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A HISTORY OF THE SCIENCES


  │Astronomy│, by Prof. GEORGE FORBES, F.R.S.

  │Chemistry, 2 volumes│, by Sir EDWARD THORPE, C.B., D Sc., F.R.S.,
      etc. (Director of Government Laboratories)

  │Old Testament Criticism│, by Prof. ARCHIBALD DUFF (Prof. of Hebrew
      and Old Testament Theology in the United College, Bradford)


_In Active Preparation_—

  │New Testament Criticism│, by F. C. CONYBEARE, M.A. (Late Fellow
      and Prælector of Univ. Coll., Oxford)

  │Geology│, by H. B. WOODWARD, F.R.S., F.G.S. (Assistant Director of
      Geological Survey)

  │Geography│, by Dr. SCOTT KELTIE, F.R.G.S., F.S.A.


G. P. PUTNAM’S SONS

    New York      London

[Illustration: JUSTUS VON LIEBIG]




    _A HISTORY OF THE SCIENCES_


    HISTORY
    OF
    CHEMISTRY


    BY
    SIR EDWARD THORPE

    C.B., LL.D., F.R.S.

    AUTHOR OF “ESSAYS IN HISTORICAL CHEMISTRY,” “HUMPHRY DAVY:
    POET AND PHILOSOPHER,” “JOSEPH PRIESTLEY,” ETC., ETC.


    TWO VOLUMES


    II.

    From 1850 to 1910


    _WITH ILLUSTRATIONS_


    G. P. PUTNAM’S SONS
    NEW YORK AND LONDON
    The Knickerbocker Press
    1910




    COPYRIGHT, 1910
    BY
    G. P. PUTNAM’S SONS


    This series is published in London by
    THE RATIONALIST PRESS ASSOCIATION, LIMITED


    The Knickerbocker Press, New York




PUBLISHERS’ NOTE


A History of the Sciences has been planned to present for the
information of the general public a historic record of the great
divisions of science. Each volume is the work of a writer who is
accepted as an authority on his own subject-matter. The books are
not to be considered as primers, but present thoroughly digested
information on the relations borne by each great division of science
to the changes in human ideas and to the intellectual development
of mankind. The monographs explain how the principal scientific
discoveries have been arrived at and the names of the workers to whom
such discoveries are due.

The books will comprise each about 200 pages. Each volume will contain
from 12 to 16 illustrations, including portraits of the discoverers and
explanatory views and diagrams. Each volume contains also a concise but
comprehensive bibliography of the subject-matter. The following volumes
will be issued during the course of the autumn of 1909.

  │The History of Astronomy.│

    By GEORGE FORBES, M.A., F.R.S., M. Inst. C.E.; author of _The
      Transit of Venus_, etc.

  │The History of Chemistry: Vol. I. circa 2000 B.C. to 1850 A.D. Vol.
      II. 1850 A.D. to date.│

    By SIR EDWARD THORPE, C.B., LL.D., F.R.S., Director of the
      Government Laboratories, London; Professor-elect and Director of
      the Chemical Laboratories of the Imperial College of Science and
      Technology; author of _A Dictionary of Applied Chemistry_.

_To be followed by_:

  │The History of Geography.│

    By Dr. JOHN SCOTT KELTIE, F.R.G.S., F.S.S., F.S.A., Hon. Mem.
      Geographical Societies of Paris, Berlin, Rome, Brussels,
      Amsterdam, Geneva, etc.; author of _Report on Geographical
      Education_, _Applied Geography_.

  │The History of Geology.│

    By HORACE B. WOODWARD, F.R.S., F.G.S., Assistant Director of
      Geological Survey of England and Wales; author of _The Geology
      of England and Wales_, etc.

  │The History of Anthropology.│

    By A. C. HADDON, M.A., Sc.D., F.R.S., Lecturer in Ethnology,
      Cambridge and London; author of _Study of Man_, _Magic and
      Fetishism_, etc.

  │The History of Old Testament Criticism.│

    By ARCHIBALD DUFF, Professor of Hebrew and Old Testament Theology
      in the United College, Bradford; author of _Theology and Ethics
      of the Hebrews_, _Modern Old Testament Theology_, etc.

  │The History of New Testament Criticism.│

    By F. C. CONYBEARE, M.A., late Fellow and Praelector of Univ.
      Coll., Oxford; Fellow of the British Academy; Doctor of Theology,
      _honoris causa_, of Giessen; Officer d’Academie; author of _Old
      Armenian Texts of Revelation_, etc.

Further volumes are in plan on the following subjects:

  │Mathematics and Mechanics.│

  │Molecular Physics, Heat, Life, and Electricity.│

  │Human Physiology, Embryology, and Heredity.│

  │Acoustics, Harmonics, and the Physiology of Hearing, together with
    Optics Chromatics, and Physiology of Seeing.│

  │Psychology, Analytic, Comparative, and Experimental.│

  │Sociology and Economics.│

  │Ethics.│

  │Comparative Philology.│

  │Criticism, Historical Research, and Legends.│

  │Comparative Mythology and the Science of Religions.│

  │The Criticism of Ecclesiastical Institutions.│

  │Culture, Moral and Intellectual, as Reflected in Imaginative
    Literature and in the Fine Arts.│

  │Logic.│

  │Philosophy.│

  │Education.│




CONTENTS


  CHAPTER I                                                         PAGE

  STATE OF CHEMISTRY IN THE MIDDLE OF THE NINETEENTH CENTURY           1
    Introductory. Some Founders of Modern Chemistry: Liebig,
    Wöhler, Dumas. Rapid Extension of Organic Chemistry after
    1850: Laurent and Gerhardt, Hofmann. Development of theory
    in Organic Chemistry. Other representative men of the middle
    period of the Nineteenth Century: Graham, Williamson, Bunsen.
    Modern Chemistry in relation to the Atomic Theory.


  CHAPTER II

  THE CHEMICAL ELEMENTS DISCOVERED SINCE 1850                         26
    Nomenclature and Classification of Elements. Numerical
    relationships. Modes of discovery. The Spectroscope. Cæsium,
    rubidium, thallium, indium, gallium, scandium, germanium. The
    rare earth elements. Industrial applications of rare elements.


  CHAPTER III

  THE INACTIVE ELEMENTS: RADIUM AND RADIO-ACTIVITY                    43
    Argon, helium, krypton, neon, and xenon. Radium.
    Disintegration theory of Rutherford and Soddy. Actinium and
    polonium. The emanations.


  CHAPTER IV

  ATOMS AND MOLECULES: ATOMIC WEIGHTS AND EQUIVALENTS                 61
    Hypothesis of Avogadro. Stanislao Cannizzaro. Determination
    of molecular weights. Applicability of law of Dulong and
    Petit. Relations of molecular weight to osmotic pressures.
    Determination of atomic weights. Hypothesis of Prout, Dumas,
    Stas, Lord Rayleigh, Leduc, Morley, Guye, Theodore Richards.
    Validity of law of conservation of mass: Landolt.


  CHAPTER V

  THE MOLECULAR THEORY OF GASES                                       79
    Interdependence of the gaseous laws. Kinetic theory of gases:
    Bernoulli, Waterston, Clausius, Maxwell, Boltzmann, Schmidt,
    Graham. Gaseous diffusion. Van der Waals’s equation. Ratio
    of specific heats: Kundt and Warburg. Liquefaction of gases.
    Critical temperatures and pressures: Andrews, Pictet and
    Cailletet, Wroblewski, Olszewki, Dewar, Kammerlingh Onnes.
    Liquefaction of air on the large scale. Research at low
    temperatures.


  CHAPTER VI

  THE PERIODIC LAW                                                   101
    Prout, Thomson, Döbereiner, Newlands, De Chancourtois.
    Statement of the Periodic Law by Mendeléeff and Lothar Meyer.
    Its importance as a system of classification.


  CHAPTER VII

  VALENCY                                                            112
    Origin of conception of Valency: Williamson, Gerhardt,
    Frankland, Couper, Kekulé. Tetravalency of carbon and linkage
    of atoms. Rational and constitutional formulæ. Dynamical
    theories of valency.


  CHAPTER VIII

  THE CHEMISTRY OF AROMATIC COMPOUNDS                                119
    Peculiarities of aromatic compounds. Kekulé’s benzene theory.
    Its applications. The essential oils. Terpenes. Camphor.
    Synthesis of perfumes. Alkaloids.


  CHAPTER IX

  STEREO-ISOMERISM: STEREO-CHEMISTRY                                 138
    Opticity: Biot, Mitscherlich, Pasteur, Wislicenus, Van ’t
    Hoff, Le Bel. Asymmetry. Racemisation. Multirotation.
    Geometrical isomerism. Geometrical inversion.
    Stereo-isomerism among nitrogen, sulphur, selenium, tin, and
    silicon compounds. Tautomerism. Steric hindrance.


  CHAPTER X

  ORGANIC SYNTHESIS: CONDENSATION: SYNTHESIS OF VITAL PRODUCTS       152
    Use of specific condensing reagents. Carbon suboxide.
    Artificial preparation of naturally occurring substances.
    Synthetic medicines. The ptomaines. Artificial alizarin.
    Indigo. The sugars and proteins: Emil Fischer. The doctrine
    of “vital force.”


  CHAPTER XI

  ON THE DEVELOPMENT OF PHYSICAL CHEMISTRY SINCE 1850                171
    Molecular volumes of liquids. Nature of solution. Van ’t
    Hoff’s application of the gas laws to phenomena of solution.
    Osmosis and osmotic pressure. Traube. Pfeffer. Semi-permeable
    membranes. Measurement of osmotic pressure. Arrhenius.
    Doctrine of ionisation. Its applicability to the explanation
    of chemical phenomena. Thermo-chemistry. Mass action.
    Nature of reversible reactions. Thermal and Electrolytic
    dissociation. Relation between chemical nature and opticity,
    magnetic rotation and viscosity. Theory of phases. Catalysis.
    Enzyme action. Relations between valency and volume.
    Photochemistry.

  BIBLIOGRAPHY                                                       187

  INDEX                                                              191




ILLUSTRATIONS


                                                                    PAGE
    JUSTUS VON LIEBIG                               _Frontispiece_

    JEAN BAPTISTE ANDRÉ DUMAS                                          9

    THOMAS GRAHAM                                                     13
        From a painting by G. F. Watts, R.A., in the possession
          of the Royal Society

    ALEXANDER WILLIAM WILLIAMSON                                      15

    BUNSEN, KIRCHHOFF, AND ROSCOE                                     19

    SIR WILLIAM RAMSAY                                                48

    MARIE CURIE (_née_ SKLODOWSKA)                                    56

    STANISLAO CANNIZZARO                                              65

    SIR JAMES DEWAR                                                   98

    DMITRI IVANOWITSCH MENDELÉEFF                                    110

    AUGUST KEKULÉ VON STRADONITZ                                     126

    JACOBUS HENRICUS VAN ’T HOFF                                     142

    EMIL FISCHER                                                     166

    SVANTE AUGUST ARRHENIUS                                          180




HISTORY OF CHEMISTRY




CHAPTER I

STATE OF CHEMISTRY IN THE MIDDLE OF THE NINETEENTH CENTURY


In the preceding volume an attempt was made to outline the significant
features in the development of chemistry, as an art and as a science,
from the earliest times down to about the middle of the last century.
Since that time chemistry has progressed at a rate and to an extent
unparalleled at any period of its history. Not only have the number
and variety of chemical products—inorganic and organic—been enormously
increased, but the study of their modes of origin, properties, and
relations has greatly extended our means of gaining an insight into
the internal structure and constitution of bodies. This extraordinary
development has carried the science beyond the limits of its own
special field of inquiry, and has influenced every department of
natural knowledge. Concurrently there has been a no less striking
extension of its applications to the prosperity and material welfare of
mankind.

With the death of Davy the era of brilliant discovery in chemistry,
wrote Edward Turner, appeared for the moment to have terminated.
Although the number of workers in the science steadily increased, the
output of chemical literature in England actually diminished for some
years; and, as regards inorganic chemistry, few first-rate discoveries
were made during the two decades prior to 1850. Chemists seemed to
be of Turner’s opinion that the time had arrived for reviewing their
stock of information, and for submitting the principal facts and
fundamental doctrines to the severest scrutiny. Their activity was
employed not so much in searching for new compounds or new elements as
in examining those already discovered. The foundations of the atomic
theory were being securely laid. The ratios in which the elements of
known compounds are united were being more exactly ascertained. The
efforts of workers, Graham excepted, seemed to be spent more on points
of detail, on the filling-in of little gaps in the chemical structure,
as it then existed, than in attempts at new developments. For a
time—during the early ’thirties—chemists struggled with the claims
of rival methods of notation, and it was only gradually that the
system of Berzelius gained general acceptance. At none of the British
universities was there anything in the nature of practical tuition in
chemistry. Thomson, at Glasgow, occasionally permitted a student to
work under him, but no systematic instruction was ever attempted. The
first impulses came from Graham in 1837, when he took charge of the
chemical teaching at the University of London, and when, in 1841, he
assisted to create the Chemical Society of London. Four years later
the Royal College of Chemistry in London was founded and placed under
the direction of August Wilhelm Hofmann—one of the most distinguished
pupils of Liebig. Under his inspiration the study of practical
chemistry made extraordinary progress, and discovery succeeded
discovery in rapid succession. In bringing Hofmann to England we had,
in fact, imported something of the spirit and power of his master,
Liebig.

Among the pupils and co-workers of Hofmann were Warren de la Rue, Abel,
Nicholson, Mansfield, Medlock, Crookes, Church, Griess, Martius, Sell,
Divers, and Perkin. Whilst at Giessen he had begun the study of the
organic bases in coal-tar with a view more especially of establishing
the identity of Fritzsche’s _anilin_ with the _benzidam_ of Zinin and
the _krystallin_ of Unverdorben. Hofmann continued to cultivate with
unremitting zeal the field thus entered. With Muspratt he discovered
_paratoluidine_ and _nitraniline_; with Cahours _allyl alcohol_. His
pupil Mansfield worked out, at the cost of his life, the methods
for the technical extraction of benzene and toluene from coal-tar,
and thereby made the coal-tar colour-industry possible. It was in
attempting to synthesise quinine by the oxidation of aniline that
Perkin, then an assistant at the college, obtained, in 1856, _aniline
purple_, or _mauve_, as it came to be called by the French, the first
of the so-called coal-tar colouring matters. In 1859 this was followed
by the discovery of _magenta_, or _fuchsine_, by Verquin. For its
manufacture Medlock, one of Hofmann’s pupils, in 1860 devised a process
by which for a time it was almost exclusively made. Hofmann studied the
products thus obtained, and showed that they were derivatives of a base
he called _rosaniline_; and he demonstrated that the colouring matters
were only produced through the concurrent presence of aniline and
toluidine. He also proved that the base of the dye, known as _aniline
blue_, was _triphenylrosaniline_. As the result of these inquiries he
obtained the violet or purple colouring matters known by his name.
Lastly, all his classical work on the amines, ammonium compounds, and
the analogous phosphorus derivatives was done at the Royal College of
Chemistry.

Prior to the establishment by Liebig, in 1826, of the Giessen
laboratory, the state of chemistry in Germany was not much, if at all,
better than with us. The creation of the Giessen school initiated a
movement which has culminated in the pre-eminent position which Germany
now occupies in the chemical world. Students from every civilised
country came to study and to work under its leader, and to carry away
with them the influence of his example, the inspiration of his genius,
and the stimulating power of his enthusiasm.

│Justus von Liebig│, was born at Darmstadt on May 12, 1803, and
after graduating at Erlangen, where he worked on the fulminates, he
repaired to Paris and entered the laboratory of Gay Lussac, with whom
he continued his inquiries. Returning to Germany, he was appointed
Professor of Chemistry at Giessen in 1826, and began those remarkable
series of scientific contributions upon which the superstructure of
organic chemistry largely rests. He investigated the _cyanates_,
_cyanides_, _ferrocyanides_, _thiocyanates_, and their derivatives.
In conjunction with Wöhler he discovered the group of the _benzoic
compounds_ and created the _radical theory_. With Wöhler also he
investigated _uric acid and its derivatives_. He discovered _hippuric
acid_, _fulminuric acid_, _chloral_, _chloroform_, _aldehyde_,
_thialdine_, _benzil_, and elucidated the _constitution of the organic
acids_ and the _amides_. He greatly improved the methods of organic
analysis, and was thereby enabled to determine the empirical formulæ of
a number of carbon compounds of which the composition was imperfectly
known. He practically laid the foundations of modern agricultural
chemistry, and to his teaching is due the establishment of an important
branch of technology—the manufacture of chemical fertilisers. He worked
on physiological chemistry, especially on the elaboration of fat, on
the nature of blood, bile, and on the juice of flesh. He studied the
processes of fermentation, and of the decay of organised matter. He was
a most prolific writer. The Royal Society’s Catalogue of Scientific
Papers enumerates no fewer than 317 contributions from his pen. He was
the founder of the _Annalen der Chemie_, which is now associated with
his name, and of the _Jahresbericht_; he published an _Encyclopædia
of Pure and Applied Chemistry_ and a _Handbook of Organic Chemistry_.
His _Familiar Letters on Chemistry_ was translated into every modern
language, and exercised a powerful influence in developing popular
appreciation of the value and utility of science. Liebig left Giessen
in 1852 to become Professor of Chemistry at the University of Munich
and President of the Academy of Sciences. He died at Munich on April
18, 1874.

With the name of Liebig that of Wöhler is indissolubly connected.
Although the greater part of their work was not published in
conjunction, what they did together exercised a profound influence on
the development of chemical theory.

│Friedrich Wöhler│ was born at Eschersheim, near Frankfort, on July 31,
1800. After studying at Marburg, where he discovered, independently of
Davy, _cyanogen iodide_, and worked on _mercuric thiocyanate_, he went
to Heidelberg and investigated _cyanic acid_ and its compounds, under
the direction of Gmelin. In 1823 he worked with Berzelius at Stockholm,
where he prepared some new tungsten compounds and practised mineral
analysis. In 1825 he became a teacher of chemistry in the Berlin Trade
School. Here he succeeded for the first time in preparing the metal
_aluminium_ and in effecting the _synthesis of urea_—one of the first
organic compounds to be prepared from inorganic materials. Jointly with
Liebig he worked upon _mellitic_ and _cyanic_ and _cyanuric acids_. In
1832 Wöhler, now appointed to the Polytechnic at Cassel, began with
Liebig their memorable investigation on _bitter-almond oil_. In 1836 he
was called to the Chair of Chemistry in the University of Göttingen,
and with Liebig attacked the constitution of _uric acid and its
derivatives_—the last great investigation the friends did in common.
Wöhler subsequently devoted himself mainly to inorganic chemistry. He
isolated _crystalline boron_, and prepared its _nitrides_, discovered
the spontaneously inflammable _silicon hydride_, _titanium nitride_,
and analysed great numbers of minerals and meteorites and compounds of
the rarer metals. He made Göttingen famous as a school of chemistry.
At the time of the one and twentieth year of his connection with the
University it was found that upwards of 8000 students had listened to
his lectures or worked in his laboratory. He died on September 23, 1882.

In France, Dumas exercised a no less powerful influence. If Liebig
could reckon among his pupils Redtenbacher, Bromeis, Varrentrapp,
Gregory, Playfair, Williamson, Gilbert, Brodie, Anderson, Gladstone,
Hofmann, Will, and Fresenius; Dumas could point to Boullay, Piria,
Stas, Melsens, Wurtz, and Leblanc—all of whom did yeoman service in
developing the rapidly expanding branch of organic chemistry.

[Illustration: JEAN BAPTISTE ANDRÉ DUMAS]

│Jean Baptiste André Dumas│ was born on July 14, 1800, at Alais, where
he was apprenticed to an apothecary. In his sixteenth year he went to
Geneva and entered the pharmaceutical laboratory of Le Royer. Without,
apparently, having received any systematic instruction in chemistry,
he commenced the work of investigation. With Coindet he established
the therapeutic value of iodine in the treatment of _goître_; with
Prevost he attempted to isolate the active principle of _digitalis_,
and studied the chemical changes in the development of the chick in
the egg. In his twenty-fourth year Dumas went to Paris and became
_Répétiteur de Chimie_ at the École Polytechnique. He joined Audouin
and Brongniart in founding the _Annales des Sciences Naturelles_, and
began his great work on _Chemistry Applied to the Arts_, of which
the first volume appeared in 1828. At about this time he devised his
_method of determining vapour densities_, and published the results
of a number of estimations made by means of it. With Boullay he
began an inquiry on the _compound ethers_, out of which grew the
_etherin theory_, which served as a stepping-stone to the theory of
compound radicals—subsequently elaborated by Liebig and Wöhler. Dumas
discovered the nature of _oxamide_ and of _ethyl oxamate_, isolated
_methyl alcohol_, and established the generic connection of groups of
similarly constituted organic substances, or, in a word, the doctrine
of _homology_. His work on the _metaleptic action of chlorine_
upon organic substances eventually effected the overthrow of the
electro-chemical theory of Berzelius and led to the theory of types,
which, in the hands of Williamson, Laurent, Gerhardt, and Odling, was
of great service in explaining the analogies and relationships of whole
groups of organic compounds. He worked in every field of chemistry.
He invented many analytical processes, established the _gravimetric
composition of water and of air_, and revised the _atomic weights_ of
the greater number of the elements then known. Dumas exercised great
influence in scientific and academic circles in France. He was an
admirable speaker, and had rare literary gifts. On the creation of the
Empire he was made a Senator, and was elected a member of the Municipal
Council of Paris, of which he became president in 1859. He died on
April 11, 1884.

It was largely through the influence of these master-minds that
chemistry took a new departure. Prior to their time organic chemistry
hardly existed as a branch of science: organic products, as a rule,
were interesting only to the pharmacist mainly by reason of their
technical or medicinal importance. But by the middle of the nineteenth
century the richness of this hitherto untilled field became manifest,
and scores of workers hastened to sow and to reap in it. The most
striking feature, indeed, of the history of chemistry during the past
sixty years has been the extraordinary expansion of the organic section
of the science. The chemical literature relating to the compounds
of carbon now exceeds in volume that devoted to all the rest of the
elements.

In the middle of the nineteenth century chemists began to concern
themselves with the systematisation of the results of the study of
organic compounds, and something like a theory of organic chemistry
gradually took shape. From this period we may date the attempts
at expressing the internal nature, constitution, and relations of
substances which, step by step, have culminated in our present
representations of the structure and spatial arrangement of molecules.
In 1850 the dualistic conceptions of Berzelius ceased to influence
the doctrines of organic chemistry. The enunciation by Dumas of the
principle of substitution, and its logical outcome in the nucleus
theory and in the theory of types, had not only effected the overthrow
of dualism, but was undermining the position of the radical theory of
Liebig and Wöhler. The teaching of Gerhardt and Laurent had spread
over Europe, and was influencing those younger chemists who, while
renouncing dualism, were not wholly satisfied with a belief in compound
radicals. Williamson’s discovery, in 1850, of the true nature of ether
and of its relation to alcohol, and his subsequent preparation of mixed
ethers, served not only to reconcile conflicting interpretations of
the process of etherification, but also to reconcile the theory of
types with that of radicals. Lastly, his method of representing the
constitution of the ethers and their mode of origin gave a powerful
stimulus to the use of type-formulæ in expressing the nature and
relations of organic compounds.

[Illustration: THOMAS GRAHAM.

From a painting by G. F. Watts, R.A., in the possession of the Royal
Society.]

Other representative men of the middle period of the nineteenth
century, in addition to Williamson, were Graham and Bunsen. The
three men were investigators of very different type, and their work
had little in common. But each was identified with discoveries of a
fundamental character, constituting turning-points in the history of
chemical progress, valuable either as regards their bearing on chemical
doctrine or as regards their influence on operative chemistry.

│Thomas Graham│ was born in Glasgow on December 21, 1805, and, after
studying under Thomas Thomson at the University of that city, attended
the lectures of Hope and Leslie in Edinburgh. In 1830 he succeeded Ure
as teacher of chemistry at Anderson’s College in Glasgow, and in 1837
was called to the Chair of Chemistry in the newly-founded University of
London, in succession to Edward Turner. In 1854 he was made Master of
the Mint. He died in London on September 16, 1869.

Graham’s work was mainly devoted to that section of the science now
known as physical chemistry. His contributions to pure chemistry
are few in number. By far the most important is his discovery of
_metaphosphoric_ acid and its relations to the other modifications of
phosphoric acid. Ortho- or ordinary phosphoric acid was known to
Boyle; pyrophosphoric acid was discovered by Clark. Graham’s work is
noteworthy as first definitely indicating the inherent property of the
acids to combine with variable but definite amounts of basic substances
by successive replacement of hydroxyl groups—the property we now term
_basicity_, and was of fundamental importance in regard to its bearing
on the constitution of acids and salts.

Graham’s fame chiefly rests upon his discovery of _the law of gaseous
diffusion_ (1829–1831), upon his work on the _diffusion of liquids_,
and upon his recognition of the condensed form of hydrogen he termed
_hydrogenium_. Questions involving the conception of molecular
mobility, indeed, constituted the main feature of his inquiries. We owe
to him, among others, the terms _crystalloid_, _colloid_, _dialysis_,
_atmolysis_, _occlusion_—all of which have taken a permanent place in
the terminology of science.

[Illustration: ALEXANDER WILLIAM WILLIAMSON.]

│Alexander William Williamson│ was born at Wandsworth, London, on
May 1, 1824. His father, a Scotchman and a fellow-clerk of James
Mill (the father of John Stuart Mill) in the East India House, took
an active share in the foundation, in 1826, of the University of
London, subsequently known as University College. In 1840 the younger
Williamson entered the University of Heidelberg with the intention
of studying medicine; but, under the influence of Leopold Gmelin, he
turned to chemistry. In 1844 he went to Giessen, to work under Liebig,
and there made his first contributions to chemical science—viz., on the
_decomposition of oxides and salts by chlorine_; _on ozone_; and _on
the blue compounds of cyanogen and iron_. After graduating at Giessen
he went, in 1846, to Paris, where he came under the influence of Comte,
with whom he studied mathematics. In 1850, at Graham’s solicitation,
he was appointed to the Chair of Practical Chemistry at University
College, vacant by the death of Fownes. He at once embarked upon those
researches which constitute his main contribution to science. In the
attempt to build up the homologous series of the aliphatic alcohols
from ordinary alcohol he succeeded in demonstrating the real nature
of ether and its genetic relation to alcohol, and in explaining the
process of etherification. The memoirs (1850–52) in which he embodied
the facts had an immediate influence on the development of chemical
theory. His explanation of the process of etherification familiarised
chemists with the idea of the essentially dynamical nature of chemical
change. He imported the conception of molecular mobility not only
into the explanation of such metathetical reactions as the formation
of the ethers, but into the interpretation of the phenomena of
chemical change in general. In these papers, as also in one on the
constitution of salts, published in 1851, he attempted to systematise
the representation of the constitution and relations of oxidised
substances—organic and inorganic—by showing how they may be regarded as
built up upon the type of water considered as

    H
     O,
    H

in which the hydrogen atoms are replaced, wholly or in part, by
other chemically equivalent atoms. This idea was immediately adopted
by Gerhardt, was further elaborated by Odling and Kekulé, and was
eventually developed into a theory of chemistry.

Williamson continued to direct the laboratory of University College
until 1887, when he retired to the country. He died at Hindhead on May
6, 1904.

│Robert Wilhelm Bunsen│ was born at Göttingen on March 31, 1811, and
after studying chemistry under Stromeyer, the discoverer of cadmium,
went to Paris and worked with Gay Lussac. In 1836 he succeeded Wöhler
as teacher of chemistry in the Polytechnic School of Cassel, and in
1842 became Professor of Chemistry in the University of Marburg. In
1852 he was called to Heidelberg, and occupied the Chair of Chemistry
there until his retirement in 1889. He died at Heidelberg on August 16,
1899.

