as being of the acetous kind, but suppose that it is a process
of fermentation peculiar to that substance.

The _putrid fermentation_ is the final operation of Nature, and her last
step towards reducing organised bodies to their simplest combinations.
All vegetables spontaneously undergo this fermentation after death,
provided there be a sufficient degree of heat and moisture, together
with access of air; for it is well known that dead plants may be
preserved by drying, or by the total exclusion of air.

CAROLINE.

But do dead plants undergo the other fermentation previous to this last;
or do they immediately suffer the putrid fermentation?

MRS. B.

That depends on a variety of circumstances, such as the degrees of
temperature and of moisture, the nature of the plant itself, &c. But if
you were carefully to follow and examine the decomposition of plants
from their death to their final dissolution, you would generally find a
sweetness developed in the seeds, and a spirituous flavour in the fruits
(which have undergone the saccharine fermentation), previous to the
total disorganisation and separation of the parts.

EMILY.

I have sometimes remarked a kind of spirituous taste in fruits that were
over ripe, especially oranges; and this was just before they became
rotten.

MRS. B.

It was then the vinous fermentation which had succeeded the saccharine,
and had you followed up these changes attentively, you would probably
have found the spirituous taste followed by acidity, previous to the
fruit passing to the state of putrefaction.

When the leaves fall from the trees in autumn, they do not (if there is
no great moisture in the atmosphere) immediately undergo a
decomposition, but are first dried and withered; as soon, however, as
the rain sets in, fermentation commences, their gaseous products are
imperceptibly evolved into the atmosphere, and their fixed remains mixed
with their kindred earth.

Wood, when exposed to moisture, also undergoes the putrid fermentation
and becomes rotten.

EMILY.

But I have heard that the _dry rot_, which is so liable to destroy the
beams of houses, is prevented by a current of air; and yet you said that
air was essential to the putrid fermentation?

MRS. B.

True; but it must not be in such a proportion to the moisture as to
dissolve the latter, and this is generally the case when the rotting of
wood is prevented or stopped by the free access of air. What is commonly
called dry rot, however, is not I believe a true process of
putrefaction. It is supposed to depend on a peculiar kind of vegetation,
which, by feeding on the wood, gradually destroys it.

Straw and all other kinds of vegetable matter undergo the putrid
fermentation more rapidly when mixed with animal matter. Much heat is
evolved during this process, and a variety of volatile products are
disengaged, as carbonic acid and hydrogen gas, the latter of which is
frequently either sulphurated or phosphorated. --When all these gases
have been evolved, the fixed products, consisting of carbon, salts,
potash, &c. form a kind of vegetable earth, which makes very fine
manure, as it is composed of those elements which form the immediate
materials of plants.

CAROLINE.

Pray are not vegetables sometimes preserved from decomposition by
petrification? I have seen very curious specimens of petrified
vegetables, in which state they perfectly preserve their form and
organisation, though in appearance they are changed to stone.

MRS. B.

That is a kind of metamorphosis, which, now that you are tolerably well
versed in the history of mineral and vegetable substances, I leave to
your judgment to explain. Do you imagine that vegetables can be
converted into stone?

EMILY.

No, certainly; but they might perhaps be changed to a substance in
appearance resembling stone.

MRS. B.

It is not so, however, with the substances that are called petrified
vegetables; for these are really stone, and generally of the hardest
kind, consisting chiefly of silex. The case is this: when a vegetable is
buried under water, or in wet earth, it is slowly and gradually
decomposed. As each successive particle of the vegetable is destroyed,
its place is supplied by a particle of siliceous earth, conveyed thither
by the water. In the course of time the vegetable is entirely destroyed,
but the silex has completely replaced it, having assumed its form and
apparent texture, as if the vegetable itself were changed to stone.

CAROLINE.

That is very curious! and I suppose that petrified animal substances are
of the same nature?

MRS. B.

Precisely. It is equally impossible for either animal or vegetable
substances to be converted into stone. They may be reduced, as we find
they are, by decomposition, to their constituent elements, but cannot be
changed to elements, which do not enter into their composition.

There are, however, circumstances which frequently prevent the regular
and final decomposition of vegetables; as, for instance, when they are
buried either in the sea, or in the earth, where they cannot undergo the
putrid fermentation for want of air. In these cases they are subject to
a peculiar change, by which they are converted into a new class of
compounds, called _bitumens_.

CAROLINE.

These are substances I never heard of before.

MRS. B.

You will find, however, that some of them are very familiar to you.
Bitumens are vegetables so far decomposed as to retain no organic
appearance; but their origin is easily detected by their oily nature,
their combustibility, the products of their analysis, and the
impressions of the forms of leaves, grains, fibres of wood, and even of
animals, which they frequently bear.

They are sometimes of an oily liquid consistence, as the substance
called _naptha_, in which we preserved potassium; it is a fine
transparent colourless fluid, that issues out of clays in some parts of
Persia. But more frequently bitumens are solid, as _asphaltum_,
a smooth, hard, brittle substance, which easily melts, and forms, in its
liquid state, a beautiful dark brown colour for oil painting. _Jet_,
which is of a still harder texture, is a peculiar bitumen, susceptible
of so fine a polish, that it is used for many ornamental purposes.


_Coal_ is also a bituminous substance, to the composition of which both
the mineral and animal kingdoms seem to concur. This most useful mineral
appears to consist chiefly of vegetable matter, mixed with the remains
of marine animals and marine salts, and occasionally containing a
quantity of sulphuret of iron, commonly called pyrites.

EMILY.

It is, I suppose, the earthly, the metallic, and the saline parts of
coals, that compose the cinders or fixed products of their combustion;
whilst the hydrogen and carbon, which they derive from vegetables,
constitute their volatile products.

CAROLINE.

Pray is not _coke_, (which I have heard is much used in some
manufactures,) also a bituminous substance?

MRS. B.

No; it is a kind of fuel artificially prepared from coals. It consists
of coals reduced to a substance analogous to charcoal, by the
evaporation of their bituminous parts. Coke, therefore, is composed of
carbon, with some earthy and saline ingredients.

_Succin_, or _yellow amber_, is a bitumen which the ancients called
_electrum_, from whence the word electricity is derived, as that
substance is peculiarly, and was once supposed to be exclusively,
electric. It is found either deeply buried in the bowels of the earth,
or floating on the sea, and is supposed to be a resinous body which has
been acted on by sulphuric acid, as its analysis shows it to consist of
ah oil and an acid. The oil is called _oil of amber_, the acid the
_succinic_.

EMILY.

That oil I have sometimes used in painting, as it is reckoned to change
less than the other kinds of oils.

MRS. B.

The last class of vegetable substances that have changed their nature
are _fossil-wood_, _peat_, and _turf_. These are composed of wood and
roots of shrubs, that are partly decomposed by being exposed to moisture
under ground, and yet, in some measure, preserve their form and organic
appearance. The peat, or black earth of the moors, retains but few
vestiges of the roots to which it owes its richness and combustibility,
these substances being in the course of time reduced to the state of
vegetable earth. But in turf the roots of plants are still discernible,
and it equally answers the purpose of fuel. It is the combustible used
by the poor in heathy countries, which supply it abundantly.

It is too late this morning to enter upon the history of vegetation. We
shall reserve this subject, therefore, for our next interview, when I
expect that it will furnish us with ample matter for another
conversation.




CONVERSATION XXII.

HISTORY OF VEGETATION.


MRS. B.

The VEGETABLE KINGDOM may be considered as the link which unites the
mineral and animal creation into one common chain of beings; for it is
through the means of vegetation alone that mineral substances are
introduced into the animal system, since, generally speaking, it is from
vegetables that all animals ultimately derive their sustenance.

CAROLINE.

I do not understand that; the human species subsists as much on animal
as on vegetable food, and there are some carnivorous animals that will
eat only animal food.

MRS. B.

That is true; but you do not consider that those that live on animal
food, derive their sustenance equally, though not so immediately, from
vegetables. The meat that we eat is formed from the herbs of the field,
and the prey of carnivorous animals proceeds, either directly or
indirectly, from the same source. It is, therefore, through this channel
that the simple elements become a part of the animal frame. We should in
vain attempt to derive nourishment from carbon, hydrogen, and oxygen,
either in their separate state, or combined in the mineral kingdom; for
it is only by being united in the form of vegetable combination, that
they become capable of conveying nourishment.

EMILY.

Vegetation, then, seems to be the method which Nature employs to prepare
the food of animals?

MRS. B.

That is certainly its principal object. The vegetable creation does not
exhibit more wisdom in that admirable system of organisation, by which
it is enabled to answer its own immediate ends of preservation,
nutrition, and propagation, than in its grand and ultimate object of
forming those arrangements and combinations of principles, which are so
well adapted for the nourishment of animals.

EMILY.

But I am very curious to know whence vegetables obtain those principles
which form their immediate materials?

MRS. B.

This is a point on which we are yet so much in the dark, that I cannot
hope fully to satisfy your curiosity; but what little I know on this
subject, I will endeavour to explain to you.

The soil, which, at first view, appears to be the aliment of vegetables,
is found, on a closer investigation, to be little more than the channel
through which they receive their nourishment; so that it is very
possible to rear plants without any earth or soil.

CAROLINE.

Of that we have an instance in the hyacinth and other bulbous roots,
which will grow and blossom beautifully in glasses of water. But I
confess I should think it would be difficult to rear trees in a similar
manner.

MRS. B.

No doubt it would, as it is the burying of the roots in the earth that
supports the stem of the tree. But this office, besides that of
affording a vehicle for food, is far the most important part which the
earthy portion of the soil performs in the process of vegetation; for we
can discover, by analysis, but an extremely small proportion of earth in
vegetable compounds.

CAROLINE.

But if earths do not afford nourishment, why is it necessary to be so
attentive to the preparation of the soil?

MRS. B.

In order to impart to it those qualities which render it a proper
vehicle for the food of the plant. Water is the chief nourishment of
vegetables; if, therefore, the soil be too sandy, it will not retain a
quantity of water sufficient to supply the roots of the plants. If, on
the contrary, it abound too much with clay, the water will lodge in such
quantities as to threaten a decomposition of the roots. Calcareous soils
are, upon the whole, the most favourable to the growth of plants: soils
are, therefore, usually improved by chalk, which, you may recollect, is
a carbonat of lime. Different vegetables, however, require different
kinds of soils. Thus rice demands a moist retentive soil; potatoes a
soft sandy soil; wheat a firm and rich soil. Forest trees grow better in
fine sand than in a stiff clay; and a light ferruginous soil is best
suited to fruit-trees.

CAROLINE.

But pray what is the use of manuring the soil?

MRS. B.

Manure consists of all kinds of substances, whether of vegetable or
animal origin, which have undergone the putrid fermentation, and are
consequently decomposed, or nearly so, into their elementary principles.
And it is requisite that these vegetable matters should be in a state of
decay, or approaching decomposition. The addition of calcareous earth,
in the state of chalk or lime, is beneficial to such soils, as it
accelerates the dissolution of vegetable bodies. Now, I ask you, what is
the utility of supplying the soil with these decomposed substances?

CAROLINE.

It is, I suppose, in order to furnish vegetables with the principles
which enter into their composition. For manures not only contain carbon,
hydrogen, and oxygen, but by their decomposition supply the soil with
these principles in their elementary form.

MRS. B.

Undoubtedly; and it is for this reason that the finest crops are
produced in fields that were formerly covered with woods, because their
soil is composed of a rich mould, a kind of vegetable earth, which
abounds in those principles.

EMILY.

This accounts for the plentifulness of the crops produced in America,
where the country was but a few years since covered with wood.

CAROLINE.

But how is it that animal substances are reckoned to produce the best
manure? Does it not appear much more natural that the decomposed
elements of vegetables should be the most appropriate to the formation
of new vegetables?

MRS. B.

The addition of a much greater proportion of nitrogen, which constitutes
the chief difference between animal and vegetable matter, renders the
composition of the former more complicated, and consequently more
favourable to decomposition. The use of animal substances is chiefly to
give the first impulse to the fermentation of the vegetable ingredients
that enter into the composition of manures. The manure of a farm-yard is
of that description; but there is scarcely any substance susceptible of
undergoing the putrid fermentation that will not make good manure. The
heat produced by the fermentation of manure is another circumstance
which is extremely favourable to vegetation; yet this heat would be too
great if the manure was laid on the ground during the height of
fermentation; it is used in this state only for hot-beds, to produce
melons, cucumbers, and such vegetables as require a very high
temperature.

CAROLINE.

A difficulty has just occurred to me which I do not know how to remove.
Since all organised bodies are, in the common course of nature,
ultimately reduced to their elementary state, they must necessarily in
that state enrich the soil, and afford food for vegetation. How is it,
then, that agriculture, which cannot increase the quantity of those
elements that are required to manure the earth, can increase its produce
so wonderfully as is found to be the case in all cultivated countries?

MRS. B.

It is by suffering none of these decaying bodies to be dissipated, but
in applying them duly to the soil. It is by a judicious preparation of
the soil, which consists in fitting it either for the general purposes
of vegetation, or for that of the particular seed which is to be sown.
Thus, if the soil be too wet, it may be drained; if too loose and sandy,
it may be rendered more consistent and retentive of water by the
addition of clay or loam; it may be enriched by chalk, or any kind of
calcareous earth. On soils thus improved, manures will act with double
efficacy, and if attention be paid to spread them on the ground at a
proper season of the year, to mix them with the soil so that they may be
generally diffused through it, to destroy the weeds which might
appropriate these nutritive principles to their own use, to remove the
stones which would impede the growth of the plant, &c. we may obtain a
produce an hundred fold more abundant than the earth would spontaneously
supply.

EMILY.

We have a very striking instance of this in the scanty produce of
uncultivated commons, compared to the rich crops of meadows which are
occasionally manured.

CAROLINE.

But, Mrs. B., though experience daily proves the advantage of
cultivation, there is still a difficulty which I cannot get over.
A certain quantity of elementary principles exist in nature, which it is
not in the power of man either to augment or diminish. Of these
principles you have taught us that both the animal and vegetable
creation are composed. Now the more of them is taken up by the vegetable
kingdom, the less, it would seem, will remain for animals; and,
therefore, the more populous the earth becomes, the less it will
produce.

MRS. B.

Your reasoning is very plausible; but experience every where contradicts
the inference you would draw from it; for we find that the animal and
vegetable kingdoms, instead of thriving, as you would suppose, at each
other's expense, always increase and multiply together. For you should
recollect that animals can derive the elements of which they are formed
only through the medium of vegetables. And you must allow that your
conclusion would be valid only if every particle of the several
principles that could possibly be spared from other purposes were
employed in the animal and vegetable creations. Now we have reason to
believe that a much greater proportion of these principles than is
required for such purposes remains either in an elementary state, or
engaged in a less useful mode of combination in the mineral kingdom.
Possessed of such immense resources as the atmosphere and the waters
afford us, for oxygen, hydrogen, and carbon, so far from being in danger
of working up all our simple materials, we cannot suppose that we shall
ever bring agriculture to such a degree of perfection as to require the
whole of what these resources could supply.

Nature, however, in thus furnishing us with an inexhaustible stock of
raw materials, leaves it in some measure to the ingenuity of man to
appropriate them to its own purposes. But, like a kind parent, she
stimulates him to exertion, by setting the example and pointing out the
way. For it is on the operations of nature that all the improvements of
art are founded. The art of agriculture consists, therefore, in
discovering the readiest method of obtaining the several principles,
either from their grand sources, air and water, or from the
decomposition of organised bodies; and in appropriating them in the best
manner to the purposes of vegetation.

EMILY.

But, among the sources of nutritive principles, I am surprised that you
do not mention the earth itself, as it contains abundance of coals,
which are chiefly composed of carbon.

MRS. B.

Though coals abound in carbon, they cannot, on account of their hardness
and impermeable texture, be immediately subservient to the purposes of
vegetation.

EMILY.

No; but by their combustion carbonic acid is produced; and this entering
into various combinations on the surface of the earth, may, perhaps,
assist in promoting vegetation.

MRS. B.

Probably it may in some degree; but at any rate the quantity of
nourishment which vegetables may derive from that source can be but very
trifling, and must entirely depend on local circumstances.

CAROLINE.

Perhaps the smoky atmosphere of London is the cause of vegetation being
so forward and so rich in its vicinity?

MRS. B.

I rather believe that this circumstance proceeds from the very ample
supply of manure, assisted, perhaps, by the warmth and shelter which the
town affords. Far from attributing any good to the smoky atmosphere of
London, I confess I like to anticipate the time when we shall have made
such progress in the art of managing combustion, that every particle of
carbon will be consumed, and the smoke destroyed at the moment of its
production. We may then expect to have the satisfaction of seeing the
atmosphere of London as clear as that of the country. --But to return to
our subject: I hope that you are now convinced that we shall not easily
experience a deficiency of nutritive elements to fertilise the earth,
and that, provided we are but industrious in applying them to the best
advantage by improving the art of agriculture, no limits can be assigned
to the fruits that we may expect to reap from our labours.

CAROLINE.

Yes; I am perfectly satisfied in that respect, and I can assure you that
I feel already much more interested in the progress and improvement of
agriculture.

EMILY.

I have frequently thought that the culture of the land was not
considered as a concern of sufficient importance. Manufactures always
take the lead; and health and innocence are frequently sacrificed to the
prospect of a more profitable employment. It has often grieved me to see
the poor manufacturers crowded together in close rooms, and confined for
the whole day to the most uniform and sedentary employment, instead of
being engaged in that innocent and salutary kind of labour, which Nature
seems to have assigned to man for the immediate acquirement of comfort,
and for the preservation of his existence. I am sure that you agree with
me in thinking so, Mrs. B.?

MRS. B.

I am entirely of your opinion, my dear, in regard to the importance of
agriculture; but as the conveniences of life, which we are all enjoying,
are not derived merely from the soil, I am far from wishing to
depreciate manufactures. Besides, as the labour of one man is sufficient
to produce food for several, those whose industry is not required in
tillage must do something in return for the food that is provided for
them. They exchange, consequently, the accommodations for the
necessaries of life. Thus the carpenter and the weaver lodge and clothe
the peasant, who supplies them with their daily bread. The greater stock
of provisions, therefore, which the husbandman produces, the greater is
the quantity of accommodation which the artificer prepares. Such are the
happy effects which naturally result from civilised society. It would be
wiser, therefore, to endeavour to improve the situation of those who are
engaged in manufactures, than to indulge in vain declamations on the
hardships to which they are too frequently exposed.

