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Transcriber's Note:

Minor inconsistencies in spelling, punctuation and formatting are
retained as in the original. Where detailed corrections have been made
on the text these are listed at the end of this document.

Disclaimer:

This is a work of historical interest only and much of the scientific
content has been superseded. There are numerous experiments described
in this book which are hazardous and should not be attempted. Advice
given on handling toxic substances, electrical apparatus etc. should
not be followed.

Do not try this at home!

         *       *       *       *       *






                  COMMON SCIENCE

              NEW-WORLD SCIENCE SERIES
            _Edited by John W. Ritchie_

         *       *       *       *       *

  SCIENCE FOR BEGINNERS
    By _Delos Fall_

  TREES, STARS, AND BIRDS
    By _Edwin Lincoln Moseley_

  COMMON SCIENCE
    By _Carleton W. Washburne_

  HUMAN PHYSIOLOGY
    By _John W. Ritchie_

  SANITATION AND PHYSIOLOGY
    By _John W. Ritchie_

  LABORATORY MANUAL FOR USE WITH "HUMAN PHYSIOLOGY"
    By _Carl Hartman_

         *       *       *       *       *

  EXERCISE AND REVIEW BOOK IN BIOLOGY
    By _J. G. Blaisdell_

  PERSONAL HYGIENE AND HOME NURSING
    By _Louisa C. Lippitt_

  SCIENCE OF PLANT LIFE
    By _Edgar Nelson Transeau_

         *       *       *       *       *

  ZOÖLOGY
    By _T. D. A. Cockerell_

  EXPERIMENTAL ORGANIC CHEMISTRY
    By _Augustus P. West_

         *       *       *       *       *




            _NEW-WORLD SCIENCE SERIES_
            _Edited by John W. Ritchie_


                  COMMON SCIENCE

                      _by_
              _Carleton W. Washburne_

  _Superintendent of Schools, Winnetka, Illinois_
  _Formerly Supervisor in Physical Sciences and
      Instructor in Educational Psychology_
               _State Normal School_
            _San Francisco, California_

                  _ILLUSTRATED_
              WITH PHOTOGRAPHS AND
                    DRAWINGS

          _Yonkers-on-Hudson, New York_
                WORLD BOOK COMPANY
                      1921


  WORLD BOOK COMPANY

  THE HOUSE OF APPLIED KNOWLEDGE

  Established, 1905, by Caspar W. Hodgson
  YONKERS-ON-HUDSON, NEW YORK
  2126 PRAIRIE AVENUE, CHICAGO

One of the results of the World War has been a widespread desire
to see the forces of science which proved so mighty in destruction
employed generally and systematically for the promotion of human
welfare. World Book Company, whose motto is The Application of the
World's Knowledge to the World's Needs, has been much in sympathy
with the movement to make science an integral part of our elementary
education, so that all our people from the highest to the lowest will
be able to use it for themselves and to appreciate the possibilities
of ameliorating the conditions of human life by its application to the
problems that confront us. We count it our good fortune, therefore,
that we are able at this time to offer _Common Science_ to the
schools. It is one of the new type of texts that are built on
educational research and not by guess, and we believe it to be a
substantial contribution to the teaching of the subject

  NWSS:WCS-2

  Copyright, 1920, by World Book Company

  Copyright in Great Britain

  _All rights reserved_




PREFACE


A collection of about 2000 questions asked by children forms the
foundation on which this book is built. Rather than decide what it is
that children ought to know, or what knowledge could best be fitted
into some educational theory, an attempt was made to find out what
children want to know. The obvious way to discover this was to let
them ask questions.

The questions collected were asked by several hundred children in the
upper elementary grades, over a period of a year and a half. They
were then sorted and classified according to the scientific principles
needed in order to answer them. These principles constitute the
skeleton of this course. The questions gave a very fair indication of
the parts of science in which children are most interested.
Physics, in simple, qualitative form,--not mathematical physics, of
course,--comes first; astronomy next; chemistry, geology, and certain
forms of physical geography (weather, volcanoes, earthquakes, etc.)
come third; biology, with physiology and hygiene, is a close fourth;
and nature study, in the ordinary school sense of the term, comes in
hardly at all.

The chapter headings of this book might indicate that the course
has to do with physics and chemistry only. This is because general
physical and chemical principles form a unifying and inclusive matrix
for the mass of applications. But the examples and descriptions
throughout the book include physical geography and the life sciences.
Descriptive astronomy and geology have, however, been omitted. These
two subjects can be best grasped in a reading course and field trips,
and they have been incorporated in separate books.

The best method of presenting the principles to the children was the
next problem. The study of the questions asked had shown that the
children's interests were centered in the explanation of a wide
variety of familiar facts in the world about them. It seemed evident,
therefore, that a presentation of the principles that would answer
the questions asked would be most interesting to the child. Experience
with many different classes had shown that it is not necessary to
subordinate these explanations of what children really wish to know to
other methods of instruction of doubtful interest value.

Obviously the quantitative methods of the high school and college were
unsuitable for pupils of this age. We want children to be attracted to
science, not repelled by it. The assumption that scientific method
can be taught to children by making them perform uninteresting,
quantitative experiments in an effort to get a result that will tally
with that given in the textbook is so palpably unfounded that it
is scarcely necessary to prove its failure by pointing to the very
unscientific product of most of our high school science laboratories.

After a good deal of experimenting with children in a number of
science classes, the method followed in this book was developed.
Briefly, it is as follows:

At the head of each section are several of the questions which, in
part, prompted the writing of the section. The purpose of these is to
let the children know definitely what their goal is when they begin a
section. The fact that the questions had their origin in the minds
of children gives reasonable assurance that they will to some extent
appeal to children. These questions in effect state the problems which
the section helps to solve.

Following the questions are some introductory paragraphs for arousing
interest in the problem at hand,--for motivating the child further.
These paragraphs are frequently a narrative description containing a
good many dramatic elements, and are written in conversational style.
The purpose is to awaken the child's imagination and to make clear the
intimate part which the principle under consideration plays in his own
life. When a principle is universal, like gravity, it is best brought
out by imagining what would happen if it ceased to exist. If a
principle is particular to certain substances, like elasticity, it
sometimes can be brought out vividly by imagining what would happen if
it were universal. Contrast is essential to consciousness. To contrast
a condition that is very common with an imagined condition that is
different brings the former into vivid consciousness. Incidentally, it
arouses real interest. The story-like introduction to many sections is
not a sugar coating to make the child swallow a bitter pill. It is
a psychologically sound method of bringing out the essential and
dramatic features of a principle which is in itself interesting, once
the child has grasped it.

Another means for motivating the work in certain cases consists
in first doing a dramatic experiment that will arouse the pupil's
interest and curiosity. Still another consists in merely calling the
child's attention to the practical value of the principle.

Following these various means for getting the pupil's interest, there
are usually some experiments designed to help him solve his problem.
The experiments are made as simple and interesting as possible. They
usually require very inexpensive apparatus and are chosen or invented
both for their interest value and their content value.

With an explanation of the experiments and the questions that arise,
a principle is made clear. Then the pupil is given an opportunity to
apply the principle in making intelligible some common fact, if the
principle has only intelligence value; or he is asked to apply the
principle to the solution of a practical problem where the principle
also has utility value.

The "inference exercises" which follow each section after the first
two consist of statements of well-known facts explainable in terms
of some of the principles which precede them. They involve a constant
review of the work which has gone before, a review which nevertheless
is new work--they review the principles by giving them new
applications. Furthermore, they give the pupil very definite training
in explaining the common things around him.

For four years a mimeographed edition of this book has been used in
the elementary department of the San Francisco State Normal School.
During that time various normal students have tried it in public
school classes in and around San Francisco and Oakland, and it has
recently been used in Winnetka, Illinois. It has been twice revised
throughout in response to needs shown by this use.

The book has proved itself adaptable to either an individual system of
instruction or the usual class methods.




TO THE TEACHER


Do not test the children on the narrative description which introduces
most sections, nor require them to recite on it. It is there merely to
arouse their interest, and that is likely to be checked if they think
it is a lesson to be learned. It is not at all necessary for them
to know everything in the introductory parts of each section. If the
children are interested, they will remember what is valuable to
them; if they are not, do not prolong the agony. The questions which
accompany and follow the experiments, the applications or required
explanations at the ends of the sections, and the extensive inference
exercises, form an ample test of the child's grasp of the principles
under discussion.

It is not necessary to have the children write up their experiments.
The experiments are a means to an end. The end is the application
of the principles to everyday facts. If the children can make these
applications, it does not matter how much of the actual experiments
they remember.

If possible, the experiments should be done by the pupils individually
or in couples, in a school laboratory. Where this cannot be done,
almost all the experiments can be demonstrated from the teacher's desk
if electricity, water, and gas are to be had. Alcohol lamps can be
substituted for gas, but they are less satisfactory.

It is a good plan to have pupils report additional exemplifications
of each principle from their home or play life, and in a quick oral
review to let the rest of the class name the principles back of each
example.

This course is so arranged that it can be used according to the
regular class system of instruction, or according to the individual
system where each child does his own work at his natural rate of
progress. The children can carry on the work with almost no assistance
from the teacher, if provision is made for their doing the experiments
themselves and for their writing the answers to the inference
exercises. When the individual system is used, the children may write
the inference exercises, or they may use them as a basis for study
and recite only a few to the teacher by way of test. In the elementary
department of the San Francisco State Normal School, where the
individual system is used, the latter method is in operation. The
teacher has a card for each pupil, each card containing a mimeographed
list of the principles, with a blank after each. Whenever a pupil
correctly explains an example, a figure 1 is placed in the blank
following that principle; when he misapplies a principle, or fails to
apply it, an _x_ is placed after it. When there are four successive
1's after any principle, the teacher no longer includes that principle
in testing that child. In this way the number of inference exercises
on which she hears any one individual recite is greatly reduced.
This plan would probably have to be altered in order to adapt it to
particular conditions.

The Socratic method can be employed to great advantage in handling
difficult inferences. The children discuss in class the principle
under which an inference comes, and the teacher guides the discussion,
when necessary, by skillfully placed questions designed to bring the
essential problems into relief.[1]

[Footnote 1: At the California State Normal School in San Francisco,
this course in general science is usually preceded by one in
"introductory science."]

The chapters and sections in this book are not of even length. In
order to preserve the unity of subject matter, it was felt desirable
to divide the book according to subjects rather than according to
daily lessons. The varying lengths of recitation periods in different
schools, and the adaptation of the course to individual instruction
as well as to class work, also made a division into lessons
impracticable. Each teacher will soon discover about how much matter
her class, if she uses the class method, can take each day. Probably
the average section will require about 2 days to cover; the longest
sections, 5 days. The entire course should easily be covered in one
year with recitations of about 25 minutes daily. Two 50-minute periods
a week give a better division of time and also ought to finish the
course in a year. Under the individual system, the slowest diligent
children finish in 7 or 8 school months, working 4 half-hours weekly.
The fastest do it in about one third that time.

Upon receipt of 20 cents, the publishers will send a manual prepared
by the author, containing full instructions as to the organization and
equipment of the laboratory or demonstration desk, complete lists of
apparatus and material needed, and directions for the construction of
a chemical laboratory.

    The latter is a laboratory course in which the children are
    turned loose among all sorts of interesting materials and
    apparatus,--kaleidoscope, microscope, electric bell, toy
    motor, chemicals that effervesce or change color when put
    together, soft glass tubing to mold and blow, etc. The teacher
    demonstrates various experiments from time to time to show the
    children what can be done with these things, but the children
    are left free to investigate to their heart's content. There
    is no teaching in this introductory course other than brief
    answers to questions. The astronomy and geology reading
    usually accompany the work in introductory science.




ACKNOWLEDGMENTS


To Frederic Burk, president of the San Francisco State Normal School,
I am most under obligation in connection with the preparation of this
book. His ideas inspired it, and his dynamic criticism did much toward
shaping it. My wife, Heluiz Chandler Washburne, gave invaluable help
throughout the work, especially in the present revision of the course.
One of my co-workers on the Normal School faculty, Miss Louise Mohr,
rendered much assistance in the classification and selection of
inferences. Miss Beatrice Harper assisted in the preparation of the
tables of supplies and apparatus, published in the manual to accompany
this book. And I wish to thank the children of the Normal School for
their patience and cooperation in posing for the photographs. The
photographs are by Joseph Marron.




CONTENTS


  CHAPTER                                               PAGE


  1. GRAVITATION                                           1

    1. A real place where things weigh nothing and
    where there is no up or down                           1

    2. "Water seeks its own level"                         6

    3. The sea of compressed air in which we live:
    Air pressure                                          10

    4. Sinking and floating: Displacement                 23

    5. How things are kept from toppling over:
    Stability                                             29


  2. MOLECULAR ATTRACTION                                 36

    6. How liquids are absorbed: Capillary attraction     36

    7. How things stick to one another: Adhesion          41

    8. The force that makes a thing hold together:
    Cohesion                                              44

    9. Friction                                           49


  3. CONSERVATION OF ENERGY                               57

    10. Levers                                            57

    11. Inertia                                           66

    12. Centrifugal force                                 72

    13. Action and reaction                               77

    14. Elasticity                                        82


  4. HEAT                                                 88

    15. Heat makes things expand                          88

    16. Cooling from expansion                            94

    17. Freezing and melting                              96

    18. Evaporation                                      100

    19. Boiling and condensing                           107

    20. Conduction of heat and convection                116


  5. RADIANT HEAT AND LIGHT                              122

    21. How heat gets here from the sun; why things
    glow when they become very hot                       122

    22. Reflection                                       129

    23. The bending of light: Refraction                 136

    24. Focus                                            142

    25. Magnification                                    150

    26. Scattering of light: Diffusion of light          158

    27. Color                                            161


  6. SOUND                                               174

    28. What sound is                                    174

    29. Echoes                                           183

    30. Pitch                                            185


  7. MAGNETISM AND ELECTRICITY                           190

    31. Magnets; the compass                             190

    32. Static electricity                               196


  8. ELECTRICITY                                         203

    33. Making electricity flow                          203

    34. Conduction of electricity                        213

    35. Complete circuits                                219

    36. Grounded circuits                                225

    37. Resistance                                       229

    38. The electric arc                                 233

    39. Short circuits and fuses                         240

    40. Electromagnets                                   247


  9. MINGLING OF MOLECULES                               259

    41. Solutions and emulsions                          259

    42. Crystals                                         265

    43. Diffusion                                        268

    44. Clouds, rain, and dew: Humidity                  274

    45. Softening due to oil or water                    290


  10. CHEMICAL CHANGE AND ENERGY                         293

    46. What things are made of: Elements                293

    47. Burning: Oxidation                               312

    48. Chemical change caused by heat                   323

    49. Chemical change caused by light                  326

    50. Chemical change caused by electricity            335

    51. Chemical change releases energy                  340

    52. Explosions                                       342


  11. SOLUTION AND CHEMICAL ACTION                       349

    53. Chemical change helped by solution               349

    54. Acids                                            351

    55. Bases                                            355

    56. Neutralization                                   360

    57. Effervescence                                    365


  12. ANALYSIS                                           370

    58. Analysis                                         370


  APPENDIXES:

    A. The Electrical Apparatus                          379

    B. Construction of the Cigar-box Telegraph           381


  INDEX                                                  383




                  COMMON SCIENCE




CHAPTER ONE

GRAVITATION


SECTION 1. _A real place where things weigh nothing and where there is
no up or down._

  Why is it that the oceans do not flow off the earth?
  What is gravity?
  What is "down," and what is "up"?

There is a place where nothing has weight; where there is no "up" or
"down"; where nothing ever falls; and where, if people were there,
they would float about with their heads pointing in all directions.
This is not a fairy tale; every word of it is scientifically true. If
we had some way of flying straight toward the sun about 160,000 miles,
we should really reach this strange place.

Let us pretend that we can do it. Suppose we have built a machine that
can fly far out from the earth through space (of course no one has
really ever invented such a machine). And since the place is far
beyond the air that surrounds the earth, let us imagine that we have
fitted out the air-tight cabin of our machine with plenty of air to
breathe, and with food and everything we need for living. We shall
picture it something like the cabin of an ocean steamer. And let us
pretend that we have just arrived at the place where things weigh
nothing:

When you try to walk, you glide toward the ceiling of the cabin and
do not stop before your head bumps against it. If you push on the
ceiling, you float back toward the floor. But you cannot tell whether
the floor is above or below, because you have no idea as to which way
is up and which way is down.

As a matter of fact there is no up or down. You discover this quickly
enough when you try to pour a glass of water. You do not know where
to hold the glass or where to hold the pitcher. No matter how you hold
them, the water will not pour--point the top of the pitcher toward the
ceiling, or the floor, or the wall, it makes no difference. Finally
you have to put your hand into the pitcher and pull the water out.
It comes. Not a drop runs between your fingers--which way can it run,
since there is no down? The big lump of water stays right on your
hand. This surprises you so much that you let go of the pitcher. Never
mind; the pitcher stays poised in mid-air. But how are you going
to get a drink? It does not seem reasonable to try to drink a large
_lump_ of water. Yet when you hold the lump to your lips and suck it
you can draw the water into your mouth, and it is as wet as ever; then
you can force it on down to (or rather _toward_) your throat with your
tongue. Still you have left in your hand a big piece of water that
will not flow off. You throw it away, and it sails through the air of
the cabin in a straight line until it splashes against the wall. It
wets the wall as much as water on the earth would, but it does not run
off. It sticks there, like a splash-shaped piece of clear, colorless
gelatin.

Suppose that for the sake of experimenting you have brought an
elephant along on this trip. You can move under him (or over
him--anyway between him and the floor), brace your feet on the floor,
and give him a push. (If he happens to step on your toes while you are
doing this, you do not mind in the least, because he does not weigh
anything, you know.) If you push hard enough to get the elephant
started, he rises slowly toward the ceiling. When he objects on the
way, and struggles and kicks and tries to get back to the floor, it
does not help him at all. His bulky, kicking body floats steadily on
till it crashes into the ceiling.

No chairs or beds are needed in this place. You can lie or sit in
mid-air, or cling to a fixture on a wall, resting as gently there as
a feather might. There is no need to set the table for meals--just lay
the dishes with the food on them in space and they stay there. If the
top of your cup of chocolate is toward the ceiling, and your plate of
food is turned the other way, no harm is done. Your feet may happen
to point toward the ceiling, while some one else's point toward the
floor, as you sit in mid-air, eating. There is some difficulty in
getting the food on the dishes, so probably you do not wish to bother
with dishes, after all. Do you want some mashed potatoes? All right,
here it is--and the cook jerks the spoon away from the potatoes,
leaving them floating before you, ready to eat.

It is literally a topsy-turvy place.

Do you want to know why all this would happen? Here is the reason:
There is a great force known as _gravitation_. It is the pull that
everything in the universe has on everything else. The more massive a
thing is, the more gravitational pull it has on other objects; but the
farther apart things are, the less pull they have on each other.

The earth is very massive, and we live right on its surface; so it
pulls us strongly toward it. Therefore we say that we weigh something.
And since every time we roll off a bed, for instance, or jump off a
chair, the earth pulls us swiftly toward it, we say that the earth is
down. "Down" simply means toward the thing that is pulling us. If we
were on the surface of the moon, the moon would pull us. "Down" would
then be under our feet or toward the center of the moon, and the earth
would be seen floating up in the sky. For "up" means _away from_ the
thing which is pulling us.

WHY WATER DOES NOT FLOW OFF THE EARTH. It was because people did not
know about gravitation that they laughed at Columbus when he said the
earth was round. "Why, if the earth were round," they argued, "the
water would all flow off on the other side." They did not know that
water flows downhill because the earth is pulling it toward its center
by gravitation, and that it does not make the slightest difference on
which side of the earth water is, since it is still pulled toward the
center.

WHY THE WORLD DOES NOT FALL DOWN. And people used to wonder "what held
the earth up." The answer, as you can see, is easy. There simply is no
up or down in space. The earth cannot fall down, because there is no
down to fall to. "Down" merely means toward the earth, and the earth
cannot very well fall toward itself, can it? The sun is pulling on it,
though; so the earth could fall into the sun, and it would do so, if
it were not swinging around the sun so fast. You will see how this
keeps it from falling into the sun when you come to the section on
centrifugal force.

WHY THERE IS A PLACE WHERE THINGS WEIGH NOTHING. Now about the place
where gravitation has no effect. Since an object near the sun is
pulled more by the sun than it is by the earth, and since down here
near the earth an object is pulled harder by the earth than by the
sun, it is clear that there must be a place between the sun and the
earth where their pulls just balance; and where the sun pulls just as
hard one way as the earth pulls the other way, things will not fall
either way, but will float. The place where the pulls of the sun and
the earth are equal is not halfway between the earth and the sun,
because the sun is so much larger and pulls so much more powerfully
than the earth, that the place where their pulls balance is much
nearer the earth than it is to the sun. As a matter of fact, it can be
easily calculated that this spot is somewhere near 160,000 miles from
the earth.

There are other spots like it between every two stars, and in the
center of the earth, and in the center of every other body. You see,
in the center of the earth there is just as much of the earth pulling
one way as there is pulling the other, so again there is no up or
down.

    _APPLICATION 1._ Explain why the people on the other side of
    the earth do not fall off; why you have weight; why rivers run
    downhill; why the world does not fall down.


SECTION 2. "_Water seeks its own level._"

  Why does a spring bubble up from the ground?
  What makes the water come up through the pipe into your house?
  Why is a fire engine needed to pump water up high?

You remember that up where the pull of the earth and the sun balance
each other, water could not flow or flatten out. Let us try to imagine
that water, here on the earth, has lost its habit of flattening out
whenever possible--that, like clay, it keeps whatever shape it is
given.

First you notice that the water fails to run out of the faucets. (For
in most places in the world as it really is, the water that comes
through faucets is simply flowing down from some high reservoir.)
People all begin to search for water to drink. They rush to the rivers
and begin to dig the water out of them. It looks queer to see a hole
left in the water wherever a person has scooped up a pailful. If
some one slips into the river while getting water, he does not drown,
because the water cannot close in over his head; there is just a deep
hole where he has fallen through, and he breathes the air that comes
down to him at the bottom of the hole. If you try to row on the water,
each stroke of the oars piles up the water, and the boat makes a deep
furrow wherever it goes so that the whole river begins to look like a
rough, plowed field.

When the rivers are used up, people search in vain for springs. (No
springs could flow in our everyday world if water did not seek its own
level; for the waters of the springs come from hills or mountains, and
the higher water, in trying to flatten out, forces the lower water up
through the ground on the hillsides or in the valleys.) So people have
to get their water from underground or go to lakes for it. And these
lakes are strange sights. Storms toss up huge waves, which remain as
ridges and furrows until another storm tears them down and throws up
new ones.

But with no rivers flowing into them, the lakes also are used up in
time. The only fresh water to be had is what is caught from the rain.
Even wells soon become useless; because as soon as you pump up the
water surrounding the pump, no more water flows in around it; and if
you use a bucket to raise the water, the well goes dry as soon as the
supply of water standing in it has been drawn.

You will understand more about water seeking its own level if you do
this experiment:

    EXPERIMENT 1. Put one end of a rubber tube over the narrow
    neck of a funnel (a glass funnel is best), and put the other
    end of the tube over a piece of glass tubing not less than
    5 or 6 inches long. Hold up the glass tube and the funnel,
    letting the rubber tube sag down between them as in Figure
    1. Now fill the funnel three fourths full of water. Raise the
    glass tube higher if the water starts to flow out of it. If
    no water shows in the glass tube, lower it until it does.
    Gradually raise and lower the tube, and notice how high the
    water goes in it whenever it is held still.

This same thing would happen with any shape of tube or funnel. You
have another example of it when you fill a teakettle: the water rises
in the spout just as high as it does in the kettle.

[Illustration: FIG. 1. The water in the tube rises to the level of the
water in the funnel.]

WHY WATER FLOWS UP INTO YOUR HOUSE. It is because water seeks its own
level that it comes up through the pipes in your house. Usually the
water for a city is pumped into a reservoir that is as high as the
highest house in the city. When it flows down from the reservoir, it
tends to rise in any pipe through which it flows, to the height at
which the water in the reservoir stands. If a house is higher than the
surface of the water in the reservoir, of course that house will get
no running water.

WHY FIRE ENGINES ARE NEEDED TO FORCE WATER HIGH. In putting out a
fire, the firemen often want to throw the water with a good deal of
force. The tendency of the water to seek its own level does not always
give a high enough or powerful enough stream from the fire hose; so a
fire engine is used to pump the water through the hose, and the stream
flows with much more force than if it were not pumped.

    _APPLICATION 2._ A. C. Wheeler of Chicago bought a little farm
    in Indiana, and had a windmill put up to supply the place with
    water. But at first he was not sure where he should put the
    tank into which the windmill was to pump the water and from
    which the water should flow into the kitchen, bathroom, and
    barn. The barn was on a knoll, so that its floor was almost as
    high as the roof of the house. Which would have been the best
    place for the tank: high up on the windmill (which stood on
    the knoll by the barn), or the basement of the house, or the
    attic of the house?

[Illustration: FIG. 2. Where is the best location for the tank?]

    _APPLICATION 3._ A man was about to open a garage in San
    Francisco. He had a large oil tank and wanted a simple way
    of telling at a glance how full it was. One of his workmen
    suggested that he attach a long piece of glass tubing to the
    side of the tank, connecting it with an extra faucet near the
    bottom of the tank. A second workman said, "No, that won't do.
    Your tank holds ever so much more than the tube would hold, so
    the oil in the tank would force the oil up over the top of the
    tube, even when the tank was not full." Who was right?

[Illustration: FIG. 3. When the tank is full, will the oil overflow
the top of the tube?]


SECTION 3. _The sea of compressed air in which we live: Air pressure._

    Does a balloon explode if it goes high in the air?

    What is suction?

    Why does soda water run up a straw when you draw on the straw?

    Why will evaporated milk not flow freely out of a can in which
    there is only one hole?

    Why does water gurgle when you pour it out of a bottle?

We are living in a sea of compressed air. Every square inch of our
bodies has about 15 pounds of pressure against it. The only reason
we are not crushed is that there is as strong pressure inside of our
bodies pushing out as there is outside pushing in. There is compressed
air in the blood and all through the body. If you were to lie down
on the ground and have all the air pumped out from under you, the air
above would crush you as flat as a pancake. You might as well let a
dozen big farm horses trample on you, or let a huge elephant roll over
you, as let the air press down on you if there were no air underneath
and inside your body to resist the pressure from above. It is hard to
believe that the air and liquids in our bodies are pressing out with
a force great enough to resist this crushing weight of air. But if you
were suddenly to go up above the earth's atmosphere, or if you were to
stay down here and go into a room from which the air were to be pumped
all at once, your body would explode like a torpedo.

When you suck the air out of a bottle, the surrounding air pressure
forces the bottle against your tongue; if the bottle is a small one,
it will stick there. And the pressure of the air and blood in your
tongue will force your tongue down into the neck of the bottle from
which part of the air has been taken.

In the same way, when you force the air out of a rubber suction cap,
such as is used to fasten reading lamps to the head of a bed, the air
pressure outside holds the suction cap tightly to the object against
which you first pressed it, making it stick there.

We can easily experiment with air pressure because we can get almost
entirely rid of it in places and can then watch what happens. A place
from which the air is practically all pumped out is called a _vacuum_.
Here are some interesting experiments that will show what air pressure
does:

[Illustration: FIG. 4. When the point is knocked off the electric
lamp, the water is forced into the vacuum.]

    EXPERIMENT 2. Hold a burned-out electric lamp in a basin of
    water, break its point off, and see what happens.

All the common electric lamps (less than 70 watts) are made with
vacuums inside. The reason for this is that the fine wire would burn
up if there were any air in the lamps. When you knock the point off
the globe, it leaves a space into which the water can be pushed. Since
the air is pressing hard on the surface of the water except in the
one place where the vacuum in the lamp globe is, the water is forced
violently into this empty space.

It really is a good deal like the way mud comes up between your toes
when you are barefoot. Your foot is pressing on the mud all around
except in the spaces between your toes, and so the mud is forced up
into these spaces. The air pressure on the water is like your foot
on the mud, and the space in the lamp globe is like the space between
your toes. Since wherever there is air it is pressing hard, the only
space into which it can force water or anything else is into a place
from which all the air has been removed, like the inside of the lamp
globe.

The reason that the water does not run out of the globe is this:
the hole is too small to let the air squeeze up past the water, and
therefore no air can take the place of the water that might otherwise
run out. In order to flow out, then, the water would have to leave an
empty space or vacuum behind it, and the air pressure would not allow
this.

WHY WATER GURGLES WHEN IT POURS OUT OF A BOTTLE. You have often
noticed that when you pour water out of a bottle it gurgles and gulps
instead of flowing out evenly. The reason for this is that when
a little water gets out and leaves an empty space behind, the air
pushing against the water starts to force it back up; but since the
mouth of the bottle is fairly wide, the air itself squeezes past the
water and bubbles up to the top.

    EXPERIMENT 3. Put a straw or a piece of glass tube down into
    a glass of water. Hold your finger tightly over the upper end,
    and lift the tube out of the water. Notice how the water stays
    in the tube. Now remove your finger from the upper end.

The air holds the water up in the tube because there is no room for
it to bubble up into the tube to take the place of the water; and the
water, to flow out of the tube, would have to leave a vacuum, which
the air outside does not allow. But when you take your finger off the
top of the straw or tube, the air from above takes the place of the
water as rapidly as it flows out; so there is no tendency to form a
vacuum, and the water leaves the tube. Now do you see why you make two
holes in the top of a can of evaporated milk when you wish to pour the
milk out evenly?

[Illustration: FIG. 5. The water is held in the tube by air pressure.]

    EXPERIMENT 4. Push a rubber suction cap firmly against the
    inside of the bell jar of an air pump. Try to pull the suction
    cap off. If it comes off, press it on again; place the bell
    jar on the plate of the air pump, and pump the air out of the
    jar. What must have been holding the suction cap against the
    inside of the jar? Does air press up and sidewise as well as
    down? Test this further in the following experiment:

[Illustration: FIG. 6. An air pump.]

    EXPERIMENT 5. Put a cork into an empty bottle. Do not use a
    new cork, but one that has been fitted into the bottle many
    times and has become shaped to the neck. Press the cork in
    rather firmly, so that it is air-tight, but do not jam it in.
    Set the bottle on the plate of the air pump, put the bell jar
    over it, and pump the air out of the jar. What makes the cork
    fly out of the bottle? What was really in the "empty" bottle?
    Why could it not push the cork out until you had pumped the
    air out of the jar?

    EXPERIMENT 6. Wax the rims of the two Magdeburg hemispheres
    (see Fig. 7). Screw the lower section into the hole in the
    plate of the air pump. Be sure that the stop valve in the
    neck of the hemisphere is open. (The little handle should be
    vertical.) Fit the other section on to the first, and pump out
    as much air as you can. _Close_ the stop valve. Unscrew the
    hemispheres from the air pump. Try to pull them apart--pull
    straight out, taking care not to slide the parts. If you wish,
    let some one else take one handle, and see if the two of you
    can pull it apart.

[Illustration: FIG. 7. The experiment with the Magdeburg hemispheres.]

Before you pumped the air out of the hemisphere, the compressed air
inside of them (you remember all the air down here is compressed) was
pushing them apart just as hard as the air outside of them was pushing
them together. When you pumped the air out, however, there was hardly
any air left inside of them to push outward. So the strong pressure of
the outside air against the hemispheres had nothing to oppose it. It
therefore pressed them very tightly together and held them that way.

This experiment was first tried by a man living in Magdeburg, Germany.
The first set of hemispheres he used proved too weak, and when the air
in them was partly pumped out, the pressure of the outside air crushed
them like an egg shell. The second set was over a foot in diameter
and much stronger. After he had pumped the air out, it took sixteen
horses, eight pulling one way and eight the opposite way, to pull the
hemispheres apart.

    EXPERIMENT 7. Fill a bottle (or flask) half full of water.
    Through a one-hole stopper that will fit the bottle, put a
    bent piece of glass tubing that will reach down to the bottom
    of the bottle. Set the bottle, thus stoppered, on the plate of
    the air pump, with a beaker or tumbler under the outer end of
    the glass tube. Put the bell jar over the bottle and glass,
    and pump the air out of the jar. What is it that forces the
    water up and out of the bottle? Why could it do this when the
    air was pumped out of the bell jar and not before?

HOW A SELTZER SIPHON WORKS. A seltzer siphon works on the same
principle. But instead of the ordinary compressed air that is all
around us, there is in the seltzer siphon a gas (carbon dioxid)
which has been much more compressed than ordinary air. This strongly
compressed gas forces the seltzer water out into the less compressed
air, exactly as the compressed air in the upper part of the bottle
forced the water out into the comparative vacuum of the bell jar in
Experiment 7.

    EXPERIMENT 8. Fill a toy balloon partly full of air by blowing
    into it, and close the neck with a rubber band so that no air
    can escape. Lay a saucer over the hole in the plate of the air
    pump, so that the rubber of the balloon cannot be sucked down
    the hole. Lay the balloon on top of this saucer, put the bell
    jar over it, and pump the air out of the jar. What makes the
    balloon expand? What is in it? Why could it not expand before
    you pumped the air out from around it?

A toy balloon expands for the same reason when it goes high in the
air. Up there the air pressure is not so strong outside the balloon,
and so the gas inside makes the balloon expand until it bursts.

[Illustration: FIG. 8. A siphon. The air pushes the water over the
side of the pan.]

    EXPERIMENT 9. Lay a rubber tube flat in the bottom of a pan of
    water, so that the tube will be filled with water. Let one end
    stay under water, but pinch the other end tightly shut with
    your thumb and finger and lift it out of the pan. Lower this
    closed end into a sink or empty pan that is lower than the
    pan of water. Now stop pinching the tube shut. This device is
    called a _siphon_ (Fig. 8).

    EXPERIMENT 10. Put the mouth of a small syringe, or better, of
    a glass model lift pump, under water. Draw the handle up. Does
    the water follow the plunger up, stand still, or go down in
    the pump?

When you pull up the plunger, you leave an empty space; you shove the
air out of the pump or syringe ahead of the plunger. The air outside,
pressing on the water, forces it up into this empty space from which
the air has been pushed. But air pressure cannot force water up even
into a perfect vacuum farther than about 33 feet. If your glass pump
were, say, 40 feet long, the water would follow the plunger up for a
little over 30 feet, but nothing could suck it higher; for by the time
it reaches that height it is pushing down with its own weight as hard
as the air is pressing on the water below. No suction pump, or siphon,
however perfect, will ever lift water more than about 33 feet, and
it will do well if it draws water up 28 or 30 feet. This is because a
perfect vacuum cannot be made. There is always some water vapor formed
by the water evaporating a little, and there is always a small amount
of air that has been dissolved in water, both of which partly fill the
space above the water and press down a little on the water within the
pump.

[Illustration: FIG. 9. A glass model suction pump.]

If you had a straw over 33 feet long, and if some one held a glass of
lemonade for you down near the sidewalk while you leaned over from
the roof of a three-story building with your long straw, you could
not possibly drink the lemonade. The air pressure would not be great
enough to lift it so high, no matter how hard you sucked,--that is, no
matter how perfect a vacuum you made in the upper part of the straw.
The lemonade would rise part way, and then your straw would be
flattened by the pressure outside.

Some days the air can force water up farther in a tube than it can
on other days. If it can force the water up 33 feet today, it will
perhaps be able to force it up only 30 feet immediately before a
storm. And if it forces water up 33 feet at sea level, it may force it
up only 15 or 20 feet on a high mountain, for on a mountain there
is much less air above to make pressure. The pressure of the air is
different in different places; where the air is heavy and pressing
hard, we say the pressure is _high_; where the air is light and not
pressing so hard, we call the pressure _low_. A place where the air is
heavy is called an area of high pressure; where it is light, an area
of low pressure. (See Section 44.)

WHAT MAKES WINDS? It is because the air does not press equally all
the time and everywhere that we have winds. Naturally, if the air is
pressing harder in one place than in another, the lower air will be
pushed sidewise in the areas of high pressure and will rush to the
areas where there is less pressure. And air rushing from one place to
another is called _wind_.

[Illustration: FIG. 10.]

    _APPLICATION 4._ A man had two water reservoirs, which stood
    at the same level, one on each side of a hill. The hill
    between them was about 50 feet high. One reservoir was full,
    and the other was empty. He wanted to get some of the water
    from the full reservoir into the empty one. He did not have a
    pump to force the water from one to the other, but he did have
    a long hose, and could have bought more. His hose was long
    enough to reach over the top of the hill, but not long enough
    to go around it. Could he have siphoned the water from one
    reservoir to the other? Would he have had to buy more hose?

    _APPLICATION 5._ Two boys were out hiking and were very
    thirsty. They came to a deserted farm and found a deep well;
    it was about 40 feet down to the water. They had no pump,
    but there was a piece of hose about 50 feet long. One boy
    suggested that they drop one end of the hose down to the water
    and suck the water up, but the other said that that would not
    work--the only way would be to lower the hose into the water,
    close the upper end, pull the hose out and let the water pour
    out of the lower end of the hose into their mouths. A stranger
    came past while the boys were arguing, and said that neither
    way would work; that although the hose was long enough, the
    water was too far down to be raised in either way. He advised
    the boys to find a bucket and to use the hose as a rope for
    lowering it. Who was right?

INFERENCE EXERCISE

    EXPLANATORY NOTE. In the inference exercises in this book,
    there is a group of facts for you to explain. They can always
    be explained by one or more of the principles studied, like
    gravitation, water seeking its own level, or air pressure. If
    asked to explain why sucking through a straw makes soda water
    come up into your mouth, for instance, you should not merely
    say "air pressure," but should tell why you think it is air
    pressure that causes the liquid to rise through the straw. The
    answer should be something like this: "The soda water comes
    up into your mouth because the sucking takes the air pressure
    away from the top of the soda water that is in the straw. This
    leaves the air pressing down only on the surface of the soda
    water in the glass. Therefore, the air pressure pushes the
    soda water up into the straw and into your mouth where the
    pressure has been removed by sucking." Sometimes, when you
    have shown that you understand the principles very well,
    the teacher may let you take a short cut and just name the
    principle, but this will be done only after you have proved by
    a number of full answers that you thoroughly understand each
    principle named.

    Some of the following facts are accounted for by air pressure;
    some by water seeking its own level; others by gravitation.
    See if you can tell which of the three principles explains
    each fact:

    1. Rain falls from the clouds.

    2. After rain has soaked into the sides of mountains it runs
    underground and rises, at lower levels, in springs.

    3. When there are no springs near, people raise the water from
    underground with suction pumps.

    4. As fast as the water is pumped away from around the bottom
    of a pump, more water flows in to replace it.

    5. After you pump water up, it flows down into your pail from
    the spout of the pump.

    6. You can drink lemonade through a straw.

    7. If a lemon seed sticks to the bottom of your straw, the
    straw flattens out when you suck.

    8. When you pull your straw out to remove the seed, there is
    no hole left in the lemonade; it closes right in after the
    straw.

    9. If you drop the seed, it falls to the floor.

    10. If you tip the glass to drink the lemonade, the surface
    of the lemonade does not tip with the glass, but remains
    horizontal.


SECTION 4. _Sinking and floating: Displacement._

    What keeps a balloon up?

    What makes an iceberg float?

    Why does cork float on the water and why do heavier substances
    sink?

    If iron sinks, why do iron ships not sink?

Again let us imagine ourselves up in the place where gravitation has
no effect. Suppose we lay a nail on the surface of a bowl of water. It
stays there and does not sink. This does not seem at all surprising,
of course, since the nail no longer has weight. But when we put a cork
in the midst of the water, it stays there instead of floating to the
surface. This seems peculiar, because the less a thing weighs the more
easily it floats. So when the cork weighs nothing at all, it seems
that it should float better than ever. Of course there is some
difficulty in deciding whether it ought to float toward the part of
the water nearest the floor or toward the part nearest the ceiling,
since there is no up or down; but one would think that it ought
somehow to get to the outside of the water and not stay exactly in the
middle. If put on the outside, however, it stays there as well.

A toy balloon, in the same way, will not go toward either the ceiling
or the floor, but just stays where it is put, no matter how light a
gas it is filled with.

The explanation is as follows: For an object to float on the water or
in the air, the water or air must be heavier than the object. It is
the water or air being pulled under the object by gravity, that pushes
it up. Therefore, if the air and water themselves weighed nothing, of
course they would be no heavier than the balloon or the cork; the
air or water would then not be pulled in under the balloon or cork by
gravity, and so would not push them up, or aside.

[Illustration: FIG. 11. The battleship is made of steel, yet it does
not sink.]

WHY IRON SHIPS FLOAT. When people first talked about building iron
ships, others laughed at them. "Iron sinks," they said, "and your
boats will go to the bottom of the sea." If the boats were solid iron
this would be true, for iron is certainly much heavier than water. But
if the iron is bent up at the edges,--as it is in a dish pan,--it has
to push much more water aside before it goes under than it would if it
were flattened out. The water displaced, or pushed aside, would have
to take up as much room as was taken up by the pan _and all the empty
space inside of it_, before the edge would go under. Naturally this
amount of water would weigh a great deal more than the empty pan.

But suppose you should fill the dish pan with water, or suppose it
leaked full. Then you would have the weight of all the water in it
added to the weight of the pan, and that would be heavy enough to push
aside the water in which it was floating and let the pan sink. This is
why a ship sometimes sinks when it springs a leak.

You may be able to see more clearly why an iron ship floats by this
example: Suppose your iron ship weighs 6000 tons and that the cargo
and crew weigh another 1000 tons. The whole thing, then, weighs 7000
tons. Now that ship is a big, bulky affair and takes up more space
than 7000 tons of water does. As it settles into the water it pushes
a great deal of water out of the way, and after it sinks a certain
distance it has pushed 7000 tons of water out of the way. Since the
ship weighs only 7000 tons, it evidently cannot push aside more than
that weight of water; so part of the ship stays above the water, and
all there is left for it to do is to float. If the ship should freeze
solid in the water where it floated and then could be lifted out of
the ice by a huge derrick, you would find that you could pour exactly
7000 tons of water into the hole where the ship had been.

But if you built your ship with so little air space in it that it took
less room than 7000 tons of water takes, it could go clear under the
water without pushing 7000 tons of water aside. Therefore a ship of
this kind would sink.

The earth's gravity is pulling on the ship and on the water. If the
ship has displaced (pushed aside) its own weight of water, gravity is
pulling down on the water as hard as it is on the ship; so the ship
cannot push any more water aside, and if there is enough air space in
it, the ship floats.

Perhaps the easiest way to say it is like this: Anything that is
lighter than the same volume of water will float; since a cubic foot
of wood weighs less than a cubic foot of water, the wood will float;
since a quart of oil is lighter than a quart of water, the oil will
float; since a pint of cream is lighter than a pint of milk, the cream
will rise. In the same way, anything that is lighter than the same
volume of air will be pushed up by the air. When a balloon with its
passengers weighs less than the amount of air that it takes the place
of at any one time, it will go up. Since a quart of warm air weighs
less than a quart of cold air, the warm air will rise.

You can see how a heavy substance like water pushes a lighter one,
like oil, up out of its way, in the following experiment:

    EXPERIMENT 11. Fill one test tube to the brim with kerosene
    slightly colored with a little iodine. Fill another test tube
    to the brim with water, colored with a little blueing. Put a
    small square of cardboard over the test tube of water, hold it
    in place, and turn the test tube upside down. You can let go
    of the cardboard now, as the air pressure will hold it up. Put
    the mouth of the test tube of water exactly over the mouth of
    the test tube of kerosene. Pull the cardboard out from between
    the two tubes, or have some one else do this while you hold
    the two tubes mouth to mouth. If you are careful, you will not
    spill a drop. If nothing happens when the cardboard is pulled
    away, gently rock the two tubes, holding their mouths tightly
    together.

[Illustration: FIG. 12. The upper tube is filled with water and the
lower with oil. What will happen when she pulls the cardboard out?]

Oil is lighter than water, as you know, because you have seen a film
of oil floating on water. When you have the two test tubes in such a
position that the oil and water can change, the water is pulled down
under the kerosene because gravity is pulling harder on the water
than it is pulling on the kerosene. The water, therefore, goes to the
bottom and this forces the kerosene up.

    _APPLICATION 6._ Three men were making a raft. For floats they
    meant to use some air-tight galvanized iron cylinders. One
    of them wanted to fill the cylinders with cork, "because," he
    said, "cork is what you put in life preservers and it floats
    better than anything I know of." "They'd be better with
    nothing in them at all," said a second. "Pump all the air
    out and leave vacuums. They're air-tight and they are strong
    enough to resist the air pressure." But the third man said,
    "Why, you've got to have some air in them to buoy them up.
    Cork would be all right, but it isn't as light as air; so air
    would be the best thing to fill them with."

    Which way would the floats have worked best?

    _APPLICATION 7._ A little girl was telling her class about
    icebergs. "They are very dangerous," she said, "and ships are
    often wrecked by running into them. You see, the sun melts
    the top off them so that all there is left is under water. The
    sailors can't see the ice under water, and so their ships
    run into it and are sunk." Another girl objected to this; she
    said, "That couldn't be; the ice would bob up as fast as the
    top melted." "No, it wouldn't," said a boy. "If that lower
    part wasn't heavier than water, it never would have stayed
    under at all. And if it was heavier at the beginning, it would
    still be heavier after the top melted off."

    Who was right?


INFERENCE EXERCISE

    Explain the following:

    11. When you wash dishes, a cup often floats on top of the
    water, while a plate made of the same sort of china sinks to
    the bottom of the pan.

    12. If you put the cup in sidewise, it sinks.

    13. The water in the cup, when lying on its side, is exactly
    as high as the water in the dish pan.

    14. If you put a glass into the water, mouth first, the water
    cannot get up into the glass; if you tip it a little, there
    are bubbles in the water and some water enters the glass.

    15. If you let a dish slip while you are wiping it, it crashes
    to the floor.

    16. It is much harder to hold a large platter while you are
    wiping it than it is to hold a small butter plate.

    17. If you set a hot glass upside down on the oilcloth table
    cover, the oilcloth bulges up into it when the hot air and
    steam shrink and leave a partial vacuum within the glass.

    18. If you spill any of the dishwater on the floor, it
    flattens out.

    19. You may use a kind of soap that is full of invisible
    little air bubbles; if you do, the soap will float on top of
    the water.

    20. When you drop a dry dishcloth into water, it floats until
    all the pores are filled with water; then it sinks.


SECTION 5. _How things are kept from toppling over: Stability._

    Why is it harder to keep your balance on stilts than on your
    feet?

    Why does a rowboat tip over more easily if you stand up in it?

In Pisa, Italy, there is a beautiful marble bell tower which leans
over as if it were just about to fall to the ground. Yet it has stood
in this position for hundreds of years and has never given a sign of
toppling. The foundations on which it rested sank down into the ground
on one side while the tower was being built (it took over 200 years
to build it), and this made it tip. But the men who were building
it evidently felt sure that it would not fall over in spite of its
tipping. They knew the law of stability.

[Illustration: FIG. 13. The Leaning Tower of Pisa.]

All architects and engineers and builders have to take this law into
consideration or the structures they put up would topple over. And
your body learned the law when you were a little over a year old, or
you never could have walked. It is worth while for your brain to know
it, too, because it is a very practical law that you can use in your
everyday life.

If you wish to understand why the Leaning Tower of Pisa does not fall
over, why it is hard to walk on stilts, why a boat tips when a person
stands up in it, why blocks fall when you build too high with them,
and how to keep things from tipping over, do the following experiment
and read the explanation that follows it:

    EXPERIMENT 12.[2] Unscrew the bell from a doorbell or a
    telephone. You will not harm it at all, and you can put it
    back after the experiment. Cut a sheet of heavy wrapping paper
    or light-weight cardboard about 5 × 9 inches. Roll this so as
    to make a cylinder about 5 inches high and as big around as
    the bell. Hold it in shape by pasting it or putting a couple
    of rubber bands around it. Cut two strips of paper about an
    inch wide and 8 inches long; lay these crosswise; lay the
    bell, round side down, on the center of the cross. Push a
    paper fastener through the hole in the bell (the kind shown in
    Figure 14) and through the crossed pieces of paper, spreading
    the fastener out so as to fasten the paper cross to the
    rounded side of the bell. Bend the arms of the cross up around
    the bell and paste them to the sides of the paper cylinder so
    that the bell makes a curved bottom to the cylinder, as shown
    in Figure 15.

[Illustration: FIG. 14.]

[Footnote 2: TO THE TEACHER. If you have a laboratory, it is well to
have this cylinder already made for the use of all classes.]

[Illustration: FIG. 15. In this cylinder the center of weight is so
high that it is not over the bottom if the cylinder is tipped to any
extent. So the cylinder falls over easily and lies quietly on its
side.]

[Illustration: FIG. 16. But in this one the center of weight is so low
that it is over the base, no matter what position the cylinder is in.]

[Illustration: FIG. 17. So even if the cylinder is laid on its side it
immediately comes to an upright position again.]

Try to tip the cylinder over. Now stuff some crumpled paper loosely
into the cylinder, filling it to the top. Tip the cylinder again. Will
it stay on its side now? Force all the crumpled paper to the bottom
of the cylinder. Now will it stay on its side? Take out the crumpled
paper and lay a flat stone in the bottom of the bell, holding it
in place by stuffing some crumpled paper in on top of it. Will the
cylinder tip over now? Take the stone out, put the crumpled paper in
the bottom of the cylinder, put the stone on top of the paper, and
again try to tip the cylinder over. Will it fall?

       *       *       *       *       *

The center of the cylinder was always in one place, of course. But the
_center of the weight_ in that cylinder was usually near the bottom,
because the bell weighed so much more than the paper. When you raised
the center of weight by putting the stone up high or filling the
cylinder with crumpled paper, just a little tipping moved the center
of weight so that it was not directly over the bell on which the
cylinder was resting. Whenever the center of weight is not over the
base of support (the bottom on which the thing is standing), an object
will topple over. Moving the center of weight up (Figs. 15 and 16)
makes an object less stable.

The two main points to remember about stability are these: the wider
the base of an object, the harder it is to tip over; and the lower the
center of the weight is, the harder it is to tip over.

If you were out in a rowboat in a storm, would it be better to sit up
straight in the seat or to lie in the bottom of the boat?

Why is a flat-bottomed boat safer than a canoe?

[Illustration: FIG. 18. Which vase would be the hardest to upset?]

Where do you suppose the center of weight of the Leaning Tower of Pisa
is,--near the bottom or near the top?

    _APPLICATION 8._ If you had a large flower to put into a vase
    and you did not want it to tip over easily, which of the three
    vases shown in Figure 18 would you choose?

    _APPLICATION 9._ Some boys made themselves a little sail-boat
    and went sailing in it. A storm came up. The boat rocked badly
    and was in danger of tipping over. "Throw out all the heavy
    things, quick!" shouted one. "No, no, don't for the life of
    you do it!" called another. "Chop down the mast--here, give me
    the hatchet!" another one said. "Crouch way down--lie on the
    bottom." "No, keep moving over to the side that is tipped up!"
    "Hold the things in the bottom of the boat still, so they'll
    not keep rolling from side to side." "Jump out and swim!"
    Every one was shouting at once. Which parts of the advice
    should you have followed if you had been on board?


INFERENCE EXERCISE

    Explain the following:

    21. A ship when it goes to sea always carries ballast (weight)
    in its bottom.

    22. If the ship springs a leak below the water line, the water
    rushes in.

    23. The ship's pumps suck the water up out of the bottom of
    the ship.

    24. The water pours back into the sea from the mouths of the
    pumps.

    25. As the sailors move back and forth on the ship during a
    storm, they walk with their legs spread far apart.

    26. Although the ship tips far from side to side, it rights
    itself.

    27. However far the ship tips, the surface of the water in the
    bottom stays almost horizontal.

    28. While the ship is in danger, the people put on life
    preservers, which are filled with cork.

    29. When the ship rocks violently, people who are standing up
    are thrown to the floor, but those who are sitting down do not
    fall over.