[Illustration: BUNSEN, KIRCHHOFF, AND ROSCOE.]

Bunsen first distinguished himself by his classical work on the
_cacodyl compounds_, obtained as the result of an inquiry into the
nature of the so-called “fuming liquor of Cadet,” an evil-smelling,
highly poisonous, inflammable liquid formed by heating arsenious oxide
with an alkaline acetate. The investigation (1837–1845) is noteworthy,
not only for the skill it exhibits in dealing with a difficult and
highly dangerous manipulative problem, but also for the remarkable
nature of its results and on account of their influence on contemporary
chemical theory. The research, in the words of Berzelius, was the
foundation-stone of the theory of compound radicals. The name _cacodyl_
or _kakodyl_ was suggested by Berzelius in allusion to the nauseous
smell of the compounds of the new radical _arsinedimethyl, As (CH3)2_,
as it was subsequently termed by Kolbe.

Bunsen greatly improved the methods of gasometric analysis; these he
applied, in conjunction with Playfair, to an examination of the gaseous
products of the blast furnace in the manufacture of iron, and thereby
demonstrated the enormous waste of energy occasioned by allowing
the gases to escape unused into the air, as was then the universal
practice. This inquiry effected a revolution in the manufacture of iron
as important, indeed, as that due to the introduction of the hot blast.

Bunsen devised methods for determining the _solubility of gases_
in liquids, for ascertaining the _specific gravity of gases_, their
_rates of diffusion_, and of combination or _inflammation_. In 1841 he
invented the _carbon-zinc battery_, and applied it to the electrolytic
production of metals, notably of _magnesium_, the properties of which
he first accurately described. In 1844 he contrived the _grease-spot
photo-meter_, which was long in general use for ascertaining the
photometric value of illuminating gas. His methods of ascertaining
_the specific heats of solids and liquids_ were simple, ingenious, and
accurate. In 1855–1863 he carried out, in conjunction with Roscoe, a
long series of investigations on the _chemical action of light_. In
1859, in association with Kirchhoff, he devised the first methods of
_spectrum analysis_, and explained the origin and significance of the
Fraunhofer lines in the solar spectrum, thus laying the foundations of
solar and stellar chemistry. The application of the _spectroscope_ to
analytical chemistry almost immediately resulted in his discovery of
_cæsium_ and _rubidium_.

Bunsen worked on problems of _chemical geology_, and made a long series
of analyses of _volcanic products_. With Schischkoff, he examined, in
1857, the products of fired gunpowder. He effected many improvements in
analytical chemistry; devised the _iodiometric method_ of volumetric
analysis, and systematised the processes of _water analysis_. Lastly,
he invented the _gas-burner_—a piece of apparatus with which his name
is inseparably associated, and which has been of inestimable service to
operative chemistry and in the arts. Bunsen was no theorist, and purely
speculative questions had little or no interest for him. At the same
time he was a great teacher, and made the chemical school of Heidelberg
no less famous than the schools of Giessen and Göttingen.

       *       *       *       *       *

The mass of material relating to the development of chemistry which
has been accumulated during the past sixty years is so vast that it
would be hopeless to attempt to survey it in detail within the limits
of such a work as this. Nor, indeed, is this required in a history of
this character. Those who desire information concerning the origin and
sequence of the facts which collectively make up the superstructure
of modern chemistry must be referred to the encyclopædias or larger
treatises—or, preferably, to the numerous monographs, dealing with
special sections, which the volume and complexity of the matter to be
dealt with seem to render increasingly necessary. All we can do here
is to attempt to show what has been the main outcome of this sixty
years of incessant effort to elucidate the mysteries of chemical
phenomena and to ascertain the nature of the conditions which control,
modify, or determine them. All this effort is ultimately directed
to the solution of the fundamental problem of the constitution of
matter. The most significant result of this endeavour has been the
elaboration and consolidation of the doctrine of chemical atoms, not
necessarily of atoms in the limited Daltonian sense, but of atoms
considered as associations of particles, or corpuscles—that is, of
entities which _may_ be divisible, but which, in the main, are not
divided in the vast number of the transformations in which they are
concerned. This modification of the original conception of Dalton has
been thought by some to destroy the basis upon which his theory really
rests. There is no necessity for such an assumption. So pronounced an
atomist as Graham, as far back as 1863, in a suggestive paper entitled
_Speculative Ideas on the Constitution of Matter_, enlarged the
conception of the Daltonian atom in precisely the sense which recent
experimental work appears to require. The present position, too, as it
affects chemists, was equally well stated by Kekulé, in 1867, in the
following terms:

  The question whether atoms exist or not has but little
  significance from a chemical point; its discussion belongs rather
  to metaphysics. In chemistry we have only to decide whether the
  assumption of atoms is an hypothesis adapted to the explanation
  of chemical phenomena. More especially have we to consider the
  question whether a further development of the atomic hypothesis
  promises to advance our knowledge of the mechanism of chemical
  phenomena.

  I have no hesitation in saying that, from a philosophical point of
  view, I do not believe in the actual existence of atoms, taking
  the word in its literal signification of indivisible particles of
  matter; I rather expect that we shall some day find for what we now
  call atoms a mathematico-mechanical explanation which will render
  an account of atomic weight, of atomicity, and of numerous other
  properties of the so-called atoms. As a chemist, however, I regard
  the assumption of atoms not only as advisable, but as absolutely
  necessary, in chemistry. I will even go further, and declare my
  belief that _chemical atoms exist_, provided the term be understood
  to denote those particles of matter which undergo no further
  division in chemical metamorphoses. Should the progress of science
  lead to a theory of the constitution of chemical atoms—important as
  such a knowledge might be for the general philosophy of matter—it
  would make but little alteration in chemistry itself. The chemical
  atoms will always remain the chemical unit; and for the specially
  chemical considerations we may always start from the constitution
  of atoms, and avail ourselves of the simplified expression thus
  obtained—that is to say, of the atomic hypothesis. We may, in
  fact, adopt the view of Dumas and of Faraday—that, _whether matter
  be atomic or not, thus much is certain: that, granting it to be
  atomic, it would appear as it now does_.[1]

    [1] _The Study of Chemical Composition_, by Ida Freund
        (Cambridge University Press), 1904.

The greater part of that which follows will be devoted, therefore,
to an exposition of certain of the great advances in knowledge—many
of them of primary importance—which have been made during the last
fifty or sixty years and which have served to strengthen this extended
conception of the atomic theory, and to establish its position as an
article of the scientific faith of the twentieth century.




CHAPTER II

THE CHEMICAL ELEMENTS DISCOVERED SINCE 1850


In 1850 the number of substances generally recognised as chemical
elements, in the sense in which that term was first employed by Boyle,
was sixty-two. Two members—viz., the _pelopium_ of Rose and the
_ilmenium_ of Hermann—were, however, subsequently shown to be identical
with metals already known. At the present time (1910) the number of the
chemical elements definitely recognised as such is eighty-two. In 1850,
as now, they were broadly classified as metals and non-metals, although
it was felt then, no less strongly than now, that no very clear line of
demarcation was traceable between the two groups. Sixty years ago the
elements usually styled non-metals were thirteen in number; to-day the
number is nineteen—the increase being due to the inclusion of arsenic
and the discovery of the so-called inactive elements, helium, argon,
krypton, neon, and xenon. In 1850 there were forty-seven elements
definitely classed as metals; in 1910 the number is sixty-three.

At all periods in the history of chemistry as a science the general
tendency has been to name substances, whenever possible, in accordance
with the theoretical conceptions of the time, and hence it has happened
that the same body at successive periods has had very dissimilar
names. But in naming the substances we term elements, theoretical
conceptions are not usually applicable. Oxygen, it is true, derives its
name from such a conception; and, etymologically, the name connotes
an error. Hydrogen, too, has no more right to be called the _water
former_ than oxygen. Davy, who invented the term chlorine, advocated
that the chemical elements should be named from some distinguishing
peculiarity, either of origin or of physical property. In the main this
principle has been adopted especially in later years although there
are numerous instances of names derived from pure arbitrary sources.
It is largely for the reason that the names of the elements are, with
rare exceptions, unconnected with theories that they have remained
unchanged, whereas names of compounds, which are far more frequently
dependent upon speculative ideas, have constantly been altered in
order to comply with the prevailing hypotheses of the period. At the
same time it is not always clear that the etymology of certain of
the elements is well ascertained. It has been recently shown, for
example that the commonly accepted origin of the word “antimony” from
_antimoine_, based on the alleged experiences of mediæval ecclesiastics
has no valid foundation. The word is, in reality, derived from the
Arabic _alhmoud_: this became latinised to _althimodium_ and eventually
to _antimonium_.

By the middle of the nineteenth century the system of symbolical
notation suggested by Berzelius was everywhere current; and, stripped
largely of its dualistic associations, this system still remains
the most generally convenient method of expressing the composition,
analogies, and numerical relations of substances. During the middle
of the last century philosophic chemists, although subscribing, with
hardly an exception, to the doctrine of definite combining proportions,
were by no means agreed as to the sufficiency of Dalton’s explanation
of the experimental laws of chemical combination; and the hypothesis of
atoms in the Daltonian sense was not universally accepted. To some the
atomic theory of Dalton, which assumed that the combining proportion
was identical with the relative weight of the atom, was unnecessary as
an explanation of the laws of combination. Or at most it was only one
out of a variety of molecular conditions in which matter might exist.
Consequently some chemists were in the habit of drawing a distinction
between _chemical_ atoms and _physical_ atoms. The chemical atom was
identical with the Daltonian atom but this was by no means the same
as the physical atom of Democritus or Leucippus. The view in 1850, in
fact, was not very dissimilar from that to which recent experimental
inquiry has led. But it can hardly be said that the doubts were
dependent upon valid experimental evidence; they arose rather from the
erroneous interpretation of imperfectly ascertained facts—upon the
supposed inconsistencies of the law of Gay Lussac with the hypotheses
of Avogadro and Ampère. As soon as the facts were clearly perceived
and the inconsistencies reconciled we heard less of the supposed
distinction between the chemical and the physical atom. It is only
within quite recent time, and as the result of entirely new lines of
inquiry, that the distinction has been revived.

In the early part of the last century attempts were made by Berzelius
to classify the chemical elements according to their electro-chemical
relations, and by Thomson according as they were “supporters” or
“non-supporters of combustion.” It was soon perceived that Thomson’s
system had no philosophical basis, and it quickly fell into disuse.
After the discovery of isomorphism, an endeavour was made by Graham to
arrange the simple bodies in accordance with their natural relations,
and even before 1850 the various elements were grouped by him very much
as now.

This scheme of classification, somewhat modified by considerations
of valency, and occasionally corrected by more accurate information
concerning true analogies (as when vanadium was transferred by Roscoe
to the nitrogen group), was in general use for practically a quarter of
a century—in fact, until it was superseded by the gradual adoption of
Mendeléeff’s arrangement based on periodicity. There can, however, be
little doubt that this attempt by Graham at a natural classification
paved the way along which Newlands and eventually Mendeléeff were led
to devise our present rational system of grouping the chemical elements.

The numerical relationships existing among the equivalents and atomic
weights of the elements of certain of these groups, pointed out by
Dumas, Pettenkofer, Odling, Gladstone, and others, gave rise to much
speculation. The values of the gradational differences, of course,
depended upon whether equivalents or atomic weights were employed; but
the immediate point is that, whichever basis was adopted, definite
numerical relations were to be perceived. Thus, in the case of the
group of the halogens, it was pointed out that the individual members
are connected together as follows:

    Fluorine.    Chlorine.       Bromine.        Iodine.
       19          35.5            80             127
       a          a + d        a + 2d + d´    2a + 2d + 2d´

where a = 19; d = 16.5; d´ = 28.

Thus, too, in the case of the nitrogen group:

    Nitrogen.  Phosphorus.   Arsenic.     Antimony.     Bismuth.
       14          31          75           119           207
       a         a + d      a + d + d´   a + d + 2d´   a + d + 4d´

where a = 14; d = 17; d´ = 44.

       *       *       *       *       *

On the basis of these and similar numerical relationships it was
surmised that, just as the successive members of a group of homologous
organic radicals are formed by increments of CH2, so the substances
in the several groups of the elements may be produced by successive
additions of some form of matter common to them all. This has its
counterpart, somewhat modified, in the modern hypothesis of the
disintegration of the elements. Dumas conceived the elements in any
particular group to be built up by successive accretions of particular
forms of matter; Rutherford and Soddy suppose them to be derived by the
successive elimination of matter from some unstable parent substance.

Since 1850 the existence of at least twenty-two new elements may
be said to have been established. Of course, many more than this
number have been announced, more or less tentatively; but subsequent
investigation has either not confirmed their existence, or has
definitely disproved it. The names, symbols, and atomic weights of the
twenty-two, arranged in alphabetical order, are as follows:

    Argon             A      39.9
    Cæsium            Cs    132.8
    Dysprosium        Dy    162.5
    Europium          Eu    152.0
    Gadolinium        Gd    157.3
    Gallium           Ga     69.9
    Germanium         Ge     72.5
    Helium            He      4.0
    Indium            In    114.8
    Krypton           Kr     83.0
    Lutecium          Lu    174.0
    Neodymium         Nd    144.3
    Neon              Ne     20.0
    Praseodymium      Pr    140.6
    Radium            Ra    226.4
    Rubidium          Rb     85.4
    Samarium          Sa    150.4
    Scandium          Sc     44.1
    Thallium          Tl    204.0
    Thulium           Tm    168.5
    Xenon             Xe    130.7
    Ytterbium      }  Yb    172.0
    (Neoytterbium) }

The additions have been due, to some extent, to the refinement of
processes of analysis already in use, but more especially to the
employment of new analytical methods; or, lastly, to the application
of a generalisation concerning the mutual relations of the elements
which has served to indicate not only the existence of new and specific
members of families of elements already known, but to point out the
probable mode of their occurrence.[2]

    [2] The substances which appear to be formed by the
        disintegration of uranium, radium, thorium—the so-called
        radio-active elements—such as _ionium_, _actinium_,
        _polonium_, and the various _emanations_ to which they
        give rise, are not here enumerated. They are dealt with in
        Chapter III.

Although the existence of the element _fluorine_ was surmised as far
back as 1771, when Scheele first recognised that the product of the
action of oil of vitriol upon fluor-spar contained a hitherto unknown
substance, it was not until 1886 that this substance was definitely
isolated by Moissan by the electrolysis of the acid potassium fluoride
in solution in hydrogen fluoride. Cerium tetrafluoride, CeF4, and
lead tetrafluoride, PbF4, when heated, were observed by Brauner to
evolve a gas having a smell resembling that of hypochlorous acid,
which was probably free fluorine. Certain violet-coloured varieties
of fluor-spar, when powdered, emit a peculiar smell, which has been
attributed to free fluorine.

Gore observed that anhydrous hydrogen fluoride would not conduct
electricity—a fact confirmed by Moissan. Moissan found, however,
that on adding potassium fluoride to the liquid it readily suffered
electrolysis with the liberation of free fluorine as a light greenish
yellow gas with a pungent, irritating smell resembling that of
hypochlorous acid. It has a vapour density corresponding with an
atomic weight 19. By the application of cold and pressure it may be
liquefied. At still lower temperatures it may be frozen to a white
solid. Fluorine is characterised by an extraordinary chemical activity,
and combines, even at ordinary temperatures, with a large number of
substances. Sulphur, phosphorus, arsenic, antimony, boron, iodine, and
silicon inflame or become incandescent in contact with it. It combines
with hydrogen with explosive violence, even in the dark and at the
lowest temperature. It unites also with the metals, occasionally with
incandescence, and decomposes water with liberation of oxygen.

The application, by Bunsen, of the _spectroscope_ to chemical analysis
almost immediately resulted in his discovery, in 1860, of _cæsium_,
and, in 1861, of _rubidium_. Cæsium was first detected in the mineral
water of Dürkheim in the Palatinate and in the mineral petalite, by
the two blue lines it forms in the spectrum, whence its name from the
Latin _cæsius_, used to designate the blue of the clear sky. Rubidium
was found in a lepidolite by means of a number of lines in different
parts of the spectrum not previously observed, two being especially
remarkable in the outermost region of the visible red portion—whence
the name of the element from the Latin _rubidus_, used to designate
the darkest red colour. The new metals were found to have the closest
analogies to potassium, with which they usually occur associated
in nature. Rubidium is found in a number of lepidolites, leucite,
spodumene, triphylite, mica, and orthoclase, and in the Stassfurt
carnallite; in sea-water and in many mineral waters. It occurs also
in the ashes of many plants such as those of beetroot, tobacco, tea,
coffee, etc. It is doubtful if it is a normal constituent of plant
food, attempts to introduce it in place of potash having failed. It is
not improbable that these elements would have remained unknown except
for spectrum analysis. At all events, one of them—cæsium—was missed
in 1846 by Plattner, in the course of the analysis of the mineral
_pollucite_, in which it occurs to the extent of one third of its
weight. After the discovery of cæsium by Bunsen, this mineral was again
analysed by Pisani, when it was found that the alkali which Plattner
had mistaken for potassium was in reality cæsium. Cæsium is found to a
very small extent in many mineral waters, in a variety of minerals, and
in the ashes of plants.

In 1861 Sir William Crookes made known the existence of a new
element which he called _thallium_. He found it in a seleniferous
deposit obtained from an oil of vitriol factory in the Harz. It was
characterised by giving a bright green line in the spectroscope—whence
its name from θαλλός, a green or budding twig. The discovery was
confirmed in the following year by Lamy. Thallium, in its general
chemical relations, has many analogies to the metals of the alkalis
although in the metallic state it has the closest resemblance to lead.
It occurs in many varieties of pyrites, in a few minerals, such as
crookesite, lorandite, zinc-blende and copper pyrites, etc., and in
certain mineral waters.

In 1863 Reich and Richter, by means of the spectroscope, detected
the presence of a new element in the zinc-blende of Freiberg.
The observation that it afforded two indigo-blue lines in the
spark-spectrum led them to give it the name _indium_. It has since been
found in numerous blendes, in various zinc and tungsten ores, and in
many iron ores. It is a silver-white, ductile, and malleable metal,
melting at 174°, and burning when heated with a violet flame. It is
related in chemical characters to aluminium and zinc. Its true place in
the natural scheme of classification of the elements was indicated by
Mendeléeff.

In 1875 Lecoq de Boisbaudran discovered a new element in the
zinc-blende of Pierrefitte in the Pyrenees, also by means of spectrum
analysis. The spark-spectrum of its salts affords two characteristic
violet lines quite different in position from those given by indium.
To the new element its discoverer gave the name of _gallium_. It has
been found in very small amounts in other blendes, but is still one
of the rarest of the chemical elements. It is a bluish-white, hard,
and slightly malleable metal fusing at a temperature not much higher
than that of a hot summer day. Its existence and main properties, as
well as its more significant chemical relationships, were predicted by
Mendeléeff in 1869 from considerations based upon his periodic law.
(See _ante_.)

In the same year Mendeléeff also predicted the existence of a new
element belonging to the group of which boron is the first member,
which he provisionally termed _eka-boron_, and described its main
properties. Mendeléeff’s prediction was verified in 1879 by Nilson’s
discovery of the element _scandium_. Scandium occurs associated with
yttrium, ytterbium, etc., in many Swedish minerals, such as _euxenite_,
_gadolinite_, _yttrotitanite_, etc. The metal itself has not been
isolated, but the properties of its compounds correspond closely
with those of the corresponding ekaboron compounds, as predicted by
Mendeléeff.

A further illustration of the value of the principle of periodicity, as
developed by Mendeléeff, in indicating the existence of new elements,
is seen in the discovery of _germanium_. In 1885 Weisbach discovered a
new Freiberg silver mineral, to which he gave the name _argyrodite_.
This on analysis by Winkler was found to contain a new element to the
extent of about seven per cent. with properties identical with those
predicted by Mendeléeff for a missing element in the fourth group of
the periodic series, consisting of silicon, tin, and lead, and which
he had provisionally termed _eka-silicon_. _Argyrodite_, in fact, is
a double sulphide of silver and germanium, 2Ag2S.GeS2. Germanium is a
greyish-white, lustrous metal of sp.gr. 5.5., melting at about 900°,
and resembling silicon and tin in its general chemical relations.

_Dysprosium_, _europium_, _gadolinium_, _lutecium_, _neodymium_,
_praseodymium_, _samarium_, _thulium_, and _ytterbium_ (_neoytterbium_)
belong, like scandium, to the group of the so-called rare earth metals.
These substances have been detected in a great variety of minerals,
many of which are extremely rare. The elements most frequently occur in
nature associated with yttrium, cerium, thorium, and zirconium.

_Dysprosium_ was first detected, in 1886, by Lecoq de Boisbaudran in
the so-called erbium earth of Mosander, in which Cleve had previously
(1880) announced the existence of two other elements, _holmium_ and
_thulium_. There is some reason to believe that the holmium of Cleve is
identical with dysprosium. _Ytterbium_ was discovered by Marignac, in
1878, in the mineral _gadolinite_. In 1906 Auer von Welsbach announced
that Marignac’s ytterbia was a mixture, which was confirmed in the
following year by Urbain, who separated it into two elements, which
he named _neoytterbium_ and _lutecium_. _Europium_ was discovered by
Demarçay in 1901. All these earths are met with in small quantities
associated with yttria in _gadolinite_, _euxenite_, _samarskite_,
_xenotime_, _cerite_, _orthite_, and other similar minerals. Their
compounds, or such of them as have been described, resemble the
corresponding compounds of yttria. They are recognised by differences
in their spectroscopic behaviour. _Gadolinium_ was detected,
independently, in 1886, by Marignac and Lecoq de Boisbaudran in the
terbium earth of Mosander.

What was long known as _didymium_ (διδυμος = a twin) was discovered by
Mosander in 1841. It owes its name to its close chemical relationship
to, and almost constant association with, _lanthanum_—both elements
occurring in many minerals, more particularly in _cerite_, _allanite_,
and _monazite_. In 1885 Auer von Welsbach announced that the didymium
of Mosander was, in reality, a mixture of two elements which could be
separated by the systematic fractional crystallisation of the double
ammonium nitrates; to these elements he gave the names _praseodymium_
(πράσινος, leek-green) and _neodymium_ (νέος, new). Neodymium salts
are rose-coloured, whereas those of praseodymium are green, and the
elements are further characterised by differences in their absorption
and spark-spectra. When mixed, the substances give the spectrum
originally considered to be characteristic of didymium.

_Samarium_ was discovered in 1879 by Lecoq de Boisbaudran in
_samarskite_. Its salts are yellow, and afford in solution
characteristic absorption bands.

It is not improbable that many of the minerals from which the so-called
rare earths are obtained contain elements hitherto unrecognised, and it
is possible that certain of the substances now assumed to be elements
may, like didymium, turn out to be mixtures. In fact, additional
elements have from time to time been announced, as for example, the
_decipium_ of Delafontaine (1878) and the _monium_ or _victorium_ of
Crookes (1899), pronounced by Urbain to be identical with gadolinium:
their individuality cannot as yet be said to be established. Didymium
itself was stated by Krüss and Nilson (1888) to be even more
complicated than the work of Auer von Welsbach would seem to indicate,
and to contain no fewer than eight elementary substances. As yet,
however, no confirmation of this surmise has been obtained.

The chemistry of the rare earths has of late years been greatly
extended owing to the employment of certain of the members of the
group in the manufacture of the “mantles” used in gas-lighting, and
which consist substantially of thoria, mixed with about one per cent.
of ceria. Large quantities of _monazite_, _thorianite_, _thorite_,
_cerite_, and other minerals, are now worked up for the sake of the
thoria and ceria they contain, and considerable amounts of residual
products, consisting largely of other members of the family, are now
available for investigation. It is reasonably certain, therefore, that
our knowledge of this section of inorganic chemistry will be largely
augmented in the immediate future. Indeed, the application of thoria
to the construction of gas-mantles may be said to have removed that
substance from the category of the rare elements. No sooner was it
discovered that it was capable of useful application than unexpected
sources of supply were found.

The same result has followed in other cases. One of the most
significant developments of modern chemistry is seen in the efforts
which are constantly being made to turn the so-called rare elements to
useful account; and when they are found to be technically valuable it
is generally observed that hitherto unknown sources of supply are soon
available. Cerium salts have been found to be useful in the colouring
of glass and porcelain, as mordants in dyeing, in photography, and in
medicine. Zirconium has been used in incandescent electric lighting,
and thallium has been employed in the manufacture of highly refractive
optical glass. Titanium, molybdenum, and vanadium are used in the
manufacture of steel of high tensile strength. Tantalum and tungsten
are employed in the construction of filaments in incandescent electric
lighting. Tantalum, indeed, has been found to occur in considerable
quantities, and to be more largely distributed than was hitherto
supposed. Alloys of tungsten and aluminium are used in automobile
construction, and alloys of tungsten, aluminium, and copper in the
manufacture of propeller blades. Tungsten steel is used in armour
plates, and to stiffen the springs of cars; in the manufacture of
piano-wire, and to increase the permanency of magnets. Even the rarer
metals of the platinum group are finding many important applications.
Osmium-iridium is used for the bearings of compasses, for the tips
of gold pens, and in the construction of standard weights. Osmium
and ruthenium enter into the composition of filaments for electric
lighting. The extraordinary influence of light on the electric
conductivity of selenium has been made use of in the transmission of
photographs by telegraph and telephone wires, and for measuring the
light intensity of the Röntgen rays in clinical work.




CHAPTER III

THE INACTIVE ELEMENTS: RADIUM AND RADIO-ACTIVITY


_Argon_, _helium_, _krypton_, _neon_, and _xenon_ belong to the group
of the so-called inactive elements, and constitute what are known as
the rare gases of the atmosphere. The existence of these bodies is
of great theoretical value and few discoveries of recent times have
exacted more interest and curiosity. Twenty years ago it was generally
assumed that practically all that was to be known concerning the
composition of atmospheric air had been ascertained. Priestley and
Cavendish had recognised that it was mainly composed of oxygen and
nitrogen, and Cavendish had definitely stated that these gases are
present in practically constant proportion, independent of season,
climate, or locality. Thénard, Saussure, and others, had determined
the limits of variation in the amount of carbon dioxide. Bunsen and
Regnault had established that the quantities of oxygen and nitrogen are
subject to slight alteration, the extent of which could be readily
determined by the exact eudiometric processes they had devised.
Lastly, it was proved beyond a doubt that the gases of the atmosphere
are simply mechanically mixed, and can be separated by a variety of
physical methods. In fact, of no single subject could it be more
confidently assumed that finality of knowledge had apparently been
reached.

In 1892, in the course of a series of determinations of the densities
of the common gases, Lord Rayleigh found that the density of nitrogen
obtained from the air was slightly greater than the density of that
gas prepared by the decomposition of ammonia and of nitric acid, the
difference in weight being about 1 part in 200—an amount far greater
than could be accounted for by errors of weighing. Various suppositions
were made in explanation of the discrepancy; but these, when tested,
were found not to account for the facts. By heating the atmospheric
nitrogen with metallic magnesium, whereby the greater portion of the
gas is absorbed to form the nitride, Sir William Ramsay found that the
density of the residual gas was still further increased, which rendered
it probable that the relatively high density of atmospheric nitrogen
as compared with that derived from ammonia, and, as Lord Rayleigh
found, from other sources also, was due to the presence of a gaseous
substance in the air of considerably greater density than nitrogen or
oxygen. Lord Rayleigh also subjected atmospheric nitrogen mixed with
oxygen to the electric discharge over a solution of caustic soda, in a
manner similar to that already employed by Cavendish, and found also
that the residual gas was considerably increased in density. At the
Oxford meeting of the British Association in August, 1894, the two
investigators were in a position to announce that the discrepancy was
actually due to the presence of a hitherto unknown gaseous constituent
of atmospheric air, considerably more soluble in water than nitrogen,
and to which, on account of its chemical inertness, the name of
_argon_ (ἀργον, idle) was given. By a special apparatus devised by
Lord Rayleigh, in which a mixture of air and oxygen is submitted to an
electric flame produced by a powerful, rapidly alternating current,
considerable quantities of argon were separated from the air. It has
also been found that by the use of metallic calcium or a mixture of
magnesium and lime, the atmospheric nitrogen is absorbed at a lower
temperature, and more rapidly than by magnesium alone.