But we must not yet take our leave of the subject of agriculture; we
have prepared the soil, it remains for us now to sow the seed. In this
operation we must be careful not to bury it too deep in the ground, as
the access of air is absolutely necessary to its germination; the earth
must, therefore, lie loose and light over it, in order that the air may
penetrate. Hence the use of ploughing and digging, harrowing and raking,
&c. A certain degree of heat and moisture, such as usually takes place
in the spring, is likewise necessary.

CAROLINE.

One would imagine you were going to describe the decomposition of an old
plant, rather than the formation of a new one; for you have enumerated
all the requisites of fermentation.

MRS. B.

Do you forget, my dear, that the young plant derives its existence from
the destruction of the seed, and that it is actually by the saccharine
fermentation that the latter is decomposed?

CAROLINE.

True; I wonder that I did not recollect that. The temperature and
moisture required for the germination of the seed is then employed in
producing the saccharine fermentation within it?

MRS. B.

Certainly. But, in order to understand the nature of germination, you
should be acquainted with the different parts of which the seed is
composed. The external covering or envelope contains, besides the germ
of the future plant, the substance which is to constitute its first
nourishment; this substance, which is called the _parenchyma_, consists
of fecula, mucilage, and oil, as we formerly observed.

The seed is generally divided into two compartments, called _lobes_, or
_cotyledons_, as is exemplified by this bean (PLATE XV. Fig. 1.)--the
dark-coloured kind of string which divides the lobes is called the
_radicle_, as it forms the root of the plant, and it is from a
contiguous substance, called _plumula_, which is enclosed within the
lobes, that the stem arises. The figure and size of the seed depend very
much upon the cotyledons; these vary in number in different seeds; some
have only one, as wheat, oats, barley, and all the grasses; some have
three, others six. But most seeds, as, for instance, all the varieties
of beans, have two cotyledons. When the seed is buried in the earth, at
any temperature above 40 degrees, it imbibes water, which softens and
swells the lobes; it then absorbs oxygen, which combines with some of
its carbon, and is returned in the form of carbonic acid. This loss of
carbon increases the comparative proportion of hydrogen and oxygen in
the seed, and excites the saccharine fermentation, by which the
parenchymatous matter is converted into a kind of sweet emulsion. In
this form it is carried into the radicle by vessels appropriated to that
purpose; and in the mean time, the fermentation having caused the seed
to burst, the cotyledons are rent asunder, the radicle strikes into the
ground and becomes the root of the plant, and hence the fermented liquid
is conveyed to the plumula, whose vessels have been previously distended
by the heat of the fermentation. The plumula being thus swelled, as it
were, by the emulsive fluid, raises itself and springs up to the surface
of the earth, bearing with it the cotyledons, which, as soon as they
come in contact with the air, spread themselves, and are transformed
into leaves. --If we go into the garden, we shall probably find some
seeds in the state which I have described--

  [Illustration: Plate XV. Vol. II. p. 250

  Germination.
  Fig. 1 & 2.
  A.B Cotyledons.
  C Envelope.
  D Radicle.

  Fig. 3.
  A.B Cotyledons.
  C Plumula.
  D Radicle.

  Fig. 4.
  A.B. Cotyledons.
  C Plumula.
  D Radicle.

  Fig. 5. Apparatus to illustrate the mechanism of breathing.
  A.A Glass Bell.
  B Bladder representing the lungs.
  C Bladder representing the Diaphragm.]

EMILY.

Here are some lupines that are just making their appearance above
ground.

MRS. B.

We shall take up several of them to observe their different degrees of
progress in vegetation. Here is one that has but recently burst its
envelope--do you see the little radicle striking downwards? (PLATE XV.
Fig. 2.) In this the plumula is not yet visible. But here is another in
a greater state of forwardness--the plumula, or stem, has risen out of
the ground, and the cotyledons are converted into seed leaves. (PLATE
XV. Fig. 3.)

CAROLINE.

These leaves are very thick and clumsy, and unlike the other leaves,
which I perceive are just beginning to appear.

MRS. B.

It is because they retain the remains of the parenchyma, with which they
still continue to nourish the young plant, as it has not yet sufficient
roots and strength to provide for its sustenance from the soil. --But,
in this third lupine (PLATE XV. Fig. 4.), the radicle had sunk deep
into the earth, and sent out several shoots, each of which is furnished
with a mouth to suck up nourishment from the soil; the function of the
original leaves, therefore, being no longer required, they are gradually
decaying, and the plumula is become a regular stem, shooting out small
branches, and spreading its foliage.

EMILY.

There seems to be a very striking analogy between a seed and an egg;
both require an elevation of temperature to be brought to life; both at
first supply with aliment the organised being which they produce; and as
soon as this has attained sufficient strength to procure its own
nourishment, the egg-shell breaks, whilst in the plant the seed-leaves
fall off.

MRS. B.

There is certainly some resemblance between these processes; and when
you become acquainted with animal chemistry, you will frequently be
struck with its analogy to that of the vegetable kingdom.

As soon as the young plant feeds from the soil, it requires the
assistance of leaves, which are the organs by which it throws off its
super-abundant fluid; this secretion is much more plentiful in the
vegetable than in the animal creation, and the great extent of surface
of the foliage of plants is admirably calculated for carrying it on in
sufficient quantities. This transpired fluid consists of little more
than water. The sap, by this process, is converted into a liquid of
greater consistence, which is fit to be assimilated to its several
parts.

EMILY.

Vegetation, then, must be essentially injured by destroying the leaves
of the plant?

MRS. B.

Undoubtedly; it not only diminishes the transpiration, but also the
absorption by the roots; for the quantity of sap absorbed is always in
proportion to the quantity of fluid thrown off by transpiration. You
see, therefore, the necessity that a young plant should unfold its
leaves as soon as it begins to derive its nourishment from the soil;
and, accordingly, you will find that those lupines which have dropped
their seed-leaves, and are no longer fed by the parenchyma, have spread
their foliage, in order to perform the office just described.

But I should inform you that this function of transpiration seems to be
confined to the upper surface of the leaves, whilst, on the contrary,
the lower surface, which is more rough and uneven, and furnished with a
kind of hair or down, is destined to absorb moisture, or such other
ingredients as the plant derives from the atmosphere.

As soon as a young plant makes its appearance above ground, light, as
well as air, becomes necessary to its preservation. Light is essential
to the development of the colours, and to the thriving of the plant. You
may have often observed what a predilection vegetables have for the
light. If you make any plants grow in a room, they all spread their
leaves, and extend their branches towards the windows.

CAROLINE.

And many plants close up their flowers as soon as it is dark.

EMILY.

But may not this be owing to the cold and dampness of the evening air?

MRS. B.

That does not appear to be the case; for in a course of curious
experiments, made by Mr. Senebier, of Geneva, on plants which he reared
by lamp-light, he found that the flowers closed their petals whenever
the lamps were extinguished.

EMILY.

But pray, why is air essential to vegetation, plants do not breathe it
like animals?

MRS. B.

At least not in the same manner; but they certainly derive some
principles from the atmosphere, and yield others to it. Indeed, it is
chiefly owing to the action of the atmosphere and the vegetable kingdom
on each other, that the air continues always fit for respiration. But
you will understand this better when I have explained the effect of
water on plants.

I have said that water forms the chief nourishment of plants; it is the
basis not only of the sap, but of all the vegetable juices. Water is the
vehicle which carries into the plant the various salts and other
ingredients required for the formation and support of the vegetable
system. Nor is this all; part of the water itself is decomposed by the
organs of the plant; the hydrogen becomes a constituent part of oil, of
extract, of colouring matter, &c. whilst a portion of the oxygen enters
into the formation of mucilage, of fecula, of sugar, and of vegetable
acids. But the greater part of the oxygen, proceeding from the
decomposition of the water, is converted into a gaseous state by the
caloric disengaged from the hydrogen during its condensation in the
formation of the vegetable materials. In this state the oxygen is
transpired by the leaves of plants when exposed to the sun's rays. Thus
you find that the decomposition of water, by the organs of the plant, is
not only a means of supplying it with its chief ingredient, hydrogen,
but at the same time of replenishing the atmosphere with oxygen,
a principle which requires continual renovation, to make up for the
great consumption of it occasioned by the numerous oxygenations,
combustions, and respirations, that are constantly taking place on the
surface of the globe.

EMILY.

What a striking instance of the harmony of nature.

MRS. B.

And how admirable the design of Providence, who makes every different
part of the creation thus contribute to the support and renovation of
each other!

But the intercourse of the vegetable and animal kingdoms through the
medium of the atmosphere extends still further. Animals, in breathing,
not only consume the oxygen of the air, but load it with carbonic acid,
which, if accumulated in the atmosphere, would, in a short time, render
it totally unfit for respiration. Here the vegetable kingdom again
interferes; it attracts and decomposes the carbonic acid, retains the
carbon for its own purposes, and returns the oxygen for ours.

CAROLINE.

How interesting this is! I do not know a more beautiful illustration of
the wisdom which is displayed in the laws of nature.

MRS. B.

Faint and imperfect as are the ideas which our limited perceptions
enable us to form of divine wisdom, still they cannot fail to inspire us
with awe and admiration. What, then, would be our feelings, were the
complete system of nature at once displayed before us! So magnificent a
scene would probably be too great for our limited and imperfect
comprehension, and it is no doubt among the wise dispensations of
Providence, to veil the splendour of a glory with which we should be
overpowered. But it is well suited to the nature of a rational being to
explore, step by step, the works of the creation, to endeavour to
connect them into harmonious systems; and, in a word, to trace in the
chain of beings, the kindred ties and benevolent design which unites its
various links, and secure its preservation.

CAROLINE.

But of what nature are the organs of plants which are endued with such
wonderful powers?

MRS. B.

They are so minute that their structure, as well as the mode in which
they perform their functions, generally elude our examination; but we
may consider them as so many vessels or apparatus appropriated to
perform, with the assistance of the principle of life, certain chemical
processes, by means of which these vegetable compounds are generated. We
may, however, trace the tannin, resins, gum, mucilage, and some other
vegetable materials, in the organised arrangement of plants, in which
they form the bark, the wood, the leaves, flowers, and seeds.

The _bark_ is composed of the _epidermis_, the _parenchyma_, and the
_cortical layers_.

The epidermis is the external covering of the plant. It is a thin
transparent membrane, consisting of a number of slender fibres, crossing
each other, and forming a kind of net-work. When of a white glossy
nature, as in several species of trees, in the stems of corn and of
seeds, it is composed of a thin coating of siliceous earth, which
accounts for the strength and hardness of those long and slender stems.
Sir H. Davy was led to the discovery of the siliceous nature of the
epidermis of such plants, by observing the singular phenomenon of sparks
of fire emitted by the collision of ratan canes with which two boys were
fighting in a dark room. On analysing the epidermis of the cane, he
found it to be almost entirely siliceous.

CAROLINE.

With iron then, a cane, I suppose, will strike fire very easily?

MRS. B.

I understand that it will. --In ever-greens the epidermis is mostly
resinous, and in some few plants is formed of wax. The resin, from its
want of affinity for water, tends to preserve the plant from the
destructive effects of violent rains, severe climates, or inclement
seasons, to which this species of vegetables is peculiarly exposed.

EMILY.

Resin must preserve wood just like a varnish, as it is the essential
ingredient of varnishes?

MRS. B.

Yes; and by this means it prevents likewise all unnecessary expenditure
of moisture.

The parenchyma is immediately beneath the epidermis; it is that green
rind which appears when you strip a branch of any tree or shrub of its
external coat of bark. The parenchyma is not confined to the stem or
branches, but extends over every part of the plant. It forms the green
matter of the leaves, and is composed of tubes filled with a peculiar
juice.

The cortical layers are immediately in contact with the wood; they
abound with tannin and gallic acid, and consist of small vessels through
which the sap descends after being elaborated in the leaves. The
cortical layers are annually renewed, the old bark being converted into
wood.

EMILY.

But through what vessels does the sap ascend?

MRS. B.

That function is performed by the tubes of the alburnum, or wood, which
is immediately beneath the cortical layers. The wood is composed of
woody fibre, mucilage, and resin. The fibres are disposed in two ways;
some of them longitudinally, and these form what is called the silver
grain of the wood. The others, which are concentric, are called the
spurious grain. These last are disposed in layers, from the number of
which the age of the tree may be computed, a new one being produced
annually by the conversion of the bark into wood. The oldest, and
consequently most internal part of the alburnum, is called heart-wood;
it appears to be dead, at least no vital functions are discernible in
it. It is through the tubes of the living alburnum that the sap rises.
These, therefore, spread into the leaves, and there communicate with the
extremities of the vessels of the cortical layers, into which they pour
their contents.

CAROLINE.

Of what use, then, are the tubes of the parenchyma, since neither the
ascending nor descending sap passes through them?

MRS. B.

They are supposed to perform the important function of secreting from
the sap the peculiar juices from which the plant more immediately
derives its nourishment. These juices are very conspicuous, as the
vessels which contain them are much larger than those through which the
sap circulates. The peculiar juices of plants differ much in their
nature, not only in different species of vegetables, but frequently in
different parts of the same individual plant: they are sometimes
saccharine, as in the sugar-cane, sometimes resinous, as in firs and
evergreens, sometimes of a milky appearance, as in the laurel.

EMILY.

I have often observed, that in breaking a young shoot, or in bruising a
leaf of laurel, a milky juice will ooze out in great abundance.

MRS. B.

And it is by making incisions in the bark that pitch, tar, and
turpentine are obtained from fir-trees. The durability of this species
of wood is chiefly owing to the resinous nature of its peculiar juices.
The volatile oils have, in a great measure, the same preservative
effects, as they defend the parts, with which they are connected, from
the attack of insects. This tribe seems to have as great an aversion to
perfumes, as the human species have delight in them. They scarcely ever
attack any odoriferous parts of plants, and it is not uncommon to see
every leaf of a tree destroyed by a blight, whilst the blossoms remain
untouched. Cedar, sandal, and all aromatic woods, are on this account of
great durability.

EMILY.

But the wood of the oak, which is so much esteemed for its durability,
has, I believe, no smell. Does it derive this quality from its hardness
alone?

MRS. B.

Not entirely; for the chesnut, though considerably harder and firmer
than the oak, is not so lasting. The durability of the oak is,
I believe, in a great measure owing to its having very little
heart-wood, the alburnum preserving its vital functions longer than in
other trees.

CAROLINE.

If incisions are made into the alburnum and cortical layers, may not the
ascending and descending sap be procured in the same manner as the
peculiar juice is from the vessels of the parenchyma?

MRS. B.

Yes; but in order to obtain specimens of these fluids, in any quantity,
the experiment must be made in the spring, when the sap circulates with
the greatest energy. For this purpose a small bent glass tube should be
introduced into the incision, through which the sap may flow without
mixing with any of the other juices of the tree. From the bark the sap
will flow much more plentifully than from the wood, as the ascending sap
is much more liquid, more abundant, and more rapid in its motion than
that which descends; for the latter having been deprived by the
operation of the leaves of a considerable part of its moisture, contains
a much greater proportion of solid matter, which retards its motion. It
does not appear that there is any excess of descending sap, as none ever
exudes from the roots of plants; this process, therefore, seems to be
carried on only in proportion to the wants of the plant, and the sap
descends no further, and in no greater quantity, than is required to
nourish the several organs. Therefore, though the sap rises and descends
in the plant, it does not appear to undergo a real circulation.

The last of the organs of plants is the _flower_, or _blossom_, which
produces the _fruits_ and _seed_. These may be considered as the
ultimate purpose of nature in the vegetable creation. From fruits and
seeds animals derive both a plentiful source of immediate nourishment,
and an ample provision for the reproduction of the same means of
subsistence.

The seed which forms the final product of mature plants, we have already
examined as constituting the first rudiments of future vegetation.

These are the principal organs of vegetation, by means of which the
several chemical processes which are carried on during the life of the
plant are performed.

EMILY.

But how are the several principles which enter into the composition of
vegetables so combined by the organs of the plant as to be converted
into vegetable matter?

MRS. B.

By chemical processes, no doubt; but the apparatus in which they are
performed is so extremely minute as completely to elude our examination.
We can form an opinion, therefore, only by the result of these
operations. The sap is evidently composed of water, absorbed by the
roots, and holding in solution the various principles which it derives
from the soil. From the roots the sap ascends through the tubes of the
alburnum into the stem, and thence branches out to every extremity of
the plant. Together with the sap circulates a certain quantity of
carbonic acid, which is gradually disengaged from the former by the
internal heat of the plant.

CAROLINE.

What! have vegetables a peculiar heat, analogous to animal heat?

MRS. B.

It is a circumstance that has long been suspected; but late experiments
have decided beyond a doubt that vegetable heat is considerably above
that of unorganised matter in winter, and below it in summer. The wood
of a tree is about sixty degrees, when the thermometer is seventy or
eighty degrees. And the bark, though so much exposed, is seldom below
forty in winter.

It is from the sap, after it has been elaborated by the leaves, that
vegetables derive their nourishment; in its progress through the plant
from the leaves to the roots, it deposits in the several sets of vessels
with which it communicates, the materials on which the growth and
nourishment of each plant depends. It is thus that the various peculiar
juices, saccharine, oily, mucous, acid, and colouring, are formed; as
also the more solid parts, fecula, woody fibre, tannin, resins, concrete
salts; in a word, all the immediate materials of vegetables, as well as
the organised parts of plants, which latter, besides the power of
secreting these from the sap for the general purpose of the plant, have
also that of applying them to their own particular nourishment.

EMILY.

But why should the process of vegetation take place only at one season
of the year, whilst a total inaction prevails during the other?

MRS. B.

Heat is such an important chemical agent, that its effect, as such,
might perhaps alone account for the impulse which the spring gives to
vegetation. But, in order to explain the mechanism of that operation, it
has been supposed that the warmth of the spring dilates the vessels of
plants, and produces a kind of vacuum, into which the sap (which had
remained in a state of inaction in the trunk during the winter) rises:
this is followed by the ascent of the sap contained in the roots, and
room is thus made for fresh sap, which the roots, in their turn, pump up
from the soil. This process goes on till the plant blossoms and bears
fruit, which terminates its summer career: but when the cold weather
sets in, the fibres and vessels contract, the leaves wither, and are no
longer able to perform their office of transpiration; and, as this
secretion stops, the roots cease to absorb sap from the soil. If the
plant be an annual, its life then terminates; if not, it remains in a
state of torpid inaction during the winter; or the only internal motion
that takes place is that of a small quantity of resinous juice, which
slowly rises from the stem into the branches, and enlarges their buds
during the winter.

CAROLINE.

Yet, in evergreens, vegetation must continue throughout the year.

MRS. B.

Yes; but in winter it goes on in a very imperfect manner, compared to
the vegetation of spring and summer.