    30. If the ship fills with water faster than the engines can
    pump it out, the ship sinks.




CHAPTER TWO

MOLECULAR ATTRACTION


SECTION 6. _How liquids are absorbed: Capillary attraction._

    Why do blotters pull water into themselves when a flat piece
    of glass will not?

    How does a towel dry your face?

Suppose you could turn off nature's laws in the way that you can turn
off electric lights. And suppose you stood in front of a switchboard
with each switch labeled with the name of the law it would shut off.
Of course, there is no such switchboard, but we know pretty well what
would happen if we _could_ shut off various laws. One of the least
dangerous-looking switches would be one labeled CAPILLARY ATTRACTION.
And now, just for fun, suppose that you have turned that switch off in
order to see the effect.

At first you do not notice any change; but after a while you begin
to feel perspiration collecting all over your body as if your clothes
were made of rubber sheeting. Soon this becomes so uncomfortable that
you decide to take a bath. But when you put your wash cloth into the
water you find that it will not absorb any water at all; it gets
a little wet on the outside, but remains stiff and is not easy or
pleasant to use. You reach for a sponge or a bath brush, but you are
no better off. Only the outside of the sponge and brush becomes wet,
and they remain for the most part harsh and dry.

Then perhaps you try to dry yourself with a towel. But that does not
work; not a drop of water will the towel absorb. You might as well try
to dry yourself on the glossy side of a piece of oilcloth.

By this time you are shivering; so you probably decide to light the
oil stove and get warm and dry over that. But the oil will not come
up the wick! As a last resort you throw a dressing gown around you
(it does not get wet) and start a fire in the fireplace. This at last
warms and dries you; but as soon as you are dressed the clammy feeling
comes again--your clothes will not absorb any perspiration. While the
capillary attraction switch is turned off you will simply have to get
used to this.

Then suppose you start to write your experience. Your fountain pen
will not work. Even an ordinary pen does not work as well as it ought
to. It makes a blot on your paper. If you use the blotter you are
dismayed to find that the blot spreads out as flat as if you were
pressing a piece of glass against it. You take your eraser and try to
remove the blot. To your delight you find that it rubs out as easily
as a pencil mark. The ink has not soaked into the paper at all.
You begin to see some of the advantages in shutting off capillary
attraction.

Perhaps you are writing at the dining-room table, and you overturn the
inkwell on the tablecloth. Never mind, it is no trouble to brush the
ink off. Not a sign of stain is left behind.

By and by you look outdoors at the garden. Everything is withering.
The moisture does not move through the earth to where the roots of the
plants can reach it. Before everything withers completely, you rush to
the switchboard and turn on the capillary attraction again.

You can understand this force of capillary attraction better if you
perform the following experiments:

    EXPERIMENT 13. Fill a glass with water and color it with a
    little blueing or red ink. Into the glass put two or three
    glass tubes, open at both ends, and with bores of different
    sizes. (One of these tubes should be so-called thermometer
    tubing, with about 1 mm. bore.) Watch the colored water and
    see in which of the tubes it is pulled highest.

    EXPERIMENT 14. Put a clean washed lamp wick into the glass of
    colored water and watch to see if the water is pulled up the
    wick. Now let the upper end of the wick hang over the side of
    the glass all night. Put an empty glass under the end that is
    hanging out. The next morning see what has happened.

[Illustration: FIG. 19. Will the water be drawn up higher in the fine
glass tube or in a tube with a larger opening?]

[Illustration: FIG. 20. The water rises through the lamp wick by
capillary attraction.]

The space between the threads of the wick, and especially the still
finer spaces between the fibers that make up the threads, act like
fine tubes and the liquid rises in them just as it did in the fine
glass tube. Wherever there are fine spaces between the particles of
anything, as there are in a lump of sugar, a towel, a blotter, a wick,
and hundreds of other things, these spaces act like fine tubes and the
liquid goes into them. The force that causes the liquid to move along
fine tubes or openings is called _capillary attraction_.

Capillary attraction--this tendency of liquids to go into fine
tubes--is caused by the same force that makes things cling to each
other (adhesion), and that makes things hold together (cohesion). The
next two sections tell about these two forces; so you will understand
the cause of capillary attraction more thoroughly after reading them.
But you should know capillary attraction when you see it now, and know
how to use it. The following questions will show whether or not you
do:

    _APPLICATION 10._ Suppose you have spilled some milk on a
    carpet, and that you have at hand wet tea leaves, dry corn
    meal, some torn bits of a glossy magazine cover, and a piece
    of new cloth the pores of which are stopped up with starch.
    Which would be the best to use in taking up the milk?

    _APPLICATION 11._ A boy spattered some candle grease on his
    coat. His aunt told him to lay a blotter on the candle grease
    and to press a hot iron on the blotter, or to put the blotter
    under his coat and the iron on top of the candle grease,--he
    was not quite sure which. While he was trying to recall his
    aunt's directions, his sister said that he could use soap and
    water to take the grease out; then his brother told him to
    scrape the spot with a knife. Which would have been the right
    thing for him to do?


INFERENCE EXERCISE

    Explain the following:

    31. A pen has a slit running down to the point.

    32. When a man smokes, the smoke goes from the cigar into his
    mouth.

    33. A blotter which has one end in water soon becomes wet all
    over.

    34. Cream comes to the top of milk.

    35. It is much harder to stand on stilts than on your feet.

    36. Oiled shoes are almost waterproof.

    37. City water reservoirs are located on the highest possible
    places in or near cities.

    38. You can fill a self-filling fountain pen by squeezing the
    bulb, then letting go.

    39. The oceans do not flow off the world.

    40. When you turn a bottle of water upside down the water
    gurgles out instead of coming out in a smooth, steady stream.


SECTION 7. _How things stick to one another: Adhesion._

    Why is it that when a thing is broken it will not stay
    together without glue?

    Why does chalk stay on the blackboard?

Now that you have found out something about capillary attraction,
suppose that you should go to the imaginary switchboard again and
tamper with some other law of nature. An innocent-looking switch,
right above the capillary attraction switch, would be labeled
ADHESION. Suppose you have turned it off:

In an instant the wall paper slips down from the walls and crumples to
a heap on the floor. The paint and varnish drop from the woodwork like
so much sand. Every cobweb and speck of dust rolls off and falls in a
little black heap below.

When you try to wash, you cannot wet your hands. But they do not need
washing, as the dirt tumbles off, leaving them cleaner than they ever
were before. You can jump into a tank of water with all your clothes
on and come out as dry as you went in. You discover by the dryness of
your clothes that capillary attraction stopped when the adhesion was
turned off, for capillary attraction is just a part of adhesion.
But you are not troubled now with the clamminess of unabsorbed
perspiration. The perspiration rolls off in little drops, not wetting
anything but running to the ground like so much quicksilver.

Your hair is fluffier than after the most vigorous shampoo. Your skin
smarts with dryness. Your eyes are almost blinded by their lack
of tears. Even when you cry, the tears roll from your eyeballs and
eyelids like water from a duck's back. Your mouth is too dry to talk;
all the saliva rolls down your throat, leaving your tongue and cheeks
as dry as cornstarch.

I think you would soon turn on the adhesion switch again.

    EXPERIMENT 15. Touch the surface of a glass of water, and then
    raise your finger slightly. Notice whether the water tends to
    follow or to keep away from your finger as you raise it. Now
    dip your whole finger into the water and draw it out. Notice
    how the water clings, and watch the drops form and fall off.
    Notice the film of water that stays on, wetting your finger,
    after all dropping stops.

[Illustration: FIG. 21. As the finger is raised the water is drawn up
after it.]

Which do you think is the stronger, the pull of gravity which makes
some of the water drip off, or the pull of adhesion which makes some
of the water cling to your finger?

If the pull of gravity is stronger, would not all the water drop off,
leaving your finger dry? If the pull of adhesion is the stronger,
would not all the water stay on your finger, none dropping off?

The truth of the matter is that gravity is stronger than adhesion
unless things are very close together; then adhesion is stronger. The
part of the water that is very close to your finger clings to it in
spite of gravity; the part that is farther away forms drops and falls
down because of the pull of gravity.

Adhesion, then, is the force that makes things cling to each other
when they are very close together.

WHY IT IS EASIER TO TURN A PAGE IF YOU WET YOUR FINGER. Water spreads
out on things so that it gets very close to them. The thin film of
water on your finger is close enough to your finger and to the page
which you are turning to cling to both; so when you move your finger,
the page moves along with it.

WHY DUST CLINGS TO THE CEILING AND WALLS. The fine particles of dust
are wafted up against the ceiling and walls by the moving air in the
room. They are so small that they can fit into the small dents that
are in plaster and paper and can get very close to the wall. Once
they get close enough, the force of adhesion holds them with a pull
stronger than that of gravity.

Oily and wet surfaces catch dust much more readily than clean, dry
ones, simply because the dust can get so much closer to the oil
or water film and because this film flows partly around each dust
particle and holds it by the force of adhesion. This is why your face
gets much dirtier when it is perspiring than when it is dry.

    _APPLICATION 12._ Explain why cobwebs do not fall from the
    ceiling; why dust clings to a wet broom; why a postage stamp
    does not fall off an envelope.


INFERENCE EXERCISE

    Explain the following:

    41. There are no springs on the tops of high mountains.

    42. People used to shake sand over their letters after writing
    them in ink.

    43. People used to make night lights for bedrooms by pouring
    some oil into a cup of water and floating a piece of wick on
    the oil. The oil always stayed on top of the water, and went
    up through the wick fast enough to keep the light burning.

    44. Your face becomes much dirtier when you are perspiring.

    45. Ink bottles are usually made with wide bases.

    46. When you spill water on the floor, you cannot wipe it up
    with wrapping paper, but you can dry it easily with a cloth.

    47. Oiled mops are used in taking up dust.

    48. Cake will stick to a pan unless the pan is greased.

    49. Although the earth turns completely over every day, we
    never fall off it.

    50. Signs are fastened sometimes to windows or to the wind
    shields of automobiles by little rubber "suction caps."


SECTION 8. _The force that makes a thing hold together: Cohesion._

    What makes rain fall in drops?

    Why are diamonds hard?

You have not yet touched any of the most dangerous switches on the
imaginary switchboard of universal laws. But if your experience in
turning off the capillary attraction and adhesion switches did not
discourage you, you might try turning off the one beside them labeled
COHESION:

[Illustration: FIG. 22. El Capitan, Yosemite Valley, California.
If the force of cohesion were suspended, a mountain like this would
immediately become the finest dust.]

Things happen too swiftly for you to know much about them. The house
you are in falls to dust instantly. You fall through the place where
the floor has been; but you do not bump on the cement basement floor
below, partly because there is no such thing as a hard floor or even
hard ground anywhere, and partly because you disintegrate--fall to
pieces--so completely that there is nothing left of you but a grayish
film of fine dust and a haze of warm water.

With a deafening roar, rocks, skyscrapers, and even mountains tumble
down, fall to pieces, and sink into an inconceivably fine dust.
Nothing stands up in the world--not a tree, not an animal, not an
island. With a wild rush the oceans flood in over the dust that has
been nations and continents, and then this dust turns to a fine muddy
ooze in the bottom of a worldwide sea.

But it is an ocean utterly different from what we have in the real
world. There are no waves. Neither are there any reflections of clouds
in its surface,--first because the clouds would fly to pieces and turn
to invisible vapor, and second, because the ocean has no surface--it
simply melts away into the air and no one can tell where the water
stops and where the air begins.

Then the earth grows larger and larger. The ocean turns to a heavy,
dense, transparent steam. The fine mud that used to be rocks and
mountains and living things turns to a heavy, dense gas.

Our once beautiful, solid, warm, living earth now whirls on through
space, a swollen, gaseous globe, utterly dead.

And the only thing that prevents all this from actually happening
right now is that there is a force called _cohesion_ that holds things
together. It is the pull which one particle of anything has on another
particle of the same material. The paper in this book, the chair on
which you are sitting, and you yourself are all made of a vast number
of unthinkably small particles called _molecules_, each of which is
pulling on its neighbor with such force that all stay in their places.
Substances in which they pull the hardest, like steel, are very hard
to break in two; that is, it is difficult to pull the molecules of
these substances apart. In liquids, such as water, the molecules do
not pull nearly so hard on each other. In a gas, such as air, they are
so far apart that they have practically no pull on each other at all.
That is why everything would turn to a gas if the force of cohesion
stopped. Why things would turn cold will be explained in Chapter 4.

Cohesion, adhesion, and capillary attraction, all are the result of
the pull of molecules on each other. The difference is that capillary
attraction is the pulling of particles of liquids up into fine
spaces, as when a lamp wick draws up oil; adhesion is the pull of the
particles of one substance or thing on the particles of another when
they are very close together, as when water clings to your hand
or when dust sticks to the ceiling; while cohesion is the clinging
together of the particles of the same substance, like the holding
together of the particles of your chair or of this paper.

When you put your hand into water it gets wet because the adhesion
of the water to your hand is stronger than the cohesion of the
water itself. The particles of the water are drawn to your hand more
powerfully than they are drawn to each other. But in the following
experiment, you have an example of cases where cohesion is stronger
than adhesion:

    EXPERIMENT 16. Pour some mercury (quicksilver) into a small
    dish and dip your finger into it. As you raise your finger,
    see if the mercury follows it up as the water did in
    Experiment 14. When you pull your finger all the way out, has
    the mercury wet it at all? Put a lamp wick or a part of your
    handkerchief into the mercury. Does it draw the mercury up as
    it would draw up water?

[Illustration: FIG. 23. The mercury does not wet the finger, and as
the finger is lifted the mercury does not follow it.]

The reason for this peculiarity of mercury is that the pull between
the particles of mercury themselves is stronger than the pull between
them and your finger or handkerchief. In scientific language, the
cohesion of the mercury is stronger than its adhesion to your finger
or handkerchief. Although this seems unusual for a liquid, it is what
we naturally expect of solid things; you would be amazed if part of
the wood of your school seat stuck to you when you got up, for you
expect the particles in solid things to cohere--to have cohesion--much
more strongly than they adhere to something else. It is because solids
have such strong cohesion that they are solids.

    _APPLICATION 13._ Explain why mercury cannot wet your fingers;
    why rain falls in _drops_; why it is harder to drive a nail
    into wood than into soap; why steel is hard.


INFERENCE EXERCISE

    Explain the following:

    51. Ink spilled on a plain board soaks in, but on a varnished
    desk it can be easily wiped off.

    52. When a window is soiled you can write on it with your
    finger; then your finger becomes soiled.

    53. A starched apron or shirt stays clean longer than an
    unstarched one.

    54. When you hold a lump of sugar with one edge just touching
    the surface of a cup of coffee, the coffee runs up the lump.

    55. A drop of water on a dry plate is not flat but rounded.

    56. It is hard to write on cloth because the ink spreads out
    and blurs.

    57. If you roughen your finger nails by cleaning them with a
    knife, they will get soiled much more quickly than if you keep
    them smooth by using an orange stick.

    58. When you dip your pen in the ink and then move it across
    the paper, it makes ink marks on the paper.

    59. If you suck the air out of a bottle, the bottle will stick
    to your tongue.

    60. You cannot break a thick piece of iron with your hands.


SECTION 9. _Friction._

    What makes ice slippery?

    How does a brake stop a car?

    Why do things wear out?

It would not be such a calamity if we were to turn off friction from
the world. Still, I doubt whether we should want to leave it off much
longer than was necessary for us to see what would happen. Suppose we
imagine the world with all friction removed:

A man on a bicycle can coast forever along level ground. Ships at
sea can shut off steam and coast clear across the ocean. No machinery
needs oiling. The clothes on your body feel smoother and softer than
the finest silk. Perpetual motion is an established fact instead of an
absolute impossibility; everything that is not going against gravity
will keep right on moving forever or until it bumps into something
else.

_But_, if there is no friction and you want to stop, you cannot.
Suppose you are in an automobile when all friction stops. You speed
along helplessly in the direction you are going. You cannot steer the
machine--your hands would slip right around on the steering wheel, and
even if you turn it by grasping the spoke, your machine still skids
straight forward. If you start to go up a hill, you slow down, stop,
and then before you can get out of the machine you start backward
down the hill again and keep on going backward until you smash into
something.

A person on foot does not fare much better. If he is walking at the
time friction ceases, the ground is suddenly so slippery that he falls
down and slides along on his back or stomach in the same direction he
was walking, until he bumps into something big or starts to slip up
a slope. If he reaches a slope, he, like the automobile, stops an
instant a little way up, then starts sliding helplessly backward.

Another man is standing still when the friction is turned off. He
cannot get anywhere. As soon as he starts to walk forward, his feet
slip out from under him and he falls on his face. He lies in the same
spot no matter how he wriggles and squirms. If he tries to push with
his hands, they slip over the rough ground more easily than they now
slip through air. He cannot push sideways enough even to turn over. If
there happens to be a rope within reach and one end is tied to a tree,
he might try to take hold of the rope to pull himself along. But no
matter how tightly he squeezes, the rope slips right through his hands
when he starts to pull. If, however, there is a loop in the rope, he
can slip his hand through the loop and try to pull. But the knots with
which the rope is tied immediately come untied and he is as helpless
as ever.

Even if he takes hold of a board fence he is no more successful.
The nails in the board slip out of their holes and he is left with a
perfectly slippery and useless board on the ground beside him for a
companion. As it grows cold toward evening he may take some matches
out of his pocket and try to start a fire. Aside from the difficulty
of his being unable to hold them except by the most careful balancing
or by shutting them up within his slippery hands, he is entirely
incapable of lighting them; they slip over the cement beneath him or
over the sole of his shoe without the least rubbing.

In the real world, however, it is fortunately as impossible to get
away from friction as it is to get away from the other laws we have
tried to imagine as being turned off. There is always some friction,
or rubbing, whenever anything moves. A bird rubs against the air,
the point of a spinning top rubs against the sidewalk on which it is
spinning. Your shoes rub against the ground as you walk and so make
it possible for you to push yourself forward. The drive wheels of
machinery rub against the belts and pull them along. There is friction
between the wheels of a car and the track they are pushing against, or
the wheels would whirl around and around uselessly.

[Illustration: FIG. 24. Hockey is a fast game because there is little
friction between the skates and the ice.]

But we can increase or decrease friction a great deal. If we make
things rough, there is more friction between them than if they are
smooth. If we press things tightly together, there is more friction
than if they touch lightly. A nail in a loose hole comes out easily,
but in a tight hole it sticks; the pressure has increased the
friction. A motorman in starting a trolley car sometimes finds the
track so smooth that the wheels whirl around without pushing the car
forward; he pours some sand on the track to make it rougher, and
the car starts. When you put on new shoes, they are so smooth on the
bottom that they slip over the ground because of the lack of friction.
If you scratch the soles, they are rougher and you no longer slip.
If you try to pull a stake out of the ground, you have to squeeze it
harder than the ground does or it will slip out of your hands
instead of slipping out of the ground. When you apply a brake to an
automobile, the brake must press tightly against the axle or wheel to
cause enough friction to stop the automobile.

There are always two results of friction: heat and wear. Sometimes
these effects of friction are helpful to us, and sometimes they are
quite the opposite. The heat from friction is helpful when it makes
it possible for us to light a fire, but it is far from helpful when it
causes a hot box because of an ungreased wheel on a train or wagon, or
burns your hands when you slide down a rope. The wear from friction is
helpful when it makes it possible to sandpaper a table, scour a pan,
scrub a floor, or erase a pencil mark; but we don't like it when
it wears out automobile tires, all the parts of machinery, and our
clothes.

    EXPERIMENT 17. Hold a nail against a grindstone while you turn
    the stone. Notice both the wear and heat. Let the nail rest
    lightly on the stone part of the time and press hard part of
    the time. Which way does the nail get hotter? Which way does
    it wear off more quickly? Run it over a pane of glass and see
    if it gets as hot as it does on the grindstone; if it wears
    down as quickly.

WHY WE OIL MACHINERY. We can decrease friction by keeping objects
from pressing tightly against each other, and by making their surfaces
smooth. The most common way of making surfaces smooth is by oiling
or greasing them. A film of oil or grease makes things so smooth and
slippery that there is very little friction. That is why all kinds of
machinery will run so smoothly if they are kept oiled. And since the
oil decreases friction, it decreases the wear caused by friction.
So well-oiled machines last much longer than machines that are not
sufficiently oiled.

[Illustration: FIG. 25. The friction of the stone heats the nail and
wears it away.]

WHY BALL BEARINGS ARE USED. There is much less friction when a round
object rolls over a surface than when two surfaces slide over one
another, unless the sliding surfaces are very smooth; think how much
easier it is to pull a wagon forward than it would be to take hold
of the wheels and pull the wagon sidewise. So when you want the least
possible friction in a machine you use ball bearings. The bearings
are located in the hub of a wheel. Then, instead of the axle rubbing
against the hub, the bearings roll inside of the hub. This causes very
little friction; and the friction is made still less by keeping the
bearings oiled.

    _APPLICATION 14._ Suppose you were making a bicycle,--in
    which of the following places would you want to increase the
    friction, and in which would you want to decrease it? Handle
    grips, axles, pedals, tires, pedal cranks, the sockets in
    which the handle bar turns, the nuts that hold the parts
    together.

    _APPLICATION 15._ A small boy decided to surprise his mother
    by oiling her sewing-machine. He put oil in the following
    places:

    On the treadle, on the large wheel over which the belt runs,
    on the axle of the same wheel, on the groove in the little
    wheel up above where the belt runs, on the joint where the
    needle runs up and down, on the little rough place under the
    needle that pushes the cloth forward. Which of these did he do
    well to oil and which should he have let alone?


INFERENCE EXERCISE

    Explain the following:

    61. Rivers flow north as well as south, although we usually
    speak of north as "up north."

    62. Tartar and bits of food stick to your teeth.

    63. Brushing your teeth with tooth powder cleans them.

    64. When a chair has gliders (smooth metal caps) on its feet,
    it slides easily across the floor.

    65. When you wet your finger, you can turn a page more easily.

    66. A lamp wick draws oil up from the lower part of a lamp to
    the burner.

    67. The sidewalks on steep hills are made of rough cement.

    68. Certain fish can rise in the water by expanding their air
    bladders, although this does not make them weigh any less.

    69. When your hands are cold, you rub them together to warm
    them.

    70. It is dangerous to stand up in a rowboat or canoe.




CHAPTER THREE

CONSERVATION OF ENERGY


SECTION 10. _Levers._

_How a big weight can be lifted with a little force; how one thing
moving slowly a short distance can make another move swiftly a long
distance._

    Why can you go so much faster on a bicycle than on foot?

    How can a man lift up a heavy automobile by using a jack?

    Why can you crack a hard nut with a nutcracker when you cannot
    crack it by squeezing it between two pieces of iron?

"Give me a lever, long enough and strong enough, and something to rest
it on, and I can lift the whole world," said an old Greek philosopher.
And as a philosopher he was right; theoretically it would be possible.
But since he needed a lever that would have been as long as from here
to the farthest star whose distance has ever been measured, and
since he would have had to push his end of the lever something like
a quintillion (1,000,000,000,000,000,000) miles to lift the earth one
inch, his proposition was hardly a practical one.

But levers are practical. Without them there would be none of our
modern machines. No locomotives could speed across the continents; no
derricks could lift great weights; no automobiles or bicycles would
quicken our travel; our very bodies would be completely paralyzed. Yet
the law back of all these things is really simple.

You have often noticed on the see-saw that a small child at one end
can be balanced by a larger child at the other end, provided that the
larger child sits nearer the middle. Why should it matter where the
larger child sits? He is always heavier--why doesn't he overbalance
the small child? It is because when the small child moves up and down
he goes a longer distance than the large child does. In Figure 26 the
large boy moves up and down only half as far as the little girl does.
She weighs only half as much as he, yet she balances him.

[Illustration: FIG. 26. The little girl raises the big boy, but in
doing it she moves twice as far as he does.]

You will begin to get a general understanding of levers and how they
work by doing the following experiment:

    EXPERIMENT 18. For this experiment there will be needed a
    small pail filled with something heavy (sand or stones will
    do), a yardstick with a hole through the middle and another
    hole near one end and with notches cut here and there along
    the edge, and a post or table corner with a heavy nail driven
    into it to within an inch of the head. The holes in the
    yardstick must be large enough to let the head of this nail
    through.

    Put the middle hole of the yardstick over the nail, as is
    shown in Figure 27. The nail is the _fulcrum_ of your lever.
    Now hang the pail on one of the notches about halfway between
    the fulcrum and the end of the stick and put your hand on the
    opposite side of the yardstick at about the same distance as
    the pail is from the fulcrum. Raise and lower the pail several
    times by moving the opposite end of the lever up and down. See
    how much force it takes to move the pail.

    Now slide your hand toward the fulcrum and lower and raise the
    pail from that position. Is it harder or easier to lift the
    pail from here than from the first position? Which moves
    farther up and down, your hand or the pail?

    Next, slide your hand all the way out to the end of the
    yardstick and raise and lower the pail from there. Is the pail
    harder or easier to lift? Does the pail move a longer or a
    shorter distance up and down than your hand?

    If you wanted to move the pail a long way without moving your
    hand as far, would you put your hand nearer to the fulcrum or
    farther from it than the pail is?

[Illustration: FIG. 27. The yardstick is a lever by which he lifts the
pail.]

[Illustration: FIG. 28. A lever with the weight between the fulcrum
and the force.]

    Suppose you wanted to lift the pail with the least possible
    effort, where would you put your hand?

    Notice another fact: when your hand is at the end of the
    yardstick, it takes the same length of time to move a long way
    as the pail takes to move a short way. Then which is moving
    faster, your hand or the pail?

    EXPERIMENT 19. Put the end hole of the yardstick on the nail,
    as shown in Figure 28. The nail is still the fulcrum of your
    lever. Put the pail about halfway between the fulcrum and the
    other end of the stick, and hold the end of the stick in your
    hands.

    Raise and lower your hand to see how hard or how easy it is
    to lift the pail from this position. Which is moving farther,
    your hand or the pail? Which is moving faster?

    Now put your hand about halfway between the fulcrum and the
    pail and raise and lower it. Is it harder or easier to raise
    than before? Which moves farther this time, your hand or the
    pail? Which moves faster?

    If you wanted to make the pail move farther and faster than
    your hand, would you put your hand nearer to the fulcrum than
    the pail is, or farther from the fulcrum than the pail? If you
    wanted to move the pail with the least effort, where would you
    put your hand?

    EXPERIMENT 20. Use a pair of long-bladed shears and fold a
    piece of cardboard once to lie astride your own or some one
    else's finger. Put the finger, protected by the cardboard,
    between the two points of the shears. Then squeeze the handles
    of the shears together. See if you can bring the handles
    together hard enough to hurt the finger between the points.

    Now watch the shears as you open and close the blades. Which
    move farther, the points of the shears or the handles? Which
    move faster?

    Next, put the finger, still protected by the cardboard,
    between the _handles_ of the shears and press the points
    together. Can you pinch the finger this way harder or less
    hard than in the way you first tried?

    Do the points or handles move farther as you close the shears?
    Which part closes with the greater force?

[Illustration: FIG. 29. You cannot pinch hard enough this way to
hurt.]

[Illustration: FIG. 30. But this is quite different.]

    EXPERIMENT 21. Use a Dover egg beater. Fasten a small piece
    of string to one of the blades, so that you can tell how many
    times it goes around. Turn the handle of the beater around
    once slowly and count how many times the blade goes around.
    Which moves faster, the handle or the blade? Where would you
    expect to find more force, in the cogs or in the blades? Test
    your conclusion this way: Put your finger between the blades
    and try to pinch it by turning the handle; then place your
    finger so that the skin is caught between the cogs and try to
    pinch the finger by turning the blades. Where is there more
    force? Where is there more motion?

    EXPERIMENT 22. Put a spool over the nail which was your
    fulcrum in the first two experiments. (Take the stick off
    the nail first, of course.) Use this spool as a pulley. Put a
    string over it and fasten one end of your string to the pail
    (Fig. 32). Lift the pail by pulling down on the other end of
    the string. Notice that it is not harder or easier to move the
    pail when it is near the nail than when it is near the floor.
    When your hand moves down from the nail to the floor, how far
    up does the pail move? Does the pail move a greater or less
    distance than your hand, or does it move the same distance?

    Next fasten one end of the string to the nail. Set the pail on
    the floor. Pass the string through the handle of the pail and
    up over the spool (Fig. 33). Pull down on the loose end of the
    string. Is the pail easier to lift in this way or in the
    way you first tried? As you pull down with your hand, notice
    whether your hand moves farther than the pail, not so far
    as the pail, or the same distance. Is the greater amount
    of motion in your hand or in the pail? Then where would you
    expect the greater amount of force?

[Illustration: FIG. 31. When the handle is turned the blades of the egg
beater move much more rapidly than the hand. Will they pinch hard enough
to hurt?]

The whole idea of the lever can be summed up like this: one end of the
contrivance _moves_ more than the other. But energy cannot be lost; so
to make up for this extra _motion_ at one end more _force_ is always
exerted at the other.

This rule is true for all kinds of levers, blocks and tackles or
pulley systems, automobile and bicycle gears, belt systems, cog
systems, derricks, crowbars, and every kind of machine. In most
machines you either put in more force than you get out and gain
motion, or you put in more motion than you get out and gain force. In
the following examples of the lever see if you can tell whether you
are applying more force and obtaining more motion, or whether you are
putting in more motion and obtaining more force:

Cracking nuts with a nut cracker.

Beating eggs with a Dover egg beater.

Going up a hill in an automobile on low gear.

Speeding on high gear.

Cutting cloth with the points of shears.

Cutting near the angle of the shears.

Turning a door knob.

Picking up sugar with sugar tongs.

Pinching your finger in the crack of a door on the hinge side.

[Illustration: FIG. 32. His hand goes down as far as the pail goes
up.]

    _APPLICATION 16._ Suppose you wanted to lift a heavy frying
    pan off the stove. You have a cloth to keep it from burning
    your hand. Would it be easier to lift it by the end of the
    handle or by the part of the handle nearest the pan?

    _APPLICATION 17._ A boy was going to wheel his little sister
    in a wheelbarrow. She wanted to sit in the middle of the
    wheelbarrow; her brother thought she should sit as near the
    handles as possible so that she would be nearer his hands.
    Another boy thought she should sit as near the wheel as
    possible. Who was right?

    _APPLICATION 18._ James McDougal lived in a hilly place. He
    was going to buy a bicycle. "I want one that will take the
    hills easily," he said. The dealer showed him two bicycles.
    On one the back wheel went around three times while the pedals
    went around once; on the other the back wheel went around
    four and a half times while the pedals went around once. Which
    bicycle should James have chosen? If he had wanted the bicycle
    for racing, which should he have chosen?

    _APPLICATION 19._ A wagon stuck in the mud. The driver got out
    and tried to help the horse by grasping the spokes and turning
    the wheel. Should he have grasped the spokes near the hub,
    near the rim, or in the middle?

[Illustration: FIG. 33. With this arrangement the pail travels more
slowly than the hand. Will it seem heavier or lighter than with the
arrangement shown in Figure 32?]


INFERENCE EXERCISE

    Explain the following:

    71. When you turn on the faucet of a distilled-water bottle,
    bubbles go up through the water as the water pours out.

    72. A clothes wringer has a long handle. It wrings the clothes
    drier than you can wring them by hand.

    73. You use a crowbar when you want to raise a heavy object
    such as a rock.

    74. Sometimes it is almost impossible to get the top from a
    jar of canned fruit unless you let a little air under the edge
    of the lid.

    75. It is much easier to carry a carpet sweeper if you take
    hold near the sweeper part than it is if you take hold at the
    end of the handle.

    76. You can make marks on a paper by rubbing a pencil across
    it.

    77. A motorman sands the track when he wishes to stop the car
    on a hill.

    78. On a faucet there is a handle with which to turn it.

    79. Before we pull candy we butter our fingers.

    80. You can scratch glass with very hard steel but not with
    wood.


SECTION 11. _Inertia._

    Why is it that if you push a miniature auto rapidly, it will
    go straight?

    Why does the earth never stop moving?

    When you jerk a piece of paper from under an inkwell, why does
    the inkwell stay still?

When you are riding in a car and the car stops suddenly, you are
thrown forward; your body tends to keep moving in the direction in
which the car was going. When a car starts suddenly, you are thrown
backward; your body tends to stay where it was before the car started.

When an automobile bumps into anything, the people in the front seat
are often thrown forward through the wind shield and are badly cut;
their bodies keep on going in the direction in which the automobile
was going.

When you jump off a moving street car, you have to run along in the
direction the car was going or you fall down; your body tries to keep
going in the same direction it was moving, and if your feet do not
keep up, you topple forward.

Generally we think that it takes force to start things to move, but
that they will stop of their own accord. This is not true. It takes
just as much force to stop a thing as it does to start it, and what
usually does the stopping is friction.

When you shoot a stone in a sling shot, the contracting rubber pulls
the stone forward very rapidly. The stone has been started and
it would go on and never stop if nothing interfered with it. For
instance, if you should go away off in space--say halfway between here
and a star--and shoot a stone from a sling shot, that stone would keep
on going as fast as it was going when it left your sling shot, forever
and ever, without stopping, unless it bumped into a star or something.
On earth the reason it stops after a while is that it is bumping into
something all the time--into the particles of air while it is in the
air, and finally against the earth when it is pulled to the ground by
gravity.

If you threw a ball on the moon, the person who caught it would have
to have a very thick mitt to protect his hand, and it would never
be safe to catch a batted fly. For there is no air on the moon, and
therefore nothing would slow the ball down until it hit something; and
it would be going as hard and fast when it struck the hand of the one
who caught it as when it left your hand or the bat.

[Illustration: FIG. 34. When the paper is jerked out, the glass of
water does not move.]

TRY THESE EXPERIMENTS:

    EXPERIMENT 23. Fill a glass almost to the brim with water.
    Lay a smooth piece of writing paper 10 or 11 inches long on a
    smooth table, placing it near the edge of the table. Set the
    glass of water on the paper near its inner edge (Fig. 34).

    Take hold of the edge of the paper that is near the edge of
    the table. Move your hand a little toward the glass so that
    the paper is somewhat bent. Then, keeping your hand near the
    level of the table, suddenly jerk the paper out from under the
    glass. If you give a quick enough jerk and keep your hand near
    the level of the table, not a drop of water will spill and the
    glass will stay almost exactly where it was.

This is because the glass of water has inertia. It was standing still,
and so it tends to remain standing still. Your jerk was so sudden that
there was not time to overcome the inertia of the glass of water; so
it stayed where it was.

    EXPERIMENT 24. Have a boy on roller skates skate down the hall
    or sidewalk toward you and have him begin to coast as he comes
    near. When he reaches you, put out your arm and try to stop
    him. Notice how much force it takes to stop him in spite of
    the fact that he is no longer pushing himself along.

    Now let the boy skate toward you again, coasting as before;
    but this time have him swing himself around a corner by taking
    hold of you as he passes. Notice how much force it takes just
    to change the direction in which he is moving.

[Illustration: FIG. 35. When a boy is moving rapidly, it takes force to
change the direction of his motion.]

You see the boy's inertia makes him tend to keep going straight ahead
at the same speed; it resists any change either in the speed or the
direction of his motion. So it takes a good deal of force either to
stop him or to turn him.

If, on the other hand, _you_ had no inertia, you could neither have
stopped him nor turned him; he would have swept you right along with
him. It was because inertia made you tend to remain still, that you
could overcome part of his inertia. At the same time he overcame part
of your inertia, for he made you move a little.

Inertia is the tendency of a thing to keep on going forever in the
same direction if once it is started, or to stand still forever unless
something starts it. If moving things did not have inertia (if they
did not tend to keep right on moving in the same direction forever or
until _something_ changed their motion), you could not throw a ball;
the second you let go of it, it would stop and fall to the ground.
You could not shoot a bullet any distance; as soon as the gases of the
gunpowder had stopped pushing against it, it would stop dead and fall.
There would be no need of brakes on trains or automobiles; the instant
the steam or gasoline was shut off, the train or auto would come to a
dead stop. But you would not be jerked in the least by the stopping,
because as soon as the automobile or train stopped, your body too
would stop moving forward. Your automobile could even crash into a
building without your being jarred. For when the machine came to a
sudden stop, you would not be thrown forward at all, but would sit
calmly in the undamaged automobile.

If you sat in a swing and some one ran under you, you would keep going
up till he let go, and then you would be pulled down by gravity just
as you now are. But just as soon as the swing was straight up and down
you would stop; there would be no inertia to make you keep on swinging
back and up.

If the inertia of moving things stopped, the clocks would no longer
run, the pendulums would no longer swing, nor the balance wheels turn;
nothing could be thrown; it would be impossible to jump; there would
cease to be waves on the ocean; and the moon would come tumbling to
the earth. The earth would stop spinning; so there would be no change
from day to night; and it would stop swinging about in its orbit and
start on a rush toward the sun.

But there is always inertia. And all things everywhere and all the
time tend to remain stock still if they are still, until some force
makes them move; and all things that are moving tend to keep on moving
at the same speed and in the same direction, until something stops
them or turns them in another direction.

    _APPLICATION 20._ Explain why you should face forward when
    alighting from a street car; why a croquet ball keeps rolling
    after you hit it; why you feel a jolt when you jump down from
    a high place.


INFERENCE EXERCISE

    Explain the following:

    81. It is much easier to erase charcoal drawings than
    water-color paintings.

    82. When an elevator starts down suddenly you feel lighter for
    a moment, while if it starts up quickly you feel heavier.

    83. You can draw a nail with a claw hammer when you could not
    possibly pull it with your hand even if you could get hold of
    it.

    84. When an automobile bumps into anything, the people in the
    front seat are often thrown forward through the wind shield.

    85. Certain weighted dolls will rise and stand upright, no
    matter in what position you lay them down.

    86. Some automobile tires have little rubber cups all over
    them which are supposed to make the tires cling to the
    pavement and thus prevent skidding.

    87. It is hard to move beds and bureaus which have no castors
    or gliders.

    88. When you jump off a moving street car, you lean back.

    89. All water flows toward the oceans sooner or later.

    90. You can skate on ice, but not on a sidewalk, with ice
    skates.


SECTION 12. _Centrifugal force._

    Why does not the moon fall down to the earth?

    Why will a lasso go so far after it is whirled?

    Why does a top stand on its point while it is spinning?

If centrifugal force suddenly stopped acting, you would at first not
notice any change. But if you happened to get into an automobile and
rode down a muddy street, you would be delighted to find that the mud
did not fly up from the wheels as you sped along. And when you went
around a slippery corner, your automobile would not skid in the least.

If a dog came out of a pool of water and shook himself while
centrifugal force was not acting, the water, instead of flying off in
every direction, would merely drip down to the ground as if the dog
were not shaking himself at all. A cowboy would find that he could no
longer throw his lasso by whirling it around his head. A boy trying to
spin his top would discover that the top would not stand on its point
while spinning, any better than when it was not spinning.

These are little things, however. Most people would be quite
unconscious of any change for some time. _Then_, as night came on and
the full moon rose, it would look as if it were growing larger and
larger. It would seem slowly to swell and swell until it filled the
whole sky. Then with a stupendous crash the moon would collide with
the earth. Every one would be instantly killed. And it would be lucky
for them that they were; for if any people survived the shock of the
awful collision, they would be roasted to death by the heat produced
by the striking together of the earth and the moon. Moreover, the
earth would be whirled swiftly toward the sun, and a little later the
charred earth would be swept into the sun's vast, tempestuous flames.

When we were talking about inertia, we said that if there were no
inertia, the moon would tumble down to the earth and the earth, too,
would fall into the sun. That was because if there were no inertia
there would be no centrifugal force. For centrifugal force is not
really a force at all, but it is one form of inertia--the inertia of
whirling things. Do this experiment:

    EXPERIMENT 25. Hold a pail half full of water in one hand.
    Swing it back and forth a couple of times; then swing it
    swiftly forward, up, and on around, bringing it down back of
    you (Fig. 36). Swing it around this way swiftly and evenly
    several times, finally stopping at the beginning of the up
    swing.

It is centrifugal force that keeps the water in the pail. It depends
entirely on inertia. You see, while the pail is swinging upward
rapidly, the water is moving up and tends by its inertia to keep right
on moving in the same upward direction. Before you get it over your
head, the tendency of the water to keep on going up is so strong that
it pulls on your arm and hand and presses against the bottom of the
pail above it. Its tendency to go on up is stronger than the downward
pull of gravity. As you swing the pail on backward, the water of
course has to move backward, too; so now it tends to keep on moving
backward; and when the pail is starting down behind you, the water is
tending to fly out in the backward direction in which it has just been
going. Therefore it still pushes against the bottom of the pail and
pulls away from your shoulder, which is in the center of the circle
about which the pail is moving. By the time you have swung the pail
on down, the water in it tends to keep going down, and it is still
pulling away from your shoulder and pressing against the bottom of the
pail.

[Illustration: FIG. 36. Why doesn't the water spill out?]

In this way, during every instant the water tends to keep going in the
direction in which it was going just the instant before. The result
is that the water keeps pulling away from your shoulder as long as you
keep swinging it around.

_All whirling things tend to fly away from the center about which they
are turning._ This is the law of centrifugal force. The earth, for
example, as it swings around the sun, tends to fly away from the
center of its orbit. This tendency of the earth--its centrifugal
force--keeps it from being drawn into the sun by the powerful pull of
the sun's gravitation. At the same time it is this gravitation of the
sun that keeps the earth from flying off into space, where we should
all be frozen to icicles and lost in everlasting night. For if the
sun's pull stopped, the earth would fly off as does a stone whirled
from the end of a string, when you let go of the string.

The moon, in like manner, would fly away from the earth and sun if
_gravitation_ stopped pulling it, but it would crash into us if its
_centrifugal force_ did not keep it at a safe distance.

Have you ever sat on a spinning platform, sometimes called "the social
whirl," in an amusement park, and tried to stay on as it spun faster
and faster? It is centrifugal force that makes you slide away from the
center and off at the edge.

[Illustration: FIG. 37. An automobile race. Notice how the track is
banked to keep the cars from overturning on the curves.]

HOW CREAM IS SEPARATED FROM MILK BY CENTRIFUGAL FORCE. The heavier
things are, the harder they are thrown out by centrifugal force. Milk
is heavier than cream, as you know from the fact that cream rises
and floats on top of the milk. So when milk is put into a centrifugal
separator, a machine that whirls it around very rapidly, the milk is
thrown to the outside harder than the cream, and the cream therefore
stays nearer the middle. As the bowl of the machine whirls faster, the
milk is thrown so hard against the outside that it flattens out and
rises up the sides of the bowl. Thus you have a large hollow cylinder
of milk on the outside against the wall of the bowl, while the
whirling cream forms a smaller cylinder inside the cylinder of
milk. By putting a spout on the machine so that it reaches the inner
cylinder, the cream can be drawn off, while a spout not put in so far
will draw off the milk.

WHY A SPINNING TOP STANDS ON ITS POINT. When a top spins, all the
particles of wood of which the top is made are thrown out and away
from the center of the top, or rather they _tend_ to go out and away.
And the pull of these particles out from the center is stronger than
the pull of gravitation on the edges of the top to make it tip over;
so it stands upright while it spins. Spin a top and see how this is.

    _APPLICATION 21._ Explain how a motor cyclist can ride on an
    almost perpendicular wall in a circular race track. Explain
    how the earth keeps away from the sun, which is always
    powerfully pulling the earth toward it.


INFERENCE EXERCISE

    Explain the following:

    91. As you tighten a screw it becomes harder to turn.

    92. There is a process for partly drying food by whirling it
    rapidly in a perforated cylinder.

    93. It is easier to climb mountains in hobnailed shoes than in
    smooth-soled ones.

    94. When you bore a hole with a brace and bit, the hand that
    turns the brace goes around a circle many times as large as
    the hole that is being bored.

    95. The hands of some persons become red and slightly swollen
    if they swing them while taking a long walk.

    96. A flywheel keeps an engine going between the strokes of
    the piston.

    97. In dry parts of the country farmers break up the surface
    of the soil frequently, as less water comes up to the surface
    through pulverized soil than would come through the fine pores
    of caked earth.

    98. After you have apparently cleaned a grease spot out of
    a suit it often reappears when you have worn the suit a few
    days.

    99. Mud flies up from the back wheel of a boy's bicycle when
    he rides along a wet street.

    100. A typewriter key goes down less than an inch, yet the
    type bar goes up nearly 5 inches.


SECTION 13. _Action and reaction._

    How can a bird fly? What makes it stay up in the air?

    What makes a gun kick?

    Why do you sink when you stop swimming?

Whenever anything moves, it pushes something else in an opposite
direction. When you row a boat you can notice this; you see the oars
pushing the water backward to push the boat forward. Also, when you
shoot a bullet forward you can feel the gun kick backward; or when
you pull down hard enough on a bar, your body rises up and you
chin yourself. But the law is just as true for things which are not
noticeable. When you walk, your feet push back against the earth; and
if the earth were not so enormous and you so small, and if no one else
were pushing in the opposite direction, you would see the earth spin
back a little for each step you took forward, just as the big ball
that a performing bear stands on turns backward as the bear tries to
walk forward.

[Illustration: FIG. 38. The horse goes forward by pushing backward on
the earth with his feet.]

The usual way of saying this is, "Action and reaction are equal and
opposite." If you climb a rope, the upward movement of your body is
the action; but you have to pull down on the rope to lift your body
up. This is the reaction.

Without this law of action and reaction no fish could swim, no
steamboat could push its way across the water, no bird could fly, no
train or machine of any kind could move forward or backward, no man or
animal could walk or crawl. The whole world of living things would be
utterly paralyzed.

[Illustration: FIG. 39. As he starts to toss the ball up, will he
weigh more or less?]

When _anything_ starts to move, it does so by pushing on something
else. When your arms start to move up, they do so by pushing your body
down a little. When you swim, you push the water back and down with
your arms and legs, and this pushes your body forward and up. When a
bird flies up into the air, it pushes its body up by beating the air
down with its wings. When an airplane whirs along, its propeller fans
the air backward all the time. Street-car tracks are kept shiny by the
wheels, which slip a little as they tend to shove the track backward
in making the car move forward. Automobile tires wear out in much the
same way,--they slip and are worn by friction as they move the earth
back in pushing the automobile forward. In fact, if there are loose
pebbles or mud on the road, you can see the pebbles or mud fly back,
as the wheels of the automobile begin to turn rapidly and give their
backward push to the earth beneath.

[Illustration: FIG. 40. Action and reaction are equal; when he pushes
forward on the ropes, he pushes backward with equal force on the
seat.]

Here are a couple of experiments that will show you action and
reaction more clearly:

    EXPERIMENT 26. Stand on a platform scale and weigh yourself.
    When the beam is exactly balanced, move your hands upward and
    notice whether you weigh more or less when they _start_ up.
    Now move them downward; when they _start_ down, do you weigh
    more or less? Toss a ball into the air, and watch your weight
    while you are tossing it. Does your body tend to go up or down
    while you are making the ball go up?

    EXPERIMENT 27. Go out into the yard and sit in a rope swing.
    Stop the swing entirely. Keep your feet off the ground all
    through the experiment. Now try to work yourself up in the
    swing; that is, make it swing by moving your legs and body and
    arms, but not by touching the ground. (Try to make it swing
    forward and backward only; when you try to swing sidewise, the
    distance between the ropes spoils the experiment.) See if you
    can figure out why the swing will not move back and forth.
    Notice your bodily motions; notice that when half of your body
    goes forward, half goes back; when you pull back with your
    hands, you push your body forward. If you watch yourself
    closely, you will see that every backward motion is exactly
    balanced by a forward motion of some part of your body.

    _APPLICATION 22._ Explain why you push forward against the
    table to shove your chair back from it; why a bird beats down
    with its wings against the air to force itself up; why you
    push back on the water with your oars to make a rowboat go
    forward.


INFERENCE EXERCISE

    Explain the following:

    101. Water comes up city pipes into your kitchen.

    102. When you try to push a heavy trunk, your feet slip out
    from under you and slide in the opposite direction.

    103. When you turn a bottle of water upside down with a small
    piece of cardboard laid over its mouth, the water stays in the
    bottle.

    104. You can squeeze a thing very tightly in a vise.

    105. There is a water game called "log rolling"; two men stand
    on a log floating in the water and roll the log around
    with their feet, each one trying to make the other lose his
    balance. Explain why the log rolls backward when the man
    apparently runs forward.

    106. The oil which fills up the spaces between the parts of
    a duck's feathers keeps the duck from getting wet when a hen
    would be soaked.

    107. Sleds run on snow more easily than wagons do.

    108. In coasting down a hill, it is difficult to stop at the
    bottom.

    109. When you light a pinwheel, the wheel whirls around as the
    powder burns, and the sparks fly off in all directions.

    110. You cannot lift yourself by your own boot straps.


SECTION 14. _Elasticity._

    What makes a ball bounce?

    How does a springboard help you dive?

    Why are automobile and bicycle tires filled with air?

Suppose there were a man who was perfectly elastic, and who made
everything he touched perfectly elastic. Fortunately there is no such
person, but suppose an elastic man _did_ exist:

He walks with a spring and a bound; his feet bounce up like rubber
balls each time they strike the earth; his legs snap back into place
after each step as if pulled by a spring. If he stumbles and falls to
the ground, he bounces back up into the air without a scar. (You see,
his skin springs back into shape even if it is scratched, so that a
scratch instantly heals.) And he bounces on and on forever without
stopping.

Suppose you, seeing his plight, try to stop him. Since we are
pretending that he makes everything he touches elastic, the instant
you touch him you bounce helplessly away in the opposite direction.

You may think your clothes will be wrinkled by all this bouncing
about, but since we are imagining that you have caught the elastic
touch from the elastic man, your clothes which touch you likewise
become perfectly elastic. So no matter how mussed they get, they
promptly straighten out again to the condition they were in when you
touched the elastic man.

If you notice that your shoe lace was untied just before you became
elastic, and you now try to tie it and tuck it in, you find it most
unmanageable. It insists upon flying out of your shoe and springing
untied again.

Perhaps your hair was mussed before you became elastic. Now it is
impossible to comb it straight; each hair springs back like a fine
steel wire.

If you take a handkerchief from your pocket to wipe your perspiring
brow, you find that it does not stay unfolded. As soon as it is spread
out on your hand, it snaps back to the shape and the folds it had
while in your pocket.

Suppose you bounce up into an automobile for a ride. The automobile,
now being made elastic by your magic touch, bounds up into the air
at the first bump it strikes, and thereafter it goes hopping down the
street in a most distressing manner, bouncing off the ground like a
rubber ball each time it comes down. And each time it bumps you are
thrown off the seat into the air.

You find it hard to stay in any new position. Your body always
tends to snap back to the position you were in when you first became
elastic. If you touch a trotting horse and it becomes elastic,
the poor animal finds that his legs always straighten out to their
trotting position, whether he wants to walk or stand still or lie
down.

Imagine the plight of a boy pitching a ball, or some one yawning and
stretching, or a clown turning a somersault, if you touch each of
these just in the act and make him elastic. Their bodies always tend
to snap back to these positions. Whenever the clown wants to rest, he
has to get in the somersault position. The boy pitcher sleeps in the
position of "winding up" to throw the ball. The one who was yawning
and stretching has to be always on the alert, because the instant he
stops holding himself in some other position, his mouth flies open,
his arms fly out, and every one thinks he is bored to death.

You might touch the clay that a sculptor is molding and make it
elastic. The sculptor can mold all he pleases, but the clay is like
rubber and always returns at once to its original shape.

If you make a tree elastic when a man is chopping it down, his ax
bounces back from the tree with such force as nearly to knock him
over, and no amount of chopping makes so much as a lasting dent in the
tree.

Suppose you step in some mud. The mud does not stick to your shoes. It
bends down under your weight, but springs back to form again as soon
as your weight is removed.

And if you try to spread some elastic butter on bread, nothing will
make the butter stay spread. The instant you remove your knife, the
butter rolls up again into the same kind of lump it was in before.