Argon has been found to exist in the gases from springs and mineral
waters, notably in those of Bath, Cauterets, Wildbad, and Harrogate. It
has also been found in a meteorite, in the gas occluded in rock-salt,
and in the minerals _malacone_, _uraninite_, _brōggerite_, etc. No
animal or vegetable substance appears to contain it. It is present in
atmospheric air to the extent of about one per cent. by volume. It is
a colourless gas of an atomic weight of 39.9: one litre of it at the
standard temperature and pressure weighs 1.7815 grams. Experiments made
by the method of Kundt and Warburg—_i.e._, by determining the ratio
of the specific heats at constant pressure and constant volume by the
velocity of sound in the gas—prove that argon, like mercury gas, is
monatomic. This of itself indicates that argon is an element, since a
monatomic compound is a contradiction in terms. The calculations from
the experimental data presuppose that argon obeys the laws of Boyle and
Dalton, which was found on trial to be the case. By the application
of cold and pressure argon can be liquefied. The liquid boils at
-186°.1 and freezes at -187°.9. The spectrum of the gas is exceedingly
complicated, consisting of a great number of lines extending throughout
the visible portion and far into the extreme red and ultra-violet. The
colour of the light emitted on sparking the gas changes with increase
of temperature from a brilliant red to a bright blue—depending on the
intensity of the discharge. All attempts to induce argon to enter
into combination with other substances have failed. The methods of
its preparation show that it does not combine with oxygen, although
Troost and Ouvrard state that it unites with magnesium vapour. It forms
no compounds with hydrogen, chlorine, phosphorus, sulphur, sodium,
tellurium, etc. Even fluorine, probably the most generally active of
the chemical elements, shows no tendency to unite with it.

In 1888 Dr. Hillebrand, of the U.S. Geological Survey, in examining
a form of uraninite known as _cleveite_, so named from the late
Professor Cleve, found that on treatment with dilute sulphuric acid
it gave off considerable quantities of a gas which was assumed to
consist only of nitrogen, as it gave the spectroscopic reactions of
that element. To test whether this gas contained argon, Ramsay, in
1895, further examined it spectroscopically. After sparking it with
oxygen in the presence of caustic soda solution, in the way already
described, it gave no indications of argon. The main characteristic
of its spectrum was a bright yellow line, known as D3, not coincident
with that afforded by sodium, but identical in position with a line
detected in the chromosphere during the solar eclipse of 1868, which
line, on examination by Frankland and Lockyer, could not be ascribed
to any known element. For this supposed new element the name _helium_,
from ἥλιος, the sun, had been suggested. This was the first occasion
on which an element observed originally only in the sun was found to
occur also on the earth. The presence of the new element in the gas
from cleveite was subsequently confirmed by Langlet working in Cleve’s
laboratory.

[Illustration: SIR WILLIAM RAMSAY.]

Helium is a monatomic gas having the atomic weight 4. It is less
soluble in water than argon. Like argon, it shows no tendency to
enter into chemical union with any other substance. It has been found
in many minerals, particularly in those containing uranium and the
so-called rare earth metals. It also occurs among the gases issuing
from certain mineral springs, such as those of Bath and at Cauterets
in the Pyrenees, and also at Adano near Padua. The spectrum of helium
contains, in addition to the characteristic yellow line—by which its
presence had been recognised not only in the solar chromosphere, but
also in certain of the fixed stars—two lines in the red, and lines in
the green, blue, and violet. The character of the light emitted by
the spark-discharge is modified by the intensity of the discharge in
a manner similar to that of argon. It has been shown by Collie that
its spectrum is altered by the presence of mercury vapour. It is the
least refractive of all the gases. Helium was liquefied by Kammerlingh
Onnes in 1908. It forms a colourless liquid of sp. gr. 0.154, boiling
at -268.5; that is, 4°.5 above the absolute zero of temperature. Its
critical temperature is about 5° absolute, and its critical pressure
above 2¼ atmospheres.

The methods now in use for obtaining liquid air, referred to in a
subsequent chapter, enable large quantities of that material to be
obtained readily; and it was in investigating spectroscopically the
residues left after volatilising a quantity of liquid air that Ramsay
and Travers, in 1898, detected the existence of two new monatomic
gaseous constituents of the air which they named respectively _krypton_
(χρυπτός, hidden) and _neon_ (νέος, new), the former heavier and the
latter lighter than argon. By fractional distillation of the argon,
simultaneously procured, a gas was obtained which in the spectroscope
showed the characteristic lines of helium—previously recognised in
atmospheric argon by Kayser and Friedländer—together with a complicated
spectrum consisting of a number of lines in the red, orange, and yellow
due to the new element neon. On cooling this mixture to -252° by means
of liquid hydrogen, the neon solidified, while the helium remained
gaseous and could thus be separated.

Krypton was obtained from the residues left on the evaporation of a
large quantity of liquid air. Mixed with the krypton was a third
gaseous constituent of air, to which the name _xenon_ (ξενος, the
stranger) was given. The boiling-point of krypton at atmospheric
pressure was found to be -152°, and its melting-point -169°; the
boiling-point of xenon was -109° and its melting-point -140°. Their
critical temperatures were respectively -62°.5 and +14°.7. Hence
xenon could be liquefied by pressure a very little below the mean
temperature of the air. Neon boils at -243° and freezes at -253°. They
form colourless liquids freezing to ice-like solids. All of them, with
the exception of argon, which is present to the extent of about 1 part
in 107 parts of air, are contained in extremely small amounts in the
atmosphere, approximately in the following proportions:

    Helium  1 part in 245,300      parts by volume.
    Neon    1  ”   ”   80,800        ”        ”
    Krypton 1  ”   ”   20 millions   ”        ”
    Xenon   1  ”   ”  170    ”       ”        ”

Many tons of liquefied air have since been systematically fractionated,
but no other gas than those above named has been obtained.

Julius Thomsen, of Copenhagen, in a paper published in 1895, entitled
_On the Probability of the Existence of a Group of Inactive Elements_,
pointed out, in relation to Mendeléeff’s Law of Periodicity (see
_ante_), that in periodic functions the change from negative to
positive value, or the reverse, can take place only by a passage
through zero or through infinity; in the first case the change
is gradual, and in the second case it is sudden. The first case
corresponds with the gradual change in electrical character with rising
atomic weight in the separate series of the periodic system, and the
second case corresponds with a passage from one series to the next.
It therefore appears that the passage from one series to the next in
the periodic system should take place through an element which is
electrically indifferent. The valency of such an element would be zero,
and therefore in this respect also it would represent a transitional
stage in the passage from the univalent electronegative elements of the
seventh to the univalent electropositive elements of the first group.
This indicates the possible existence of a group of inactive elements
with the atomic weights 4, 20, 36, 84, 132—numbers corresponding fairly
closely with the atomic weights respectively of helium, neon, argon,
krypton, and xenon.

       *       *       *       *       *

No discovery of recent years has created more widespread interest than
that of the radio-active elements.

In 1896 Henri Becquerel found that uranium salts emitted an invisible
radiation which had the power of affecting a photographic plate,
even though not directly exposed to it, exactly in the same way as
the Röntgen or X-rays. Since that time a number of substances have
been shown to possess a similar property. Such substances are said
to be radio-active. The radiation emitted by them is not uniform in
character. It has been found to be of three distinct types, known
respectively as the α, β, and γ radiations. The α rays consist of
positively electrified particles moving with a velocity equal to about
a fifteenth of that of light. These rays have little penetrative power,
and are capable of being deflected by a magnet.

The β rays consist of negatively electrified particles of a mass not
greater than one thousandth of that of the hydrogen atom, and they
move with a velocity approximating to that of light. The β rays have a
greater penetrative power than the α rays, and are even more readily
deflected by a magnet.

The γ rays are analogous to, if not identical with, the X or Röntgen
rays; they move with the velocity of light, have a high penetrative
power, but are not affected by the magnet. All three forms of
radiation render gases electrically conductive, excite luminescence or
fluorescence in certain substances, change the colour of glass, convert
oxygen into ozone and yellow phosphorus into red phosphorus, and act
upon photographic plates.

According to the disintegration theory of Rutherford and Soddy,
the radio-active elements are forms of matter undergoing changes
resulting in the formation of new forms possessing chemical and
physical properties differing from those of the parent substance,
these changes being accompanied by the production of sensible
heat, or some other manifestation of energy, due to the process of
transformation of the changing atoms. The rate of change is found to
be different for each radio-active element, but to be constant for
the same element irrespective of its particular form of combination.
The relative radio-activity of the various chemical combinations of a
given radio-active element is directly proportional to the quantity
of the element contained in them. The process of disintegration
may be carried through a number of intermediate products until
a stable form is produced. Uranium, in which the phenomenon of
radio-activity was first perceived, is supposed to give rise to no
fewer than seventeen different forms of matter, including _radium_,
_actinium_, and _polonium_. Thorium, another radio-active element,
is supposed to disintegrate into eight different forms of matter.
Uranium disintegrates with extreme slowness; it is calculated that
in a year not more than one ten-billionth part of the uranium is
transformed. The first disintegration product is termed uranium │x│.
If a quantity of dehydrated uranium nitrate be treated with ordinary
ether, a slight residue is obtained which is found to contain uranium
│x│. It emits β and γ rays, and is relatively rapidly transformed
into other substances. Ordinary uranium, freed from uranium │x│, only
emits α rays. Uranium salts can be freed from uranium │x│ by repeated
crystallisation, uranium │x│ remaining in the mother liquors.

[Illustration: MARIE CURIE (_née_ SKLODOWSKA).]

The existence of radium was first made known by │Mme. Curie│ in 1898.
In examining certain uranium minerals and uranium products, Mme. Curie
observed that their radio-activity was apparently greater than that
corresponding with the amount of uranium contained in them, and she
was led to surmise that this might be due to the presence of some
constituent more strongly radio-active than uranium. This supposition
proved to be well founded, and she eventually succeeded in isolating
a new element termed _radium_, forming compounds with characters and
relationships akin to those of barium. The richest source of radium at
present known consists of certain residues occurring at Joachimsthal,
in Bohemia, left after the extraction of uranium from pitch-blende, in
which radium occurs to the extent of 0.2 gram per ton. These residues
are mainly sulphates of lead and calcium, mixed with a great variety
of other metallic compounds. To obtain the radium the mixture is
heated with concentrated caustic soda solution, the residue washed with
water and treated with hydrochloric acid which dissolves the greater
portion of the material. Nearly the whole of the radium is left in
the insoluble portion. This, after washing with water, is boiled with
a solution of sodium carbonate so as to transform the alkali-earths
into carbonates. These are converted into chlorides or bromides from
which, by repeated crystallisation, barium chloride or bromide is
obtained, containing the greater portion of the radium as a halide
salt. The radium and barium salts are then separated by fractional
crystallisation, the radium salts being slightly less soluble in water
and alcohol, and in solutions containing the halogen acid, than the
barium salt.

Pure radium chloride (RaCl2) is a white crystalline salt, resembling
barium chloride, with which it appears to be isomorphous. Radium,
like barium, forms an insoluble carbonate and sulphate, but a soluble
nitrate and bromide. The bromide is much less stable than the chloride;
on standing it evolves bromine and becomes basic. Radium has as yet
been obtained in such small quantities that very few of its compounds
have been prepared.

The rays from radium salts burn the skin, and are found to be useful
in the destruction of rodent ulcers; they appear to act upon proteids,
destroy bacteria, bleach chlorophyll, and affect the germinative power
of seeds. A pure and freshly-prepared salt of radium seems to emit only
α rays, but it soon forms disintegration products, and then gives out,
in addition, the β and γ rays.

In the process of disintegration the salts emit heat corresponding to
about 75 gram calories per hour for each gram of radium present; their
temperature is thus uniformly higher than that of their environment.
One product of the change probably connected with the emission of the α
rays, is the gas helium.

Radium has an atomic weight of 226.5. It is regarded as a product
of the disintegration of uranium, the atomic weight of which is
238.5. It is believed to have been formed through an intermediate
product known as _ionium_, a radio-active element discovered by
Boltwood in the mineral _carnolite_. The atomic weight of ionium is
surmised to be about 230. Radium itself is supposed to form at least
eight disintegration products, the first of which is the so-called
_emanation_, discovered by Dorn in 1900, an inactive gas with an
atomic weight of about 180, giving a bright line spectrum, decomposing
into helium, liberating oxygen and hydrogen from water, and capable
of being condensed to a liquid and solidified at a low temperature.
Ramsay and Gray have determined its physical constants. The liquid
is phosphorescent and shines with a colour depending on the nature
of the glass of the vessel which contains it. The solid is also
phosphorescent, the colour varying with the temperature. It gives out
only α rays and in its disintegration, like radium, evolves heat. Its
position in the Periodic Table is probably above that of xenon. Other
products are known as _radio-lead_ and _polonium_. The latter substance
was identified by M. and Mme. Curie in 1898, and was the first of the
strongly radio-active substances to be recognised. In the periodic
system it seems to follow bismuth and to be a member of Group VI., with
a possible atomic weight of 210. Its spectroscopic characters have
recently been examined by Mme. Curie and Debierne, who have shown that
in its decay it evolves helium.

The rate of disintegration of radium is relatively slow; it has been
calculated that the time required for half of any given quantity of
radium to change completely into other products is about 2000 years.
Rutherford has calculated that in 26,000 years a kilogram of radium
would be reduced to one milligram of active substance, the remainder
having passed into degradation products.

In 1899 Debierne announced the existence of another radio-active
element contained in uranium minerals, which he termed _actinium_. This
is probably a disintegration product of uranium and identical with the
_emanium_ of Giesel. It occurs associated with the rare earths which
can be separated from the pitch-blende residues, and is eventually
found in the lanthanum salts. Nothing is known as to its atomic
weight or its chemical relationships. It undergoes change, and forms,
apparently, a gaseous emanation which rapidly disintegrates and can
be condensed to a liquid at a low temperature. Four other successive
products have been identified by the character of the radiation they
emit, their degradation constants, and the time required for one half
of any given quantity to disintegrate into other forms of matter.

_Thorium_ was shown to contain a radio-active element by Mme. Curie
and Schmidt, independently, in 1898. Whether thorium is itself
active is doubtful. The rate of disintegration of _radio-thorium_
is probably greater than that of uranium. It, too, seems to form a
gaseous emanation which can be condensed at the temperature of liquid
air and appears to be an inert gas of high molecular weight with the
characteristics of the argon family.

The type of radiation emitted by the several products has been
observed, and their constants of change and half-value periods
calculated; but little or nothing is known at present concerning their
atomic weights, spectroscopic or chemical characters.




CHAPTER IV

ATOMS AND MOLECULES: ATOMIC WEIGHTS AND EQUIVALENTS


It has already been pointed out that the discovery by Gay Lussac, and
independently by Dalton, that gases combine in simple proportions by
volume, and that the volume of the gaseous product, measured under
comparable conditions of temperature and pressure, stand in simple
relation to the volumes of the constituents, seemed to most of Dalton’s
contemporaries, but not to Dalton himself, to afford strong evidence
of the validity of his explanation of the essential nature of chemical
combination. It appeared obvious from the facts that there must exist
some simple relation between the densities, or specific gravities,
of the elementary gases and their atomic weights. When, however, the
principle underlying Gay Lussac’s law was extended so as to include
gases in general—both simple and compound—difficulties were met with
which were only satisfactorily cleared away during the latter half of
the nineteenth century. The first rational attempt to explain the
facts observed by Dalton and Gay Lussac, concerning the volumetric
relations of gases, was made in 1813 by Amedeo Avogadro by the
assumption that a given volume of all gases—simple or compound—contains
the same number of integral molecules; hence the relative weights
of these volumes represent the relative weights of the molecules.
According to Avogadro, in the case of the simple gases the integral
molecules are composed of a certain number of elementary molecules
of _the same kind_, whereas the integral molecules of compound gases
and vapours are made up of elementary molecules _of different kinds_.
The _elementary molecule_ of Avogadro is now termed the _atom_; his
_integral molecule_ we call simply a _molecule_. Similar conceptions
were published independently by Ampère in 1814. It follows from the
doctrine of Avogadro and Ampère that, as the number of integral
molecules is the same in equal volumes of all gases, these molecules
must be equidistant from each other, their mutual distances depending
upon pressure and temperature. This at once serves to explain the laws
of Boyle and Dalton that gases, no matter what their chemical nature,
behave identically, as regards change of volume, when compressed by
pressure or expanded by heat.

The true significance of the hypotheses of Avogadro and Ampère was
long obscured, first on account of their imperfect appreciation by
the great leaders of chemical thought during the first half of the
nineteenth century—Berzelius, Gay Lussac, Wollaston, and Gmelin—and,
secondly, on account of the almost universal practice of deducing
atomic weights from purely chemical considerations of equivalence.
At the same time, it must be admitted that the recognition of the
value of these hypotheses was still further retarded by the seeming
anomalies which resulted from a more extended knowledge of the vapour
densities of elements and compounds. Thus the vapour densities of
mercury, sulphur, phosphorus, and arsenic, as ascertained by Dumas and
Mitscherlich, were plainly inconsistent with chemical analogies and
the law of Dulong and Petit. So, too, what appeared to be the vapour
densities of sal-ammoniac, phosphorus pentachloride, sulphuric acid,
calomel, and of other substances that might be mentioned, were not in
accordance with the values demanded by other well-ascertained facts.

By the middle of the nineteenth century the hypothesis of Avogadro
was practically forgotten and the law of volumes ignored. The atomic
weights of the elements, and the system of notation universally
employed in England and Germany, were based wholly upon equivalents.
The anomalies thus created were clearly pointed out by Gerhardt, and
subsequently by Laurent who showed how a consistent and harmonious
explanation of the facts could be reached by regarding as true
equivalents equal volumes—for example, of steam, ammonia, hydrogen
chloride, carbon dioxide, marsh gas, etc.; by assuming, in other words,
that equal numbers of the molecules of these various substances are
contained in equal volumes of the gases, as contended by Avogadro and
Ampère. The simplicity and consistency of the new notation gradually
won for it the adhesion of chemists. This adhesion was facilitated by
the memorable researches of Williamson on etherification, of Gerhardt
on the anhydrides, and by the work of Frankland on the radicals; and
it reached its logical conclusion when │Cannizzaro│—on the basis of
Avogadro’s hypothesis—discussed, in 1858, the true atomic weights of
the metallic elements as distinguished from their equivalent values.
In certain of its aspects the new table of atomic weights drawn up by
Cannizzaro resembled that originally proposed by Berzelius; but the
numbers adopted by the Swedish chemist were founded on no uniform or
rational basis, and were frequently inconsistent.

[Illustration: STANISLAO CANNIZZARO.]

Our present tables of atomic weight bring, therefore, the values
for the several elements into harmony with the doctrine of Avogadro
and Ampère, with the law of Dulong and Petit, and with the facts of
isomorphism. The values are, in fact, in unison with all the criteria
which serve to indicate the atomic weights of the elements. As a
result, our present system of notation, which is, of course, based upon
these atomic weights, assigns formulæ to compounds which indicate their
true relative molecular weights and simplify the accurate expression
of their relationships and chemical transformations. One by one the
instances of anomalous vapour density, which were so many stumbling
blocks to the universal acceptance of a system based upon the law of
gaseous volumes, have been shown to be not only not inconsistent with
it, but actually so many corroborative proofs. Thus, in the case of
ammonium chloride, the observed vapour density of which was found to
be practically half its calculated value, it has been proved that
the vapour of this salt, when heated to the temperature at which the
observations were made, is mainly resolved into molecules of ammonia
and hydrogen chloride, which together occupy double the space of the
ammonium chloride molecule. Phosphorus pentachloride vapour, on being
sufficiently heated, is similarly more or less resolved into phosphorus
trichloride and chlorine.

Moreover, it has been found, by a more accurate study of the action
of heat upon the vapour of ammonium chloride and of phosphorus
pentachloride, that within certain narrow limits of temperature these
substances can actually exist as such in the gaseous state, and that
the density of their vapours does actually conform to that demanded
by theory. Moreover, phosphorus pentafluoride, the analogue of the
pentachloride, is gaseous at ordinary temperatures, and has a normal
density. It may be heated to a high temperature without showing any
sign of decomposition.

Limitations of space will not allow of a fuller explanation of the
apparent anomalies already alluded to. They have each in turn been
experimentally attacked and satisfactorily explained. No valid
exception is now known to the universal applicability of the principle.

To-day the chemical history of a substance, whether elementary or
compound, if vaporisable, is not complete until its vapour density is
known, since a knowledge of this constant affords the most certain
means of establishing the relative weight of its molecule. Accordingly
many chemists have endeavoured to simplify and render more convenient
the modes of determining vapour densities. Thanks to the efforts of
Hofmann and Victor Meyer, the processes associated with the names of
Dumas, Gay Lussac, Deville, and Troost, which have furnished us with
valuable information in the past, have now given way to comparatively
simple and rapid methods, which, although not necessarily more
accurate, furnish the required information with less expenditure of
time and trouble; that is to say, they serve to indicate which of
two, or more, presumed molecular weights is correct, and so enable
us to establish the molecular formula of the substance. The chemical
formula of a substance is a condensed expression of a number of facts
connected with its history. Thus the expression H2O—the chemical
formula for water—indicates that the substance is composed of hydrogen
and oxygen, in the proportion, using round numbers, of 2 parts by
weight of hydrogen and 16 parts by weight of oxygen; or, in other
words, of 2 atoms of hydrogen, each weighing 1, and 1 atom of oxygen
weighing 16. The formula, moreover, connotes the fact that when the
gases combine 2 volumes of hydrogen unite with 1 volume of oxygen to
form 2 volumes of water-vapour (steam). So, too, the formula HCl—which
represents hydrogen chloride—means that the substance is a compound of
1 atom of hydrogen weighing 1 united with 1 atom of chlorine weighing
35.5; it also denotes the fact that in the act of union 1 volume of
hydrogen combines with 1 volume of chlorine to form 2 volumes of
hydrogen chloride. Lastly, the formula NH3 signifies that the molecule
of ammonia is composed of 1 atom of nitrogen weighing 14, and 3 atoms
of hydrogen, each weighing 1; it further indicates that when ammonia
gas is resolved into its constituents, as it can be when sufficiently
heated, 2 volumes of ammonia gas are increased to 4 volumes of a
mixture made up of 1 volume of nitrogen and 3 volumes of hydrogen.

In short, all the formulæ are what are called _two-volume formulæ_;
that is, the relative molecular weights of the substances occupy
the same volume as two relative parts by weight of hydrogen. Hence
their vapour densities—referred to hydrogen as unity—are the halves
respectively of their molecular weights. Steam is 9 times, hydrogen
chloride 18.25 times, and ammonia 8.5 times heavier than hydrogen, when
measured under identical conditions of temperature and pressure. The
quantities expressed by the formulæ H2, H2O, HCl, NH3, occupy the same
volume. This is equally true of the quantities expressed by O2, N2,
Cl2, etc. These expressions signify that the molecules of the elements
of hydrogen, oxygen, nitrogen, and chlorine consist each of 2 atoms of
the respective substances: the molecule of water consists of 3 atoms—2
of hydrogen and 1 of oxygen; the molecule of hydrogen chloride of 2
atoms—1 of hydrogen and 1 of chlorine; whereas the molecule of ammonia
contains 4 atoms—1 atom of nitrogen and 3 atoms of hydrogen.

Certain of the elementary bodies are, as already stated, capable of
existing in different allotropic states. Thus there is a modification
of oxygen known as _ozone_. This substance has long been recognised as
being formed from air under the influence of the electric discharge. It
was the subject of study by Schönbein as far back as 1839; but that it
was a condensed form of oxygen and not a peroxide of hydrogen—as was at
one time surmised—was first established by Andrews and Tait. The degree
of its condensation was definitely ascertained by Soret by observing
its rate of diffusion, from which, on the basis of Graham’s law, its
density could be inferred. It was found to be 1½ times that of ordinary
oxygen. Hence, if the molecule of oxygen consists of 2 atoms, that of
ozone consists of 3 atoms. The chemical symbol of ozone is, therefore,
O3.

It has been found also that the molecule of sulphur, in the state of
solution, contains eight atoms. This complex molecule in the gaseous
state gradually breaks down as the temperature is increased, and at
temperatures above 850° contains, like its analogue oxygen, only
two atoms. The molecules of phosphorus and arsenic, in the gaseous
state, are each found to consist of four atoms. On the other hand, the
molecules of mercury, zinc, and cadmium each consist of only one atom.
As will be shown later, Kundt and Warburg established, in 1875, the
fact that mercury vapour is a monatomic gas, by determining the rate at
which sound is propagated through it. In the same way it was shown that
helium, argon, and its congeners are also, as already stated, monatomic
gases.

The applicability of the law of Dulong and Petit to the determination
of atomic weights has been frequently exemplified during the last
sixty years; and a number of these constants have been rectified
by its means—_e.g._, of thallium, uranium, glucinum, indium, etc.
The anomalies presented by the cases of elements of low atomic
weight—_e.g._, carbon, boron, silicon—have been further inquired into;
and it has been shown by Weber, and independently by Dewar, that in the
case of these substances the specific heat rapidly increases with the
temperature, and approximates at high temperatures to a value required
by the law of Dulong and Petit.

Within recent years other methods of ascertaining molecular weights
have been put at the disposal of chemists. These methods are especially
valuable in the case of bodies which cannot be volatilised. They depend
upon the influence of the substance (1) upon the freezing-point and
(2) upon the boiling-point of a solvent. It has long been known that a
substance in solution affects the freezing-point of the solvent, and
in the great majority of cases depresses it. Sir Charles Blagden,
as far back as 1788, showed that in aqueous solutions of inorganic
salts the depression was proportional to the amount dissolved. It was
subsequently found by Coppet that, in a number of solutions of similar
salts where these were present in the ratio of their molecular weights,
the solutions froze at practically the same temperature: the molecular
depressions of the freezing-points differ from group to group but are
nearly equal in groups of similar compounds. Raoult further observed
that, when certain quantities of the same substance are successively
dissolved in a solvent on which it exerts no chemical action, there
is a progressive lowering of the point of solidification of the
solution, and this depression is proportional to the weight of the
substance dissolved in a constant weight of the solvent. In the case
of a large number of solvents the depressions of the freezing-point,
calculated for amounts proportional to the molecular weights of the
dissolved substance, were nearly constant. Raoult pointed out that
these relations between the molecular weights and the lowering in
the freezing-point may be employed to determine the molecular weight
of a soluble substance. The molecular weight _m_ is found from the
expression _m_ = K/A, where A is the quotient obtained by dividing
the observed depression in the freezing-point of the solvent by the
percentage content of the solution, and K (the molecular depression)
is a constant dependent on the solvent. Thus in the case of phosphorous
oxide it was found that 0.6760 gram added to 20.698 grams of benzene—in
which the oxide is soluble without change—lowered the freezing-point of
the 3.16 per cent. benzene solution by 0°.68. Since the value of K for
benzene is 49, we have (3.16 × 49)/0.68 = 227, which serves to indicate
that P4O6 is the true molecular formula for phosphorous oxide. This
result is confirmed by vapour-density observations.

The effect of adding a substance to a solvent is to diminish the
vapour pressure of the liquid. Hence, since the boiling-point of a
liquid is that temperature at which the vapour pressure is equal to
the atmospheric pressure, the effect of adding the soluble substance
is to raise the boiling-point, since a higher temperature is required
in order that the pressure of the vapour shall equal that of the
atmosphere. It has been proved that equal volumes of solutions in the
same solvent which have the same boiling-point contain an equal number
of molecules of the dissolved substance.