We have dwelt much longer on the history of vegetable chemistry than I
had intended; but we have at length, I think, brought the subject to a
conclusion.

CAROLINE.

I rather wonder that you did not reserve the account of the
fermentations for the conclusion; for the decomposition of vegetables
naturally follows their death, and can hardly, it seems, be introduced
with so much propriety at any other period.

MRS. B.

It is difficult to determine at what point precisely it may be most
eligible to enter on the history of vegetation; every part of the
subject is so closely connected, and forms such an uninterrupted chain,
that it is by no means easy to divide it. Had I begun with the
germination of the seed, which, at first view, seems to be the most
proper arrangement, I could not have explained the nature and
fermentation of the seed, or have described the changes which manure
must undergo, in order to yield the vegetable elements. To understand
the nature of germination, it is necessary, I think, previously to
decompose the parent plant, in order to become acquainted with the
materials required for that purpose. I hope, therefore, that, upon
second consideration, you will find that the order which I have adopted,
though apparently less correct, is in fact the best calculated for the
elucidation of the subject.




CONVERSATION XXIII.

ON THE COMPOSITION OF ANIMALS.


MRS. B.

We are now come to the last branch of chemistry, which comprehends the
most complicated order of compound beings. This is the animal creation,
the history of which cannot but excite the highest degree of curiosity
and interest, though we often fail in attempting to explain the laws by
which it is governed.

EMILY.

But since all animals ultimately derive their nourishment from
vegetables, the chemistry of this order of beings must consist merely in
the conversion of vegetable into animal matter.

MRS. B.

Very true; but the manner in which this is effected is, in a great
measure, concealed from our observation. This process is called
_animalisation_, and is performed by peculiar organs. The difference of
the animal and vegetable kingdoms does not however depend merely on a
different arrangement of combinations. A new principle abounds in the
animal kingdom, which is but rarely and in very small quantities found
in vegetables; this is nitrogen. There is likewise in animal substances
a greater and more constant proportion of phosphoric acid, and other
saline matters. But these are not essential to the formation of animal
matter.

CAROLINE.

Animal compounds contain, then, four fundamental principles; oxygen,
hydrogen, carbon, and nitrogen?

MRS. B.

Yes; and these form the immediate materials of animals, which are
_gelatine_, _albumen_, and _fibrine_.

EMILY.

Are those all? I am surprised that animals should be composed of fewer
kinds of materials than vegetables; for they appear much more
complicated in their organisation.

MRS. B.

Their organisation is certainly more perfect and intricate, and the
ingredients that occasionally enter into their composition are more
numerous. But notwithstanding the wonderful variety observable in the
texture of the animal organs, we find that the original compounds, from
which all the varieties of animal matter are derived, may be reduced to
the three heads just mentioned. Animal substances being the most
complicated of all natural compounds, are most easily susceptible of
decomposition, as the scale of attractions increases in proportion to
the number of constituent principles. Their analysis is, however, both
difficult and imperfect; for as they cannot be examined in their living
state, and are liable to alteration immediately after death, it is
probable that, when submitted to the investigation of a chemist, they
are always more or less altered in their combinations and properties,
from what they were, whilst they made part of the living animal.

EMILY.

The mere diminution of temperature, which they experience by the
privation of animal heat, must, I should suppose, be sufficient to
derange the order of attractions that existed during life.

MRS. B.

That is one of the causes, no doubt: but there are many other
circumstances which prevent us from studying the nature of living animal
substances. We must therefore, in a considerable degree, confine our
researches to the phenomena of these compounds in their inanimate state.

These three kinds of animal matter, gelatine, albumen, and fibrine, form
the basis of all the various parts of the animal system; either solid,
as the _skin_, _flesh_, _nerves_, _membranes_, _cartilages_, and
_bones_; or fluid, as _blood_, _chyle_, _milk_, _mucus_, the _gastric_
and _pancreatic juices_, _bile_, _perspiration_, _saliva_, _tears_, &c.

CAROLINE.

Is it not surprising that so great a variety of substances, and so
different in their nature, should yet all arise from so few materials,
and from the same original elements?

MRS. B.

The difference in the nature of various bodies depends, as I have often
observed to you, rather on their state of combination, than on the
materials of which they are composed. Thus, in considering the chemical
nature of the creation in a general point of view, we observe that it is
throughout composed of a very small number of elements. But when we
divide it into the three kingdoms, we find that, in the mineral, the
combinations seem to result from the union of elements casually brought
together; whilst in the vegetable and animal kingdoms, the attractions
are peculiarly and regularly produced by appropriate organs, whose
action depends on the vital principle. And we may further observe, that
by means of certain spontaneous changes and decompositions, the elements
of one kind of matter become subservient to the reproduction of another;
so that the three kingdoms are intimately connected, and constantly
contributing to the preservation of each other.

EMILY.

There is, however, one very considerable class of elements, which seems
to be confined to the mineral kingdom: I mean metals.

MRS. B.

Not entirely; they are found, though in very minute quantities, both in
the vegetable and animal kingdoms. A small portion of earths and sulphur
enters also into the composition of organised bodies. Phosphorus,
however, is almost entirely confined to the animal kingdom; and
nitrogen, but with few exceptions, is extremely scarce in vegetables.

Let us now proceed to examine the nature of the three principal
materials of the animal system.

_Gelatine_, or _jelly_, is the chief ingredient of skin, and of all the
membranous parts of animals. It may be obtained from these substances,
by means of boiling water, under the forms of glue, size, isinglass, and
transparent jelly.

CAROLINE.

But these are of a very different nature; they cannot therefore be all
pure gelatine.

MRS. B.

Not entirely, but very nearly so. Glue is extracted from the skin of
animals. Size is obtained either from skin in its natural state, or from
leather. Isinglass is gelatine procured from a particular species of
fish; it is, you know, of this substance that the finest jelly is made,
and this is done by merely dissolving the isinglass in boiling water,
and allowing the solution to congeal.

EMILY.

The wine, lemon, and spices, are, I suppose, added only to flavour the
jelly?

MRS. B.

Exactly so.

CAROLINE.

But jelly is often made of hartshorn shavings, and of calves' feet; do
these substances contain gelatine?

MRS. B.

Yes. Gelatine may be obtained from almost any animal substance, as it
enters more or less into the composition of all of them. The process for
obtaining it is extremely simple, as it consists merely in boiling the
substance that contains it with water. The gelatine dissolves in water,
and may be attained of any degree of consistence or strength, by
evaporating this solution. Bones in particular produce it very
plentifully, as they consist of phosphat of lime combined or cemented by
gelatine. Horns, which are a species of bone, will yield abundance of
gelatine. The horns of the hart are reckoned to produce gelatine of the
finest quality; they are reduced to the state of shavings in order that
the jelly may be more easily extracted by the water. It is of hartshorn
shavings that the jellies for invalids are usually made, as they are of
very easy digestion.

CAROLINE.

It appears singular that hartshorn, which yields such a powerful
ingredient as ammonia, should at the same time produce so mild and
insipid a substance as jelly?

MRS. B.

And (what is more surprising) it is from the gelatine of bones that
ammonia is produced. You must observe, however, that the processes by
which these two substances are obtained from bones are very different.
By the simple action of water and heat, the gelatine is separated; but
in order to procure the ammonia, or what is commonly called hartshorn,
the bones must be distilled, by which means the gelatine is decomposed,
and hydrogen and nitrogen combined in the form of ammonia. So that the
first operation is a mere separation of ingredients, whilst the second
requires a chemical decomposition.

CAROLINE.

But when jelly is made from hartshorn shavings, what becomes of the
phosphat of lime which constitutes the other part of bones?

MRS. B.

It is easily separated by straining. But the jelly is afterwards more
perfectly purified, and rendered transparent, by adding white of egg,
which being coagulated by heat, rises to the surface along with any
impurities.

EMILY.

I wonder that bones are not used by the common people to make jelly;
a great deal of wholesome nourishment, might, I should suppose, be
procured from them, though the jelly would perhaps not be quite so good
as if made from hartshorn shavings?

MRS. B.

There is a prejudice among the poor against a species of food that is
usually thrown to the dogs; and as we cannot expect them to enter into
chemical considerations, it is in some degree excusable. Besides, it
requires a prodigious quantity of fuel to dissolve bones and obtain the
gelatine from them.

The solution of bones in water is greatly promoted by an accumulation of
heat. This may be effected by means of an extremely strong metallic
vessel, called _Papin's digester_, in which the bones and water are
enclosed, without any possibility of the steam making its escape. A heat
can thus be applied much superior to that of boiling water; and bones,
by this means, are completely reduced to a pulp. But the process still
consumes too much fuel to be generally adopted among the lower classes.

CAROLINE.

And why should not a manufacture be established for grinding or
macerating bones, or at least for reducing them to the state of
shavings, when I suppose they would dissolve as readily as hartshorn
shavings?

MRS. B.

They could not be collected clean for such a purpose, but they are not
lost, as they are used for making hartshorn and sal ammoniac; and such
is the superior science and industry of this country, that we now send
sal ammoniac to the Levant, though it originally came to us from Egypt.

EMILY.

When jelly is made of isinglass, does it leave no sediment?

MRS. B.

No; nor does it so much require clarifying, as it consists almost
entirely of pure gelantine, and any foreign matter that is mixed with
it, is thrown off during the boiling in the form of scum. --These are
processes which you may see performed in great perfection in the
culinary laboratory, by that very able and most useful chemist the cook.

CAROLINE.

To what an immense variety of purposes chemistry is subservient!

EMILY.

It appears, in that respect, to have an advantage over most other arts
and sciences; for these, very often, have a tendency to confine the
imagination to their own particular object, whilst the pursuit of
chemistry is so extensive and diversified, that it inspires a general
curiosity, and a desire of enquiring into the nature of every object.

CAROLINE.

I suppose that soup is likewise composed of gelatine; for, when cold, it
often assumes the consistence of jelly?

MRS. B.

Not entirely; for though soups generally contain a quantity of gelatine,
the most essential ingredient is a mucous or extractive matter,
a peculiar animal substance, very soluble in water, which has a strong
taste, and is more nourishing than gelatine. The various kinds of
portable soup consist of this extractive matter in a dry state, which,
in order to be made into soup, requires only to be dissolved in water.

Gelatine, in its solid state, is a semiductile transparent substance,
without either taste or smell. --When exposed to heat, in contact with
air and water, it first swells, then fuses, and finally burns. You may
have seen the first part of this operation performed in the carpenter's
glue-pot.

CAROLINE.

But you said that gelatine had no smell, and glue has a very
disagreeable one.

MRS. B.

Glue is not pure gelatine; as it is not designed for eating, it is
prepared without attending to the state of the ingredients, which are
more or less contaminated by particles that have become putrid.

Gelatine may be precipitated from its solution in water by alcohol. --We
shall try this experiment with a glass of warm jelly. --You see that the
gelatine subsides by the union of the alcohol and the water.

EMILY.

How is it, then, that jelly is flavoured with wine, without producing
any precipitation?

MRS. B.

Because the alcohol contained in wine is already combined with water,
and other ingredients, and is therefore not at liberty to act upon the
jelly as when in its separate state. Gelatine is soluble both in acids
and in alkalies; the former, you know, are frequently used to season
jellies.

CAROLINE.

Among the combinations of gelatine we must not forget one which you
formerly mentioned; that with tannin, to form leather.

MRS. B.

True; but you must observe that leather can be produced only by gelatine
in a membranous state; for though pure gelatine and tannin will produce
a substance chemically similar to leather, yet the texture of the skin
is requisite to make it answer the useful purposes of that substance.

The next animal substance we are to examine is _albumen_; this, although
constituting a part of most of the animal compounds, is frequently found
insulated in the animal system; the white of egg, for instance, consists
almost entirely of albumen; the substance that composes the nerves, the
serum, or white part of the blood, and the curds of milk, are little
else than albumen variously modified.

In its most simple state, albumen appears in the form of a transparent
viscous fluid, possessed of no distinct taste or smell; it coagulates at
the low temperature of 165 degrees, and, when once solidified, it will
never return to its fluid state.

Sulphuric acid and alcohol are each of them capable of coagulating
albumen in the same manner as heat, as I am going to show you.

EMILY.

Exactly so. --Pray, Mrs. B., what kind of action is there between
albumen and silver? I have sometimes observed, that if the spoon with
which I eat an egg happens to be wetted, it becomes tarnished.

MRS. B.

It is because the white of egg (and, indeed, albumen in general)
contains a little sulphur, which, at the temperature of an egg just
boiled, will decompose the drop of water that wets the spoon, and
produce sulphurated hydrogen gas, which has the property of tarnishing
silver.

We may now proceed to _fibrine_. This is an insipid and inodorous
substance, having somewhat the appearance of fine white threads adhering
together; it is the essential constituent of muscles or flesh, in which
it is mixed with and softened by gelatine. It is insoluble both in water
and alcohol, but sulphuric acid converts it into a substance very
analogous to gelatine.


These are the essential and general ingredients of animal matter; but
there are other substances, which, though not peculiar to the animal
system, usually enter into its composition, such as oils, acids,
salts, &c.

_Animal oil_ is the chief constituent of fat; it is contained in
abundance in the cream of milk, whence it is obtained in the form of
butter.

EMILY.

Is animal oil the same in its composition as vegetable oils?

MRS. B.

Not the same, but very analogous. The chief difference is that animal
oil contains nitrogen, a principle which seldom enters into the
composition of vegetable oils, and never in so large a proportion.

There are a few animal acids, that is to say, acids peculiar to animal
matter, from which they are almost exclusively obtained.

The animal acids have triple bases of hydrogen, carbon, and nitrogen.
Some of them are found native in animal matter; others are produced
during its decomposition.

Those that we find ready formed are:

The _bombic acid_, which is obtained from silk-worms.

The _formic acid_, from ants.

The _lactic acid_, from the whey of milk.

The _sebacic_, from oil or fat.

Those produced during the decomposition of animal substances by heat,
are the _prussic_ and _zoonic_ acids. This last is produced by the
roasting of meat, and gives it a brisk flavour.

CAROLINE.

The class of animal acids is not very extensive?

MRS. B.

No; nor are they, generally speaking, of great importance. The _prussic
acid_ is, I think, the only one sufficiently interesting to require any
further comment. It can be formed by any artificial process, without the
presence of any animal matter; and it may likewise be obtained from a
variety of vegetables, particularly those of the narcotic kind, such as
poppies, laurel, &c. But it is commonly obtained from blood, by strongly
heating that substance with caustic potash; the alkali attracts the acid
from the blood, and forms with it a _prussiat of potash_. From this
state of combination the prussic acid can be obtained pure by means of
other substances which have the power of separating it from the alkali.

EMILY.

But if this acid does not exist ready formed in blood, how can the
alkali attract it from it?

MRS. B.

It is the triple basis only of this acid that exists in the blood; and
this is developed and brought to the state of acid, during the
combustion. The acid therefore is first formed, and it afterwards
combines with the potash.

EMILY.

Now I comprehend it. But how can the prussic acid be artificially made?

MRS. B.

By passing ammoniacal gas over red-hot charcoal; and hence we learn that
the constituents of this acid are hydrogen, nitrogen, and carbon. The
two first are derived from the volatile alkali, the last from the
combustion of the charcoal.

CAROLINE.

But this does not accord with the system of oxygen being the principle
of acidity.

MRS. B.

The colouring matter of prussian blue is called an acid, because it
unites with alkalies and metals, and not from any other characteristic
properties of acids; perhaps the name is not strictly appropriate. But
this circumstance, together with some others of the same kind, has
induced several chemists to think that oxygen may not be the exclusive
generator of acids. Sir H. Davy, I have already informed you, was led by
his experiments on dry acids to suspect that water might be essential to
acidity. And it is the opinion of some chemists that acidity may
possibly depend rather on the arrangement than on the presence of any
particular principles. But we have not yet done with the prussic acid.
It has a strong affinity for metallic oxyds, and precipitates the
solutions of iron in acids of a blue colour. This is the prussian blue,
or prussiat of iron, so much used in the arts, and with which I think
you must be acquainted.

EMILY.

Yes, I am; it is much used in painting, both in oil and in water
colours; but it is not reckoned a permanent oil-colour.

MRS. B.

That defect arises, I believe, in general, from its being badly
prepared, which is the case when the iron is not so fully oxydated as to
form a red oxyd. For a solution of green oxyd of iron (in which the
metal is more slightly oxydated), makes only a pale green, or even a
white precipitate, with prussiat of potash; and this gradually changes
to blue by being exposed to the air, as I can immediately show you.

CAROLINE.

It already begins to assume a pale blue colour. But how does the air
produce this change?

MRS. B.

By oxydating the iron more perfectly. If we pour some nitrous acid on
it, the prussian blue colour will be immediately produced, as the acid
will yield its oxygen to the precipitate, and fully saturate it with
this principle, as you shall see.

CAROLINE.

It is very curious to see a colour change so instantaneously.

MRS. B.

Hence you perceive that prussian blue cannot be a permanent colour,
unless prepared with red oxyd of iron, since by exposure to the
atmosphere it gradually darkens, and in a short time is no longer in
harmony with the other colours of the painting.

CAROLINE.

But it can never become darker, by exposure to the atmosphere, than the
true prussian blue, in which the oxyd is perfectly saturated?

MRS. B.

Certainly not. But in painting, the artist not reckoning upon partial
alterations in his colours, gives his blue tints that particular shade
which harmonises with the rest of the picture. If, afterwards, those
tints become darker, the harmony of the colouring must necessarily be
destroyed.

CAROLINE.

Pray, of what nature is the paint called _carmine_?

MRS. B.

It is an animal colour prepared from _cochineal_, an insect, the
infusion of which produces a very beautiful red.

CAROLINE.

Whilst we are on the subject of colours, I should like to learn what
_ivory black_ is?

MRS. B.

It is a carbonaceous substance obtained by the combustion of ivory.
A more common species of black is obtained from the burning of bone.

CAROLINE.

But during the combustion of ivory or bone, the carbon, I should have
imagined, must be converted into carbonic acid gas, instead of this
black substance?

MRS. B.

In this, as in most combustions, a considerable part of the carbon is
simply volatilised by the heat, and again obtained concrete on cooling.
This colour, therefore, may be called the soot produced by the burning
of ivory or bone.




CONVERSATION XXIV.

ON THE ANIMAL ECONOMY.


MRS. B.

We have now acquired some idea of the various materials that compose the
animal system; but if you are curious to know in what manner these
substances are formed by the animal organs, from vegetable, as well as
from animal substances, it will be necessary to have some previous
knowledge of the nature and functions of these organs, without which it
is impossible to form any distinct idea of the process of
_animalisation_ and _nutrition_.

CAROLINE.

I do not exactly understand the meaning of the word animalisation?