As for chewing your bread, you might as well try to chew a rubber
band. You force your jaws open, and they snap back on the bread all
right; then they spring open again, and snap back and keep this up
automatically until you make them stop. But for all this vigorous
chewing your bread looks as if it had never been touched by a tooth.

Sewing is about as difficult. The thread springs into a coil in the
shape of the spool. No hem stays turned; the cloth you try to sew
springs into its original folds in a most exasperating manner.

On the whole, a perfectly elastic world would be a hopeless one to
live in.

_Elasticity is the tendency of a thing to go back to its original
shape or size whenever it is forced into a different shape or size._

A thing does not have to be soft to be elastic. Steel is very elastic;
that is why good springs are almost always made of steel. Glass
is elastic; you know how you can bounce a glass marble. Rubber is
elastic, too. Air is elastic in a different way; it does not go back
to its original shape, since it has no shape, but if it has been
compressed and the pressure is removed it immediately expands again;
so a football or any such thing filled with air is decidedly elastic.
That is why automobile and bicycle tires are filled with air; it makes
the best possible "springs."

Balls bounce because they are elastic. When a ball strikes the ground,
it is pushed out of shape. Since it is elastic it tries immediately to
come back to its former shape, and so pushes out against the ground.
This gives it such a push upward that it flies back to your hand.

Sometimes people confuse elasticity with action and reaction. But the
differences between them are very clear. Action and reaction happen at
the same time; your body goes up at the same time that you pull down
on a bar to chin yourself; while in elasticity a thing moves first one
way, then the other; you throw a ball down, _then_ it comes back up to
you. Another difference is that in action and reaction one thing moves
one way and another thing is pushed the other way; while in elasticity
the same thing moves first one way, then the other. If you press
down on a spring scale with your hand, you are lifting up your body a
little to do it; that is action and reaction. But after you take your
hand off the scale the pan springs back up: first it was pushed down,
then it springs back to its original position; it does this because of
the elasticity of its spring.

    _APPLICATION 23._ Explain why basket balls are filled with
    air; why springs are usually made of steel; why we use rubber
    bands to hold papers together; why a toy balloon becomes small
    again when you let the air out.


INFERENCE EXERCISE

    Explain the following, being especially careful not to confuse
    action and reaction with elasticity:

    111. When you want to push your chair back from a table, you
    push forward against the table.

    112. The pans in which candy is cooled must be greased.

    113. Good springs make a bed comfortable.

    114. Paper clips are made of steel or spring brass.

    115. A spring door latch acts by itself if you close the door
    tightly.

    116. On a cold morning, you rub your hands together to warm
    them.

    117. If an electric fan is not fastened in place and has not a
    heavy base, it will move backward while it is going.

    118. Doors with springs on them will close after you.

    119. When you jump down on the end of a springboard, it throws
    you into the air.

    120. You move your hands backward to swim forward.

    NOTE. There are really two kinds of elasticity, which have
    nothing to do with each other. Elasticity of _form_ is the
    tendency of a thing to go back to its original shape, as
    rubber does. If you make a dent in rubber, it springs right
    back to the shape it had before. Elasticity of _volume_ is the
    tendency of a substance to go back to its original _size_, as
    lead does. If you manage to squeeze lead into a smaller space,
    it will spring right back to the same size as soon as you stop
    pressing it on all sides. But a dent in lead will stay there;
    it has little elasticity of form.

    Air and water--all liquids, in fact--have a great deal of
    elasticity of _volume_, but practically no elasticity of form.
    They do not tend to keep their shape, but they do tend to fill
    the same amount of space. Putty and clay likewise have very
    little elasticity of form; when you change their shape, they
    stay changed.

    Jelly and steel and glass have a great deal of elasticity of
    _form_. When you dent them or twist them or in any way change
    their shape, they go right back to their first shape as soon
    as they can.

    When we imagined a man with an "elastic touch," we were
    imagining a man who gave everything he touched perfect
    elasticity of _form_. It is elasticity of _form_ that most
    people mean when they talk about elasticity.




CHAPTER FOUR

HEAT


SECTION 15. _Heat makes things expand._

    How does a thermometer work? What makes the mercury rise in
    it?

    Why does heat make things get larger?

When we look at objects through a microscope, they appear much larger
and in many cases we are able to see the smaller parts of which they
are made. If we had a microscope so powerful that it made a tiny
speck of dust look as big as a mountain (of course no such microscope
exists), and if we looked through this imaginary microscope at a piece
of iron, we should find to our surprise that the particles were not
standing still. The iron would probably look as if it were fairly
alive with millions of tiny specks moving back and forth, back and
forth, faster than the flutter of an insect's wings.

These tiny moving things are _molecules_. Everything in the world is
made of them. It seems strange that we should know this, since there
really are no microscopes nearly powerful enough to show the molecules
to us. Yet scientists know a great deal about them. They have devised
all sorts of elaborate experiments--very accurate ones--and have
tested the theories about molecules in many ways. They have said, for
instance, "Now, if this thing _is_ made of molecules, then it will
grow larger when we make the molecules move faster by heating it."
Then they heated it--in your next experiment you will see what
happened. This is only one of thousands of experiments they have
performed, measuring over and over again, with the greatest care,
exactly _how much_ an object expanded when it was heated a certain
amount; exactly how much heat was needed to change water to steam;
exactly how far a piece of steel of a certain size and shape could
bend without breaking; exactly how crystals form--and so on and so on.
And they have always found that everything acts as if it were made
of moving molecules. Their experiments have been so careful
and scientists have found out so much about what _seem_ to be
molecules,--how large they are, what they probably weigh, how fast
they move, and even what they are made of,--that almost no one has any
doubt left that fast-moving molecules make up everything in the world.

[Illustration: FIG. 41. A thermometer.]

To go back, then: if we looked at a piece of iron under a microscope
that would show us the molecules,--and remember, no such powerful
microscope could exist,--we should see these quivering particles, and
nothing more. Then if some one heated the iron while we watched the
molecules, or if the sun shone on it, we should see the molecules move
faster and faster and separate farther and farther. That is why heat
expands things. When the molecules in an object move farther apart,
naturally the object expands.

_Heat is the motion of the molecules._ When the molecules move faster
(that is, when the iron grows hotter), they separate farther and the
iron swells.

[Illustration: FIG. 42. A thermometer made of a flask of water. It
does not show the exact degree of heat of the water, but it does show
whether the water is hot or cold.]

HOW WE CAN TELL THE TEMPERATURE BY READING A THERMOMETER. The mercury
(quicksilver) in the bulb of the thermometer like everything else
expands (swells) when it becomes warm. It is shut in tightly on all
sides by the glass, except for the little opening into the tube above.
When it expands it must have more room, and the only space into which
it can move is up in the tube. So it rises in the tube.

[Illustration: FIG. 43. Will the hot ball go through the ring?]

Water will do the same thing. You can make a sort of thermometer,
using water instead of mercury, and watch the water expand when you
heat it. Here are the directions for doing this:

[Illustration: FIG. 44. When the wire is cold, it is fairly tight.]

    EXPERIMENT 28. Fill a flask to the top with water. Put a piece
    of glass tubing through a stopper, letting the tube stick 8 or
    10 inches above the top of the stopper. Put the stopper into
    the flask, keeping out all air; the water may rise 2 or 3
    inches in the glass tube. Dry the flask on the outside and put
    it on a screen on the stove or ring stand, and heat it. Watch
    the water in the tube. What effect does heat have on the
    water?

Here are two interesting experiments that show how solid things expand
when they are heated:

    EXPERIMENT 29. The brass ball and brass ring shown in Figure
    43 are called the expansion ball and ring. Try pushing the
    ball through the ring. Now heat the ball over the flame for a
    minute or two--it should not be red hot--and try again to pass
    it through the ring.

    Heat both ball and ring for a short time. Does heating expand
    the ring?

    EXPERIMENT 30. Go to the electric apparatus (described on page
    379) and turn on the switch that lets the electricity flow
    through the long resistance wire. Watch the wire as it becomes
    hot.

    _APPLICATION 24._ A woman brought me a glass-stoppered bottle
    of smelling salts and asked me if I could open it. The stopper
    was in so tightly that I could not pull it out. I might have
    done any of the following things: Tried to pull the stopper
    out with a pair of pliers; plunged the bottle up to the neck
    in hot water; plunged it in ice-cold water; tried to loosen
    the stopper by tapping it all around. Which would have been
    the best way or ways?

[Illustration: FIG. 45. But notice how it sags when it is hot.]

    _APPLICATION 25._ I used to buy a quart of milk each evening
    from a farmer just after he had milked. He cooled most of the
    milk as soon as it was strained, to make it keep better. He
    asked me if I wanted my quart before or after it was cooled.
    Either way he would fill his quart measure brim full. Which
    way would I have received more milk for my money?


INFERENCE EXERCISE

    Explain the following:

    121. Billiard balls will rebound from each other and from the
    edges of the table again and again and finally stop.

    122. In washing a tumbler in hot water it is necessary to lay
    it in sidewise and wet it all over, inside and out, to keep
    it from cracking; if it is thick in some parts and thin in
    others, like a cut-glass tumbler, it is not safe to wash it in
    hot water at all.

    123. The swinging of the moon around the earth keeps the moon
    from falling to the earth.

    124. A fire in a grate creates a draft up the chimney.

    125. Telegraph wires and wire fences put up in the summer must
    not be strung too tightly.

    126. Candy usually draws in somewhat from the edge of the pan
    as it hardens.

    127. A meat chopper can be screwed to a table more tightly
    than you can possibly push it on.

    128. A floor covered with linoleum is more easily kept clean
    than a plain wood floor.

    129. Rough seams on the inside of clothes chafe your skin.

    130. You can take the top off a bottle of soda pop with an
    opener that will pry it up, but you cannot pull it off with
    your fingers.


SECTION 16. _Cooling from expansion._

    We get our heat from the sun; then why is it so cold up on the
    mountain tops?

    What is coldness?

Here is an interesting and rather strange thing about heat and
expansion. Although heat expands things, yet expansion does not heat
them. On the contrary, if a thing expands without being heated from an
outside source, it actually gets cold! You see, in order to expand, it
has to push the air or something else aside, and it actually uses up
the energy of its own heat to do this. You will understand this better
after you do the next experiment.

    EXPERIMENT 31. Wet the inside of a test tube. Hold the mouth
    of the test tube against the opening of a carbon dioxid
    tank. Open the valve of the tank with the wrench and let the
    compressed gas rush out into the test tube until the mouth
    of the test tube is white. Shut off the valve. Feel your test
    tube.

What has happened is this: The gas was tightly compressed in the tank.
It was not cold; that is, it had some heat in it, as everything has.
When you let it loose, it used up much of its heat in pushing the air
in the test tube and all around it out of the way. In this way it lost
its heat, and then it became cold. _Cold means absence of heat_, as
dark means absence of light. So when the compressed gas used up its
heat in pushing the air out of its way, it became so cold that it
froze the water in your test tube.

[Illustration: FIG. 46. The expansion of the compressed gas freezes
the moisture on the tube.]

ONE REASON WHY IT IS ALWAYS COLD HIGH UP IN THE AIR. Even on hot
summer days aviators who fly high suffer from the cold. You might
think that they would get warmer as they went up nearer the sun; one
reason that they get colder instead is this:

As you saw in the last experiment, a gas that expands gets very cold.
Air is a kind of gas. And whenever air rises to where there is not so
much air crowding down on it from above, it expands. So the air that
rises high and expands gets very cold. Consequently mountains which
reach up into this high, cold air are snow covered all the year round;
and aviators who fly high suffer keenly from the cold. There are
several reasons for this coldness of the high air. This is just _one_
of them.

    _APPLICATION 26._ Explain why air usually cools when it rises;
    why high mountain tops are always covered with snow.


INFERENCE EXERCISE

    Explain the following:

    131. You should not fill a teakettle brim full of cold water
    when you are going to put it on the stove.

    132. It is harder to erase an ink mark than a pencil mark.

    133. Bearings of good watches, where there is constant rubbing
    on the parts, are made of very hard jewels.

    134. You feel lighter for an instant when you are in an
    elevator which starts down suddenly.

    135. When men lay cement sidewalks, they almost always make
    cracks across them every few feet.

    136. To cool hot coffee one sometimes blows on it.

    137. It is much easier to turn the latch of a door with the
    knob than with the spindle when the knob is off.

    138. Patent-leather shoes do not soil as easily as plain
    leather shoes.

    139. We use rubber bands to hold things together tightly.

    140. As air goes up it usually cools.


SECTION 17. _Freezing and melting._

    When water freezes in a pipe, why does the pipe burst?

    What is liquid air?

    Why does not the wire in an electric lamp melt when it is red
    hot?

Suppose we looked at a piece of ice through the imaginary microscope
that shows us the molecules. The ice molecules would be different from
the iron molecules in size, but they would be vibrating back and forth
in exactly the same way, only with less motion. It is because they
have less motion that we say the ice is colder than the iron. Then let
us suppose that the sun was shining on the ice while we watched the
ice molecules.

First we should see movements of the ice molecules become gradually
more rapid, just as the iron molecules did when the iron was warmed.
Then, as they moved faster and faster, they would begin to bump into
each other and go around every which way, each molecule bumping first
into one neighbor, then into another, and bouncing back in a new
direction after each collision. This is what causes the ice to melt.
When its molecules no longer go back and forth in the same path all
the time, the ice no longer keeps its shape, and we call it water--a
liquid.

Almost all solid substances will melt when they are heated. Or, to
put it the other way around, every liquid will freeze solid if it
gets cold enough. Even liquid air (which is ordinary air cooled and
compressed until it runs like water) can be frozen into a solid chunk.
Some things will melt while they are still very cold; solid air, for
instance, melts at a temperature that would freeze you into an icicle
before you could count ten. Other things, such as stones, are melted
only by terrific heat.

When the little particles of water that make up the clouds become
very cold, they freeze as they gather and so make snowflakes. When the
little particles of water in the air, that usually make dew, freeze
while they are gathering on a blade of grass, we call it frost.
When raindrops are carried up into colder, higher air while they are
forming, they freeze and turn to hail. When snow or frost or hail or
ice is heated, it melts and turns back to water.

[Illustration: FIG. 47. Why did the bottle break when the water in it
turned to ice?]

But here is a strange fact: although heat usually expands things,
water expands when it _freezes_. Like everything else, however, water
also expands when it becomes hot, as you found when you made a kind
of thermometer, using a flask of water and a glass tube. But if you
should put that flask into a freezing mixture of ice and salt, you
would find that when the water became very cold it would begin to
expand a little immediately before it froze.

And it is very lucky for us that water does expand when it freezes,
because if it did not, ice would be heavier than water is. But since
the water expands as it freezes, ice weighs less than water and
floats. And that is why lakes and oceans and rivers freeze over the
top and do not freeze at the bottom. If they froze from the bottom up,
as they would if the ice sank as it formed, every river and lake would
be solid ice in the winter. All the harbors outside the tropics would
probably be ice-bound all winter long. And the ice in the bottom of
the lakes and rivers and in the ocean would probably never melt.

So in the case of freezing water, and in the case of a couple of
metals, there is a point where coldness, not heat, makes things
expand.

    EXPERIMENT 32. Take a ketchup bottle with a screw cap and a
    cork that fits tightly. Fill it to the top with water; put
    a long pin beside the cork while you insert it, so that the
    water can be crowded out as the cork goes down; then when you
    have pushed the cork in tightly, pull out the pin. Screw the
    cap on the bottle so as to hold the cork fast. Put the bottle
    in a pail or box, and pack ice and salt around it. Within an
    hour you should be able to see what the freezing water does to
    the bottle.

    _APPLICATION 27._ Explain why ice is lighter than water; why
    we have no snow in summer.


INFERENCE EXERCISE

    Explain the following:

    141. Sealing wax is held over a candle flame before it is
    applied to a letter.

    142. Automobile tires tighten upon a sudden change from cold
    weather to hot.

    143. When paper has been rolled, it tends to curl up again
    after being unrolled.

    144. Seats running across a car are much more comfortable
    when a car starts and stops, than are seats running along the
    sides.

    145. You cannot siphon water from a low place to a higher one.

    146. Candles get soft in hot weather.

    147. Meteorites fall to the earth from the sky.

    148. When you preserve fruit and pour the hot fruit into
    the jars, you fill the jars brim full and screw on the cap
    air-tight; yet a few hours later the fruit does not fill the
    jars; there is some empty space between the top of the fruit
    and the cover.

    149. Water pipes burst in the winter when it is very cold.

    150. When people want to make iron castings, they first melt
    the iron, then pour it into molds. They leave it in the molds
    until cold. After that the iron holds the shape of the molds.
    Explain why the iron changes from a liquid to a solid.


SECTION 18. _Evaporation._

    Why is it that when ink is spilled it dries up, but when it is
    in the bottle it does not dry up?

    What put the salt into the ocean?

    Why do you feel cold when you get out of the bathtub?

Wet clothes get dry when they are hung on the clothes-line. The water
in them _evaporates_. It turns to invisible vapor and disappears into
the air. Water and all liquids evaporate when they are long exposed to
the air. If they didn't--well, let us imagine what the world would be
like if all evaporation should suddenly stop:

You find that your face is perspiring and your hands as well. You wipe
them on your handkerchief, but soon they are moist again, no matter
how cool the weather. After wiping them a few more times your
handkerchief becomes soaking wet, and you hang it up to dry. There may
be a good breeze stirring, yet your handkerchief does not get dry.
By this time the perspiration is running off your face and hands, and
your underclothes are getting drenched with perspiration.

[Illustration: FIG. 48. An evaporating dish.]

You hurry into the house, change your clothes, bathe and wipe yourself
dry with a towel. When you find that your wet things are not drying,
and that your dry ones are rapidly becoming moist, you hastily build
a fire and hang your clothes beside it. No use, your clothes remain as
wet as ever. If you get them very hot the moisture in them will boil
and turn to steam, of course, but the steam will all turn back to
water as soon as it cools a little and the drops will cling to your
clothes and to everything around the room. You will have to get used
to living in wet clothes. You won't catch cold, though, since there is
no evaporation to use up your heat.

But the water problem outside is not one of mere inconvenience.
It never rains. How can it when the water from the oceans cannot
evaporate to form clouds? Little by little the rivers begin to run
dry--there is no rain to feed them. No fog blows in from the sea; no
clouds cool the sun's glare; no dew moistens the grass at night; no
frost shows the coming of cold weather; no snow comes to cover the
mountains. In time there is no water left in the rivers; every lake
with an outlet runs dry. There are no springs, and, after a while,
no wells. People have to live on juicy plants. The crops fortunately
require very little moisture, since none evaporates from them or from
the ground in which they grow. And the people do not need nearly as
much water to drink.

Little by little, however, the water all soaks too deep into the
ground for the plants to get it. Gradually the continents become great
deserts, and all life perishes from the land.

All these things would really happen, and many more changes besides,
if water did not evaporate. Yet the evaporation of water is a very
simple occurrence. As the molecules of any liquid bounce around, some
get hit harder than others. These are shot off from the rest up into
the air, and get too far away to be drawn back by the pull of the
molecules behind. This shooting away of some of the molecules is
evaporation. And since it takes heat to send these molecules
flying off, the liquid that is left behind is colder because of the
evaporation. That is why you are always cold after you leave the
bathtub until you are dry. The water that evaporates from your body
uses up a good deal of your heat.

[Illustration: FIG. 49. Diagram illustrating how in the evaporation of
water some of the molecules shoot off into the air.]

Gasoline evaporates more quickly than water. That is why your hands
become so cold when you get them wet with gasoline.

Since heat is required to evaporate a liquid, the quickest way to dry
anything is to warm it. That is why you hang clothes in the sun or by
the stove to dry.

Try these experiments:

    EXPERIMENT 33. Read a thermometer that has been exposed to
    the room air. Now dip it in water that is warmer than the
    air, taking it out again at once. Watch the mercury. Does the
    thermometer register a higher or a lower temperature than
    it did at the beginning? What is taking up the heat from the
    mercury?

    EXPERIMENT 34. Put a few drops of water in each of two
    evaporating dishes. Leave one cold; warm the other over the
    burner, but do not heat it to boiling. Which evaporates more
    quickly?

WHY THE SEA IS SALT. You remember various fairy stories about why the
sea is salt. For a long time the saltness of the sea puzzled people.
But the explanation is simple. As the water from the rains seeps
through the soil and rocks, it dissolves the salt in them and
continually carries some of it into the rivers. So the waters of the
rivers always carry a very little salt with them out to sea. The water
in the ocean evaporates and leaves the salt behind. For millions of
years this has been going on. So the rivers and lakes, which have only
a little salt in them, keep adding their small amounts to the sea, and
once in the sea the salt never can get out. The oceans never get any
fuller of water, because water only flows into the ocean as fast as it
evaporates from the ocean. Yet more salt goes into the ocean all the
time, washed down by thousands of streams and rivers. So little by
little the ocean has been growing more and more salty since the world
began.

[Illustration: FIG. 50. A view of the Dead Sea.]

Great Salt Lake and the Dead Sea, unlike most lakes, have no rivers
flowing out of them to carry the salt and water away, but rivers flow
into them and bring along small amounts of salt all the time. Then the
water evaporates from Great Salt Lake and the Dead Sea, leaving the
salt behind; and that is why they are so very salty.

When people want to get the salt out of sea water, they put the sea
water in shallow open tanks and let the water evaporate. The salt is
left behind.

    EXPERIMENT 35. Dissolve some salt in warm water until no more
    will dissolve. Pour the clear liquid off into an evaporating
    dish, being careful not to let any solid particles of the
    salt go over. Either set the dish aside uncovered, for several
    days, or heat it almost to boiling and let it evaporate to
    dryness. What is left in the dish?

    _APPLICATION 28._ Some girls were heating water for tea, and
    were in a hurry. They had only an open stew pan to heat the
    water in.

    "Cover the pan with something; you'll let all the heat out!"
    Helen said.

    "No, you want as much heat to go through the water as
    possible. Leave the lid off so that the heat can flow through
    easily," said Rose.

    "The water will evaporate too fast if the lid is off, and all
    the heat will be used up in making it evaporate; it will take
    it much longer to get hot without the lid," Louise argued.

    "That's not right," Rose answered. "Boiling water evaporates
    fastest of all. We want this to boil, so let it evaporate;
    leave the lid off."

    What should they have done?

    _APPLICATION 29._ Two men were about to cross a desert. They
    had their supply of water in canvas water bags that leaked
    just enough to keep the outside of the bags wet. Naturally
    they wanted to keep the water as cold as possible.

    "I'm going to wrap my rubber poncho around my water bag and
    keep the hot desert air away from the water," said one.

    "I'm not. I'm going to leave mine open to the air," the other
    said.

    Which man was right? Why?


INFERENCE EXERCISE

    Explain the following:

    151. When you go up high in an elevator, you feel the pressure
    of the air in your ears.

    152. Water is always flowing into Great Salt Lake; it has no
    outlet; yet it is getting more nearly empty all the time.

    153. A nail sinks while a cork floats in water.

    154. Steep hillsides are paved with cobblestones instead of
    asphalt.

    155. If you place one wet glass tumbler inside another you can
    pull them apart only with difficulty, and frequently you break
    the outer one in the attempt.

    156. Sausages often break their skins when they are being
    cooked.

    157. A drop of water splashed against a hot lamp chimney
    cracks it.

    158. When you shoot an air gun, the air is compressed at
    first; then when it is released it springs out to its original
    volume and throws the bullet ahead of it.

    159. Leather soles get wet through in rainy weather, while
    rubbers remain perfectly dry on the inside.

    160. When you want to clean a wooden floor, you scrub it with
    a brush.


SECTION 19. _Boiling and condensing._

    What makes a geyser spout?

    How does a steam engine go?

Once more let us imagine we are looking at molecules of water through
our magical microscope. But this time suppose that the water has been
made very hot. If we could watch the molecules smash into each other
and bound about more and more madly, suddenly we should see large
numbers of them go shooting off from the rest like rifle bullets,
and they would fly out through the seemingly great spaces between the
slower molecules of air. This would mean that the water was boiling
and turning to steam.

Here are a couple of experiments that will show you how much more room
water takes when it turns to steam than while it remains just water:

    EXPERIMENT 36. Pour a half inch of water into the bottom of a
    test tube. Put a cork in the test tube so tightly that it will
    not let any steam pass it, but not too tightly. Hold the test
    tube with a test-tube clamp at arm's length over a flame,
    pointing the cork away from you. Wait for results.

The reason the cork flew out of the test tube is this: Steam takes
a great deal more room than water does,--many times as much room; so
when the water in the test tube turned to steam, the steam had to get
out and pushed the cork out ahead of it.

[Illustration: FIG. 51. In a minute the cork will fly out.]

    EXPERIMENT 37. Pour about half an inch of water into the
    bottom of a flask. Bring it to a vigorous boil over the burner
    and let it boil half a minute. Now take the flask off the
    flame and quickly slip the mouth of a toy balloon over the
    mouth of the flask. Watch what happens. If things go too
    slowly, you can speed them up by stroking the outside of the
    flask with a cold, wet cloth.

    When the balloon has been drawn into the flask as far as it
    will go, you can put the flask back on the burner and heat the
    water till it boils. When the balloon has been forced out of
    the flask again and begins to grow large, take the flask off
    the burner. Do this before the balloon explodes.

The reason the balloon was drawn into the flask was that the steam in
the flask turned back to water as it cooled, and took very much less
space. This left a vacuum or empty space in the flask. What pushed the
balloon into the empty space?

[Illustration: FIG. 52. A toy balloon has been slipped over the mouth
of a flask that is filled with steam.]

[Illustration: FIG. 53. As the steam condenses and leaves a vacuum,
the air pressure forces the balloon into the flask.]

HOW STEAM MAKES AN ENGINE GO. The force of steam is entirely due to
the fact that steam takes so much more room than the water from which
it is made. A locomotive pulls trains across continents by using
this force, and by the same force a ship carries thousands of tons of
freight across the ocean. The engines of the locomotive and the ship
are worked by the push of steam. A fire is built under a boiler. The
water is boiled; the steam is shut in; the only way the steam can get
out is by pushing the piston ahead of it; the piston is attached to
machinery that makes the locomotive or ship move.

ONE THEORY ABOUT THE CAUSE OF VOLCANOES. The water that sinks deep
down into some of the hot parts of the earth turns to steam, takes
up more room, and forces the water above it out as a geyser. It is
thought by some scientists that volcanoes may be started by the water
in the ocean seeping down through cracks to hot interior parts of
the world where even the stone is melted; then the water, turning to
steam, pushes its way up to the surface, forcing dust and stone ahead
of it, and making a passage up for the melted stone, or lava. The
persons who hold this view call attention to the fact that volcanoes
are always in or near the sea. If this is the true explanation of
volcanoes, then we should have no volcanoes if steam did not take more
room than does the water from which it comes.

Here is a very practical fact about boiling water that many people do
not know; and their gas bills would be much smaller if they knew it.
Try this experiment:

[Illustration: FIG. 54. Will boiling water get hotter if you make it
boil harder?]

    EXPERIMENT 38. Heat some water to boiling. Put the
    boiling-point thermometer into the water (the thermometer
    graduated to 110° Centigrade and 220° Fahrenheit), and note
    the temperature of the boiling water. Turn up the gas and make
    the water boil as violently as possible. Read the thermometer.
    Does the water become appreciably hotter over the very hot
    fire than it does over the low fire, if it is boiling in both
    cases? But in which case is more steam given off? Will a very
    hot fire make the water boil away more rapidly than a low
    fire?

When you are cooking potatoes, are you trying to keep them very hot or
are you trying to boil the water away from them? Which are you trying
to do in making candy, to keep the sugar very hot or to boil the water
away from it?

All the extra heat you put into boiling water goes toward changing the
water into steam; it cannot raise the water's temperature, because
at the moment when water gets above the boiling point it ceases to be
water and becomes steam. This steam takes up much more room than
the water did, so it passes off into the air. You can tell when a
teakettle boils by watching the spout to see when the steam[3] pours
forth from it in a strong, steady stream. If the steam took no more
room than the water, it could stay in the kettle as easily as the
water.

[Footnote 3: What you see is really not the steam, but the vapor
formed as the steam condenses in the cool room. The steam itself is
invisible, as you can tell by looking at the mouth of the spout of a
kettle of boiling water. You will see a clear space before the white
vapor begins. The clear space is steam.]

DISTILLING. When liquids are mixed together and dissolved in each
other, it looks as if it would be impossible to take them apart. But
it isn't. They can usually be separated almost perfectly by simply
boiling them and collecting their vapor. For different substances boil
at different temperatures just as they melt at different temperatures.
Liquid air will boil on a cake of ice; it takes the intense heat of
the electric furnace to boil melted iron. Alcohol boils at a lower
temperature than water; gasoline boils at a lower temperature than
kerosene. And people make a great deal of practical use of these facts
when they wish to separate substances which have different boiling
temperatures. They call this distilling. You can do some distilling
yourself and separate a mixture of alcohol and water in the following
manner:

    EXPERIMENT 39. First, pour a little alcohol into a cup--a
    few drops is enough--and touch a lighted match to it. Will it
    burn? Now mix two teaspoonfuls of alcohol with about half a
    cup of water and enough blueing to color the mixture. Pour a
    few drops of this mixture into the cup and try to light it.
    Will it burn?

    Now pour this mixture into a flask. Pass the end of the long
    bent glass rod (the "worm") through a one-hole rubber stopper
    that will fit the flask (Fig. 55). Put the flask on a ring
    stand and, holding it steady, fasten the neck of the flask
    with a clamp that is attached to the stand. Put the stopper
    with the worm attached into the flask, and support the worm
    with another clamp. Put a dry cup or beaker under the lower
    end of the worm. Set a lighted burner under the flask. When
    the mixture in the flask begins to boil, turn the flame
    down so that the liquid will just barely boil; if it boils
    violently, part of the liquid splashes up into the lower end
    of the worm.

    As the vapor rises from the mixture and goes into the worm,
    it cools and condenses. When several drops have gone down into
    the cup, try lighting them. What is it that has boiled and
    then condensed: the water, the alcohol, or the blueing? Or is
    it a mixture of them?

[Illustration: FIG. 55. By distillation clear alcohol can be separated
from the water and red ink with which it was mixed.]

Alcohol is really made in this way, only it is already mixed in the
water in which the grains fermented and from which people then distil
it. Gasoline and kerosene are distilled from petroleum; there is a
whole series of substances that come from the crude oil, one after
the other, according to their boiling points, and what is left is the
foundation for a number of products, including paraffine and vaseline.

    EXPERIMENT 40. Put some dry, fused calcium chlorid on a saucer
    and set it on the plate of the air pump. This is to absorb the
    moisture when you do the experiment. (This calcium chlorid
    is _not_ the same as the chlorid of lime which you buy for
    bleaching or disinfecting.) Fill a flask or beaker half full
    of water and bring it to a boil over a Bunsen burner. Quickly
    set the flask on the plate of the air pump. The water will
    stop boiling, of course. Cover the flask and the saucer of
    calcium chlorid with the bell jar immediately, and pump the
    air out of the jar. Watch the water.

The water begins to boil again because water will boil at a lower
temperature when there is less air pressure on its surface. So
although the water is too cool to boil in the open air, it is still
hot enough to boil when the air pressure is partially removed. It is
because of this that milk is evaporated in a vacuum for canning; it is
not necessary to make it so hot that it will be greatly changed by
the heat, if the boiling is done in a vacuum. On a high mountain the
slight air pressure lets the water boil at so low a temperature that
it never becomes hot enough to cook food.

    _APPLICATION 30._ Two college students were short of money and
    had to economize greatly. They got an alcohol lamp to use in
    cooking their own breakfasts. They planned to boil their eggs.

    "Let's boil the water gently, using a low flame," one said;
    "we'll save alcohol."

    "It would be better to boil the eggs fast and get them done
    quickly, so that we could put the stove out altogether," the
    other replied.

    Which was right?

    _APPLICATION 31._ Two girls were making candy. They put a
    little too much water into it.

    "Let us boil the candy hard so that it will candy more
    quickly," said one.

    "Why, you wasteful girl," said the other. "It cannot get any
    hotter than the boiling point anyhow, so you can't cook it any
    faster. Why waste gas?"

    Which girl was right?


INFERENCE EXERCISE

    Explain the following:

    161. Warm air rises.

    162. The lid of a teakettle rattles.

    163. Heating water makes a steam engine go.

    164. When an automobile with good springs and without shock
    absorbers goes over a rut, the passengers do not get a jolt,
    but immediately afterward bounce up into the air.

    165. Comets swing around close to the sun, then off again into
    space; how do they get away from the sun?

    166. When you wish to pour canned milk out, you need two holes
    in the can to make it flow evenly.

    167. Liquid air changes to ordinary air when it becomes even
    as warm as a cake of ice.

    168. Skid chains tend to keep automobiles from skidding on wet
    pavement.

    169. A warm iron and a blotter will take candle grease out of
    your clothes.

    170. Candies like fudge and nougat become hard and dry when
    left standing several days open to the air.


SECTION 20. _Conduction of heat and convection._

    Why does a feather comforter keep you so warm?

    When you heat one end of a nail, how does the heat get through
    to the other end?

    How does a stove make the whole room warm?

Here is a way to make heat run a race. See whether the heat that goes
through an iron rod will beat the heat that goes through a glass rod,
or the other way round:

[Illustration: FIG. 56. The metal balls are fastened to the iron and
glass rods with drops of wax.]

    EXPERIMENT 41. Take a solid glass rod and a solid iron rod,
    each about a quarter inch in diameter and about 6 inches long.
    With sealing wax or candle grease stick three ball bearings or
    pieces of lead, all the same size, to each rod, about an inch
    apart, beginning 2 inches from the end. Hold the rods side by
    side with their ends in a flame, and watch the balls fall off
    as the heat comes along through the rods. The heat that first
    melts off the balls beats.

[Illustration: FIG. 57. Does the heat travel faster through the iron
or through the glass?]

What really happens down among the molecules when the heat travels
along the rods is that the molecules near the flame are made to move
more quickly; they joggle their neighbors and make them move faster;
these joggle the ones next to them, and so on down the line. Heat
that travels through things in this way is called _conducted_ heat.
Anything like iron, that lets the heat travel through it quickly, is
called a _good conductor_ of heat. Anything like glass, that allows
the heat to travel through it only with difficulty, is called a _poor
conductor_ of heat, or an _insulator_ of heat.

A silver spoon used for stirring anything that is cooking gets so
hot all the way up the handle that you can hardly hold it, while the
handle of a wooden spoon never gets hot. Pancake turners usually have
wooden handles. Metals are good conductors of heat; wood is a poor
conductor.

An even more obvious example of the conducting of heat is seen in a
stove lid; your fire is under it, yet the top gets so hot that you can
cook on it.

When anything feels hot to the touch, it is because heat is being
conducted to and through your skin to the sensitive little nerve ends
just inside. But when anything feels cold, it is because heat is being
conducted away from your skin into the cold object.

AIR CARRIES HEAT BY CONVECTION. One of the poorest conductors of heat
is air; that is, one particle of air can hardly give any of its heat
to the next particle. But particles of air move around very easily and
carry their heat with them; and they can give the heat they carry with
them to any solid thing they bump into. So when air can move around,
the part that is next to the stove, for instance, becomes hot; this
hot air is pushed up and away by cold air, and carries its heat with
it. When it comes over to you in another part of the room, some of its
heat is conducted to your body. When air currents--or water currents,
which work the same way--carry heat from one place to another like
this, we say that the heat has traveled by _convection_.

[Illustration: FIG. 58. Convection currents carrying the heat of the
stove about the room.]

Since heat is so often carried to us by convection,--by warm winds,
warm air from the stove, warm ocean currents, etc.,--it _seems_ as if
air must be a good conductor of heat. But if you shut the air up into
many tiny compartments, as a bird's feathers do, or as the hair on an
animal's back does, so that it cannot circulate, the passage of heat
is almost completely stopped. When you use a towel or napkin to lift
something hot, it is not so much the fibers of cotton which keep the
heat from your hand; it is principally the very small pockets of air
between the threads and even between the fibers of the threads.

COLD THE ABSENCE OF HEAT. Cold is merely the absence of heat; so if
you keep the heat from escaping from anything warm, it cannot become
cold; while if you keep the heat from reaching a cold thing it cannot
become warm. A blanket is just as good for keeping ice from melting,
by shutting the heat out, as it is for keeping you warm, by holding
heat in.

    _APPLICATION 32._ Explain why ice is packed in straw or
    sawdust; why a sweater keeps you warm.

    Select from the following list the good conductors of heat
    from the poor conductors (insulators): glass, silver, iron,
    wood, straw, excelsior, copper, asbestos, steel, nickel,
    cloth, leather.

[Illustration: FIG. 59. Diagram of a hot-water heater. What makes the
water circulate?]


INFERENCE EXERCISE

    Explain the following:

    171. If the axle of a wheel is not greased, it swells until it
    sticks fast in the hub; this is a hot box.

    172. When you have put liquid shoe polish on your shoes, your
    feet become cold as it dries.

    173. The part of an ice-cream freezer which holds the cream
    is usually made of metal, while that which goes outside and
    contains the ice and salt is usually made of wood.

    174. The steam in a steam radiator rises from a boiler in the
    basement to the upper floors.

    175. When you throw a ball, it keeps going for a while after
    it leaves your hand.

    176. Clothes keep you warm, especially woolen clothes.

    177. The Leaning Tower of Pisa does not fall over.

    178. It is almost impossible to climb a greased pole.

    179. Heat goes up a poker that is held in a fire.

    180. A child can make a bicycle go rapidly without making his
    feet go any faster than if he were walking.




CHAPTER FIVE

RADIANT HEAT AND LIGHT


SECTION 21. _How heat gets here from the sun; why things glow when
they become very hot._

If we were to go back to our imaginary switchboard we should find a
switch, between the heat and the light switches, labeled RADIATION.
Suppose we turn it off:

Instantly the whole world becomes pitch dark; so does the sky. We
cannot see the sun or a star; no electric lights shine; and although
we can "light" a match, it gives no light. The air above the burning
match is hot, and we can burn our fingers in the invisible flame, but
we can see nothing whatever.

Yet the world does not get cold. If we leave the switch off for years,
while the earth remains in darkness and we all live like blind people,
it never gets cold. Winter and summer are alike, day and night are
just the same. Gradually, after many ages, the ice and snow in the
north and in the far south begin to melt as the warmth from the rest
of the world is conducted to the polar regions. And the heat from
the interior of the earth makes all the parts of the earth's surface
warmer. Winds almost stop blowing. Ocean currents stop flowing. The
land receives less rainfall, until finally everything turns to a
desert; almost the only rain is on the ocean. Animals die even before
the rivers dry up, for the flesh eaters are not able to see their
prey, and since, without light, all green things die, the animals that
live on plants soon starve. Men have to learn to live on mushrooms,
which grow in the dark. The world is plunged into an eternal warm,
pitch-black night.

[Illustration: FIG. 60. It is by radiation that we get all our heat
and light from the sun.]

Turning off the radiation would cause all these things to happen,
because it is by radiation that we get all our heat from the sun and
all our light from any source. And it is by radiation that the earth
loses heat into space in the night and loses still more heat into
space during the winter.

We do not get our heat from the sun by conduction; we cannot, because
there is nothing between us and the sun to conduct it. The earth's
air, in amounts thick enough to count, goes up only a hundred miles
or so. It is really just a thin sort of blanket surrounding the earth.
The sun is 93,000,000 miles away. Between us and the sun there is
empty space. There are no molecules to speak of in that whole vast
distance. So if heat traveled only by conduction,--that is, if
radiation stopped,--we should be so completely shut off from the sun
that we should not know there was such a thing.

But even if we filled the space between us and the sun with copper or
silver, which are about the best conductors of heat in the world, it
would take the heat from the sun years and years to be conducted down
to us. Yet we know that the sun's heat really gets to us in a few
minutes. This is because heat can travel in a very much quicker way
than by conduction. It _radiates_ through space, just as light does.
And it can come the whole 93,000,000 miles from the sun in about
8 minutes. This is so fast that if it were going around the world
instead of coming from the sun, it would go around 7-1/2 times before
you could say "Jack Robinson,"--really, because it takes you at least
one second to say "Jack Robinson."

We are not absolutely sure how heat gets here so fast. But what most
scientists think nowadays is that there is a sort of invisible rigid
stuff, not made of molecules or of anything but just itself, called
_ether_. (This ether, if there really is such a thing, is not related
at all to the ether that doctors use in putting people to sleep. It
just happens to have the same name.) The ether is supposed to fill
all space, even the tiny spaces between molecules. The fast moving
particles of the sun joggle the ether up there, and make ripples that
spread out swiftly all through space. When those ripples strike our
earth, they make the molecules of earth joggle, and that is heat. The
ripples that spread out from the sun are called _ether waves_.

But the important and practical fact to know is that there is a kind
of heat, called _radiant heat_, that can pass through empty space with
lightning-like quickness. And when this radiant heat strikes _things_,
it is partly absorbed and changed to the usual kind of heat.

This radiant heat is closely related to light. As a matter of fact,
light is only the special kind of ether waves that affect our eyes.
Radiant heat is invisible. The ether waves that are visible we call
light. In terms of ether waves, the only difference between light and
radiant heat is that the ripples in light are shorter. So it is no
wonder that when we get a piece of iron hot enough, it begins to give
off light; and we say it is red hot. What happens to the ether is
this: As the molecules of iron go faster and faster (that is, as the
iron gets hotter and hotter), they make the ripples in the ether move
more frequently until they get short enough to be _light_ instead of
radiant heat. Objects give off radiant heat without showing it at all;
the warmth that you feel just below a hot flatiron is mainly radiant
heat.

When anything becomes hot enough to glow, we say it is _incandescent_.
That is why electric lamps are called _incandescent lamps_. The
fine wires--called the _filament_--in the lamp get so hot when the
electricity flows through them that they glow or become incandescent,
throwing off light and radiant heat.

It is the absorbing of the radiant heat by your hand that makes you
feel the heat the instant you turn an electric lamp on. Try this
experiment:

    EXPERIMENT 42. Turn on an incandescent lamp that is cold. Feel
    it with your hand a second, then turn it off at once. Is the
    glass hot? (The lamp you use should be an ordinary 25, 40, or
    60 watt vacuum lamp.)

The radiant heat from the incandescent filament in the lamp passed
right out through the vacuum of the lamp, and much of it went on
through the glass to your hand. You already know what a poor conductor
of heat glass is; yet it lets a great deal of radiant heat pass
through it, just as it does light. As soon as the lamp stops glowing,
the heat stops coming; the glass is not made hot and you no longer
feel any heat. In one way the electric filament shining through a
vacuum is exactly like the sun shining through empty space: the heat
from both comes to us by radiation.

If a lamp glows for a long time, however, the glass really does become
hot. That is partly because there is not a perfect vacuum within it
(there is a little gas inside that carries the heat to the glass by
convection), and it is partly because the glass does not let quite
all of the radiant heat and light go through it, but absorbs some and
changes it to the regular conducted heat.

One practical use that is made of a knowledge of the difference
between radiant and conducted heat is in the manufacture of thermos
bottles.

    EXPERIMENT 43. Take a thermos bottle apart. Examine it
    carefully. If it is the standard thermos bottle, with the name
    "thermos" on it, you will find that it is made of two layers
    of glass with a vacuum between them. The vacuum keeps any
    _conducted_ heat from getting out of the bottle or into it.
    But, as you know, _radiant_ heat can flash right through a
    vacuum. So to keep it from doing this the glass is silvered,
    making a mirror out of it. Just as a mirror sends light back
    to where it comes from, it sends practically all radiant heat
    back to where it comes from. Heat, therefore, cannot get
    into the thermos bottle or out of it either by radiation or
    conduction. And that is why thermos bottles will keep things
    very hot or ice-cold for such a long time.

    Fill the thermos bottle with boiling water, stopper it, and
    put it aside till the next day. See whether the water is still
    hot.

[Illustration: FIG. 61. How a thermos bottle is made. Notice the double
layer of glass in the broken one.]

If we could make the vacuum perfect, and surround all parts of the
bottle, even the mouth, with the perfect vacuum, and if the mirror
were perfect, things put into a thermos bottle would stay boiling hot
or icy cold forever and ever.

WHY IT IS COOL AT NIGHT AND COLD IN WINTER. It is the radiation of
heat from the earth into space that makes the earth cooler at night
and cold in winter. Much of the heat that the earth absorbs from
the sun in the daytime radiates away at night. And since it keeps on
radiating away until the sun brings us more heat the next day, it is
colder just before dawn than at midnight, more heat having radiated
into space.

For the same reason it is colder in January and February than in
December. It is in December that the days are shortest and the sun
shines on us at the greatest slant, so that we get the least heat from
it; but we still have left some of the heat that was absorbed in the
summer. And we keep losing this heat by radiation faster than we get
heat from the sun, until almost spring.

    _APPLICATION 33._ Distinguish between radiant and conducted
    heat in each of the following examples:

    (a) The sun warms a room through the window. (b) A room is
    cooler with the shades down than up, when the sun shines on
    the window. (c) But even with the shades down a room on the
    sunny side of the house is warmer than a room on the shady
    side. (d) When a mirror is facing the sun, the back gets hot.
    (e) If you put your hand in front of a mirror held in the sun,
    the mirror reflects heat to your hand. (f) If you put a plate
    on a steam radiator, the top of the plate gradually becomes
    hot. (g) If anything very hot or cold touches a gold or
    amalgam filling of a sensitive tooth, you feel it decidedly.
    (h) The handle of your soup spoon becomes hot when the bowl of
    it is in the hot soup. (i) The moon is now very cold, although
    it probably was once very hot.


INFERENCE EXERCISE

    Explain the following:

    181. Trees bend in the wind, then straighten up again. Why do
    they straighten up?

    182. A cloth saturated with kerosene and placed in the bottom
    of a clock will oil the clockworks above it.

    183. In cold weather the doorknob _inside_ the front door is
    cold.

    184. It is cool in the shade.

    185. Clothes get hot when you iron them.

    186. Potatoes fried in deep fat cook more quickly than those
    boiled in water.

    187. If you hold your hand near a vacuum electric lamp globe
    that is glowing, some of the heat will go out to your hand at
    once.

    188. Rubbing silver with fine powder polishes it.

    189. A mosquito can suck your blood.

    190. A hot-water tank becomes hot at the top first, then
    gradually heats downward. When you light the gas under an
    ordinary hot-water heater, the hot water circulates to the
    top of the boiler, while the cold water from the boiler pushes
    into the bottom part of the heater, as shown in Figure 59.
    What causes this circulation?


SECTION 22. _Reflection._

    How is it that you can see yourself in a mirror?

    What makes a ring around the moon?

    Why can we see clouds and not the air?

    Why is a pair of new shoes or anything smooth usually shiny?

If we turn off a switch labeled REFLECTION OF LIGHT on our imaginary
switchboard, we think at first that we have accidentally turned off
RADIATION again, for once more everything instantly becomes dark
around us. We cannot see our hands in front of our faces. Although it
is the middle of the day, the sky is jet black. But this time we see
bright stars shining in it. And among them is the sun, shining as
brightly as ever and dazzling our eyes when we look at it. But its
light does no good. When we look down from the sky toward the earth,
everything is so black that we should think we were blind if we had
not just seen the stars and sun.

Groping our way along to an electric lamp, we turn it on. It shines
brightly, but it does not make anything around it light; everything
stays absolutely invisible. It is as if all things in the world except
the lights had put on some sort of magic invisible caps.

We can strike a match and see its flame. We can see a fire on the
hearth. We may feel around for the invisible poker, and when we find
it, we may put it in the fire. When it becomes hot enough, it will
glow red and become visible. We can make a match head glow by rubbing
it on a wet finger. We can even see a firefly, if one comes around.
But only those things which are glowing of themselves, like flames,
and red-hot pokers, and fireflies, will be visible.

The reason why practically everything would be invisible if there were
no reflection of light is this: When you look at anything, as a man,
for instance, what you really see is the light that hits him and
bounces back (reflects) into your eyes. Suppose you go into a dark
room and turn on an electric light. Instantly ripples of light flash
out from the lamp in every direction. As soon as they strike the
object you are looking at, they reflect (bounce back) from it to your
eyes. When light strikes your eyes, you see.

Of course, when you look at an electric lamp, or a star, or the sun,
or anything that is incandescent (so hot that it shines by its own
light), you can see it, whether reflection exists or not. But most
things you look at do not shine by their own light. This book that
you are reading simply reflects the light in the room to your eyes;
it would not give any light in a dark room. The paper reflects a
good deal of light that strikes it, so it looks very light; the print
reflects practically none of the light that strikes it, so it looks
dark, or black, just as a keyhole looks black because it does not
reflect any light to your eyes. But without reflection, the book would
be entirely invisible. The only kind of print you could read if
there were no reflection would be the electric signs made out of
incandescent lamps arranged to form letters.

WHAT THE RING AROUND THE MOON IS; WHAT SUNBEAMS ARE. The reason you
sometimes see a ring around the moon is that some of the moonlight
reflects from tiny droplets of water in the air, making them visible.
In the same way, the dust in the air of a room becomes visible when
the sun shines through it and is reflected by each speck of dust; we
call it a _sunbeam_. But we are not really looking directly at the
sunlight; we are seeing the part of the sunlight that is reflected by
the dust specks.

Have you ever noticed that when you stand a little to one side of a
mirror where you cannot see your own image in it, you can sometimes
see that of another person clearly, while he cannot see his own image
but can see yours? It is easy to understand this by comparing the
reflection of the light from your face to his eye and from his face
to your eye, to the bouncing of a ball from one person to another.
Suppose you and a friend are standing a little way apart on sandy
ground where you cannot bounce a ball, but that between you there is
a plank. If each of you is standing well away from the plank, neither
one of you can possibly bounce the ball on it in such a way that he
can catch it himself. Yet you can easily bounce it to your friend and
he can bounce it to you.

[Illustration: FIG. 62. The ball bounces from one boy to the other,
but it does not return to the one who threw it.]

The mirror is like that plank; it is something that will reflect
(bounce) the light directly. The light from your face goes into the
mirror, just as you may throw the ball against the plank, and the
light is reflected to your friend just as the ball is bounced to him;
so he sees your image in the mirror. If he can see you, you can see
him, just as when you bounce the ball to him he can bounce it to you.
But you may be unable to see yourself, just as you may be unable to
bounce the ball on the plank so that you yourself can catch it.

In other words, when light strikes against something it bounces away,
just as a rubber ball bounces from a smooth surface. If you throw a
ball straight down, it comes straight up; if light shines straight
down on a flat, smooth surface, it reflects straight up. If you
throw a ball down at a slant, it bounces up at the same slant in the
opposite direction; if light strikes a smooth surface at a slant, it
reflects at the same slant in the opposite direction.

[Illustration: FIG. 63. In the same way, the light bounces (reflects)
from one boy to the other. It does not return to the point from which
it started and neither boy can see himself.]

But to reflect light directly and to give a clear image, the surface
the light strikes _must_ be extremely smooth, just as a tennis court
must be fairly smooth to make a tennis ball rebound accurately.
Any surface that is smooth enough will act like a mirror, although
naturally, if it lets most of the light go through, it will not
reflect as well as if it sends all the light back. A pane of glass is
very smooth, and you can see yourself in it, especially if there is
not much light coming through the glass from the other side to mix up
with your reflection. But if the pane of glass is silvered so that no
light can get through, you have a real mirror; most of the light that
leaves your face is reflected to your eyes again.

WHY SMOOTH OR WET THINGS ARE SHINY. When a surface is very smooth,
we say it is shiny or glossy. Even black shoes, if they are polished,
become smooth enough to reflect much of the light that strikes them;
of course the parts where the light is being reflected do not look
black but white, as any one who has tried to paint or draw a picture
of polished shoes knows. Anything wet is likely to be shiny, because
the surface of water is usually smooth enough to reflect light rather
directly.