The equation for the molecular increment of the boiling-point for
a solvent is _d_ = 0.02T²/_w_, in which _d_ is the increment of
the boiling-point caused by the solution of one gram-molecule of a
substance in 100 grams of the solvent, T the _absolute_ boiling-point
of the solvent, and _w_ the heat of vaporisation of the solvent for one
gram. The molecular rise of the boiling-point is therefore independent
of the nature of the dissolved substance.

The molecular weight of the substance _m_ is obtained from the
formula _m_ = _pd_/Δ, in which _p_ = the percentage weight of the
dissolved substance, _d_ = the molecular increment in boiling-point
(0.02T²/_w_), Δ = the observed rise in boiling-point. If the latent
heat of vaporisation of the liquid is unknown, the value of _d_ may be
obtained by preliminary experiments with a substance of known molecular
weight; in this case _d_ = _m_Δ/_p_.

The calculation of the molecular weight _m_ may also be made by the
formula _m_ = K(_s_/ΔL) × 100, in which Δ is the rise in boiling-point,
_s_ the weight of dissolved substance, L the weight of solvent, and K
the molecular boiling-point increment. Convenient forms of apparatus
for using these methods have been devised by Beckmann, and are now in
general use.

       *       *       *       *       *

From the time of Berzelius, each successive generation of chemists has
striven to better the example of that master of determinative chemistry
in the effort to obtain accurate values for the atomic weights of the
elements.

Among the immediate successors of Berzelius in this work should be
mentioned Turner, Penny, Dumas, and Marignac. Dumas in 1859 published
the results of an extensive revision of the atomic weights of the
elements. On this he based the far-reaching generalisation that, in
the language of Prout, “the combining or atomic weights of bodies bear
certain simple relations to one another, frequently by multiple, and
consequently that many of them must necessarily be multiples of some
one unit.” Dumas further agreed with Prout that “there seems to be no
reason why bodies still lower in the scale than hydrogen (similarly,
however, related to one another, as well as to those above hydrogen)
may not exist, of which other bodies may be multiples, without being
actually multiples of the intermediate hydrogen.”

The Belgian chemist, Stas, who had been associated with Dumas in a
classical determination of the atomic weight of carbon, set himself
to determine, with the highest degree of precision then possible,
the atomic weights of about a dozen of the elements, with a view of
ascertaining (1) whether an atomic weight is a definite and constant
quantity, or whether, as suggested by Marignac, and subsequently by
Crookes, an atomic weight represents “a mean value around which the
actual weights of the atoms vary within certain narrow limits”;
(2) whether, if the atomic weights of the elements are respectively
definite and invariable, the numbers are commensurable as alleged by
Prout and Dumas; and (3) if it should turn out that the numbers are
severally fixed and commensurable, whether this necessarily indicates
that the elements are built up of a primordial matter, the πρώτη ιλη of
the ancients, referred to by Prout in 1816.

Stas devoted many years to the solution of these questions, working
on a scale and with an accuracy and manipulative skill previously
unapproached. The main results of his labour appeared in 1865. He
concluded (1) that the atomic weights of the elements are absolutely
constant values, and are not affected by the nature of the compounds in
which they occur, or the physical conditions of their existence; (2)
that the numbers so obtained are not commensurable: to quote his own
words: “_On doit considérer la loi de Prout comme une pure illusion._”
Hence the elements must, on the basis of Stas’s experimental evidence,
be regarded as “_individualités à part_,” as he expressed it—each a
primordial and unalterable substance.

The appearance of this monumental work, which will ever remain one
of the classics of chemistry, created a great impression. Its effect
persists to this day. It constituted a model and furnished a standard
which each successive worker has striven to emulate, with the result
that atomic weights to-day are among the best ascertained of physical
constants.

Space will not permit of any detailed account of the work done in
connection with atomic weights during the forty-five years which have
elapsed since the publication of Stas’s memoirs; and the reader who
desires fuller information must be referred to the special treatises on
the subject, such as the _Constants of Nature_ of F. W. Clarke, or the
monographs of Meyer and Seubert, Becker, Sebelien, and Van der Plaats.

Reference, however, must be made to the determinations by Lord
Rayleigh, Leduc, Morley, Noyes, Guye, Dixon, and Edgar of values which,
like those of oxygen, hydrogen, nitrogen, silver, and the halogens, are
largely made use of as fiduciary values in atomic-weight work.

Lastly, it should be mentioned, the re-determination of the atomic
weights of the elements with the highest attainable precision and by
the most refined and most modern methods has for some years past been
a special feature of the work of the Harvard Laboratory, under the
direction of Theodore Richards; and some of the most trustworthy and
best established values we possess have been ascertained by him and
his pupils. Atomic weights are of such fundamental importance that the
various nations interested in the pursuit of chemistry have consented
to the establishment of an International Committee, which will take
cognisance of the work done from time to time in this department of
operative chemistry, examine and assess its value, and draw up an
annual report on the subject.

Despite the accumulated testimony of this work in relation to the
validity of the law of the conservation of mass, the sufficiency of the
evidence has now and again been impugned. This aspect of the matter
has within recent years been directly investigated by Landolt; and
as the result of a painstaking series of experiments, in which every
recognised source of error was removed or allowed for, it would appear
that there is absolutely no ground for the belief that there is any
dissipation of mass in the course of, or as the result of, a chemical
change.




CHAPTER V

THE MOLECULAR THEORY OF GASES


The more obvious physical phenomena of gases were, of course, well
known by the middle of the nineteenth century; and the so-called
gaseous laws—the laws of Boyle, Dalton, and Gay Lussac—were universally
accepted by chemists and physicists at that time as fundamental. That
the first two laws were only approximations to truth in a mathematical
sense was also well known; and the experimental labours of Regnault and
Magnus had not only established limits within which they were inexact,
but had, to some extent, indicated the cause of their departure from
the ideal condition. The hypothesis of Avogadro, as already stated,
was practically ignored at this period, or at least its value was
unappreciated until the time of Gerhardt and Laurent, and more
particularly Cannizzaro, who in 1858 pointed out its real meaning and
made it the keystone in the edifice of modern chemistry.

One of the most significant achievements of the last half-century has
been the demonstration that these gaseous laws are interdependent.
Their further study, and in particular the study of their variation
from exact mathematical expression, has led to a conception of the real
nature of a gas which not only comprehends and knits together these
laws, but affords a rational explanation of them. If the laws of Boyle
and Dalton concerning the relations between pressure, temperature, and
gaseous volume are, and must be from the very nature of the case, only
approximations, it follows that the same is equally true of the laws of
Gay Lussac and Avogadro, since these are dependent on the others. The
definite experimental proof that gases do not actually combine in the
precise ratios demanded by the law of Gay Lussac has been forthcoming
only during the last twenty years. It has been found that, instead
of oxygen and hydrogen combining in the exact ratio of one volume of
oxygen to two volumes of hydrogen to form water, as stated by Gay
Lussac, one volume of oxygen combines with, according to Scott, 2.00245
vols.; according to Leduc, 2.0024 vols.; according to Morley, 2.00268
vols. of hydrogen. What is true of the volume-ratios in which oxygen
and hydrogen are actually found to combine, under ordinary conditions,
is no doubt equally true of analogous instances, such as the union
of hydrogen and chlorine to form hydrogen chloride. It follows also
that the extent of the variation from the mathematical expression of
Gay Lussac’s law must get smaller and smaller as the combining gases
approach the condition of the ideal gas—as they do, for example, under
very low pressures. The precise degree of departure from Gay Lussac’s
law is therefore in a sense accidental, and is dependent upon the
conditions under which combination takes place.

The more exact study of the physical phenomena of gases, and in
particular the clearer recognition of the causes which determine the
extent of their departure from the ideal gaseous laws, have afforded
valuable assistance in ascertaining the atomic weights of certain
elements independently of chemical considerations. The processes of
physical measurement have been so refined within recent years that
physical methods of arriving at molecular—and, inferentially, at
atomic—weights, in the case of all elements and compounds which can
be brought into a condition approaching that of the ideal gas, are to
be preferred to the gravimetric methods of analysis or synthesis as
affording the most probable values of the true atomic weights of the
elements. The work of Lord Rayleigh, Leduc, and of Guye and his pupils
on the densities of the gases has furnished us with a series of values
for the atomic weights of a number of the elements which, in point of
accuracy, are as superior to the values of Stas as the values of Stas
were superior to those of his predecessors. Daniel Berthelot pointed
out in 1898 that the true molecular weight of a gas can be deduced
from its density and its observed variations from Boyle’s law under
atmospheric pressure and at very low pressures. Incidentally, the study
of gaseous phenomena has served to place the theory of atoms upon a far
more stable foundation than it occupied half a century ago. How halting
was the adhesion which even some of the most eminent chemists then gave
to this theory was well exemplified by the remarkable lecture given
before the Chemical Society of London in 1869, in which Williamson—one
of the most sturdy champions of Dalton’s doctrine—set forth its true
value.

That a gas may be looked upon as an association of particles—hard
elastic spheres—moving backwards and forwards in right lines with great
velocity, and possessing in the aggregate a very small proportion of
the space through which they travel, was first conceived by Daniel
Bernoulli in 1738. By means of this hypothesis he explained the direct
proportionality between the density and pressure of a gas. If the
gas consists of moving particles, and the pressure which it exerts
on the sides of the containing vessel is due to the impacts of these
particles, it is obvious that by halving the original volume of the
containing space we halve the space through which the particles travel,
and therefore double the number of their impacts in a given time; in
other words, by compressing the gas to half its initial volume we
double the pressure it exerts, which is nothing else than the law of
Boyle. This conception of the nature of a gas is known as the _kinetic
theory of gases_; it was further developed by Waterston in 1845, still
more fully by Clausius in 1857, and was subsequently placed in its
present position by Maxwell and Boltzmann.

That gases actually do move, and at rates depending on their specific
nature, was rendered probable—apart from this explanation of Boyle’s
law—by many phenomena observed by chemists and physicists in the
eighteenth and early part of the nineteenth century. It was known from
the observations of Leslie in 1804 that specifically light gases moved
or diffused faster than heavy gases. Attempts to determine these rates
were made by Schmidt in 1820, and by Graham in 1846, both of whom found
that the rate of movement of a gas was independent of its chemical
nature, and was determined solely by its mass: _gases move at rates
inversely proportional to the square roots of their densities_. The
following table given by Graham shows the experimentally ascertained
relative rates for a number of gases compared with the rates demanded
by the “_law of gaseous diffusion_.” Column one gives the name of the
gas; column two, the observed rate of diffusion; and column three, the
square root of the density of the gas (air = 1):

       _Gas._    _Time of diffusion._  _√density._

    Air               1                   1
    Hydrogen          0.276               0.263
    Marsh gas         0.753               0.745
    Ethylene          0.987               0.985
    Nitrogen          0.986               0.986
    Oxygen            1.053               1.051
    Carbon dioxide    1.203               1.237

Nitrogen and ethylene are, chemically, totally dissimilar gases, but
they have the same density and hence the same rate of movement. As
Graham showed, it is possible to separate more or less completely a
mixture of gases, if the constituents are of different densities,
by taking advantage of their different rates of movement. Such an
_atmolytic_ method was employed by Rayleigh and Ramsay to prove that
atmospheric nitrogen contained argon.

The fact that all gaseous substances, however different their chemical
nature, conform in the main to certain simple “laws” indicates
the probability that their mechanical structure is similar and
comparatively simple. The so-called gaseous “laws”—the laws of Boyle,
Dalton, Gay Lussac, Avogadro, and Graham—are to-day explained on
the assumption that a gas consists of an aggregation of molecules,
moving incessantly in straight lines and with great rapidity. The
rate of movement of the particles is variable by reason of their
mutual encounters; at the same instant some are moving rapidly, others
more slowly. As already explained, to this ceaseless movement of the
molecules is to be ascribed the pressure they exert; the pressure which
a gas exerts on any containing surface is the aggregate effect of the
impact of its molecules. The law of Boyle states that the product of
the volume V and pressure P of a given mass of gas is invariable so
long as the temperature is unchanged: PV = constant. It was found
by Regnault, Magnus, Natterer, and Amagat that all gases, with the
exception of hydrogen, show a departure from Boyle’s law in the sense
that PV is less than theory demands. In the case of hydrogen PV is
greater than theory. This exception, however, is only apparent. Every
gas, if maintained above a certain temperature, shows, after a certain
pressure has been reached, a deviation in the same sense as that
exhibited by hydrogen.

The deviations from Boyle’s law are probably due to two causes: (1) to
the effect of cohesion among the molecules, whereby the volume, and
hence PV, is less than theory requires; (2) the molecules are not
mathematical points—they have a certain volume; hence, with increasing
pressure, PV is greater than theory demands. The effect of the molecule
having a certain magnitude will be clear from the following figure: Let
M be a molecule moving backwards and forwards within a certain space,
_a b_:

        | M                             |
    _a_ |                               | _b_
        | •                             |
        |                               |

Assume, now, we halve the containing space:

        | M             |
    _a_ |               | _b_
        | •             |
        |               |

It will be seen that M, since its volume is unchanged, will have less
than half the original distance to travel or, in other words, it
will strike the boundaries of the containing space _more_ than twice
as frequently in the same interval of time as before; hence P, and
therefore PV, becomes greater than Boyle’s law demands.

It will be noticed, then, that the two causes tending to bring
about deviations from Boyle’s law act in contrary directions. In
the greater number of gases the effect due to cohesion at ordinary
pressure is greater than the effect due to the actual space occupied
by the molecules. In the case of hydrogen at ordinary temperature the
contrary is the case; if, however, hydrogen is strongly cooled, it
shows variations similar to those exhibited by other gases at ordinary
temperatures. By heating these gases the effect due to cohesion—to
the mutual attraction of the molecules—becomes less and less; in such
circumstances these gases show departures from theory in the same sense
that hydrogen does at ordinary temperatures.

The effect of mutual attraction among the molecules is to make the
volume of the gas less than the theoretical value; the cohesive force
may therefore be regarded as equal in effect to a certain additional
pressure; that is, (P + A) V = constant, in which A is the measure of
the force of cohesion. A, of course, must have relation to the number
of molecules mutually attracted: _A is proportional to the square of
the number of the molecules_. But the number of the molecules in the
unit volume is proportional to the density of the gas, and in a given
mass of gas the density is inversely proportional to the volume. Hence
A is inversely proportional to the square of the volume—A = _a_/V2,
hence (P + _a_/V2) V = constant. Now let us trace the effect of the
second cause of variation from the mathematical exactitude of Boyle’s
law. The fact that the molecules are not mathematical points means that
V in the foregoing expression is not identical with the space in which
the molecules move. That space is V - _b_, in which _b_ is the measure
of the aggregate volume of the molecules. Hence the true expression
becomes (P + _a_/V2) (V - _b_) = constant.

The law of Dalton (Charles) also receives its simplest explanation by
the kinetic theory of gases; and, moreover, the departures from the
mathematical truth of the statement follow as a necessary consequence
of the facts that the molecules have sensible magnitudes and are
mutually attracted. We can measure the effect of heat upon a gas
in two ways. We can either keep the pressure of the gas constant,
and measure the increase in volume; or we can prevent the gas from
expanding, and measure the elastic force or pressure it exerts. If the
law of Dalton were mathematically true, it would follow that, _if the
volume of the gas were maintained constant during the heating, its
pressure would increase in the same proportion as the volume would have
increased if the gas had been allowed to expand, but maintained at a
constant pressure_. In other words, the expansion-coefficient and the
pressure-coefficient should be the same. Experiment shows, however,
that they are not identical.

The following table gives the results of a number of measurements by
Regnault:

                       _Expansion     _Pressure
                       (Pressure       (Volume
                       Constant)._    Constant)._

    Hydrogen            .003661        .003667
    Air                 .003670        .003665
    Carbon dioxide      .003710        .003688
    Sulphur dioxide     .003903        .003845

Variations in the same sense have since been observed by Jolly and
Chappuis. With the exception of hydrogen, and probably also helium, all
the gases show greater values for the coefficient of expansion than
for the coefficient of pressure, and the differences are greater the
greater the coefficient of expansion of the gas.

Since, as we have already stated, the law of Boyle is directly related
to the law of Dalton, both being dependent on molecular movements, the
same course of reasoning used to account for the variations in the case
of Boyle’s law applies equally to the case of Dalton’s law. The “law”
of Avogadro follows also as a necessary consequence of this explanation
of the laws of gaseous pressure and temperature. If all gases show
approximately the same increase in pressure when heated under constant
volume, and if the increased pressure is due only to the increased
energy with which the molecules strike the sides of the containing
vessel, it follows that all gases must contain the same number of
molecules in unit volume. But as, from the very nature of the case, the
laws of Boyle and Dalton cannot be mathematically true, it follows that
the laws of Avogadro and Gay Lussac must be only approximations in the
same sense.

The law of Graham, connecting the rate of diffusion of a gas with its
density, follows also as a necessary consequence of this explanation
of the laws of Boyle, Dalton, Gay Lussac, and Avogadro. If the number
of molecules in the unit volume of any gas, whatever be its nature and
whatever be their mass, is approximately the same, it follows that the
mean velocity of the molecules must be variable; their mean velocities
must be in the inverse ratio of the square roots of their densities.

The mean velocity with which the molecules of a gas move can be
calculated if we know the pressure it exerts, the weight of a definite
volume, and the value of the acceleration due to gravity. The square
of this velocity in metres per second of time at 0°C. is given by the
expression U² = 3_pg_/_q_ in which _p_ = pressure per square metre =
10,333 kilograms; _g_ = the gravitation constant = 9.81; _q_ = weight
of a cubic metre of the gas at 0°C. and one atmosphere of pressure.

For hydrogen we have

U² = 3 × 10,333 × 9.81/0.0899

whence U = 1842 metres per second; for oxygen we have U² = 3 × 10,333
× 9.81/1.430, whence U = 461. These numbers accord with those demanded
by Graham’s law. The density of H being taken as 1, that of oxygen is
16 and √16 = 4; the numbers 1842 and 461 are in the ratio of 4 to 1.

The amount of heat required to raise the temperature of the unit mass
of a gas through a definite interval depends, as Laplace first pointed
out, upon whether the gas is allowed to expand or not; in other words,
the specific heat of a gas varies as the heating is at constant volume
or at constant pressure. If, having raised the temperature of the unit
mass, and so expanded it, we then compress it until it occupies its
initial volume, a further rise of temperature takes place without any
external heat having been applied. This rise of temperature is, in
fact, due to the liberation of the amount of heat required merely to
expand the gas without increasing its temperature. The quantity of heat
needed to raise the temperature of a gas through a definite interval is
therefore greater when it is allowed to expand than when its volume is
kept constant; in other words, the specific heat at constant pressure
is greater than the specific heat at constant volume. The ratio of
the two specific heats can be calculated: on the assumption that
the energy imparted to the molecules simply accelerates their mean
rectilinear velocity, and that no energy is absorbed in doing internal
work among them, it is found that, when the gas is permitted to expand,
the amount of heat required is 1.67 times greater than that needed
when its volume is kept constant. This ratio has been experimentally
determined for a number of gases. For oxygen under normal conditions it
is 1.408, for hydrogen 1.414, for carbon dioxide 1.264, for methane
1.269—all numbers notably below the value 1.67. The direct experimental
determination of this ratio by thermometric measurements is a matter of
some difficulty. It was, however, demonstrated by Dulong that it can
be ascertained with comparative ease from observations on the velocity
of sound in the gas—the velocity being probably a direct function of
this ratio. As carried out experimentally, the method consists in
sending a sound-wave through the gas contained in a glass tube along
the horizontal length of which is strewn a quantity of a light powder
such as the spores of lycopodium or finely divided silica. The glass
tube is fitted at one end with a glass rod; by rubbing this a series
of longitudinal vibrations is set up and communicated to the gas
whereby the light powder is thrown up into little heaps along the tube,
the distance between the heaps being equal to half a wave length. By
comparative measurements with air and the gas under examination, data
are obtained from which the ratio of the specific heats can be deduced.

By experiments conducted on this principle Kundt and Warburg found that
mercury vapour gave numbers agreeing with the theoretical ratio 1.67.
Now, its vapour density shows that mercury vapour is a monatomic gas;
it actually fulfils the conditions prescribed for a gas which theory
indicates should give the value 1.67. All the energy imparted to its
molecules on heating simply accelerates their translational velocity.
On the other hand, all the gases above named as giving values below
1.67 are diatomic gases; in their case the energy imparted to them
is employed partly in augmenting the translational velocity of the
molecules, and partly in bringing about internal changes within them.
By experiments made in like manner Ramsay and Travers succeeded in
showing that the inert gases of the atmosphere are monatomic.

No attempt can be made here to explain the various methods by which
it has been sought to obtain an estimate of the absolute size of
gaseous molecules or to determine their number in a definite volume. By
observations on their viscosity, rates of diffusion, conductivity for
heat, variations from the law of Boyle, dielectric constants, electric
charges, etc., Maxwell, O. E. Meyer, Loschmidt, Lothar Meyer, Van der
Waals, Mossotti, Planck, Sir J. J. Thomson, and others, have arrived
at estimates of the magnitude and number of molecules in a gas. These
estimates necessarily vary with the hypotheses made in deducing them.
It would serve no useful purpose to give the results, since the figures
convey no impression to the mind of the minuteness of molecules, or
even as to the extraordinary number of them in, say, so small a volume
as one cubic centimetre. As an example, it has been calculated that
there are about 640 trillions of hydrogen molecules in one milligram of
the gas.[3]

    [3] O. E. Meyer, _The Kinetic Theory of Gases_, 1899.

       *       *       *       *       *

In the preceding volume a short account has been given of the history
of the early attempts to effect the liquefaction of the gases. These
resulted in their division into the two classes of _liquefiable_ and
_permanent_ gases. One of the most notable achievements of the latter
half of the last century was to sweep away this arbitrary distinction.
The fundamental condition needed to effect the liquefaction of a
gas, although surmised by Faraday, was first clearly indicated by
Andrews about 1863. He showed that, in order to liquefy a gas, its
temperature must be lowered to a point peculiar to each gas, when, on
the application of sufficient pressure, it will become a liquid. Thus,
in the case of gaseous carbon dioxide, Andrews found that, if its
temperature were maintained above 31° C., no amount of pressure would
cause it to liquefy; if the temperature were lowered just below this
point—termed the _critical point_—a pressure of 75 atmospheres would
effect its liquefaction. On the other hand, if the temperature of the
liquid carbon dioxide be slowly raised to about 31°, the surface of
demarcation between the liquid and the gas becomes gradually fainter
and eventually disappears. Carbon dioxide may thus be made to pass
from the state of liquid to that of gas without any sudden alteration
of volume. If a given volume of the gas, say at 50°, be exposed to
gradually increasing pressure, say up to 150 atmospheres, the volume
is gradually diminished with the increment of pressure, but no sudden
contraction indicating liquefaction occurs. If the gas under the high
pressure be allowed to cool down to the ordinary temperature, no
sudden contraction is observed to follow. The carbon dioxide, at the
outset a gas, in the end becomes a liquid by a gradual and continuous
transition, unaccompanied by any abrupt change of volume. These
observations show that what we style the liquid and gaseous states
are simply separated manifestations of the same condition of matter.
There is a definite temperature for every gaseous substance at which it
ceases to be liquefiable under pressure; and the reason that Faraday
failed to liquefy certain gases was that he was unable, with the means
at his command, to lower their temperatures sufficiently and so reach
their critical points; hence the enormous pressures which he and other
investigators applied were unavailing. These facts were definitely
made known by Andrews in 1869, were theoretically developed by Van
der Waals in 1873, and practically applied to the liquefaction of
oxygen in 1877, independently and almost simultaneously, by Pictet, of
Geneva, and Cailletet, of Châtillon-sur-Seine. Pictet exposed oxygen,
under great pressure, to the cold produced by the rapid evaporation
of liquid carbon dioxide; Cailletet brought about the same result by
suddenly diminishing the tension of the strongly compressed oxygen, the
rapid expansion of the gas effecting the reduction of its temperature
below the critical point. Other workers took up the subject, notably
Wroblewski and Olszewki in Poland, Dewar in England, and Kammerlingh
Onnes in Holland; and the liquefaction of all the gases has now been
accomplished.

The following table shows the _absolute_ boiling-(B.P.) and
melting-points (M.P.), critical points (C.P.), and pressures
(C.Press.), together with the density (D) at their boiling-points of a
number of liquefied gases:

                                                     C.
                   B. P.       M. P.       C. P.    Press.    D.
                 _degrees_   _degrees_  _degrees_   _m._

    Helium          4.5           ——        ——       ——     0.15
    Hydrogen       20             15        35      11.6    0.06
    Oxygen         90.5     below 50       154      44      1.131
    Nitrogen       77.5           60       124      20.9    0.791
    Methane       108.3           ——       191      42.4    0.416
    Ethylene      169.5          104       282      44      0.571
    Fluorine      186             40        ——       ——     1.11
    Chlorine      239.6           ——        ——       ——     1.507
    Ammonia       234.5          197.5     404      85.9     ——
    Neon           30.40          ——  below 65       ——      ——
    Argon          86.90          ——       155.6    40.2    1.212
    Krypton       121.33          ——       210.5    41.2    2.155
    Xenon         163.9           ——       287.8    43.5    3.52

The principle of the Cailletet method of effecting the liquefaction of
oxygen had been theoretically and experimentally studied by Joule and
Lord Kelvin many years previously. It was extended by Siemens and has
been applied by Linde and Hampson to the construction of machinery for
the production of liquid air on a large scale, without the use of any
intermediate refrigerant.

It is now readily possible to procure considerable quantities of liquid
air, and even of liquid hydrogen. By the evaporation of liquid hydrogen
temperatures approaching the _absolute zero_—that is, 273° C. below
the melting-point of ice—can now be reached. Incidentally there has
been developed a special field of inquiry relating to the behaviour of
substances at low temperatures.

[Illustration: SIR JAMES DEWAR.]

The pioneers in this field have been Dewar in England and Kammerlingh
Onnes in Holland. Research at low temperatures, indeed, has been
the main feature of the work of the Royal Institution of Great
Britain during the last twenty years. It has included observations
at temperatures approaching the absolute zero, on the electrical
resistivity of metals and alloys, on the behaviour of so-called
insulators, on changes in the cohesive force of metals, on the
dielectric constants of frozen electrolytes, on the influence of cold
on magnetisation and on magnetic permeability, and on the optical
behaviour of bodies, on vital phenomena at low temperatures, and on the
influence of cold on chemical change.

Dewar has succeeded in liquefying and solidifying large quantities
of hydrogen, and has studied its properties at low temperatures.
Liquid hydrogen is transparent and colourless. It is a non-conductor
of electricity, and gives no absorption spectrum. It freezes into an
ice-like solid, devoid of metallic properties. Dewar has made use of
the property possessed by charcoal of occluding gases, especially
at low temperatures, in the production of high vacua, and in the
separation of gases; and he has also determined the molecular heat
of absorption by charcoal of various gases. He has employed liquid
air, liquid nitrogen, and liquid hydrogen as calorimetric agents, and
has determined by means of them the heat capacities of a number of
substances at very low temperatures. Lastly, his ingenious contrivance
of silvered vacuum protected vessels, now introduced into commerce
under the name of “Thermos flasks,” has greatly facilitated the
manipulation of liquefied gases for experimental purposes.




CHAPTER VI

THE PERIODIC LAW


In an anonymous essay “On the Relation between the Specific Gravities
of Bodies in their Gaseous State and the Weights of their Atoms,”
published in Thomson’s _Annals of Philosophy_ in 1815, the attempt
was made to indicate certain consequences which seem to follow from
Dalton’s law of gaseous volumes, as generalised by Gay Lussac. The
author of this essay was subsequently discovered to be a medical
student named William Prout, noteworthy as having been one of the
first to point out the suggestiveness of the numerical relationships
which occur among the atomic weights of the elements. This paper is
usually assumed to contain the statement that the atomic weights of the
elements are multiples of that of hydrogen. As a fact, however, this
hypothesis is nowhere explicitly stated in the paper. The inference was
practically due to Thomson, who strove to support it by experimental
proof of so weak a character as to draw forth the remark of Berzelius
that much of it appeared to have been made at the writing-desk.