MRS. B.

Animalisation is the process by which the food is _assimilated_, that is
to say, converted into animal matter; and nutrition is that by which the
food thus assimilated is rendered subservient to the purposes of
nourishing and maintaining the animal system.

EMILY.

This, I am sure, must be the most interesting of all the branches of
chemistry!

CAROLINE.

So I think; particularly as I expect that we shall hear something of the
nature of respiration, and of the circulation of the blood?

MRS. B.

These functions undoubtedly occupy a most important place in the history
of the animal economy. --But I must previously give you a very short
account of the principal organs by which the various operations of the
animal system are performed. These are:

The _Bones_;
    _Muscles_,
    _Blood vessels_,
    _Lymphatic vessels_,
    _Glands_, and
    _Nerves_.

The _bones_ are the most solid part of the animal frame, and in a great
measure determine its form and dimensions. You recollect, I suppose,
what are the ingredients which enter into their composition?

CAROLINE.

Yes; phosphat of lime, cemented by gelatine.

MRS. B.

During the earliest period of animal life, they consist almost entirely
of gelatinous membrane having the form of the bones, but of a loose
spongy texture, the cells or cavities of which are destined to be filled
with phosphat of lime; it is the gradual acquisition of this salt which
gives to the bones their subsequent hardness and durability. Infants
first receive it from their mother's milk, and afterwards derive it from
all animal and from most vegetable food, especially farinaceous
substances, such as wheat-flour, which contain it in sensible
quantities. A portion of the phosphat, after the bones of the infant
have been sufficiently expanded and solidified, is deposited in the
teeth, which consist at first only of a gelatinous membrane or case,
fitted for the reception of this salt; and which, after acquiring
hardness within the gum, gradually protrude from it.

CAROLINE.

How very curious this is; and how ingeniously nature has first provided
for the solidification of such bones as are immediately wanted, and
afterwards for the formation of the teeth, which would not only be
useless, but detrimental in infancy!

MRS. B.

In quadrupeds the phosphat of lime is deposited likewise in their horns,
and in the hair or wool with which they are generally clothed.

In birds it serves also to harden the beaks and the quills of their
feathers.

When animals are arrived at a state of maturity, and their bones have
acquired a sufficient degree of solidity, the phosphat of lime which is
taken with the food is seldom assimilated, excepting when the female
nourishes her young; it is then all secreted into the milk, as a
provision for the tender bones of the nursling.

EMILY.

So that whatever becomes superfluous to one being, is immediately wanted
by another; and the child acquires strength precisely by the species of
nourishment which is no longer necessary to the mother. Nature is,
indeed, an admirable economist!

CAROLINE.

Pray, Mrs. B., does not the disease in the bones of children, called the
rickets, proceed from a deficiency of phosphat of lime?

MRS. B.

I have heard that this disease may arise from two causes; it is
sometimes occasioned by the growth of the muscles being too rapid in
proportion to that of the bones. In this case the weight of the flesh is
greater than the bones can support, and presses upon them so as to
produce a swelling of the joints, which is the great indication of the
rickets. The other cause of this disorder is supposed to be an imperfect
digestion and assimilation of the food, attended with an excess of acid,
which counteracts the formation of phosphat of lime. In both instances,
therefore, care should be taken to alter the child's diet, not merely by
increasing the quantity of aliment containing phosphat of lime, but also
by avoiding all food that is apt to turn acid on the stomach, and to
produce indigestion. But the best preservative against complaints of
this kind is, no doubt, good nursing: when a child has plenty of air and
exercise, the digestion and assimilation will be properly performed, no
acid will be produced to interrupt these functions, and the muscles and
bones will grow together in just proportions.

CAROLINE.

I have often heard the rickets attributed to bad nursing, but I never
could have guessed what connection there was between exercise and the
formation of the bones.

MRS. B.

Exercise is generally beneficial to all the animal functions. If man is
destined to labour for his subsistence, the bread which he earns is
scarcely more essential to his health and preservation than the
exertions by which he obtains it. Those whom the gifts of fortune have
placed above the necessity of bodily labour are compelled to take
exercise in some mode or other, and when they cannot convert it into an
amusement, they must submit to it as a task, or their health will soon
experience the effects of their indolence.

EMILY.

That will never be my case: for exercise, unless it becomes fatigue,
always gives me pleasure; and, so far from being a task, is to me a
source of daily enjoyment. I often think what a blessing it is, that
exercise, which is so conducive to health, should be so delightful;
whilst fatigue, which is rather hurtful, instead of pleasure, occasions
painful sensations. So that fatigue, no doubt, was intended to moderate
our bodily exertions, as satiety puts a limit to our appetites.

MRS. B.

Certainly. --But let us not deviate too far from our subject. --The
bones are connected together by ligaments, which consist of a white
thick flexible substance, adhering to their extremities, so far as to
secure the joints firmly, though without impeding their motion. And the
joints are moreover covered by a solid, smooth, elastic, white
substance, called _cartilage_, the use of which is to allow, by its
smoothness and elasticity, the bones to slide easily over one another,
so that the joints may perform their office without difficulty or
detriment.

Over the bones the _muscles_ are placed; they consist of bundles of
fibres which terminate in a kind of string, or ligament, by which they
are fastened to the bones. The muscles are the organs of motion; by
their power of dilatation and contraction they put into action the
bones, which act as levers, in all the motions of the body, and form the
solid support of its various parts. The muscles are of various degrees
of strength or consistence in different species of animals. The
mammiferous tribe, or those that suckle their young, seem in this
respect to occupy an intermediate place between birds and cold-blooded
animals, such as reptiles and fishes.

EMILY.

The different degrees of firmness and solidity in the muscles of these
several species of animals proceed, I imagine, from the different nature
of the food on which they subsist?

MRS. B.

No; that is not supposed to be the case: for the human species, who are
of the mammiferous tribe, live on more substantial food than birds, and
yet the latter exceed them in muscular strength. We shall hereafter
attempt to account for this difference; but let us now proceed in the
examination of the animal functions.

The next class of organs is that of the _vessels_ of the body, the
office of which is to convey the various fluids throughout the frame.
These vessels are innumerable. The most considerable of them are those
through which the blood circulates, which are of two kinds: the
_arteries_, which convey it from the heart to the extremities of the
body, and the _veins_, which bring it back into the heart.

Besides these, there are a numerous set of small transparent vessels,
destined to absorb and convey different fluids into the blood; they are
generally called the _absorbent_ or _lymphatic_ vessels: but it is to a
portion of them only that the function of conveying into the blood the
fluid called _lymph_ is assigned.

EMILY.

Pray what is the nature of that fluid?

MRS. B.

The nature and use of the lymph have, I believe, never been perfectly
ascertained; but it is supposed to consist of matter that has been
previously animalised, and which, after answering the purpose for which
it was intended, must, in regular rotation, make way for the fresh
supplies produced by nourishment. The lymphatic vessels pump up this
fluid from every part of the system, and convey it into the veins to be
mixed with the blood which runs through them, and which is commonly
called venous blood.

CAROLINE.

But does it not again enter into the animal system through that channel?

MRS. B.

Not entirely; for the venous blood does not return into the circulation
until it has undergone a peculiar change, in which it throws off
whatever is become useless.

Another set of absorbent vessels pump up the _chyle_ from the stomach
and intestines, and convey it, after many circumvolutions, into the
great vein near the heart.

EMILY.

Pray what is chyle?

MRS. B.

It is the substance into which food is converted by digestion.

CAROLINE.

One set of the absorbent vessels, then, is employed in bringing away the
old materials that are no longer fit for use; whilst the other set is
busy in conveying into the blood the new materials that are to replace
them.

EMILY.

What a great variety of ingredients must enter into the composition of
the blood?

MRS. B.

You must observe that there is also a great variety of substances to be
secreted from it. We may compare the blood to a general receptacle or
storehouse for all kinds of commodities, which are afterwards fashioned,
arranged, and disposed of as circumstances require.

There is another set of absorbent vessels in females which is destined
to secrete milk for the nourishment of the young.

EMILY.

Pray is not milk very analogous in its composition to blood; for, since
the nursling derives its nourishment from that source only, it must
contain every principle which the animal system requires?

MRS. B.

Very true. Milk is found, by its analysis, to contain the principal
materials of animal matter, albumen, oil, and phosphat of lime; so that
the suckling has but little trouble to digest and assimilate this
nourishment. But we shall examine the composition of milk more fully
afterwards.

In many parts of the body numbers of small vessels are collected
together in little bundles called _glands_, from a Latin word meaning
acorn, on account of the resemblance which some of them bear in shape to
that fruit. The function of the glands is to _secrete_, or separate
certain matters from the blood.

The secretions are not only mechanical, but chemical separations from
the blood; for the substances thus formed, though contained in the
blood, are not ready combined in that fluid. The secretions are of two
kinds, those which form peculiar animal fluids, as bile, tears, saliva,
&c.; and those which produce the general materials of the animal system,
for the purpose of recruiting and nourishing the several organs of the
body; such as albumen, gelatine, and fibrine; the latter may be
distinguished by the name of _nutritive secretions_.

CAROLINE.

I am quite astonished to hear that all the secretions should be derived
from the blood.

EMILY.

I thought that the bile was produced by the liver?

MRS. B.

So it is; but the liver is nothing more than a very large gland, which
secretes the bile from the blood.

The last of the animal organs which we have mentioned are the _nerves_;
these are the vehicles of sensation, every other part of the body being,
of itself, totally insensible.

CAROLINE.

They must then be spread through every part of the frame, for we are
every where susceptible of feeling.

EMILY.

Excepting the nails and the hair.

MRS. B.

And those are almost the only parts in which nerves cannot be
discovered. The common source of all the nerves is the brain; thence
they descend, some of them through different holes of the skull, but the
greatest part through the back bone, and extend themselves by
innumerable ramifications throughout the whole body. They spread
themselves over the muscles, penetrate the glands, wind round the
vascular system, and even pierce into the interior of the bones. It is
most probably through them that the communication is carried on between
the mind and the other parts of the body; but in what manner they are
acted on by the mind, and made to re-act on the body, is still a
profound secret. Many hypotheses have been formed on this very obscure
subject, but they are all equally improbable, and it would be useless
for us to waste our time in conjectures on an enquiry, which, in all
probability, is beyond the reach of human capacity.

CAROLINE.

But you have not mentioned those particular nerves that form the senses
of hearing, seeing, smelling, and tasting?

MRS. B.

They are considered as being of the same nature as those which are
dispersed over every part of the body, and constitute the general sense
of feeling. The different sensations which they produce arise from their
peculiar situation and connection with the several organs of taste,
smell, and hearing.

EMILY.

But these senses appear totally different from that of feeling?

MRS. B.

They are all of them sensations, but variously modified according to the
nature of the different organs in which the nerves are situated. For, as
we have formerly observed, it is by contact only that the nerves are
affected. Thus odoriferous particles must strike upon the nerves of the
nose, in order to excite the sense of smelling; in the same manner that
taste is produced by the particular substance coming in contact with the
nerves of the palate. It is thus also that the sensation of sound is
produced by the concussion of the air striking against the auditory
nerve; and sight is the effect of the light falling upon the optic
nerve. These various senses, therefore, are affected only by the actual
contact of particles of matter, in the same manner as that of feeling.

The different organs of the animal body, though easily separated and
perfectly distinct, are loosely connected together by a kind of spongy
substance, in texture somewhat resembling net-work, called the cellular
membrane; and the whole is covered by the skin.

The _skin_, as well as the bark of vegetables, is formed of three coats.
The external one is called the _cuticle_ or _epidermis_; the second,
which is called the _mucous membrane_, is of a thin soft texture, and
consists of a mucous substance, which in negroes is black, and is the
cause of their skin appearing of that colour.

CAROLINE.

Is then the external skin of negroes white like ours?

MRS. B.

Yes; but as the cuticle is transparent, as well as porous, the blackness
of the mucous membrane is visible through it. The extremities of the
nerves are spread over this skin, so that the sensation of feeling is
transmitted through the cuticle. The internal covering of the muscles,
which is properly the skin, is the thickest, the toughest, and most
resisting of the whole; it is this membrane which is so essential in the
arts, by forming leather when combined with tannin.

The skin which covers the animal body, as well as those membranes that
form the coats of the vessels, consists almost exclusively of gelatine;
and is capable of being converted into glue, size, or jelly.

The cavities between the muscles and the skin are usually filled with
fat, which lodges in the cells of the membranous net before mentioned,
and gives to the external form (especially in the human figure) that
roundness, smoothness, and softness, so essential to beauty.

EMILY.

And the skin itself is, I think, a very ornamental part of the human
frame, both from the fineness of its texture, and the variety and
delicacy of its tints.

MRS. B.

This variety and harmonious graduation of colours, proceed, not so much
from the skin itself, as from the internal organs which transmit their
several colours through it, these being only softened and blended by the
colour of the skin, which is uniformly of a yellowish white.

Thus modified, the darkness of the veins appears of a pale blue colour,
and the floridness of the arteries is changed to a delicate pink. In the
most transparent parts, the skin exhibits the bloom of the rose, whilst
where it is more opake its own colour predominates; and at the joints,
where the bones are most prominent, their whiteness is often
discernible. In a word, every part of the human frame seems to
contribute to its external grace; and this not merely by producing a
pleasing variety of tints, but by a peculiar kind of beauty which
belongs to each individual part. Thus it is to the solidity and
arrangement of the bones that the human figure owes the grandeur of its
stature, and its firm and dignified deportment. The muscles delineate
the form, and stamp it with energy and grace; and the soft substance
which is spread over them smooths their ruggedness, and gives to the
contours the gentle undulations of the line of beauty. Every organ of
sense is a peculiar and separate ornament; and the skin, which polishes
the surface, and gives it that charm of colouring so inimitable by art,
finally conspires to render the whole the fairest work of the creation.

But now that we have seen in what manner the animal frame is formed, let
us observe how it provides for its support, and how the several organs,
which form so complete a whole, are nourished and maintained.

This will lead us to a more particular explanation of the internal
organs: here we shall not meet with so much apparent beauty, because
these parts were not intended by nature to be exhibited to view; but the
beauty of design, in the internal organisation of the animal frame, is,
if possible, still more remarkable than that of the external parts.

We shall defer this subject till our next interview.




CONVERSATION XXV.

ON ANIMALISATION, NUTRITION, AND RESPIRATION.


MRS. B.

We have now learnt of what materials the animal system is composed, and
have formed some idea of the nature of its organisation. In order to
complete the subject, it remains for us to examine in what manner it is
nourished and supported.

Vegetables, we have observed, obtain their nourishment from various
substances, either in their elementary state, or in a very simple state
of combination; as carbon, water, and salts, which they pump up from the
soil; and carbonic acid and oxygen, which they absorb from the
atmosphere.

Animals, on the contrary, feed on substances of the most complicated
kind; for they derive their sustenance, some from the animal creation,
others from the vegetable kingdom, and some from both.

CAROLINE.

And there is one species of animals, which, not satisfied with enjoying
either kind of food in its simple state, has invented the art of
combining them together in a thousand ways, and of rendering even the
mineral kingdom subservient to its refinements.

EMILY.

Nor is this all; for our delicacies are collected from the various
climates of the earth, so that the four quarters of the globe are often
obliged to contribute to the preparation of our simplest dishes.

CAROLINE.

But the very complicated substances which constitute the nourishment of
animals, do not, I suppose, enter into their system in their actual
state of combination?

MRS. B.

So far from it, that they not only undergo a new arrangement of their
parts, but a selection is made of such as are most proper for the
nourishment of the body, and those only enter into the system, and are
animalised.

EMILY.

And by what organs is this process performed?

MRS. B.

Chiefly by the stomach, which is the organ of digestion, and the prime
regulator of the animal frame.

_Digestion_ is the first step towards nutrition. It consists in reducing
into one homogeneous mass the various substances that are taken as
nourishment; it is performed by first chewing and mixing the solid
aliment with the saliva, which reduces it to a soft mass, in which state
it is conveyed into the stomach, where it is more completely dissolved
by the _gastric juice_.

This fluid (which is secreted into the stomach by appropriate glands) is
so powerful a solvent that scarcely any substances will resist its
action.

EMILY.

The coats of the stomach, however, cannot be attacked by it, otherwise
we should be in danger of having them destroyed when the stomach was
empty.

MRS. B.

They are probably not subject to its action; as long, at least, as life
continues. But it appears, that when the gastric juice has no foreign
substance to act upon, it is capable of occasioning a degree of
irritation in the coats of the stomach, which produces the sensation of
hunger. The gastric juice, together with the heat and muscular action of
the stomach, converts the aliment into an uniform pulpy mass called
chyme. This passes into the intestines, where it meets with the bile and
some other fluids, by the agency of which, and by the operation of other
causes hitherto unknown, the chyme is changed into chyle, a much thinner
substance, somewhat resembling milk, which is pumped by immense numbers
of small absorbent vessels spread over the internal surface of the
intestines. These, after many circumvolutions, gradually meet and unite
into large branches, till they at length collect the chyle into one
vessel, which pours its contents into the great vein near the heart, by
which means the food, thus prepared, enters into the circulation.

CAROLINE.

But I do not yet clearly understand how the blood, thus formed,
nourishes the body and supplies all the secretions?

MRS. B.

Before this can be explained to you, you must first allow me to complete
the formation of the blood. The chyle may, indeed, be considered as
forming the chief ingredient of blood; but this fluid is not perfect
until it has passed through the lungs, and undergone (together with the
blood that has already circulated) certain necessary changes that are
effected by RESPIRATION.

CAROLINE.

I am very glad that you are going to explain the nature of respiration:
I have often longed to understand it, for though we talk incessantly of
_breathing_, I never knew precisely what purpose it answered.

MRS. B.

It is indeed one of the most interesting processes imaginable; but, in
order to understand this function well, it will be necessary to enter
into some previous explanations. Tell me, Emily, --what do you
understand by respiration?

EMILY.

Respiration, I conceive, consists simply in alternately _inspiring_ air
into the lungs, and _expiring_ it from them.

MRS. B.

Your answer will do very well as a general definition. But, in order to
form a tolerably clear notion of the various phenomena of respiration,
there are many circumstances to be taken into consideration.

In the first place, there are two things to be distinguished in
respiration, the _mechanical_ and the _chemical_ part of the process.

The mechanism of breathing depends on the alternate expansions and
contractions of the chest, in which the lungs are contained. When the
chest dilates, the cavity is enlarged, and the air rushes in at the
mouth, to fill up the vacuum formed by this dilatation; when it
contracts, the cavity is diminished, and the air forced out again.

CAROLINE.

I thought that it was the lungs that contracted and expanded in
breathing?

MRS. B.