If a surface is uneven, like a pool with ripples on it, the light
reflects unevenly, and you see a distorted image; your face seems to
be rippling and moving in the water.

[Illustration: FIG. 64. How should the mirror be placed?]

    _APPLICATION 34._ Some boys were playing war and were in a
    ditch that they called a trench. They wanted to make a simple
    periscope so that they could look out of the ditch at the
    "enemy" without being in danger. They had an old stovepipe and
    a mirror. Practically all of them agreed that if the mirror
    were fixed in the top of the stovepipe and if they looked up
    through the bottom, they would be able to see over the side
    of the ditch. But they had an argument as to how the mirror
    should be placed. Each drew a diagram to show how he thought
    the mirror should be arranged, using dotted lines to show how
    the light would come from the enemy to their eyes. Three of
    the diagrams are shown in Figure 64.

    The boy who drew the first said: "If you want to see the
    enemy, the mirror's got to face him. Then it will reflect the
    light down to your eyes."

    The boy who drew the second said: "No, the light would just
    go back to him again. The mirror must slant so that the light
    that strikes it at a slant will be reflected to your eye at
    the same slant."

    "How could it get to your eye at all," the third boy said,
    "if the mirror didn't face you? You've got to have the mirror
    reflect right down toward your face. Then all the light that
    strikes it will come down to you."

    Which arrangement would work?


INFERENCE EXERCISE

    Explain the following:

    191. Your hands do not get wet when you put them into mercury.

    192. When beating hot candy, we sometimes put it in a pan of
    water.

    193. Electric stoves frequently have bright reflectors.

    194. We put ice in the _top_ of a refrigerator.

    195. You can jack up the back part of an automobile when you
    could not possibly lift it up.

    196. The sun shines up into your face and sunburns you when
    you are on the water.

    197. People in the tropics dress largely in white.

    198. Menthol rubbed into your skin makes it feel very cold
    afterward.

    199. We feel the heat of the sun almost as soon as the sun
    rises.

    200. You can shoot a stone far and hard with a sling shot.


SECTION 23. _The bending of light: Refraction._

    How do glasses help your eyes?

    On a hot day, how is it that you see "heat waves" rising from
    the street?

    What makes the stars twinkle?

Light usually travels in straight lines. If the light from an object
comes from straight in front of you, you know that the object is
straight in front of you. But you can bend light so that it seems to
come from a different place, thus making things seem to be where they
are not.

    EXPERIMENT 44. Hold a triangular glass prism vertically
    (straight up and down) in front of one eye, closing the other
    eye. Look through the prism, turning it or your head around
    until you see a chair through it. Watch only the chair through
    the prism. When you are sure you know just where it is, try to
    sit down in it.

    Now look for a pencil or a piece of chalk through the prism,
    in the same way. When you think you know where it is, try to
    pick it up.

The reason the chalk and chair seem to be where they are not is that
the prism bends the light that comes from them and makes the light
seem to come from somewhere else.

As you already know, when you look at a chair you see the light that
reflects from it. You judge where the chair is by the direction from
which the light is coming when it reaches your eye. But if the light
is bent on its way, so that it comes to your eye as it ordinarily
comes from an object off to one side, naturally you think the thing
you are looking at is off to one side. Maybe the diagram (Fig. 65)
will make this clearer.

[Illustration: FIG. 65. In passing through the prism the light is bent
so that an object at _b_ appears to be at _c_.]

Here in _a_ is an object the same height as the eye. The light comes
straight to the eye, and one knows that the object is level with the
eye. In _b_ the object is in the same position as in _a_, but the
prism bends the light so that it strikes the eye with an upward slant.
So the person thinks the object is below the eye at _c_.

Here is another experiment with bending light:

    EXPERIMENT 45. Fill a china cup with water. Put a pencil in
    it, letting the pencil rest at a slant from left to right.
    Lower your head until it is almost level with the surface of
    the water. How does the pencil look?

[Illustration: FIG. 66. The pencil is not bent, but the light that
comes from it is.]

The reason the pencil looks bent is because the light from the part of
it under the water is bent when it passes from the water into the air
on its way to your eye; so the slant at which it comes to your eye is
the same slant at which it ordinarily would come from a bent pencil.

    EXPERIMENT 46. Fill a glass with water. Put the pencil into
    it in the same way you put it in the cup in the previous
    experiment, letting the pencil slant from left to right. Lower
    your head this time until it is on a level with the water in
    the glass, and look through the glass and water at the pencil.
    Notice what happens where the pencil goes into the water.

What you see is explained in the same way as are the things that took
place in the other experiments in refraction, or bending of light. The
light from the part of the pencil above the water comes straight to
your eye, of course; so you see it just as it is. But the light from
the part of the pencil in the water is bent when it comes out of the
water into the air on its way to your eye. This makes it come to your
eye from a different direction and makes the lower part of the pencil
seem to be in a place to one side of the place where it _really_ is.
The pencil, therefore, looks broken.

[Illustration: FIG. 67. The bending of the light by the water in the
glass causes the pencil to look broken.]

Whenever light passes first through something dense like water or
glass, and then through something rare or thin like air, it is bent
one way; whenever it passes from a rare medium into a dense one, it is
bent the other way. Light passing from a fish to your eye is bent one
way; light passing from you to the fish's eye is bent the other way,
but the main point is that it is bent. And when light is bent before
reaching your eyes it usually makes things seem to be where they are
not.

If light goes through a perfectly smooth, flat pane of glass, it is
bent one way when it goes into the glass and back the other way when
it comes out; so it seems to be perfectly straight and we see things
practically as they are through a good window. But if the window glass
has flaws in it, so that some parts are a little thicker than others,
the uneven parts act like prisms and bend the light to one side.
This makes anything we look at through a poor window seem bent out of
shape. Of course the things are not bent any more than your pencil
in the water was bent, but they look misshapen because the light from
them is bent; the reflected light is all we see of things anyway.

[Illustration: FIG. 68. The light is bent when it enters a window pane
and is bent again in the opposite direction when it leaves it.]

The air itself is uneven in a way. The parts of the air that are warm,
as you already know, are thinner and more expanded than are the cold
parts. So light going from cold air into warm or from warm air into
cold, will be bent. And this is why you see what are called "heat
waves" above a stove or rising from a hot beach or sidewalk. Really
these are just waves of hot air rising, and they bend the light
that comes through them so as to give everything behind them a wavy
appearance.

Stars twinkle for much the same reason. As the starlight comes down
through the cold air and then through the warm air it is bent, and the
star seems to be to one side of where it really is; but the air does
not stand still,--sometimes it bends the light more and sometimes
less. So the star seems to move a little back and forth. And this is
what we call "twinkling." Really it is the bending of light.

    _APPLICATION 35._ Explain why an unevenness in your eye will
    keep you from seeing clearly; how glasses can help this; why
    good mirrors are made from plate glass, which is very smooth,
    instead of from the cheaper and more uneven window glass; why
    fishes in a glass tank appear to be where they are not.


INFERENCE EXERCISE

    Explain the following:

    201. The fire in the open fireplace ventilates a room well by
    making air go up the chimney.

    202. A drop of water glistens in the sun.

    203. Dust goes up to the ceiling and clings there.

    204. When you look at a person under moving water, his face
    seems distorted.

    205. You sit in the sun to dry your hair.

    206. Paste becomes hard and unfit for use when left open to
    the air.

    207. In laundries clothes are partly dried by whirling them in
    perforated cylinders.

    208. Circus balloons are filled by building a big fire under
    them.

    209. Unevenness in a window pane makes telephone wires seen
    through it look crooked and bent.

    210. You can see the image of a star even in a shallow puddle.

[Illustration: FIG. 69. When the light from one point goes through
the lens, it is bent and comes together at another point called the
focus.]


SECTION 24. _Focus._

    How can you take pictures with a camera?

    What causes the picture in the camera to be inverted?

    Why is a magnifying glass able to set things on fire when you
    let the sun shine through it?

In your eye, right back of the pupil, there is a flattened ball, as
clear as glass, called the _lens_. If the lens were left out of your
eye, you never could see anything except blurs of light and shadow.
If you looked at the sun it would dazzle you practically as much as it
does now. However, you would not see a round sun, but only a blaze of
light. You could tell night from day as well as any one, and you could
tell when you stepped into the shade. If some one stepped between you
and the light, you would know that some one was between you and the
light or that a cloud had passed over the sun,--you could not be quite
sure which. In short, you could tell all degrees of light and dark
apart nearly as well as you can now, but you could not see the form of
anything.

In the front of a camera there is a flattened glass ball called
the _lens_. If you were to remove it, the camera would not take any
pictures; it would take a blur of light and shade and nothing more.

[Illustration: FIG. 70. The light from each point of the candle flame
goes out in all directions.]

In front of a moving-picture machine there is a large lens, a piece of
glass rounded out toward the middle and thinner toward the edges.
If you were to take that lens off while the machine was throwing the
motion pictures on the screen, you would have a flicker of light and
shade, but no picture.

It is the lens that forms the pictures in your eye, on a photographic
plate or film, and on a moving-picture screen. And a lens is usually
just a piece of glass or something glassy, rounded out in such a way
as to make all the spreading light that reaches it from one point come
together in another point, as shown in Figure 69.

As you know, when light goes out from anything, as from a candle flame
or an incandescent lamp, or from the sun, it goes in all directions.
If the light from the point of a candle flame goes in all directions,
and if the light from the base of the flame also goes in all
directions, the light from the point will get all mixed up with the
light from the base, as shown in Figure 70. Naturally, if the light
from the point of the candle flame is mixed up with the light from
the base and the beams are all crisscross, you will not get a clear
picture of the flame.

[Illustration: FIG. 71. The reading glass is a lens which focuses the
light from the candle flame and forms an image.]

    EXPERIMENT 47. Fasten a piece of paper against a wall and
    place a lighted candle about 4 feet in front of it. Look at
    the paper. Is there any picture of the candle flame on it?
    Now hold a magnifying glass (reading glass) near the candle,
    between the candle and the paper, so that the light will shine
    through the lens on to the paper. (The magnifying glass is a
    lens.) Move the lens slowly toward the paper until you get
    a clear picture of the candle flame. Is it right side up or
    upside down?

The lens has brought the light from the candle flame to a _focus_; all
the light that goes through the lens from one point of the flame has
been brought together at another point (Fig. 72). In the diagram you
see all the light from the _point_ of the candle flame spreading out
in every direction. But the part that goes through the lens is brought
together at one point, called the focus. Of course the same thing
happens to the light from the base of the candle flame (Fig. 73). Just
as before, all the light from the base of the flame is brought to a
focus. The light spreads out until it reaches the lens. Then the lens
bends it together again until it comes to a point.

[Illustration: FIG. 72. The light from the tip of the candle flame is
focused at one point.]

[Illustration: FIG. 73. And the light from the base of the flame is
focused at another point.]

[Illustration: FIG. 74. The light from the tip and base (and from
every other point) of the flame is, of course, focused at the same
time. In this way an image of the flame is formed.]

But of course the light from the base of the flame is focused at the
same time as the light from the point; so what really happens is that
which is illustrated in Figure 74. In this diagram, we have drawn
unbroken lines to show the light from the point of the candle flame
and dotted lines to show the light from the base of the flame. This is
so that you can follow the light from each part and see where it
goes. Compare this diagram with the one where the light is shown all
crisscrossed (Fig. 70), and you will see why the lens makes an image,
while you have no image without it.

By looking at the last diagram (Fig. 74) you can also see how the
image happens to be upside down.

    EXPERIMENT 48. Set up the candle and piece of paper as you did
    for the last experiment, but move the magnifying glass back
    and forth between the paper and the candle. Notice that there
    is one place where the image of the candle is very clear. Does
    the image become clearer or less clear if you move the lens
    closer to the candle? if you move it farther from the candle?

The explanation is this: After the light comes together into a point,
it spreads out again beyond the point, as shown in Figure 75. So if
you hold the lens in such a way that the light comes to a focus before
it reaches the paper, the paper will catch the spreading light and you
will get a blur instead of a sharp image. It is as shown in Figure 76.

[Illustration: FIG. 75. The light spreads out again beyond the focus.]

[Illustration: FIG. 76. So if the light comes to a focus before it
reaches the paper, the image will be blurred.]

On the other hand, if you hold your lens in such a way that the light
has not yet come to a focus when it reaches the paper, naturally you
again have a blur of light instead of a point, and the image is not
sharp and definite (Fig. 77).

[Illustration: FIG. 77. Or if the light reaches the paper before it
comes to a focus, the image will be blurred.]

And that is why good cameras have the front part, in which the lens
is set, adjustable; you can move the lens back and forth until a
sharp image is formed on the plate. Motion-picture machines and
stereopticons likewise have lenses that can be moved forward and back
until they form a sharp focus on the screen. Even the lens in your eye
has muscles that make it flatter and rounder, so that it can make a
clear image on the sensitive retina in the back of your eye. The lens
in the eyes of elderly people often becomes too hard to be regulated
in this way, and so they have to wear one kind of glasses to see
things near them clearly and another kind to see things far away.

The kind of lens we have been talking about is the _convex_ lens.
"Convex" means bulging out in the middle. There are other kinds of
lenses, some flat on one side and bulging out on the other, some
hollowed out toward the middle instead of bulging, and so on. But the
only lens that most people make much use of (except opticians) is the
convex lens that bulges out toward the center. The convex lens makes a
clear image and it is the only kind of lens that will do this.

[Illustration: FIG. 78. Lenses of different kinds.]

WHY YOU CAN SET FIRE TO PAPER WITH A MAGNIFYING GLASS. A convex lens
brings light to a focus, and it also brings radiant heat to a focus.
And that is why you can set fire to things by holding a convex lens in
the sunlight so that the light and heat are focused on something that
will burn. All the sun's radiant heat that strikes the lens is brought
practically to one point, and all the light which is absorbed at this
point is changed to heat. When so much heat is concentrated at one
point, that point becomes hot enough to catch fire.

    _APPLICATION 36._ Explain why there is a lens in a
    moving-picture machine; why a convex lens will burn your hand
    if you hold it between your hand and the sun; why the front
    of a good camera is made so that it can be moved closer to the
    plate or farther away from it, according to the distance of
    the object you are photographing; why there is a lens in your
    eye.


INFERENCE EXERCISE

    Explain the following:

    211. Cut glass ware sparkles.

    212. An unpainted floor becomes much dirtier and is harder to
    clean than a painted one.

    213. If you sprinkle wet tea leaves on a rug before sweeping
    it, not so much dust will be raised.

    214. Food leaves a spoon when the spoon is struck sharply upon
    the edge of a stewpan.

    215. An image is formed on the photographic plate of a camera.

    216. Ripples in a pool distort the image seen in it.

    217. Cream rises to the top of a bottle of milk.

    218. Your eyes have to adjust themselves differently to see
    things near by and to see things at a distance.

    219. A vacuum cleaner does not wear out a carpet nearly as
    quickly as a broom or a carpet sweeper does.

    220. You can see a sunbeam in a dusty room.


SECTION 25. _Magnification._

    Why is it that things look bigger under a magnifying glass
    than under other kinds of glass?

    How does a telescope show you the moon, stars, and planets?

    How does a microscope make things look larger?

Everybody knows, of course, that a convex lens in the right position
makes things look larger. People use convex lenses to make print
look larger when they read, and for that reason such lenses are often
called _reading glasses_. For practical purposes it is not necessary
to understand how a convex lens magnifies; the important thing is the
fact that it does magnify. But you may be curious to know just how a
magnifying glass works.

First, you should realize that the image formed by a convex lens is
not always larger than the object. Repeat Experiment 41, but this time
move the lens from near the candle toward the paper, past the point
where it makes its first clear image. Keep moving the lens slowly
toward the paper until a second image is formed. Which image is larger
than the flame? Which is smaller?

[Illustration: FIG. 79. A section of the eye.]

The important point in this experiment is for you to see that if the
lens is nearer to the image on the paper than it is to the candle, the
image is smaller than the candle. That is why a photograph is usually
smaller than the thing photographed; it would be impossible to take
a picture of a house or a mountain if the lens in the camera gave a
_magnified_ image.

[4]Your eye is a small camera. It has a lens in the front; it is lined
with black; and at the back there is a sensitive part on which the
picture is formed. This sensitive part of the eye is called the
_retina_. It is in the back part of your eyeball and is made of many
very sensitive nerve endings. When the light strikes these nerve
endings, it sends an impulse through the nerves to the back part of
the brain; then you know that the image is formed. And, of course,
since your eyeball is small and many of the things you see are large,
the image on the retina must be much smaller than the object itself,
and this is because the lens is so much nearer to the retina than it
is to the object.

[Footnote 4: The following explanation may be omitted by any children
who are not interested in it. Let such children skip to the foot of
page 156.]

[Illustration: FIG. 80. How an image is formed on the retina of the
eye.]

[Illustration: FIG. 81. A simpler diagram showing how an image is
formed in the eye.]

[Illustration: FIG. 82. A diagram showing how a reading glass causes
things to look larger by making the image on the retina larger.]

[Illustration: FIG. 83. Diagram showing how a reading glass enlarges
the image on the retina. More lines are drawn in than in Figure 82.]

You can understand magnification best by looking at Figures 80, 81,
82, and 83.

In Figure 80 there are a candle flame, the lens of an eye, and the
retina on which the image is being formed.

Figure 81 is the same as Figure 80, with all the lines left out except
the outside ones that go to the lens. It is shown in this way merely
for the sake of simplicity. All the lines really belong in this
diagram as in the first. In both diagrams the size of the image on the
retina is the distance between the point where the top line touches it
and the point where the bottom line touches it.

In order to make anything look larger, we must make the image on the
retina larger. A magnifying glass, or convex lens, if put in the right
place, will do this. In the next diagram, Figure 82, we shall include
the magnifying glass, leaving out all lines except the two outside
ones shown in Figure 81.

You will notice that the magnifying glass starts to bend the lines
together, and that the lens in the eye bends them farther together;
so they cross sooner, and the image is larger. Figure 83 shows more of
the lines drawn in.

[Illustration: FIG. 84. Diagram of a microscope.]

The two important points to notice are these: First, the magnifying
glass is too close to the eye for the light to be brought to a focus
before it reaches the eye; the light is bent toward a focus, but it
reaches the eye before the focus is formed. The focus is formed for
the first time on the retina itself. Second, the magnifying glass
bends the light on its way to your eye so that the light crosses
sooner in your eye and spreads out farther before it comes to a focus.
This forms the larger image, as you see in the simple diagram, Figure
82.

[Illustration: FIG. 85. This is the way a concave mirror forms a
magnified image.]

[Illustration: FIG. 86. The concave mirror forms an image of the
burning candle.]

HOW THE MICROSCOPE WORKS. But the microscope is different. It works
like this: The first lens is put very near the object which you are
examining. This lens brings the light from the object to a focus and
forms an image, much larger than the object itself, high up in the
tube. If you held a piece of paper there you would see the image.
But since there is nothing there to stop the light, it goes on up the
tube, spreading as it goes. Then there is another lens which catches
this light and bends it inward on its way to your eye, just as any
magnifying glass does. Next the lens in the eye forms an image on
the retina. The diagram (Fig. 84) will make this clearer. (A real
microscope is not so simple, of course, and usually has two lenses
wherever the diagram shows one.) What actually happens is that the
first lens makes an image many times as big as the object; then you
look at this image through a magnifying glass, so that the object is
made to look very much larger than it really is. That is why you can
see blood corpuscles and germs and cells through a microscope, when
you cannot see them at all with your naked eye.

[Illustration: FIG. 87. The great telescope of the Yerkes Observatory
at Lake Geneva, Wisconsin.]

A MIRROR THAT MAGNIFIES. A convex lens is not the only thing that can
magnify. A concave mirror, which is one that is hollowed out toward
the middle, does the same thing. When light is reflected by such a
mirror, it acts exactly as if it had gone through a convex lens (Fig.
85).

    EXPERIMENT 49. Place the lighted candle and the paper about 4
    feet apart, as you did in Experiment 47. Hold a concave mirror
    _back_ of the candle (so that the candle is between the mirror
    and the paper); then move the mirror back, the mirror casting
    the reflection of the candle light on the paper, until a clear
    image of the candle is formed.

    Look at your image in the concave mirror. Does it look larger
    or smaller than you?

HOW TELESCOPES ARE MADE. Astronomers use convex lenses in some of
their telescopes; in others, called _reflecting telescopes_, they use
concave mirrors. Both do the same work, making the moon, the planets,
and the sun look much larger than they otherwise would.

    _APPLICATION 37._ Explain how a reading glass makes print look
    larger; how you can see germs through a microscope; what kind
    of mirror will magnify; what kind of lens will magnify.


INFERENCE EXERCISE

    Explain the following:

    221. The water that forms rain comes from the ocean, yet the
    rain is not salty.

    222. Iron glows when it is very hot.

    223. You can start a fire with sunlight by holding a reading
    glass at the right distance above the fuel.

    224. Big telescopes make it possible for us to see in detail
    the surface structure of the moon.

    225. A room is lighter if it has white walls than if it has
    dark walls.

    226. Iron is heated by a blacksmith before he shapes it.

    227. A dentist's mirror is concave; he sees your teeth
    enlarged in it.

    228. Good penholders usually have cork or rubber tips.

    229. A man's suit becomes shiny when it gets old.

    230. When you look at a window from the sidewalk, you
    frequently see images of the houses across the street.


SECTION 26. _Scattering of light: Diffusion._

    Why is it that on a dark day the sun cannot be seen through
    light clouds?

    Why do not the stars come out in the daytime?

If you were on the moon, you could see the stars in the daytime. The
sun would be shining even more brightly than it does here, but the sky
around the sun would be pitch black, except for the stars shining out
of its blackness. The reason is that there is no air on the moon to
scatter the light.

WHY WE CANNOT SEE THE STARS IN THE DAYTIME. Most of the sun's light
that comes to the earth reaches us rather directly; that is why we
can see the image of the sun. But part of the sunlight is scattered
by particles of air, and that is why the whole sky is bright in the
daytime. You know, of course, that the blue sky is only the air that
surrounds the earth. Enough of the light is scattered around to make
the sky as bright as the stars look from here; so we cannot see the
stars through the sky in the daytime.

HOW A CLOUD CAN HIDE THE SUN WITHOUT CUTTING OFF ALL ITS LIGHT. When a
cloud drifts between us and the sun, we no longer see the sun; yet
the earth does not become dark. The sun's light is evidently still
reaching us. The cloud is made of millions of very tiny droplets of
water. When the sunlight strikes the curved sides of these droplets,
it is reflected at all angles according to the way it strikes, as
shown in Figure 89.

[Illustration: FIG. 88. The sunlight is scattered (diffused) by
the clouds. The photograph shows in the foreground the Parliament
Buildings, London, England.]

Some of the light is reflected back into the sky; that is why
everything becomes darker when the sun goes behind a cloud; but much
of the light comes through to us, at all sorts of slants. When it
comes all higgledy-piggledy and crisscross like this, no lens can put
it together again; it is as hopelessly broken up as Humpty-Dumpty was.
But much of the light gets here just the same; so we see it without
seeing the form of the sun. Light that cannot be brought to a focus is
called _scattered_ or _diffused light_.

When you look through a ground-glass electric lamp, you cannot see the
filament; the light passing through all the rough parts of the glass
gets so scattered that you cannot bring it to a focus. Therefore, no
image of the filament in the incandescent lamp can be formed on the
retina of your eye.

[Illustration: FIG. 89. How the droplets in a cloud scatter the rays
of light.]

A piece of white paper reflects practically all the light that strikes
it. Yet you cannot see yourself in a piece of ordinary white paper.
The trouble is that the paper is too rough; there are too many little
uneven places that reflect the light at all sorts of angles; the light
is scattered and the lens in your eye cannot bring it to a focus.

    _APPLICATION 38._ Explain why a scrim curtain will keep people
    from seeing into a room, but will not shut the light out; why
    curtains soften the light of a room; why indirect lighting
    (i.e. light thrown up against the ceiling and then reflected
    down into the room by the rough ceiling) is better for your
    eyes than is the old-time direct lighting.


INFERENCE EXERCISE

    Explain the following:

    231. The alcohol formed by the yeast in making bread light is
    practically all gone by the time the bread is baked.

    232. The oceans do not flow off the earth at the south pole.

    233. Lamp globes often have frosted bottoms.

    234. A damp dust cloth will take up the dust, without making
    it fly.

    235. The stars twinkle when their light passes through the
    moving air currents that surround the earth.

    236. Shears for cutting tin and metal have long handles and
    short blades.

    237. A coin at the bottom of a glass of water seems raised
    when you look at it a little from one side.

    238. You have to brace your feet to row well.

    239. Light from the northern part of the sky, where the sun is
    not, does not make sharp shadows.

    240. Pokers and lifters for stove lids often have open spiral
    handles.


SECTION 27. _Color._

    What makes the ocean look green in some places and blue in
    others?

    What makes the sky blue?

    What causes material to be colored?

    What makes a rainbow?

    What is color?

[Illustration: FIG. 90. Making a rainbow on the wall.]

Color is merely a kind of light. We say that a sweater is red; really
the sweater is not red, but the light that it reflects to our eyes is
red. We speak of a piece of red glass, but the glass is not red; it is
the light that it lets pass through it that is red.

White is not really a color; _all_ colors put together make white.
Experiments 50 and 51 will prove this.

[Illustration: FIG. 91. The prism separates the white light into the
rainbow colors.]

    EXPERIMENT 50. Hold a prism in the sunlight by the window
    and make a "rainbow" on the wall. The diagram here shown
    illustrates how the prism breaks up the single beam of white
    light into different-colored beams of light.

[Illustration: FIG. 92. When the wheel is rapidly whirled the colors
blend to make white.]

    EXPERIMENT 51. Rotate the color disk on the rotator and watch
    it. Make it go faster and faster until all the colors are
    perfectly merged. What color do you get by combining all
    the colors of the rainbow? If the colors on the disk were
    perfectly clear rainbow colors, in exactly the same proportion
    as in the rainbow, the whirling would give a white of dazzling
    purity.

Since you can break up pure white light into all the colors, and since
you can combine all the colors and get pure white light, it is clear
that white light is made up of all the colors.

       *       *       *       *       *

As we have already said, light is probably vibrations or waves of
ether. Light made of the longest waves that we can see is red. If the
waves are a little shorter, the light is orange; if they are shorter
yet, it is yellow; still shorter, green; shorter still, blue; while
the shortest waves that we can see are those of violet light. Black
is not a color at all; it is the absence of light. We say the night
is black when we cannot see anything. A deep hole looks black because
practically no light is reflected up from its depths. When you "see"
anything black, you really see the things around it and the parts of
it that are not perfectly black. A pair of shoes, for instance, has
particles of gray dust on them; or if they are very shiny they reflect
part of the light that strikes them as a white high-light. But the
really black part of your shoes would be invisible against an equally
black background.

A black thing absorbs the light that strikes it and turns it to heat.
Here is an experiment that will prove this to you:

    EXPERIMENT 52. (a) On a sunny day, take three bottles, all of
    the same size and shape, and pour water out of a pitcher or
    pan into each bottle. Do not run the water directly from the
    faucet into the bottle, because sometimes that which comes
    out of the faucet first is warmer or colder than that which
    follows; in the pitcher or pan it will all be mixed together,
    and so you can be sure that the water in all three bottles is
    of the same temperature to begin with. Wrap a piece of white
    cotton cloth twice around one bottle; a piece of red or
    green cotton cloth of the same weight twice around the second
    bottle, and a piece of black cotton cloth of the same weight
    twice around the third bottle, fastening each with a rubber
    band. Set all three bottles side by side in the sunlight, with
    2 or 3 inches of space between them. Leave them for about an
    hour. Now put a thermometer into each to see which is warmest
    and which is least warm.

    From which bottle has most of the light been reflected back
    into the air by the cloth around it? Which cloth absorbed most
    of the light and changed it into heat? Does the colored cloth
    absorb more or less light than the white one? than the black
    one?

[Illustration: FIG. 93. Which color is warmest in the sunlight?]

    (b) On a sunny day when there is snow on the ground, spread
    three pieces of cotton cloth, all of the same size and
    thickness, one white, one red or green, and one black, on top
    of the snow, where the sun shines on them. Watch them for a
    time. Under which does the snow melt first?

    The white cloth is white because it reflects _all_ colors back
    at once. It therefore absorbs practically no light. But the
    reason the black cloth looks black is that it reflects almost
    none of the colors--it absorbs them all and changes them to
    heat. The colored cloth reflects just the red or the green
    light and absorbs the rest.

Maybe you will understand color better if it is explained in another
way. Suppose I throw balls of all colors to you, having trained you to
keep all the balls except the red ones. I throw you a blue ball; you
keep it. I throw a red ball; you throw it back. I throw a green ball;
you keep it. I throw a yellow ball; you keep it. I throw two balls at
once, yellow and red; you keep the yellow and throw back the red. I
throw a blue and yellow ball at the same time; you keep both balls.

Now suppose I change this a little. Instead of throwing balls, I shall
throw lights to you. You are trained always to throw red light back
to me and always to keep (absorb) all other kinds of light. I throw a
blue light; you keep it, and I get no light back. I throw a red light;
you throw it back to me. I throw a green light; you keep it, and I get
no light back. I throw a yellow light; you keep it, and I get no light
back. I throw two lights at the same time, yellow and red; you keep
the yellow and throw back only the red. But yellow and red together
make orange; so when I throw an orange light, you throw back the red
part of it and keep the yellow.

Now if we suppose that instead of throwing lights to _you_ I throw
them to molecules of dye which are "trained" to throw back the red
lights and keep all the other kinds (absorb them and change them to
heat), we can understand what the dye in a red sweater does. The dye
is not really trained, of course, but for a reason which we do not
entirely understand, some kinds of dye always throw back (reflect) any
red that is in the light that shines on them, but they keep all other
kinds of light, changing them to heat. Other dyes or coloring matter
always throw back any green that is in the light that shines on them,
keeping the other colors. Blue coloring matter throws back only the
blue part of the light, and so on through all the colors.

So if you throw a white light, which contains all the colors, on a
"red" sweater, the dye in the sweater picks out the red part of the
white light and throws that back to your eyes (reflects it to you) but
it keeps the rest of the colors of the white light, changing them to
heat; and since only the red part of the light is reflected to your
eyes, that is the only part of it that you can see; so the sweater
looks red. The "green" substance (chlorophyll) in grass acts in the
same way; only it throws the green part of the sunlight back to your
eyes, keeping the rest; so the part of the light that reaches you from
the grass is the green light, and the grass looks green.

Anything white, like a piece of paper, reflects all the light that
strikes it; so if all the colors (white light) strike it, all are
reflected to your eyes and the object looks white.

You have looked at people under the mercury-vapor lights in
photo-postal studios, have you not? The lights are long, inclined
tubes which glow with a greenish-violet light. No matter how good the
color of a person is in ordinary light, in that light it is ghastly.

[Illustration: FIG. 94. A mercury-vapor lamp.]

Go into the kitchen tonight, light a burner of the gas stove, turn out
the light and sprinkle salt on the blue gas flame. The flame will leap
up, yellow. Look at your hands, at some one's lips, at a piece of red
cloth, in this light. Does anything look red?

The reason why nothing looks pink or red in these two kinds of light
is this: The light given by glowing salt vapor or mercury vapor has no
red in it; if you tried to make a "rainbow" from it with a prism, you
would find no red or orange color in it. A thing looks red when it
absorbs all the parts of the light that are not red and reflects the
red light to your eyes. If there is no red in the light to reflect,
obviously a thing cannot look red in that light.

When you look through a piece of colored glass, the case is somewhat
different. A piece of blue glass, for instance, acts as a sort of
strainer. The coloring matter in it lets the blue light through it,
but it holds back (absorbs) the other kinds of light. So if you look
through a piece of blue glass you see everything blue; that is, only
the blue part of the light from different objects can reach your eyes
through this kind of glass. Anything that is transparent and colored
acts in a similar way.

WHY THE SKY IS BLUE. And that is why the sky looks blue. Air holds
back all colors of light except blue; that is, it holds them back a
little. A room full of air holds the colors back hardly at all. A
few miles of air hold them back more; mountains in the distance look
bluish because only the blue light from them can reach you through
the air. The hundred or more miles of air above you hold back a
considerable amount of the other colors of light, letting through much
more of blue than of any other color. So the sky looks blue; that is,
when the air scatters the sunlight above you, it is chiefly the blue
parts of the sunlight that it allows to reach your eyes.

WHY BODIES OF WATER LOOK GREEN IN SOME PLACES AND BLUE IN OTHERS.
Water acts in a similar way, but it lets the green light through
instead of the blue. A little water holds back (absorbs) the other
colors so slightly that you cannot notice the effect in a glass of
water. But in a swimming tank full of water, or in a lake or an ocean,
you can notice it decidedly when you look straight down into the water
itself.

When you look at a smooth body of water at a slant on a clear day,
the blue sky is reflected to you and the water looks blue instead of
green. And it may even look blue when you look straight down in it if
it is too deep for you to see the bottom and the sky is reflected from
the surface.

WHY THE SKY IS OFTEN RED AT SUNSET. Dust lets more of red and yellow
light through than of any other color, and for this reason there is
much red and yellow in the sunset. Just before the sun sets, it shines
through the low, dusty air. The dust filters out most of the light
except the red and yellow. The red light and yellow light are
reflected by the clouds (for the clouds are themselves "white"; that
is, they reflect all the colors that strike them), and you have the
beautiful sunset clouds. Sometimes there is a purple in the sunset,
and even green. But since the air itself is blue (that is, it lets
mostly blue light go through), it is easy to see how this blue can
combine with the red or yellow that the dust lets through, to form
purple or green.

But we could not have sunset colors or all the colors we see on earth,
if it were not that the sunlight is mostly white--that it contains all
colors. And that, too, is why we can have a rainbow.

HOW RAINBOWS ARE FORMED. You already know fairly well how a rainbow
is formed, since you made an imitation of one with a prism. A rainbow
appears in the sky when the sun shines through the rain; the plain
white light of the sun is divided up into red, orange, yellow, green,
blue, indigo, and violet. As the white light of the sun passes through
the raindrops, the violet part of the light is bent more than any of
the rest, the indigo part is bent not quite so much, and so on to the
red, which is bent least of all. So all the colors fan out from the
single beam of white light and form a band of color, which we call the
rainbow.

[Illustration: FIG. 95. Explain why the breakers are white and the sea
green or blue.]

HOW WE CAN TELL WHAT THE SUN AND STARS ARE MADE OF. When a gas or
vapor becomes hot enough to give off light (when it is incandescent),
it does not give off white light but light of different colors. An
experiment will let you see this for yourself.

    EXPERIMENT 53. Sprinkle a little copper sulfate (bluestone)
    in the flame of a Bunsen burner. What color does it make the
    flame?

Copper vapor always gives this greenish-blue light when it is heated.
The photographer's mercury-vapor light gave a greenish-violet glow.
When you burn salt or soda in a gas flame, you remember that you get a
clear yellow light. By breaking up these lights, somewhat as you broke
up the sunlight with the prism, chemists and astronomers can tell what
kind of gas is glowing. The instrument they use to break up the light
into its different colors is called a _spectroscope_, and the band
of colors formed is called the _spectrum_. With the spectroscope they
examine the light that comes from the sun and stars and by the colors
of the spectra they can tell what these far-distant bodies are made
of.

    _APPLICATION 39._ If you were going to the tropics, would it
    be better to wear outside clothes that were white or black?

    _APPLICATION 40._ A dancer was to dance in a spotlight on the
    stage. The light was to change colors constantly. She wanted
    her robe to reflect each color that was thrown on it. Should
    she have worn a robe of red, yellow, white, green, or blue?

    _APPLICATION 41._ If you looked through a red glass at a
    purple flower (purple is red mixed with blue), would the
    flower look red, blue, purple, black, or white?


INFERENCE EXERCISE

    Explain the following:

    241. Mercury is separated from its ore by heating the ore so
    strongly that the mercury rises from it as a vapor.

    242. Hothouses are built of glass.

    243. A "rainbow" is sometimes seen in the spray of a garden
    hose.

    244. Your feet become hot when your shoes are being polished.

    245. Doors into offices usually have windows of ground glass
    or frosted glass.

    246. Opera glasses are of value to those sitting at a distance
    from the stage.

    247. In order to see clearly through opera glasses, you have
    to adjust them.

    248. It is warm inside an Eskimo's hut although it is built of
    ice and snow.

    249. It is usually cooler on a lawn than on dry ground.

    250. Black clothes are warmer in the sunlight than clothes of
    any other color.




CHAPTER SIX

SOUND


SECTION 28. _What sound is._

    What makes a dictaphone or a phonograph repeat your words?

    What makes the wind howl when it blows through the branches of
    trees?

    Why can you hear an approaching train better if you put your
    ear to the rail?

If you were to land on the moon tonight, and had with you a tank
containing a supply of air which you could breathe (for there is no
air to speak of on the moon), you might _try_ to speak. But you would
find that you had lost your voice completely. You could not say a
word. You would open and close your mouth and not a sound would come.

Then you might try to make a noise by clapping your hands; but that
would not work. You could not make a sound. "Am I deaf and dumb?" you
might wonder.

The whole trouble would lie in the fact that the moon has practically
no air. And sound is usually a kind of motion of the air,--hundreds of
quick, sharp puffs in a second, so close together that we cannot feel
them with anything less sensitive than the tiny nerves in our ears.

If you can once realize the fact that sound is a series of quick,
sharp puffs of air, or to use a more scientific term, _vibrations_ of
air, it will be easy for you to understand most of the laws of sound.

A phonograph seems almost miraculous. Yet it works on an exceedingly
simple principle. When you talk, the breath passing out of your throat
makes the vocal cords vibrate. These and your tongue and lips make the
air in front of you vibrate.

When you talk into a dictaphone horn, the vibrating air causes the
needle at the small end of the horn to vibrate so that it traces a
wavy line in the soft wax of the cylinder as the cylinder turns. Then
when you run the needle over the line again it follows the identical
track made when you talked into the horn, and it vibrates back and
forth just as at first; this makes the air in the horn vibrate exactly
as when you talked into the horn, and you have the same sound.

All this goes back to the fundamental principle that sound is
vibrations of air; different kinds of sounds are simply different
kinds of vibrations. The next experiments will prove this.

    EXPERIMENT 54. Turn the rotator rapidly, holding the corner of
    a piece of stiff paper against the holes in the disk. As
    you turn faster, does the sound become higher or lower? Keep
    turning at a steady rate and move your paper from the inner
    row of holes to the outer row and back again. Which row has
    the most holes in it? Which makes the highest sound? Hold your
    paper against the teeth at the edge of the disk. Is the pitch
    higher or lower than before? Blow through a blowpipe against
    the different rows of holes while the disk is being whirled.
    As the holes make the air vibrate do you get any sound?

This experiment shows that by making the air vibrate you get a sound.

The next experiment will show that when you have sound you are getting
vibrations.

    EXPERIMENT 55. Tap a tuning fork against the desk, then
    hold the prongs lightly against your lips. Can you feel them
    vibrate? Tap it again, and hold the fork close to your ear.
    Can you hear the sound?

[Illustration: FIG. 96. An interesting experiment in sound.]

The experiment which follows will show that we usually must have air
to do the vibrating to carry the sound.

    EXPERIMENT 56. Make a pad of not less than a dozen thicknesses
    of soft cloth so that you can stand an alarm clock on it on
    the plate of the air pump. The pad is to keep the vibrations
    of the alarm from making the plate vibrate. A still better way
    would be to set a tripod on the plate of the air pump and to
    suspend the alarm clock from the tripod by a rubber band. Set
    the alarm so that it will ring in 3 or 4 minutes, put it under
    the bell jar, and pump out the air. Before the alarm goes off,
    be sure that the air is almost completely pumped out of the
    jar. Can you hear the bell ring? Distinguish between a dull
    trilling sound caused by the jarring of the air pump when the
    alarm is on, and the actual _ringing_ sound of the bell.

[Illustration: FIG. 97. When the air is pumped out of the jar, you
cannot hear the bell ring.]

The experiment just completed shows how we know there would be no
sound on the moon, since there is practically no air around it. The
next experiment will show you more about the way in which phonographs
work.

    EXPERIMENT 57. Put a blank cylinder on the dictaphone, adjust
    the recording (cutting) needle and diaphragm at the end of the
    tube, start the motor, and talk into the dictaphone. Shut
    off the motor, remove the cutting needle, and put on the
    reproducing needle (the cutting needle, being sharp, would
    spoil the cylinder). Start the reproducing needle where the
    recording needle started, turn on the motor, and listen to
    your own voice.

Notice that in the dictaphone the air waves of your voice are all
concentrated into a small space as they go down the tube. At the end
of the tube is a diaphragm, a flat disk which is elastic and vibrates
back and forth very easily. The air waves from your voice would not
vibrate the needle itself enough to make any record; but they vibrate
the diaphragm, and the needle, being fastened rigidly to it, vibrates
with it.

[Illustration: FIG. 98. Making a phonograph record on an old-fashioned
phonograph.]

In the same way, when the reproducing needle vibrates as it goes over
the track made by the cutting needle, it would make air vibrations too
slight for you to hear if it were not fastened to the diaphragm. When
the diaphragm vibrates with the needle, it makes a much larger surface
of air vibrate than the needle alone could. Then the tube, like an ear
trumpet, throws all the air vibrations in one direction, so that you
hear the sound easily.

    EXPERIMENT 58. Put a clean white sheet of paper around the
    recording drum, pasting the two ends together to hold it in
    place. Put a small piece of gum camphor on a dish just under
    the paper, light it, and turn the drum so that all parts will
    be evenly smoked. Be sure to turn it rapidly enough to keep
    the paper from being burned.

    Melt a piece of glass over a burner and draw it out into a
    thread. Break off about 8 inches of this glass thread and tie
    it firmly with cotton thread to the edge of one prong of a
    tuning fork. Clamp the top of the tuning fork firmly above
    the smoked drum, adjusting it so that the point of the glass
    thread rests on the smoked paper. Turn the handle slightly to
    see if the glass is making a mark. If it is not, adjust it so
    that it will. Now let some one turn the cylinder quickly and
    steadily. While it is turning, tap the tuning fork on the
    prong which has _not_ the glass thread fastened to it. The
    glass point should trace a white, wavy line through the smoke
    on the paper. If it does not, keep on trying, adjusting the
    apparatus until the point makes a wavy line.

[Illustration: FIG. 99. A modern dictaphone.]

Making a record in this way is, on a large scale, almost exactly like
the making of a phonograph record. The smoked paper on which a tracing
can easily be made as it turns is like the soft wax cylinder. The
glass needle is like the recording needle of a phonograph. The chief
difference is that you have struck the tuning fork to make it and
the needle vibrate, instead of making it vibrate by air waves set in
motion by your talking. It is because these vibrations of the tuning
fork are more powerful and larger than are those of the recording
needle of a phonograph that you can see the record on the recording
drum, while you cannot see it clearly on the phonograph cylinder.

[Illustration: FIG. 100. How the apparatus is set up.]

In all ordinary circumstances, sound is the vibration of _air_. But
in swimming we can hear with our ears under water, and fishes hear
without any air. So, to be accurate, we should say that sound is
vibrations of any kind of matter. And the vibrations travel better in
most other kinds of matter than they do in air. Vibrations move rather
slowly in air, compared with the speed at which they travel in other
substances. It takes sound about 5 seconds to go a mile in air; in
other words, it would go 12 miles while an express train went one.
But it travels faster in water and still faster in anything hard like
steel. That is why you can hear the noise of an approaching train
better if you put your ear to the rail.

[Illustration: FIG. 101. When the tuning fork vibrates, the glass
needle makes a wavy line on the smoked paper on the drum.]

WHY WE SEE STEAM RISE BEFORE WE HEAR A WHISTLE BLOW. But even through
steel, sound does not travel with anything like the speed of light.
In the time that it takes sound to go a mile, light goes hundreds of
thousands of miles, easily coming all the way from the moon to the
earth. That is why we see the steam rise from the whistle of a train
or a boat before we hear the sound. The sound and the light start
together; but the light that shows us the steam is in our eyes almost
at the instant when the steam leaves the whistle; the sound lags
behind, and we hear it later.

    _APPLICATION 42._ Explain why a bell rung in a vacuum makes
    no noise; why the clicking of two stones under water sounds
    louder if your head is under water, than the clicking of the
    two stones in the air sounds if your head is in the air; why
    you hear a buzzing sound when a bee or a fly comes near you;
    how a phonograph can reproduce sounds.


INFERENCE EXERCISE

    Explain the following:

    251. The paint on woodwork blisters when hot.

    252. You can screw a nut on a bolt very much tighter with a
    wrench than with your fingers.

    253. When a pipe is being repaired in the basement of a house,
    you can hear a scraping noise in the faucets upstairs.

    254. Sunsets are unusually red after volcanic eruptions.

    255. Thunder shakes a house.

    256. Shooting stars are really stones flying through space.
    When they come near the earth, it pulls them swiftly down
    through the air. Explain why they glow.

    257. At night it is difficult to see out through a closed
    window of a room in which a lamp is lighted.

    258. When there is a breeze you cannot see clear reflections
    in a lake.

    259. Rubbing with coarse sandpaper makes rough wood smooth.

    260. A bow is bent backward to make the arrow go forward.


SECTION 29. _Echoes._

    When you put a sea shell to your ear, how is it that you hear
    a roar in the shell?

    Why can you sometimes hear an echo and sometimes not?

If it were not for the fact that sound travels rather slowly, we
should have no echoes, for the sound would get back to us practically
at the instant we made it. An echo is merely a sound, a series of air
vibrations, bounced back to us by something at a distance. It takes
time for the vibration which we start to reach the wall or cliff that
bounces it back, and it takes as much more time for the returning
vibration to reach our ears. So you have plenty of time to finish your
shout before the sound bounces back again. The next experiment shows
pretty well how the waves, or vibrations, of sound are reflected; only
in the experiment we use waves of water because they go more slowly
and we can watch them.

    EXPERIMENT 59. Fill the long laboratory sink (or the bathtub
    at home) half full of water and start a wave from one end.
    Watch it move along the side of the sink. Notice what happens
    when it reaches the other end.

Air waves do the same thing; when they strike against a flat surface,
they bounce back like a rubber ball. If you are far enough away from
a flat wall or cliff, and shout, the sound (the air vibrations you
start) is reflected back to you and you hear the echo. But if you are
close to the walls, as in an empty room, the sound _reverberates_; it
bounces back and forth from one wall to the other so rapidly that no
distinct echo is heard, and there is merely a confusion of sound.

[Illustration: FIG. 102. When the wave reaches the end of the sink, it
is reflected back. Sound waves are reflected in the same way.]

When you drop a pebble in water, the ripples spread in all directions.
In the same way, when you make a sound in the open air, the air waves
spread in all directions. But when you shout through a megaphone the
air waves are all concentrated in one direction and consequently they
are much stronger in that direction. However, while the megaphone
intensifies sound, the echoing from the sides of the megaphone makes
the sound lose some of its distinctness.

WHY IT IS HARD TO UNDERSTAND A SPEAKER IN AN EMPTY HALL. A speaker can
be heard much more easily in a room full of people than in an empty
hall. The sound does not reflect well from the soft clothes of the
audience and the uneven surfaces of their bodies, just as a rubber
ball does not bounce well in sand. So the sound does not reverberate
as in an empty hall.

    _APPLICATION 43._ Explain why a carpeted room is quieter than
    one with a bare floor; why you shout through your hands when
    you want to be heard at a distance.


INFERENCE EXERCISE

    Explain the following:

    261. It is harder to walk when you shuffle your feet.

    262. The air over a lamp chimney, or over a register in a
    furnace-heated house, is moving upward rapidly.

    263. The shooting of a gun sounds much louder within a room
    than it does outdoors.

    264. A drum makes a loud, clear sound when the tightened head
    is struck.

    265. The pull of the moon causes the ocean tides.

    266. Sand is sometimes put in the bottom of vases to keep them
    from falling over.

    267. It is difficult to understand clearly the words of one
    who is speaking in an almost empty hall.

    268. The ridges in a washboard help to clean the clothes that
    are rubbed over them.

    269. One kind of mechanical toy has a heavy lead wheel inside.
    When you start this to whirling, the toy runs for a long time.

    270. If you raise your finger slightly after touching the
    surface of water, the water comes up with your finger.


SECTION 30. _Pitch._

    What makes the keys of a piano give different sounds?

    Why does the moving of your fingers up and down on a violin
    string make it play different notes?

    Why is the whistle of a peanut roaster so shrill, and why is
    the whistle of a boat so deep?

Did you ever notice how tiresome the whistle on a peanut roaster gets?
Well, suppose that whenever you spoke you had to utter your words in
exactly that pitch; that every time a car came down the street its
noise was like the whistle of the peanut roaster, only louder; that
every step you took sounded like hitting a bell of the same pitch;
that when you went to the moving-picture theater the orchestra played
only the one note; that when any one sang, his voice did not rise and
fall; in short, that all the sounds in the world were in one pitch.
That is the way it would be if different kinds of air vibrations did
not make different kinds of notes,--if there were no differences in
pitch.

PITCH DUE TO RAPIDITY OF VIBRATION. When air vibrations are slow,--far
apart,--the sound is low; when they are faster, the sound is higher;
when they are very quick indeed, the sound is very shrill and high. In
various ways, as by people talking and walking and by the running of
street cars and automobiles, all sorts of different vibrations are
started, giving us a pleasant variety of high and low and medium
pitches in the sounds of the world around us.

An experiment will show how pitch varies and how it is regulated:

    EXPERIMENT 60. Move the slide of an adjustable tuning fork
    well up from the end of the prongs, tap one prong lightly on
    the desk, and listen. Move the slide somewhat toward the
    end of the prongs, and repeat. Is a higher or a lower sound
    produced as the slide shortens the length of the prongs?

    Whistle a low note, then a high one. Notice what you do with
    your lips; when is the opening the smaller? Sing a low note,
    then a high one. When are the cords in your throat looser?
    Fill a drinking glass half full of water, and strike it. Now
    pour half the water out, and strike the glass again. Do you
    get the higher sound when the column of water is shorter or
    when it is longer? Stretch a rubber band across your thumb and
    forefinger. Pick the band as you make it tighter, not making
    it longer, but pulling it tighter with your other fingers.
    Does it make a higher or a lower sound as you increase the
    tightness? Stretch the band from your thumb to your little
    finger and pick it; now put your middle finger under the band
    so as to divide it in halves, and pick it again. Does a short
    strand give a higher or lower pitch than a long strand?

[Illustration: FIG. 103. When the prongs of the tuning fork are made
longer or shorter, the pitch of the sound is changed.]

A violinist tunes his violin by tightening the strings; the tighter
they are and the thinner they are, the higher the note they give.
Some of the strings are naturally higher than others; the highest is
a smaller, finer string than the lowest. When the violinist plays, he
shortens the strings by holding them down with his fingers, and the
shorter he makes them the higher the note. A bass drum is much larger
than a high-pitched kettledrum. The pipes of an organ are long and
large for the low notes, shorter and smaller for the high ones.

In general, the longer or larger the object is that vibrates, the
slower the rate of vibration and consequently the lower the pitch. But
the shorter or finer the object is that vibrates, the higher the rate
of vibration and the higher the pitch.

All musical instruments contain devices which can be made to
vibrate,--either strings or columns of air, or other things that swing
to and fro and start waves in the air. And by tightening them, or
making them smaller or shorter, the pitch can be made higher; that is,
the number of vibrations to each second can be increased.

    _APPLICATION 44._ Explain why a steamboat whistle is usually
    of much lower pitch than is a toy whistle; why a banjo player
    moves his fingers toward the drum end of the banjo when he
    plays high notes; why the sound made by a mosquito is higher
    in pitch than that made by a bumblebee.

    _APPLICATION 45._ A boy had a banjo given him for Christmas.
    He wanted to tune it. To make a string give a higher note,
    should he have tightened or loosened it? Or could he have
    secured the same result by moving his finger up and down the
    string to lengthen or shorten it?