Nevertheless, the occurrence of such numerical relationships continued,
as already stated, to excite speculation. Döbereiner, in 1829, pointed
out that in certain groups of correlated elements, consisting each
of three members, the middle member had an atomic weight practically
identical with the arithmetic mean of the atomic weights of the others;
and similar observations were made by Gmelin, Dumas, Gladstone, and
Strecker. An approach to the recognition of the general law underlying
these facts was made by Newlands in England, and independently by
De Chancourtois in France, who were the first to indicate that the
properties of the elements are related to their atomic weights. This
conception was developed by the Russian chemist, Mendeléeff. In
Mendeléeff’s arrangement, first published in 1869, the elements are so
grouped that their properties are periodic functions of their atomic
weights. The general statement of what is now known as the Periodic
Law may be put in this form: If the elements are arranged in order
of increasing atomic weight, the properties of these elements vary
from member to member of the series, but return more or less nearly
to the same value at certain fixed points in the series. This is
observed to occur in the atomic value, or valency, of the several
members; also in their specific volumes, melting-points, ductility,
hardness, volatility, crystalline form, thermal expansion, refraction
equivalents, and conductivities for heat and electricity, in their
magnetic properties and electro-chemical behaviour, and in their heats
of chemical combination, etc.

The first chemist of note to grasp the significance of Mendeléeff’s
generalisation was Lothar Meyer, who, dealing at the outset with one of
the characteristic properties of the elements—viz., their specific or
atomic volumes (that is, the values obtained by dividing their specific
gravities into their respective atomic weights)—greatly developed
the principle of periodicity, representing it graphically in a most
striking and suggestive manner, leading up to a classification almost
identical with that of Mendeléeff.

Since the date of its promulgation the scheme of classification
of the elements in accordance with the principle of periodicity
has experienced certain minor modifications necessitated by fuller
knowledge; but in its essential features it remains very much in the
form devised by Mendeléeff. The discovery of the so-called inert and
radio-active elements required that their relations to the periodic law
should be defined. Their inclusion raises no fundamental difficulty.
Indeed, the generalisation seems to adapt itself to the far-reaching
considerations which spring from modern views of the nature of the
atom, its electro-chemical relationships, and the orderly arrangement
of the corpuscles of which it may be composed. In the 1905 edition
of the English translation of his famous _Principles of Chemistry_,
Mendeléeff has given a table which may be said to embody his final
views concerning the systematic classification of the elements. This
is reproduced on p. 105. In this table he postulates the existence of
two hypothetical elements, _x_ and _y_, the former of which he regards
as identical with the physical ether; while the latter is an analogue
of helium, possibly identical with the “coronium” of the solar coronal
atmosphere, with a molecular weight of about 0.4.

The striking feature of Mendeléeff’s generalisation is its
universality. In this respect it differs from all previous attempts
at natural classifications of the elements; these were limited and
partial, and therefore unsatisfactory. Nevertheless, it is easy to
trace in them fundamental conceptions upon which Mendeléeff built.
Mendeléeff, in fact, gave a great extension to ideas with which the
chemical world of half a century ago was more or less familiar; and
doubtless it was this circumstance, combined with the remarkable
boldness and comprehensiveness of this extension, followed almost
immediately by a most striking series of confirmations of his own
previsions, as logical consequences of his generalisation, that
secured for it attention, and ultimately universal adoption.

    -------+------------+------------+------------+------------+-
    Series.|Zero group. |  Group I.  | Group II.  | Group III. |
    -------+------------+------------+------------+------------+-
       0   |     _x_    |     ——     |     ——     |     ——     |
           |            |            |            |            |
       1   |     _y_    | Hydrogen.  |            |            |
           |            | H = 1.008  |     ——     |     ——     |
           |            |            |            |            |
       2   |  Helium.   |  Lithium.  | Beryllium. |   Boron.   |
           |  He = 4.0  | Li = 7.03  |  Be = 9.1  |  B = 11.0  |
           |            |            |            |            |
       3   |   Neon.    |  Sodium.   | Magnesium. | Aluminium. |
           | Ne = 19.9  | Na = 23.05 | Mg = 24.1  | Al = 27.0  |
           |            |            |            |            |
           |            |            |            |            |
           |            |            |            |            |
       4   |   Argon.   | Potassium. |  Calcium.  | Scandium.  |
           |  Ar = 38   |  K = 39.1  | Ca = 40.1  | Sc = 44.1  |
           |            |            |            |            |
           |            |            |            |            |
           |            |            |            |            |
       5   |     ——     |  Copper.   |   Zinc.    |  Gallium.  |
           |            | Cu = 63.6  | Zn = 65.4  | Ga = 70.0  |
           |            |            |            |            |
           |            |            |            |            |
           |            |            |            |            |
       6   |  Krypton.  | Rubidium.  | Strontium. |  Yttrium.  |
           |  Kr=81.8   | Rb = 85.4  | Sr = 87.6  |  Y = 89.0  |
           |            |            |            |            |
           |            |            |            |            |
           |            |            |            |            |
       7   |     ——     |  Silver.   |  Cadmium.  |  Indium.   |
           |            | Ag = 107.9 | Cd = 112.4 | In = 114.0 |
           |            |            |            |            |
       8   |   Xenon.   |  Cæsium.   |  Barium.   | Lanthanum. |
           |  Xe = 128  | Cs = 132.2 | Ba = 137.4 |  La = 139  |
           |            |            |            |            |
       9   |     ——     |     ——     |     ——     |     ——     |
           |            |            |            |            |
           |            |            |            |            |
           |            |            |            |            |
           |            |            |            |            |
       10  |     ——     |     ——     |     ——     | Ytterbium. |
           |            |            |            |  Yb = 173  |
           |            |            |            |            |
           |            |            |            |            |
           |            |            |            |            |
       11  |     ——     |   Gold.    |  Mercury.  | Thallium.  |
           |            | Au = 197.2 | Hg = 200.0 | Tl = 204.1 |
           |            |            |            |            |
       12  |     ——     |    ——      |  Radium.   |     ——     |
           |            |            |  Rd = 224  |            |
    -------+------------+------------+------------+------------+-

    -------+------------+------------+------------+------------+-
    Series.| Group IV.  |  Group V.  | Group VI.  | Group VII. |
    -------+------------+------------+------------+------------+-
       0   |     ——     |     ——     |     ——     |     ——     |
           |            |            |            |            |
       1   |            |            |            |            |
           |     ——     |     ——     |     ——     |     ——     |
           |            |            |            |            |
       2   |  Carbon.   | Nitrogen.  |  Oxygen.   | Fluorine.  |
           |  C = 12.0  | N = 14.04  |  O = 16.0  |  F = 19.0  |
           |            |            |            |            |
       3   |  Silicon.  |Phosphorus. |  Sulphur.  | Chlorine.  |
           | Si = 28.4  |  P = 31.0  | S = 32.06  | Cl = 35.45 |
           |            |            |            |            |
           |            |            |            |            |
           |            |            |            |            |
       4   | Titanium.  | Vanadium.  | Chromium.  | Manganese. |
           | Ti = 48.1  |  V = 51.4  | Cr = 52.1  | Mn = 55.0  |
           |            |            |            |            |
           |            |            |            |            |
           |            |            |            |            |
       5   | Germanium. |  Arsenic.  | Selenium.  |  Bromine.  |
           | Ge = 72.3  | As = 75.0  | Se = 79.0  | Br = 79.95 |
           |            |            |            |            |
           |            |            |            |            |
           |            |            |            |            |
       6   | Zirconium. |  Niobium.  |Molybdenum. |     ——     |
           | Zr = 90.6  | Nb = 94.0  | Mo = 96.0  |            |
           |            |            |            |            |
           |            |            |            |            |
           |            |            |            |            |
       7   |    Tin.    | Antimony.  | Tellurium. |  Iodine.   |
           | Sn = 119.0 | Sb = 120.0 |  Te = 127  |  I = 127   |
           |            |            |            |            |
       8   |  Cerium.   |     ——     |     ——     |     ——     |
           |  Ce = 140  |            |            |            |
           |            |            |            |            |
       9   |     ——     |     ——     |     ——     |     ——     |
           |            |            |            |            |
           |            |            |            |            |
           |            |            |            |            |
           |            |            |            |            |
       10  |     ——     | Tantalum.  | Tungsten.  |     ——     |
           |            | Ta = 183.0 |  W = 184   |            |
           |            |            |            |            |
           |            |            |            |            |
           |            |            |            |            |
       11  |   Lead.    |  Bismuth.  |     ——     |     ——     |
           | Pb = 206.9 |  Bi = 208  |            |            |
           |            |            |            |            |
       12  |  Thorium.  |     ——     |  Uranium.  |     ——     |
           |  Th = 232  |            |  U = 239   |            |
    -------+------------+------------+------------+------------+-

    -------+----------------
    Series.|  Group VIII.
    -------+----------------
       0   |       --
           |
       1   |
           |       --
           |
       2   |       --
           |
           |
       3   |       --
           |
           |{     Iron.
           |{   Fe = 55.9
           |{
       4   |{    Cobalt.
           |{    Co = 59
           |{
           |{    Nickel.
           |{  Ni = 59(Cu)
       5   |       --
           |
           |{  Ruthenium.
           |{  Ru = 101.7
           |{
       6   |{   Rhodium.
           |{  Rh = 103.0
           |{
           |{  Palladium.
           |{Pd = 106.5(Ag)
       7   |      --
           |
           |
       8   |      --
           |
           |
       9   |      --
           |
           |{    Osmium.
           |{   Os = 191
           |{
       10  |{   Iridium.
           |{   Ir = 193
           |{
           |{   Platinum.
           |{Pt = 194.9(Au)
       11  |      --
           |
           |
       12  |      --
           |
    -------+-----------------

The periodic law, in the words of its author, is the “direct outcome
of the stock of generalisations of established facts which had been
accumulated by the end of the decade 1860–1870.” It is founded
wholly on experiment, and is as much the embodiment of fact as are
the laws of chemical combination. It was based upon the adoption of
the definite numerical values of the atomic weights, as indicated
by Cannizzaro, as a consequence of the hypothesis of Avogadro, and
upon the assumption that the relations between the atomic weights of
analogous elements must be governed by a general law. The application
of the periodic law immediately led to the re-determination of certain
atomic weights and to the correction of their assumed atomic values.
At the time of its enunciation the determination of the valency of
an element was purely empirical, with no apparent necessary relation
to that of other elements. We find now that the valency is a matter
of _a priori_ knowledge, just as much as any other property of the
element. The amended values for the atomic weight and valency of a
number of elements thus demanded by the law have been confirmed by all
the experimental criteria employed by chemists. The generalisation
further indicated the existence of new elements; it pointed out their
probable sources, and foretold their properties. Instances of this
power of divination in the law are to be seen, as already mentioned,
in the discovery of _gallium_ by Lecoq de Boisbaudran, of _scandium_
by Nilson, and of _germanium_ by Winkler, the existence and main
properties of which were severally foretold by Mendeléeff in 1871.

The promulgation of the law was heralded as a proof of the validity of
the conception of a primordial matter. It was held that it can find
a rational explanation only in the idea of unity in the formative
material. But its author would not admit that his generalisation had
any relation to the Pythagorean hypothesis:

  The periodic law, based as it is on the solid and wholesome ground
  of experimental research, has been evolved independently of any
  conception as to the nature of the elements. It does not in the
  least originate in the idea of an unique matter, and it has no
  historical connection with that relic of the torments of classical
  thought; and therefore it affords no more indication of the unity
  of matter or of the compound nature of the elements than do the
  laws of Avogadro, and Gerhardt, or the law of specific heats, or
  even the conclusions of spectrum analysis. None of the advocates
  of a unique matter has ever tried to explain the law from the
  standpoint of ideas taken from a remote antiquity, when it was
  found convenient to admit the existence of many gods—and of a
  unique matter.

The reader who desires a fuller exposition of the principles of the
periodic law must be referred to special treatises on the subject, or
to the larger manuals on general chemistry. It must, however, be stated
that, while many facts discovered since the original promulgation of
the principle and since its development by Lothar Meyer, Carnelley,
Thomsen, and others, are consistent with the law, other facts, some
of which were known before 1870, are apparently out of harmony with
it, or at all events await a fuller interpretation. For example,
tellurium is not in its proper place in the scheme if its atomic
weight, 127.5, has been correctly ascertained. Cobalt (58.97) and
nickel (58.68) have atomic weights so closely accordant that their
properties and those of their corresponding compounds should be very
similar, and, in fact, almost identical; but such is not the case.
Indeed, it has been said, no prevision of the periodic law would
have led to the discovery of nickel. Similar considerations apply to
manganese, chromium, and iron; the atomic weights of these elements
are less widely different than the differences in their properties and
the divergence in their chemical relationships would seem to require.
The relative positions of argon and potassium are also not consistent
with the law. There are difficulties, too, connected with what we
know at present concerning the atomic weights of the so-called rare
earth metals. In spite, however, of these seeming anomalies, it can
hardly be doubted that the periodic law is as much the expression of
a natural law as is the law of gravitation; although it is possible,
and indeed probable, that, as we now define it, it is only the first
approximation to the truth, and that, as our knowledge becomes more
precise, Mendeléeff’s classification, in its present form, will require
modification and extension, just as Mendeléeff’s own scheme may be said
to be a modification and extension of the attempts at the rational
classification of the chemical elements made by his predecessors.

[Illustration: DMITRI IVANOWITSCH MENDELÉEFF.]

│Dmitri Ivanowitsch Mendeléeff│, with whose name this fruitful
generalisation is indissolubly connected, was born February 7, 1834
(N.S.), at Tobolsk, in Siberia, and was the fourteenth and youngest
child of Ivan Mendeléeff, the Director of the gymnasium at that place.
Soon after the birth of Dmitri his father became blind, and the
family were practically dependent upon the mother, Maria Dmitrievna
Mendeleeva, who established a glass works near Tobolsk, on the profits
of which she brought up and educated her large family. At the age of
fifteen Mendeléeff was taken by his mother to St. Petersburg, and
began the study of natural science at the Physico-Mathematical Faculty
of the Institute. After serving as a science master at Simferopol
in the Crimea and at Odessa, in 1856 he became a _privat-docent_ in
the University; then, following a short period of study in France
and Germany, he returned to St. Petersburg, and in 1866 he was made
Professor of General Chemistry in the University. His reputation mainly
rests upon his contributions to chemical philosophy and physical
chemistry, notably on specific volumes, on critical temperatures, on
the thermal expansion of liquids, on the nature of solutions, on the
elasticity of gases, and the origin and nature of petroleum. He died on
January 31, 1907.




CHAPTER VII

VALENCY


Chemical formulæ, from the time of Berzelius onwards, have been
regarded as rational expressions—that is, they serve to represent
the relations and analogies of the substance they are employed to
designate, and indicate in the simplest and at the same time the most
comprehensive manner the chemical changes in which the substances take
part. In the words of Gerhardt, those formulæ are “the best that make
evident the greatest number of such relations and analogies,” and that
serve to express the greatest number of the chemical changes in which
they are concerned.

In such concrete expressions of chemical change it was frequently
observed that a definite group of some or all of the constituent
elements of the substance hung together, as it were, and passed,
apparently unchanged, into the products of its transformation. These
groups were not necessarily radicals in the sense in which Liebig and
Wöhler used the term; to Gerhardt and to Kekulé they were simply
_residues_, remaining unattacked in a chemical metamorphosis, and
passing as such into the products of the change. They might or might
not be capable of isolation as definite entities. Thus, for example, we
may represent the composition of the following sulphur compounds so as
to show that they all contain the group SO2, or _sulphuryl_:

          SO2{Cl                    SO2{Cl
             {Cl                       {OH
    Sulphuryl chloride.    Chlorosulphonic acid.

        SO2{NO2               SO2{NO2           SO2{OH
           {Cl                   {OH               {OH
    Sulphuryl nitryl      Leaden chamber     Sulphuric
       chloride.             crystals.         acid.

These formulæ serve to show how the several substances are mutually
related, and that they may be derived from one another by the
substitution of atoms of chlorine for hydroxyl (OH), or nitryl (NO2),
or _vice versa_.

It was pointed out in 1851 by Williamson, and subsequently by Gerhardt,
that these groups are characterised by differences in their power of
combining with or replacing atoms of hydrogen, or of groups or elements
which, like chlorine, are chemically equivalent to hydrogen. Such a
radical or residue as _ethyl_ (C2H5) is chemically equivalent to _one_
atom of hydrogen, as is shown when we compare the formula for ether, as
established by Williamson, with that of ordinary alcohol:

     C2H5}O           C2H5}O
      H  }            C2H5}
    Alcohol.          Ether.

_Sulphuryl_, SO2, is chemically equivalento _two_ atoms of hydrogen;
_phosphoryl_, PO, as suggested by Odling, to _three_ atoms of hydrogen.
Gerhardt therefore proposed to designate these and similar groups as
_monatomic_, _diatomic_, _triatomic_, according to their respective
hydrogen-replacing power.

This conception of the definite atom-fixing or replacing power of
groups or compound radicals was extended by Frankland, in 1852, so as
to include the simple radical—that is, the elements. In the memoir
in which he announced the existence of the organometallic compounds
he pointed out that the elements may be classified according to
their combining power, or, as he expressed it, according as “their
affinities are best satisfied.” This idea was independently developed
by Couper and Kekulé in 1858; it is from that period that the definite
introduction of the conception of _atomicity_, _atomic-value_, or
_valency_, into chemical doctrine may be said to date.

The memoir in Liebig’s _Annalen der Chemie und Pharmacie_, in which
Kekulé announced his views, deals particularly with the tetravalency
of carbon and the doctrine of linking of atoms in terms of their
valency. As formulated by Kekulé and as subsequently developed in his
famous text-book, this doctrine exercised an immediate effect on the
progress of the chemistry of carbon compounds. Like every fruitful
hypothesis, it stimulated inquiry, and brought out analogies; and
the more it was applied the more apparent became its suggestiveness
and utility. The scope of chemical formulæ was greatly extended.
Rational formulæ grew into dissected or constitutional formulæ; and
on the system of constitutional formulæ have been grafted successive
attempts to elucidate the manner in which the constituents of a
molecule are grouped and held together. It is interesting to note
that the proximate effect of the theory of chemical structure which
grew out of Kekulé’s doctrine was to assimilate what was sound in the
seemingly antagonistic theories of types and radicals. As a mode of
exposition, Kekulé used models to illustrate the manner in which the
affinity-values of compounds are satisfied; these were not intended
to represent the actual spatial distribution of the atoms in a
molecule, but they nevertheless familiarised the mind with the idea
first clearly recognised by Wollaston and Berzelius that this is the
ultimate aim of chemistry. It was probably their use, either actually
or by visualisation, that led Kekulé in 1865 to his theory of the
constitution of benzene, as developed in his paper on the constitution
of the aromatic compounds—a theory no less fruitful in its consequences
than that of the tetravalency of carbon and of the linkage of atoms.
Such models, too, in the hands of Van ’t Hoff, subsequently served to
elucidate the connection between optical characters and crystalline
form, and to explain the isomerism of certain organic substances.

Kekulé was of opinion that the valency, or affinity-value, of an
element was a definite and invariable quantity—a fundamental property
of the atom as immutable as its atomic weight. Many facts appear to
show that such is not the case. Thus phosphorus and nitrogen are
sometimes trivalent and at other times pentavalent; tin, in certain
of its compounds, is divalent; in others, tetravalent. Sulphur may
be a dyad, a tetrad, or a hexad. It will be seen that the valency of
these particular elements varies by two units: this was at one time
held to be a natural law, and the various elements were divided by
Frankland into the two main groups of (1) _perissads_, or elements of
odd atomic value, and (2) _artiads_, or elements of even atomic value.
Experience has demonstrated that a rigid classification on this basis
is not possible. Many instances are known of elements not only varying
in valency by two units, but even by one unit. Thus nitrogen, which
is usually a perissad, is apparently an artiad in nitric oxide and in
gaseous nitrogen peroxide. Roscoe has shown that uranium and tungsten,
originally regarded as artiads, form pentachlorides.

To what the difference in the affinity-value of an element is due, and
why different elements should manifest different values, is at present
unknown. Valency, like other properties, appears to be a periodic
function of atomic weight; from the behaviour of such analogous
compounds as phosphorus pentafluoride, phosphorus pentachloride,
phosphorus pentabromide, it seems, too, to be related to the weights
of the atoms in combination. Further, it would appear that the mutual
affinities of substances vary with temperature—_i.e._, with the energy
imparted to their molecules; numerous instances appear to indicate
that the atom-fixing power of an element decreases when it is strongly
heated—that is, when the internal energy imparted to its combinations
exceeds a certain limiting value. Van ’t Hoff has attempted a
mechanical explanation of valency depending on the shape of the atoms,
as affected by variation in their vibratory motions resulting from
differences of temperature. Helmholtz suggested that the different
charges of electricity associated with the atoms may determine their
affinity-values—that, for example, a monad carries a single charge, a
dyad two, a triad three charges. Many considerations go to show that
the affinity-value of an element is not capable of definite numerical
expression in the sense which the doctrine of valency as generally
understood implies, and that the variations are not of the _per saltum_
character assumed by saying that the affinity-value is sometimes 1,
sometimes 2, at other times 3, and so on. When we have apparently
satisfied the accepted atomic value of an element by allocating to it
what we regard as the necessary complement of atoms of other bodies, it
is frequently evident that the capacity for combination of the whole
molecule is not satisfied. Many apparently saturated molecules have
the power of combining with other equally saturated molecules. Sulphur
trioxide (SO3) and barium monoxide (BaO) would appear each to have
their affinity-values satisfied; nevertheless they combine with great
readiness to form barium sulphate, BaSO4.




CHAPTER VIII

THE CHEMISTRY OF AROMATIC COMPOUNDS


The suggestions of Couper and Kekulé that an explanation of the
properties of chemical compounds should be sought in the nature and
mutual affinities of their constituent elements rather than of their
radicals were not wholly accepted at the time they were first made.
Speculative ideas have to justify themselves by facts. The value of
an hypothesis depends upon its usefulness and expediency, and on
its power of indicating the lines of future inquiry. How far it is
inductively sound and deductively useful is a matter for individual
judgment. Consequently the tendency to pass from purely rational and
constitutional formulæ to formulæ which sought to symbolise the inner
structure—the very skeleton, as it were—of a molecule, was resisted for
a time, and by no one more strongly than by Kolbe.

Kolbe’s attitude to the new doctrine may be said to have had its
justification in his own work. His remarkable prediction, based on
considerations which had nothing in common with Kekulé’s doctrine, of
the existence of the secondary and tertiary alcohols, so soon to be
confirmed by Friedel’s discovery of secondary propyl alcohol, and by
Butlerow’s isolation of tertiary butyl alcohol, served to retard the
general adoption of Kekulé’s views by showing that apparently they
were no more fruitful in suggestiveness than those they were intended
to supplant. But it was exactly in their suggestiveness with regard to
the development of isomerism that structural formulæ based upon valency
were gradually found to be most useful. It was perceived that it was
now possible not only to foretell the existence of isomers, but to
determine their number, and to some extent to forecast their properties
and modes of decomposition. Cayley, for example, calculated the number
of possible isomers of the hydrocarbons of the generic formula CnH2n+2
up to C6H14 all those that theory predicted have been discovered. In
no single case have more been obtained than the number indicated by
theory. The accumulated weight of this and similar testimony ultimately
established the doctrine of chemical structure on a firm basis.

This conception received a great extension as the result of Kekulé’s
application of his ideas to the explanation of the chemical
constitution of the group of substances of vegetable origin—consisting
of essential oils, balsams, resins, and their products, which, on
account of their characteristic odours, were collectively known to
the older chemists as the _aromatic compounds_. Some of these, like
oil of bitter almonds, gum benzoin, coumarin, oil of wintergreen, oil
of anise, oil of cinnamon, oil of cumin, balsam of tolu, phenol, and
certain of their derivatives, such as benzene, aniline, salicylic acid,
cinnamic acid, toluene, cymene, had already been investigated with
important theoretical results; but as a class they had received far
less attention than the derivatives of the great group of homologous
radicals of which methyl is the initial member. Of course it was part
of the doctrine of Liebig—the discoverer of benzoyl—that the aromatic
compounds also contained specific radicals; but the relation of these
compounds to those we now call aliphatic (fatty) compounds was not
understood, although certain analogies were recognised.

In 1866 Kekulé drew attention to the following significant
peculiarities of the aromatic compounds: (1) All aromatic compounds,
even the simplest, are comparatively richer in carbon than the
corresponding class of fatty (aliphatic) compounds; (2) among the
aromatic substances, as among fatty compounds, numerous homologous
compounds exist; (3) the simplest aromatic substances contain at
least six atoms of carbon; (4) all decomposition products of aromatic
substances show a certain family resemblance; the main product of the
decomposition contains at least six atoms of carbon—_e.g._, benzene
C6H6, phenol C6H6O, etc., which would seem to indicate that all
aromatic substances contain a nucleus or atomic grouping containing
six carbon atoms. Within this nucleus the carbon atoms are in closer
connection or denser combination, from which it follows that all
aromatic compounds are comparatively rich in carbon. More carbon atoms
can then be added to this nucleus according to the same laws that
govern the fatty compounds. In this way the existence of homologous
compounds may be explained.

On the assumption that carbon is tetravalent and that its valency is
constant, Kekulé showed how, by linking together six carbon atoms by
alternate single and double bonds, six affinity units may be left
free. If we assume that six carbon atoms are attached to one another
according to this law of symmetry, we obtain a group which, regarded as
an _open chain_, contains _eight_ unsaturated units of affinity:

    ——C══C——C══C——C══C——
      |  |  |  |  |  |

By making the further assumption that the two carbon atoms at the
ends of the chain are linked together by one unit of affinity each,
a _closed chain_ (a symmetrical ring) is obtained which now contains
_six_ unsaturated units of affinity:

    +--------------+
    |              |
    C══C——C══C——C══C
    |  |  |  |  |  |

From this _closed chain_ all the substances usually designated as
“aromatic compounds” are derived. In these a common nucleus may be
assumed: it is the closed chain C6A6, where A denotes an unsaturated
affinity. The six affinities of this nucleus may be satisfied by six
monovalent elements. They may also, wholly or in part, be satisfied by
one affinity of polyvalent elements, the latter necessarily bringing
with them other atoms into the compound, thus producing one or more
side chains, which in their turn may be lengthened by the addition of
other atoms.

If each of the free units is satisfied by an atom of hydrogen, we
obtain benzene, which, as Kekulé demonstrated, becomes the centre round
which the great group of aromatic compounds might be said to revolve.
Benzene was discovered by Faraday in 1825 among the volatile liquids
condensed from the oil-gas made by the Portable Gas Company. It had
already played a notable part in the development of chemical theory
in connection with the discovery of isomerism. It was now to play a
far more important rôle: to become, in fact, the progenitor of a great
family of substances, not only of theoretical value, but of great
economic importance.