They do likewise; but their action is only the consequence of that of
the chest. The lungs, together with the heart and largest blood vessels,
in a manner fill up the cavity of the chest; they could not, therefore,
dilate if the chest did not previously expand; and, on the other hand,
when the chest contracts, it compresses the lungs and forces the air out
of them.

CAROLINE.

The lungs, then, are like bellows, and the chest is the power that works
them.

MRS. B.

Precisely so. Here is a curious little figure (PLATE XV. Fig. 5.), that
will assist me in explaining the mechanism of breathing.

CAROLINE.

What a droll figure! a little head fixed upon a glass bell, with a
bladder tied over the bottom of it!

MRS. B.

You must observe that there is another bladder within the glass, the
neck of which communicates with the mouth of the figure--this represents
the lungs contained within the chest; the other bladder, which you see
is tied loose, represents a muscular membrane, called the _diaphragm_,
which separates the chest from the lower part of the body. By the chest,
therefore, I mean that large cavity in the upper part of the body
contained within the ribs, the neck, and the diaphragm; this membrane is
muscular, and capable of contraction and dilatation. The contraction may
be imitated by drawing the bladder tight over the bottom of the
receiver, when the air in the bladder, which represents the lungs, will
be forced out through the mouth of the figure--

EMILY.

See, Caroline, how it blows the flame of the candle in breathing!

MRS. B.

By letting the bladder loose again, we imitate the dilatation of the
diaphragm, and the cavity of the chest being enlarged, the lungs expand,
and the air rushes in to fill them.

EMILY.

This figure, I think, gives a very clear idea of the process of
breathing.

MRS. B.

It illustrates tolerably well the action of the lungs and diaphragm; but
those are not the only powers that are concerned in enlarging or
diminishing the cavity of the chest; the ribs are also possessed of a
muscular motion for the same purpose; they are alternately drawn in,
edgeways, to assist the contraction, and stretched out, like the hoops
of a barrel, to contribute to the dilatation of the chest.

EMILY.

I always supposed that the elevation and depression of the ribs were the
consequence, not the cause of breathing.

MRS. B.

It is exactly the reverse. The muscular action of the diaphragm,
together with that of the ribs, are the _causes_ of the contraction and
expansion of the chest; and the air rushing into, and being expelled
from the lungs, are only _consequences_ of those actions.

CAROLINE.

I confess that I thought the act of breathing began by opening the mouth
for the air to rush in, and that it was the air alone, which, by
alternately rushing in and out, occasioned the dilatations and
contractions of the lungs and chest.

MRS. B.

Try the experiment of merely opening your mouth; the air will not rush
in, till by an interior muscular action you produce a vacuum--yes, just
so, your diaphragm is now dilated, and the ribs expanded. But you will
not be able to keep them long in that state. Your lungs and chest are
already resuming their former state, and expelling the air with which
they had just been filled. This mechanism goes on more or less rapidly,
but, in general, a person at rest and in health will breathe between
fifteen and twenty-five times in a minute.

We may now proceed to the chemical effects of respiration; but, for this
purpose, it is necessary that you should previously have some notion of
the _circulation_ of the blood. Tell me, Caroline, what do you
understand by the circulation of the blood?

CAROLINE.

I am delighted that you come to that subject, for it is one that has
long excited my curiosity. But I cannot conceive how it is connected
with respiration. The idea I have of the circulation is, that the blood
runs from the heart through the veins all over the body, and back again
to the heart.

MRS. B.

I could hardly have expected a better definition from you; it is,
however, not quite correct, for you do not distinguish the _arteries_
from the _veins_, which, as we have already observed, are two distinct
sets of vessels, each having its own peculiar functions. The arteries
convey the blood from the heart to the extremities of the body; and the
veins bring it back into the heart.

This sketch will give you an idea of the manner in which some of the
principal veins and arteries of the human body branch out of the heart,
which may be considered as a common centre to both sets of vessels. The
heart is a kind of strong elastic bag, or muscular cavity, which
possesses a power of dilating and contracting itself, for the purposes
of alternately receiving and expelling the blood, in order to carry on
the process of circulation.

EMILY.

Why are the arteries in this drawing painted red, and the veins purple?

MRS. B.

It is to point out the difference of the colour of the blood in these
two sets of vessels.

CAROLINE.

But if it is the same blood that flows from the arteries into the veins,
how can its colour be changed?

MRS. B.

This change arises from various circumstances. In the first place,
during its passage through the arteries, the blood undergoes a
considerable alteration, some of its constituent parts being gradually
separated from it for the purpose of nourishing the body, and of
supplying the various secretions. The consequence of this is, that the
florid arterial colour of the blood changes by degrees to a deep purple,
which is its constant colour in the veins. On the other hand, the blood
is recruited during its return through the veins by the fresh chyle, or
imperfect blood, which has been produced by food; and it receives also
lymph from the absorbent vessels, as we have before mentioned. In
consequence of these several changes, the blood returns to the heart in
a state very different from that in which it left it. It is loaded with
a greater proportion of hydrogen and carbon, and is no longer fit for
the nourishment of the body, or other purposes of circulation.

EMILY.

And in this state does it mix in the heart with the pure florid blood
that runs into the arteries?

MRS. B.

No. The heart is divided into two cavities or compartitions, called the
_right_ and _left ventricles_. The left ventricle is the receptacle for
the pure arterial blood previous to its circulation; whilst the venous,
or impure blood, which returns to the heart after having circulated, is
received into the right ventricle, previous to its purification, which I
shall presently explain.

CAROLINE.

For my part, I always thought that the same blood circulated again and
again through the body, without undergoing any change.

MRS. B.

Yet you must have supposed that the blood circulated for some purpose?

CAROLINE.

I knew that it was indispensable to life; but had no idea of its real
functions.

MRS. B.

But now that you understand that the blood conveys nourishment to every
part of the body, and supplies the various secretions, you must be
sensible that it cannot constantly answer these objects without being
proportionally renovated and purified.

CAROLINE.

But does not the chyle answer this purpose?

MRS. B.

Only in part. It renovates the nutritive principles of the blood, but
does not relieve it from the superabundance of water and carbon with
which it is encumbered.

EMILY.

How, then, is this effected?

MRS. B.

By RESPIRATION. This is one of the grand mysteries which modern
chemistry has disclosed. When the venous blood enters the right
ventricle of the heart, it contracts by its muscular power, and throws
the blood through a large vessel into the lungs, which are contiguous,
and through which it circulates by millions of small ramifications. Here
it comes in contact with the air which we breathe. The action of the air
on the blood in the lungs is, indeed, concealed, from our immediate
observation; but we are able to form a tolerably accurate judgment of it
from the changes which it effects not only in the blood, but also on the
air expired.

The air, after passing through the lungs, is found to contain all the
nitrogen inspired, but to have lost part of its oxygen, and to have
acquired a portion of watery vapour and of carbonic acid gas. Hence it
is inferred, that when the air comes in contact with the venous blood in
the lungs, the oxygen attracts from it the superabundant quantity of
carbon with which it has impregnated itself during the circulation, and
converts it into carbonic acid. This gaseous acid, together with the
redundant moisture from the lungs*, being then expired, the blood is
restored to its former purity, that is, to the state of arterial blood,
and is thus again enabled to perform its various functions.

    [Footnote *: The quantity of moisture discharged by the lungs in
    24 hours, may be computed at eight or nine ounces.]

CAROLINE.

This is truly wonderful! Of all that we have yet learned, I do not
recollect any thing that has appeared to me so curious and interesting.
I almost believe that I should like to study anatomy now, though I have
hitherto had so disgusting an idea of it. Pray, to whom are we indebted
for these beautiful discoveries?

MRS. B.

Priestley and Crawford, in this country, and Lavoisier, in France, are
the principal inventors of the theory of respiration. Of late years the
subject has been farther illustrated and simplified by the accurate
experiments of Messrs. Allen and Pepys. But the still more important and
more admirable discovery of the circulation of the blood was made long
before by our immortal countryman Harvey.

EMILY.

Indeed I never heard any thing that delighted me so much as this theory
of respiration. But I hope, Mrs. B., that you will enter a little more
into particulars before you dismiss so interesting a subject. We left
the blood in the lungs to undergo the salutary change: but how does it
thence spread to all the parts of the body?

MRS. B.

After circulating through the lungs, the blood is collected into four
large vessels, by which it is conveyed into the left ventricle of the
heart, whence it is propelled to all the different parts of the body by
a large artery, which gradually ramifies into millions of small arteries
through the whole frame. From the extremities of these little
ramifications the blood is transmitted to the veins, which bring it back
to the heart and lungs, to go round again and again in the manner we
have just described. You see, therefore, that the blood actually
undergoes two circulations; the one, through the lungs, by which it is
converted into pure arterial blood; the other, or general circulation,
by which nourishment is conveyed to every part of the body; and these
are both equally indispensable to the support of animal life.

EMILY.

But whence proceeds the carbon with which the blood is impregnated when
it comes into the lungs?

MRS. B.

Carbon exists in a greater proportion in blood than in organised animal
matter. The blood, therefore, after supplying its various secretions,
becomes loaded with an excess of carbon, which is carried off by
respiration; and the formation of new chyle from the food affords a
constant supply of carbonaceous matter.

CAROLINE.

I wonder what quantity of carbon may be expelled from the blood by
respiration in the course of 24 hours?

MRS. B.

It appears by the experiments of Messrs. Allen and Pepys that about
40,000 cubic inches of carbonic acid gas are emitted from the lungs of a
healthy person, daily; which is equivalent to _eleven ounces_ of solid
carbon every 24 hours.

EMILY.

What an immense quantity! And pray how much of carbonic acid gas do we
expel from our lungs at each expiration?

MRS. B.

The quantity of air which we take into our lungs at each inspiration, is
about 40 cubic inches, which contain a little less than 10 cubic inches
of oxygen; and of those 10 inches, one-eighth is converted into carbonic
acid gas on passing once through the lungs*, a change which is
sufficient to prevent air which has only been breathed once from
suffering a taper to burn in it.

    [Footnote *: The bulk of carbonic acid gas formed by respiration,
    is exactly the same as that of the oxygen gas which disappears.]

CAROLINE.

Pray, how does the air come in contact with the blood in the lungs?

MRS. B.

I cannot answer this question without entering into an explanation of
the nature and structure of the lungs. You recollect that the venous
blood, on being expelled from the right ventricle, enters the lungs to
go through what we may call the lesser circulation; the large trunk or
vessel that conveys it branches out, at its entrance into the lungs,
into an infinite number of very fine ramifications. The windpipe, which
conveys the air from the mouth into the lungs, likewise spreads out into
a corresponding number of air vessels, which follow the same course as
the blood vessels, forming millions of very minute air-cells. These two
sets of vessels are so interwoven as to form a sort of net-work,
connected into a kind of spongy mass, in which every particle of blood
must necessarily come in contact with a particle of air.

CAROLINE.

But since the blood and the air are contained in different vessels, how
can they come into contact?

MRS. B.

They act on each other through the membrane which forms the coats of
these vessels; for although this membrane prevents the blood and the air
from mixing together in the lungs, yet it is no impediment to their
chemical action on each other.

EMILY.

Are the lungs composed entirely of blood vessels and air vessels?

MRS. B.

I believe they are, with the addition only of nerves and of a small
quantity of the cellular substance before mentioned, which connects the
whole into an uniform mass.

EMILY.

Pray, why are the lungs always spoken of in the plural number? Are there
more than one?

MRS. B.

Yes; for though they form but one organ, they really consist of two
compartments called lobes, which are enclosed in separate membranes or
bags, each occupying one side of the chest, and being in close contact
with each other, but without communicating together. This is a beautiful
provision of nature, in consequence of which, if one of the lobes be
wounded, the other performs the whole process of respiration till the
first is healed.

The blood, thus completed, by the process of respiration, forms the most
complex of all animal compounds, since it contains not only the numerous
materials necessary to form the various secretions, as saliva, tears,
&c. but likewise all those that are required to nourish the several
parts of the body, as the muscles, bones, nerves, glands, &c.

EMILY.

There seems to be a singular analogy between the blood of animals and
the sap of vegetables; for each of these fluids contains the several
materials destined for the nutrition of the numerous class of bodies to
which they respectively belong.

MRS. B.

Nor is the production of these fluids in the animal and vegetable
systems entirely different; for the absorbent vessels, which pump up the
chyle from the stomach and intestines, may be compared to the absorbents
of the roots of plants, which suck up the nourishment from the soil. And
the analogy between the sap and the blood may be still further traced,
if we follow the latter in the course of its circulation; for, in the
living animal, we find every where organs which are possessed of a power
to secrete from the blood and appropriate to themselves the ingredients
requisite for their support.

CAROLINE.

But whence do these organs derive their respective powers?

MRS. B.

From a peculiar organisation, the secret of which no one has yet been
able to unfold. But it must be ultimately by means of the vital
principle that both their mechanical and chemical powers are brought
into action.

I cannot dismiss the subject of circulation without mentioning
_perspiration_, a secretion which is immediately connected with it, and
acts a most important part in the animal economy.

CAROLINE.

Is not this secretion likewise made by appropriate glands?

MRS. B.

No; it is performed by the extremities of the arteries, which penetrate
through the skin and terminate under the cuticle, through the pores of
which the perspiration issues. When this fluid is not secreted in
excess, it is _insensible_, because it is dissolved by the air as it
exudes from the pores; but when it is secreted faster than it can be
dissolved, it becomes _sensible_, as it assumes its liquid state.

EMILY.

This secretion bears a striking resemblance to the transpiration of the
sap of plants. They both consist of the most fluid part, and both exude
from the surface by the extremities of the vessels through which they
circulate.

MRS. B.

And the analogy does not stop there; for, since it has been ascertained
that the sap returns into the roots of the plants, the resemblance
between the animal and vegetable circulation is become still more
obvious. The latter, however, is far from being complete, since, as we
observed before, it consists only in a rising and descending of the sap,
whilst in animals the blood actually _circulates_ through every part of
the system.

We have now, I think, traced the process of nutrition, from the
introduction of the food into the stomach to its finally becoming a
constituent part of the animal frame. This will, therefore, be a fit
period to conclude our present conversation. What further remarks we
have to make on the animal economy shall be reserved for our next
interview.




CONVERSATION XXVI.

ON ANIMAL HEAT; AND ON VARIOUS ANIMAL PRODUCTS.


EMILY.

Since our last interview, I have been thinking much of the theory of
respiration; and I cannot help being struck with the resemblance which
it appears to bear to the process of combustion. For in respiration, as
in most cases of combustion, the air suffers a change, and a portion of
its oxygen combines with carbon, producing carbonic acid gas.

MRS. B.

I am much pleased that this idea has occurred to you: these two
processes appear so very analogous, that it has been supposed that a
kind of combustion actually takes place in the lungs; not of the blood,
but of the superfluous carbon which the oxygen attracts from it.

CAROLINE.

A combustion in our lungs! that is a curious idea indeed! But, Mrs. B.,
how can you call the action of the air on the blood in the lungs
combustion, when neither light nor heat are produced by it?

EMILY.

I was going to make the same objection. --Yet I do not conceive how the
oxygen can combine with the carbon, and produce carbonic acid, without
disengaging heat?

MRS. B.

The fact is, that heat is disengaged.* Whether any light be evolved,
I cannot pretend to determine; but that heat is produced in considerable
and very sensible quantities is certain, and this is the principal, if
not the only source of ANIMAL HEAT.

    [Footnote *: It has been calculated that the heat produced by
    respiration in 12 hours, in the lungs of a healthy person, is such
    as would melt about 100 pounds of ice.]

EMILY.

How wonderful! that the very process which purifies and elaborates the
blood, should afford an inexhaustible supply of internal heat?

MRS. B.

This is the theory of animal heat in its original simplicity, such
nearly as it was first proposed by Black and Lavoisier. It was equally
clear and ingenious; and was at first generally adopted. But it was
objected, on second consideration, that if the whole of the animal heat
was evolved in the lungs, it would necessarily be much less in the
extremities of the body than immediately at its source; which is not
found to be the case. This objection, however, which was by no means
frivolous, is now satisfactorily removed by the following
consideration:-- Venous blood has been found by experiment to have _less
capacity for heat_ than arterial blood; whence it follows that the
blood, in gradually passing from the arterial to the venous state,
during the circulation, parts with a portion of caloric, by means of
which heat is diffused through every part of the body.

EMILY.

More and more admirable!

CAROLINE.

The cause of animal heat was always a perfect mystery to me, and I am
delighted with its explanation. --But pray, Mrs. B., can you tell me
what is the reason of the increase of heat that takes place in a fever?

EMILY.

Is it not because we then breathe quicker, and therefore more heat is
disengaged in the system?

MRS. B.

That may be one reason: but I should think that the principal cause of
the heat experienced in fevers, is, that there is no vent for the
caloric which is generated in the body. One of the most considerable
secretions is the insensible perspiration; this is constantly carrying
off caloric in a latent state; but during the hot stage of a fever, the
pores are so contracted, that all perspiration ceases, and the
accumulation of caloric in the body occasions those burning sensations
which are so painful.

EMILY.

This is, no doubt, the reason why the perspiration that often succeeds
the hot stage of a fever affords so much relief. If I had known this
theory of animal heat when I had a fever last summer, I think I should
have found some amusement in watching the chemical processes that were
going on within me.

CAROLINE.

But exercise likewise produces animal heat, and that must be quite in a
different manner.

MRS. B.

Not so much so as you think; for the more exercise you take, the more
the body is stimulated, and requires recruiting. For this purpose the
circulation of the blood is quickened, the breath proportionably
accelerated, and consequently a greater quantity of caloric evolved.

CAROLINE.

True; after running very fast, I gasp for breath, my respiration is
quick and hard, and it is just then that I begin to feel hot.

EMILY.

It would seem, then, that violent exercise should produce fever.

MRS. B.

Not if the person is in a good state of health; for the additional
caloric is then carried off by the perspiration which succeeds.

EMILY.

What admirable resources nature has provided for us! By the production
of animal heat she has enabled us to keep up the temperature of our
bodies above that of inanimate objects; and whenever this source becomes
too abundant, the excess is carried off by perspiration.

MRS. B.

It is by the same law of nature that we are enabled, in all climates,
and in all seasons, to preserve our bodies of an equal temperature, or
at least very nearly so.

CAROLINE.

You cannot mean to say that our bodies are of the same temperature in
summer, and in winter, in England, and in the West-Indies.

MRS. B.

Yes, I do; at least if you speak of the temperature of the blood, and
the internal parts of the body; for those parts that are immediately in
contact with the atmosphere, such as the hands and face, will
occasionally get warmer, or colder, than the internal or more sheltered
parts. But if you put the bulb of a thermometer in your mouth, which is
the best way of ascertaining the real temperature of your body, you will
scarcely perceive any difference in its indication, whatever may be the
difference of temperature of the atmosphere.