    _APPLICATION 46._ A man was tuning a piano for a concert. The
    hall was cold, yet he knew it would be warm at the time of the
    concert. Should he have tuned the piano to a higher pitch than
    he wanted it to have on the concert night, to the exact pitch,
    or to a lower pitch?


INFERENCE EXERCISE

    Explain the following:

    271. A cowboy whirls his lasso around and around his head
    before he throws it.

    272. Furnaces are always placed in the basements of buildings,
    never on top floors.

    273. A rather slight contraction of a muscle lifts your arm a
    considerable distance.

    274. A player on a slide trombone changes the pitch of the
    notes by lengthening and shortening the tube while he blows
    through it.

    275. Rain runs off a tar roof in droplets, while on shingles
    it soaks in somewhat and spreads.

    276. There is a sighing sound as the wind blows through the
    branches of trees, or through stretched wires or ropes.

    277. Sometimes a very violent noise breaks the membrane in the
    drum of a person's ear.

    278. As a street car goes faster and faster, the hum of its
    motor is higher and higher.

    279. If a street is partly dry, the wet spots shine more than
    the dry spots do.

    280. Molten type metal, when poured into a mold, becomes hard,
    solid type when it cools.




CHAPTER SEVEN

MAGNETISM AND ELECTRICITY


SECTION 31. _Magnets; the compass._


    What makes the needle of a compass point north?

    What causes the Northern Lights?

For many hundreds of years sailors have used the compass to determine
directions. During all this time men have known that one point of the
needle always swings toward the north if there is no iron near to pull
it some other way, but until within the past century they did not know
why. Now we have found the explanation in the fact that the earth is
a great big magnet. The experiment which follows will help you to
understand why the earth's being a magnet should make the compass
needle point north and south.

    EXPERIMENT 61. Lay a magnetic compass flat on the table.
    Notice which point swings to the north. Now hold a horseshoe
    magnet, points down, over the compass. Turn the magnet around
    and watch the compass needle; see which end of the magnet
    attracts the north point; hold that end of it toward the south
    point and note the effect. Hold the magnet, ends up, under the
    table directly below the compass and turn the magnet, watching
    the compass needle.

The earth is a magnet, and it acts just as your magnet does: one end
attracts one point of the compass, and the other end attracts the
other point. That ought to make it clear why the compass points
north. But how is the compass made? The next experiment will show this
plainly.

    EXPERIMENT 62. Take a long shoestring and make a loop in one
    end of it. Slip the magnet through the loop and suspend it,
    ends down. Fasten the shoestring to the top of a doorway so
    that the magnet can swing easily. Steady the magnet and let it
    turn until it comes to a rest. Mark the end that swings to the
    north. Turn this end around to the south; let go and watch it.
    Place the magnet the other way around in the loop so that you
    can be sure that it is not twisting of the shoestring that
    makes the magnet turn in this direction.

    Now stroke a needle several times along one arm of the magnet,
    _always in the same direction_, as shown in Figure 105. Hold
    the needle over some iron filings or touch any bit of iron
    or steel with it. What has the needle become? Lay it on a
    cardboard milk-bottle top of the flat kind, and on that float
    it in the middle of a glass or earthenware dish of water.
    Notice which end turns north. Turn this end to the south and
    see what happens. Hold your magnet, ends up, under the dish,
    and turn the magnet. What does the needle do?

[Illustration: FIG. 104. The compass needle follows the
magnet.]

Now it should be easy to understand why the compass points north. One
end of any magnet pulls on _one_ end of another magnet and drives the
_other_ end away. The earth is a big magnet. So if you make a magnet
and balance it in such a way that it is free to swing, the north end
of the big earth magnet pulls one end of the little magnet toward
it and pushes the other end away. Therefore one end of your compass
always points north.

OTHER EFFECTS OF THE EARTH'S MAGNETISM. Another interesting effect
of the earth's being a big magnet is to be seen if you lay a piece of
steel so that it points north and south, and then pound it on one end.
It becomes magnetized just as your needle became magnetized when it
was rubbed on the small magnet.

[Illustration: FIG. 105. Magnetizing a needle.]

[Illustration: FIG. 106. A compass made of a needle and a piece of
cardboard.]

And still another effect of the earth's magnetism is this: Tiny
particles of electricity, called _electrons_, are probably shooting
through space from the sun. It is believed that as they come near the
earth, the magnetism of the north and south polar regions attracts
them toward the poles, and that as they rush through the thin, dry
upper air, they make it glow. And this is probably what causes the
Northern Lights or Aurora Borealis.

WHAT HAPPENS WHEN A NEEDLE IS MAGNETIZED. The reason that a needle
becomes magnetic if it is rubbed over a magnet is probably this:
Every molecule of iron may be an extremely tiny magnet; if it is, each
molecule has a north and south pole like the needle of a compass. In
an ordinary needle (or in any unmagnetized piece of iron or steel)
these molecules would be facing every way, as shown in Figure 107.

[Illustration: FIG. 107. Diagram of molecules in unmagnetized iron.
The north and south poles of the molecules are supposed to be pointing
in all directions.]

[Illustration: FIG. 108. Diagram of magnetized iron. The north and
south poles of the molecules are all supposed to point in the same
direction.]

But when a piece of steel or iron that is already magnetized is
brought near the unmagnetized needle, all the north poles of the
molecules of the needle are pulled in the same direction--it is almost
like combing tangled hair to stroke a needle over a magnet. Then
the molecules are arranged more as shown in Figure 108. When all the
molecules, each of which is a tiny magnet, pull in the same direction,
they make a strong magnet, and they magnetize any iron that comes near
them just as they were magnetized.

Steel will stay magnetized a long time; but ordinary soft iron loses
magnetism almost as soon as another magnet is taken away from it,--the
molecules become all disarranged again.

In a later section you will find that whenever electricity flows
through a wire that is coiled around a piece of iron, the iron becomes
magnetized just as when it is rubbed with a magnet.

    _APPLICATION 47._ An explorer lost his compass. In clear
    weather he could tell the directions by the sun and stars, but
    in cloudy weather he was badly handicapped. He had with him a
    gun, plenty of ammunition, a sewing kit, a hunting knife, and
    some provisions. How could he have made a compass?


INFERENCE EXERCISE

    Explain the following:

    281. Snow turns to water in the first warm weather.

    282. A person's face looks ghastly by the greenish light of a
    mercury-vapor lamp.

    283. If a red-hot coal is touched with a cold poker, the coal
    turns black at the place touched.

    284. Stereopticon slides are put in upside down, yet the
    picture on the screen is right side up.

    285. If the vocal cords of your throat did not vibrate, you
    could not talk out loud.

    286. A watch is sometimes put out of order if it is held near
    a magnet.

    287. The water will be no higher on the inside of a leaky boat
    than it is on the outside.

    288. A bass viol is considerably larger than a violin.

    289. Ships that are used by men testing the earth's magnetism
    carry very sensitive compasses. Explain why such ships are
    made entirely of wood and brass.

    290. Thunder rolls; that is, after the first peal there is a
    reverberating sound that becomes less and less distinct.


SECTION 32. _Static electricity._

    What is electricity?

    What makes thunder and lightning?

    Why does the barrel or cap of a fountain pen pick up small
    bits of paper after it has been rubbed on your coat sleeve?

    Why do sparks fly from the fur of a cat when you stroke it in
    the dark?

The Greeks, 2000 years ago, knew that there was such a thing as
electricity, and they used to get it by rubbing amber with silk. In
the past century men have learned how to make electricity do all sorts
of useful work: making boats and cars and automobiles go, ringing
bells, furnishing light, and, in the telephone and telegraph, carrying
messages. But no one knew what electricity really was until, within
the last 25 years, scientists found out.

ATOMS AND ELECTRONS. When we talked about molecules, we said that they
were as much smaller than a germ as a germ is smaller than a mountain.
Well, a molecule is made up, probably, of some things that are much
smaller still,--so small that people thought that nothing could be
smaller. Those unthinkably tiny things are called _atoms_; you will
hear more about them when you come to the parts of this book that tell
about chemistry.

But if you took the smallest atom in the world and divided it into
1700 pieces, each one of these would be about the size of a piece of
electricity.

Electricity is made up of the tiniest things known to man--things so
small that nobody really can think of their smallness. These little
pieces of electricity are called _electrons_, and for all their
smallness, scientists have been able to find out a good deal about
them. They have managed to get one electron all by itself on a droplet
of oil and they have seen how it made the oil behave. Of course
they could not see the electron, but they could tell from various
experiments that they had just one. Scientists know how many trillions
of electrons flow through an incandescent electric lamp in a second
and how many quadrillions of them it would take to weigh as much as a
feather. They know what the electrons do when they move, how fast they
can move, and what substances let electrons move through them easily
and what substances hold them back; and they know perfectly well
how to set them in motion. How the scientists came to know all these
things you will learn in the study of physics; it is a long story.
But you can find out some things about electrons yourself. The first
experiment is a simple one such as the Greeks used to do with amber.

    EXPERIMENT 63. Rub a hard rubber comb on a piece of woolen
    cloth. The sleeve of a woolen coat or sweater will do. Rub the
    comb quickly in the same direction several times. Now hold it
    over some small bits of paper or sawdust. What does it do
    to them? Hold it over some one's hair. The rest of this
    experiment will work well only on cool, clear days. Rub the
    comb again, a dozen or more times in quick succession. Now
    touch it gently to the lobe of your ear. Do you hear the snap
    as the small spark jumps from the comb to your ear?

    Pull a dry hair out of your head and hold it by one end.
    Charge your comb by rubbing it again, and bring it near the
    loose end of the hair. If the end of the hair clings to the
    comb at first, leave it clinging until it flies off. Now try
    to touch the hair with the comb. Next, pinch the end of the
    hair between your thumb and finger and again bring the
    charged comb near it. Is the hair attracted or repelled? After
    touching the comb what does it do?

    You can get the same effects by rubbing glass or amber on
    silk.

[Illustration: FIG. 109. When the comb is rubbed on the coat, it becomes
charged with electricity.]

OBJECTS NEGATIVELY AND POSITIVELY CHARGED WITH ELECTRICITY. There are
probably electrons in everything. But when there is just the usual
number of electrons in an object, it acts in an ordinary way and we
say that it is not charged with electricity. If there are more than
the usual number of electrons on an object, however, we say that it is
_negatively charged_, or that it has a negative charge of electricity
on it. But if there are fewer electrons than usual in an object, we
say that it has a positive charge of electricity on it, or that it is
_positively charged_.

You might expect a "negative charge" to indicate fewer electrons than
usual, not more. But people called the charge "negative" long before
they knew anything about electrons; and it is easier to keep the
old name than to change all the books that have been written about
electricity. So we still call a charge "negative" when there are
unusually _many_ electrons, and we call it "positive" when there are
unusually _few_. A _negative charge_ means that more electrons are
present than usual. A _positive charge_ means that fewer electrons are
present than usual.

[Illustration: FIG. 110. The charged comb picks up pieces of paper.]

Before you rubbed your comb on wool, neither the comb nor the wool
was charged; both had just the usual number of electrons. But when you
rubbed them together, you rubbed some of the electrons off the wool on
to the comb. Then the comb had a negative charge; that is, it had too
many electrons--too many little particles of electricity.

When you brought the comb near the hair, the hair had fewer electrons
than the comb. Whenever one object has more electrons on it than
another, the two objects are pulled toward each other; so there was
an attraction between the comb and the hair, and the hair came over to
the comb. As soon as it touched the comb, some of the extra electrons
jumped from the comb to the hair. The electrons could not get off the
hair easily, so they stayed there. Electrons repel each other--drive
each other away. So when you had a number of electrons on the end of
the comb and a number on the end of the hair, they pushed each other
away, and the hair flew from the comb. But when you pinched the
hair, the electrons could get off it to your moist hand, which
lets electricity through it fairly easily. Then the comb had extra
electrons on it and the hair did not; so the comb pulled the hair over
toward it again.

When you brought the charged comb near your ear, some of the electrons
on the comb pushed the others off to your ear, and you heard them snap
as they rushed through the air, making it vibrate.

HOW LIGHTNING AND THUNDER ARE CAUSED. In thunderstorms the strong
currents of rising air blow some of the forming raindrops in the
clouds into bits of spray. The tinier droplets get more than their
share of electrons when this happens and are carried on up to higher
clouds. In this way clouds become charged with electricity. One
cloud has on it many more electrons than another cloud that is made,
perhaps, of lower, larger droplets. The electricity leaps from the
cloud that has the greater number of electrons to the cloud that has
the less number, or it leaps from the heavily charged cloud down to a
tree or house or the ground. You see the electricity leap and call it
_lightning_. Much more leaps, however, than leaped from the comb to
your ear, and so it makes a very much louder snap. The snap is caused
in this way: As the electric spark leaps through the air, it leaves
an empty space or vacuum immediately behind it. The air from all sides
rushes into the vacuum and collides there; then it bounces back. This
again leaves a partial vacuum; so the air rushes in once more, coming
from all sides at once, and again bounces back. This starts the air
vibrations which we call _sound_. Then the sound is echoed from cloud
to cloud and from the clouds to the earth and back again, and we call
it _thunder_.

The electricity you have been reading about and experimenting with in
this section is called _static electricity_. "Static" means standing
still. The electricity you rubbed up to the surface of the comb or
glass stayed still until it jumped to the bit of paper or hair; then
it stayed still on that. This was the only kind of electricity most
people knew anything about until the nineteenth century; and it is
not of any great use. Electricity must be flowing through things to do
work. That is why people could not invent electric cars and electric
lights and telephones before they knew how to make electricity flow
steadily rather than just to stand still on one thing until it jumped
across to another and stood there. In the next chapter we shall take
up the ways in which electrons are made to flow and to do work.

    _APPLICATION 48._ Explain why the stroking of a cat's back
    will sometimes cause sparks and make the cat's hairs stand
    apart; why combing sometimes makes your hairs fly apart. Both
    of these effects are best secured on a dry day, because on a
    damp day the water particles in the air will let the electrons
    pass to them as fast as they are rubbed up to the surface of
    the hair.


INFERENCE EXERCISE

    Explain the following:

    291. If you shuffle your feet on a carpet in clear, cold
    weather and then touch a person's nose or ear, a slight spark
    passes from your finger and stings him.

    292. If you stay out in the cold long, you get chilled
    through.

    293. The air and earth in a greenhouse are warmed by the sun
    through the glass even when it is cold outside and when the
    glass itself remains cold.

    294. When you hold a blade of grass taut between your thumbs
    and blow on it, you get a noise.

    295. Shadows are usually black.

    296. Some women keep magnets with which to find lost needles.

    297. You can grasp objects much more firmly with pliers than
    with your fingers.

    298. If the glass in a mirror is uneven, the image of your
    face is unnatural.

    299. A sweater clings close to your body.

    300. Kitchens, bathrooms, and hospitals should have painted
    walls.




CHAPTER EIGHT

ELECTRICITY


SECTION 33. _Making electricity flow._

    What causes a battery to produce electricity?

    What makes electricity come into our houses?

The kind of electricity you get from rubbing (friction) is not of much
practical use, you remember. Men had to find a way to get a steady
current of electricity before they could make electricity do any work
for them. The difference between static electricity--when it leaps
from one thing to another--and flowing electricity is a good deal like
the difference between a short shower of rain and a river. Both rain
and river are water, and the water of each is moving from one place to
another; but you cannot get the raindrops to make any really practical
machine go, while the rivers can do real work by turning the wheels in
factories and mills.

Within the past century two devices for making electricity flow and
do work have been perfected: One of these is the electric battery; the
other is the dynamo.

THE ELECTRIC BATTERY. A battery consists of two pieces of different
kinds of metal, or a metal and some carbon, in a chemical solution.
If you hang a piece of zinc and a carbon, such as comes from an arc
light, in some water, and then dissolve sal ammoniac in the water, you
will have a battery. Some of the molecules of the sal ammoniac divide
into two parts when the sal ammoniac gets into the water, and the
molecules continue to divide as long as the battery is in use or
until it "wears out." One part of each molecule has an unusually large
number of electrons; the other part has unusually few. The parts with
unusually large numbers of electrons gather around the zinc; so the
zinc is _negatively charged_,--it has more than the ordinary number of
electrons. The part of the sal ammoniac with unusually few electrons
goes over to the carbon; so the carbon is _positively charged_,--it
has fewer than the ordinary number of electrons.

MAKING THE CURRENT FLOW. Now if we can make some kind of bridge
between the carbon and the zinc, the electrons will flow from the
place where there are many to the place where there are few. Electrons
can flow through copper wire very easily. So if we fasten one end
of the copper wire to the carbon and the other end to the zinc, the
electrons will flow from the zinc to the carbon as long as there are
more electrons on the zinc; that is, until the battery wears out.
Therefore we have a steady flow of electricity through the wire. While
the electricity is flowing from one pole to the other, we can make it
do work.

    EXPERIMENT 64. Set up two or three Samson cells. They consist
    of a glass jar, an open zinc cylinder, and a smaller carbon
    cylinder. Dissolve a little over half a cup of sal ammoniac
    in water and put it into the glass jar; then fill the jar with
    water up to the line that is marked on it. Put the carbon and
    zinc which are attached to the black jar cover into the jar.
    Be careful not to let the carbon touch the zinc. One of these
    cells will probably not be strong enough to ring a doorbell
    for you; so connect two or three together in series as
    follows:

    Fasten a piece of copper wire from the carbon of the first
    cell to the zinc of the second. If you have three cells,
    fasten another piece of wire from the carbon of the second
    cell to the zinc of the third, as shown in Figure 111.

    Fasten one end of a copper wire to the zinc of the first
    cell and the other end of this wire to one binding post of an
    electric bell. Fasten one end of another piece of copper wire
    to the carbon of the third cell, if you have three, and touch
    the other end of this wire to the free binding post of the
    electric bell. If you have everything connected rightly, the
    bell should ring.

[Illustration: FIG. 111. A wet battery of three cells connected to ring
a bell.]

DIFFERENT KINDS OF BATTERIES. There are many different kinds of
batteries. The one you have just made is a simple one frequently used
for doorbells. Other batteries are more complicated. Some are made
with copper and zinc in a solution of copper sulfate; some, even, are
made by letting electricity from a dynamo run through a solution from
one lead plate to another until a chemical substance is stored on one
of them; then, when the two lead plates are connected by a wire, the
electrons run from one to the other. This kind of battery is called a
_storage battery_, and it is much used in submarines and automobiles.

[Illustration: FIG. 112. A battery of three dry cells.]

But all the different batteries work on the same general principle: A
chemical solution divides into two parts, one with many electrons and
the other with a less number. One part of the solution gathers on one
pole (piece of metal in the solution) and charges it positively; the
other part gathers on the other pole and charges it negatively. Then
the electricity flows from one pole to the other.

POSITIVE AND NEGATIVE POLES. Before people knew anything about
electrons, they knew that electricity flowed from one pole of a
battery to the other. But they always said that it flowed from the
carbon to the zinc; and they called the carbon the positive pole and
the zinc the negative. Although we now know that the electrons flow
from the zinc to the carbon, it is much more convenient to use the old
way of speaking, as was explained on page 199. Practically, it makes
no difference which way the electrons are going as long as a current
of electricity is flowing through the wire from one pole of the
battery to the other pole. So every one speaks of electricity as
flowing from the positive pole of a battery (usually the carbon
or copper) to the negative pole (usually the zinc), although the
electrons actually move in the other direction.

[Illustration: FIG. 113. A storage battery.]

Batteries make enough electricity flow to do a good deal of work. But
they are rather expensive, and it takes a great many to give a flow of
electricity sufficient for really heavy work, such as running street
cars or lighting a city. Fortunately there is another way of getting
large amounts of electricity to flow. This is by means of dynamos.

HOW A DYNAMO MAKES A CURRENT FLOW. To understand a dynamo, you must
first realize that there are countless electrons in the world--perhaps
all things are made entirely of them. But you remember that when we
want to get these electrons to do work we must make them flow. This
can be done by spinning a loop of wire between the poles of a magnet.
Whenever a loop of wire is turned between the two poles of a magnet,
the magnetism pushes the electrons that are already in the wire around
and around the loop. As long as we keep the loop spinning, a current
of electricity flows.

[Illustration: FIG. 114. Spinning loops of wire between the poles of a
magnet causes a current of electricity to flow through the wire.]

If only one loop of wire is spun between the poles of a magnet, the
current is very feeble. If you loop the wire around twice, as shown in
Figure 114, the magnet acts on twice as much of the wire at the same
time; so the current is stronger. If a very long piece of wire is used
and is looped around many times, and the whole coil is spun rapidly
between the poles of a powerful magnet, myriads of the electrons
in the wire rush around and around the loops--a powerful current of
electricity flows through the wire.

[Illustration: FIG. 115. The more loops there are, the stronger the
current.]

Now suppose you bring one loop of the long wire out, as shown in
Figure 115, and suppose you spin the rest of the loops between the
poles of the magnet. Then, to flow through the loops by the magnet the
electricity will have to go clear out through the long loop and back
again. While it is flowing through this long loop, we can make it
work. We can cut the long loop and attach one broken end to one part
of an electric lamp and the other end to the other part, so that the
electricity has to flow through the lamp in order to get back to the
spinning coil of wire, as shown in Figure 116. Such an arrangement as
this is really an extremely simple dynamo.

[Illustration: FIG. 116. If the electricity passes through a lamp on
its way around the circuit the filament of the lamp glows.]

[Illustration: FIG. 117. A dynamo in an electric light plant.]

You could make a dynamo that would actually work, by arranging such an
apparatus so that the coil would spin between the poles of the
magnet. But of course the big commercial dynamos are very much more
complicated in their construction. Figure 116 shows only the general
principle on which they work. The main point to note is that by
spinning a coil of wire between the poles of a magnet, you can make
electricity flow rapidly through the wire. And it is in this way that
most of the electricity we use is made.

The power spinning the coil of wire is sometimes steam, and sometimes
gasoline or distillate; and water power is very often used. A
large amount of our electricity comes from places where there are
waterfalls. Niagara, for instance, turns great dynamos and generates
an enormous amount of electricity.

WHY MANY AUTOMOBILES HAVE TO BE CRANKED. In an automobile, the magneto
is a little dynamo that makes the sparks which explode the gasoline.
While the automobile is going the engine spins the coil of wire
between the magnets, but at starting you have to spin the coil
yourself; and doing that is called "cranking" the automobile.
"Self-starters" have a battery and motor to spin the coil for you
until the engine begins to go; then the engine turns the coil of the
magneto.

HOW OLD-FASHIONED TELEPHONES ARE RUNG. The old-fashioned telephones,
still often used in the country, have little cranks that you turn to
ring for central. The crank turns a coil of wire between the poles of
the magnet and generates the electricity for ringing the bell.
These little dynamos, like those in automobiles, are usually called
magnetos.

[Illustration: FIG. 118. The magneto in an automobile is a small
dynamo.]

ALTERNATING CURRENT. For the sake of simplicity and convenience we
speak of electricity as always flowing in through one wire and out
through the other. With batteries this is actually the case. It is
also the case where people have what is called _direct-current_ (d.
c.) electricity. But it is easier to raise and lower the voltage
(pressure) of the current if instead of being direct it is
_alternating_; that is, if for one instant the electricity flows in
through one wire and out through the other, the next instant flowing
the opposite way, then the first way again, and so on. This kind of
current is called _alternating current_ (a. c.), because the current
alternates, coming in the upper wire and out of the lower for a
fraction of a second; then coming in the lower and out of the upper
for the next fraction of a second; then coming in the upper again
and out of the lower for a fraction of a second; and so on, back
and forth, all the time. For heating and lighting, this alternating
current is just as good as the direct current, and it is probably what
you have in your own home. For charging storage batteries and making
electromagnets, separating water into two gases, and for running
certain kinds of motors, however, the direct current is necessary.
Find out whether the current in your laboratory is direct or
alternating.

    _APPLICATION 49._ Explain why we need fuel or water to
    generate large currents of electricity; how we can get small
    amounts of electricity to flow without using dynamos; why
    automobiles must be cranked unless they have batteries to
    start them.


INFERENCE EXERCISE

    Explain the following:

    301. Mexican water jars are made of porous clay; the water
    that seeps through keeps the water inside cool.

    302. When you crank an automobile, electricity is generated.

    303. Potatoes will not cook any more quickly in water that is
    boiling violently than in water that is boiling gently.

    304. When you brush your hair on a winter morning, it
    sometimes stands up and flies apart more and more as you
    continue to brush it.

    305. You cannot see a person clearly through a ground-glass
    window, although it lets most of the light through.

    306. There is a layer of coarse, _light-colored_ gravel over
    the tar on roofs, to keep the tar from melting.

    307. It is very easy to slip on a well-waxed hardwood floor.

    308. If you have a silver filling in one of your teeth and
    you touch the filling with a fork or spoon, you get a slight
    shock.

    309. You can shake a thing down into a bottle when it will not
    slip down by itself.

    310. If you rub a needle across one pole of a magnet three or
    four times in the same direction, then float it on a cork in
    water one end of the needle will point north.


SECTION 34. _Conduction of electricity._

    How does electricity travel?

    Why do you get a shock if your hands are wet when you touch a
    live wire?

If you were to use a piece of string instead of a copper wire to go
from one pole of a battery to another or to spin between the poles
of the magnet of the dynamo, you could get no flow of electricity to
speak of. Electrons do not flow through string easily, but they flow
through a copper wire very easily. Anything that carries, or conducts,
electricity well is called a _good conductor_. Anything that
carries it poorly is called a _poor conductor_. Anything that
allows practically no electricity to pass through it is called an
_insulator_.

    EXPERIMENT 65.[5] Turn on an electric lamp. Turn it off by
    opening the knife switch. Cover the blade of the knife switch
    with a fold of paper and close it. Will the lamp glow? Try a
    fold of dry cloth; a fold of the same cloth wet. Connect the
    blade to the slot with a piece of iron; with a piece of glass;
    with porcelain; with rubber; with dry wood; with wood that is
    soaking wet; with a coin. Which of these are good conductors
    of electricity? Which could be used as insulators?

[Footnote 5: Read footnote, page 226, before doing this experiment.]

[Illustration: FIG. 119. Electricity flows through the coin.]

HOW YOU CAN GET AN ELECTRICAL SHOCK. A person's body is not a very
good conductor of electricity, but will conduct it somewhat. When
electricity goes through your body, you get a shock. The shock from
the ordinary current of electricity, 110 volts, is not enough to
injure you at all; in fact, if you were standing on dry wood, it would
be _safe_, although you would get a slight shock, to connect the blade
of a knife switch to the slot of the switch, through your hand or
body. Your body would not allow enough current to pass through it to
light the lamp. Stronger currents, like those of power lines and even
trolley wires, are extremely dangerous.

All the electric wires entering your house are made of copper. They
are all covered with cloth and rubber and are fastened with glass
or porcelain knobs. The reason is simple: Copper and practically all
other metals are very good conductors of electricity; that is, they
allow electricity to pass through them very easily. Cloth, rubber,
glass, and porcelain are very poor conductors, and they are therefore
used as insulators,--to keep the electricity from going where you do
not want it to go.

[Illustration: FIG. 120. Will electricity go through the glass?]

    EXPERIMENT 66. To each binding post of an electric bell fasten
    a piece of insulated copper wire with bare ends and at least 4
    feet long. Connect the free end of one of these wires with one
    pole of a battery, using a regular laboratory battery or one
    you made yourself. Attach one end of another piece of wire
    a foot or so long, with bare ends, to the other pole of the
    battery. Touch the free end of this short wire to the free end
    of the long wire, as shown in Figure 120. Does the bell ring?
    If it does not, something is wrong with the connection or with
    the battery; fix them so that the bell will ring. Now leave a
    gap of about an inch between the free end of the long wire
    and the free end of the short wire. Try making the electricity
    flow from the short wire into the long one through a number of
    different things, such as string, a key, a knife, a piece of
    glass tubing, wet cloth, dry cloth, rubber, paper, a nail,
    a dish of mercury (dip the ends of the wire into the dish so
    that they both touch the mercury at the same time), a dish of
    water, a stone, a pail, a pin, and anything else that you may
    like to try.

[Illustration: FIG. 121. Electrical apparatus: _A_, plug fuse; _B_,
cartridge fuse; _C_, knife switch; _D_, snap switch; _E_, socket with
nail plug in it; _F_, fuse gap; _G_, flush switch; _H_, lamp socket;
_I_, _J_, _K_, resistance wire.]

Each thing that makes the bell ring is a good conductor. Each one that
will not make it ring is a poor conductor or an insulator. Make a list
of the things you have tried; in one column note the good conductors,
and in another column note the insulators and poor conductors.

The water and wet cloth did not ring the bell, but this is because the
pressure or voltage of the electricity in the batteries is not very
high. In dealing with high-power wires it is much safer to consider
water, or anything wet, as a pretty good conductor of electricity.
Absolutely pure, distilled water is an extremely poor conductor; but
most water has enough minerals dissolved in it to make it conduct
electricity fairly well. In your list you had better put water and wet
things in the column with the good conductors.

    _APPLICATION 50._ Robbers had cut the telegraph line between two
    railroad stations (Fig. 122). The broken ends of the wire fell to
    the ground, a number of feet apart. A farmer caught sight of the men
    speeding away in an automobile and he saw the cut wires on the
    ground. He guessed that they had some evil purpose and decided to
    repair the damage. He could not bring the two ends of the wire
    together. He ran to his barn and found the following things there:

    A ball of cord, a pickax, a crowbar, some harness, a wooden wagon
    tongue, a whip, a piece of iron wire around a bale of hay (the wire
    was not long enough to stretch the whole distance between the two
    ends of the telegraph wire, even if you think he might have used it
    to patch the gap), a barrel with four iron hoops, and a rope.

    Which of these things could he have made use of in connecting the
    broken ends of the telegraph wire?

[Illustration: FIG. 122. Which should he choose to connect the broken
wires?]

    _APPLICATION 51._ A man was about to put in a new socket for an
    electric lamp in his home. He did not want to turn off the current
    for the whole house, as it was night and there was no gas to furnish
    light while he worked.

    "I've heard that if you keep your hands wet while you work, the film
    of water on them will keep you from getting a shock," his wife said.

    "Don't you wet your hands, Father," said his 12-year-old boy; "keep
    them dry, and handle the wires with your pliers, so that you won't
    have to touch it."

    "I advise you to put on your canvas gloves while you work; then you
    can't get a shock," added another member of the family.

    "That's a good idea," said the wife, "but wet the gloves, then you
    will have the double protection of the water and the cloth."

    The man laughed and went to work on the socket. He did not get a
    shock. Which advice, if any, do you think he followed?


INFERENCE EXERCISE

    Explain the following:

    311. A red postage stamp looks greenish gray in the green
    light of a mercury-vapor lamp.

    312. Cracks are left between sections of the roadbed in cement
    auto highways.

    313. Electricity goes up a mountain through a wire.

    314. It is impossible to stand sidewise against a wall on one
    foot, when that foot touches the wall.

    315. A charged storage battery will run an electric
    automobile.

    316. An empty house is noisier to walk in and talk in than is
    a furnished one.

    317. Lightning rods are made of metal.

    318. It is harder to hold a frying pan by the end of the
    handle than by part of the handle close to the pan.

    319. Diamonds flash many colors.

    320. In swimming, if you have hold of a fastened rope and try
    to pull it toward you, you go toward it.


SECTION 35. _Complete circuits._

    Why does a doorbell ring when you push a button?

    Why is it that when you touch one electric wire you feel no
    shock, while if you touch two wires you sometimes get a shock?

    When a wire is broken in an electric light, why does it not
    light?

Suppose you have some water in an open circular trough like the one
shown in Figure 123. Then suppose you have a paddle and keep pushing
the water to your right from one point. The water you push pushes
the water next to it, that pushes the water next to it, and so on all
around the trough until the water just behind your paddle pushes in
toward the paddle; the water goes around and around the trough in
a complete circuit. There never is too much water in one place; you
never run out of water. But then suppose a partition is put across the
trough somewhere along the circuit. When the water reaches that, it
cannot pass; it has no place to flow to, and the current of water
stops.

THE ELECTRIC CIRCUIT. The flow of electricity in an electric circuit
may be compared to the flow of the water in the tank we have been
imagining. The long loop of wire extending out from the dynamo to
your house and back again corresponds to the tank. The electricity
corresponds to the water. Your dynamo pushes the electricity around
and around the circuit, as the paddle pushes the water. But let some
one break the circuit by putting a partition between two parts of
it, and the electricity immediately stops flowing. One of the most
effective partitions we can put into an electric circuit is a gap of
air. It is very difficult for any electricity to flow through the air;
so if we simply cut the wire in two, electricity can no longer flow
from one part to the other, and the current is broken.

[Illustration: FIG. 123. Electricity flows around a completed circuit
somewhat as water might be made to flow around this trough.]

BREAKING AND MAKING THE CIRCUIT. The most convenient way to put an air
partition into an electric circuit and so to break it, or to close the
circuit again so it will be complete, is to use a switch.

    EXPERIMENT 67. In the laboratory, examine the three different
    kinds of switches where the electricity flows into the lamp
    and resistance wire and then out again. Trace the path the
    electricity must take from the wire coming into the building
    down to the first switch that it meets; then from one end
    of the wire through the brass or copper to which the wire is
    screwed, through the switch and on out into the end of the
    next piece of wire. Turn the first switch off and see how
    a partition of air is made between the place where the
    electricity comes in and the place where it would get out if
    it could. Turn the switch on and notice how this gives the
    electricity a complete path through to the next piece of wire.
    In this way follow the circuit on through all the switches to
    the electric lamp.

If you examine the socket into which the lamp screws and examine the
lamp itself, you will see that electricity which goes to the outer
part of the socket passes into the rim of the lamp; from here it goes
into one end of the filament. It passes through the filament to the
other end, which is connected to the little brass disk at the end of
the lamp. From this you can see that it goes into the center point
of the socket, and then on into the second wire that connects to the
socket. Trace the current on back through this other wire until you
see where this wire leads toward the dynamo. You should understand
that the electric lamp, the switches, the fuses, all things along the
circuit, are simply parts of the long loop from the dynamo, as shown
in Figure 124.

CONNECTING IN PARALLEL. The trouble with Figure 124 is that it is a
little too simple. From looking at it you might think that the loop
entered only one building. And it might seem that turning off one
switch would shut off the electricity all along the line. It would,
too, if the circuit were arranged exactly as shown above. To avoid
this, and for other reasons, the main loop from the dynamo has
branches so that the electricity can go through any or all of them at
the same time and so that shutting off one branch will not affect the
others. Electricians call this _connecting in parallel_; there are
many parallel circuits from one power house.

[Illustration: FIG. 124. Diagram of the complete circuit through the
laboratory switches.]

Figure 125 illustrates the principle just explained. As there
diagrammed, the electricity passes out from the dynamo along the lower
wire and goes down the left-hand wire of circuit _A_ through one of
the electric lamps that is turned on, and then it goes back through
the right-hand wire of the _A_ circuit to the upper wire of the
main circuit and then on back to the dynamo. But only a part of the
electricity goes through the _A_ circuit; part goes on to the _B_
circuit, and there it passes partly through the electric iron. Then it
goes back through the other wire to the dynamo. No electricity can get
through the electric lamp on the _B_ circuit, because the switch
to the lamp is open. The switch on the _C_ circuit is open; so no
electricity can pass through it.

The purpose of the diagram is to show that electricity from the
dynamo may go through several branch circuits and then get back to the
dynamo, and that shutting off the electricity from one branch circuit
does not shut it off from the others. And the purpose of this section
is to make it clear that electricity can flow only through a complete
circuit; it must have an unbroken path from the dynamo back to the
dynamo again or from one pole of the battery back to the other pole.
If the electricity does not have a complete circuit, it will not flow.

    _APPLICATION 52._ A small boy disconnected the doorbell
    batteries from the wires that ran to them, and when he wanted
    to put the wires back, he could not remember how they had been
    connected. He tried fastening both wires to the carbon part of
    the battery, connecting one wire to the carbon and one to the
    zinc, and connecting both to the zinc. Then he decided that
    one wire was all that had to be connected anyway, that the
    second was simply to make it stronger. Which of the ways he
    tried, if any, would have been right?

[Illustration: FIG. 125. Parallel circuits.]

[Illustration: FIG. 126. How should he connect them?]

    _APPLICATION 53._ Dorothy was moving. "When they took out our
    telephone," she said to her chum, Helen, "the electrician just
    cut the wires right off."

    "He must have turned off the electricity first," Helen
    answered, "or else it would all have run out of the cut ends
    of the wire and gone to waste."

    "Why, it couldn't," Dorothy said. "Electricity won't flow off
    into the air."

    "Of course it can if there is nothing to hold it in," Helen
    argued.

    Which was right?


INFERENCE EXERCISE

    Explain the following:

    321. It is very easy to get chilled when one is perspiring.

    322. Ice cream becomes liquid if you leave it in your dish too
    long.

    323. You should face forward when alighting from a street car.

    324. There are always at least two electric wires going into a
    building that is wired.

    325. Woolen sweaters keep you warm.

    326. Steel rails are not riveted to railroad ties but the
    spikes are driven close to each rail so that the heads hook
    over the edge and hold the rail down without absolutely
    preventing its movement forward and backward. Why should rails
    be laid in this way?

    327. The earth keeps whirling around the sun without falling
    into it, although the pull from the sun is very great.

    328. Electricity is brought down from power houses in the
    mountains by means of cables.

    329. White clothes are cooler than black when the person
    wearing them is out in the sun.

    330. All the street cars along one line are stopped when a
    trolley wire breaks.


SECTION 36. _Grounded circuits._

    Why can a bird sit on a live wire without getting a shock,
    while a man would get a shock if he reached up and took hold
    of the same wire?

We have just been laying emphasis on the fact that for electricity to
flow out of a dynamo or battery, it must have a complete circuit back
to the battery or dynamo. Yet only one wire is needed in order to
telegraph between two stations. Likewise, a single wire could be made
to carry into a building the current for electric lights. This is
because the ground can carry electricity.

If you make all connections from a battery or dynamo just as for any
complete circuit, but use the earth for one wire, the electricity will
flow perfectly well (Fig. 127). To connect an electric wire with the
earth, the wire must go down deep into the ground and be well packed
with earth; but since water pipes go down deep and the earth is
already packed around them, the most convenient way to ground a
circuit is to connect the wire that should go into the ground with the
water pipe. The next experiment, the grounding of a circuit, should be
done by the class with the help of the teacher.

[Illustration: FIG. 127. The ground can be used in place of a wire to
complete the circuit.]

EXPERIMENT 68. _Caution: Keep the switches turned off throughout this
experiment._[6]

[Footnote 6: All through this chapter it is assumed that the
electrical apparatus described in the appendix is being used. In this
apparatus all the switches are on one wire, the other wire being alive
even when the switches are turned off.]

    (a) Put a piece of fuse wire across the fuse gap. Screw the
    plug with nails in it into the lamp socket. Connect the bare
    end of a piece of insulated wire to the water faucet and touch
    the other end to one nail of the plug. If nothing happens,
    touch it to the other nail instead. The electricity has gone
    down into the ground through the water pipe, instead of into
    the other wire. The ground carries the electricity back to the
    dynamo just as a wire would.

    (b) Put a new piece of fuse wire across the gap. _Keep
    switches turned off._ Touch the brass disk at the bottom of
    an electric lamp to the nail which worked, and touch the wire
    from the faucet to the other brass part of the lamp (Fig.
    129). What happens?

    _Caution: Under no circumstances allow the switch to be turned
    on while you are doing any part of this experiment. Under no
    circumstances touch the wire from the faucet to the binding
    posts of the fuse gap. Do only as directed._ Explain what
    would happen if you disobeyed these rules.

[Illustration: FIG. 128. Grounding the circuit. The faucet and water
pipe lead the electricity to the ground.]

WHY A BIRD IS NOT ELECTROCUTED WHEN IT SITS ON A LIVE WIRE. If a man
accidentally touches a live wire that carries a strong current of
electricity he is electrocuted; yet birds perch on such a wire in
perfect safety. If a man should leap into the air and grasp a live
wire, hanging from it without touching the ground, he would be no more
hurt by it than a bird is. A person who is electrocuted by touching
such a wire must at the same time be standing on the ground or on
something connected with it. The ground completes the electric circuit
which passes through the body. An electric circuit can always be
completed through the ground, and when this is done, it is called
_grounding a circuit_.

[Illustration: FIG. 129. How the lamp and wire are held to ground the
circuit.]

    _APPLICATION 54._ Explain why only one wire is needed to
    telegraph between two stations; why you should not turn an
    electric light on or off while standing in a tub of water.

    _APPLICATION 55._ In a house in the country, the electric wires
    passed through a double wall. They were separated from each
    other and well covered with insulation, but they were not
    within an iron pipe, as is now required in many cities. The
    current was alternating. One night when the lights were out a
    rat in the wall gnawed through the insulation of the wire
    and also gnawed clear through one of the wires. Did he get a
    shock? The next morning, the woman of the house wanted to use
    the electric iron in the kitchen and it would not work. The
    kitchen had in it a gas stove, a sink with running water, a
    table, a couple of chairs, and the usual kitchen utensils.
    There was also a piece of wire about long enough to reach
    across the kitchen. The electrician could not come out for
    several hours, and the woman wanted very much to do her
    ironing. Figure 130 is a diagram of the wires and the kitchen.
    Show what the woman might have done in order to use her iron
    until the electrician arrived.

[Illustration: FIG. 130. How can the electric iron be used after one
wire has been cut?]

    _APPLICATION 56._ A man wanted to change the location of the
    wiring in his cement-floored garage. While he was working,
    would it have been best for him to stand on the bare cement
    floor, on a wire mat, on an old automobile tire, on a wet rug,
    or on some skid chains that were there?


INFERENCE EXERCISE

    Explain the following:

    331. An ungreased wheel squeaks.

    332. Lightning rods extend into the earth.

    333. A banjo player moves his fingers toward the drum end of
    the banjo when he plays high notes.

    334. When the filament breaks, an electric lamp will no longer
    glow.

    335. An inverted image is formed by the lens of a camera.

    336. A blown-out fuse may be replaced temporarily with a
    hairpin or with a copper cent.

    337. Sparks fly when a horse's shoe hits a stone.

    338. A room requires less artificial light if the wall paper
    is light than if it is dark.

    339. Phonographs usually have horns, either inside or outside.

    340. An electric car needs only one wire to make it go.


SECTION 37. _Resistance._

What makes an electric heater hot?

Why does lightning kill people when it strikes them?

What makes an electric light glow?

We have talked about making electricity work when it flows in a steady
stream, and everybody knows that it makes lights glow, makes toasters
and electric stoves hot, and heats electric irons. But did it
ever strike you as remarkable that the same electricity that flows
harmlessly through the wires in your house without heating them,
suddenly makes the wire in your toaster or the filament in your
incandescent lamp glowing hot? The insulation is not what keeps the
wire cool, as you can see by the next experiment.

    EXPERIMENT 69. Between two of the laboratory switches you will
    find one piece of wire which has no insulation. Turn on the
    electricity and make the lamp glow; see that you are standing
    on dry wood and are not touching any pipes or anything
    connected to the ground. Feel the bare piece of wire with your
    fingers. Why does this not give you a shock? What would happen
    if you touched your other hand to the gas pipe or water pipe?
    _Do not try it!_ But what would happen if you did?

The reason that the filament of the electric lamp gets white hot
while the copper wire stays cool is this: All substances that conduct
electricity resist the flow somewhat; there is something like friction
between the wire and the electricity passing through it. The smaller
around a wire is, the greater resistance it offers to the passing of
an electric current. The filament of an electric lamp is very fine
and therefore offers considerable resistance. However, if the filament
were made of copper, even as fine as it is, it would take a much
greater flow of electricity to make it white hot, and it would be
very expensive to use. So filaments are not made of copper but of
substances which do not conduct electricity nearly as well and which
therefore have much higher resistance. Carbon was once used, but now
a metal called _tungsten_ is used for most incandescent lamps. Both
carbon and tungsten resist an electric current so much that they are
easily heated white hot by it. On the other hand, they let so little
current through that what does pass flows through the larger copper
wires very easily and does not heat them noticeably.

[Illustration: FIG. 131. Feeling one live wire does not give her a
shock, but what would happen if she touched the gas pipe with her
other hand?]

    EXPERIMENT 70. Turn on the switch that lets the electricity
    flow through the long resistance wire that passes around the
    porcelain posts. Watch the wire.

The resistance wire you are using is an alloy, a mixture of metals
that will resist electricity much more than ordinary metals will. This
is the same kind of wire that is used in electric irons and toasters
and heaters. It has so great a resistance to the electricity that it
is heated red hot, or almost white hot, by the electricity passing
through it.

    _APPLICATION 57._ A power company wanted to send large
    quantities of electricity down from a mountain. Should the
    company have obtained resistance wire or copper wire to carry
    it? Should the wire have been large or fine?

    _APPLICATION 58._ A firm was making electric toasters. In
    the experimental laboratory they tried various weights of
    resistance wire for the toasters. They tried a very fine wire,
    No. 30; a medium weight wire, No. 24; and a heavy wire, No.
    18. One of these wires did not get hot enough, and it took so
    much electricity that it would have been too expensive to
    run; another got so hot that it soon burned out. One worked
    satisfactorily. Which of the three sizes burned out? Which was
    satisfactory?


INFERENCE EXERCISE

    Explain the following:

    341. If you attach one end of a wire to a water faucet and
    connect the other end to an electric lamp in place of one of
    the regular lighting wires, the lamp will light.

    342. The needle of a sewing machine goes up and down many
    times to each stroke of the treadle.

    343. Trolley wires are bare.

    344. If you had rubbers on your feet, you could take hold
    of one live wire with perfect safety, provided you touched
    nothing else.

    345. If you were on the moon, you would look up at the earth.

    346. Toy balloons burst when they go high up where the air is
    thin.

    347. You have to put on the brakes to stop a car quickly.

    348. Telephone wires are strung on glass supporters.

    349. If you pour boiling water into a drinking glass,
    frequently the glass will crack.

    350. An asbestos mat tends to keep food from burning.

[Illustration: FIG. 132. Pencils ready for making an arc light.]


SECTION 38. _The electric arc._

    How can electricity set a house on fire?

THIS IS ONE OF THE MOST IMPORTANT SECTIONS IN THE BOOK.

Do you know that you can make an arc light with two ordinary pencils?
The next experiment, which should be done by the class with the help
of the teacher, shows how to do it.

    EXPERIMENT 71. Sharpen two pencils. About halfway between the
    point and the other end of each pencil cut a notch all the way
    around and down to the "lead," or burn a notch down by means
    of the glowing resistance wire. What you call the "lead" of
    the pencil is really graphite, a form of carbon. The leads of
    your two pencils are almost exactly like the carbons used in
    arc lights, except, of course, that they are much smaller.
    Turn off the electricity both at the snap switch and at the
    knife switch. Fasten the bare end of a 2-foot piece of fine
    insulated wire (about No. 24) around the center of the lead in
    each pencil so that you get a good contact, as shown in Figure
    132. Fasten the other bare end of each wire to either side
    of the open knife switch so that when this switch is open the
    electricity will have to pass down one wire to the lead of one
    pencil, from that to the lead of the other pencil, and from
    that back through the second wire to the other side of the
    knife switch and on around the circuit, as shown in Figure
    133. Keep the two pencils apart and off the desk, while some
    one turns on the snap switch and the "flush" switch that lets
    the electricity through the resistance wire. Now bring the
    pencil points together for an instant, immediately drawing
    them apart about half an inch. You should get a brilliant
    white arc light.

    _Caution: Do not look at this brilliant arc for more than a
    fraction of a second unless you look through a piece of smoked
    or colored glass._

    Blow out the flame when the wood catches fire. After you have
    done this two or three times, the inside of the wood below the
    notches will be burned out so completely that you can pull it
    off with your fingers, leaving the lead bare all the way up to
    the wires.

    Let the class stand well back and watch the teacher do the
    next part of the experiment.

    Connect two heavy insulated copper wires, about No. 12, to
    the sides of the knife switch just as you connected the
    fine wires. But this time bring the ends of the copper wires
    themselves together for an instant, then draw them apart. Hold
    the ends of the wires over the zinc of the table while you do
    this, as melted copper will drop from them.

[Illustration: FIG. 133. The pencil points are touched together and
immediately drawn apart.]

[Illustration: FIG. 134. A brilliant arc light is the result.]

WHAT HAPPENS WHEN AN ARC IS FORMED. What happens when you form an
electric arc is this: As you draw the two ends of the pencils apart,
only a speck of the lead in each touches the other. The electricity
passing for an instant through the last speck at the end of the pencil
makes it so hot that it turns to vapor. The vapor will let electricity
go through it, and makes a bridge from one pencil point to the other.
But the vapor gets very hot, because it has a rather high resistance.
This heat vaporizes more carbon and makes more vapor for the
electricity to pass through, and so on. The electricity passing
through the carbon vapor makes it white hot, and that is what causes
the brilliant glow. Regular arc lights are made exactly like this
experimental one, except that the carbons used are much bigger and are
made to stand the heat better than the small carbons in your pencil.

Carbon is one of those substances that turn directly from a solid to
a gas without first melting. That is one reason why it is used for arc
lights. But copper melts when it becomes very hot, as you saw when you
made an arc light with the copper wires. So copper cannot be used for
practical arc lights.

FIRES CAUSED BY ARCS. There is one extremely important point about
this experiment with arcs: most fires that result from defective
wiring are caused by the forming of arcs. You see, if two wires touch
each other while the current is passing and then move apart a little,
an arc is formed. And you have seen how intensely hot such an arc is.
Two wires rubbing against each other, or a wire not screwed tightly to
its connection, can arc. A wire broken, but with its ends close enough
together to touch and then go apart, can cause an arc. And an arc is
very dangerous in a house if there is anything burnable near it.

Wires should never be just twisted together and then bound with tape
to form a joint. Twisted wires sometimes break and sometimes come
loose; then an arc forms, and the house catches fire. Good wiring
always means soldering every joint and screwing the ends of the wires
tightly into the switches or sockets to which they lead.

[Illustration: FIG. 135. An arc lamp. The carbons are much larger than
the carbons in the pencils, and the arc gives an intense light.]

KEEPING ARCS FROM FORMING. Well-wired houses have the wires brought
in through iron pipes, called _conduits_, and the conduits are always
grounded; so if an arc should form anywhere along the line, the house
would be protected by an iron conduit and if one of the loose ends of
wire came in contact with the conduit, the current would rush to the
ground through it, blowing out a fuse. The next section tells about
the purpose of fuses.

The directions that usually come with electric irons, toasters, and
stoves say that the connection should be broken by pulling out the
plug rather than by turning off the switch. This is because the switch
in the electric-light socket sometimes loses its spring and instead
of snapping all the way around and quickly leaving a big gap, it moves
only a little way around and an arc is formed in the socket; if you
hear a sizzling sound in a socket, you may be pretty sure that an arc
has been formed. But when you pull the plug entirely out of the iron
or stove, the gap is too big for an arc to form and you are perfectly
safe.

Fire commissions usually condemn extension lights, because if the
insulation wears out on a lamp cord so that the two wires can come
in contact, a dangerous arc may easily form. And the insulation might
suddenly be scraped off by something heavy moving across the cord.
This can happen whether the light at the end of the cord is turned on
or off. So it is best if you have an extension light always to turn it
off at the socket from which the cord leads, not at the lamp itself.
Many people do not do this, and go for years without having a fire.
But so might you live for years with a stick of dynamite in your
bureau drawer and never have an explosion. Still, it is not wise to
keep dynamite in your bureau.

Arc lights themselves, of course, are no more dangerous than is a
fire in a kitchen stove. For an arc light is placed in such a way
that nothing can well come near it to catch fire. The danger from
the electric arc is like the danger from gasoline spilled and
matches dropped where you are not expecting them, so that you are not
protected against them.

Fortunately ordinary batteries have not enough voltage to cause
dangerous arcs. So you do not have to be as careful in wiring for
electric bells and telegraph instruments. It requires the high voltage
of a city power line to make a dangerous electric arc.