The limits of this work preclude any attempt to trace in detail the
development of the conception with which Kekulé enriched science, or to
dilate upon the great extension of benzenoid or cyclic chemistry which
has resulted from it during the past forty years. It has been said
that Kekulé’s memoir on the benzene theory is the most brilliant piece
of scientific prediction to be found in the whole range of organic
chemistry. Of course, on its promulgation it had to run the gauntlet
of criticism; and an army of eager, active workers was soon engaged in
testing its sufficiency and in developing the rich province which it
first made known. As the facts multiplied, other statical formulæ were
suggested by Dewar, Ladenburg, and Claus, but they have not proved more
adequate to explain the facts as these have become better understood.
Observations which seemed to contradict Kekulé’s theory, or which
seemed to be imperfectly explained by it, have, in the light of fuller
knowledge, been shown to be in harmony with it; and such additional
proofs of agreement have thereby served to strengthen its position.
Its capacity for development is, indeed, as in the case of every
other hypothesis of the first rank, one of its cardinal qualities. It
adequately explains the constitution of great numbers of derivatives
whose analogies and relations, apart from it, would have remained
obscure and in many cases unintelligible. The symmetrical distribution
of the carbon and hydrogen atoms in the benzene molecule, assumed by
Kekulé on indirect grounds, has been established by the independent
investigations of Ladenburg and others, and its ring structure has been
demonstrated by Baeyer and Perkin. Purely physical evidence, based
upon its thermo-chemical and optical characters can be adduced in its
support. Determinations of the molecular volume and magnetic rotation
of its compounds further serve to substantiate it.

[Illustration: AUGUST KEKULÉ VON STRADONITZ.]

│Friedrich August Kekulé│ was born at Darmstadt on September 7, 1829.
After passing through the gymnasium of his native town, he entered,
in 1847, the University of Giessen, with the intention of becoming an
architect. Attracted by Liebig’s teaching, he turned to chemistry,
and worked with Will on _amyl sulphuric acid_ and its salts. In 1851
he went to Paris, heard Dumas’s lectures, and formed a friendship
with Gerhardt, whose _Traité de Chimie Organique_ largely moulded his
views. He became an assistant to Von Planta, occupying himself with
the chemistry of the alkaloids. Subsequently he came to London,
worked under Stenhouse, and made the acquaintance of Williamson, then
in the full vigour of his scientific activity. Here he discovered
_thioacetic acid_. It was at this time, also, that his ideas with
regard to structural chemistry began to take shape. Returning to
Germany, he attached himself to the University of Heidelberg as a
_privat-docent_, and had for a pupil Baeyer, who took up the study of
the organo-arsenic compounds. In 1858 he published his memorable paper
“On the Constitution and Metamorphoses of Chemical Compounds and on
the Chemical Nature of Carbon,” in which he developed his views on the
linking of atoms, out of which has grown our system of constitutional
formulæ. The immediate result of this celebrated memoir was a call
to the chemical chair of the University of Ghent, where Kekulé had
among other students Baeyer, Hübner, Körner, Ladenburg, Linnemann, and
Dewar. Here he remained nine years, and here he published his classical
_Lehrbuch der Organischen Chemie_. The years he spent in Ghent were the
most productive time of his career, and it was there that he developed
his benzene theory—a conception as fruitful as that of his doctrine
of atom-linking. In 1867 Kekulé was called to Bonn to take charge of
the newly erected laboratory which Hofmann had designed. Although he
continued to work, mostly in collaboration with his pupils, among whom
may be named Anschütz, Bernthsen, Thorpe, Carnelley, Claisen, Dittmar,
Franchimont, Van ’t Hoff, Japp, Schultz, Wallach, and Zincke, his
health after 1876 began to fail. He died on July 13, 1896.

       *       *       *       *       *

Of course no statical formula can be the ultimate representation of
the constitution of benzene. However convenient and suggestive such a
formula may be, it can be only a transitional phase in its complete
chemical and physical history. Kekulé was early conscious of this
fact, and suggested a dynamical hypothesis based upon a mechanical
conception of valency. This he imagined might represent the number
of contacts with other atoms which a vibrating atom experienced in
the unit of time. Two atoms of tetravalent carbon, each linked by
one combining unit, will experience four oscillations, striking each
other and three other atoms in the unit of time, while the monovalent
hydrogen atom makes only one oscillation. The doubly linked carbon will
collide with its neighbouring atom twice, and also with two other atoms
within the same period. The assumption that the carbon atom has a more
rapid motion than the hydrogen atom is, however, not warranted by the
kinetic theory. Other dynamic formulæ have been proposed by Knorr and
by Collie. Collie and Baly have further suggested that the absorption
bands of benzene observed in the ultra violet of its spectrum point to
synchronous oscillations of its molecule, due to dynamic changes in
the making and breaking of the links between the several pairs of the
carbon atoms, setting up vibrations in the benzene ring comparable with
those of an elastic ring in the act of expanding and contracting.

       *       *       *       *       *

The large group of the _essential oils_, containing hydrocarbons
similar to oil of turpentine, and classed under the generic term
of _terpenes_, might, from their origin and mode of occurrence, be
expected to be allied in constitution to the aromatic compounds; and
such is found to be the case. The terpenes are isomeric hydrocarbons
of the formula C10H16. They are found sometimes singly, at other times
mixed, in a great variety of plants, associated with _sesquiterpenes_
C15H24, and oxygenated substances, such as camphor, borneol, menthol,
etc., some of which have long been known and valued for their medicinal
properties and technical applications. The elucidation of their
constitution has taxed the skill of many workers during the past
thirty years; but, thanks to the labours of Wallach, Baeyer, Perkin,
Tiemann, Bredt, Komppa, and others, an insight has been gained into
their nature and analogies. They are apparently all cyclic compounds
with certain attributes which connect them with hydrocarbons of the
aliphatic series. _Pinene_, the characteristic constituent of oil
of turpentine, obtained by distilling the resinous exudations of
many species of pines, exists in two modifications, distinguished by
differences in optical activity, known respectively as _australene_,
found in American, Russian, and Swedish turpentine, and _terebenthene_,
found mainly in French turpentine. It would seem from their empirical
formulæ, as well as from their association in nature, that the terpenes
and _camphor_, which Dumas first showed to have the composition
C10H16O, should be closely allied in constitution, and that it ought
to be readily possible to effect their mutual transformation. The
constitution of camphor was long one of the standing problems of
organic chemistry, and dozens of formulæ have been suggested at
various times during the last twenty years in explanation of its
structure. That it contained a benzene nucleus seemed to be proved
by the ease with which it yielded toluene, cymene, and other benzene
homologues. The first real insight into its structure was gained
when Bredt ascertained the constitution of _camphoronic acid_, C6H11
(CO2H)4—a product, together with _camphoric acid_, of the oxidation
of camphor—which he found broke up into trimethylsuccinic acid and
_iso_butyric acid, and the structure of which was established by Perkin
and J. F. Thorpe.

The result of the Japanese monopoly has been to greatly enhance the
price of natural camphor; during the last ten years it has practically
trebled. This has naturally stimulated endeavours to prepare this
substance by synthetical means. _Artificial camphor_ is now made from
_pinene_ by transforming the hydrocarbon into bornyl chloride by the
action of hydrochloric acid. From this _camphene_ is prepared; by
treatment with glacial acetic acid it forms _isobornyl acetate_. On
hydrolysis this is transformed into _isoborneol_, which by oxidation
yields _camphor_, differing from the naturally occurring variety only
in the fact that it is optically inactive. All so-called aromatic
compounds are not necessarily cyclic systems, for it has been
recognised within the past few years that some of the most valuable
natural perfumes, such as that of the rose, lavender, and orange
blossom, lemon-grass, geranium, ylang-ylang, neroli, etc., owe their
characteristic aroma to the presence of terpenes and camphors, which
are not strictly benzenoid or cyclic compounds, but “ruptured rings”
behaving like open-chain or aliphatic substances. To judge from past
experience, it may confidently be stated that, now the constitution
of these substances is understood, their synthetical preparation on
an industrial scale is practicable. The discovery by Cahours in 1844
that _oil of wintergreen_ is substantially methyl salicylate led to
its artificial production from synthetically prepared salicylic acid.
Sir William Perkin in 1868 effected the synthesis of _coumarin_, the
aromatic principle in woodruff and hay. Fittig and Mielck in 1869
synthesised _heliotropin_, and in 1871 Tiemann and Haarmann obtained
_vanillin_, the characteristic aromatic body in the vanilla pod, by
synthetic means, and established its manufacture on a commercial scale.
The chemical nature of the characteristic odoriferous substances in oil
of cumin, anise, rue, cinnamon, heliotrope, jasmine, violet, parsley,
etc., has now been established and some of them are made industrially.
The artificial essence of violets known as _ionone_, prepared by
Tiemann in 1893, and now made commercially, is similar but not
identical in structure with the true perfume—_irone_. What is known as
_artificial musk_ is a trinitro-butyl toluol. _Artificial orange-flower
oil_ is a methyl ester of anthranilic acid.

In Vol. I. a short account has been given of the early history of
the large and important group of vegetable products known as the
_alkaloids_. Many of these have long been valued on account of their
powerful physiological action. As they all contain nitrogen and are
generally basic, they were regarded by Berzelius, and subsequently
by Liebig and Hofmann, as akin to ammonia in constitution, and were
classed as amines. The first experimental evidence of their nature
was obtained by Gerhardt, who found that, when strychnine and certain
of the alkaloids belonging to the quinine group are treated with
potash, an oil was obtained which he termed _quinoline_, and which was
recognised by Hofmann as identical with a substance obtained in 1834
from coal-tar by Runge, and at that time known as _leucol_. By other
modes of treatment certain alkaloids—_e.g._, nicotine and conine—are
found to yield pyridine, a basic substance found by Anderson, in 1846,
in the fœtid liquor obtained by distilling bones, and since found in
coal-tar. Others of them—_e.g._, papaverine, narcotine, etc.—yield
_isoquinoline_, an oil also discovered in coal-tar, by Hoogewerff, and
Van Dorp, in 1885. These three substances—quinoline, _iso_quinoline,
and pyridine—constitute so many nuclei in the constitution of a large
number of alkaloids. Pyridine resembles benzene in being a cyclic
compound, consisting of five carbon atoms and one nitrogen atom.
Quinoline stands to pyridine in much the same relation that naphthalene
stands to benzene. It can be obtained synthetically, as first shown by
Koenigs and Skraup, and subsequently by Doebner and Von Miller, from
benzene derivatives.

_Iso_quinoline, isomeric with quinoline, differs from that substance
in the position of the nitrogen atom. It, too, has been synthetically
prepared from benzene derivatives in a number of ways.

Among the naturally occurring pyridine alkaloids may be named
_piperine_, found in black pepper, and _conine_, the poisonous
principle of hemlock (_conium maculatum_). The latter alkaloid was
prepared synthetically by Ladenburg in 1886; as first obtained
it differed from the naturally occurring product, which is
dextro-rotatory, in being optically inactive. Ladenburg surmised that
the synthetic preparation stood to the naturally occurring compound
in the same relation that racemic acid stood to tartaric acid, and
that, by treatment in the manner employed by Pasteur, the racemic
modification of conine might be separated into its dextro- and
lævo-constituents. This was found to be the case; but the separated
dextro component was now found to be distinctly more optically
active than the pure, natural variety. It was, in fact, an isomeric
modification—_iso_-conine. By heating this to 300° it was transformed
into ordinary conine, identical in all respects with the natural
alkaloid. Ladenburg has also effected the synthesis of piperine by
condensing piperidine and piperinic acid.

_Nicotine_, the alkaloid of tobacco, was discovered by Posselt and
Reimann in 1828. Its constitution was first ascertained by Pinner, and
it was synthetically obtained by Amé Pictet, in 1904, as an inactive
substance, capable of being resolved by the crystallisation of its
tartrates into a dextro- and lævo-modification, the latter of which was
identical with that found in the tobacco leaf.

_Atropine_ and _hyoscyamine_—the poisonous principles of belladonna
and henbane—are isomeric alkaloids, the former of which is optically
inactive, and the latter is lævo-rotatory. Atropine is, in fact, the
racemic modification. The constitution of both alkaloids is known, and
their synthesis is now possible.

The successive steps may be thus indicated:

1. _Synthesis of glycerin_ (Faraday, Kolbe, Melsens, Boerhave, Friedel,
and Silva).

2. _Glycerin to glutaric acid_ (Berthelot and De Luca, Cahours and
Hofmann, Erlenmeyer, Lermantoff, and Markownikoff).

3. _Glutaric acid to suberone_ (Brown and Walker, Boussingault).

4. _Suberone to tropidine_ (Willstätter).

5. _Tropidine to tropine_ (Willstätter, Ladenburg).

6. _Synthesis of tropic acid_ (Berthelot, Fittig and Tollens, Friedel,
Ladenburg, and Rügheimer).

7. _Tropine and tropic acid: atropine_ (Ladenburg).

The alkaloid _cocaïne_, contained in the leaves of _erythroxylon
coca_ and now employed as a local anæsthetic, was discovered by
Niemann in 1860. It has been shown to be closely related to atropine
in constitution, and has now been synthetically prepared in the
dextro-modification.

The alkaloids _papaverine_, _narcotine_, _narceïne_, contained in
opium, are derivatives of _iso_quinoline, as also is _berberine_, found
in the common barberry (_berberis vulgaris_). Papaverine, which occurs
in opium to the extent of about one per cent., was first isolated by
Merck in 1848. Its constitution has been established by Goldschmidt.
_Narcotine_ is, next to morphine, the most abundant constituent of
opium. The study of the products of its hydrolysis and oxidation—viz.,
_opianic acid_ and _cotarnine_, both of which substances have long
been known—has indicated its probable structure. _Narceïne_ is closely
allied to narcotine, and can, indeed, be obtained from it by combining
the latter alkaloid with methyl iodide and treating the compound with
caustic potash. The constitution of _berberine_, which is one of the
few coloured vegeto-alkaloids known, has been worked out by Perkin. As
yet nothing definite is known concerning the structure of the most
important and largest constituent of opium—viz., _morphine_; or of its
congeners _codeïne_ and _thebaine_. Grimaux, however, in 1881 converted
morphine into codeïne by treatment with methyl iodide and potash; hence
the two alkaloids stand in a relation somewhat similar to that in which
narceïne stands to narcotine. There is very little doubt that the
three alkaloids are very closely related, and that a knowledge of the
constitution of one of them would immediately elucidate the structure
of the others. They are probably phenanthrene derivatives.

_Quinine_ and _cinchonine_, the most important of the cinchona
alkaloids, are quinoline compounds, and are closely related in
constitution. But the many attempts to unravel their structure have
yielded no definite results up to the present.




CHAPTER IX

STEREO-ISOMERISM: STEREO-CHEMISTRY


The first gropings in the search for light on the inner structure
of molecular groupings may be said to date from Biot’s work
on polarisation. In 1815 Biot, a pupil of Malus, made the
remarkable discovery that a number of naturally occurring organic
compounds—_e.g._, sugar, tartaric acid, oil of turpentine, camphor,
etc., are _optically active_—that is, rotate the plane of polarisation
in one direction or the other. The property had previously been
observed in quartz, and was assumed to be connected with the
crystalline character of that substance. Biot, however, pointed out
that the case of oil of turpentine which is a liquid, and the cases
of the other substances when in solution, showed that crystalline
character had no necessary connection with the phenomenon, but that it
must be dependent upon the internal or molecular arrangement of the
optically active substance.

In 1844 Mitscherlich, who first demonstrated the relation between
atomic constitution and crystalline form, drew attention to the
fact that the salts of the isomeric modifications of tartaric
acid, studied by Berzelius, although possessing the same chemical
composition, the same crystalline form, with the same angles, the
same double refraction, and therefore the same angles between their
optical axes, nevertheless behave quite differently as regards their
optical activity, solutions of the tartrates rotating the plane of
polarisation, whereas those of the racemates are inactive. In 1848
this remarkable circumstance engaged the attention of Louis Pasteur,
a young man who had just completed his course at the École Normale in
Paris, and was acting as assistant to Balard, the discoverer of the
element bromine. Pasteur, on examining the crystals of the two forms
of tartaric acid, and of some of their salts, detected the presence,
on some of them, of certain facets—so-called hemihedral faces—which
had hitherto been unrecognised, but were similar to facets which Haüy
had observed on quartz. Haüy had, in fact, divided quartz crystals
into two classes—right-handed and left-handed, depending upon the
side on which these facets occurred. The forms were, as it is termed,
enantiomorphous. Biot, moreover, found that some quartz crystals, cut
parallel to the axis, turned the plane of polarisation to the right,
whereas others turned it to the left; and Herschel suggested that the
phenomena were probably connected, and such was found to be the case.

Mindful of Herschel’s observation, Pasteur found that the crystals of
certain of the optically active tartrates showed hemihedral faces,
whereas those of the corresponding racemates showed no trace of them.
On recrystallising the racemates, however, it was noticed that two sets
of crystals were formed—enantiomorphic forms—the first set of crystals
having hemihedral forms on the right-hand side, and the second set
on the left-hand side. The forms, in fact, were so related that one
appeared, as if it were the image, as seen in a mirror, of the other.
When solutions of these crystals were examined, one set was found to
rotate to the right, the other to an equal degree to the left. The
dextro-rotatory salt yielded ordinary tartaric acid; the corresponding
lævo-rotatory acid was a hitherto unknown modification: the two
together, in equal proportions, constituted racemic acid.

In 1863 Wislicenus published a remarkable memoir on the synthesis of
lactic acid. The acid in sour milk was discovered by Scheele in 1780.
In 1807 Berzelius discovered a similar acid, called _sarcolactic acid_,
in muscle juice; this was erroneously pronounced by Liebig to be
identical with that of sour milk. Other forms of lactic acid were made
known, the structural character of which was not to be explained by
current hypotheses. Wislicenus concluded that their differences could
be due only to different arrangements of their atoms in space.

In 1874 the conception of atomic grouping received a remarkable
development by the publication of two memoirs—one by Van ’t Hoff, and
the other by Le Bel—which served to connect molecular structure with
optical activity. Confining their attention to carbon compounds, they
inferred that all optically active substances contained at least one
multivalent atom, united to other atoms or groups, so as to form in
space an unsymmetrical arrangement. Van ’t Hoff regarded the carbon
atom as occupying the centre of a tetrahedron, to the summits of which
its valencies were directed. If different groupings are attached to
these summits, the structure is _asymmetrical_, and is optically
active. The two forms of lactic acid, for example, may be represented
by the following space formulæ:

[Illustration]

It will be seen from an inspection of the figures that the one is the
image-form of the other, and, no matter how they are turned, they are
not superposable; they are right- and left-handed, or, as it is termed,
enantiomorphs.

There is no fundamental distinction between the hypothesis of Van ’t
Hoff and Le Bel as to the effect of asymmetry on optical behaviour. Le
Bel regards the effect of asymmetry simply as a necessary consequence
of the presence of four dissimilar groupings, and as independent of
valency and the geometrical form of the molecule.

[Illustration: JACOBUS HENRICUS VAN ’T HOFF.]

It was surmised by Pasteur that every liquid or solid in solution
showing optical activity, if crystallisable, would be found to manifest
hemihedral faces; but this has not been generally established.
Further, it does not always happen that an optically active substance
in solution is so when solid. Lastly, optical activity may be latent
even in asymmetric carbon compounds if dextro- or lævo-modifications
are present in equal proportions, as in racemic acid. Such compounds
are, in fact, termed “racemic,” or _racemoids_; and they may be
separated occasionally by crystallisation, as in Pasteur’s method with
the tartrates; or as shown by him by the action of the racemoid upon
another optically active substance; or, lastly, by taking advantage
of the specific action (specific assimilation) of organisms—Pasteur’s
so-called biochemical method.

It is a physiological fact of great interest that the behaviour of
enantiomorphs towards the animal organism is frequently markedly
different. Lævo-tartaric acid administered to guinea-pigs is found to
be twice as poisonous as the dextro-acid; dextro-asparagine possesses
a sweet taste, but lævo-asparagine is tasteless; lævo-nicotine is more
poisonous than the dextro-alkaloid.

The ferments known as _enzymes_ are also found to possess the power
of selection, behaving differently towards the different optically
active modifications of the same substance. It is frequently observed
that an optically active substance may be rendered inactive by the
conversion of half the substance into its enantiomorph. This operation
was first performed by Pasteur, and may be brought about by heating
the substance, either alone or with water, under pressure. Indeed, it
is occasionally observed to take place at the ordinary temperature
(_autoracemisation_).

By the action of various reagents the derivatives of an optically
active substance are found not unfrequently to change the direction of
their optical activity. Indeed, by such means one enantiomorph may be
changed into another. Thus _lævo_-menthol may be converted into the
_dextro_-modification by treatment with sulphuric acid.

The rotatory power of a substance is frequently modified by the
character of its solvent, and varies with the temperature and
concentration of the solution. Landolt and Oudemans found that the
specific rotation of dilute solutions of tartrates and of salts of the
active alkaloids was independent of the nature of the base and acid
respectively present—a fact which finds its explanation in the theory
of electrolytic dissociation. It has been known for some years past
that the specific rotation of solutions of certain sugars changes with
time, being sometimes less and sometimes more than the initial amount.
This phenomenon is now known as _multirotation_, or _mutarotation_. It
seems to be connected with an alteration in the configuration of the
molecules.

There is a special case of stereo-isomerism, differing from that
of optical isomerism and of structural isomerism (with which we
have hitherto been alone concerned), which was predicted by Van ’t
Hoff in his remarkable work _La Chimie dans l’Espace_, published in
1877—noteworthy as being the first serious attempt to grapple with
the problem of spatial molecular grouping, foreshadowed by Wollaston,
Berzelius, and, indeed, all the early philosophic thinkers who accepted
the atomic theory. The special form of stereo-isomerism now referred
to, which has been more particularly investigated by Wislicenus, is
distinguished as _geometrical isomerism_; not, perhaps, a sufficiently
descriptive term, since, comprehensively, all forms of isomerism are
really cases of geometrical isomerism. Instances of it are to be
met with among the isomeric acids existing as glycerides in certain
fats, in cinnamic acid, in stilbene and its derivatives, etc. It was
first observed in _maleic_ and _fumaric acids_—isomeric acids of the
empirical formula C2H2 (COOH)2, obtained by the distillation of malic
acid, the characteristic acid met with in the apple and other fruits
and in certain other vegetal products. These acids may be represented
by the following space formulæ:

    COOH——C——H       COOH——C——H
          ║                ║
    COOH——C——H          H——C——COOH
    Maleic acid.     Fumaric acid.

which show no asymmetry, and hence no possibility of optical activity
or enantiomorphous modifications.

In the case of maleic acid it will be seen that the same groups (COOH
or H) are represented on the same side of the molecule—in other words,
they are placed symmetrically in a plane—whereas in fumaric acid they
are placed diagonally or are axially symmetrical. Isomers of the first
case are classified as _malenoid_ or _cis_-forms, while those of the
latter are termed _fumaroid_ or _trans_-forms.

Substances of the character referred to are, as a rule, mutually
convertible with more or less ease; they are susceptible of what is
called _geometrical inversion_. Thus fumaric acid may be readily
converted into maleic acid by heating; maleic chloride is gradually
transformed into fumaric chloride at ordinary temperatures. Sunlight,
or a particular solvent, or the presence of some substance which acts
as a catalyst, may effect the inversion. _Cis_ and _trans_ isomerism is
also met with among cyclic compounds; it occurs among the terpenes; and
certain alkaloids, as, for example, cocaïne, exhibit it.

Although the doctrine of stereo-chemistry was first enunciated in
the case of carbon, and was, indeed, for a time solely confined to
compounds in which carbon was the nucleal element, there is no _a
priori_ reason why the phenomenon should be so restricted. Van ’t
Hoff, in fact, in 1878, discussed the question in relation to nitrogen
compounds. Stereo-isomeric nitrogen derivatives were first obtained
by Victor Meyer and his pupils, and the stereo-chemistry of nitrogen
has since proved to be a very fruitful field of investigation, notably
in the hands of Goldschmidt, Beckmann, Hantzsch and Werner, Le Bel,
Ladenburg, Bamberger, Kipping, H. O. Jones, Pope, and others. The
stereo-chemistry of nitrogen differs from that of carbon, inasmuch as
variation of valency plays a far more important part in the case of
nitrogen than it has hitherto been observed to do in that of carbon;
the spatial representation of the trivalent nitrogen atom differs from
that of the pentavalent atom. Le Bel, in 1891, succeeded in obtaining
an optically active nitrogen enantiomorph by the application of
Pasteur’s biochemical method. Optically active compounds have since
been prepared by Pope and Peachey and H. O. Jones. Pope and Peachey
have also prepared optically active compounds of sulphur, selenium, and
tin; and Kipping has obtained an asymmetric compound of silicon.

In 1863 Geuther, and, independently, Frankland and Duppa, made known
the existence of _aceto-acetic ester_. By Geuther this compound was
termed _ethyl-di-acetic acid_—

    CH3.C(OH): CHCOOC2H5

by Frankland and Duppa it was considered to be _acetone-carboxylic
acid_—

    CH3.CO.CH2.COOC2H5.

The essential difference in these formulæ, as the two names
respectively indicate, is that the first implies that the ester has an
acidic or hydroxylic character, proved by its forming characteristic
salts; the other that it contains the group CO, proved by its yielding
acetone and the usual reactions of the ketones. The attempt to settle
the constitution of this substance gave rise to much controversy,
and, as it was found to be very reactive, led to a great amount
of conflicting experimental work. The ultimate result was to show
that both formulæ are correct: at the time of reaction the ester
is sometimes hydroxylic, at other times ketonic, or, adopting the
terminology of Brühl, it sometimes shows the _enol_ form, at other
times the _keto_ form. Other substances were subsequently found to
behave in the same way. In 1885 the question was discussed by Laar,
who suggested the term _tautomerism_ (ταὐτό, the same; μέρος, a
part) to denote the fact that one and the same substance could have
structural formulæ varying with conditions of reaction and depending
upon the migrations of certain of its atoms within the molecule. During
the last twenty years a large number of examples of the kind have
been discovered. They are found to occur, not only among aliphatic
substances, but in cyclic and heterocyclic compounds. We now know that
such intermolecular changes may occur by the migration of any of the
elements or groups present in the molecule. Thus, to confine ourselves
to simple and well-known examples, the transformation of sodium phenyl
carbonate into sodium salicylate, discovered by Kolbe, is due to the
wandering of an atom of hydrogen from the benzene residue to oxygen,
thus:

                       OH
                      /
    C6H5.O.COONa→C6H4
                      \
                       COONa.

The conversion of the nitriles into the cyanides by heating is due to
the transference of the alkyl radical from the nitrogen atom to the
carbon—

    R.NC→NC.R.

Alkyl groups may also be transferred from oxygen to nitrogen; a radical
may detach itself from a carbon atom and wander to a nitrogen atom;
radicals in cyclic compounds may be transferred from the side chains to
the nucleus, etc.

The phenomenon, in fact, is now so general that grave doubts have
been thrown upon the uniform value of deducing the structural formula
of a substance from the study of its decomposition products, or from
the nature of its derivatives, owing to the readiness with which
tautomerism may occur. The change may be brought about by variation of
temperature, by the reagent itself, by the action of a solvent or the
presence of a catalyst—that is, of a substance which _apparently_ plays
no part in the metamorphosis. Hence the value of specific reagents as
clues to constitution is considerably weakened, since the results may
be equivocal. Fortunately, the great extension, within recent years,
of the application of physical methods has considerably strengthened
our means of gaining an insight into molecular structure; and the
investigations of Brühl on refraction and dispersion, of Perkin on
magnetic rotation, of Hantzsch on electrical conductivity, of Lowry on
solubility, of Lowry and E. F. Armstrong on optical activity, of Knorr
and Findlay on melting-points, and, lastly, of Hartley, Dobbie, Lauder,
Baly, and Desch on absorption spectra, have collectively afforded
valuable information on the mechanism of isomeric change based upon
dynamical considerations.

Space will not permit of a more extended treatment of the subject of
stereo-chemistry; and certain matters relating to it, as, for example,
the phenomena classed under the term _steric hindrance_, must be left
unnoticed. This term has reference to the hindrance which certain
groups, or the particular distribution in space of certain atoms, exert
on the progress or extent of a reaction, as, for example, of hydrolysis
or esterification, etc. The influence of special groupings in retarding
chemical change is apparently well established, but no comprehensive
theory of the subject is yet possible. Until such a theory is
forthcoming a dynamical theory of stereo-chemistry is incomplete.