CAROLINE.

And when I feel overcome by heat, I am really not hotter than when I am
shivering with cold?

MRS. B.

When a person in health feels very hot, whether from internal heat, from
violent exercise, or from the temperature of the atmosphere, his body is
certainly a little warmer than when he feels very cold; but this
difference is much smaller than our sensations would make us believe;
and the natural standard is soon restored by rest and by perspiration.
It is chiefly the external parts that are warmer, and I am sure that you
will be surprised to hear that the internal temperature of the body
scarcely ever descends below ninety-five or ninety-six degrees, and
seldom attains one hundred and four or one hundred and five degrees,
even in the most violent fevers.

EMILY.

The greater quantity of caloric, therefore, that we receive from the
atmosphere in summer, cannot raise the temperature of our bodies beyond
certain limits, as it does that of inanimate bodies, because an excess
of caloric is carried off by perspiration.

CAROLINE.

But the temperature of the atmosphere, and consequently that of
inanimate bodies, is surely never so high as that of animal heat?

MRS. B.

I beg your pardon. Frequently in the East and West Indies, and sometimes
in the southern parts of Europe, the atmosphere is above ninety-eight
degrees, which is the common temperature of animal heat. Indeed, even in
this country, it occasionally happens that the sun's rays, setting full
on an object, elevate its temperature above that point.

In illustration of the power which our bodies have to resist the effects
of external heat, Sir Charles Blagden, with some other gentlemen, made
several very curious experiments. He remained for some time in an oven
heated to a temperature not much inferior to that of boiling water,
without suffering any other inconvenience than a profuse perspiration,
which he supported by drinking plentifully.

EMILY.

He could scarcely consider the perspiration as an inconvenience, since
it saved him from being baked by giving vent to the excess of caloric.

CAROLINE.

I always thought, I confess, that it was from the heat of the
perspiration that we suffered in summer.

MRS. B.

You now find that you are quite mistaken. Whenever evaporation takes
place, cold, you know, is produced in consequence of a quantity of
caloric being carried off in a latent state; this is the case with
perspiration, and it is in this way that it affords relief. It is on
that account also that we are so apt to _catch cold_, when in a state of
profuse perspiration. It is for the same reason that tea is often
refreshing in summer, though it appears to heat you at the moment you
drink it.

EMILY.

And in winter, on the contrary, tea is pleasant on account of its heat.

MRS. B.

Yes; for we have then rather to guard against a deficiency than an
excess of caloric, and you do not find that tea will excite perspiration
in winter, unless after dancing, or any other violent exercise.

CAROLINE.

What is the reason that it is dangerous to eat ice after dancing, or to
drink any thing cold when one is very hot?

MRS. B.

Because the loss of heat arising from the perspiration, conjointly with
the chill occasioned by the cold draught, produce more cold than can be
borne with safety, unless you continue to use the same exercise after
drinking that you did before; for the heat occasioned by the exercise
will counteract the effects of the cold drink, and the danger will be
removed. You may, however, contrary to the common notion, consider it as
a rule, that cold liquids may, at all times, be drunk with perfect
safety, however hot you may feel, provided you are not at the moment in
a state of great perspiration, and on condition that you keep yourself
in gentle exercise afterwards.

EMILY.

But since we are furnished with such resources against the extremes of
heat or cold, I should have thought that all climates would have been
equally wholesome.

MRS. B.

That is true, in a certain degree, with regard to those who have been
accustomed to them from birth; for we find that the natives of those
climates, which we consider as most deleterious, are as healthy as
ourselves; and if such climates are unwholesome to those who are
habituated to a more moderate temperature, it is because the animal
economy does not easily accustom itself to considerable changes.

CAROLINE.

But pray, Mrs. B., if the circulation preserves the body of an uniform
temperature, how does it happen that animals are sometimes frozen?

MRS. B.

Because, if more heat be carried off by the atmosphere than the
circulation can supply, the cold will finally prevail, the heart will
cease to beat, and the animal will be frozen. And, likewise, if the body
remained long exposed to a degree of heat, greater than the perspiration
could carry off, it would at last lose the power of resisting its
destructive influence.

CAROLINE.

Fish, I suppose, have no animal heat, but only partake of the
temperature of the water in which they live?

EMILY.

And their coldness, no doubt, proceeds from their not breathing?

MRS. B.

All kinds of fish breathe more or less, though in a much smaller degree
than land animals. Nor are they entirely destitute of animal heat,
though, for the same reason, they are much colder than other creatures.
They have comparatively but a very small quantity of blood, therefore
but very little oxygen is required, and a proportionally small quantity
of animal heat is generated.

CAROLINE.

But how can fish breathe under water?

MRS. B.

They breathe by means of the air which is dissolved in the water, and if
you put them into water deprived of air by boiling, they are soon
suffocated.

If a fish is confined in a vessel of water closed from the air, it soon
dies; and any fish put in afterwards would be killed immediately, as all
the air had been previously consumed.

CAROLINE.

Are there any species of animals that breathe more than we do?

MRS. B.

Yes; birds, of all animals, breathe the greatest quantity of air in
proportion to their size; and it is to this that they are supposed to
owe the peculiar firmness and strength of their muscles, by which they
are enabled to support the violent exertion of flying.

This difference between birds and fish, which may be considered as the
two extremes of the scale of muscular strength, is well worth observing.
Birds residing constantly in the atmosphere, surrounded by oxygen, and
respiring it in greater proportions than any other species of animals,
are endowed with a superior degree of muscular strength, whilst the
muscles of fish, on the contrary, are flaccid and oily; these animals
are comparatively feeble in their motions, and their temperature is
scarcely above that of the water in which they live. This is, in all
probability, owing to their imperfect respiration; the quantity of
hydrogen and carbon, that is in consequence accumulated in their bodies,
forms the oil which is so strongly characteristic of that species of
animals, and which relaxes and softens the small quantity of fibrine
which their muscles contain.

CAROLINE.

But, Mrs. B., there are some species of birds that frequent both
elements, as, for instance, ducks and other water fowl. Of what nature
is the flesh of these?

MRS. B.

Such birds, in general, make but little use of their wings; if they fly,
it is but feebly, and only to a short distance. Their flesh, too,
partakes of the oily nature, and even in taste sometimes resembles that
of fish. This is the case not only with the various kinds of water
fowls, but with all other amphibious animals, as the otter, the
crocodile, the lizard, &c.

CAROLINE.

And what is the reason that reptiles are so deficient in muscular
strength?

MRS. B.

It is because they usually live under ground, and seldom come into the
atmosphere. They have imperfect, and sometimes no discernible organs of
respiration; they partake therefore of the soft oily nature of fish;
indeed, many of them are amphibious, as frogs, toads, and snakes, and
very few of them find any difficulty in remaining a length of time under
water. Whilst, on the contrary, the insect tribe, that are so strong in
proportion to their size, and alert in their motions, partake of the
nature of birds, air being their peculiar element, and their organs of
respiration being comparatively larger than in other classes of animals.

I have now given you a short account of the principal animal functions.
However interesting the subject may appear to you, a fuller
investigation of it would, I fear, lead us too far from our object.

EMILY.

Yet I shall not quit it without much regret; for of all the branches of
chemistry, it is certainly the most curious and most interesting.

CAROLINE.

But, Mrs. B., I must remind you that you promised to give us some
account of the nature of _milk_.

MRS. B.

True. There are several other animal productions that deserve likewise
to be mentioned. We shall begin with milk, which is certainly the most
important and the most interesting of all the animal secretions.

Milk, like all other animal substances, ultimately yields by analysis
oxygen, hydrogen, carbon, and nitrogen. These are combined in it under
the forms of albumen, gelatine, oil, and water. But milk contains,
besides, a considerable portion of phosphat of lime, the purposes of
which I have already pointed out.

CAROLINE.

Yes; it is this salt which serves to nourish the tender bones of the
suckling.

MRS. B.

To reduce milk to its elements, would be a very complicated, as well as
useless operation; but this fluid, without any chemical assistance, may
be decomposed into three parts, _cream_, _curds_, and _whey_. These
constituents of milk have but a very slight affinity for each other, and
you find accordingly that cream separates from milk by mere standing. It
consists chiefly of oil, which being lighter than the other parts of the
milk, gradually rises to the surface. It is of this, you know, that
butter is made, which is nothing more than oxygenated cream.

CAROLINE.

Butter, then, is somewhat analogous to the waxy substance formed by the
oxygenation of vegetable oils.

MRS. B.

Very much so.

EMILY.

But is the cream oxygenated by churning?

MRS. B.

Its oxygenation commences previous to churning, merely by standing
exposed to the atmosphere, from which it absorbs oxygen. The process is
afterwards completed by churning; the violent motion which this
operation occasions brings every particle of cream in contact with the
atmosphere, and thus facilitates its oxygenation.

CAROLINE.

But the effect of churning, I have often observed in the dairy, is to
separate the cream into two substances, butter and butter-milk.

MRS. B.

That is to say, in proportion as the oily particles of the cream become
oxygenated, they separate from the other constituent parts of the cream
in the form of butter. So by churning you produce, on the one hand,
butter, or oxygenated oil; and, on the other, butter-milk, or cream
deprived of oil. But if you make butter by churning new milk instead of
cream, the butter-milk will then be exactly similar in its properties to
creamed or skimmed milk.

CAROLINE.

Yet butter-milk is very different from common skimmed milk.

MRS. B.

Because you know it is customary, in order to save time and labour, to
make butter from cream alone. In this case, therefore, the butter-milk
is deprived of the creamed milk, which contains both the curd and whey.
Besides, in consequence of the milk remaining exposed to the atmosphere
during the separation of the cream, the latter becomes more or less
acid, as well as the butter-milk which it yields in churning.

EMILY.

Why should not the butter be equally acidified by oxygenation?

MRS. B.

Animal oil is not so easily acidified as the other ingredients of milk.
Butter, therefore, though usually made of sour cream, is not sour
itself, because the oily part of the cream had not been acidified.
Butter, however, is susceptible of becoming acid by an excess of oxygen;
it is then said to be rancid, and produces the sebacic acid, the same as
that which is obtained from fat.

EMILY.

If that be the case, might not rancid butter be sweetened by mixing with
it some substance that would take the acid from it?

MRS. B.

This idea has been suggested by Sir H. Davy, who supposes, that if
rancid butter were well washed in an alkaline solution, the alkali would
separate the acid from the butter.

CAROLINE.

You said just now that creamed milk consisted of curd and whey. Pray how
are these separated?

MRS. B.

They may be separated by standing for a certain length of time exposed
to the atmosphere; but this decomposition may be almost instantaneously
effected by the chemical agency of a variety of substances. Alkalies,
rennet*, and indeed almost all animal substances, decompose milk by
combining with the curds.

Acids and spirituous liquors, on the other hand, produce a decomposition
by combining with the whey. In order, therefore, to obtain the whey
pure, rennet, or alkaline substances, must be used to attract the curds
from it.

But if it be wished to obtain the curds pure, the whey must be separated
by acids, wine, or other spirituous liquors.

    [Footnote *: Rennet is the name given to a watery infusion of the
    coats of the stomach of a sucking calf. Its remarkable efficacy in
    promoting coagulation is supposed to depend on the gastric juice
    with which it is impregnated.]

EMILY.

This is a very useful piece of information; for I find white-wine whey,
which I sometimes take when I have a cold, extremely heating; now, if
the whey were separated by means of an alkali instead of wine, it would
not produce that effect.

MRS. B.

Perhaps not. But I would strenuously advise you not to place too much
reliance on your slight chemical knowledge in medical matters. I do not
know why whey is not separated from curd by rennet, or by an alkali, for
the purpose which you mention; but I strongly suspect that there must be
some good reason why the preparation by means of wine is generally
preferred. I can, however, safely point out to you a method of obtaining
whey without either alkali, rennet, or wine; it is by substituting lemon
juice, a very small quantity of which will separate it from the curds.

Whey, as an article of diet, is very wholesome, being remarkable light
of digestion. But its effect, taken medicinally, is chiefly, I believe,
to excite perspiration, by being drunk warm on going to bed.

From whey a substance may be obtained in crystals by evaporation, called
_sugar of milk_. This substance is sweet to the taste, and in its
composition is so analogous to common sugar, that it is susceptible of
undergoing the vinous fermentation.

CAROLINE.

Why then is not wine, or alcohol, made from whey?

MRS. B.

The quantity of sugar contained in milk is so trifling, that it can
hardly answer that purpose. I have heard of only one instance of its
being used for the production of a spirituous liquor, and this is by the
Tartan Arabs; their abundance of horses, as well as their scarcity of
fruits, has introduced the fermentation of mares' milk, by which they
produce a liquor called _koumiss_. Whey is likewise susceptible of being
acidified by combining with oxygen from the atmosphere. It then produces
the _lactic acid_, which you may recollect is mentioned amongst the
animal acids, as the acid of milk.

Let us now see what are the properties of curds.

EMILY.

I know that they are made into cheese; but I have heard that for that
purpose they are separated from the whey by rennet, and yet this you
have just told us is not the method of obtaining pure curds?

MRS. B.

Nor are pure curds so well adapted for the formation of cheese. For the
nature and flavour of the cheese depend, in a great measure, upon the
cream or oily matter which is left in the curds; so that if every
particle of cream be removed from the curds, the cheese is scarcely
eatable. Rich cheeses, such as cream and Stilton cheeses, derive their
excellence from the quantity, as well as the quality, of the cream that
enters into their composition.

CAROLINE.

I had no idea that milk was such an interesting compound. In many
respects there appears to me to be a very striking analogy between milk
and the contents of an egg, both in respect to their nature and their
use. They are, each of them, composed of the various substances
necessary for the nourishment of the young animal, and equally destined
for that purpose.

MRS. B.

There is, however, a very essential difference. The young animal is
formed, as well as nourished, by the contents of the egg-shell; whilst
milk serves as nutriment to the suckling, only after it is born.


There are several peculiar animal substances which do not enter into the
general enumeration of animal compounds, and which, however, deserve to
be mentioned.

_Spermaceti_ is of this class; it is a kind of oily substance obtained
from the head of the whale, which, however, must undergo a certain
preparation before it is in a fit state to be made into candles. It is
not much more combustible than tallow, but it is pleasanter to burn, as
it is less fusible and less greasy.

_Ambergris_ is another peculiar substance derived from a species of
whale. It is, however, seldom obtained from the animal itself, but is
generally found floating on the surface of the sea.

_Wax_, you know, is a concrete oil, the peculiar product of the bee,
part of the constituents of which may probably be derived from flowers,
but so prepared by the organs of the bee, and so mixed with its own
substance, as to be decidedly an animal product. Bees' wax is naturally
of a yellow colour, but it is bleached by long exposure to the
atmosphere, or may be instantaneously whitened by the oxy-muriatic acid.
The combustion of wax is far more perfect than that of tallow, and
consequently produces a greater quantity of light and heat.

_Lac_ is a substance very similar to wax in the manner of its formation;
it is the product of an insect, which collects its ingredients from
flowers, apparently for the purpose of protecting its eggs from injury.
It is formed into cells, fabricated with as much skill as those of the
honey-comb, but differently arranged. The principal use of lac is in the
manufacture of sealing-wax, and in making varnishes and lacquers.

_Musk_, _civet_, and _castor_, are other particular productions, from
different species of quadrupeds. The two first are very powerful
perfumes; the latter has a nauseous smell and taste, and is only used
medicinally.

CAROLINE.

Is it from this substance that castor oil is obtained?

MRS. B.

No. Far from it, for castor oil is a vegetable oil, expressed from the
seeds of a particular plant; and has not the least resemblance to the
medicinal substance obtained from the castor.

_Silk_ is a peculiar secretion of the silk-worm, with which it builds
its nest or cocoon. This insect was originally brought to Europe from
China. Silk, in its chemical nature, is very similar to the hair and
wool of animals; whilst in the insect it is a fluid, which is
coagulated, apparently by uniting with oxygen, as soon as it comes in
contact with the air. The moth of the silk-worm ejects a liquor which
appears to contain a particular acid, called _bombic_, the properties of
which are but very little known.

EMILY.

Before we conclude the subject of the animal economy, shall we not learn
by what steps dead animals return to their elementary state?

MRS. B.

Animal matter, although the most complicated of all natural substances,
returns to its elementary state by one single spontaneous process, the
_putrid fermentation_. By this, the albumen, fibrine, &c. are slowly
reduced to the state of oxygen, hydrogen, nitrogen, and carbon; and thus
the circle of changes through which these principles have passed is
finally completed. They first quitted their elementary form, or their
combination with unorganised matter, to enter into the vegetable system.
Hence they were transmitted to the animal kingdom; and from this they
return, again to their primitive simplicity, soon to re-enter the sphere
of organised existence.

When all the circumstances necessary to produce fermentation do not take
place, animal, like vegetable matter, is liable to a partial or
imperfect decomposition, which converts it into a combustible substance
very like spermaceti. I dare say that Caroline, who is so fond of
analogies, will consider this as a kind of animal bitumen.

CAROLINE.

And why should I not, since the processes which produce these substances
are so similar?

MRS. B.

There is, however, one considerable difference; the state of bitumen
seems permanent, whilst that of animal substances, thus imperfectly
decomposed, is only transient; and unless precautions be taken to
preserve them in that state, a total dissolution infallibly ensues. This
circumstance, of the occasional conversion of animal matter into a kind
of spermaceti, is of late discovery. A manufacture has in consequence
been established near Bristol, in which, by exposing the carcases of
horses and other animals for a length of time under water, the muscular
parts are converted into this spermaceti-like substance. The bones
afterwards undergo a different process to produce hartshorn, or, more
properly, ammonia, and phosphorus; and the skin is prepared for leather.

Thus art contrives to enlarge the sphere of useful purposes, for which
the elements were intended by nature; and the productions of the several
kingdoms are frequently arrested in their course, and variously
modified, by human skill, which compels them to contribute, under new
forms, to the necessities or luxuries of man.

But all that we enjoy, whether produced by the spontaneous operations of
nature, or the ingenious efforts of art, proceed alike from the goodness
of Providence. --To GOD alone man owes the admirable faculties which
enable him to improve and modify the productions of nature, no less than
those productions themselves. In contemplating the works of the
creation, or studying the inventions of art, let us, therefore, never
forget the Divine Source from which they proceed; and thus every
acquisition of knowledge will prove a lesson of piety and virtue.




INDEX.