So many fires are caused by electric arcs forming in buildings, that
you had better go back to the beginning of this section and read it
all through again carefully. It may save your home and even your life.

After you have reread this section, test your understanding of it by
answering the following questions:

1. How can you make an electric arc?

2. Why should wires not be twisted together to make electric
connections?

3. Why should wires be brought into houses and through walls in iron
conduits?

4. Why should you pull out the plug of an electric iron, percolator,
toaster, heater, or stove?

5. Why do fire commissions condemn extension lights?

6. If you use an extension light, where should it be turned off?

7. If you hear a sizzling and sputtering in your electric-light
socket, what does it mean? What should you do?

8. Is there any danger in defective sockets with switches that do not
snap off completely? What is the danger?

9. In Application 55, page 228, if the rat had gnawed the wire in two
while the electric iron was being used, would anything have happened
to the rat? Would there have been any danger to the house?

10. Where a wire is screwed into an electric-light socket, what harm,
if any, might result from not screwing it in tightly?

11. How can a wire be safely spliced?

12. Why is an electric arc in a circuit dangerous?


INFERENCE EXERCISE

    Explain the following:

    351. White objects look blue when seen through a blue glass.

    352. When you pull the plug out of an electric iron, the iron
    cools.

    353. People who do not hear well sometimes use speaking
    trumpets.

    354. The sounding board of a piano is roughly triangular; the
    longest strings are the extreme left, and those to the right
    get shorter and shorter.

    355. Birds can sit on live wires without getting a shock.

    356. Deaf people can sometimes identify musical selections by
    holding their hands on the piano.

    357. An electric toaster gets hot when a current passes
    through it.

    358. The cord of an electric iron sometimes catches fire while
    the iron is in use, especially if the cord is old.

    359. If a live wire touches the earth or anything connected
    with it, the current rushes into the earth.

    360. When you stub your toe, you have to run forward to keep
    from falling.


SECTION 39. _Short circuits and fuses._

    Why does a fuse blow out?

Sometimes during the evening when the lights are all on in your home,
some one tinkers with a part of the electric circuit or turns on an
electric heater or iron, and suddenly all the lights in that part of
the house go out. A fuse has blown out. If you have no extra fuses on
hand, it may be necessary to wait till the next day to replace the one
that is blown out. It is always a good idea to keep a couple of extra
fuses; they cost only 10 cents each. And if you do not happen to know
how fuses work or how to replace them when they blow out, it will cost
a dollar or so to get an electrician to put in a new fuse. The next
three experiments will help you to understand fuses.

[Illustration: FIG. 136. _A_, the "fuse gap" and _B_, the "nail
plug."]

    EXPERIMENT 72. On the lower wire leading to the electric lamp
    in the laboratory you will find a "gap," a place where the
    wire ends in a piece of a knife switch, and then begins again
    about an inch away in another piece of the switch, as shown in
    Figure 136. There must be some kind of wire or metal that will
    conduct electricity across this gap. But the gap is there to
    prevent as much electricity from flowing through as might flow
    through copper wire. So never put copper wire across this gap.
    If you do, you will have to pay for the other fuses which may
    blow out. Always keep a piece of fuse wire stretched across
    the gap. Fuse wire is a soft leadlike wire, which melts as
    soon as too much electricity passes through it.

    Unscrew the lamp, and into the socket where it was, screw
    the plug with the two nails sticking out of it. Turn the
    electricity on. Does anything happen? Turn the electricity
    off. Now touch the heads of the two nails together, or connect
    them with a piece of any metal, and turn on the electricity.
    What happens? Examine the pieces of the fuse wire that are
    left.

It was so easy for the electricity to pass through the nails and wire,
that it gushed through at a tremendous rate. This melted the fuse
wire, or blew out the fuse. If the fuse across the gap by the socket
had not been the more easily burned out, one or perhaps both of the
more expensive fuses up above, where the wire comes in, would have
blown out. These cost about 10 cents each to replace, while the fuse
wire you burned out costs only a fraction of a cent. If there were
no fuses in the laboratory wirings and you had "short circuited" the
electricity (given it an easy enough path), it would have blown
out the much more expensive fuses where the electricity enters the
building. If there were no big fuses where the electricity enters
the building, the rush of electricity would make all the copper wires
through which it flowed inside the building so hot that they would
melt and set fire to the building. As long as you keep a piece of fuse
wire across the gap, there is no danger from short circuits.

WHY FUSE WIRE MELTS. For two reasons, the fuse wire melts when
ordinary wire would not. First, it has enough resistance to
electricity so that if many amperes (much current) flow through, it
gets heated. It has not nearly as much resistance, however, as the
filament in an electric lamp or even as has the long resistance wire.
It does not become white hot as they do.

Second, it has a low melting point. It melts immediately if you hold a
match to it; try this and see. Consequently, long before the fuse wire
becomes red hot, it melts in two. It has enough resistance to make it
hot as soon as too many amperes flow through; and it has such a low
melting point that as soon as it gets hot it melts in two, or blows
out. This breaks the circuit, of course, so that no more electricity
can flow. In this way the fuse protects houses from catching fire
through short circuits.

[Illustration: FIG. 137. What will happen when the pin is thrust
through the cords and the electricity turned on?]

Unfortunately, however, the fuse is almost no protection against an
electric arc. The copper vapor through which the electricity passes
in an arc has enough resistance to keep the amperage (current) low;
so the arc may not blow out the fuse at all. But if it were not for
fuses, there would be about as much danger of houses being set on
fire by short circuits as by arcs. Perhaps there would be more danger,
because short circuits are the more common.

    EXPERIMENT 73. Put a new piece of fuse wire across the fuse
    gap. Leave the "nail plug" screwed in the socket. Use a piece
    of flexible lamp cord--the kind that is made of two strands of
    wire twisted together (see Fig. 137). Fasten one bared end of
    each wire around each nail of the "nail plug." See that the
    other ends of the lamp cord are not touching each other.
    Turn on the electricity. Does anything happen? Turn off the
    electricity. Now put a pin straight through the middle of the
    two wires. Turn on the electricity again. What happens?

There is not much resistance in the pin, and so it allows the
electricity to rush through it. People sometimes cause fuses to blow
out by pinning pictures to electric lamp wires or by pinning the wires
up out of the way.

A SHORT CIRCUIT AN "EASY CIRCUIT." You always get a short circuit when
you give electricity an easy way to get from one wire to the other.
But you get no current unless you give it some way to pass from one
wire to the other, thus completing the circuit. Therefore you should
always complete the circuit through something which resists the flow
of electricity, like an electric lamp, a heater, or an iron. Remember
this and you will have the key to an understanding of the practical
use of electricity.

The term "short circuit" is a little confusing, in that electricity
may have to go a longer way to be short circuited than to pass through
some resistance, such as a lamp. Really a short circuit should be
called an "easy circuit" or something like that, to indicate that it
is the path of least resistance. Wherever the electricity has a
chance to complete its circuit without going through any considerable
resistance, no matter how _far_ it goes, we have a short circuit. And
since everything resists electricity a little, a large enough flow
of electricity would even heat a _copper_ wire red hot; that is why a
short circuit would be dangerous if you had no fuses.

    _APPLICATION 59._ To test your knowledge of short circuits and
    fuses, trace the current carefully from the upper wire as it
    enters the laboratory, through the plug fuse. Show where it
    comes from to enter the plug fuse, exactly how it goes through
    the fuse, where it comes out, and where it goes from there.
    Trace it on through the cartridge fuse in the same way,
    through all the switches into the lamp socket, through the
    lamp, out of the lamp socket to the fuse gap, across this to
    the other wire, and on out of the room.

    It goes on from there through more fuses and back to the
    dynamo from which the other wire comes.

Test yourself further with the following questions:

1. Where in this circuit is the resistance supposed to be?

2. What happens when you put a good conductor in place of this
resistance if the electricity can get from one wire to the other
without passing through this resistance?

3. Why do we use fuses?

4. What is a short circuit?

5. What makes an electric toaster get hot?

6. Why should you not stick pins through electric cords?

    EXPERIMENT 74. Take the fuse wire out of the fuse gap and put
    a single strand of zinc shaving in its place. Instead of the
    nail plug, screw the lamp into the socket. Do not turn on the
    switch that lets the electricity flow through the resistance
    wire, but turn on the electricity so that the lamp will glow.
    Does the zinc shaving work satisfactorily as a fuse wire?
    Now turn the electricity on through the resistance wire. What
    happens?

    When are the greater number of amperes of electricity flowing
    through the zinc shaving? (NOTE. "Amperes" means the amount of
    current flowing.) Can the zinc shaving stand as many
    amperes as the fuse wire you ordinarily use? Which lets more
    electricity pass through it, the lamp or the resistance wire?
    Why do electric irons and toasters often blow out fuses? If
    this happens at your home, examine the fuse and see how many
    amperes (how much current) it will allow to flow through it.
    It will say _6A_ if it allows 6 amperes to pass through it;
    _25A_ if it allows 25 amperes to pass through it, etc. The
    fuse wire across the fuse gap allows about 8 amperes to pass
    through before it melts. The zinc shaving allows only about
    2. Read the marks on the cartridge and plug fuses. How many
    amperes will they stand?

    _APPLICATION 60._ A family had just secured an electric
    heater. The first night it was used, the fuse blew out.

    The boy said: "Let's put a piece of copper wire across the
    fuse socket; then there can't be any more trouble."

    The father said that they had better get a new fuse to replace
    the old one. The old fuse was marked _10A_.

    Was the boy or was the father right? If the father was right,
    should they have got a fuse marked _6A_, one marked _10A_, or
    one marked _15A_?

    _APPLICATION 61._ The family were putting up an extension
    light. They wanted the cord held firmly up out of the way.
    One suggested that they drive a nail through both parts of the
    cord and into the wall. Another thought it would be better to
    put a loop of string around the cord and fasten the loop to
    the wall. A third suggested the use of a double-pointed carpet
    tack that would go across the wires, but not through them,
    and if driven tightly into the wall would hold the wire more
    firmly than would the loop.

    Which way was best?


INFERENCE EXERCISE

    Explain the following:

    361. If the insulation wears off both wires of a lamp cord,
    the fuse will blow out.

    362. Street cars are heated by electricity.

    363. The handles of pancake turners are often made of wood.

    364. Glue soaks into the pores of pieces of wood and gradually
    hardens.

    365. The glue then holds the pieces tightly together.

    366. You need a fuse of higher amperage, as a 10-ampere fuse,
    instead of a 6-ampere one, where you use electricity for an
    iron, and one of still higher amperage for an electric stove.

    367. You should be careful about turning on electric lights or
    doing anything with electric wires when you are on a cement,
    iron, or earthen floor, or if you are standing in water.

    368. The keys and buttons with which you turn on electric
    lights are usually made of a rubber composition.

    369. Defective wiring, because of which bare wires may touch,
    has caused many fires.

    370. A person wearing glasses can sometimes see in them the
    image of a person behind him.


SECTION 40. _Electromagnets._

    How is a telegram sent?

    What carries your voice when you telephone?

So far we have talked about electricity only making heat and light by
being forced through something that resists it. But everybody knows
that electricity can be made to do another kind of work. It can
be made to move things,--to run street cars, to click telegraph
instruments, to vibrate the thin metal disk in a telephone receiver,
and so on. The following experiments will show you how electricity
moves things:

[Illustration: FIG. 138. The magnetized bolt picks up the iron
filings.]

    EXPERIMENT 75. Bare an inch of each end of a piece of
    insulated wire about 10 feet long. Fasten one end to the zinc
    of your battery or to one wire from the storage battery; wrap
    the wire around and around an iron machine bolt, leaving the
    bolt a foot or so from the battery, until you have only about
    a foot of wire left. Hold your bolt over some iron filings. Is
    it a magnet? Now touch the free end of your wire to the carbon
    of your battery or to the other wire from the storage battery,
    and hold the bolt over the iron filings. Is it a magnet now?

    You have completed the circuit by touching the free end of
    the wire to the free pole of your battery; so the electricity
    flows through the wire, around the bolt, and back to the
    battery.

    Disconnect one end of the wire from the battery. You have now
    broken the circuit, and the electricity can no longer flow
    around the bolt to magnetize it. See if the bolt will pick
    up the iron filings any more; it may keep a little of its
    magnetism even when no electricity is flowing, but the
    magnetism will be noticeably less. When you disconnect the
    wire so that the electricity can no longer flow through a
    complete circuit from its source back to its source again, you
    are said to _break the circuit_.

[Illustration: FIG. 139. Sending a message with a cigar-box
telegraph.]

    EXPERIMENT 76. Examine the cigar-box telegraph (see Appendix
    B) and notice that it is made on the same principle as was the
    magnetized bolt in Experiment 75. Complete the circuit through
    the electromagnet (the bolt wound with wire) by connecting
    the two ends of the wire that is wrapped around the bolt,
    with wires from the two poles of the battery. By making and
    breaking the circuit (connecting and disconnecting one of the
    wires) you should be able to make the lower bolt jump up
    and down and give the characteristic click of the telegraph
    instrument.

    In this experiment it does not matter how long the wires are
    if the batteries are strong enough. Of course it makes no
    difference where you break the circuit. So you could have the
    batteries in the laboratory and the cigar box a hundred miles
    away, with the wire going from the batteries to the bolt and
    back again. Then if you made and broke the circuit at the
    laboratory, the instrument would click a hundred miles away.
    If you want to, you may take the cigar-box telegraph out into
    the yard, leaving the batteries in the laboratory, while you
    try to telegraph this short distance.

    Examine a regular telegraph instrument. Trace the wire from
    one binding post, around the coil and through the key, back
    to the other binding post, and notice how pushing down the
    key completes the circuit and how raising it up breaks the
    circuit.

[Illustration: FIG. 140. Connecting up a real telegraph instrument.]

    EXPERIMENT 77. Connect two regular telegraph instruments,
    leaving one at each end of the long laboratory table. Make the
    connections as follows:

    Take a wire long enough to go from one instrument to the
    other. Fasten the bare ends of this wire into the right-hand
    binding post of the instrument at your left, and into the
    left-hand binding post of the instrument at your right; that
    is, connect the binding posts that are nearest together, as in
    Figure 141.

    Now connect one wire from the laboratory battery to the free
    post of the right-hand instrument. Connect the other wire
    from the laboratory battery to the ground through a faucet,
    radiator, or gas pipe, making the connection firm and being
    sure that there is a good, clear contact between the bare end
    of the wire and the metal to which the wire is attached.

    Make another ground connection near the left-hand instrument;
    that is, take a wire long enough to reach from some pipe or
    radiator to the left-hand telegraph instrument, bind one bare
    end of this wire firmly to a clean part of the pipe and bring
    the other end toward the instrument. Before attaching
    this other end to the free binding post of the left-hand
    instrument, be sure to open the switch beside the telegraph
    key by pushing it to your right. Close the switch on the
    other instrument. Now attach the free ground wire to the free
    binding post of your telegraph instrument, and press the key.
    Does the other instrument click? If not, disconnect the ground
    wire and examine all connections. Also press the sounder of
    each instrument down and see if it springs back readily. It
    may be that some screw is too tight, or too loose, or that a
    spring has come off; tinker awhile and see if you cannot make
    the instrument work. If you are unable to do so, ask for help.

    Figure 141 is a diagram of all the connections.

    When you want to telegraph, open the switch of the instrument
    you want to send from and close the switch of the instrument
    which is to receive the message.

    Holding the key down a little while, then letting it up, makes
    a "dash," while letting it spring up instantly, makes a "dot."

    Practice making dots and dashes. Telegraph the word "cat,"
    using the alphabet shown on the next page. Telegraph your own
    name; your address.

[Illustration: FIG. 141. Diagram showing how to connect up two
telegraph instruments. The circles on the tables represent the binding
posts of the instruments.]

[Illustration: FIG. 142. Telegraphing across the room.]

Here is the Morse telegraph code in dots and dashes:

  LETTERS

   A     B     C     D     E     F     G
  ·-  -···   ·· ·   -··    ·    ·-·   --·

   H     I     J     K     L     M     N
  ····  ··   -·-·  -·-   --    - -    -·

   O     P     Q     R     S     T     U
  · ·  ·····  ··-·  · ··  ···    -    ··-

   V     W     X     Y     Z      &
  ···-  ·--  ·-··  ·· ··  ··· ·  · ···

  NUMERALS

   1       2      3      4     5
  ·--·  ··-··  ···-·  ····-  ---

   6       7      8      9     0
  ······  --··  -····  -··-   ----

By using the Morse code, telegraph and cable messages are sent all
over the world in a few seconds. The ability to send messages in this
way arose from the simple discovery that when an electric current
passes around a piece of iron, it turns the iron into a magnet.

HOW A TELEPHONE WORKS. A telephone is much like a delicate and
complicated telegraph in which the vibrations started by your voice
press the "key," and in which the sounder can vibrate swiftly in
response to the electric currents passing through the wire. The "key"
in the telephone is a thin metal disk that vibrates easily, back of
the rubber mouthpiece. Each time an air vibration from your voice
presses against it, it increases the current flowing in the circuit.
And each time the current in the circuit is increased, the disk in the
receiver is pulled down, just as the sounder of a telegraph is pulled
down. So every vibration of the disk back of the mouthpiece causes
a vibration of the disk in the receiver of the other telephone; this
makes the air over it vibrate just as your voice made the mouthpiece
vibrate, and you get the same sound.

To make a difference between slight vibrations and larger ones in
telephones, there are some carbon granules between the mouthpiece disk
and a disk behind it; and there are various other complications,
such as the bell-ringing apparatus and the connections in the central
office. But the principle of the telephone is almost exactly the same
as the principle of the telegraph. Both depend entirely on the fact
that an electric current passing around a piece of iron magnetizes the
iron.

    EXPERIMENT 78. By means of your battery, make an electric
    bell ring. Examine the bell and trace the current through
    it. Notice how the current passes around two iron bars and
    magnetizes them, as it did in the telegraph instrument.
    Notice that the circuit is completed through a little metal
    attachment on the base of the clapper, and that when the
    clapper is pulled toward the electromagnet the circuit is
    broken. The iron bars are then no longer magnetized. Notice
    that a spring pulls the clapper back into place as soon as the
    iron stops attracting it. This completes the circuit again and
    the clapper is pulled down. That breaks the circuit and
    the clapper springs back. See how this constant making and
    breaking of the circuit causes the bell clapper to fly back
    and forth.

[Illustration: FIG. 143. The bell is rung by electromagnets.]

The electric bell, like the telephone and telegraph, works on the
simple principle that electricity flowing through a wire that is
wrapped around and around a piece of iron will turn that piece of iron
into a magnet as long as the electricity flows.

THE ELECTRIC MOTOR. The motor of a street car is a still more
complicated carrying out of the same principle. In the next experiment
you will see the working of a motor.

    EXPERIMENT 79. Connect the wires from the laboratory battery
    to the two binding posts of the toy motor, and make the motor
    run. Examine the motor and see that it is made of several
    electromagnets which keep attracting each other around and
    around.

Motors, and therefore all things that are _moved_ by electricity,
including trolley cars and electric railways, submarines while
submerged, electric automobiles, electric sewing machines, electric
vacuum cleaners, and electric player-pianos, are moved by magnetizing
a piece of iron and letting this pull on another piece of iron. And
the iron is magnetized by letting a current of electricity flow around
and around it.

[Illustration: FIG. 144. A toy electric motor that goes.]

The making of various kinds of electromagnets and putting currents
of electricity to work is becoming one of the great industries of
mankind. Waterfalls are being hitched up to dynamos everywhere, and
the water power that once turned the mill wheels now turns millions
of coils of wire between the poles of powerful magnets. The current
generated in this way is used for all kinds of work--not only for
furnishing light to cities, and cooking meals, heating homes, and
ironing clothes, but for running powerful motors in factories, for
driving interurban trains swiftly across the country, for carrying
people back and forth to work in city street cars, for lifting great
pieces of iron and steel in the yards where huge electromagnets are
used,--for countless pieces of work in all parts of the globe. Yet the
use of electricity is still only in its beginning. Tremendous amounts
of water power are still running to waste; there is almost no limit to
the amount of electricity we shall be able to generate as we use the
world's water power to turn our dynamos.

[Illustration: FIG. 145. An electric motor of commercial size.]

    _APPLICATION 62._ Explain how pressing a telegraph key can
    make another instrument click hundreds of miles away, and how
    you can hear over the telephone. Is it vibrations of sound
    or of electricity that go through the telephone wire, or does
    your voice travel over it, or does the wire itself vibrate?
    Explain how electricity can make a car go.


INFERENCE EXERCISE

    Explain the following:

    371. When a fuse blows out, you can get no light.

    372. If you lay your ear on a desk, you hear the sounds in the
    room clearly.

    373. If you touch a live wire with wet hands, you get a much
    worse shock than if you touch it with dry hands.

    374. A park music stand is backed by a sounding board.

    375. The clapper of an electric bell is pulled against the
    bell when you push the button.

    376. A hot iron tire put on a wagon wheel fits very tightly
    when it cools.

    377. Candy will cool more rapidly in a tin plate than in a
    china plate.

    378. When a trolley wire breaks and falls to the ground it
    melts and burns at the point at which it touches the ground.

    379. By allowing the electricity from the trolley wire to flow
    down through an underground coil of wire, a motorman can open
    a switch in the track.

    380. The bare ends of the two wires leading to your electric
    lamp should never be allowed to touch each other.




CHAPTER NINE

MINGLING OF MOLECULES


SECTION 41. _Solutions and emulsions._

    How does soap make your hands clean?

    Why will gasoline take a grease spot out of your clothes?

If we were to go back to our convenient imaginary switchboard to turn
off another law, we should find near the heat switches, and not far
from the chemistry ones, a switch labeled SOLUTION. Suppose we turned
it off:

The fishes in the sea are among the first creatures to be surprised
by our action. For instantly all the salt in the ocean drops to the
bottom like so much sand, and most salt-water fishes soon perish in
the fresh water.

If some one is about to drink a cup of tea and has sweetened it just
to his taste, you can imagine his amazement when, bringing it to his
lips, he finds himself drinking tasteless, white, milky water. Down
in the bottom of the cup is a sediment of sugar, like so much fine
gravel, with a brownish dust of tea covering it.

To see whether or not the trouble is with the sugar itself, he may
take some sugar out of the bowl and taste it,--it is just like white
sand. Wondering what has happened, and whether he or the sugar is
at fault, he reaches for the vinegar cruet. The vinegar is no longer
clear, but is a colorless liquid with tiny specks of brown floating
about in it. Tasting it, he thinks it must be dusty water. Salt,
pepper, mustard, onions, or anything he eats, is absolutely tasteless,
although some of the things _smell_ as strong as ever.

To tell the truth, I doubt if the man has a chance to do all of this
experimenting. For the salt in his blood turns to solid hard grains,
and the dissolved food in the blood turns to dustlike particles. His
blood flows through him, a muddy stream of sterile water. The cells of
his body get no food, and even before they miss the food, most of the
cells shrivel to drops of muddy water. The whole man collapses.

Plants are as badly off. The life-giving sap turns to water with
specks of the one-time nourishment floating uselessly through it. Most
plant cells, like the cells in the man, turn to water, with fibers
and dust flecks making it cloudy. Within a few seconds there is not
a living thing left in the world, and the saltless waves dash up on a
barren shore.

Probably we had better let the SOLUTION switch alone, after all.
Instead, here are a couple of experiments that will help to make clear
what happens when anything dissolves to make a _solution_.

    EXPERIMENT 80. Fill a test tube one fourth full of cold water.
    Slowly stir in salt until no more will dissolve. Add half a
    teaspoonful more of salt than will dissolve. Dry the outside
    of the test tube and heat the salty water over the Bunsen
    burner. Will hot water dissolve things more readily or less
    readily than cold? Why do you wash dishes in hot water?

    EXPERIMENT 81. Fill a test tube one fourth full of any kind
    of oil, and one fourth full of water. Hold your thumb over the
    top of the test tube and shake it hard for a minute or two.
    Now look at it. Pour it out, and shake some prepared cleanser
    into the test tube, adding a little more water. Shake the test
    tube thoroughly and rinse. Put it away clean.

When you shake the oil with the water, the oil breaks up into tiny
droplets. These droplets are so small that they reflect the light that
strikes them and so look white, or pale yellow. This milky mixture is
called an _emulsion_. Milk is an emulsion; there are tiny droplets of
butter fat and other substances scattered all through the milk. The
butter fat is _not_ dissolved in the rest of the milk, and the oil is
_not_ dissolved in the water. But the droplets may be so small that an
emulsion acts almost exactly like a solution.

[Illustration: FIG. 146. Will heating the water make more salt
dissolve?]

But when you shake or stir salt or sugar in water, the particles
divide up into smaller and smaller pieces, until probably each piece
is just a single molecule of the salt or sugar. And these molecules
get into the spaces between the water molecules and bounce around
among them. They therefore act like the water and let the light
through. This is a solution. The salt or sugar is _dissolved_ in the
water. Any liquid mixture which remains clear is a solution, no matter
what the color. Most red ink, most blueing, clear coffee, tea, and
ocean water are solutions. If a liquid is _clear_, no matter what
the color, you can be sure that whatever things may be in it are
dissolved.

[Illustration: FIG. 147. Will the volume be doubled when the alcohol
and water are poured together?]

    EXPERIMENT 82. Pour alcohol into a test tube (square-bottomed
    test tubes are best for this experiment), standing the tube
    up beside a ruler. When the alcohol is just 1 inch high in the
    tube, stop pouring. Put exactly the same amount of water
    in another test tube of the same size. When you pour them
    together, how many inches high do you think the mixture will
    be? Pour the water into the alcohol, shake the mixture a
    little, and measure to see how high it comes in the test tube.
    Did you notice the warmth when you shook the tube?

    If you use denatured alcohol, you are likely to have an
    emulsion as a result of the mixing. The _alcohol_ part of the
    denatured alcohol dissolves in the water well enough, but the
    _denaturing_ substance _in_ the alcohol will not dissolve
    in water; so it forms tiny droplets that make the mixture of
    alcohol and water cloudy.

The purpose of this experiment is to show that the molecules of water
get into the spaces between the molecules of alcohol. It is as if you
were to add a pail of pebbles to a pail of apples. The pebbles would
fill in between the apples, and the mixture would not nearly fill two
pails.

The most important difference between a solution and an emulsion is
that the particles in an emulsion are very much larger than those in
a solution; but for practical purposes that often does not make
much difference. You dissolve a grease spot from your clothes with
gasoline; you make an emulsion when you take it off with soap and
water; but by either method you remove the spot. You dissolve part of
the coffee or tea in boiling water; you make an emulsion with cocoa;
but in both cases the flavor is distributed through the liquid. Milk
is an emulsion, vinegar is a solution; but in both, the particles
are so thoroughly mixed with the water that the flavor is the same
throughout. Therefore in working out inferences that are explained in
terms of solutions and emulsions, it is not especially important for
you to decide whether you have a solution or an emulsion if you know
that it is one or the other.

HOW PRECIOUS STONES ARE FORMED. Colored glass is made by dissolving
coloring matter in the glass while it is molten. Rubies, sapphires,
emeralds, topazes, and amethysts were colored in the same way, but by
nature. When the part of the earth where they are found was hot enough
to melt stone, the liquid ruby or sapphire or emerald, or whatever
the stone was to be, happened to be near some coloring matter that
dissolved in it and gave it color. Several of these stones are made
of exactly the same kind of material, but different kinds of coloring
matter dissolved in them when they were melted.

Many articles are much used chiefly because they are good
_emulsifiers_ or good _solvents_ (dissolve things well). Soap is a
first-rate emulsifier; water is the best solvent in the world; but it
will not dissolve oil and gummy things sufficiently to be of use when
we want them dissolved. Turpentine, alcohol, and gasoline find one of
their chief uses as solvents for gums and oils. Almost all cleaning is
simply a process of dissolving or emulsifying the dirt you want to get
rid of, and washing it away with the liquid. Do not forget that heat
helps to dissolve most things.

    _APPLICATION 63._ Explain why clothes are washed in hot suds;
    why sugar disappears in hot coffee or tea; why it does not
    disappear as quickly in cold lemonade; why you cannot see
    through milk as you can through water.


INFERENCE EXERCISE

    Explain the following:

    381. A kind of lamp bracket is made with a rubber cup. When
    you press this cup against the wall or against a piece of
    furniture and exhaust the air from the cup, the cup sticks
    fast to the wall and supports the lamp bracket.

    382. You can take a vaseline stain out with kerosene.

    383. If the two poles of an electric battery are connected
    with a copper wire, the battery soon becomes discharged.

    384. Electric bells have iron bars wound around and around
    with insulated copper wire.

    385. Piano keys may be cleaned with alcohol.

    386. Linemen working with live wires wear heavy rubber gloves.

    387. Crayon will not write on the smooth, glazed parts of a
    blackboard.

    388. Varnish and shellac may be thinned with alcohol.

    389. Filtering will take mud out of water, but it will not
    remove salt.

    390. Explain why only one wire is needed to telegraph between
    two stations.


SECTION 42. _Crystals._

    How is rock candy made?

    Why is there sugar around the mouth of a syrup jug?

    How are jewels formed in the earth?

You can learn how crystals are formed--and many gems and rock candy
and the sugar on a syrup jug are all crystals--by making some. Try
this experiment:

    EXPERIMENT 83. Fill a test tube one fourth full of powdered
    alum; cover the alum with boiling water; hold the tube over
    a flame so that the mixture will boil gently; and slowly add
    boiling-hot water until all of the alum is dissolved. Do not
    add any more water than you have to, and keep stirring the
    alum with a glass rod while you are adding the water. Pour
    half of the solution into another test tube for the next
    experiment. Hang a string in the first test tube so that
    it touches the bottom of the tube. Set it aside to cool,
    uncovered. The next day examine the string and the bottom of
    the tube.

    EXPERIMENT 84. While the solution of alum in the second test
    tube (Experiment 83) is still hot, hold the tube in a pan of
    cold water and shake or stir it until it cools. When white
    specks appear in the clear solution, pour off as much of the
    clear part of the liquid as you can; then pour a little of
    the rest on a glass slide, and examine the specks under a
    microscope.

[Illustration: FIG. 148. Alum crystals.]

In both of the above experiments, the hot water was able to dissolve
more of the alum than the cold water could possibly hold. So when the
water cooled it could no longer hold the alum in solution. Therefore
part of the alum turned to solid particles.

When the string was in the cooling liquid, it attracted the particles
of alum as they crystallized out of the solution. The force of
adhesion drew the near-by molecules to the string, then these drew the
next, and these drew more, and so on until the crystals were formed.
But when you kept stirring the liquid while it cooled, the crystals
never had time to grow large before they were jostled around to
some other part of the liquid or were broken by your stirring rod.
Therefore they were small instead of large. Stirring or shaking a
solution will always make crystals form more quickly, but it will also
make them smaller.

HOW ROCK CANDY IS MADE. Rock candy is made by hanging a string in a
strong sugar solution or syrup and letting the water evaporate slowly
until there is not enough water to hold all the sugar in solution.
Then the sugar crystals gather slowly around the string, forming the
large, clear pieces of rock candy. The sugar around the mouth of a
syrup jug is formed in the same way.

You always get crystallization when you make a liquid too cool to
hold the solid thing in solution, or when you evaporate so much of
the liquid that there is not enough left to keep the solid thing
dissolved.

When you make fudge, the sugar forms small crystals as the liquid
cools. When a boat has been on the ocean, salt crystals form on the
sails when the spray that has wet them evaporates.

But crystals may form also in the air. There is always some moisture
in the air, and when it becomes very cold, some of this moisture forms
crystals of ice. If they form up in the clouds, they fall as snow.
If they form around blades of grass or on the sidewalk, as the alum
crystals formed on the string, we have frost.

Still another place that crystals occur is in the earth. When the
rocks in the earth were hot enough to be melted and then began to
cool, certain substances in the rocks crystallized. Some of these
crystals that are especially hard and clear constitute precious and
semi-precious stones.

    _APPLICATION 64._ Explain why you beat fudge as it cools; why
    the paper around butter becomes encrusted with salt if it is
    exposed to the air for some time.


INFERENCE EXERCISE

    Explain the following:

    391. Dynamos have _copper_ brushes to lead the current from
    the coils of wire to the line wires.

    392. A megaphone makes the voice carry farther than usual.

    393. Copper wire is used to conduct electricity, although iron
    wire costs much less.

    394. A flute gives notes that differ in pitch according to the
    stops that are opened.

    395. There are usually solid pieces of sugar around the mouth
    of a syrup jar.

    396. You can beat eggs quickly with a Dover egg beater.

    397. When ocean water stands in shallow open tanks for some
    time, salt begins to form before the water has all evaporated.

    398. In a coffee percolator the boiling water goes up through
    a tube. As this water drips back through the ground coffee
    beans, it becomes brown and flavored, and the coffee is made.

    399. Kerosene will clean off the rim of soap and grease that
    forms in bathtubs.

    400. Beating cake frosting or candy causes it to sugar.


SECTION 43. _Diffusion._

    How does food get into the blood?

    Why can you so quickly smell gas that is escaping at the
    opposite side of a room?

On our imaginary switchboard the DIFFUSION switch would not be safe
to tamper with. It would be near the SOLUTION switch, and almost as
dangerous. For if you were to make diffusion cease in the world, the
dissolved food and oxygen in your blood would do no good; it could not
get out of the blood vessels or into the cells of your body. You might
breathe all you liked, but breathing would not help you; the air could
not get through the walls of your lungs into the blood. Plants would
begin to wither and droop, although they would not die quite as
quickly as animals and fishes and people. But no sap could enter their
roots and none could pass from cell to cell. The plants would be as
little able to breathe through their leaves as we through our lungs.

If gas escaped in the room where you were, you could not smell it even
if you stayed alive long enough to try; the gas would rise to the top
of the room and stay there. All gases and all liquids would stay as
they were, and neither would ever form mixtures.

It would not make so much difference in the dead parts of the world
if diffusion ceased; the rocks, mountains, earth, and sea would not be
changed at all at first. To be sure, the rivers where they flowed into
the oceans would make big spaces of saltless water; and when water
evaporated from the ocean the vapor would push aside the air and stay
in a layer over the ocean, instead of mixing with the air and rising
to great heights. But the real disaster would be to living things. All
of them would be smothered and starved to death as soon as diffusion
ceased.

Here is an experiment that shows how gases diffuse:

    EXPERIMENT 85. Take two test tubes with mouths of the same
    size so that you can fit them snugly against each other when
    you want to. Fill one to the brim with water and hold your
    thumb or a piece of cardboard over its mouth while you place
    it upside down in a pan of water. Take the free end of a
    rubber tube that is attached to a gas pipe and put it into the
    test tube a short distance, so that the gas will go up into
    the tube, as shown in Figure 149. Now turn on the gas gently.
    When all the water has been forced out of the tube and the gas
    bubbles begin to come up on the outside, turn off the gas.
    Put a piece of cardboard, about an inch or so square, over the
    mouth of the tube so that no air can get into it, and take the
    tube out of the water, _keeping the mouth down and covered_.
    Bring the empty test tube, which of course is full of air,
    mouth up under the test tube full of gas, making the mouths of
    the two tubes meet with the cardboard between them, as shown
    in Figure 150. Now have some one pull the cardboard gently
    from between the two test tubes, so that the mouths of
    the tubes will be pressed against each other and so that
    practically no gas will escape. Hold them quietly this way,
    the tube of gas uppermost, for not less than one full minute
    by the clock. A minute and a half is not too much time. Now
    have some one light a match for you, or else go to a lighted
    Bunsen burner.

    Take the test tubes apart gently and hold the lower one, which
    was full of air, with its mouth to the flame. What has the gas
    in the upper tube done? Now hold the flame to the upper test
    tube, which was full of gas. What happens? Has all the gas
    gone out of it?

[Illustration: FIG. 149. Filling a test tube with gas.]

[Illustration: FIG. 150. The lower test tube is full of air; the upper,
of gas. What will happen when the cardboard is withdrawn?]

As you well know, gas is much lighter than air; you can make a balloon
rise by filling it with gas. Yet part of the gas went _down_ into the
lower tube. The explanation is that the molecules of gas and those of
air were flying around at such a rate that many of the gas molecules
went shooting down among the air molecules, and many of the molecules
of air went shooting up among those of gas, so that the gas and the
air became mixed.

DIFFUSION IN LIQUIDS. Diffusion takes place in liquids, as you know.
For when you put sugar in coffee or tea and do _not_ stir it, although
the upper part of the tea or coffee is not sweetened, the part nearer
the sugar is very sweet. If you should let the coffee or tea, with the
sugar in the bottom, stand for a few months, it would get sweet all
through. Diffusion is slower in liquids than in gases, because the
molecules are so very much closer together.

OSMOSIS. One of the most striking and important facts about diffusion
is that it can take place right through a membrane. Try this
experiment:

    EXPERIMENT 86. With a rubber band fasten a piece of parchment
    paper, made into a little bag, to the end of a piece of glass
    tubing about 10 inches long. Or make a small hole in one end
    of a raw egg and empty the shell; then, to get the hard
    part off the shell, soak it overnight in strong vinegar or
    hydrochloric acid diluted about 1 to 4. This will leave a
    membranous bag that can be used in place of the parchment bag.
    Fill a tumbler half full of water colored with red ink, and
    add enough cornstarch to make the water milky. Pour into the
    tube enough of a strong sugar solution to fill the membranous
    bag at its base and to rise half an inch in the tube. Put the
    membranous bag down into the pink, milky water, supporting the
    tube by passing it through a square cardboard and clamping
    it with a spring clothespin as shown in Figure 151. Every few
    minutes look to see what is happening. Does any of the red
    ink pass through the membrane? Does any of the cornstarch pass
    through?

This is an example of diffusion through a membrane. The process is
called _osmosis_, and the pressure that forces the liquid up the tube
is called _osmotic pressure_. It is by this sort of diffusion that
chicks which are being incubated get air, and that growing plants get
food. It is in this way that the cells of our body secure food and
oxygen and get rid of their wastes. There are no little holes in our
blood vessels to let the air get into them from our lungs. The air
simply diffuses through the thin walls of the blood vessels. There
are no holes from the intestinal tract into the blood vessels. Yet the
dissolved food diffuses right through the intestinal wall and through
the walls of the blood vessels. And later on, when it reaches the body
cells that need nourishment, the dissolved food diffuses out through
the walls of the blood vessels again and through the cell walls into
the cells. Waste is taken out of the cells into the blood and passes
from the blood into the lungs and kidneys by this same process
of diffusion. So you can readily see why everything would die if
diffusion stopped.

[Illustration: FIG. 151. Pouring the syrup into the "osmosis tube."]

    _APPLICATION 65._ Explain how the roots of a plant can take in
    water and food when there are no holes from the outside of
    the root to the inside; how bees can smell flowers for a
    considerable distance.


INFERENCE EXERCISE

    Explain the following:

    401. A shell in the bottom of a teakettle gathers most of
    the scale around it and so keeps the scale from caking at the
    bottom of the kettle.

    402. There is oxygen dissolved in water. When the water comes
    in contact with the fine blood vessels in a fish's gills, some
    of this oxygen passes through the walls of the blood vessels
    into the blood. Explain how it does so.

    403. Asphalt becomes soft in summer.

    404. When the trolley comes off the wire the car soon stops.

    405. You cannot see stars in the daytime on earth, yet you
    could see them in the daytime on the airless moon.

    406. Although the carbon dioxid you breathe out is heavier
    than the rest of the air, part of it goes up and mixes with
    the air above.

    407. On a cold day wood does not feel as cold as iron.

    408. To make mayonnaise dressing, the oil, egg, and vinegar
    are thoroughly beaten together.

    409. A solution of iodin becomes stronger if it is allowed to
    stand open to the air.

    410. A drop of milk in a glass of water clouds all the water
    slightly.


SECTION 44. _Clouds, rain, and dew: Humidity._

    Why is it that you can see your breath on a cold day?

    Where do rain and snow come from?

    What makes the clouds?

There is water vapor in the air all around us--invisible water
vapor, its molecules mingling with those of the air--water that has
evaporated from the oceans and lakes and all wet places.

This water vapor changes into droplets of water when it gets cool
enough. And those droplets of water make up our clouds and fogs; they
join together to form our rain and snow high in the air, or gather as
dew or frost on the grass at night.

If the water vapor should suddenly lose its power of changing into
droplets of water when it cooled,--well, let us pretend it has lost
this power but that any amount of water can evaporate, and see what
happens:

What fine weather it is! There is not a cloud in the sky. As evening
closes in, the stars come out with intense brightness. The whole sky
is gleaming with stars--more than we have ever seen at night before.

The next morning we find no dew or frost on the grass. All the green
things look dry. As the day goes on, they begin to wilt and wither.
We all wish the day were not quite so fine--a little rain would help
things wonderfully. Not a cloud appears, however, and we water as
much of our gardens as we can. They drink the water greedily, and that
night, again no dew or fog, and not the faintest cloud or mist to dim
the stars. And the new day once more brings the blazing sun further
to parch the land and plants. Day after day and night after night the
drought gets worse. The rivers sink low; brooks run dry; the edges of
the lakes become marshes. The marshes dry out to hardened mud. The
dry leaves of the trees rustle and crumble. All the animals and
wood creatures gather around the muddy pools that once were lakes or
rivers. People begin saving water and buying it and selling it as the
most precious of articles.

As the months go by, winter freezes the few pools that remain. No snow
falls. Living creatures die by the tens of thousands. But the winter
is less cold than usual, because there is now so much water vapor in
the air that it acts like a great blanket holding in the earth's heat.

With spring no showers come. The dead trees send forth no buds. No
birds herald the coming of warm weather. The continents of the world
have become vast, uninhabitable deserts. People have all moved to the
shores of the ocean, where their chemists are extracting salt from the
water in order to give them something to drink. By using this saltless
water they can irrigate the land near the oceans and grow some food to
live on. Each continent is encircled by a strip of irrigated land and
densely populated cities close to the water's edge.

It is many years before the oceans disappear. But in time they too
are transformed into water vapor, and no more life as we know it is
possible in the world. The earth has become a great rocky and sandy
ball, whirling through space, lifeless and utterly dry.

That which prevents this from really happening is very simple: In the
world as it is, water vapor condenses and changes to drops of water
whenever it gets cool enough.

HOW WATER VAPOR GETS INTO THE AIR. The water vapor gets into the air
by evaporation. When we say that water evaporates, we mean that it
changes into water vapor. As you already know, it is heat that makes
water evaporate; that is why you hang wet clothes in the sun or by the
fire to dry: you want to change the water in them to water vapor. The
sun does not suck up the water from the ocean, as some people say; but
it warms the water and turns part of it to vapor.

What happens down among the molecules when water evaporates is this:
The heat makes the molecules dance around faster and faster; then the
ones with the swiftest motion near the top shoot off into the air. The
molecules that have shot off into the air make up the water vapor.

The water vapor is entirely invisible. No matter how much of it there
is, you cannot see it. The weather is just as clear when there is a
great deal of water vapor in the air as when there is very little, as
long as none of the vapor condenses.

HOW CLOUDS ARE FORMED. But when water vapor condenses, it forms into
extremely small drops of real water. Each of these drops is so small
that it is usually impossible to see one; they are so tiny that you
could lay about 3000 of them side by side in one inch! Yet, small as
they are, when there are many of them they become distinctly visible.
We see them floating around us sometimes and call them fog or mist.
And when there are millions of them floating in the air high above us,
we call them a cloud.

The reason clouds form so high in the air is this: When air or any gas
expands, it cools. Do you remember Experiment 31, where you let the
gas from a tank expand into a wet test tube and it became so cold that
the water on the test tube froze? Well, it is much the same way with
rising air. When air rises, there is less air above it to keep it
compressed; so it expands and cools. Then the water vapor in it
condenses into droplets of water, and these form a cloud.

Each droplet forms a gathering place for more condensing water vapor,
and therefore grows. When the droplets of water in a cloud are very
close together, some may be jostled against one another by the wind.
And when they touch each other, they stick together, forming a larger
drop. When a drop grows large enough it begins to fall through the
cloud, gathering up the small droplets as it goes. By the time it gets
out of the cloud it has grown to a full-sized raindrop, and falls to
earth. The complete story of rain, then, is this:

HOW RAIN IS CAUSED. The surface of the oceans and lakes is warmed by
the sun. The water evaporates, turning to invisible water vapor. This
water vapor mingles with the air. After a while the air is caught in a
rising current and swept up high, carrying the water vapor with it. As
the air rises, there is less air above it to press down on it; so
it expands. When air expands it cools, and the water vapor which is
mingled with it likewise cools. When the water vapor gets cool enough
it condenses, changing to myriads of extremely small drops of water.
These make a cloud.

A wind comes along; that is, the air in which the cloud is floating
moves. The wind carries the cloud along with it. More rising air, full
of evaporated water from the ocean, joins the cloud and cools, and
the water forms into more tiny droplets. The droplets get so close
together that they shut out the sun's light from the earth, and people
say that the sky is darkening.

Meanwhile some of the droplets begin to touch each other and to stick
together. Little by little the drops grow bigger by joining together.
Pretty soon they get so big and heavy that they can no longer float
high in the air, and they fall to the ground as rain.

Part of the rain soaks into the ground. Some of it gradually seeps
down through the ground to an underground stream. This has its outlet
in a spring or well, or in an open lake or the ocean. But the rain
does not all soak in. After the storm, some of the water again
evaporates from the top of the ground and mixes with the warm air, and
it goes through the same round. Other raindrops join on the ground
to form rivulets that trickle along until they meet and join other
rivulets; and all go on together as a brook. The brook joins others
until the brooks form a river; and the river flows into a lake or into
the ocean.

Then again the sun warms the surface of the ocean or lake; the water
evaporates and mixes with the air, which rises, expands, and cools;
the droplets form and make clouds; the droplets join, forming big
drops, and they fall once more as rain. The rain soaks into the
ground or runs off in rivulets, and sooner or later it is once more
evaporated. And so the cycle is repeated again and again.

And all this is accounted for by the simple fact that when water
evaporates its vapor mingles with the air; and when this vapor is
sufficiently cooled it condenses and forms droplets of water.

THE BAROMETER. In predicting the weather a great deal of use is made
of an instrument called the _barometer_. The barometer shows how
hard the air around it is pressing. If the air is pressing hard, the
mercury in the barometer rises. If the air is not pressing hard the
mercury sinks. Just before a storm, the air usually does not press so
hard on things as at other times; so usually, just before a storm,
the mercury in the barometer is lower than in clear weather. You
will understand the barometer better after you make one. Here are the
directions for making a barometer:

    EXPERIMENT 87. _To be done by the class with the aid of the
    teacher._ Use a piece of glass tubing not less than 32 inches
    long, sealed at one end. Fill this tube to the brim with
    mercury (quicksilver), by pouring the mercury into it through
    a paper funnel. Have the sealed end of the tube in a cup, to
    catch any mercury that spills.[7] When the tube is full, pour
    mercury into the cup until there is at least half an inch of
    it at the bottom. Now put your forefinger very tightly over
    the open end of the tube, take hold of the sealed end with
    your other hand, and turn the tube over. Lower the open end,
    with your finger over it, into the cup. When the mercury in
    the cup completely covers your finger and the end of the tube,
    remove your finger carefully so that no air can get up into
    the tube of mercury. Let the open end of the tube rest gently
    on the bottom of the cup, and hold the tube upright with your
    hand or by clamping it to a ring stand. Hold a yardstick or
    meter stick beside the tube, remembering to keep the tube
    straight up and down. Measure accurately the height of the
    mercury column from the _surface_ of the mercury in the cup.
    Then go to the regular barometer hanging on the wall, and read
    it.

    [Footnote 7: If mercury spills on the floor or table during
    this experiment, gather it all into a piece of paper by
    brushing even the tiny droplets together with a soft brush;
    squeeze it through a towel into a cup to clean it. It is
    expensive; so try not to lose any of it.]

    The reason your barometer may not read exactly the same as the
    expensive laboratory instrument is that a little air and
    water vapor stick to the inside of the tube and rise into the
    "vacuum" above the mercury; also, the tube may not be quite
    straight up and down. Otherwise the readings would be the
    same.

[Illustration: FIG. 152. Filling the barometer tube with mercury.]

Of course you understand what holds the mercury up in the tube. If you
could put the cup of mercury into a vacuum, the mercury in the tube
would sink down into the cup. But the pressure of the air on the
surface of the mercury in the cup keeps the mercury from flowing out
of the tube and so leaving a vacuum in there. If the air pushes down
hard on the mercury in the cup, the mercury will stand high in the
tube. This is called _high pressure_. If the air does not press hard
on the mercury in the cup, the mercury stands low in the tube. This is
called _low pressure_.

[Illustration: FIG. 153. Inverting the filled tube in the cup of
mercury.]

HOW WEATHER IS FORECAST. Weather forecasters make a great deal of use
of the barometer, for storms are usually accompanied by low pressure,
and clear weather nearly always goes with high pressure.

The reason storms are usually accompanied by low pressure is this: A
storm is almost always due to the rising of air, for the rising
air expands and cools, and if there is much water vapor in it, this
condenses when it cools and forms clouds and rain. Now air rises only
when there is comparatively little pressure from above. Therefore,
before and during a storm there is not so much pressure on the mercury
of the barometer and the barometer is low.

Clear weather, on the other hand, is often the result of air being
compressed, for compressing air warms it. When air is being warmed,
the water vapor in it will not condense; so the air remains clear. But
when the air is being compressed, it presses hard on the mercury of
the barometer; the pressure is high, and the mercury in the barometer
rises high. Therefore when the mercury in the barometer is rising, the
weather is usually clear.

[Illustration: FIG. 154. Finding the pressure of the air by measuring
the height of the mercury in the tube.]

These two statements are true only in a very general way, however. If
weather forecasters had only their own barometers to go by, they would
not be of much value; for one thing, they could not tell us that a
storm was coming much before it reached us. But there are weather
stations all over the civilized world, and they keep in touch with
each other by telegraph. It is known that storms travel from west to
east in our part of the world. If one weather man reports a storm at
his station, and tells how his barometer stands, the weather men to
the east of him know that the storm is coming their way. From several
such reports the weather men to the east can tell how fast the storm
is traveling and exactly which way it is going. Then they can tell
when it will reach their station and can make the correct prediction.

[Illustration: FIG. 155. The kind of mercury barometer that you buy.]

Weather men do not have to wait for an actual storm to be reported.
If the reports from the west show that the air is rising as it swirls
along--that is, if the barometer readings in the west are low--they
know that this low-pressure air is approaching them. And they know
that low pressure usually means air that is rising and cooling
and therefore likely to drop its moisture. In the same way, if the
barometers to the west show high pressure, the eastern weather men
know that the air that is blowing toward them is being compressed and
warmed, and is therefore not at all likely to drop its moisture; so
they predict fair weather.

The weather man is not ever certain of his forecasts, however.
Sometimes the air will begin to rise just before it gets to him. Then
there may be a shower of rain when he has predicted fair weather. Or
sometimes the air that has been rising to the west, and which has made
him predict bad weather, may stop rising; the storm may be over before
it reaches his station. Then his prediction of bad weather is wrong.
Or sometimes the storm unexpectedly changes its path. There are many
ways in which a weather prophecy may go wrong; and then we blame the
weather man. We are likely to remember the times that his prophecy is
mistaken and to forget the many, many times when it is right.

[Illustration: FIG. 156. An aneroid barometer is more convenient than
one made with mercury. The walls are forced in or spring back out
according to the pressure of the air. This movement of the walls
forces the hand around.]

HOW SNOW IS FORMED. The difference between the ways in which snow and
rain are formed is very slight. In both cases water evaporates and its
vapor mingles with the warm air. The warm air rises and expands. It
cools as it expands, and when it gets cool enough the water vapor
begins to condense. _But_ if the air as it expands becomes _very_
cold, so cold that the droplets of water freeze as they form and
gather together to make delicate crystals of ice, snow is formed.
The ice crystals found in snow are always six-sided or six-pointed,
because, probably, the water or ice molecules pull from six directions
and therefore gather each other together along the six lines of this
pull. At any rate, the tiny crystals of frozen water are formed and
come floating down to the ground; and we call them _snowflakes_. After
the snow melts it goes through the same cycle as the rain, most of it
finally getting back to the ocean through rivers, and there, in time,
being evaporated once more.