CHAPTER X

ORGANIC SYNTHESIS: CONDENSATION: THE SYNTHESIS OF VITAL PRODUCTS


In its widest sense, the term “synthesis,” as used in organic
chemistry, means the building-up of carbon compounds, either from
their constituent elements or from groups of differently constituted
molecules. At one period this term was confined to cases in which
the organic compound was prepared from inorganic materials, or from
combinations which themselves could be formed from their elements; but
latterly it has lost, in large measure, this restricted signification.
At the same time, the attempt has been made to indicate by special
terms certain classes of synthetical reactions. Thus the special
case of the formation of an organic compound by the union of two or,
it may be, more molecular groupings is now frequently spoken of as
_condensation_.

Organic chemistry has been largely developed by the discovery from time
to time of special reagents and special types of reactions which have
shown themselves to be capable of extensive application. Such, for
example, was Frankland’s discovery, in 1852, of zinc-ethyl—the first of
the organo-metallic compounds, and the type of a series of substances
of great theoretical importance, and of great practical value by reason
of their reactive powers. They led to the synthesis of the paraffins,
the secondary and tertiary alcohols, and ketones. A few years later
Wurtz introduced the use of metallic sodium as a condensing agent, and
showed thereby how the hydrocarbon _butane_ could be produced from
ethyl iodide:

    2C2H5I + Na2 = C4H10 + 2NaI.

Use was made of the same agent by Fittig, in 1863, in effecting the
synthesis of the homologues of benzene by the action of an alkyl iodide
upon bromobenzene:

    C6H5Br + CH3I + Na2 = C6H5.CH3 + NaI + NaBr.

In like manner Kekulé, in 1866, obtained benzoic acid by the action of
carbon dioxide upon bromobenzene:

    C6H5Br + CO2 + Na2 = C6H5COONa + NaBr.

The readiness with which magnesium can now be obtained, mainly as the
result of Sonstadt’s efforts to develop its metallurgy, has led to its
application, at the suggestion of Barbier, in 1899, in place of zinc.
The particular form of magnesium compound now employed as a reagent was
prepared by Grignard in 1900, and is known by his name. It is obtained
by bringing an ethereal solution of an alkyl iodide or bromide into
contact with magnesium, when the metal is dissolved, forming, in the
case of methyl iodide,

    MgCH3I.(C2H5)2O.

Grignard’s reagent has shown itself to be extraordinarily reactive, and
a great number of condensations—of hydrocarbons, alcohols, aldehydes,
acids, ketones, amides, and additive compounds—have been effected by
means of it.

Other condensing reagents of value are aceto-acetic ester, sodium
amalgam, sodamide, sodium ethoxide, dimethyl sulphate, zinc chloride,
aluminium chloride, fused caustic potash, hydrogen chloride,
phenyl-hydrazine, hydrogen peroxide in presence of a ferrous salt
(Fenton’s reagent), ammonia, and various amines. The application of
these reagents has led to the discovery of a variety of new compounds,
the mode of origin of which has served to elucidate their constitution.

The great majority of organic syntheses, especially when they start
by the use of inorganic materials, consist in passing from simple to
complex molecular groupings by condensation processes. An interesting
example of the reverse process is seen in the production of _carbon
suboxide_, or _carbon carbonyl_, C3O2, obtained from various malonyl
compounds, but most conveniently prepared by the action of phosphoric
oxide on malonic acid under diminished pressure, or by treating an
ethereal solution of dibromomalonyl chloride with zinc:

            COOH            CO
           /               //
    (1) CH2      = 2H2O + C
           \               \\
            COOH            CO

                                             CO
                                            //
    (2) CBr2(COCl.)2 + Zn2 = ZnCl2 + ZnBr2 C
                                            \\
                                             CO

Carbon suboxide is a colourless, extremely mobile, refractive,
poisonous liquid, of sp. gr. 1.11, with a strong and peculiar
smell. It boils at 7°, and freezes at -107°. It is stable only at
low temperatures; at ordinary temperatures it polymerises to a red
solid, which dissolves in water, forming a solution of the colour of
eosin. The change is almost instantaneous at 100°. Carbon suboxide is
inflammable, burning with a blue but smoky flame: C3O2 + 2O2 = 3CO2.
Its low boiling-point and the high value of its molecular refraction
and dispersion, its general resemblance to the metallic carbonyls and
ketones, etc., indicate that this remarkable oxide of carbon is, in
all probability, the anhydride of malonic acid. Indeed, by the action
of water upon it, it is reconverted into malonic acid.

In point of principle, and viewed as chemical operations, the synthesis
of vital products is in nowise different from the synthesis of any
other group of organic compounds; and the special interest, and even
astonishment, at one time created by the artificial preparation of such
products has largely died away. The synthetical production of some of
the substances formerly known only to be formed by vital action, either
in the animal or the plant, has already been incidentally referred
to. But it may be convenient to treat the subject of the artificial
production of this group of bodies rather more comprehensively and as a
sub-section of this chapter on organic synthesis, since their formation
by such means constitutes a phase in the development of chemistry, and
has undoubtedly exercised a profound influence on scientific thought
and on philosophical and even theological doctrine. During the past
fifty or sixty years the chemist has been enabled to form the active
principles or characteristic products of many plants and animals.
He has built up substances which were formerly regarded as capable
of being made only by the very process of living. He has prepared
compounds which were at one time considered as only producible by
changes in organised matter after death.

Since the date of Wöhler’s epoch-making discovery, already referred
to[4] _urea_ has been synthetically prepared by many reactions,
notably by Regnault and Natanson by the action of ammonia on carbonyl
chloride, and by Basarow and Dexter from ammonium carbamate. Both these
substances can be formed directly or indirectly from their elements. It
may also be obtained by the hydrolysis of lead cyanate:

Pb(CNO)2 + 2H2O = PbCO3 + CO(NH2)2.

    [4] Vol. I., p. 163.

The successive steps in its production from inorganic materials by this
method are:

    K + C + N → KCN → KCNO
    → Pb(CNO)2 → CO(NH2)2.

Associated with urea as products of metabolism are _uric acid_,
_xanthine_, and _sarcine_. Urea was first artificially transformed into
uric acid by Horbaczewski, and its synthesis was effected by Behrend
and Roosen. Closely related in chemical composition to these substances
are _theobromine_ and _caffeine_, the characteristic principles
respectively of cocoa (the fruit of _theobroma cacao_); and of coffee,
tea, maté (the leaves of _ilex paraguayensis_); “guarana,” obtained
from the seeds of the South American plant _paullinia sorbilis_,
and the kola-nut of Central Africa. Strecker, in 1860, showed how
theobromine may be converted into caffeine; and Emil Fischer, by
similar means, transformed xanthine into theobromine. Since that time
xanthine itself has been prepared artificially. Caffeine can now be
built up from its elements by a series of transformations effected by a
succession of chemists, as follows:

  1. Carbon and oxygen give carbonic oxide.—_Priestley_, _Cruickshank_.

  2. Carbonic oxide and chlorine give carbonyl chloride.—_J. Davy._

  3. Carbonyl chloride and ammonia give urea.—_Natanson._

  4. Urea gives uric acid.—_Horbaczewski_; _Behrend and Roosen_.

  5. Uric acid gives xanthine.—_E. Fischer._

  6. Xanthine gives theobromine.—_Strecker._

  7. Theobromine gives caffeine.—_E. Fischer._

Synthetic theobromine is now made on the large scale, and introduced as
a soda compound, in combination with sodium acetate, into medicine as a
diuretic under the name of agurin. Synthetic caffeine is also prepared
on a manufacturing scale from uric acid through the medium of the
methylxanthines. The close relationship of xanthine to uric acid is of
great physiological significance, since there is little doubt that the
xanthine bases are the most important sources of uric acid within the
organism.

In this connection reference may be made to the large number of
synthetic organic products which have been introduced into medicine
during the past few years. The investigation of the constitution of
the alkaloids has served to show in many cases to what particular
molecular grouping the physiological action of the drug is mainly due,
and this has led to the production of substances containing these
groups, but not necessarily existing as natural products. Among these
may be mentioned _antipyrin_, a derivative of the pyrazol group,
discovered by Knorr in 1883, and of which upwards of 17,000 kilos, of
the approximate value of £35,000, were produced in 1899. This substance
is a _phenyl-dimethyl-pyrazolone_.

_Acetanilide_ C6H5NH.COCH3, an aniline derivative, was discovered by
Gerhardt in 1853. _Phenacetin_ is a derivative of _para_-aminophenol:

         OC2H5
        /
    C6H4
        \
         NH.COCH3.

An extraordinary number of synthetical soporifics have been introduced
at various times during recent years—_e.g._, _chloral hydrate_,
_veronal_, _sulphonal_, _trional_ and _tetronal_, etc. The three
last-named substances are closely related, as the following formulæ
indicate:

    CH3                 CH3
       \                   \
        C(SO2C2H5)2         C(SO2C2H5)2
       /                   /
    CH3                C2H5
         Sulphonal.          Trional.

    C2H5
        \
         C(SO2C2H5)2
        /
    C2H5
        Tetronal.

Sulphonal is prepared by the oxidation of a substance obtained by
the combination of acetone and ethylmercaptan. _Veronal_ is an ethyl
compound of barbituric acid, obtained by the condensation of urea and
diethyl malonyl chloride:

       NH.CO
      /     \
    CO       C(C2H5)2
      \     /
       NH.CO
        Veronal.

Attempts have been made to connect the physiological working of local
anæsthetics with particular constitutional groupings, as, for example,
in cocaïne; and these have led to the introduction of such substances
as the _orthoforms_, _nirvanine_, _stovaïne_, _alyhine_, _novocaïne_,
and _adrenaline_ into medicine. Adrenaline, used in conjunction with
cocaïne, has proved itself a most valuable agent in producing what is
called _lumbal anæsthesia_, whereby large sections of the lower half
of the body may be rendered completely insensitive to pain.

The study of the putrefactive changes of albuminous substances of
animal origin, induced by the activity of micro-organisms, has revealed
the existence of a number of basic nitrogenous compounds, some of
which are highly poisonous. These were classed by Selmi under the
generic name of _ptomaines_ (πτῶμα, a corpse). Brieger found that the
typhoid bacillus yielded a poisonous substance—_typhotoxine_, and that
the bacillus of tetanus forms a highly toxic basic body, _tetanine_.
All the ptomaines, however, are not poisonous. Some of them, like
_choline_ (χυλὴ, bile)—originally discovered by Strecker in bile, in
the brain, in yolk of egg, and now found to be among the products of
the putrefaction of meat and fish—have been known for some time past.
Choline was first synthetically prepared by Wurtz. _Neurine_ (νεὺρον,
nerve), a derivative of brain substance, is related to choline, and
is readily transformed into it, but differs from it in being very
poisonous. It has been synthesised by Hofmann and by Baeyer. Another
of the so-called corpse-alkaloids—_cadaverine_—has been synthetically
formed by Ladenburg. Schmiedeberg and Kopp isolated the poisonous
principle of the fungus _agaricus muscarius_, which they named
_muscarine_. It occurs with choline, from which it can be readily
obtained, among the products of the putrefaction of flesh, as well as
in many fungi.

The synthesis of the alkaloids _conine_, _atropine_, _cocaïne_,
_piperine_, and _nicotine_ has been already referred to[5] as also that
of _vanillin_, the aromatic principle of the dried fermented pods of
certain orchids; _coumarin_, the odoriferous principle of woodruff and
of the tonka bean; of _salicylic acid_, _oil of wintergreen_, _oil of
mustard_, _bitter-almond oil_, and _camphor_. _Acetic_, _succinic_,
_tartaric_, and _citric acids_ have also been artificially obtained,
and may, indeed, be built up from their elements.

    [5] P. 133.

No synthesis of recent years created more widespread interest than that
of _alizarin_, first effected by Graebe and Liebermann in 1868. Its
successful commercial manufacture by Sir William Perkin in this country
and by Caro in Germany created nothing less than a revolution in one
of our leading industries, and completely destroyed a staple trade of
France, Holland, Italy, and Turkey. To procure alizarin, anthraquinone
is treated with sulphuric acid, and the product is fused with alkali
and potassium chlorate.

The remarkable industrial results attending the synthetical formation
of this madder-product naturally led to attempts to procure other
important vegetable dye-stuffs artificially, notably _indigo_.
The synthetical production of indigo has been accomplished by
the joint labours of many chemists, notably Baeyer, Heumann, and
Heymann, and the substance is now prepared on an industrial scale.
The starting-point is _naphthalene_, obtained from coal-tar. This
is converted into _phthalic acid_, which is then transformed into
_phthalimide_. The last-named substance is converted into _anthranilic
acid_, which, on treatment with monochloracetic acid, is changed into
_phenylglycin_-ortho-_carbonic acid_. On melting this with caustic
potash it yields _indoxyl acid_, which is transformed into _indoxyl_,
and thence into _indigo_.

Another method is to treat the sodium salt of phenylglycin with
sodamide, whereby _indoxyl_ is at once obtained, and this by
condensation yields _indigo blue_:

    C6H5.NH.CH2.COONa + Na.NH2
     Sodium salt of    Sodamide.
      phenylglycin.

           CO
          /  \
    →C6H4    CH2
          \  /
           NH
         Indoxyl.

           CO    CO
          /  \  /  \
    →C6H4   C:C     C6H4
          \  /  \  /
           NH    NH
         Indigo blue.

Phenylglycin is obtained by the action of monochloracetic acid
on aniline, which in its turn is obtained through nitrobenzene
from benzene. Since benzene can be synthetically prepared by the
condensation of acetylene, which can be obtained by the direct union of
carbon and hydrogen at a high temperature, it is theoretically possible
to build up indigo blue from inorganic materials.

Synthetical indigo blue was placed on the market in 1897 with an
almost immediate effect on the production and price of the natural
variety, and to-day the output of Bengal indigo has fallen by more
than fifty per cent. In 1902 the amount of the natural product was
probably not greater than three million kilos, whereas in the same
year the production of synthetic indigo was upwards of five million
kilos. Before the introduction of the artificial variety the price of
pure indigo blue ranged from sixteen to twenty shillings per kilo; by
the end of 1905 it had fallen to seven or eight shillings. Mention
should be made also of _thio-indigo red_ and the _thionaphthene_
derivatives, some of which promise to be important colouring matters.
In recent years the so-called _sulphur colouring matters_ have acquired
considerable importance. Space will not permit of any fuller treatment
of the development of the manufacture of the artificial organic
colouring matters. This industry had its beginnings in England, but it
is now mainly carried on in Germany. Its importance may be gleaned
from the fact that the value of the production at the present time
amounts to not less than £12,500,000 per annum, two thirds of the
output being exported. It demands the services of battalions of skilled
chemists, and gives employment to many thousands of artisans.

Some of the most notable achievements of modern synthetical chemistry
are to be found in the work of Emil Fischer on the _sugars_ and the
_proteins_. Although the sugars have from the earliest times been
reckoned among the most characteristic products of plant life, and have
long been used as food and as sources of alcohol, comparatively little
was known until lately of their real nature and mutual relations,
in spite of numerous attempts to elucidate their constitution. Much
of the mystery surrounding their chemical history has now been
dispelled. Not only has the molecular structure of the more important
naturally occurring sugars been unravelled, but a large number of
hitherto unknown members of the various groups of the great family
to which they belong have been prepared. The first insight into the
constitution of these bodies may be said to date from the researches
of Kiliani, made about a quarter of a century ago. In 1887 Fischer
effected the synthesis of a form of fructose (fruit sugar), and
immediately afterwards of ordinary dextro-glucose (grape sugar) and
its enantiomorph lævo-glucose, and the two optically active forms
of natural fruit sugar. Since that time such sugars as arabinose,
xylose, fucose, mannose, sorbose, cane-sugar, maltose, lactose, etc.,
and the sugars existing as glucosides, have been examined, their
stereo-chemical relations defined, and synthetic methods of production
devised. Incidentally, their behaviour towards enzymes has been
studied, and the remarkable selective action of these ferments on the
various groups, due apparently to differences of configuration, has
been established, with the result that much light has been thrown
on the mechanism of enzyme action and on the general theory of
fermentation.

[Illustration: EMIL FISCHER.]

The study of the proteins by Fischer constitutes a new chapter in
bio-chemistry. Although long recognised as among the most important
of vital products, from the circumstance that they enter into the
composition of animal tissues and secretions and are essential
constituents of protoplasm, the proteins are among the worst defined
substances known to the chemist. They are difficult to separate, as
they closely resemble one another, and afford no certain indications of
individuality. Very few of them have been obtained in a form in which
their identity could be established. _Oxyhæmoglobin_ was isolated some
years ago, but the proteins of serum albumin and of egg albumin have
only recently been obtained in definite crystalline shape. All the
proteins—even the simplest of them—are of great complexity, and possess
apparently very high molecular weights. _Hæmoglobin_, for example,
appears to have approximately the formula C158H123N195O218FeS3, with
a minimum molecular weight of 16,600. Indeed, there is experimental
evidence to show that it is even considerably higher than this.

The main clues to the nature of these substances have been gained by
the systematic study of their hydrolysis, induced by reagents, or
by the action of enzymes, whereby they are found to break down into
proteoses, peptones, and a great variety of amino acids, some of which
have been synthesised. Among the proteins of simplest constitution are
the _protamines_, found in the spermatozoa of fish. They are basic
substances, especially rich in nitrogen, forming salts with platinum
chloride and certain metallic oxides. The best investigated member of
the group is _salmine_, obtained from the testicle of the salmon. The
products of its hydrolysis have been fairly well ascertained, and their
quantitative relation is such that the substance must have at least a
molecular weight of 2045, corresponding to the formula C81H155N45O18.
Many of the albumins and globulins—coagulable proteins contained in the
animal tissues—have been isolated in a more or less definite form, and
some of them have been found to yield substances akin to carbohydrates.
_Thyreoglobulin_, the globulin of the thyroid gland, has been found to
contain iodine, apparently as a normal constituent of a body which can
be isolated as a definite proximate principle. The presence of this
element is possibly connected with the curative value of the globulin
in crétinism. A considerable amount of work on the vegetable albumins
has also been done of late years; and some of them, as _edestin_ from
hemp seed and _zein_ from maize, have been obtained in definite form.

The limits of this work preclude a more detailed account of one of
the most interesting but at the same time one of the most obscure
departments of chemistry. The field has hitherto been tilled in a
somewhat intermittent and partial manner. Now that it has been entered
by chemists of experience and resourcefulness, armed with modern
methods of cultivation, it will doubtless soon yield a rich harvest of
facts, valuable alike to the physiologist and the physician.

There can be no reasonable doubt that the chemical processes of organic
life are essentially similar to those of the laboratory. The doctrine
that a special “vital force” is concerned in the production of vital
products receives no support from the teaching of modern science,
and is, indeed, contradicted by it. At the same time, it must be
admitted that we know very little as yet of the real agencies at work
in the elaboration and mutations of chemical products in the living
organism. Because we have effected the putting together of such a
product by purely laboratory processes—it may be, indeed, by a variety
of different and dissimilar processes—it by no means follows that
any one of them is identical with that actually occurring in nature.
The building up of materials in the plant by the agency of light,
for example, has not yet been imitated in the laboratory. Many plant
products are produced by the action of unorganised ferments—so-called
enzymes—none of which the chemist has succeeded in creating.

Processes akin to condensation undoubtedly occur in the living
organism; but the means by which they are effected are, in all
probability, very different from anything known to the chemist at
present. Many laboratory condensations are only accomplished at
relatively high temperatures or under considerable pressure—or, in
other words, under totally different conditions from those which obtain
in the organism.




CHAPTER XI

ON THE DEVELOPMENT OF PHYSICAL CHEMISTRY SINCE 1850


Chemistry and physics are each complementary to the other: that region
of inquiry in which they mutually overlap is known as _physical
chemistry_. Its beginnings are practically contemporaneous with those
of chemistry itself. Its main development has occurred, however, during
the last twenty-five years. Certain of its leading features have
been referred to already in connection with the establishment of the
fundamental principles of chemistry, the explanation of the so-called
gaseous laws, the constitution of gases, the relations of their volumes
to heat and pressure, and the conditions affecting their transition to
the liquid state.

As regards the molecular volumes of gases it has been shown that
simple relations are obtained when quantities represented by their
respective molecular weights are compared under identical conditions of
temperature and pressure—that is, under circumstances in which equal
numbers of molecules form the basis of comparison. The investigation
of the molecular volumes of liquids is complicated by the uncertainty
as to what constitutes in their case a valid condition of comparison.
Kopp’s assumption that a comparable condition was the temperature
at which the vapour pressures of the liquids are equal to the mean
atmospheric pressure was justified by the fact that the boiling-points
of liquids are approximately two thirds of their respective critical
temperatures. His conclusions have been confirmed and extended by
Lossen, Thorpe, and Schiff. It has been shown that the molecular
volume of a liquid—that is, the product of its relative density at the
boiling-point into its molecular weight—is in the main an additive
function modified by constitutive influences. Definite values have
thus been obtained for a number of the elements from a comparison of
homologous or similarly constituted compounds; and in certain cases
these are found to be practically identical with the values of the
elements in the uncombined state.

Considerable light has been gained during the last two decades
concerning the nature of _solution_. In its most comprehensive sense
solution means the homogeneous mixture of two or more substances: thus
the gases which exert no chemical action on each other are mutually
soluble; gases, liquids, and solids may be soluble in liquids; and,
lastly, solids maybe soluble in solids, forming what are known as
_solid solutions_. The mutual solubility of gases was studied by Dalton
who enunciated the _law of partial pressures_, which states that the
total pressure of a mixture of gases is the sum of the pressures
exerted by the individual components. This, like all the so-called
gaseous laws, is necessarily not strictly accurate under ordinary
conditions, but approximates to truth in proportion as the gases are
rarefied. Van ’t Hoff pointed out that the true partial pressures of
the components of a gaseous mixture might be experimentally ascertained
by the use of a membrane capable of effecting their separation, and on
this principle Ramsay measured the partial pressures of a mixture of
hydrogen and nitrogen contained in a palladium vessel connected with
a manometer. The palladium, at a sufficiently high temperature, is
permeable to hydrogen to the exclusion of the nitrogen. The conditions
affecting the solubility of gases in liquids were experimentally
studied by Dalton and Henry, and what is known as Henry’s law implies
that the volume of a gas dissolved by a definite volume of a liquid
is independent of the pressure; or, in other words, the density
(concentration) of the gas in solution is proportional to that in
the space above the liquid. Gases are dissolved by liquids in very
different amounts, but nothing definite is known as yet concerning the
relation between the nature of the gas and its solubility, although
certain broad generalisations are possible. Thus neutral gases—_e.g._,
hydrogen and nitrogen—are sparingly soluble, whereas gases which
show acidic or basic properties, such as the hydrogen halides, etc.,
ammonia, etc., are freely soluble. Easily liquefiable gases are also
comparatively soluble as noted by Graham.

Comparatively little is known definitely concerning the conditions
of solubility of liquids in liquids. Some liquids are wholly, others
partially miscible; and temperature and pressure appear to affect the
proportions in which the components form a homogeneous mixture. As
regards the solubility of solids in liquids, our knowledge is more
extensive, and a considerable body of literature exists on the subject,
chiefly concerning solubility of solids in water. The solubility of
a solid depends on the temperature of the solvent, and, as a rule,
increases with the temperature until a certain amount of the solid
has been dissolved, when the solution is said to be _saturated_. If
the clear saturated solution be slowly cooled, say, to a particular
temperature, it is frequently observed that more of the solid remains
in solution than is normal to that temperature; such a solution is
said to be _supersaturated_. On adding some of the solid to the
supersaturated solution the excess of the _solute_ is precipitated.
In certain cases of solubility of substances in water, increase of
temperature appears to diminish the amount dissolved. In nearly all
such cases the difference in solubility is due to differences in the
hydration of the solute. The phenomena of solid solutions have been
less perfectly investigated, but the facts appear to show that such
solutions in general tend to obey the laws regulating the solution of
liquids in liquids. Alloys may be looked upon as solid solutions; and
Roberts-Austen has shown that metals are capable of intradiffusion,
like liquids and gases respectively.

The general question of solution was greatly developed in 1885 by
Van ’t Hoff, by specially considering the case of dilute solutions. The
gaseous laws are capable of their simplest expression when the gases
are rarefied to such an extent that their molecules exert no sensible
mutual influence. The case of dilute solutions is analogous. If the
solute is present only in very small amount, the mutual influence of
its molecules is practically negligible. Under such conditions it obeys
the laws hitherto supposed to be applicable only to matter in the
gaseous state.

It may be desirable to explain how this fundamental fact was
recognised. It has long been known to the physiologist that certain
membranes are _semi-permeable_—that is, they allow of the passage of
certain liquids, and of substances in solution, to the exclusion of
others. This phenomenon is termed _osmosis_, and is of great biological
significance. It was first studied by plant-physiologists, notably by
Traube and Pfeffer. Many such semi-permeable membranes can be formed
artificially, but the most generally convenient is found to be one
consisting of copper ferrocyanide deposited on the walls of a porous
vessel.

If a vessel so prepared be filled with a solution of sugar, and be
then placed in water, the water is found to pass through the membrane,
but the membrane is impermeable to the sugar. In consequence pressure,
termed _osmotic pressure_, is found to occur within the pot, and may
be measured by suitable means. These osmotic pressures may at times be
very large: thus a 1 per cent. solution of sugar may exert a pressure
of half an atmosphere, and in the case of a solution of potassium
nitrate of the same concentration it may amount to a couple of
atmospheres.

Pfeffer determined the relation of the osmotic pressures to the
concentration of solutions of these substances, measuring the pressures
in centimetres of mercury by a manometer attached to the closed porous
vessel. His results in the case of sugar were as follows:

     _Percentage      _Pressure in cm.
    strength (C)._    of mercury (P)._    _P/C._

          1                53.5            53.5
          2               101.6            50.8
          4               208.2            52.1
          6               307.5            51.3

It will be seen from these numbers that the ratio P/C is practically
constant—that is, _the osmotic pressure varies directly as the
concentration_. It was further found that the osmotic pressure exerted
by a solution of uniform strength increases with the temperature.

The importance, of these observations in relation to the general theory
of solution was first recognised by Van ’t Hoff. Osmotic pressure was
regarded by him as analogous to gaseous pressure. Since P/C is constant
for any one substance, and since for a definite weight of the solute
the concentration is inversely as the volume of the solution, we obtain
an equation analogous to the statement of Boyle’s law, PV = constant.
Van ’t Hoff also found that the _osmotic pressure is proportional
to the absolute temperature_, like the gaseous pressure. From these
results, in conjunction with Avogadro’s hypothesis, it follows that
_the osmotic pressure exerted by any substance in solution is the
same as it would exert if present as gas in the same volume as that
occupied by the solution, provided that the solution is so dilute that
the volume occupied by the solute is negligible in comparison with
that occupied by the solvent_. Another important consequence is that
_solutes, when present in the ratio of their molecular weights in equal
volumes of the same solvent, exert the same osmotic pressure_. Such
solutions are said to be _isomotic_ or _isotonic_. It can be proved by
thermodynamical reasoning that depression of the vapour pressure and
freezing-point of a solution is proportional to its osmotic pressure.
The significance of this relation in connection with the determination
of the molecular weight of a soluble substance has already been
referred to.[6]

    [6] See pp. 70–73.