A

  Absorbent vessels, ii. 304
  Absorption of caloric, i. 59. 66
  Acetic acid, ii. 75. 197
  Acetous fermentation, ii. 232
  ---- acid, ii. 193. 232
  Acidulous gaseous mineral waters, ii. 129
  ---- salts, ii. 200
  Acids, i. 262. ii. 69
  Aeriform, i. 36
  Affinity, i. 19. ii. 1
  Agate, ii. 51
  Agriculture, ii. 252
  Air, i. 182. ii. 262
  Albumen, ii. 277. 288
  Alburnum, ii. 267
  Alchemists, i. 4
  Alcohol, or spirit of wine, ii. 215. 222
  Alembic, i. 258
  Alkalies, ii. 19
  Alkaline earths, ii. 50. 58
  Alloys, i. 344
  Alum, or sulphat of alumine, ii. 55. 95
  Alumine, ii. 54
  Alumium, i. 13
  Amalgam, i. 347
  Ambergris, ii. 358
  Amethyst, ii. 58
  Amianthus, ii. 66
  Ammonia, or volatile alkali, i. 363. ii. 20. 35
  Ammoniacal gas, ii. 36
  Ammonium, i. 13
  Analysis, i. 287
  ---- of vegetables, ii. 165
  Animals, ii. 276
  Animal acids, ii. 75. 290
  ---- colours, ii. 292
  ---- heat, ii. 337
  ---- oil, ii. 178. 283
  Animalization, ii. 276. 297. 315
  Antidotes, ii. 41. 87
  Antimony, i. 14
  Aqua fortis, ii. 105
  ---- regia, i. 340. ii. 144
  Arrack, ii. 220
  Argand's Lamp, i. 208
  Arsenic, i. 14. 340. 348
  Arteries, ii. 304. 323
  Arterial blood, ii. 305. 326. 338
  Asphaltum, ii. 240
  Assafoetida, ii. 188
  Assimilation, ii. 298
  Astringent principle, ii. 198
  Atmosphere, i. 90. 181. ii. 262
  Atmospherical air, i. 182
  Attraction of aggregation, or cohesion, i. 16. ii. 2
  ---- of composition, i. 16. ii. 1
  Azot, or nitrogen, i. 182, ii. 100
  Azotic gas, i. 182

B

  Balsams, ii. 165. 188
  Balloons, i. 245
  Bark, ii. 193. 265
  Barytes, ii. 44. 58. 61
  Bases of acids, i. 263. ii. 69
  ---- gases, i. 183
  ---- salts, ii. 5
  Beer, ii. 212. 220
  Benzoic acid, ii. 74. 197
  Bile, ii. 308
  Birds, ii. 347
  Bismuth, i. 14
  Bitumens, ii. 239
  Black lead, or plumbago, i. 304
  Bleaching, i. 32. ii. 89. 140.
  Blow-pipe, i. 324. ii. 226
  Blood, ii. 306. 317
  Blood-vessels, ii. 298
  Boiling water, i. 93
  Bombic acid, ii. 75. 290
  Bones, ii. 298, 299
  Boracic acid, i. 365. ii. 131
  Boracium, i. 13. ii. 132
  Borat of soda, ii. 133
  Brandy, ii. 218
  Brass, i. 344
  Bread, ii. 233
  Bricks, ii. 56
  Brittle-metals, i. 14
  Bronze, i. 341
  Butter, ii. 351
  Butter-milk, ii. 352

C

  Calcareous earths, ii. 65
  ---- stones, ii. 123
  Calcium, i. 13
  Caloric, i. 12. 33
  ----, absorption of, i. 66
  ----, conductors of, i. 70
  ----, combined, i. 122
  ----, expansive power of i. 35
  ----, equilibrium of, i. 50
  ----, reflexion of, i. 54. 67
  ----, radiation of, i. 52. 61
  ----, solvent power of, i. 96. 102
  ----, capacity for, i. 124
  Calorimeter, i. 156
  Calx, i. 183
  Camphor, ii. 165. 185
  Camphoric acid, ii. 74. 197
  Caoutchouc, ii. 165. 189
  Carbonats, ii. 25. 129
  Carbonat of ammonia, ii. 41
  ---- lead, i. 320
  ---- lime, ii. 59. 130
  ---- magnesia, ii. 67
  ---- potash, ii. 25
  Carbonated hydrogen gas, i. 302
  Carbon, i. 282. ii. 329
  Carbonic acid, i. 290. 359. ii. 327
  Carburet of iron, i. 304. 342
  Carmine, ii. 295
  Cartilage, ii. 303
  Castor, ii. 359
  Cellular membrane, ii. 311
  Caustics, i. 349
  Chalk, ii. 62. 123
  Charcoal, i. 282
  Cheese, ii. 356
  Chemical attraction, i. 15. ii. 9
  Chemistry, i. 3
  Chest, ii. 318
  China, ii. 54
  Chlorine, i. 214
  Chrome, i. 14. 340
  Chyle, ii. 305. 317
  Chyme, ii. 316
  Citric acid, ii. 74. 197
  Circulation of the blood, ii. 322
  Civet, ii. 359
  Clay, i. 48. ii. 55
  Coke, ii. 241
  Coal, ii. 240. 252
  Cobalt, i. 14
  Cochineal, ii. 295
  Cold, i. 50. 58
  ---- from evaporation, i. 102. 113. 150
  Colours of metallic oxyds, i. 319
  Columbium, i. 14. 340. 348
  Combined caloric, i. 122
  Combustion, i. 190
  ----, volatile products of, i. 207
  ----, fixed products of, i. 207
  ----, of alcohol, ii. 225
  ----, of ammoniacal gas, ii. 42
  ----, of boracium, ii. 133
  ----, by oxymuriatic acid or chlorine, ii. 142
  ----, of carbon, i. 289
  ----, of coals, i. 207. 297
  ----, of charcoal by nitric acid, ii. 102
  ----, of candles, i. 236. 309. ii. 179
  ----, of diamonds, i. 292
  ----, of ether, ii. 230
  ----, of hydrogen, i. 229.
  ----, of iron, i. 200. 322
  ----, of metals, i. 321
  ----, of oils, i. 208. ii. 178. 309
  ----, of oil of turpentine by nitrous acid, ii. 6
  ----, of phosphorus, i. 272
  ----, of sulphur, i. 261
  ---- of potassium, i. 358. ii. 132. 138, 139
  Compound bodies, i. 9. ii. 14
  ---- or neutral salts i. 333. ii. 4
  Conductors of heat, i. 71
  ----, solids, i. 73
  ----, fluids, i. 78
  ----, Count Rumford's theory, i. 79
  Constituent parts, i. 9
  Copper, i. 14. 331
  Copal, ii. 187. 224
  Cortical layers, ii. 265. 267
  Cotyledons, or lobes, ii. 256
  Cream, ii. 351
  Cream of tartar, or tartrit of potash, ii. 200. 222
  Cryophorus, i. 154
  Crystallisation, i. 338. ii. 47
  Cucurbit, i. 258
  Culinary heat, i. 88
  Curd, ii. 351. 354
  Cuticle, or epidermis, ii. 310

D

  Decomposition, i. 8. 20
  ---- of atmospherical air, i. 181. 209
  ---- of water by the Voltaic battery, i. 220
  ---- of salts by the Voltaic battery, ii. 14
  ---- of water by metals, i. 225. 334
  ---- ---- by carbon, i. 301
  ---- of vegetables, ii. 202
  ---- of potash, i. 356
  ---- of soda, i. 56
  ---- of ammonia, i. 363. ii. 37
  ---- of the boracic acid, ii. 132
  ---- of the fluoric acid, ii. 136
  ---- of the muriatic acid, ii. 139
  Deflagration, ii. 118
  Definite proportions, ii. 13
  Deliquescence, ii. 95
  Detonation, i. 219. ii. 116
  Dew, i. 105
  Diamond, i. 285
  Diaphragm, ii. 320
  Digestion, ii. 316
  Dissolution of metals, i. 165. 316. 333
  Distillation, i. 259. ii. 218
  ---- of red wine, ii. 218
  Divellent forces, ii. 12
  Division, i. 7
  Drying oils, ii. 181
  Dying, ii. 191

E

  Earths, ii. 44
  Earthen-ware, ii. 53. 57
  Effervescence, i. 298
  Efflorescence, ii. 94
  Elastic fluids, i. 37
  Electricity, i. 12. 25. 160. 220. ii. 139
  Electric machine, i. 169
  Elective attractions, ii. 9
  Elementary bodies, i. 8. 12
  Elixirs, tinctures, or quintessences, ii. 225
  Enamel, ii. 57
  Epidermis of vegetables, ii. 269
  ---- of animals, ii. 310
  Epsom salts, ii. 63. 95
  Equilibrium of caloric, i. 50
  Essences, i. 307. ii. 183. 224
  Essential, or volatile oils, i. 307. ii. 183
  Ether, i. 111. ii. 229
  Evaporation, i. 103
  Evergreens, ii. 274
  Eudiometer, i. 276
  Expansion of caloric, i. 36
  Extractive colouring matter, ii. 165. 190

F

  Falling stones, i. 319
  Fat, i. 306. ii. 311
  Feathers, ii. 300
  Fecula, ii. 176
  Fermentation, ii. 205
  Fibrine, ii. 277. 289
  Fire, i. 7. 27
  Fish, ii. 346
  Fixed air, or carbonic acid, i. 290. ii. 125
  ---- alkalies, ii. 20
  ---- oils, i. 307. ii. 165. 177
  ---- products of combustion, i. 207
  Flame, i. 237
  Flint, ii. 30. 51
  Flower or blossom, ii. 271
  Fluoric acid, ii. 54. 134
  Fluorium, or Fluorine, i. 12. ii. 136
  Formic acid, ii. 290
  Fossil wood, ii. 242
  Frankincense, ii. 187
  Free or radiant caloric, or heat of temperature, i. 33
  Freezing mixtures, i. 142
  ---- by evaporation, i. 104. 150, &c.
  Frost, i. 94
  Fruit, ii. 271
  Fuller's earth, ii. 55
  Furnace, i. 304

G

  Galls, ii. 199
  Gallat of iron, ii. 98
  Gallic acid, ii. 74. 197, 198
  Galvanism, i. 163
  Gas, i. 182
  Gas-lights, i. 240
  Gaseous oxyd of carbon, i. 296
  ---- nitrogen, ii. 111
  Gastric juice, ii. 316
  Gelatine, or jelly, ii. 277. 280
  Germination, ii. 256
  Gin, ii. 221
  Glands, ii. 298. 307
  Glass, ii. 30
  Glauber's salts, or sulphat of soda, ii. 92
  Glazing, ii. 57
  Glucium, i. 13
  Glue, ii. 281. 287
  Gluten, ii. 165. 177
  Gold, i. 14. 323
  Gum, ii. 170
  ---- arabic, ii. 170
  ---- elastic, or caoutchouc, ii. 189
  ---- resins, ii. 165. 188
  Gunpowder, ii. 116
  Gypsum, or plaister of Paris, or sulphat of lime, ii. 95

H

  Hair, ii. 300
  Harrogate water, i. 268. 341
  Hartshorn, ii. 35. 39. 281. 285
  Heart, ii. 323
  ---- wood, ii. 268
  Heat, i. 26. 33
  ---- of capacity, i. 127. 135
  ---- of temperature, i. 33
  Honey, ii. 175
  Horns, ii. 282. 300
  Hydro-carbonat, i. 241. 303
  Hydrogen, i. 214
  ---- gas, i. 215

I

  Jasper, ii. 51
  Ice, i. 138
  Jelly, ii. 281
  Jet, ii. 240
  Ignes fatui, i. 277
  Ignition, i. 119
  Imponderable agents, i. 12
  Inflammable air, i. 215
  Ink, ii. 98. 199
  Insects, ii. 349
  Integrant pans, i. 9
  Iridium, i. 14
  Iron, i. 14. 319. 328
  Isinglass, ii. 194. 285
  Ivory black, ii. 295
  Iodine, i. 214. ii. 157

K

  Kali, ii. 34
  Koumiss, ii. 356

L

  Lac, ii. 358
  Lactic acid, ii. 75. 290. 356
  Lakes, colours, ii. 190
  Latent heat, i. 133
  Lavender water, ii. 184. 224
  Lead, i. 14. 318. 330
  Leather, ii. 193. 287
  Leaves, ii. 260
  Life, ii. 159. 168
  Ligaments, ii. 303
  Light, i. 12. 26. ii. 261
  Lightning, i. 248
  Lime, ii. 59
  ---- water, ii. 61
  Limestone, ii. 60
  Linseed oil, ii. 178
  Liqueurs, ii. 224
  Liver, ii. 308
  Lobes, ii. 256. 332
  Lunar caustic, or nitrat of silver, i. 350. ii. 119
  Lungs, ii. 319. 330
  Lymph, ii. 304
  Lymphatic vessels, ii. 304

M

  Magnesia, ii. 44. 66
  Magnium, i. 13
  Malic acid, ii. 74. 197
  Malt, ii. 211
  Malleable metals, i. 14
  Manganese, i. 14. 317
  Manna, ii. 176
  Manure, ii. 247
  Marble, ii. 123
  Marine acid, or muriatic acid, ii. 136
  Mastic, ii. 187. 224
  Materials of animals, ii. 277
  ---- of vegetables, ii. 165
  Mercury, i. 14. 346
  ----, new mode of freezing, i. 155. 347
  Metallic acids, i. 340
  ---- oxyds, i. 316
  Metals, i. 12. 314
  Meteoric stones, i. 342
  Mica, ii. 66
  Milk, ii. 299. 306. 350
  Minerals, i. 315. ii. 44. 158
  Mineral waters, i. 296. ii. 129
  ---- acids, ii. 73
  Miner's lamp, i. 249
  Mixture, i. 99
  Molybdena, i. 14. 340
  Mordant, ii. 165. 192
  Mortar, ii. 53. 65
  Mucilage, ii. 170
  Mucous acid, ii. 74. 171. 197
  ---- membrane, ii. 311
  Muriatic acid, or marine acid, ii. 136
  Muriats, ii. 151
  Muriat of ammonia, ii. 35. 152
  ---- lime, i. 100
  ---- soda, or common salt, ii. 136. 151
  ---- potash, ii. 138
  Muriatium, i. 13
  Muscles of animals, ii. 298. 303
  Musk, ii. 359
  Myrrh, ii. 188

N.

  Naphtha, i. 357. ii. 240
  Negative electricity, i. 25. 161. 185
  Nerves, ii. 279. 298. 308
  Neutral, or compound salts, i. 333. ii. 4. 22. 69
  Nickel, i. 13. 343
  Nitre, or nitrat of potash, or saltpetre, ii. 32. 104. 116
  Nitric acid, ii. 100
  Nitrogen, or azot, i. 181. ii. 100
  ---- gas, i. 182. 211
  Nitro-muriatic acid, or aqua regia, ii. 144
  Nitrous acid gas, ii. 101. 106
  ---- air, or nitrit oxyd gas, ii. 107
  Nitrats, ii. 116
  Nitrat of copper, ii. 5
  ---- ammonia, ii. 113. 118
  ---- potash, or nitre, or saltpetre, ii. 32. 104. 116
  ---- silver, or lunar caustic, ii. 19
  Nomenclature of acids, i. 264. ii. 69
  ---- compound salts, ii. 4. 22
  ---- other binary compounds, i. 278
  Nut-galls, ii. 98. 199
  Nut-oil, ii. 178
  Nutrition, ii. 297

O

  Ochres, i. 320
  Oils, i. 285. ii. 306
  Oil of amber, ii. 241
  ---- vitriol, or sulphuric acid, ii. 80
  Olive oil, ii. 178
  Ores, i. 315
  Organized bodies, ii. 159
  Organs of animals, ii. 290. 310
  ---- vegetables, ii. 159. 265. 271
  Osmium, i. 14. 348
  Oxalic acid, ii. 74. 197
  Oxyds, i. 198
  Oxyd of manganese, i. 117. 317
  ---- iron, i. 204. 319
  ---- lead, i. 319
  ---- sulphur, ii. 91
  Oxydation, or oxygenation, i. 196
  Oxygen, i. 11. 181. 201. 211
  ---- gas, or vital air, i. 182. 201
  Oxy-muriatic acid, ii. 140
  Oxy-muriats, ii. 153
  Oxy-muriat of potash, ii. 155

P

  Palladium, i. 13. 348
  Papin's digester, i. 120. ii. 284
  Parenchyma, ii. 256. 266
  Particles, i. 16
  Pearlash, ii. 24
  Peat, ii. 242
  Peculiar juice of plants, ii. 268
  Perfect metals, i. 14. 324
  Perfumes, i. 308. ii. 183
  Perspiration, ii. 333. 329
  Petrification, ii. 237
  Pewter, i. 344
  Pharmacy, i. 14
  Phosphat of lime, ii. 99. 299
  Phosphorated hydrogen gas, i. 277
  Phosphorescence, i. 29
  Phosphoric acid, i. 273. ii. 99
  Phosphorous acid, i. 274. ii. 99
  Phosphorus, i. 270
  Phosphoret of lime, i. 278. 341
  ---- sulphur, i. 279. 341
  Pitch, ii. 187
  Plaster, ii. 65
  Platina, i. 14. 323
  Plating, i. 345
  Plumbago, or black lead, i. 304
  Plumula, ii. 257
  Porcelain, ii. 56
  Positive electricity, i. 25. 161. 185
  Potassium, i. 13. 357. ii. 15
  Pottery, ii. 56
  Potash, i. 356. ii. 22
  Precipitate, i. 22
  Pressure of the atmosphere, i. 112. 116
  Printer's ink, ii. 144
  Prussiat of iron, or prussian blue, ii. 291
  ---- potash, ii. 291
  Prussic acid, ii. 75. 290
  Putrid fermentation, ii. 235. 360
  Pyrites, i. 341. ii. 97
  Pyrometer, i. 38. 42

Q

  Quick lime, ii. 59
  Quiescent forces, ii. 12

R

  Radiation of caloric, i. 52
  ----, Prevost's theory, i. 52
  ----, Pictet's explanations, i. 54
  ----, Leslie's illustrations, i. 61
  Radicals, ii. 5. 69
  Radicle; or root, ii. 257
  Rain, i. 104
  Rancidity, ii. 182
  Rectification, ii. 223
  Reflexion of caloric, i. 54. 64
  Reptiles, ii. 349
  Resins, ii. 165, 186. 266
  Respiration, ii. 317. 326
  Reviving of metals, i. 327
  Rhodium, i. 14. 348
  Roasting metals, i. 316
  Rock crystal, ii. 61
  Ruby, ii. 53
  Rum, ii. 219
  Rust, i. 318. 328