[Illustration: FIG. 157. Different forms of snowflakes. Each snowflake
is a collection of small ice crystals.]

Hail is rain that happens to be caught in a powerful current of rising
air as it forms, and is carried up so high that it freezes in the
cold, expanding air into little balls of ice, or hail stones, which
fall to the ground before they have time to melt.

WHY ONE SIDE OF A MOUNTAIN RANGE USUALLY HAS RAINFALL. When air that
is moving along reaches a mountain range, it either would have to
stop, or rise and go over the mountain. The pressure of the air behind
it, moving in the same direction, keeps it from stopping, and so it
has to go up the slopes and over the range. But as it goes up, there
is less air above it to push down on it; so it expands. This makes it
cool, and the water vapor in it begins to condense and form snow or
rain. Therefore the side of mountain ranges against which the wind
usually blows, almost always has plenty of rainfall.

It is different on the farther side of the mountain range. For here
the air is sinking. As it sinks it is being compressed. And as it is
compressed it is heated. If you hold your finger over the mouth of a
bicycle pump and compress the air in the pump by pushing down on the
handle, you will find that the pump is decidedly warmed. When the air,
sinking down on the farther side of the mountain range, is heated,
the water vapor in it is not at all likely to condense. Therefore rain
seldom falls on the side of the mountains which is turned away from
the prevailing winds.

HOW DEW AND FROST ARE FORMED. The heat of the earth radiates out into
the air and on out into space. At night, when the earth loses its heat
this way and does not receive heat from the sun, it becomes cooler.
When the air, carrying its water vapor, touches the cool leaves and
flowers, the water vapor is condensed by the coolness and forms drops
of dew upon them. Or, if the night is colder, the droplets freeze as
they form, and in the morning we see the grass and shrubs all covered
with frost.

THE CAUSE OF FOGS. When warm air is cooled while it is down around us,
the water vapor in it condenses into myriads of droplets that float
in the air and make it foggy. The air may be cooled by blowing in from
the warm lake or ocean in the early morning, for at night the land
cools more rapidly than the water does. This accounts for the early
morning fogs in many cities that are on the coasts.

Likewise when the wind has been blowing over a warm ocean current,
the surface of the warm water evaporates and fills the air with water
vapor. Then when this air passes over a cold current, the cold current
cools the air so much that the moisture in it condenses and forms fog.
That is why there are fog banks, dangerous to navigation, in parts of
the ocean, particularly off Labrador.

WHY YOU CAN SEE YOUR BREATH ON COLD DAYS. You really make a little fog
when you breathe on a cold morning. The air in your lungs is warm. The
moisture in the lungs evaporates into this warm air, and you breathe
it out. If the outside air is cold, your breath is cooled; so some of
the water vapor in it condenses into very small droplets, and you see
your breath.

Here are two experiments in condensing water vapor by cooling the air
with which it is mixed. Both work best if the weather is warm or the
air damp.

    EXPERIMENT 88. Put the bell jar on the plate of the air pump
    and begin to pump the air out of it. Watch the air in the jar.
    If the day is warm or damp, a slight mist will form.

As part of the air is pumped out, the rest expands and cools, as warm
air does when it rises and is no longer pressed on so hard by the air
above it. And as in the case of the rising warm air, the water vapor
condenses when it cools, and forms the mist that you see. This mist,
like all clouds and fog, consists of thousands of extremely small
droplets.

    EXPERIMENT 89. Hold a saucer of ice just below your mouth.
    Open your mouth wide and breathe gently over the ice. Can you
    see your breath?

    Now put the ice into half a glass of water and cover the
    glass. Be sure the outside of the glass is thoroughly dry. Set
    it aside and look at it again in a few minutes.

    What caused the mist when you breathed across the ice?

    Where did the water on the outside of the glass of ice water
    come from? What made it condense?

[Illustration: FIG. 158. If you blow gently over ice, you can see your
breath.]

    _APPLICATION 66._ Explain why clouds are formed high in
    the atmosphere; why we have dew at night instead of in the
    daytime; why clothes dry more quickly in a breeze than in
    still air; why clothes dry more quickly on a sunny day than on
    a foggy one.

[Illustration: FIG. 159. The glass does not leak; the moisture on it
comes from the air.]


INFERENCE EXERCISE

    Explain the following:

    411. A gas-filled electric lamp gets hotter than a vacuum
    lamp.

    412. You can remove a stamp from an envelope by soaking it in
    water.

    413. We see our breath on cold days and not on warm days.

    414. The electric arc is exceedingly hot.

    415. Rock candy is made by hanging a string in a strong syrup
    left open to the air.

    416. Dishes in which candy has been made should be put to
    soak.

    417. Moisture gathers on eyeglasses when the wearer comes from
    a cold room into a warm one.

    418. Sprinkling the street on a hot day makes the air cool.

    419. You cannot see things in a dark room.

    420. Where air is rising there is likely to be rain.


SECTION 45. _Softening due to oil or water._

    Why does fog deaden a tennis racket?

    How does cold cream keep your face from becoming chapped?

Let us now imagine that animal and plant substances have suddenly lost
their ability to be softened by oil or water.

All living things soon feel very uncomfortable. Your face and hands
sting and crack; the skin all over your body becomes harsh and dry;
your mouth feels parched. The shoes you are wearing feel as if they
had been dried over a radiator after being very wet, only they are
still harder and more uncomfortable.

A man driving a horse feels the lines stiffening in his hands; and
the harness soon becomes so dry and brittle that it cracks and perhaps
breaks if the horse stops suddenly.

The leaves on the trees begin to rattle and break into pieces as the
wind blows against them. Although they keep their greenness, they act
like the driest leaves of autumn.

I doubt whether you or any one can stay alive long enough to notice
such effects. For the muscles of your body, including those that make
you breathe and make your heart beat, probably become so harsh
and stiff that they entirely fail to work, and you drop dead among
thousands of other stiff, harsh-skinned animals and people.

So it is well that in the real world oil and water soften practically
all plant and animal tissues. Of course, in living plants and animals
the oil and water come largely from within themselves. Your skin is
kept moist and slightly oily all the time by little glands within it,
some of which, called _sweat glands_, secrete perspiration and others
of which secrete oil. But sometimes the oil is washed off the surface
of your hands, as when you wash an article in gasoline or strong soap.
Then you feel that your skin is dry and harsh.

And when you want to soften it again you rub into it oily substances,
like cold cream or vaseline.

In the same way if harness or shoes get wet and then are dried out,
they can be made properly flexible by oiling. You could wet them, of
course, and this would soften them as long as they stayed wet. But
water evaporates rather quickly; so when you want a thing to _stay_
soft, you usually apply some kind of oil or grease.

Just as diffusion and the forming of solutions are increased by heat,
this softening by oil and water works better if the oil or water
is warm. That is why you soak your hands in _warm_ water before
manicuring your nails.

    _APPLICATION 67._ Explain why women dampen clothes before
    ironing them; why crackers are put up in waterproof cartons;
    why an oil shoe polish is better than one containing water.


INFERENCE EXERCISE

    Explain the following:

    421. You can shorten your finger nails by filing them.

    422. You can do it more quickly after washing them than
    before.

    423. After a flashlight picture is taken, the smoke soon
    reaches all parts of the room.

    424. A jeweler wears a convex lens on his eye when he works
    with small objects.

    425. Shoemakers soak the leather before half-soling shoes.

    426. Lightning often sets fire to houses or trees that it
    strikes.

    427. The directions on many bottles of medicine and of
    preparations for household use say, "Shake well before using."

    428. If you set a cold tumbler inside of one that has just
    been washed in hot water, the outer one will crack in a few
    minutes.

    429. A dry cloth hung out at night becomes wet, while a wet
    cloth hung out on a clear day dries.

    430. Putting cold cream or tallow around the roots of your
    finger nails will help to prevent hangnails.




CHAPTER TEN

CHEMICAL CHANGE AND ENERGY


SECTION 46. _What things are made of: Elements._

    What is water made of?

    What is iron made of?

    Is everything made out of dust?

One of the most natural questions in the world is, "What is this
made of?" If we are talking about a piece of bread, the answer is, of
course, "flour, water, milk, shortening, sugar, salt, and yeast." But
what is each of these made of? Flour is made of wheat, and the wheat
is made of materials that the plant gets from the earth, water, and
air. Then what are the earth, water, and air made of? A chemist is
a person who can answer these questions and who can tell what almost
everything is made of. And a strange thing that chemists have found
out is this: Everything in the world is made out of one or more of
about eighty-five simple substances called _elements_.

WHAT AN ELEMENT IS. An element is a substance that is not made of
anything else but itself. Gold is one of the eighty-five elements;
there are no other substances known to man that you can put together
to make gold. It is made of gold and that is all. There is a theory
that maybe all the elements are made of electrons in different
arrangements, or of electrons and one other thing; but we do not know
that, it is only a theory. Carbon is another element; pure charcoal
is carbon. The part of the air that we use when we breathe or when we
burn things is called _oxygen_. Oxygen is an element; it is not made
of anything but itself. There is another gas which is often used to
fill balloons that are to go very high; it is the lightest in the
world and is called _hydrogen_. Hydrogen is an element.

For a long time people thought that water was an element. Water
certainly looks and seems as if it were made only of itself. Yet
during the thousands of years that people believed water was an
element, they were daily putting two elements together and making
water out of them. When you put a kettle, or anything cold, over a
fire, tiny drops of water always form on it. These are not drops of
water that were dissolved in the air, and that condense on the sides
of the cold kettle; if they were, they would gather on the kettle
better in the open air than over the hot fire. Really there is some
of that very light gas, hydrogen, in the wood or coal or gas that
you use, and this hydrogen joins the oxygen in the air to make water
whenever we burn ordinary fuel.

But the best way to prove that water is made of two gases is to take
the water apart and get the gases from it. Here are the directions for
doing this:

    EXPERIMENT 90. A regular bought electrolysis apparatus may be
    used, or you can make a simple one as follows:

    Use a tumbler and two test tubes. If the test tubes are rather
    small (3/8'' X 3'') they will fill more quickly. Dissolve a
    little lye (about 1/8 teaspoonful) in half a pint of water
    to make the water conduct electricity easily, or you may use
    sulfuric acid in place of lye. Pour half of this solution
    into the tumbler. Pour as much more as possible into the test
    tubes, filling both tubes brim full. Cover the mouth of each
    test tube with a small square of dry paper or cardboard, and
    turn it upside down, lowering it into the tumbler.

    The "electrodes" are two 3/4'' pieces of platinum wire (#30),
    which are soldered to two pieces of insulated copper wire,
    each about 2 feet long.[8] The other ends of the copper wire
    are bare. Fasten the bare end of one copper wire to one nail
    of the nail plug if you have direct current (d. c.) in the
    laboratory, and fasten the bare end of the other wire to the
    other nail; then turn on the electricity. If you do not have
    direct current in the laboratory, attach the copper wires to
    the two poles of a battery instead.

    [Footnote 8: If the copper wire is drawn through a piece of
    1/4-inch soft glass tubing so that only the platinum wire
    projects from the end of the tube, and the tube is then sealed
    around the platinum by holding it in a Bunsen burner a few
    minutes, your electrodes will be more permanent and more
    satisfactory. The pieces of glass tubing should be about 6
    inches long (see Fig. 160).]

    Bend the platinum electrodes up so that they will stick up
    into the test tubes from below. Bubbles should immediately
    begin to gather on the platinum wire and to rise in the test
    tubes. As the test tubes fill with gas, the water is forced
    out; so you can tell how much gas has collected at any time by
    seeing how much water is left in each tube.

    One tube should fill with gas twice as fast as the other. The
    gas in this tube is hydrogen; there is twice as much hydrogen
    as there is oxygen in water. The tube that fills more slowly
    contains oxygen.

    When the faster-filling tube is full of hydrogen--that is,
    when all of the water has been forced out of it--take the
    electrode out and let it hang loose in the glass. Put a piece
    of cardboard about 1 inch square over the mouth of the test
    tube; take the test tube out of the water and turn it right
    side up, keeping it covered with the cardboard. Light a match,
    remove the cardboard cover, and hold the match over the open
    test tube. Does the hydrogen in it burn?

    When the tube containing the oxygen is full, take it out,
    covered, just as you did the hydrogen test tube. But in this
    case make the end of a stick of charcoal glow, remove the
    cardboard from the tube, and then plunge the glowing charcoal
    into the test tube full of oxygen.

[Illustration: FIG. 160. The electrodes are made of loops of platinum
wire sealed in glass tubes.]

Only oxygen will make charcoal burst into flame like this.

When people found that they could take water apart in this way and
turn it into hydrogen and oxygen, and when they found that whenever
they combined hydrogen with oxygen they got water, they knew, of
course, that water was not an element. Maybe some day they will find
that some of the eighty-five or so substances that we now consider
elements can really be divided into two or more elements; but so far
the elements we know show no signs of being made of anything except
themselves.

The last section of this book will explain something about the way
the chemist goes to work to find out what elements are hidden in
compounds.

[Illustration: FIG. 161. Water can be separated into two gases by a
current of electricity.]

THE QUICK WAY CHEMISTS WRITE ABOUT ELEMENTS. Since everything in the
world is made of a combination or a mixture of elements, chemists have
found it very convenient to make abbreviations for the names of the
elements so that they can quickly write what a thing is made of.
They indicate hydrogen by the letter H. O always means oxygen to the
chemist; C means carbon; and Cl means chlorine, the poison gas so much
used in the World War. The abbreviation stands for the Latin name of
the element instead of for the English name, but they are often almost
alike. The Latin name for the metal sodium, however, is _natrum_, and
chemists always write Na when they mean sodium; this is fortunate,
because S already stands for the element sulfur. Fe means iron (Latin,
_ferrum_). But I stands for the element iodine. (The iodine you use
when you get scratched is the element iodine dissolved in alcohol.) It
is not necessary for you to remember the chemical symbols unless
you mean to become a chemist or unless you read a good deal about
chemistry. But almost every one knows at least that H means hydrogen,
O means oxygen, and C means carbon.

When a chemist wants to show that water is made of hydrogen two
parts and oxygen one part, he writes it very quickly like this: H_2O
(pronounced "H two O"). "H_2O" means to a chemist just as much as
"w-a-t-e-r" means to you; and it means even more, because it tells
that water is made of two parts hydrogen and one part oxygen. If a
chemist wanted to write, "You can take water apart and it will give
you two parts of hydrogen and also one part of oxygen," this is what
he would put down:

  H_2O -> 2H+O.

If he wanted to show that you could combine two parts of hydrogen and
one part of oxygen to form water, he would write it quickly like this:

  2H+O -> H_2O.

These are called _chemical equations_. You do not need to remember
them; they are put here merely so that you will know what they look
like. Some of them are much longer and more complicated, like this:

  HC_2H_3O_2+NaHCO_3 -> H_2O+CO_2+NaC_2H_3O_2.

This is the chemist's way of saying, "Vinegar is made of one part
of hydrogen gas that will come off easily and that gives it its sour
taste, two parts of carbon, three parts of hydrogen that does not come
off so easily, and two parts of oxygen. When you put this with baking
soda, which is made of one part of the metal sodium, one part of
hydrogen, one part of carbon, and three parts of oxygen, you get water
and carbon dioxid gas and a kind of salt called sodium acetate." Or,
more briefly, "If you put baking soda with vinegar, you get water, a
gas called carbon dioxid, and a salt." You can see how much shorter
the chemist's way of writing it is.

SOME ELEMENTS YOU ALREADY KNOW. Here is a list of some elements that
you are already pretty well acquainted with. The abbreviation is put
after the name for each. This list is only for reference and need not
be learned.

  Aluminum   (Al)
  Carbon     (C)     Charcoal, diamonds, graphite (the lead in a
                       pencil is graphite), hard coal, and soot are
                       all made of carbon.
  Chlorine   (Cl)    A poison gas that was used in the war.
  Copper     (Cu)
  Gold       (Au)
  Hydrogen   (H)     The lightest gas in the world; you got it from
                       water in the last experiment and will get it
                       from an acid in the next.
  Iodine     (I)     It is a solid; what you use is iodine dissolved
                       in alcohol.
  Iron       (Fe)
  Lead       (Pb)
  Mercury    (Hg)    This is another name for quicksilver.
  Nickel     (Ni)
  Nitrogen   (N)     About four fifths of the air is pure nitrogen.
  Oxygen     (O)     This is the part of the air we use in breathing.
                       You got some out of water, and you will
                       have it to deal with in another experiment.
  Phosphorus (P)     Phosphorus makes matches glow in the dark,
                       and it makes them strike easily.
  Platinum   (Pt)
  Radium     (Ra)
  Silver     (Ag)
  Sodium     (Na)    You are not acquainted with sodium by itself,
                       but when it is combined with the poison
                       gas, chlorine, it makes ordinary table salt.
  Sulfur     (S)
  Tin        (Sn)
  Zinc       (Zn)

For the rest of the elements you can refer to any textbook on
chemistry.

HOW ELEMENTS HIDE IN COMPOUNDS. One strange thing about an element
is that it can hide so completely, by combining with another
element, that you would never know it was present unless you took the
combination apart. Take the black element carbon, for instance. Sugar
is made entirely of carbon and water. You can tell this by making
sugar very hot. When it is hot enough, it turns black; the water part
is driven off and the carbon is left behind. Yet to look at dry, white
sugar, or to taste its sweetness, one would never suspect that it was
made of pure black, tasteless carbon and colorless, tasteless water.
Mixing carbon and water would never give you sugar. But combining them
in the right proportions into a chemical compound does produce sugar.

Not only is carbon concealed in sugar, but it is present in all plant
and animal matter. That is why burning almost any kind of food makes
it black. You drive off most of the other elements and separate the
food into its parts by getting it too hot; the water evaporates and so
does the nitrogen; what is left is mainly black carbon.

MAKING HYDROGEN COME OUT OF HIDING. The light gas, hydrogen, conceals
itself as perfectly as carbon does by combining with other elements.
It is hiding in everything that is sour and in many things that are
not sour. And you can get it out of sour things with metals. In
some cases it is harder to separate than in others; and some metals
separate it better than others do. But one sour compound that you can
easily get the hydrogen out of is hydrochloric acid (HCl), which
is hydrogen combined with the poison gas, chlorine. One of the best
metals to get the hydrogen out with is zinc. Here are the directions
for doing it and incidentally for making a toy balloon:

    EXPERIMENT 91. _Do this experiment on the side of the
    laboratory farthest from any flames or fire. Do not let any
    flame come near the flask in which you are making hydrogen._

    In the bottom of a flask put two or three wads of zinc
    shavings, each about the size of your thumb. Fit a one-hole
    rubber stopper to the flask. Take the stopper out and put a
    piece of glass tubing about 5 inches long through the hole of
    the stopper, letting half an inch or so stick down into the
    flask when the stopper is in place (Fig. 162). With a rubber
    band fasten the mouth of a rubber balloon over the end of the
    glass tube that will be uppermost. Fill the balloon by blowing
    through the glass tube to see if all connections are tight,
    and to see how far it may be expanded without danger of
    breaking. You can tell when the balloon has about all it will
    hold, by pressing gently with your fingers. If the rubber
    feels tight, do not blow any more. Let the air out of the
    balloon again.

    Now get some hydrochloric acid (HCl) diluted with three parts
    of water. Find the bottle marked "HCl, dilute 1-3," in which
    the acid is already diluted. Before you open the bottle,
    get some solution of soda, and keep it near you; if in this
    experiment or any other you spatter acid on your hands or
    face or clothes, wash it off _immediately_ with soda solution.
    _Remember this._ Ammonia will do as well as the soda solution
    to wash off the acid, but be careful not to get it into your
    eyes.

    Pour the hydrochloric acid (HCl) on the zinc shavings in the
    bottom of the flask, until the acid stands about an inch deep.
    Then quickly put the rubber stopper with its attachments into
    the flask, so that the gas that bubbles up will blow up the
    balloon.

    If the bubbles do not form rapidly, ask the teacher to pour a
    little strong hydrochloric acid into the flask; but this will
    probably not be necessary. Let the balloon keep filling until
    it is as large as you blew it. But if the bubbles stop coming
    before it gets as large as that, close the neck of the balloon
    by pinching it tightly, and take the stopper out. Let some
    one add more zinc shavings and more acid to the flask; put the
    stopper back in, and stop pinching the neck of the balloon.
    _In this and all other experiments when you use strong acids,
    pour the used acids into the crockery jar that is provided
    for such wastes. Do not pour them into the sink, as acids ruin
    sink drainpipes._

    When the balloon is full, close the neck by slipping the
    rubber band up from the part of the neck that is over the
    glass tube on to the upper part of the neck. Pull the balloon
    off the glass tube and pinch the neck firmly shut. Take the
    stopper out and rinse the flask several times with running
    water. Any zinc that is left should be rinsed thoroughly,
    dried, and set aside so that it may be used again. Now tie one
    end of a long thread firmly around the mouth of the balloon
    and let the balloon go. Does it rise? If it does not, the
    reason is that you did not get it full enough. In that case
    make more hydrogen and fill it fuller, as explained above.

[Illustration: FIG. 162. Filling a balloon with hydrogen.]

[Illustration: FIG. 163. Adding more acid without losing the gas.]

Here is another experiment with hydrogen:

    EXPERIMENT 92. Put a wad of zinc shavings, about the size of
    the end of your little finger, into the bottom of a test tube.
    Cover it with hydrochloric acid (HCl) diluted one to three,
    as in the preceding experiment. After the bubbles have been
    rising for a couple of minutes, take the test tube to the side
    of the laboratory where the burners are, and hold a lighted
    match at its mouth. Will hydrogen burn?

Remember that the hydrogen which the zinc is driving out of the acid
is exactly the same as the hydrogen you drove out of water with an
electric current. There is a metal called _sodium_ (Na) and another
called _potassium_ (K) which are as soft as stiff putty and as shiny
as silver; if you put a tiny piece of sodium (Na) or potassium (K) on
water, it will drive the hydrogen out of the water just as zinc drove
it out of the acid. The action is so swift and violent and releases so
much heat that the hydrogen which is set free catches fire. This makes
it look as if the metal were burning as it sputters around on top
of the water. There is so much sputtering that the experiment
is dangerous; people have been blinded by the hot alkaline water
spattering into their eyes. So you cannot try this until sometime when
you take a regular course in chemistry.

[Illustration: FIG. 164. Trying to see if hydrogen will burn.]

GETTING OXYGEN, A GAS, FROM TWO SOLIDS. Oxygen (O) can hide just as
successfully as hydrogen. Practically all elements can do the same by
combining with others. Here is an experiment in which you can get the
gas, oxygen, out of a couple of solids. If you went to the moon or
some other place where there is no air, you could carry oxygen
very conveniently locked up in these solid substances. Oxygen, you
remember, is the part of the air that keeps us alive when we breathe
it.

    EXPERIMENT 93. In a test tube mix about one half teaspoonful
    each of white potassium chlorate crystals and black grains of
    manganese dioxid. Put a piece of glass tubing through a cork
    so that the tubing will stick down a little way into the test
    tube. _Do not put the glass tubing through the cork while the
    cork is in the test tube: insert the glass tubing first, then
    put the cork into the test tube._ Put one end of a 2-foot
    piece of rubber tubing over the glass tube and put the other
    end into a pan of water.

    Fill a flask or bottle to the brim with water, letting it
    overflow a little; hold a piece of cardboard firmly over the
    mouth of the bottle; turn the bottle upside down quickly,
    putting the mouth of it under water in the pan; take the
    cardboard away. The water should all stay in the bottle.

    Now shove the rubber tube into the neck of the bottle until it
    sticks up an inch or two. During this experiment, be careful
    not to let the neck of the bottle or flask pinch the rubber
    tubing; small pieces of wood or glass tubing laid beside the
    rubber tubing where it goes under the run of the neck will
    prevent this.

    Hold, the test tube, tightly corked, over the flame of a
    burner, keeping the tube at a slant and moving it slightly
    back and forth so that all the material in it will be
    thoroughly heated. If you stop heating the test tube even for
    a couple of seconds, take the cork out; if you do not remove
    the cork, the cooling gas in the test tube will shrink and
    allow the water from the pan to be forced through the rubber
    tube into the test tube, breaking it into pieces.

    When enough gas has bubbled up into the bottle to force all
    the water out, and when bubbles begin to come up outside the
    bottle, uncork the test tube and lay it aside where it will
    not burn anything; then slide the cardboard under the mouth of
    the bottle and turn it right side up; leave the cardboard on
    the bottle.

    Light a piece of charcoal, or let a splinter of wood burn a
    few minutes and then blow it out so that a glowing coal will
    be left on the end of it. Lift the cardboard off the bottle
    and plunge the glowing stick into it for a couple of seconds.
    Cover the bottle after taking out the stick, and repeat, using
    a lighted match or a burning piece of wood instead of the
    glowing stick. If you dip a piece of iron picture wire in
    sulfur and light it, and then plunge it into the bottle, you
    will see iron burn.

[Illustration: FIG. 165. Filling a bottle with oxygen.]

[Illustration: FIG. 166. The iron really burns in the jar of oxygen.]

Both manganese dioxid and potassium chlorate have a great deal of
oxygen bound up in them. When they join together, as they do when you
heat them, they cannot hold so much oxygen, and it escapes as a gas.
In the experiment, the escaping oxygen passed through the tube, filled
the bottle, and forced the water out.

WHAT BURNING IS. When anything burns, it is simply joining oxygen.
When a thing burns in air, it cannot join the oxygen of the air very
fast, for every quart of oxygen in the air is diluted with a gallon of
a gas called _nitrogen_. Nitrogen will not burn and it will not
help anything else to burn. But when you have pure oxygen, as in the
bottle, the particles of wood or charcoal or picture wire can join it
easily; so there is a very bright blaze.

Although free oxygen helps things to burn so brilliantly, a match
applied to the solids from which you got it would go out. And while
hydrogen burns very easily, you cannot burn water although it is
two-thirds hydrogen. Water is H_2O, you remember.

WHAT COMPOUNDS ARE. When elements are combined with other elements,
the new substances that are formed are called _compounds_. Water
(H_2O) is a compound, because it is made of hydrogen and oxygen
combined.

When elements unite to form compounds, they lose their original
qualities. The oxygen in water will not let things burn in it; the
hydrogen in water will not burn. Salt (NaCl) is a compound. It is made
of the soft metal sodium (Na), which when placed on water sputters and
drives hydrogen out of the water, and the poison gas chlorine (Cl),
combined with each other. And salt is neither dangerous to put in
water like sodium, nor is it a greenish poison gas like chlorine.

MIXTURES. But sometimes elements can be mixed without their combining
to form compounds, in such a way that they keep most of their original
properties. Air is a mixture. It is made of oxygen (O) and nitrogen
(N). If they were _combined_, instead of _mixed_, they might form
laughing gas,--the gas dentists use in putting people to sleep when
they pull teeth. So it is well for us that air is only a _mixture_ of
oxygen and nitrogen, and _not_ a compound.

You found that things burned brilliantly in oxygen. Well, things burn
in air too, because a fifth of the air is oxygen and the oxygen of
the air has all its original properties left. Things do not burn as
brightly in air as they do in pure oxygen for the same reason that a
teaspoonful of sugar mixed with 4 teaspoonfuls of boiled rice does not
taste as sweet as pure sugar. The sugar itself is as sweet, but it is
not as concentrated. Likewise the oxygen in the air is as able to help
things burn as pure oxygen is; but it is diluted with four times its
own volume of nitrogen.

A solution is a mixture, too; for although substances disappear when
they dissolve, they keep their own properties. Sugar is sweet whether
it is dissolved or not. Salt dissolved in water makes brine; but the
water will act in the way that it did before. It will still help to
make iron rust; and salt will be salty, whether or not it is dissolved
in water. That is why solutions are only mixtures and are not chemical
compounds.

EVERYTHING IN THE WORLD IS MADE OF ATOMS. Everything in the world is
either an element or a compound or a mixture. Most plant and
animal matter is made of very complicated compounds, or mixtures of
compounds. All pure metals are elements; but metals, when they are
melted, can be dissolved in each other to form alloys, which really
are mixtures. Most of the so-called gold and silver and nickel
articles are really made of alloys; that is, the gold, silver, or
nickel has some other elements dissolved in it to make it harder,
or to impart some other quality. Bronze and brass are always alloys;
steel is generally an alloy made chiefly of iron but with other
elements such as tungsten, of which electric lamp filaments are made,
dissolved in it to make it harder. An alloy is a special kind of
solution not quite like an ordinary solution.

You remember that in the opening chapters we often spoke of molecules,
the tiny particles of matter that are always moving rapidly back
and forth. Well, if you were to examine a molecule of water with the
microscope which we imagined could show us molecules, you would find
that the molecule of water was made of three still smaller particles,
called _atoms_. Two of these would be atoms of hydrogen and would
probably be especially small; the third would be larger and would be
an oxygen atom.

In the same way if you looked at a _molecule_ of salt under this
imaginary microscope, you would probably find it made of _two atoms_,
one of sodium (Na) and one of chlorine (Cl), held fast together in
some way which we do not entirely understand.

The smallest particle of an _element_ is called an _atom_.

The smallest particle of a _compound_ is called a _molecule_.

Molecules are usually made of two or more atoms joined together.

    _APPLICATION 68._ In the following list tell which things
    are elements, which are compounds, and which are mixtures,
    remembering that both solutions and alloys are mixtures:

    Air, water, salt, gold, hydrogen, milk, oxygen, radium,
    nitrogen, sulfur, baking soda, sodium, diamonds, sweetened
    coffee, phosphorus, hydrochloric acid, brass.


INFERENCE EXERCISE

    Explain the following:

    431. Although in most electric lamps there is a vacuum between
    the glowing filaments and the glass, the glass nevertheless
    becomes warm.

    432. Clothes left out on the line overnight usually become
    damp.

    433. You can separate water into hydrogen and oxygen, yet
    you cannot separate the hydrogen or the oxygen into any other
    substances.

    434. Wet paper tears easily.

    435. Windows are soiled on the outside much more quickly in
    rainy weather than in clear weather.

    436. If you stir iron and sand together, the iron will rust as
    if alone.

    437. Rust is made of iron and oxygen, yet you cannot separate
    the iron from the oxygen with a magnet.

    438. A reading glass helps you to read fine print.

    439. Stretching the string of a musical instrument more
    tightly makes the note higher.

    440. Mayonnaise dressing, although it contains much oil, can
    readily be washed off a plate with cold water.


SECTION 47. _Burning: Oxidation._

    What makes smoke?

    What makes fire burn?

    Why does air keep us alive?

    Why does an apple turn brown after you peel it?

If oxygen should suddenly lose its power of combining with other
things to form compounds, every fire in the world would go out at
once. You could go on breathing at first, but your breathing would be
useless. You would shiver, begin to struggle, and death would come,
all within a minute or two. Plants and trees would die, but they would
remain standing until blown down by the wind. Even the fish in the
water would all die in a few minutes,--more quickly than they usually
do when we take them out of the water. In a very short time everything
in the world would be dead.

Then suppose that this condition lasted for hundreds and hundreds of
years, the oxygen remaining unable to combine with other elements.
During all that time nothing would decay. The trees would stay as they
fell. The corpses of people would dry and shrivel, but they would lie
where they dropped as perfectly preserved as the best of mummies. The
dead fish would float about in the oceans and lakes.

This is all because life is kept up by burning. And burning is simply
the combining of different things with oxygen. If oxygen ceased to
combine with the wood or gas or whatever fuel you use, that fuel could
not burn; how could it when "burning" _means_ combining with oxygen?
The heat in your body and the energy with which you move come entirely
from the burning (oxidation) of materials in your body; and that is
why you have to breathe; you need to get more and more oxygen into
your body all the time to combine with the carbon and hydrogen in the
cells of which your body is made. Plants breathe, too. They do not
need so much oxygen, since they do not keep warm and do not move
around; but each plant cell needs oxygen to live; there is burning
(oxidation) going on in every living cell. Fishes breathe oxygen
through their gills, absorbing the oxygen that is dissolved in the
water. They do not take the water apart to get some of the combined
oxygen from it; there is always some free oxygen dissolved in any
water that is open to the air. It is clear that fires would all go out
and everything would die if burning (combining with oxygen) stopped.

The reason things would not decay is that decay usually is a slow
kind of oxidation (burning). When it is not this, it is the action
of bacteria. But bacteria themselves could not live if they had _no_
oxygen; so they could not make things decay.

Not only would the dead plants and animals remain in good condition,
but the clothes people were wearing when they dropped dead would stay
unfaded and bright colored through all the storms and sunshine. And
the iron poles and car tracks and window bars would remain unrusted.
For bleaching and rusting are slow kinds of oxidation or burning
(combining with oxygen).

Here are two experiments which show that you cannot make things burn
unless you have oxygen to combine with them:

    EXPERIMENT 94. Light a candle not more than 4 inches long and
    stand it on the plate of the air pump. Cover it with the bell
    jar and pump the air out. What happens to the flame?

    EXPERIMENT 95. Fasten a piece of candle 3 or 4 inches long to
    the bottom of a pan. Pour water into the pan until it is about
    an inch deep. Light the candle. Turn an empty milk bottle
    upside down over the candle. Watch the flame. Leave the bottle
    over the candle until the bottle cools. Watch the water around
    the bottom of the bottle. Lift the bottle partly out of the
    water, keeping the mouth under water.

The bubbles that came out for a few seconds at the beginning of the
experiment were caused by the air in the bottle being heated and
expanded by the flame. Soon, however, the oxygen in the air was used
so fast that it made up for this expansion, and the bubbles stopped
going out. When practically all the oxygen was used, the flame went
out.

The candle is made mostly of a combination of hydrogen and carbon. The
hydrogen combines with part of the oxygen in the air that is in the
bottle to form a little water. The carbon combines with the rest of
the oxygen to make carbon dioxid, much of which dissolves in the water
below. So there is practically empty space in the bottle where the
oxygen was, and the air outside forces the water up into this space.
The rest of the bottle is filled with the nitrogen that was in the air
and that has remained unchanged.

About how much of the air was oxygen is indicated by the space that
the water filled after the oxygen was combined with the candle.

[Illustration: FIG. 167. The water rises in the bottle after the
burning candle uses up the oxygen.]

CARBON AND HYDROGEN THE CHIEF ELEMENTS IN FUEL. Carbon and hydrogen
make up the larger part of almost every substance that is used for
fuel, including gas, gasoline, wood, and soft coal; alcohol, crude
oil, kerosene, paper, peat, and the acetylene used in automobile and
bicycle lamps. Hard coal, coke, and charcoal are, however, chiefly
plain carbon. Since burning is simply the combining of things with
oxygen, it is plain that when the carbon of fuel joins oxygen we shall
get carbon dioxid (CO_2). When the hydrogen in the fuel joins oxygen,
what must we get?

When things do not burn up completely, the carbon may be left behind
as charcoal. That is what happens when food "burns" on the stove.
But if anything burns up entirely, the carbon or charcoal burns too,
passing off as the invisible gas, carbon dioxid, just as the hydrogen
burns to form steam or water.

It is because almost every fuel forms water when it burns, that we
find drops of water gathering on the outside of a cold kettle or cold
flatiron if either is put directly over a flame. The hydrogen in the
fuel combines with the oxygen of the air to form steam. As the steam
strikes the cold kettle or iron, it condenses and forms drops of
water.

NOTHING EVER DESTROYED. One important result of the discovery that
burning is only a combining of oxygen with the fuel was that people
began to see that nothing is ever destroyed. There is exactly as much
carbon in the carbon dioxid that floats off from a fire as there was
in the wood that was burned up; and there is exactly as much hydrogen
in the water vapor that floats off from the fire as there was in the
wood. Chemists have caught all the carbon dioxid and the water vapor
and weighed them and added their weight to the weight of the ashes;
and they have found them to weigh even more than the original piece of
wood, because of the presence of the oxygen that combined with them in
the burning.

If everything in the world were to burn up, using the oxygen that is
already here, the world would not weigh one ounce more or less than it
does now. All the elements that were here before would still be here;
but they would be combined in different compounds. Instead of wood
and coal and oxygen we should have water and carbon dioxid; instead of
diamonds, we should have just carbon dioxid; and so on with everything
that can burn.

WHY WATER PUTS OUT A FIRE. Water puts out a fire because it will not
let enough free oxygen get to the wood, or whatever is burning, to
combine with it. The oxygen that is locked up in a compound, like
water, you remember, has lost its ability to combine with other
things. Sand puts out a fire in the same way that water does. Most
fire extinguishers make a foam of carbon dioxid (CO_2) which covers
the burning material and keeps the free oxygen in the air from coming
near enough to combine with it.

Water will not put out burning oil, however, as the oil floats up on
top of the water and still combines with the oxygen in the air.

WHY ELECTRIC LAMPS ARE USUALLY VACUUMS. Electric lamps usually have
vacuums inside because the filament gets so hot that it would burn up
if there were any oxygen to combine with it. But in a globe containing
no oxygen the filament may be made ever so hot and it cannot possibly
burn.

High-power electric lamps are not made with vacuums but are
"gas-filled." The gas that is oftenest put into lamps is
nitrogen,--the same gas that is mixed with the oxygen in air. By
taking all the oxygen out of a quantity of air, the lamp manufacturers
can use in perfect safety the nitrogen that is left. It will not
combine with the glowing filament. There is no oxygen to combine with
the filament; so the lamp does not burn out.

WHAT FLAMES ARE. When you look at a flame, it seems as if fire were
a real thing and not merely a process of combining something with
oxygen. The flame is a real thing. It is made up of hot gases,
rising from the hot fuel, and it is usually filled with tiny glowing
particles of carbon. When you burn a piece of wood, the heat partly
separates its elements, just as heating sugar separates the carbon
from the water. Some of the hydrogen gas in the wood and some of the
carbon too are separated from the wood by the heat. These are pushed
up by the cooler air around and combine with the oxygen as they rise.
The hydrogen combines more easily than the carbon; part of the carbon
may remain behind as charcoal if your wood does not all burn up,
and many of the smaller carbon particles only glow in the burning
hydrogen, instead of burning. That is what makes the flame yellow. If
you hold anything white over a yellow flame, it will soon be covered
with black soot, which is carbon.

WHAT SMOKE IS. Smoke consists mostly of little specks of unburned
carbon. That is why it is gray or black. When you have black smoke,
you may always be sure that some of the carbon particles are not
combining properly with oxygen.

Yellow flames are usually smoky; that is, they usually are full of
unburned bits of carbon that float off above the flame. But by letting
enough air in with the flame, it is possible to make all the little
pieces of carbon burn (combine with the oxygen of the air) before they
leave the heat of the burning hydrogen. That is why kerosene lamps do
not smoke when the chimney is on. The chimney keeps all the hot gases
together, and this causes a draft of fresh air to blow up the chimney
to push the hot gases on up. The fresh air blowing up past the flame
gives plenty of oxygen to combine with the carbon. The drum part of
an oil heater acts in the same way; when the drum is open, the heater
smokes badly; when it is closed up, enough air goes past the flame
to burn up all the carbon. But if you turn either lamp or heater
too high, it will smoke anyway; you cannot get enough air through to
combine with all the carbon.

The hottest flames are the blue flames. That is because in a blue
flame all the carbon is burning up along with the hydrogen of the
fuel--both are combining with the oxygen of the air as rapidly as
possible. A gas or gasoline stove is so arranged that air is fed
into the burner with the gas. You will see this in the following
experiment:

    EXPERIMENT 96. Light the Bunsen burner in the laboratory. Open
    wide the little valve in the bottom. Now put your finger and
    thumb over the hole in the bottom of the burner. What happens
    to the flame? Turn the valve so that it will close the hole
    in the same way. Now hold a white saucer over the flame, being
    careful not to get it hot enough to break. What is the black
    stuff on the bottom of the saucer?

    Examine the gas plate (small gas stove) in the laboratory.
    Light it, and see if you can find the place where the air is
    fed in with the gas. Close this place and see what happens.
    Open it wider and see what happens. If the air opening is too
    large, the flame "blows"; there is too much cold air coming in
    with the gas, and your flame is not as hot as it would be if
    it did not "blow." Also, the stove is liable to "back-fire"
    (catch fire at the air opening) when the air opening is too
    wide.

[Illustration: FIG. 168. The Bunsen burner smokes when the air holes are
closed.]

    _APPLICATION 69._ An oil lamp tipped over and the burning oil
    spread over the floor. Near by were a pail of water, a pan of
    ashes, a rug, and a seltzer siphon. Which of these might have
    been used to advantage in putting out the fire?

    _APPLICATION 70._ My finger was burned. I wanted the flesh
    around it to heal and new skin cells to live and grow rapidly
    around the burn.

    "Put a rubber finger cot on the finger and keep all air out,"
    one friend advised me. "Air causes decay and will therefore be
    bad for the burn."

    "He's wrong; you should bandage it with clean cloth; you want
    air to reach the finger, I've heard," said another friend.

    "Oh, no, you don't; air makes things burn, and the burn will
    therefore get worse," still another one said. What should I
    have done?

    _APPLICATION 71._ Two students were discussing how coal was
    formed.

    "The trees must have fallen into water and been completely
    covered by it, or they would have decayed," said one.

    "Water makes things decay more quickly; there must have been
    a drought and the trees must have fallen on dry ground," said
    the second.

    Which was right?

[Illustration: FIG. 169. Regulating the air opening in a gas stove.]

    _APPLICATION 72._ A gas stove had a yellow flame. In front,
    by the handles, was a metal disk with holes so arranged that
    turning it to the left allowed air to mix with the gas on the
    way to the flame, and turning it to the right shut the air off
    (see Fig. 170).

    One member of the family said, "Turn the disk to the left and
    let more air mix with the gas."

    But another objected. "It has too much air already; that's why
    the flame is yellow. Turn it to the right and shut off the air
    from below."

    "You're both wrong. Why do you want to change it?" said a
    third member of the family. "The yellow flame is the hottest,
    anyway. Can't you see that the yellow flame gives more light?
    And don't you know that light is just a kind of radiant heat?
    Of course the yellow flame is the hottest. Leave the stove
    alone."

    Who was right?

[Illustration: FIG. 170. The air openings in the front of a gas stove.]


INFERENCE EXERCISE

    Explain the following:

    441. Iron tracks are welded together with an electric arc.

    442. The cool mirror in a bathroom becomes covered with
    moisture when you take a hot bath.

    443. This prevents you from seeing yourself in the mirror.

    444. Carbon dioxid has oxygen in it, yet a burning match
    dropped into a bottle of it will go out.

    445. A ship that sinks to the bottom of the ocean does not
    decay.

    446. When women put their hair in curlers, they usually
    moisten the hair slightly.

    447. To dry a pan after washing it, a person often sets it on
    the hot stove for a few minutes.

    448. When you put a kettle of cold water over a gas flame,
    drops of water appear on the lower part of the sides of the
    kettle.

    449. Electric power plants are often situated where running
    water will turn the dynamo. Explain the necessity of turning
    the dynamo.

    450. We make carbon dioxid by burning carbon, but you cannot
    put different things together to make carbon.


SECTION 48. _Chemical change caused by heat._

    Why do you have to strike a match to make it burn?

    How does pulling the trigger make a gun go off?

    What makes cooked foods taste different from raw ones?

Has it struck you as strange that we do not all burn up, since burning
is a combining with oxygen, and we are walking around in oxygen
all the time? The only reason we do not burn up is that it usually
requires heat to start a chemical change. You already know this in a
practical way. You know that you have to rub the head of a match and
get it hot before it will begin to burn; that gunpowder does not go
off unless you heat it by the sudden blow of the gun hammer which you
release when you pull the trigger; that you have to concentrate the
sun's rays with a magnifying glass to make it set a piece of paper on
fire; and that to change raw food into food that tastes pleasant you
have to heat it. If heat did not start chemical change, you could
never cook food,--partly because the fire would not burn, and
partly because the food would not change its taste even if heated by
electricity or concentrated sunlight.

Here is an experiment to show that gas will not burn unless it gets
hot enough:

    EXPERIMENT 97. Hold a wire screen 2 or 3 inches above the
    mouth of a Bunsen burner. Turn on the gas and light a match,
    holding the lighted match _above_ the screen. Why, do you
    suppose, does the gas below the screen not burn? Hold a
    lighted match to the gas below the screen. Does it burn now?

The reason the screen kept the gas below it from catching fire
although the gas above it was burning was this: The heat from the
flame above was conducted out to the sides by the wire screen as soon
as it reached the screen; so very little heat could get through the
screen to the gas below. Therefore the gas below the screen never got
hot enough for the chemical change of oxidation, or burning, to take
place. So the gas below it did not catch fire.

Another simple experiment with the Bunsen burner, that shows the same
thing in a different way, is this:

[Illustration: FIG. 171. Why doesn't the flame above the wire gauze
set fire to the gas below?]

    EXPERIMENT 98. Light the Bunsen burner. Open the air valve at
    the bottom all the way. Hold the wood end of a match (not the
    head) in the center of the inner greenish cone of flame, about
    half an inch above the mouth of the burner. Does the part of
    the match in the center of the flame catch fire? Does the part
    on the edge? What do you suppose is the reason for this? Where
    are the cold gas and air rushing in? Can they get hot all at
    once, or will they have to travel out or up a way before they
    have time to get hot enough to combine?

[Illustration: Fig. 172. The part of the match in the middle of the
flame does not burn.]

    _APPLICATION 73._ Explain why boiled milk has a different
    taste from fresh milk; why blowing on a match will put it out;
    why food gets black if it is left on the stove too long.


INFERENCE EXERCISE

    Explain the following:

    451. When you want bread dough to rise, you put it in a warm
    place.

    452. Ink left long in an open inkwell becomes thick.

    453. A ball bounces up when you throw it down.

    454. When the warm ocean air blows over the cool land in the
    early morning, there is a heavy fog.

    455. Striking a match makes it burn.

    456. When you have something hard to cut, you put it in the
    part of the scissors nearest the handles.

    457. A magnet held over iron filings makes them leap up.

    458. Dishes in which flour thickening or dough has been mixed
    should be washed out with cold water.

    459. A woolen sweater is liable to stretch out of shape after
    being washed.

    460. When a telegraph operator presses a key in his set, a
    piece of iron is pulled down in the set of another operator.


SECTION 49. _Chemical change caused by light._

    How can a camera take a picture?

    Why does cloth fade in the sun?

    What makes freckles?

If light could not help chemical change, nothing would ever fade when
hung in the sun; wall paper and curtains would be as bright colored
after 20 years as on the day they were put up, if they were kept
clean; you would never become freckled, tanned, or sunburned; all
photographers and moving-picture operators would have to go out of
business; but worst of all, every green plant would immediately stop
growing and would soon die. Therefore, all cows and horses and other
plant-eating animals would die; and then the flesh-eating animals
would have nothing to eat and they would die; and then all people
would die.

You will be able better to understand why all this would happen after
you do the following experiments, the first of which will show that
light helps the chemical change called bleaching or fading.

    EXPERIMENT 99. Rinse two small pieces of light-colored cloth.
    (Lavender is a good color for this experiment.) Lay one piece
    in the bright sun to dry; dry the other in a dark cabinet or
    closet. The next day compare the two cloths. Which has kept
    its color the better? If the difference is not marked, repeat
    the experiment for 2 or 3 days in succession, putting the same
    cloth, wet, in the sun each time.

Bleaching is usually a very slow kind of burning. It is the dye that
is oxidized (burned), not the cloth. Most dyes will combine with the
oxygen in the air _if they are exposed to the sunlight_. The dampness
quickens the action.

WHY SOME PEOPLE FRECKLE IN THE SUN. When the sunlight falls for a long
time on the skin, it often causes the cells in the under part of
the skin to produce some dark coloring matter, or pigment. This dark
pigment shows through the outer layer of skin, and we call the little
spots of it _freckles_. Some people are born with these pigment spots;
but when the freckles come out from long exposure to the sunlight,
they are an example right in our own skins of chemical change caused
by the action of light. Tan also is due to pigment in the skin and is
caused by light.

The next experiments with their explanations will show you how
cameras can take pictures. If you are not interested in knowing how
photographs are made, do the experiments and skip the explanations
down to the middle of page 332.

    EXPERIMENT 100. Dissolve a small crystal of silver nitrate
    (AgNO_3) in about half an inch of pure water in the bottom
    of a test tube. Distilled water is best for this purpose.
    Now add one drop of hydrochloric acid (HCl). The white powder
    formed is a silver salt, called _silver chlorid_ (AgCl); the
    rest of the liquid is now a diluted nitric acid (HNO_3).

    Pour the suspension of silver chlorid (AgCl) on a piece of
    blotting paper or on a paper towel, so that the water will be
    absorbed. Spread the remaining white paste of silver chlorid
    (AgCl) out over the blotter as well as you can. Cover part of
    it with a key (or anything that will shut off the light), and
    leave the other part exposed. If the sun is shining, put the
    blotter in the sunlight for 5 minutes. Otherwise, let as much
    daylight fall on it as possible for about 10 minutes. Now take
    the key off the part of the silver chlorid (AgCl) that it was
    covering and compare this with the part that was exposed
    to the light. What has the light done to the silver chlorid
    (AgCl) that it shone on?

What has happened is that the light has made the silver (Ag)
_separate_ from the chlorine (Cl) of the silver chlorid (AgCl).
Chemists would write this:

  AgCl -> Ag + Cl.

That is, silver chlorid (AgCl) has changed into silver (Ag) and
chlorine (Cl). Chlorine, as you know, is a poisonous gas, and it
floats off in the air, leaving the fine particles of silver behind.
When silver is divided into very tiny particles, it absorbs light
instead of reflecting it; so it looks dark gray or black.

HOW PHOTOGRAPHS ARE MADE. All photography depends on this action of
light. The plates or films are coated with a silver salt,--usually a
more sensitive salt than silver chlorid. This is exposed to the light
that shines through the lens of the camera. As you have learned, the
lens brings the light from the object to a focus and makes an image
on the film or plate. The light parts of this image will change the
silver salt to silver; the dark parts will not change it. So wherever
there is a white place on the object you are photographing, there will
be a dark patch of silver on the film or plate, and wherever there
is a dark spot on the object, there will be no change on the film or
plate.

[Illustration: FIG. 173. The silver salt on the paper remains white
where it was shaded by the key.]

As a matter of fact, the film or plate is exposed such a short
time that there is not time for the change to be completed. So the
photographer develops the negative; he washes it in some chemicals
that finish the process which the light started.

If he exposed the whole plate to the light now, however, all the
_unchanged_ parts of the silver salt would also be changed by the
light, and there would be no picture left. So before he lets any light
shine on it, except red light which has no effect on the silver salt,
he dissolves off all the white unchanged part of the silver salt,
in another kind of chemical called the _fixing bath_. This is called
"fixing" the negative.

The only trouble with the picture now is that wherever there should
be a patch of white, there is a patch of dark silver particles; and
wherever there should be a dark place, there is just the clear glass
or celluloid, with all the silver salt dissolved off. This kind of
picture is called a _negative_; everything is just the opposite shade
from what it should be. A white man dressed in a black suit looks like
a negro dressed in a white suit.

HOW A PHOTOGRAPHIC PRINT IS MADE. The negative not only has the lights
and shadows reversed, but it is on celluloid or glass, and except
for moving pictures and stereopticons, we usually want the picture on
paper. So a print is made of the negative. The next experiment will
show you how this is done.