Determinations of molecular freezing-point depressions by Raoult
and others showed that certain substances exerted only about half
the osmotic pressure calculated from their known formulæ, whereas
others have abnormally high osmotic pressures. The explanation of
the discrepancies in the latter case was given in 1887 by Arrhenius,
who pointed out that _only those solutions which have abnormally
high osmotic pressures are electrically conductive_. This pregnant
observation proved to be very fruitful in suggestiveness; and the
connection between conductivity and Van ’t Hoff’s theory of solution
was developed by Arrhenius into the doctrine of _electrolytic
dissociation_ or _ionisation_—one of the most important consequences
of Faraday’s electrolytic laws, the work of Hittorf, and the kinetic
conceptions of Williamson and Clausius to which the last quarter
of a century has given rise. Arrhenius showed that not only were
free ions present in an electrically conductive solution before
electrolysis, as maintained by Clausius, but that the proportion of
molecules dissociated into ions could be calculated from measurements
of electrical conductivity, as well as from measurements of osmotic
pressure. Both methods give concordant results—a strong confirmation of
the validity of the theory. In a solution of common salt, containing a
gramme equivalent of that substance in a litre, Arrhenius calculated
that only about three tenths of the salt exists as NaCl, the remaining
seven tenths being resolved into independent ions of chlorine
(chloridion) and sodium (sodion): NaCl⇄[Na·] + Cl´, each moving freely
in all directions, like gaseous molecules. On passing the current,
electrodes placed in the solution exert a directive action on the free
ions, these alone being concerned in determining the conductivity, the
un-ionised molecules or the solvent itself exercising no influence.
Methods of determining the migration velocity of the ions have been
worked out by Hittorf, Kohlrausch, Lodge, and others.

[Illustration: SVANTE AUGUST ARRHENIUS.]

The theory of ionisation affords a satisfactory explanation of many
chemical phenomena. It accounts for the characteristic properties of
acids, and explains why different acids have varying “strengths” and
why a “weak” acid has the same “strength” as the “strong” acid at
high equivalent dilutions: in each case the acid is nearly completely
ionised—in other words, the “strength” of an acid depends on the
concentration of its hydrogen ions. So, too, the “strength” of a base
is related to the number of its hydroxyl ions. Aqueous ammonia is
relatively a “weak” base—its solution contains few hydroxyl ions. On
the other hand, caustic potash is a “strong” base—its solution, on
moderate dilution, is almost completely ionised: KOH = K· + OH´, the
positive ion being represented by one or more dots, and the negative
ion by one or more dashes. The theory accounts, too, for many phenomena
in analytical chemistry—such as why magnesia is precipitated by ammonia
only in the absence of ammonium chloride, and why sulphuretted hydrogen
throws down zinc sulphide in the absence of hydrochloric acid. It
also serves to explain many thermo-chemical facts observed by Hess,
Thomsen, and others, such as the fact that the heat of neutralisation
of the “strong” acids and bases is independent of their nature, and has
the uniform value of 13,700 calories, in agreement with the value, as
calculated by Van ’t Hoff, for the reaction H· + OH´ = H2O, deduced
from Kohlrausch’s measurements of the conductivity of water at varying
temperatures.

Certain phenomena relative to the effect of concentration (mass action)
in determining chemical change—many of which have been studied by
Ostwald and his pupils, as, for example, why two dilute solutions can
be mixed together without thermal disturbance; numerous hydrolytic
actions; the alkalinity and acidity of salts on solution; the behaviour
of the “indicators” in analysis; such phenomena as the precipitability
of common salt in aqueous solution by hydrogen chloride; the influence
of an excess of a precipitant; the varying behaviour of reagents; the
varying colour of salt solutions; the reason why water is formed in so
many reactions; why a potential difference occurs at the surface of
two electrolytic solutions, etc.—phenomena for the most part otherwise
unintelligible, are all capable of explanation by means of it.

Although, in the above statement, we have been mainly concerned with
aqueous solutions, it should be said that the theory of ionisation
is applicable to other solvents, organic and inorganic. Moreover,
it should be added, the theory has not been universally accepted as
accounting for all the phenomena of solution. Many substances form
definite hydrates which can be isolated, and it is a moot point
whether such hydrates are capable of existing in aqueous solution, as
contended by Mendeléeff, Pickering, Kahlenberg, Armstrong, and others.
Such hydrates are, however, unstable compounds, affected by temperature
changes, and dissociable on dilution in accordance with the law of
concentration (mass action). Further, there is evidence, largely based
on the work of Kohlrausch, H. C. Jones, and Lowry, to show that the
ions in aqueous solutions of electrolytes are themselves hydrated.

Limitations of space preclude further attempts to deal with the
development of physical chemistry during the last half-century, and
many important matters must remain practically unnoticed.

The subject of thermo-chemistry is mainly the creation of the last
half-century, elaborated by the labours of Hess, Andrews, Thomsen,
Favre and Silbermann, and Berthelot. The work of Wenzel and Berthollet
on the influence of molecular concentration on chemical change has
been greatly extended by Berthelot, Guldberg and Waage, Julius
Thomsen, Van ’t Hoff, Harcourt and Esson, and Le Chatelier; and the
theory of mass action and the nature of reversible processes are now
capable of definite expression, and can be proved independently by
thermo-dynamical and kinetic reasoning. The phenomena of catalysis
and the action of enzymes and of fermentation in general have
received attention from many investigators. The phenomena of gaseous
transpiration have been studied by Graham, Maxwell, and O. E. Meyer.
Thermal dissociation has been experimentally observed by Deville,
Troost, and others, and mathematically investigated by Willard Gibbs
and Van der Waals; and its analogy to electrolytic dissociation
has been established. The nature of gaseous explosions has been
investigated by Berthelot, Le Chatelier, Abel, and Dixon. Important
work has been done by Gladstone, Lorentz, Landolt, Nasini, Brühl,
and others, on the connection between the nature and constitution of
substances and their optical characters. Similar work has been done by
Sir William Perkin as regards their magnetic rotation, and by Thorpe
and Rodger with reference to their viscosity. The theory of phases,
originating with Gibbs and developed by Van der Waals and Roozeboom,
has been greatly extended. Sir J. J. Thomson and Sir J. Larmor have
elaborated an electrical theory of the atom. Barlow and Pope have
traced the relation between valency and volume, and the accurate
measurements of Groth and of Tutton have extended our knowledge of the
crystallographic relations of correlated substances.

Lastly, the whole subject of photo-chemistry, although originating with
the observations of Ingenhousz, Scheele, and Senebier, may be said
to have been studied only within our own time, notably by Bunsen and
Roscoe, Pringsheim, Pfeffer, Vogel, and Abney.




BIBLIOGRAPHY

RELATING TO THE PERIOD COVERED BY VOL II.


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Price. Longmans, 1907.

Beilstein’s _Handbuch der Organischen Chemie_. Nine vols. Leopold Voss,
Hamburg, 1901–1906.

Bischoff’s _Materialen der Stereochemie_. Vieweg and Son, Brunswick,
1904.

Bischoff and Walden’s _Handbuch der Stereochemie_. H. Beckhold,
Frankfort-on-the-Main, 1894.

Cain, J. C., and Thorpe, J. F. _Synthetic Dyestuffs and Intermediate
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Chemical Society’s Annual Reports. Gurney and Jackson.

Chemical Society. _Memorial Lectures, 1893–1900._ Gurney and Jackson,
1901.

Cohen, Julius B. _Organic Chemistry._ Edward Arnold, 1907.

Curie, Marie. _Radio-Active Substances._ “Chemical News,” London, 1903.

Findlay, A. _The Phase Rule._ Longmans, 1904.

Fischer, Emil. _Die Aminosäuren, Polypeptide und Proteine._ Julius
Springer, Berlin, 1906.

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Berlin, 1907.

Fischer, Emil. _Untersuchungen über Kohlenhydrate und Fermente._ Julius
Springer, Berlin, 1909.

Freund, Ida. _Study of Chemical Composition._ Cambridge University
Press, 1904.

Garrett, A. E. _The Periodic Law._ Kegan Paul, 1909.

Ladenburg, Albert. _Development of Chemistry since the Time of
Lavoisier._ Translated by Leonard Dobbin. Alembic Club, Edinburgh, 1900.

Landolt, H. _Optical Activity and Chemical Composition._ Translated by
J. McCrae. Whittaker, 1900.

Laurent, A. _Chemical Method._ Translated by W. Odling. Cavendish
Society’s Publications, London, 1855.

Mann, Gustav. _Chemistry of the Proteids._ Macmillan, 1906.

Maxwell, Clerk. _Theory of Heat._ With corrections and additions by
Lord Rayleigh. Longmans.

Meldola, Raphael. _Chemical Synthesis of Vital Products._ Edward
Arnold, 1904.

Mendeléeff, D. _Principles of Chemistry._ Translated by Kamensky and
Greenaway. Longmans.

Meyer, Lothar. _Outlines of Theoretical Chemistry._ Translated by
Bedson and Williams. Longmans, 1892.

Meyer, Lothar. _Modern Theories of Chemistry._ Translated by Bedson and
Williams. Longmans, 1888.

Meyer and Jacobson’s _Lehrbuch der Organischen Chemie_. Veit and Co.,
Leipzig.

Meyer, O. E. _The Kinetic Theory of Gases._ Translated by R. E. Baynes.
Longmans, 1899.

Muir, M. M. Pattison. _History of Chemical Theories and Laws._ John
Wiley and Sons, New York, 1907.

Nernst, Walter. _Theoretical Chemistry._ Translated by C. S. Palmer.
Macmillan, 1895.

Ostwald’s _Klassiker der Exakten Wissenschaften_.

Pictet, Amé. _Vegetable Alkaloids._ Translated by H. C. Biddle. John
Wiley and Sons, New York, 1904.

Richter’s _Lexikon der Kohlenstoff Verbindungen_. Leopold Voss, Hamburg
and Leipzig.

Roscoe and Schorlemmer, _Treatise on Chemistry_. Macmillan.

Rutherford, E. _Radio-Activity._ Cambridge University Press, 1904.

Schorlemmer, Carl. _Rise and Development of Organic Chemistry._ Edited
by Arthur Smithells. Macmillan.

Schryver, S. B. _Chemistry of the Albumens._ Murray, 1906.

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Co., London, 1904.

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Thorpe, T. E. _Essays in Historical Chemistry._ Macmillan, 1902.

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INDEX


    Abel, 184

    Abney, 185

    Absolute zero, 97

    Acetanilide, 159

    Aceto-acetic ester, 148

    Actinium, 59

    Adrenaline, 160

    Agurin, 158

    Alizarin, 162

    Alkaloids, 132

    Allyl alcohol, 4

    Alyhine, 160

    Ammonium chloride, dissociation of, 66

    Ampère, 62

    Anderson, 133

    Andrews, 70, 95, 183

    Aniline blue, 4

    Aniline purple, 4

    Antimony, origin of name, 28

    Antipyrin, 159

    Argon, 43, 45, 46

    Argyrodite, 37

    Armstrong, E. F., 151

    Armstrong, H. E., 183

    Aromatic compounds, 121, 123

    Arrhenius, 179, 180

    Arsine-dimethyl, 20

    Artiads, 116

    Asymmetry, 141

    Atmolysis, 84

    Atomic value, 114

    Atomic weight, determination of, 75

    Atomicity, 114

    Atropine, 135, 162
      its synthesis 135

    Australene, 130

    Autoracemisation, 144

    Avogadro, 62
      his hypothesis, 63


    Baeyer, 125, 129, 161, 163

    Balard, 139

    Baly, 129, 151

    Bamberger, 147

    Barbier, 154

    Barlow, 184

    Basarow, 157

    Basicity, 16

    Becquerel, Henri, 52

    Behrend, 157

    Benzene, 123
      its constitution, 123 _et seq._, 128

    Benzidam, 3

    Berberine, 136

    Bernoulli, Daniel, 82

    Berthelot, Daniel, 82, 183, 184

    Berthollet, 183

    Berzelius, 63

    Biot, 138

    Bitter almond oil, 7

    Blagden, Sir Charles, 72

    Boisbaudran, Lecoq de, 36, 38, 39

    Boltzmann, 83

    Bone oil, 133

    Boron, specific heat of, 71

    Boyle’s law, 84–87

    Brauner, 33

    Bredt, 129, 130

    Brieger, 161

    Brühl, 149, 151

    Bunsen, Robert W., 18, 34, 43, 185

    Bunsen burner, 22


    Cacodyl, 20

    Cadaverine, 161

    Cæsium, 21, 34

    Caffeine, 157

    Cahours, 4, 132

    Cailletet, 96

    Camphene, 131

    Camphor, 130, 131, 162

    Cannizzaro, Stanislao, 64, 79

    Carbon, atomic weight of, 75
      specific heat of, 71
      tetravalency of, 115

    Carbon carbonyl, 155

    Carbon suboxide, 155

    Carnelley, 108

    Carnolite, 58

    Cavendish, 43, 45

    Cayley, 120

    de Chancourtois, 102

    Chloral hydrate, 159

    Choline, 161

    Cinchonine, 137

    Clausius, 83, 179

    Cleve, 38, 47

    Cleveite, 47

    Cocaïne, 136, 162

    Codeine, 137

    Coindet, 8

    Collie, 49, 128

    Condensation, 152

    Conine, 162

    Conservation of mass, law of, 78

    Coumarin, 132, 162

    Critical point, 94

    Critical pressure, 95

    Crookes, Sir William, 35, 40, 75

    Curie, Marie, 55

    Cyanic acid, 7

    Cyanuric acid, 7


    Debierne, 59

    Decipium, 40

    Delafontaine, 49

    Demarçay, 30

    Desch, 151

    Deville, 184

    Dewar, 71, 96, 127

    Dexter, 157

    Didymium, 40

    Digitalis, 10

    Disintegration theory, 54

    Dixon, 184

    Dobbie, 151

    Doebner, 134

    Dumas, Jean B. A., 8, 9, 30, 31, 63, 75, 102, 130

    Duppa, 148

    Dysprosium, 38


    Edestin, 169

    Electrolysis, 179

    Elements, nomenclature of, 27

    Emanation from Radium, 58

    Emanium, 60

    Enantiomorphism, 140

    Enzymes, 144

    Ether, discovery of its constitution, 12, 17

    Etherin theory, 10

    Europium, 39


    Favre, 183

    Fenton’s reagent, 154

    Findlay, 151

    Fischer, Emil, 158, 165, _et seq._

    Fittig, 153

    Fluorine, 33, 34

    Formulæ, chemical, significance of, 68

    Frankland, 114, 116, 148

    Fuchsine, 4

    Fumaric acid, 146


    Gadolinium, 39

    Gallium, 36, 107

    Gases, kinetic theory of, 83
      law of diffusion of, 84, 89
      liquefaction of, 94
      molecular theory of, 79

    Gas-mantles, 40

    Gay Lussac, 63, 80

    Geometrical isomerism, 146, 147

    Gerhardt, 79, 113

    Germanium, 37, 107

    Geuther, 148

    Gibbs, Willard, 184

    Giesel, 60

    Gladstone, 30, 102, 184

    Gmelin, 7, 63, 102

    Gore, 33

    Graebe, 162

    Graham, Thomas, 2, 14, 23, 29, 83

    Grignard’s reagent, 154

    Groth, 184

    Guldberg, 183

    Guye, 81


    Haarmann, 132

    Hæmoglobin, 168

    Hantzsch, 147

    Hartley, 151

    Haüy, 139

    Heliotropin, 132

    Helium, 43, 47, 49

    Helmholtz, 117

    Henry’s law, 173

    Hermann, 26

    Herschel, 140

    Hess, 181, 183

    Heumann, 163

    Heymann, 163

    Hillebrand, 47

    Hittorf, 179

    Hofmann, August W., 3, 67, 133, 161

    Holmium, 38

    Hoogewerff, 133

    Horbaczewski, 157

    Hübner, 127

    Hydrogenium, 16

    Hyoscyamine, 135


    Ilmenium, 26

    Indigo, 162, 164

    Indigo blue, 163

    Indium, 36

    Ingenhousz, 185

    Ionisation, 178

    Ionium, 58

    Ionone, 132

    Irone, 132

    Isoborneol, 131

    Isoconine, 134

    Isoquinoline, 133

    Isotonic solutions, 178


    Jones, H. C., 183

    Jones, H. O., 147


    Kahlenberg, 183

    Kammerlingh Onnes, 49, 97

    Kekulé, Friedrich August, 23, 113, 114, 121, 123, 125, 153

    Kiliani, 165

    Kipping, 147

    Kirchhoff, 21

    Knorr, 128, 151, 159

    Koenigs, 133

    Kohlrausch, 182, 183

    Komppa, 129

    Kopp, 172

    Körner, 127

    Krüss, 40

    Krypton, 43, 50

    Krystallin, 3

    Kundt, 46


    Laar, 149

    Lactic acid, spatial representation of, 141

    Ladenburg, 127, 134, 147, 161

    Lamy, 35

    Landolt, 78, 184

    Langlet, 49

    Larmor, Sir J., 184

    Lauder, 151

    Laurent, 79

    Le Bel, 143, 147, 148

    Leduc, 80, 81

    Le Royer, 8

    Leucol, 133

    Liebermann, 162

    Liebig, Justus von, 5

    Linnemann, 127

    Liquid diffusion, 16

    Liquids, molecular volumes of, 172

    Lossen, 172

    Lowry, 151, 183

    Low temperature research, 97, 39

    Lutecium, 89


    Magenta, 4

    Magnus, 79

    Maleic acid, 146

    Mansfield, 3

    Marignac, 39, 75

    Mauve, 4

    Maxwell, 83

    Medlock, 3

    Mellitic acid, 7

    Membranes, semi-permeable, 176

    Mendeléeff, 30, 102, 109, 183

    Mercury forms a monatomic gas, 71, 92

    Metaphosphoric acid, 14

    Meyer Lothar, 103

    Meyer, Victor, 67, 147

    Mitscherlich, 63, 138

    Moissan, 33

    Molecule, integral, 62

    Molecules, their number, 93
      their size, 93

    Monium, 40

    Morley, 80

    Morphine, 137

    Mosander, 39

    Multirotation, 145

    Muscarine, 161

    Muspratt, 4

    Mutarotation, 145


    Naphthalene, 163

    Narceïne, 136

    Narcotine, 136

    Nasini, 184

    Natanson, 157

    Neodymium, 39

    Neon, 43, 50

    Neoytterbium, 38

    Neurine, 161

    Newlands, 102

    Nicotine, 135, 162

    Nilson, 37, 40, 107

    Nirvanine, 160

    Nitrogen compounds, stereo-isomerism of, 147

    Novocaïne, 160


    Odling, 30, 114

    Oil of wintergreen, 162

    Olszewki, 96

    Opianic acid, 136

    Organic synthesis, 152

    Orthoforms, 160

    Osmotic pressure, 176

    Ostwald, 182

    Oxyhæmoglobin, 167

    Ozone, 70
      its symbol, 70


    Papaverine, 136

    Partial pressure, law of, 173

    Pasteur, 139

    Peachey, 148

    Pelopium, 26

    Penny, 75

    Periodic law, 101

    Perissads, 116

    Perkin, Sir William, 3, 129, 131, 132, 162

    Pettenkofer, 30

    Pfeffer, 176, 185

    Phenacetin, 159

    Phenylglycin, 163

    Phosphorous oxide, molecular weight of, 73

    Phosphorus pentachloride, 66

    Phosphorus pentafluoride, 67

    Phthalic acid, 163

    Pickering, 183

    Pictet, 96, 135

    Pinene, 130, 131

    Pinner, 135

    Piperine, 134, 162

    Plattner, 35

    Playfair, 20

    Pollucite, 35

    Pope, 147, 184

    Posselt, 135

    Praseodymium, 38

    Pringsheim, 185

    Protamines, 168

    Proteins, 165

    Prout’s law, an illusion, 76

    Ptomaines, 161

    Pyridine, 133


    Quinine, 137

    Quinoline, 133


    Racemic acid, 140

    Racemism, 143

    Radio-thorium, 60

    Radium, atomic weight of, 58
      discovery of, 55
      disintegration of, 58
      emanation, 58
      extraction of, 55

    Radium chloride, 57

    Ramsay, Sir William, 44, 47, 173

    Raoult, 72, 178

    Rayleigh, Lord, 44, 81

    Regnault, 43, 79, 88, 157

    Reich, 36

    Reimann, 135

    Richards, Theodore, 77

    Richter, 36

    Rodger, 184

    Roosen, 157

    Roozeboom, 184

    Rosaniline, 4

    Roscoe, 21, 30, 117, 185

    Rose, 26

    Rotatory power, 144

    Rubidium, 21, 34

    Runge, 133

    Rutherford, 59


    Salicylic acid, 162

    Salmine, 168

    Samarium, 40

    Sarcine, 157

    Sarcolactic acid, 140

    Saturated solutions, 174

    Saussure, 43

    Scandium, 37, 107

    Scheele, 33, 140, 185

    Schiff, 172

    Schischkoff, 21

    Schmidt, 83

    Schmiedeberg, 161

    Scott, 80

    Selmi, 161

    Senebier, 185

    Sesquiterpenes, 129

    Silbermann, 183

    Silicon, specific heat of, 71

    Skraup, 133

    Solute, 175
      influence of, on boiling-point, 71
      on freezing-point, 71
      on vapour pressure, 73

    Sonstadt, 153

    Soret, 70

    Spectrum analysis, 21

    Stas, 75

    Steric hindrance, 151

    Stovaïne, 160

    Strecker, 102, 158, 161

    Sugars, 165

    Sulphonal, 159

    Supersaturated solutions, 174


    Tait, 70

    Tautomerism, 149

    Terebenthene, 130

    Terpenes, 129

    Tetanine, 161

    Tetronal, 159

    Thallium, 35

    Thebaine, 137

    Thénard, 43

    Theobromine, 157

    Thioacetic acid, 127

    Thomsen, Julius, 51, 108, 183

    Thomson, Sir J. J., 184

    Thomson, Thomas, 3

    Thorium, 60

    Thorpe, 172, 184

    Thulium, 38

    Thyreoglobulin, 169

    Tiemann, 129, 132

    Traube, 176

    Travers, 50

    Trional, 159

    Triphenylrosaniline, 4

    Troost, 184

    Turner, Edward, 2, 14, 75

    Turpentine, oil of, 129

    Tutton, 184

    Type theory, 18

    Typhotoxine, 161


    Unverdorben, 3

    Uranium, 54

    Uranium │x│, 54

    Urbain, 40

    Urea, synthesis of, 157, 158

    Uric acid, 158


    Valency, 112

    Vanadium, 30

    Van der Waals, 95, 184

    Van Dorp, 133

    Vanillin, 132, 162

    Van ’t Hoff, 117, 141, 145, 147, 173, 177

    Veronal, 159

    Verquin, 4

    Victorium, 40

    “Vital force” doctrine, 169

    Vogel, 185

    Von Miller, 134


    Waage, 183

    Wallach, 129

    Warburg, 46

    Waterston, 83

    Weber, 71

    Weisbach, 37

    Welsbach, Auer von, 38

    Werner, 147

    Williamson, Alexander W., 14, 16, 113

    Winkler, 107

    Wintergreen, oil of, 132

    Wislicenus, 140

    Wöhler, Friedrich, 5, 7

    Wollaston, 63

    Wroblewski, 96

    Wurtz, 153


    Xanthine, 157

    Xenon, 43, 51


    Zein, 169

    Zinin, 3




A History of the Sciences


¶ Hitherto there have been few, if any, really popular works touching
the historical growth of the various great branches of knowledge.
The ordinary primer leaves unexploited the deep human interest which
belongs to the sciences as contributing to progress and civilization,
and calling into play the faculties of many of the finest minds.
Something more attractive is wanted.

¶ The above need in literature has now been met. Each volume in _The
History of Sciences_ is written by an expert in the given subject,
and by one who has studied the history as well as the conclusions of
his own branch of science. The monographs deal briefly with the myths
or fallacies which preceded the development of the given science, or
include biographical data of the great discoverers. Consideration is
given to the social and political conditions and to the attitudes
of rulers and statesmen in furthering or in hindering the progress
of the given science. The volumes record the important practical
application of the given science to the arts and life of civilized
mankind, and also contain a carefully-edited bibliography of the
subject. Each volume contains from twelve to sixteen carefully-prepared
illustrations, including portraits of celebrated discoverers, many from
originals not hitherto reproduced, and explanatory views and diagrams.
The series as planned should cover in outline the whole sphere of human
knowledge.

¶ Science is to be viewed as a product of human endeavor and mental
discipline, rather than taken in its purely objective reference to
facts. The essential purpose has been to present as far as practicable
the historical origins of important discoveries, also to indicate the
practical utility of the sciences to human life.

    G. P. Putnam’s Sons
    New York      London




A History of the Sciences

Each volume is adequately illustrated, attractively printed, and
substantially bound.

_16mo. Each, net, 75 cents. By mail, 85 cents. 12 illustrations_


History of Astronomy

By George Forbes, M.A., F.R.S., M.Inst. C.E.

Formerly Professor of Natural Philosophy, Anderson’s College, Glasgow

I thank you for the copy of Forbes’s _History of Astronomy_ received.
I have run it over, and think it very good indeed. The plan seems
excellent, and I would say the same of your general plan of a series of
brief histories of the various branches of science. The time appears
to be ripe for such a series, and if all the contributions are as good
as Prof. Forbes’s, the book will deserve a wide circulation, and will
prove very useful to a large class of readers.—_Extract from a letter
received from Garrett P. Serviss, B. S._


History of Chemistry

By Sir Edward Thorpe, C.B., LL.D., F.R.S.

Author of “Essays in Historical Chemistry,” “Humphry Davy: Poet and
Philosopher,” “Joseph Priestley,” etc.

_12 illustrations. Two vols. Vol. I—circa 2000 B.C. to 1850 A.D. Vol.
II—1850 A.D. to date_

The author traces the evolution of intellectual thought in the progress
of chemical investigation, recognizing the various points of view of
the different ages, giving due credit even to the ancients. It has
been necessary to curtail many parts of the History, to lay before the
reader in unlimited space enough about each age to illustrate its tone
and spirit, the ideals of the workers, the gradual addition of new
points of view and of new means of investigation.


The History of Old Testament Criticism

By Archibald Duff

Professor of Hebrew and Old Testament Theology in the United College,
Bradford

The author sets forth the critical views of the Hebrews concerning
their own literature, the early Christian treatment of the Old
Testament, criticism by the Jewish rabbis, and criticism from Spinoza
to Astruc, and from Astruc until the present.


_In Preparation_

  The History of Geography.

    By Dr. JOHN SCOTT KELTIE, F.R.G.S., F.S.A., Hon. Mem. Geographical
      Societies of Paris, Berlin, Rome, Brussels, Amsterdam, Geneva,
      etc.


The History of Geology.

    By HORACE B. WOODWARD, F.R.S., F.G.S., Assistant Director of
      Geological Survey of England and Wales.


The History of Anthropology.

    By A. C. HADDON, M.A., Sc.D., F.R.S., Lecturer in Ethnology,
      Cambridge and London.


The History of New Testament Criticism.

    By F. C. CONYBEARE, M.A., late Fellow and Praelector of Univ.
      Coll., Oxford; Fellow of the British Academy; Doctor of Theology,
      _honoris causa_, of Giessen; Officer d’Academie.


_Further volumes are in plan on the following subjects_:

  Mathematics and Mechanics—Molecular Physics, Heat, Light, and
  Electricity—Human Physiology, Embryology, and Heredity—Acoustics,
  Harmonics, and the Physiology of Hearing, together with
  Optics, Chromatics, and Physiology of Seeing—Psychology,
  Analytic, Comparative, and Experimental—Sociology and
  Economics—Ethics—Comparative Philology—Criticism, Historical
  Research, and Legends—Comparative Mythology and the Science of
  Religions—The Criticism of Ecclesiastical Institutions—Culture,
  Moral and Intellectual, as Reflected in Imaginative Literature and
  in the Fine Arts—Logic—Philosophy—Education.

    New York      G. P. Putnam’s Sons      London




Transcriber’s Notes


Punctuation and spelling were made consistent when a predominant
preference was found in this book; otherwise they were not changed.

Simple typographical errors were corrected; occasional unbalanced
quotation marks retained.

Ambiguous hyphens at the ends of lines were retained; occurrences of
inconsistent hyphenation have not been changed.

Index not checked for proper alphabetization or correct page references.

The table on page 105 has been split into three parts to keep it narrow.