S

  Saccharine fermentation, ii. 208
  Sal ammoniac, or muriat of ammonia, ii. 35
  ---- polychrest, or sulphat of potash, ii. 91
  ---- volatile, or carbonat of ammonia, ii. 41
  Salifiable bases, ii. 5
  Salifying principles, ii. 5
  Saltpetre, or nitre, or nitrat of potash, ii. 32. 104. 116
  Salt, ii. 91
  Sand, ii. 30. 51
  Sandstone, ii. 51
  Sap of plants, ii. 165. 260. 262. 270. 272
  Sapphire, ii. 58
  Saturation, i. 101.
  Sapphire, ii. 58
  Saturation, i. 101
  Seas, temperature of, i. 33.
  Sebacic acid, ii. 75. 182. 290. 353
  Secretions, ii. 307
  Seeds of plants, ii. 210. 271
  Seltzer water, i. 289. ii. 63. 129
  Senses, ii. 310
  Silex, or silica, ii. 30. 51
  Silicium, i. 13.
  Silk, ii. 359
  Silver, i. 321
  Simple bodies, i. 10. 12
  Size, ii. 281
  Skin, ii. 279. 310. 193
  Slakeing of lime, i. 147. ii. 56
  Slate, ii. 51. 66
  Smelting metals, i. 316
  Smoke i. 208
  Soap, ii. 24
  Soda, i. 363. ii. 33
  ---- water, i. 299
  Sodium, i. 13. 363
  Soils, i. 42. ii. 245
  Soldering, i. 345
  Solubility, ii. 92
  Solution, i. 96
  ---- by the air, i. 102
  ---- of potash, ii. 28
  Specific heat, i. 126
  Spermaceti, ii. 358
  Spirits, ii. 313
  Steam, i. 140. 182
  Steel, i. 305
  Stomach, ii. 315
  Stones, ii. 46
  Stucco, ii. 65
  Strontites, ii. 44. 68
  Strontium, i. 13
  Suberic acid, ii. 74. 197
  Sublimation, i. 257
  Succin, or yellow amber, ii. 241
  Succinic acid, ii. 74. 197. 241
  Sugar, ii. 165. 174. 208
  ---- of milk, ii. 355
  Sulphats, ii. 5. 91
  Super oxygenated sulphuric acid, ii. 70.
  Sulphat of alumine, or alum, ii. 54. 95
  ---- barytes, ii. 58
  ---- iron, ii. 96
  ---- lime, or gypsum, or plaster of Paris, ii. 95
  ---- magnesia, or Epsom salt, ii. 67. 95
  ---- potash, or sal polychrest, ii. 91
  ---- soda, or Glauber's salts, ii. 92
  Sulphur, i. 256
  ---- flowers of, i. 257
  Sulphurated hydrogen gas, i. 165. 268
  Sulphurets, i. 341
  Sulphurous acid, i. 254. ii. 88
  Sulphuric acid, i. 74. ii. 265
  Sympathetic ink, i. 354
  Synthesis, i. 287

T

  Tan, ii. 192
  Tannin, ii. 165. 192
  Tar, ii. 187
  Tartarous acid, ii. 74. 197
  Tartrit of potash, ii. 222
  Teeth, ii. 300
  Tellurium, i. 14
  Temperature, i. 33
  Thaw, i. 158
  Thermometers, i. 40
  ----, Fahrenheit's, i. 42
  ----, Reaumur's, i. 42
  ----, Centigrade, i. 43
  ----, air, i. 44
  ----, differential, i. 46
  Thunder, i. 248
  Tin, i. 14. 344
  Titanium, i. 14. 348
  Turf, ii. 242
  Turpentine, ii. 187
  Transpiration of plants, ii. 260
  Tungsten, i. 14. 340

V

  Vapour, i. 36. 49. 93. 182
  Vaporisation, i. 103
  Varnishes, ii. 187
  Vegetables, ii. 158
  Vegetable acid, i. 310. ii. 74. 197
  ---- colours, ii. 190
  ---- heat, ii. 272
  ---- oils, ii. 177
  Veins, ii. 304. 323.
  Venous blood, ii. 305. 326. 338
  Ventricles, ii. 324
  Verdigris, i. 352
  Vessels, ii. 304
  Vinegar, ii. 232
  Vinous fermentation, ii. 212
  Vital air, or oxygen gas, i. 182
  Vitriol, or sulphat of iron, ii. 81
  Volatile oils, i. 307. ii. 165. 183. 224. 269
  ---- products of combustion, i. 207
  ---- alkali, i. 363. ii. 20. 35
  Voltaic battery, i. 164. 220. 356. ii. 15

U

  Uranium, i. 14

W

  Water, i. 215. ii. 262
  ----, decomposition of, by electricity, i. 200. 225
  ----, condensation of, i. 32
  ---- of the sea, i. 86
  ----, boiling, i. 93
  ----, solution by, i. 96
  ---- of crystallisation, i. 339
  Wax, i. 309. ii. 180. 358
  Whey, ii. 351
  Wine, ii. 212
  Wood, ii. 267
  Woody fibre, ii. 156. 196. 267
  Wool, ii. 300

Y

  Yeast, ii. 234.
  Yttria, ii. 44.
  Yttrium, i. 13.

Z

  Zinc, ii. 14. 344
  Zirconia, ii. 44
  Zirconium, i. 14
  Zoonic acid, ii. 75. 220


END.


  Printed by A. Strahan,
  Printers-Street, London

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Terminology

  oxy-muriatic acid = chlorine
    (proposed as an element in 1815: see Conversation XIX)
  "columbium or tantalium" = niobium and tantalum
    (the two elements always occur together, and were not recognized
    as separate until much later in the 19th century)
  phosphat of lime = calcium diphosphate _or_ calcium
    (the element calcium was isolated in 1808, but is named only once
    in this 1817 edition)
  glucium = beryllium (Humphry Davy's name for the element)

  muriatic acid = hydrochloric acid
    (still called "muriatic acid" for some commercial uses)
  muriat of lime = calcium chloride
  oxymuriate of potash = potassium chlorate
  muriat of soda = sodium chloride (table salt)
  carbonic acid = carbon dioxide

Note also:

  simple body, fundamental principle = element
  fecula = starch (usually spelled "faecula")
  spirit of wine = alcohol
  philosopher = scientist
  arts = industry, manufacture, crafts etc. (seldom "fine arts")

Some essential concepts relating to living things--photosynthesis,
microorganisms, the cell, proteins--are either unknown or not mentioned.
The atom theory had been proposed, but not by Humphry Davy; it is not
mentioned in this book.

The word "explode" is used at least once in its orginal, figurative
sense ("a word that should be exploded in chemistry") but far more
often in its later, concrete one. The word "explosion" is always used
concretely ("an explosion, or a _detonation_ as chemists commonly call
it").

Calculated Values:

"the point of zero, or the absolute privation of heat, must consequently
be 1260 degrees below 32 degrees"

  [-1228 deg. F. The calculation is based on wrong premises; the correct
  figure is about -460 deg. F or -273 deg. C.]

"Mercury congeals only at seventy-two degrees below the freezing point."

  [-40 deg. F, which is also -40 deg. C. This figure is correct, though
  approximate.]

"The proportion stated by Sir H. Davy, in his Chemical Researches, is as
1 to 2.389."

"[ammonia] consisted of about one part of hydrogen to four parts of
nitrogen.... and from the latest and most accurate experiments, the
proportions appear to be, one volume of nitrogen gas to three of
hydrogen gas"

  [These and similar calculations involving weight and volume make more
  sense when one knows the elements' atomic weights. For nitric acid,
  HNO_3, the figures are 1:14:48, giving a proportion closer to 1:3.5.
  For ammonia, NH_3 (not 4), the figures are 14:3.]

"The _oxalic acid_, distilled from sorrel, is the highest term of
vegetable acidification; for, if more oxygen be added to it, it loses
its vegetable nature, and is resolved into carbonic acid and water;"

  [Oxalic acid = H_2C_2O_4; carbonic acid (carbon dioxide) = CO_2.
  H_2C_2O_4 + O becomes H_2O + CO_2 + CO_2.]

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Contents: Numbering and Changes

The 3rd and 4th editions used the same Conversation numbering.
Changes between the 4th and 5th (present text) edition are shown.
Some illustrations were also changed.

_Volume I: On Simple Bodies_

(I, II, III no change)

   IV.        On Specific Heat, Latent Heat, and Chemical Heat.
         IV.  On Combined Caloric, Comprehending Specific Heat
                and Latent Heat.

   --     V.  [New Chapter] On The Chemical Agencies Of Electricity.

    V.   VI.  On Oxygen And Nitrogen.

   VI.  VII.  On Hydrogen.
              [5th edn: adds sections on Gas lights and Miner's Lamp]

  VII. VIII.  On Sulphur And Phosphorus.
              [5th edn: adds section on Decomposition of Sulphur]

 VIII.        On Carbone.
         IX.  On Carbon.
              [4th edn: Section "Diamond is Carbon in a State of perfect
              purity"; later edn: "Diamond" alone]

   IX.    X.  On Metals.

    X.  XIV.  On Alkalies.
   XI.   XV.  On Earths.
              [5th edn: both moved to Vol. II.]

_Volume II. On Compound Bodies_

  XII. XIII.  On The Attraction Of Composition.

              [5th edn. XIV, XV = 4th edn. X, XI]

 XIII.        On Compound Bodies.
        XVI.  On Acids.
              [Most of XIII, On Compound Bodies, became XVI, On Acids.
              Some introductory material was moved to XIV, On Alkalies.]

  XIV.        On The Combinations of Oxygen with Sulphur and with
                Phosphorus; and of the Sulphats And Phosphats.
       XVII.  Of the Sulphuric and Phosphoric Acids: or, The
                Combinations of ....

   XV.        On The Combination of Oxygen With Nitrogen and with
                Carbone; and of The Nitrats And Carbonats.
      XVIII.  Of The Nitric And Carbonic Acids: Or The Combination ...

  XVI.        On Muriatic And Oxygenated Muriatic Acids; and on Muriats.
        XIX.  On The Boracic, Fluoric, Muriatic, and Oxygenated Muriatic
                Acids; and on Muriats.

 XVII.   XX.  On The Nature And Composition Of Vegetables.

(remaining Conversations: 4th edn. + 3 = 5th edn.)

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ERRATA

Inconsistencies are generally unchanged; see end of Errata list.

Two items are noted in the printed Errata, immediately after the
Contents for Vol. I:

  Vol. I. page 56. last line but one, for "caloric," read "calorific."
              179. Note, for "Plate XII." r. "Plate XIII."

  I.56
    The principal use of the mirrors in this experiment is, to prove
    that the calorific emanation ...
  I.179 fn.
    A model of this mode of construction is exhibited in PLATE XIII.
    Fig. 1.


Errata Noted by Transcriber:

Contents
  II ... Dr. Herschel's Experiments
    [_body text has "Herschell"_]
  XIV ... Hartshorn and Sal Volatile  [ad]

Conversation I
  [Emily] the electric spark which is visible, and  [aud]

Conversation II
  [Mrs. B., parenthesis] (PLATE I. Fig. 1.)  [Fig. I.]
  [Mrs. B.] it has been called _differential_ thermometer
     [_missing "the"?_]
  [Mrs. B., parenthesis] (PLATE III. Fig. 1.)  [Fig. I.]
  [Emily] the tin surface should radiate the least caloric  [carolic]

Conversation III
  [Emily] the glass skreen  [_spelling unchanged_]
  [Mrs. B., parenthesis] (PLATE IV. Fig. 1.)  [_error for Fig. 2._]
  [Plate IV caption] Thermometers one in the Ether, the other
    [_invisible comma after "Thermometers"?_]
  [Mrs. B.] he found that it was considerably colder  [is was]

Conversation IV
  [Caroline] But how can you reverse this experiment?
    [_printed "expe-/periment" at line break_]
  [Mrs. B.] instead of being 75 degrees, will be 80 degrees
    [_error for 88?_]
  [Emily, footnote] See page 102.  [_in Conversation III_]
  [Mrs. B.] then soke it in ether  [_spelling unchanged_]

Conversation V
  [Mrs. B.] at regular distances in wooden troughs  [throughs]
  [Caroline] the nature of the action of the Voltaic battery  [Votaic]

Conversation VI
  [Caroline] the nature of OXYGEN, which come next in our table
    [_error for "comes"?_]

Conversation VII
  [Mrs. B., parenthesis] (c, d, PLATE VIII. fig. 2.)  [fig. 2,]
  [Caroline] be soon adopted every where, [every where.]
  [Plate X] C. apperture for supplying Oil.  [_spelling unchanged_]

Conversation VIII
  [Mrs. B.] sulphur is a very combustible substance  [sulpur]
  [Mrs. B.] I now put into the receiver  [_missing "it"?_]
  [Emily] What is ... after its detonation?  [. for ?]

Conversation IX
  [Mrs. B.] we are to burn the carbon  [bnrn]
  [Mrs. B.] since they may be prepared  [thay]
  [Mrs. B.] artificial Seltzer water  [artifical]

Conversation X
  [Mrs. B.] increase the rapidity of its combustion  [of of]
  [Caroline] a pair of scissars  [_spelling unchanged_]
  [Mrs. B.] as well as by evaporating the liquid.  [? for .]
  [Mrs. B., footnote] page 155. of this volume
    [_near the end of Conversation IV_]
  [Caroline] amongst the metals. I had no notion  [, for .]
  [Emily] But is it not very singular  [singulr]
  [Mrs. B.] Thenard and Gay Lussac  [_usually hyphenated: "Gay-Lussac"_]

Conversation XIII
  [Emily] ... render that decomposition perceptible?  [. for ?]

Conversation XIV
  [Mrs. B.] an acrid burning taste  [on acrid]
  [Caroline] according to which heat is disengaged
    [_t in "heat" invisible_]
  [Mrs. B.] one volume of nitrogen gas to three of hydrogen gas
    [_text has "oxygen" for "hydrogen"_]

Conversation XV
  [Mrs. B.] so many interesting and important compounds  [interesing]
  [Caroline] And of what nature ... painting porcelain?  [. for ?]
    [_speaker's name missing; supplied from other editions_]

Conversation XVI
  [Mrs. B.] combined with acidifiable radicals  [acidificiable]
  [Mrs. B.] this power of charring wood  [charing]

Conversation XVIII
  [Caroline] not mentioning this acid
    [_printed "mention-/this" at line break_]
  [Mrs. B. (footnote)] 1 to 2.389.]
    [_printed "2,389": no other decimal numbers occur in the text,
    but a comma appears once as a thousands separator_]
  [Mrs. B.] You, understand, now, I hope,  [_all commas in original_]

Conversation XX
  [Mrs. B.] of vegetables; we shall, therefore  [, for ;]

Conversation XXI
  [Mrs. B.] the compound last formed will be destroyed;  [detroyed]
  [Mrs. B.] in such climates great part of the water
    [_missing "a" before "great"?_]
  [Mrs. B.] a state of debility and languor  [langour]
  [Mrs. B., parenthesis] (PLATE XIV. Fig. 2.)  [PLATE XIII.]
  [Mrs. B.] burns at so slow a temperature
    [_text unchanged: error for "low" or correct as printed?_]
  [Mrs. B.] the poor in heathy countries
    [_not an error: "heath-y", not "healthy"_]

Conversation XXII
  [Mrs. B., parenthesis] (PLATE XV. Fig. 4.)  [PLATE XIV.]
  [Mrs. B.] so many vessels or apparatus
    [_not an error: "apparatus" is the Latin plural form_]
  [Mrs. B.] chesnut  [_common variant spelling_]

Conversation XXIII
  [Mrs. B.] pure gelatine  [gelantine]

Conversation XXV
  [Mrs. B.] The muscular action of the diaphragm  [diaphram]

Conversation XXVI
  [Mrs. B.] the air had been previously consumed  [previouly]
  [Mrs. B.] this is by the Tartan Arabs
    [_text unchanged: Tartar?_]
  [Mrs. B.] or studying the inventions of art, let us, therefore
    [; for ,]

Index
  Arrack  [_body text has "arack"_]
  [Cold] ---- from evaporation, i. 102. 113. 150
    [_volume number missing_]
  Culinary heat, i. 88  [_volume number missing_]
  [Decomposition] ---- of ammonia, i. 363. ii. 37
    [ammonnia; _"i" invisible_]
  Frankincense  [Francincense]
  [Freezing mixtures] ---- by evaporation, i. 104. 150, &c.
    [_volume number invisible_]
  Glue, ii. 281. 287  [_volume number missing_]
  N. [_anomalous . unchanged_]
  Phosphorous acid, i. 274. ii. 99
    [_volume I body text always has "Phosphorus acid"_]
  [Sulphat] ---- lime, or gypsum, or  [gypsum of]
  [Sulphur] ---- flowers of, i. 257  [_volume number missing_]
  [Thermometers] ----, Centigrade, i. 43  [Centrigade]
  [Thermometers] ----, differential, i. 46  [differentiial]
  V, U  [_alphabetized as shown_]
  Zirconia, ii. 44  [Zicornia]
  Zirconium, i. 14  [Zicornium ... 13]


Inconsistencies and variant spellings:

Standard spellings in this book include:
  bason, judgment, embrio, volcanos (plural), potatoe (singular)

Inconsistencies include:
  capitalization of "Fig." or "fig."
  hyphenization of words such as "oxy-muriatic"
  "glauber salt" and "Glauber's salt" both occur

Variant forms include:
  opake, opaque
  aeriform, aeriform (with and without dieresis)
  gasses, gases
  phosphoret, phosphuret (but always carburet)
  Libya, Lybia
  dy(e)ing  [from "dye"]
  nap(h)tha
  pla(i)ster
  slak(e)ing
  earthen-ware, earthen ware
  "sulphurous", "naphtha" are used in the Contents and the Index;
    "sulphureous", "naptha" in the body text
  forms in "-xion" (such as "connexion") appear only in the Contents
    and the Index

Volume I has more archaic forms than Volume II:
  "shew", "inclose" are sometimes used instead of "show", "enclose"
  "carbone" with final "e" appears in one Plate caption. (In the
    same plate's header, the "e" appears to have been removed by
    the engraver.)
  "develope(ment)" is more common in Volume I, "develop(ment)" in
    Volume II
  "-ize" and "-yze" forms (for later "-ise" and "-yse") are common in
    Volume I, rare in Volume II except in the Index


The "Dr. Marcet" mentioned in a few footnotes and figure captions is the
author's husband. Humphry Davy ("Sir H. Davy") was knighted in 1812,
between the 3rd and 4th editions of the book.


Reminder:

DO NOT TRY THIS AT HOME.


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End of Project Gutenberg's Conversations on Chemistry, V. 1-2, by Jane Marcet