    EXPERIMENT 101. In a dark room or closet, take a sheet of
    blueprint paper from the package, afterwards closing the
    package carefully so that no light can get to the papers
    inside. Hold the piece of blueprint paper under your waist or
    coat, to keep it dark when you go into the light. Now lay it,
    greenish side downward, on a negative. Hold the two together,
    or place them in a printing frame, and turn them over so that
    the light will shine through the negative upon the greenish
    side of the blueprint paper. Be sure that the paper is held
    firmly against the negative and not moved around. Let the sun
    shine through the negative upon the paper for 1 or 2 minutes
    according to the brightness of the sun, or let the gray light
    of the sky, if it is cloudy, shine on it for 5 or 10 minutes.
    Now quickly put the blueprint paper (not the negative) into a
    basin of water, face down. Wash for a couple of minutes. Turn
    it over and examine it. If it has been exposed to the light
    too long, it will be dark; if it has been exposed too short
    a time, it will be too light; in either case, if the print
    is not clear, repeat with a fresh piece of blueprint paper,
    altering the time of exposure to the sunlight to improve the
    print.

    You can make pretty outline pictures of leaves and pressed
    flowers, or of lace, by laying these on the blueprint paper in
    place of the negative and in other respects doing as directed
    above.

[Illustration: FIGS. 174 and 175. Where the negative is dark, the print
is light.]

In making blueprints you are changing an iron salt instead of a silver
salt, by the action of light. Regular photographic prints are usually
made on paper treated with a silver salt rather than with iron salt,
and sometimes a gold or platinum salt is used. But these other salts
have to be washed off with chemicals since they do not come off in
water, as the unchanged part of the iron salt comes off when you fix
the blueprint paper in the water bath.

Since the light cannot get through the black part of a negative, the
coating on the paper behind that part is not affected and it stays
light colored; and since the light can get through the clear parts
of the negative, the coating on the paper back of those parts _is_
affected and becomes dark. Therefore, the print is "right side
out,"--there is a light place on the print for every white place on
the object photographed, and there is a dark place on the print for
every black place on the object.

Moving-picture films are printed from one film to another, just as you
printed from a negative to a piece of paper. The negative is taken
on one film, then this is printed on another film. The second film is
"right side out."

LIGHT AND THE MANUFACTURE OF FOOD IN PLANTS. Much the most important
chemical effect of light, however, is not in making photographs, in
bleaching things, or in "burning" your skin. It is in the putting
together of carbon and water to make sugar in plants. Plants get water
(H_2O) from the earth and carbon dioxid (CO_2) from the air. When
the sun shines on chlorophyll, the green substance in plants, the
chlorophyll puts them together and makes sugar. The plant changes this
sugar into starch and other foods, and into the tissues of the plant
itself. Nothing in the world can put carbon dioxid and water together
and make food out of them except certain bacteria and the chlorophyll
of plants. And light is absolutely necessary for this chemical action.
Try this experiment:

    EXPERIMENT 102. Pin together two pieces of cork on opposite
    sides of a leaf that is exposed to the sun. The next day take
    this leaf from the plant and heat it in a beaker of alcohol
    until the green coloring matter is removed from the leaf. Then
    place the leaf in a glass of water that contains iodine.
    The iodine will color the leaf dark where the cells contain
    starch. (See Experiment 115, page 373.) Is starch formed where
    the light does not reach the leaf?

No plant can make food except with the help of light. The part of the
plant that can put carbon dioxid and water together is the green stuff
or chlorophyll, and this can work only when light is shining on it. So
all plants would die without light.

But if all plants should die, all animals would die also, for animals
cannot make food out of carbon dioxid and water, as they do not have
the chlorophyll that puts these things together. A lion does not live
on leaves, it is true, but he lives on deer and other animals that
do live on leaves and plants. If the plants died, all plant-eating
animals would die. Then there would be nothing for the flesh-eating
animals to eat except each other, and in time no animals would be left
in the world. The same thing would happen to the fish. And man, of
course, could no longer exist. The food supply of the world depends on
the fact that light can start chemical change.

OXYGEN RELEASED IN THE MANUFACTURE OF PLANT FOOD. Besides in one way
or another giving us all of our food, plants, helped by light, also
give us most of the free oxygen that we breathe. We and all animals
get the energy by which we live by _combining_ oxygen with the
hydrogen of our food (forming water) and by combining oxygen with the
carbon in our food (forming carbon dioxid). This combining (burning
or oxidizing) gives us our body heat and the energy to move. The free
oxygen is carried to the different parts of our bodies by the red
blood corpuscles that float in the liquid part of the blood. The
liquid part of the blood also carries the food to the different parts
of the body, and the food contains the carbon and hydrogen that is to
be burned. Then in a muscle, for instance, the oxygen that has been
carried by the corpuscles combines with the carbon to form carbon
dioxid, and with the hydrogen to form water. The corpuscles carry part
of the carbon dioxid back to the lungs, and the water is carried with
other wastes and the rest of the carbon dioxid in the liquid part of
the blood. In the lungs the carbon dioxid is exchanged for the free
oxygen we have just inhaled, and we exhale the carbon dioxid. A good
deal of water is also breathed out, as you can tell from the way the
mist gathers on a window pane when you blow on it.

If there were only animals (including people) in the world, all the
free oxygen in the air would in time be combined by the animals with
hydrogen to make water and with carbon to make carbon dioxid (CO_2).
As animals cannot breathe water and cannot get any good from carbon
dioxid, they would all smother.

But the plants, as we have already said, use carbon dioxid (CO_2)
and water (H_2O) to make food. They do not need so much oxygen, and
so they set some of it free. The countless plants in the world set
the oxygen free as rapidly as the countless animals combine it with
hydrogen to make water and with carbon to make carbon dioxid. Since
the water and carbon dioxid are the main things a plant needs to
make its food, the animals really are as helpful to the plants as the
plants are to the animals. For the animals furnish the materials to
the plants for making their food in exchange for the ready-made food
furnished by the plant. And both plants and animals would die if light
stopped helping to bring about chemical change.

    _APPLICATION 74._ Explain why the heart of a cabbage is white
    instead of green like the outside leaves; why a photographer
    works in a dark room with only a ruby light; why you get
    freckled in the sun.


INFERENCE EXERCISE

    Explain the following:

    461. If a pin is put through a lamp cord, a fuse is likely to
    blow out.

    462. The wall paper back of a picture is often darker than
    that on the rest of the wall.

    463. If you wet an eraser, it rubs through the paper.

    464. Clothes are hot after being ironed.

    465. If you drop candle grease on your clothes, you can remove
    the grease by placing a blotter over it and pressing the
    blotter with a warm iron.

    466. Milliners cover hats that are on display in windows where
    the sun shines in on the hats.

    467. You pull down on a rope when you try to climb it.

    468. In taking a picture, you expose the sensitive film or
    plate to the light for a short time.

    469. Good cameras have an adjustable front part so that the
    lens may be moved nearer to the plate or film, or farther
    from it, according to the distance of the object to be
    photographed.

    470. A pencil has to be resharpened frequently when it is much
    used.


SECTION 50. _Chemical change caused by electricity._

    How are storage batteries charged?

    How is silver plating done by electricity?

You have already done an experiment showing that electricity can start
chemical change, for you changed water into hydrogen and oxygen by
passing a current of electricity through the water.

The plating of metals is made possible by the fact that electricity
helps chemical change. You can nickel plate a piece of copper in the
following manner:

    EXPERIMENT 103. Dissolve a few green crystals of "double
    nickel salts" in water, until the water is a clear green. The
    water should be about 2 or 3 inches deep in a glass or china
    bowl that is not less than 5 inches across.

    Lay two bare copper wires across the bowl, about 3 inches
    apart, as shown in Figure 177. Connect the positive wire from
    a storage battery, or the wire from the carbon of a battery of
    three or four cells, to an end of one bare wire. Connect the
    negative wire from the storage or the negative wire from the
    zinc of the other battery to an end of the second bare wire.

    Now fasten a fine bare wire 5 or 6 inches long around a small
    piece of copper, and another like it around a piece of nickel,
    as shown in Figure 176. Then put the piece of copper in the
    bottom of an evaporating dish, with the wire hanging out, as
    in Figure 177.

[Illustration: FIG. 176. The copper and the nickel cube ready to hang in
the cleansing solution.]

[Illustration: FIG. 177. Cleaning the copper in acids.]

Pour over the piece of copper enough of the cleansing solution to
cover it.[9] _The cleansing solution contains strong acids. If you get
any on your skin or clothes, wash it off immediately with ammonia or
soda._ As soon as the copper is bright and clean, take it out of the
cleansing solution and suspend it by the _negative_ wire in the green
nickel solution. You can tell if you have it on the negative wire, for
in that case bubbles will rise from it during the experiment. The copper
should be entirely covered by the nickel solution, but should not touch
the bottom or sides of the bowl. Pour the cleansing solution from the
evaporating dish back into the bottle. Suspend the nickel, in the same
way as the copper, from the _positive_ wire crossing the bowl. When
set up, the apparatus should appear as shown in Figure 178.

[Footnote 9: The formula for making the cleansing solution is as
follows:

1 cup water.

1 cup concentrated sulfuric acid.

1 cup concentrated nitric acid.

1 teaspoonful concentrated hydrochloric acid.

The sulfuric and nitric acids must be measured in glass or china cups,
and the hydrochloric acid must be measured in a silver-plated spoon
or in glass--not in tin.]

[Illustration: FIG. 178. Plating the copper by electricity.]

Turn on the electricity. If the copper becomes black instead of
silvery, clean it again in the cleansing solution, and move the two
bare wires much farther apart,--practically the full width of
the bowl. If the copper still turns black, it means that too much
electricity is flowing. In that case use fewer batteries.

The electricity has started two chemical changes. It has made part of
the piece of nickel combine with part of the solution of nickel salt
to form more nickel salt, and it has made some of the nickel salt
around the copper change into metallic nickel. Then the negative
electricity in the copper has attracted the positive bits of nickel
metal made from the nickel salt, and made them cling to the copper. If
there is no dirt or grease on the copper, the particles of nickel get
so close to it that they stick by adhesion, even after the electric
attraction has ceased. This leaves the copper nickel-plated, but to
make it shiny the nickel plating must be polished.

Silver plating and gold plating are done substantially in the way that
you have done the nickel plating, only gold salt or silver salt is
used instead of nickel salt.

Just as electricity helps chemical changes in plating, it helps
changes in a storage battery. But in the storage battery the new
compounds formed by "charging" the battery change back again and
generate electricity when the poles of the battery are connected with
each other by a good conductor.

    _APPLICATION 75._ Explain how spoons can be silver plated; how
    water can be changed into hydrogen and oxygen.


INFERENCE EXERCISE

    Explain the following:

    471. Clothes dry best in the sun and wind.

    472. Proofs of photographs that have not been thoroughly
    "fixed" fade if left out of their envelope.

    473. Blowing a match puts it out, yet a good draft is
    necessary for a hot fire.

    474. A cup does not naturally fall apart, yet after it is
    broken it falls apart even if you fit the pieces together
    again.

    475. Crayon leaves marks on a blackboard.

    476. A baked potato tastes very different from a raw one.

    477. An air-filled automobile tire is harder at noon than in
    the early morning.

    478. When a live trolley wire breaks and falls to the street,
    it becomes so hot that it burns.

    479. Glass jars of fruit should be kept in a fairly dark
    place.

    480. You wash dishes in _hot_ water.


SECTION 51. _Chemical change releases energy._

    Why is fire hot?

    What makes glowworms glow?

    Why does cold quicklime boil when you pour cold water on it?

If no energy were released by chemical change, we should run down like
clocks, and could never be wound up again. We could breathe, but to do
so would do us no more good than it would if oxygen could not combine
with things. Oxidation would go on in our bodies, but it would neither
keep us warm nor help us to move. A few spasmodic jerks of our hearts,
a few gasps with our lungs, and they would stop, as the muscles would
have no energy to keep them going.

The sunlight _might_ continue to warm the earth, as we are not sure
that the sun gets any of its heat from chemical change. But fires,
while they would burn for an instant, would be absolutely cold; no
energy would be given out by the fuel combining with oxygen. But the
fires could not burn long, because there would be nothing to keep the
gases and fuel hot enough to make them combine with the oxygen.

Even during the instant that a fire lasted it would be invisible, for
it would give off no light if no energy were released by the chemical
change. Only electric lights and heaters would continue to work, and
even some of these would fail. The electric motors in submarines and
electric automobiles would instantly stop; battery flashlights would
go out as quickly as the fire; no doorbells would ring. In short, all
forms of electric batteries would stop sending currents of electricity
out through their wires, and everything depending upon batteries would
stop running.

A fire gives out heat and light; both are kinds of energy. And it is
the electric energy caused by the chemical change in batteries that
runs submarines, electric automobiles, flashlights, and doorbells.
Since burning (oxidation) is simply a form of chemical change, it is
not difficult to realize that chemical change releases energy.

WHY GLOWWORMS GLOW. When a glowworm glows at night, or when the head
of a match glows as you rub it on your wet hand in the dark, we
call the light _phosphorescence_. The name "phosphorus" means
light-bearing, and anything like the element phosphorus, that glows
without actively burning, is said to be phosphorescent. Match heads
have phosphorus in them. Phosphorescence is almost always caused
by chemical change. The energy released is a dim light, not heat or
electricity. Sometimes millions of microscopic sea animals make
the sea water in warm regions phosphorescent. They, like fireflies,
glowworms, and will-o'-the-wisps, have in them some substance that is
slowly changing chemically, and energy is released in the form of dim
light as the change takes place. Most luminous paint is phosphorescent
for the same reason,--there is a chemical change going on that
releases energy in the form of light.

When you poured the hydrochloric acid on the zinc to make hydrogen,
the flask became warm; the chemical change going on in the flask
released heat energy.

    _APPLICATION 76._ Explain why pouring cold water on cold
    quicklime makes the slaked lime that results boiling hot; why
    a cat's eyes shine in the dark; why a piece of carbon and a
    piece of zinc placed in a solution of sal ammoniac will make
    electricity run through the wire that connects them; why fire
    is hot.


INFERENCE EXERCISE

    Explain the following:

    481. A baking potato sometimes bursts in the oven.

    482. Turpentine is used in mixing paint.

    483. Sodium is a metal; chlorine is a poisonous gas; yet salt,
    which is made up of these two, is a harmless food.

    484. When bricklayers mix water with cement and lime, the
    resulting mortar boils and steams.

    485. Green plants will not grow in the dark.

    486. Parts of the body are constantly uniting with oxygen.
    This keeps the body warm.

    487. Water will not always put out a kerosene fire.

    488. Delicately colored fabrics should be hung in the shade to
    dry.

    489. A match glows when you rub it in the dark.

    490. Candy hardens when it cools.


SECTION 52. _Explosions._

    What makes a gun shoot?

    What makes an automobile go?

Usually we think of explosions as harmful, and they often are, of
course. Yet without them we could no longer run automobiles; gasoline
launches would stop at once; motorcycles would no longer run; gasoline
engines for pumping water or running machinery would not be of any
use; and all aviation would immediately cease. Tunneling through
mountains, building roads in rocky places, taking up tree stumps,
and preparing hard ground for crops would all be made very much more
difficult. War would have to be carried on much as it was during
the Middle Ages; soldiers would use spears and bows and arrows;
battleships would be almost useless in attacking; modern forts would
be of little value; cannon, guns, rifles, howitzers, mortars, and
revolvers would all be so much junk.

[Illustration: FIG. 179. The explosion of 75 pounds of dynamite. A
"still" from a motion-picture film.]

[Illustration: FIG. 180. Diagram of the cylinder of an engine. The
piston is driven forward by the explosion of the gasoline in the
cylinder.]

WHAT MAKES AN AUTOMOBILE GO. In all the above cases the explosions
are caused by chemical action. When gasoline mixed with air is sprayed
into the cylinder of an automobile, an electric spark makes the
gasoline combine with the oxygen of the air; the gasoline suddenly
burns and changes to steam and carbon dioxid. As you already know,
when a liquid like gasoline turns to gases such as steam and carbon
dioxid, the gases take much more room. But that is not all that
happens. Much heat is released by the burning of the gasoline spray,
and heat causes expansion. So the gases formed by the burning gasoline
are still further expanded by the heat released by the burning.
Therefore they need a great deal more room; but they are shut up in a
small place in the top of a cylinder. The only thing to hold them up
in this small space, however, is a piston (Fig. 180), and the suddenly
expanding gases shove this piston down and escape. The piston is
attached to the drive wheel of the automobile, and when the piston is
pushed down it gives the automobile a push forward. If it were not for
the expansion of a gas in the cylinder, this gas being confined to a
small space, the piston would not be pushed down.

An explosion is simply the sudden pushing of a confined gas expanding
on its way to freedom. The gasoline vapor and air were the confined
gas. Their chemical combining made them expand; by pushing the piston
out of its way the newly formed gas suddenly freed itself. This was
an explosion, and it gave the automobile one forward push. But the
automobile engine is so arranged that the piston goes up into the
cylinder again, and is pulled down again, drawing a spray of gasoline
and air into the cylinder after it. Then it goes up a second time,
an electric spark explodes the gasoline, the piston is forced down
violently once more, and so it goes on. There are several cylinders,
of course, and the explosions take place within them one after the
other so as to keep the automobile going steadily.

HOW A GUN SHOOTS. Pulling a trigger makes a gun shoot by causing an
explosion. There is a spring on the hammer of a gun. This drives
the hammer down suddenly when you release the spring by pulling the
trigger. The hammer jars the chemicals in the cap and causes them to
explode. The heat and flame then cause the oxygen in the gunpowder to
combine with some of the other elements in the powder to make a gas.
The gas requires more room than the powder and is further expanded
by the heat released by the chemical change. The expanding gas frees
itself by pushing the bullet out of its way. The bullet gets such a
push through the exploding of the gunpowder that it may fly to a mark
and pierce it.

[Illustration: FIG. 181. The most powerful explosions on earth occur
in connection with volcanic activity. The photograph shows Mt. Lassen,
California, the only active volcano in the United States.]

There is a slight explosion even when you shoot an air gun. First you
compress some air in the upper part of the barrel of the air gun; then
you suddenly release it. The only thing in the way of the expanding
air is the bullet; so the air pushes this out in front of it.

In Experiment 36, where you stoppered a test tube containing a little
water and then held the tube over a flame until the cork flew out, you
were causing an explosion. As the water changed to steam, the steam
was an expanding gas. It was at first confined to the test tube by the
cork. Then there was an explosion; the gas freed itself by blowing out
the cork.

Steam boilers have safety valves to prevent explosions. These are
valves so arranged that when the steam expands and presses hard enough
to endanger the boiler, the steam will open the valves and escape
instead of bursting the boiler to get free.

EXPLOSIVES. Dynamite, gunpowder, and most explosives are mixtures of
solids or liquids that will combine easily and will form gases that
expand greatly as a result of the combination. One of the essentials
in explosives is some compound of oxygen (such as the manganese dioxid
or potassium chlorate you used to make oxygen in Experiment 93) which
will easily set its oxygen free. This oxygen combines very swiftly
with something else in the explosive, releasing heat and forming a
gas that takes much more room. In its effort to free itself, this
expanding gas will blast rocks out of the way, shoot cannon balls, or
do any similar work.

But if gunpowder does not have to push anything of much importance out
of its way to expand, there is no explosion. That is why a firecracker
merely fizzes when you break it in two and light the powder. The
cardboard no longer confines the expanding gas; so there is nothing to
burst or to push violently out of the way.

Useful explosions are generally caused by a chemical action which
suddenly releases a great deal of heat and combines solid things into
expanding gases. But the bursting of a steam boiler, or the "blow
out" of an automobile tire, or the bursting of a potato in the oven,
although not caused by chemical action, still are real explosions. An
explosion is the _sudden_ release of a confined gas.

    _APPLICATION 77._ Explain how gasoline makes a motorcycle go,
    and why it goes "pop, pop, pop." Explain why a paper bag
    will burst with a bang, when you blow it up and then clap it
    between your hands; why a Fourth-of-July torpedo "goes off"
    when you throw it on the pavement.


INFERENCE EXERCISE

    Explain the following:

    491. The engine of an automobile is cooled by the water that
    passes over it from the radiator.

    492. When you light a firecracker, the flame travels down the
    wick until it reaches the gunpowder, and then the firecracker
    bursts with a bang.

    493. If you light a small pile of gunpowder in the open, as
    you do when you make a squib by breaking the firecracker in
    two, you merely have a blaze.

    494. Air-filled tires make bicycles ride much more evenly than
    solid tires would.

    495. When clay has once been baked into brick, you can never
    change it back to clay.

    496. A photographic negative turns black all over if it is
    exposed to the light before it is "fixed."

    497. The outside of a window shade fades.

    498. A vacuum electric lamp globe feels hot instantly when
    turned on, but if turned off again at once, it immediately
    feels cold.

    499. Coal gas is made by heating coal very hot in an air-tight
    chamber.

    500. White straw turns yellow when it is long exposed to the
    sunlight.




CHAPTER ELEVEN

SOLUTION AND CHEMICAL ACTION


SECTION 53. _Chemical change helped by solution._

    Why does iron have to get wet to rust?

    Is it good to drink water with your meals?

When iron rusts, it is really slowly burning (combining with oxygen).
If your house is on fire, you throw water on it to stop the burning.
Yet if you throw water on iron it rusts, or burns, better than if you
leave it dry. What do you suppose is the reason for this?

The answer is not difficult. You know perfectly well that iron does
not burn easily; we could not make fire grates and stoves out of iron
if it did. But when iron is wet, a little of it dissolves in the water
that wets it. There is also a little oxygen dissolved in the water,
as we know from the fact that fish can breathe under the water. This
_dissolved_ oxygen can easily combine with the _dissolved_ iron;
the _solution_ helps the chemical change to take place. The
chemical change that results is oxidation,--the iron combining with
oxygen,--which is a slow kind of burning; and in iron this is usually
called _rusting_.[10] But when we pour water on burning wood, the wood
_stops_ burning, for there is not nearly enough oxygen dissolved in
water to combine rapidly with burning wood; and the water shuts off
the outside air from burning wood and therefore puts the fire out.

[Footnote 10: The rusting of iron is not quite as simple as this, as
it probably undergoes two or three changes before finally combining
with oxygen. But the solution helps all these changes.]

Another chemical change, greatly helped by solution, is the combining
of the two things that baking powder is made of, and the setting
free of the carbon dioxid (CO_2) that is in one of them. Try this
experiment:

    EXPERIMENT 104. Put half a teaspoonful of baking powder in the
    bottom of a cup and add a little water. What happens?

The chemical action which takes place in the baking powder and
releases the gas in bubbles--the gas is carbon dioxid (CO_2)--will not
take place while the baking powder is dry; but when it is dissolved,
the chemical change takes place in the solution.

If you ate your food entirely dry, you would have a hard time
digesting it; and this would be for the same reason that baking powder
will not work without water. Perhaps you can drink too much water with
a meal and dilute the digestive juices too much; certainly you should
not use water to wash down your food and take the place of the saliva,
for the saliva is important in the digestion of starch. But you need
also partly to dissolve the food to have it digest well. Crackers and
milk are usually more easily digested than are plain crackers, for the
milk partly dissolves the crackers, and drinking one or two glasses of
water with a meal hastens the digestion of the food.

    _APPLICATION 78._ Explain why paint preserves wood; why iron
    will rust more quickly in a wet place than it will either
    under water or in a dry place; why silver salts must be
    dissolved in order to plate a spoon by electricity.


INFERENCE EXERCISE

    Explain the following:

    501. There is dew on the grass early in the morning.

    502. Cold cream makes your hands and face soft.

    503. Glowworms and fireflies can be seen on the darkest
    nights.

    504. A lake looks gray on a cloudy day and blue on a clear
    day.

    505. Dried fruit will keep much longer than fresh fruit.

    506. If you scratch a varnished surface, you can rub the
    scratch out completely by using a cloth wet with alcohol.

    507. Soda is usually dissolved in a little water before it is
    added to a sour-milk batter.

    508. Iron rusts when it gets wet.

    509. Peroxide is usually kept in brown bottles.

    510. Dry lye may be kept in tin cans, but if the lye is
    _moistened_ it will eat the can.


SECTION 54. _Acids._

    Why are lemons sour?

    How do acids act?

Some acids are very powerful. There is one, called _hydrofluoric
acid_, that will eat through glass and has to be kept in wax bottles;
and all acids tend to eat or corrode metals. You saw what hydrochloric
acid did to the zinc shavings when you wanted to make a balloon; or,
to be more accurate, you saw what the zinc shavings did to the acid,
for the hydrogen gas that bubbled off was driven out of the acid by
the zinc. Then the zinc combined with the rest of the acid to form
what chemists call a _salt_.

If we were to let the soft metal, sodium, act on hydrochloric acid,
we should get hydrogen also; but the salt that formed would be regular
table salt (NaCl). You cannot do this experiment, however, as the
sodium does its work so violently that it is dangerous.

    EXPERIMENT 105. _To be done by the teacher before the class.
    If acid spatters on any one's skin or clothes, wash it of
    immediately with ammonia or a strong soda solution._

    Drop a little candle grease on a piece of copper about 3/4
    inch wide and 2 or 3 inches long. In the flame of a Bunsen
    burner, gently heat the end of the copper that has the candle
    grease (paraffin) on it, so that the paraffin will spread out
    all over the end. Let it harden. With a nail, draw a design in
    the paraffin on the copper, scratching through the thin coat
    of paraffin to the copper below. Pour a couple of drops of
    concentrated nitric acid on the paraffin-covered end of the
    piece of copper, and spread the acid with a match so that
    it can get down into the scratches. Let it stand by an open
    window for 5 or 10 minutes. Do not inhale the brown fumes
    that are given off. They are harmless in small amounts, but if
    breathed directly they are very irritating. Now wash off the
    acid by holding the copper under the hydrant, and scrape off
    the paraffin.

[Illustration: FIG. 182. Etching copper with acid.]

The nitric acid did to the copper in this experiment exactly what
the hydrochloric acid did to the zinc shavings when you made the toy
balloon. The copper drove the hydrogen out of the nitric acid and
incidentally broke down some of the nitric acid to make the brown gas,
and then the copper joined the rest of the nitric acid to make a
salt called _copper nitrate_. This salt is green, and it dissolves in
water. When you washed the copper, the green salt was washed away and
a dent remained in the copper where the copper salt had been.

Here is a more practical experiment showing the action of acid on
metal:

    EXPERIMENT 106. Use two knives, one of bright steel and the
    other a silver-plated one. If the steel knife is not bright,
    scour it until it is. Drop a little lemon juice on each knife
    and let it stand for a few minutes, while the teacher does the
    next experiment. Then rinse both knives and examine them. What
    has the lemon juice done to the silver knife? to the steel
    one?

The lemon juice acts in this way because it is acid. Acids act on the
taste nerves in the tongue and give the taste of sourness; everything
sour is an acid. The black stuff formed on the steel knife by the
lemon juice is an iron salt. The iron in the knife drove the hydrogen
out of the lemon juice, but there was not enough for you to see it
coming off; then the iron combined with the rest of the lemon juice to
form an iron salt.

Whenever an acid acts on a metal, the metal drives off the hydrogen
and forms a salt. The salt is not always good to eat; for instance,
the salt that tin forms with acids is poisonous.

ACTION OF ACIDS ON OTHER SUBSTANCES. Acids do not act on metals only,
however. Watch the next experiment to see what a strong acid will do
to cloth.

    EXPERIMENT 107. _To be done by the teacher._ Put a drop of
    concentrated nitric or sulfuric acid on a piece of colored
    wool cloth, or on a piece of colored silk. Let it stand for
    a few minutes, then rinse it thoroughly. Test the cloth where
    the acid has been to see whether or not it is as strong as the
    rest of the cloth. How has the acid affected the color?

[Illustration: FIG. 183. Strong acids will eat holes like this in
cloth.]

ACTION OF ACIDS ON THE NERVES OF TASTE. Acids act on the taste nerves
in the tongue and give the taste of sourness; everything sour is an
acid. Lemon juice, sour milk, and sour fruits are all too weak acids
to injure clothes or skin, but their sour taste is a result of the
acid in them acting on the nerves of taste.

    _APPLICATION 79._ A girl wanted to make lemonade. She did
    not know which of two knives to use, a steel-bladed one or a
    silver-plated one. Which should she have used?

    _APPLICATION 80._ A woman was going to put up some tomatoes.
    She needed something large to cook them in. She had a shiny
    new tin dish pan, an older enamelware dish pan, a galvanized
    iron water pail, and an old-fashioned copper kettle. Which
    would have been best for her to use?

    Make a list of as many acids as you can think of.


INFERENCE EXERCISE

    Explain the following:

    511. Sugar dissolves readily in _hot_ coffee.

    512. The sugar disappears, yet the coffee flavor remains and
    so does the sweetness of the sugar.

    513. A tin spoon left overnight in apple sauce becomes black.

    514. If one's clothes are on fire, rolling over on the ground
    is better than running.

    515. Lemon juice bleaches straw hats.

    516. Will-o'-the-wisps glow at night, deceiving travelers by
    their resemblance to moving lanterns.

    517. Tomatoes should never be left in a tin can after it has
    been opened.

    518. Boiled milk tastes different from ordinary milk.

    519. Your hands become very cold after you have washed things
    in gasoline.

    520. Wood decays more quickly when wet than when dry.


SECTION 55. _Bases._

    Why does strong soap make your face sting?

    How is soap made?

"Contains no free alkali," "Will not injure the most delicate of
fabrics," "99-44/100% pure,"--such phrases as these are used in
advertising soaps. What is meant by 99-44/100% pure? What is free
alkali? Why should any soap injure fabrics? What makes a soap
"strong"?

The answer to all these questions is that there are some substances
called _bases_, which are the opposites of acids, and some of which
are as powerful as acids. Lye, ammonia, caustic soda, and baking and
washing soda are common bases. The strong bases, like lye and caustic
soda, are also called _alkalies_. If you want to see what a strong
base--an alkali--will do to "the most delicate of fabrics," and to
fabrics that are not so delicate, for that matter, try the following
experiment:

    EXPERIMENT 108. _To be done by the teacher._ If you get any
    alkali on your skin or clothes, wash it off immediately with
    vinegar or lemon juice.

    Put half a teaspoonful of lye and a quarter of a cup of water
    into a beaker, a small pan, or an evaporating dish. Bring it
    to a _gentle_ boil. Drop a small piece of woolen cloth and a
    small piece of silk cloth into it and let them boil gently
    for a couple of minutes. What happens to them? Try a piece of
    plain cotton cloth, and then a piece of cloth that is mixed
    wool and cotton or mixed silk and cotton. What happens to
    them? This is a very good test to determine whether any goods
    you buy are pure silk or wool, or whether there is a cotton
    thread mixed with them. Drop one end of a long hair into the
    hot lye solution. What happens to it? Drop a speck of meat or
    a piece of finger nail into it.

From this experiment you can readily see why lye will burn your skin
and ruin your clothes. You can also see how it softens the food that
sticks to the bottom of the cooking pan and makes the pan easy to
clean. Lye is one of the strongest bases or alkalies in the world.

[Illustration: FIG. 184. The lye has changed the wool cloth to a
jelly.]

HOW SOAP IS MADE. When lye and grease are boiled together, they form
soap. You cannot very well make soap in the laboratory now, as the
measurements must be exact and you need a good deal of strong lye to
make it in a quantity large enough to use. But the fact that soap is
made with oil, fat, or grease boiled with lye, or caustic soda, which
is almost the same thing, shows why a soap must be 99-44/100% pure,
or something like that, if it is not to injure "the most delicate
fabric." If a little too much lye is used there will be free alkali
in the soap, and it will make your hands harsh and sore and spoil the
clothes you are washing. A "pure" soap is one with no free alkali
in it. A "strong" soap is one that does have some free alkali in it;
there is a little too much lye for the oil or fat, so some lye is left
uncombined when the soap is made. This free alkali cleans things well,
but it injures hands and clothes.

When the drainpipe of a kitchen sink is stopped up, you can often
clear it by sprinkling lye down it, and then adding boiling water.
_If you ever do this, stand well back so that no lye will spatter into
your face; it sputters when the boiling water strikes it._ The grease
in the drainpipe combines with the lye when the hot water comes
down; then the soap that is formed is carried down the pipe, partly
dissolved by the hot water.

When you sponge a grease spot with ammonia, the same sort of chemical
action takes place. The ammonia is a base; it combines with the grease
to form soap, and this soap rinses out of the cloth.

THE LITMUS TEST. To tell what things are bases and what are acids,
a piece of paper dyed with litmus is ordinarily used. Litmus is made
from a plant (lichen). This paper is called _litmus paper_. Try the
following experiment with litmus paper:

    EXPERIMENT 109. Pour a few drops of ammonia, a base, into a
    cup. Into another cup pour a few drops of vinegar, an acid.
    Dip your litmus paper first into one, then into the other, and
    then back into the first. What color does the vinegar turn
    it? the ammonia? Try lemon juice; diluted hydrochloric acid; a
    _very_ dilute lye solution.

This is called the _litmus test_. All ordinary acids, if not too
strong, will turn litmus pink. All bases or alkalies will turn it
blue. If it is already pink when you put it into an acid, it will stay
pink, of course; if it is already blue when you put it into a base, it
will stay blue. But if you put a piece of litmus paper into something
that is neither an acid nor a base, like sugar or salt, it will still
stay the same color. So, to test for a base, use a piece of litmus
paper that is pink and see if it turns blue, or if you want to test
for an acid, use blue litmus paper. Do this experiment:

    EXPERIMENT 110. With pink and blue litmus paper, test the
    different substances named below to see which are acids and
    which are bases. Make a list of all the acids and another list
    for all the bases. Do not put down anything that is neither
    acid or base. You cannot be sure a thing is an acid unless it
    turns _blue_ litmus _pink_. A piece of pink litmus would stay
    pink in an acid, but it would also stay pink in things that
    were neither acid nor base, like salt or water. In the same
    way you cannot be sure a thing is a base unless it turns
    _pink_ litmus _blue_. Here is a list of things to try: 1,
    sugar; 2, orange; 3, dilute sulfuric acid; 4, baking soda in
    water; 5, alum in water; 6, washing soda in water; 7, ammonia;
    8, dilute lye; 9, lemon juice; 10, vinegar; 11, washing powder
    in water; 12, sour milk; 13, cornstarch in water; 14, wet
    kitchen soap; 15, oil; 16, salt in water.

You may have to make the orange and sour milk test at home. You may
take two pieces of litmus paper home with you and test anything else
that you may care to. If you have a garden, try the soil in it. If it
is acid it needs lime.

    _APPLICATION 81._ A boy spilled some greasy soup on his best
    dark blue coat. Which of the following methods would have
    served to clean the coat? to sponge it (a) with cold water;
    (b) with water (hot) and ammonia; (c) with hot water and
    vinegar; (d) with concentrated nitric acid; to sprinkle lye on
    the spot and pour boiling water over it.

    _APPLICATION 82._ A woman scorched the oatmeal she was cooking
    for breakfast. When she wanted to wash the pan, she found that
    the blackened cereal stuck fast to the bottom. Which of the
    following things would have served best to loosen the burned
    oatmeal from the pan: lye and hot water, ammonia, vinegar,
    salt water, lemon juice?


INFERENCE EXERCISE

    Explain the following:

    521. After clothes have been washed with washing soda or
    strong soap, they should be thoroughly rinsed. Otherwise they
    will be badly eaten as they dry.

    522. Carbon will burn; oxygen will support combustion; yet
    carbon dioxid (CO_2), which is made of both these elements,
    will neither burn nor support combustion.

    523. You can clean silver by putting it in hot soda solution
    in contact with aluminum.

    524. When you stub your toe while walking, you tend to fall
    forward.

    525. Electric lamps glow when you turn on the switch.

    526. If you use much ammonia in washing clothes or cleaning,
    your hands become harsh and dry.

    527. If a person swallows lye or caustic soda, he should
    immediately drink as much vegetable oil or animal oil as
    possible.

    528. Water is made of hydrogen and oxygen; air is made of
    nitrogen and oxygen; yet while things will not burn in water,
    they will burn easily in air.

    529. The backs of books that have been kept in cases for
    several years are not as bright colored as the side covers.

    530. If you try to burn a book or magazine in a grate, only
    the outer pages and edges burn.


SECTION 56. _Neutralization._

    When you put soda in vinegar, what makes the vinegar less
    sour?

    When we use sour milk for cooking, why does the food not taste
    sour?

One of the most interesting and important facts about acids and bases
is that if they are put together in the right proportions they turn
to salt and water. Strong hydrochloric acid (HCl), for instance, will
attack the skin and clothes, as you know; if you should drink it, it
would kill you. Caustic soda (NaOH), a kind of lye, is such a strong
alkali that it would dissolve the skin of your mouth in the way
that lye dissolved hair in Experiment 108. Yet if you put these two
strongly poisonous chemicals together, they promptly turn to ordinary
table salt (NaCl) and water (H_2O). Or, as the chemists write it:

  NaOH+HCl -> NaCl+H_2O.

You can make this happen yourself in the following experiment, using
the acid and base dilute enough so that they will not hurt you:

    EXPERIMENT 111. Although strong hydrochloric acid and strong
    caustic soda are dangerous, if they are diluted with enough
    water they are perfectly harmless. You will find two bottles,
    one labeled "_caustic soda_ (NaOH) diluted for tasting,"
    and the other labeled "_hydrochloric acid_ (HCl) diluted
    for tasting." From one bottle take a little in the medicine
    dropper and let a drop fall on your tongue. Taste the contents
    of the other bottle in the same way. _It is not usually safe
    to taste things in the laboratory. Taste only those things
    which are marked "for tasting."_

    Now put a teaspoonful of the same hydrochloric acid into a
    clean evaporating dish. Lay a piece of litmus paper in the
    bottom of the dish. With a medicine dropper gradually add the
    dilute caustic soda (NaOH), stirring as you add it. Watch the
    litmus paper. When the litmus paper begins to turn blue, add
    the dilute caustic soda drop by drop until the litmus paper
    stays blue when you stir the mixture. Now add a drop or two
    more of the acid until the litmus turns pink again. Taste the
    mixture.

    Put the evaporating dish on the wire gauze over a Bunsen
    burner, and bring the liquid to a boil. Boil it gently until
    it begins to sputter. Then take the Bunsen burner in your hand
    and hold it under the dish for a couple of seconds; remove it
    for a few seconds, and then again hold it under the dish for a
    couple of seconds; remove it once more, and keep this up until
    the water has all evaporated and left dry white crystals and
    powder in the bottom of the dish. As soon as the dish is cool,
    taste the crystals and powder. What are they?

    Is salt an acid or a base?

Whenever you put acids and bases together, you get some kind of salt
and water. Thus the chlorine (Cl) of the hydrochloric acid (HCl)
combines with the sodium (Na) of caustic soda (NaOH) to form ordinary
table salt, sodium chloride (NaCl), while the hydrogen (H) of the
hydrochloric acid (HCl) combines with the oxygen and hydrogen (OH) of
the caustic soda (NaOH) to form water (H_2O). Chemists write this as
follows:

  NaOH+HCl -> NaCl+H_2O.

WHY SOUR MILK PANCAKES ARE NOT SOUR. It is because bases neutralize
acids that you put baking soda with sour milk when you make sour milk
pancakes or muffins. The soda is a weak base. The sour milk is a weak
acid. The soda neutralizes the acid, changing it into a kind of salt
and plain water. Therefore the sour milk pancakes or muffins do not
taste sour.

In the same way a little soda keeps tomatoes from curdling the milk
when it is added to make cream of tomato soup. It is the acid in the
tomatoes that curdles milk. If you neutralize the acid by adding a
base, there is no acid left to curdle the milk; the acid and base turn
to water and a kind of salt.

When you did an experiment with strong acid, you were advised to have
some ammonia at hand to wash off any acid that might get on your skin
or clothes. The ammonia, being a base, would immediately neutralize
the acid and therefore keep it from doing any damage. Lye also would
neutralize the acid, but if you used the least bit too much, the lye
would do as much harm as the acid. That is why you should use a weak
base, like ammonia or baking soda or washing soda, to neutralize any
acid that spills on you. Then if you get too much on, it will not do
any harm.

In the same way you were warned to have vinegar near at hand while you
worked with lye. Strong nitric acid also would neutralize the lye, but
if you happened to use a drop too much, the acid would be worse than
the lye. Vinegar, of course, would not hurt you, no matter how much
you put on.

_Any_ acid will neutralize _any_ base. But it would take a great deal
of a weak acid to neutralize a strong base or alkali; you would have
to use a great deal of vinegar to neutralize concentrated lye. In the
same way it would take a great deal of a weak base to neutralize a
strong acid; you would have to use a large amount of baking soda or
ammonia to neutralize concentrated nitric acid.

    _APPLICATION 83._ A woman was cleaning kettles with lye. Her
    little boy was playing near, and some lye splashed on his
    hand. She looked swiftly around and saw the following things:
    soap, oil, lemon, flour, peroxide, ammonia, iodine, baking
    soda, essence of peppermint. Which should she have put on the
    boy's hand?

    _APPLICATION 84._ A teacher spilled some nitric acid on her
    apron. On the shelf there were: hydrochloric acid, vinegar,
    lye, caustic soda, baking soda, ammonia, salt, alcohol,
    kerosene, salad oil. Which should she have put on her apron?

    _APPLICATION 85._ A boy had "sour stomach." His sister said,
    "Chew some gum." His aunt said, "Drink hot water with a little
    peppermint in it." His mother told him to take a little baking
    soda in water. His brother said, "Try some hot lemonade."
    Which advice should he have followed?

    _APPLICATION 86._ Two women were bleaching a faded pair of
    curtains. The Javelle water which they had used was made of
    bleaching powder and washing soda. Before hanging the curtains
    out to dry, one of them said that she was afraid the Javelle
    water would become so strong as the water evaporated from the
    curtains that it would eat the curtains. They decided they had
    better rinse them out with something that would counteract
    the soda and lime in the Javelle water, and in the laundry and
    pantry they found: ammonia, blueing, starch, washing powder,
    soap, vinegar, and gasoline. Which of them, if any, would it
    have been well to put in the rinsing water?


INFERENCE EXERCISE

    Explain the following:

    531. Solid pieces of washing soda disappear in hot water.

    532. Greasy clothes put into hot water with washing soda
    become clean.

    533. If you hang these clothes up to dry without rinsing them,
    the soda will weaken the cloth.

    534. Lemon juice in the rinsing water will prevent washing
    soda from injuring the clothes.

    535. If you hang them in the sun, the color will fade.

    536. A piece of soot blown against them will stick.

    537. A drop of oil that may spatter against them will spread.

    538. The clothes will be easier to iron if dampened.

    539. The creases made in ironing the clothes will reappear
    even if you flatten the creases out with your hand.

    540. After they have been worn, washed, and ironed a number of
    times, clothes are thinner than they were when they were new.


SECTION 57. _Effervescence._

    What makes baking powder bubble?

    What makes the foam on soda water?

Did you ever make soda lemonade? It is easy to make and is rather
good. Try making it at home. Here are the directions:

    EXPERIMENT 112. Make a glass of ordinary lemonade (half
    a lemon, 1-1/2 teaspoonfuls of sugar; fill the glass with
    water). Pour half of this lemonade into another cup or glass.
    Into the remaining half glass stir half a teaspoonful of soda.
    Drink it while it fizzes. Does it taste sour?

When anything fizzes or bubbles up like this, we say that it
_effervesces_. Effervescence is the bubbling up of a gas from a
liquid. The gas that bubbled up from your lemonade was carbon dioxid
(CO_2), and this is the gas that usually bubbles up out of things when
they effervesce.

When you make bread, the yeast turns the sugar into carbon dioxid
(CO_2) and alcohol. The carbon dioxid tries to bubble up out of the
dough, and the bubbles make little holes all through the dough.
This makes the bread light. When bread rises, it really is slowly
effervescing.

HOW SODA WATER IS MADE. Certain firms make pure carbon dioxid
(commercially known as _carbonic acid_ _gas_) and compress it in
iron tanks. These iron tanks of carbon dioxid (CO_2) are shipped to
soda-water fountains and soda-bottling works. Here the compressed
carbon dioxid is dissolved in water under pressure,--this is called
"charging" the water. When the charged water comes out of the faucet
in the soda fountains, or out of the spout of a seltzer siphon, or out
of a bottle of soda pop, the carbon dioxid that was dissolved in
the water under pressure bubbles up and escapes,--the soda water
effervesces.

Sometimes there is compressed carbon dioxid down in the ground. This
dissolves in the underground water, and when the water bubbles up from
the ground and the pressure is released, the carbon dioxid foams
out of the water; it effervesces like the charged water at a soda
fountain.

But the most useful and best-known effervescence is the kind you got
when you stirred the baking soda in the lemonade. Baking soda is made
of the same elements as caustic soda (NaOH), with carbon dioxid (CO_2)
combined with them. The formula for baking soda could be written
NaOHCO_2, but usually chemists put all of the O's together at the end
and write it NaHCO_3. Whenever baking soda is mixed with any kind
of acid, the caustic soda part (NaOH) is used up in neutralizing
the acid. This leaves the carbon dioxid (CO_2) part free, so that it
bubbles off and we have effervescence. Baking soda mixed with an acid
always effervesces. That is why sour milk muffins and pancakes are
light as well as not sour. The effervescing carbon dioxid makes
bubbles all through the batter, while the caustic soda (NaOH) in the
baking soda neutralizes the acid of the sour milk.

[Illustration: FIG. 185. Making a glass of soda lemonade.]

EFFERVESCENCE GENERALLY DUE TO THE FREEING OF CARBON DIOXID. Since
baking soda is so much used in the home for neutralizing acids, people
sometimes get the idea that whenever there is neutralization there is
effervescence. Of course this is not true. Whenever you neutralize an
acid with baking soda or washing soda, the carbon dioxid in the soda
bubbles up and you have effervescence. But if you neutralize an
acid with ammonia, lye, or plain caustic soda, there is not a bit of
effervescence. Ammonia, lye, and plain caustic soda have no carbon
dioxid in them to bubble out.

Baking _powder_ is merely a mixture of baking soda and dry acid (cream
of tartar or phosphates in the better baking powders, alum in the
cheap ones). These dry acids cannot act on the soda until they go into
solution. As long as the baking powder remains dry in the can, there
is no effervescence. But when the baking powder is stirred into the
moist biscuit dough or cake batter, the baking powder dissolves; so
the acid in it can act on the baking soda and set free the carbon
dioxid.

In most cases it is the freeing of carbon dioxid that constitutes
effervescence, but the freeing of any gas from liquid is
effervescence. When you made hydrogen by pouring hydrochloric acid
(HCl) on zinc shavings, the acid effervesced,--the hydrogen gas was
set free and it bubbled up.

Stirring or shaking helps effervescence, just as it does
crystallization. As the little bubbles form, the stirring or shaking
brings them together and lets them join to form big bubbles that pass
quickly up through the liquid. That is why soda pop will foam so much
if you shake it before you pour it, or if you stir it in your glass.

    _APPLICATION 87._ Explain why we do not neutralize the acid in
    sour milk gingerbread with weak caustic soda instead of with
    baking soda; why soda water which is drawn with considerable
    force from the fine opening at a soda fountain makes so much
    more foam than does the same charged water if it is drawn
    from a large opening, from which it flows gently; why there is
    _always_ baking soda and dry acid in baking powder.

    _APPLICATION 88._ A woman wanted to make gingerbread. She had
    no baking powder and no sour milk, but she had sweet milk and
    all the other articles necessary for making gingerbread. She
    had also baking soda, caustic soda, lemons, oranges, vanilla,
    salad oil, vinegar, and lye. Was there any way in which she
    might have made the gingerbread light without spoiling it?


INFERENCE EXERCISE

    Explain the following:

    541. Harness is oiled to keep it flexible.

    542. When you pour nitric acid on copper filings, there is a
    bubbling up of gas.

    543. The flask or dish in which the action takes place becomes
    very hot.

    544. The copper disappears and a clear green solution is left.

    545. In making cream of tomato soup, soda is added to the
    tomatoes before the milk is, so that the milk will not curdle
    How does the soda prevent curdling?

    546. The soda makes the soup froth up.

    547. A wagon squeaks when an axle needs greasing.

    548. Seidlitz powders are mixed in only _half_ a glass of
    water.

    549. The work of developing photographs is all done with a
    ruby light for illumination.

    550. Coal slides forward off the shovel into a furnace when
    you stop the shovel at the furnace door.




CHAPTER TWELVE

ANALYSIS


SECTION 58. _Analysis._

    How can people tell what things are made of?

If it were not for chemical analysis, most of the big factories would
have to shut down, much of our agricultural experimentation would
stop, the Pure Food Law would be impossible to enforce, mining would
be paralyzed, and the science of chemistry would almost vanish.

Analysis is finding out what things are made of. In order to make
steel from ore, the ore has to be analyzed; and factories could not
run very well without steel. In order to test soil, to test cow's
milk, or to find the food value of different kinds of feed, analysis
is essential. As to the Pure Food Law, how could the government find
out that a firm was using artificial coloring matter or preservatives
if there were no way of analyzing the food? In mining, the ore must be
assayed; that is, it must be analyzed to show what part of it is gold,
for instance, and what part consists of other minerals. Also, the
analysis must show what these substances are, so that they can be
treated properly. And the science of chemistry is largely the science
of analyzing--finding out what things are made of and how they will
act on each other.

The subject of chemical analysis is extremely important. But in this
course it is impossible and unnecessary for you to learn to analyze
everything; the main thing is for you to know what analysis is and to
have a general notion of how a chemist analyzes things.

[Illustration: FIG. 186. The platinum loop used in making the borax
bead test.]

When you tested a number of substances with litmus paper to find out
which of them were acids, you were really doing some work in chemical
analysis. Chemists actually use litmus paper in this way to find out
whether a substance is an acid or a base.

THE BORAX BEAD TEST. This is another chemical test, by which certain
substances can be recognized:

    EXPERIMENT 113. Make a loop of wire about a quarter of an inch
    across, using light-weight platinum wire (about No. 30). Seal
    the straight end of the wire into the end of a piece of glass
    tubing by melting the end of the tube around the wire.

    Hold the loop of wire in the flame of a Bunsen burner for
    a few seconds, then dip the looped end in borax powder. Be
    careful not to get borax on the upper part of the wire or on
    the handle. Some of the borax will stick to the hot loop. Hold
    this in the flame until it melts into a glassy bead in the
    loop. You may have to dip it into the borax once or twice more
    to get a good-sized bead.

    When the bead is all glassy, and while it is melted, touch it
    lightly to _one small grain_ of one of the chemicals on the
    "jewel-making plate." This jewel-making plate is a plate
    with six small heaps of chemicals on it. They are: manganese
    dioxid, copper sulfate, cobalt chlorid, nickel salts, chrome
    alum, and silver nitrate. Put the bead back into the flame
    and let it melt until the color of the chemical has run all
    through it. Then while it is still melted, shake the bead out
    of the loop on to a clean plate. If it is dark colored and
    cloudy, try again, getting a still smaller grain of the
    chemical. You should get a bead that is transparent, but
    clearly colored, like an emerald, topaz, or sapphire.

    Repeat with each of the six chemicals, so that you have a set
    of six different-colored beads.

[Illustration: FIG. 187. Making the test.]

This is a regular chemical test for certain elements when they are
combined with oxygen. The cobalt will always change the borax bead to
the blue you got; the chromium will make the bead emerald green or, in
certain kinds of flame, ruby red; etc. If you wanted to know whether
or not certain substances contained cobalt combined with oxygen, you
could really find out by taking a grain on a borax bead and seeing if
it turned blue.

THE HYDROCHLORIC ACID TEST FOR SILVER. The experiment in which you
tested the action of light in effecting chemical change, and in which
you made a white powder or precipitate in a silver nitrate solution
by adding hydrochloric acid (page 327), is a regular chemical test
to find out whether or not a thing has silver in it. If any silver
is dissolved in nitric acid, you will get a precipitate (powder) when
hydrochloric acid is added. Make the test in the following experiment:

    EXPERIMENT 114. _Use distilled water all through this
    experiment if possible._ First wash two test tubes and an
    evaporating dish thoroughly, rinsing them several times. Into
    one test tube pour some nitric acid diluted 1 to 4. Heat this
    to boiling, then add a few drops of hydrochloric acid diluted
    1 to 10. Does anything happen? Pour out this acid and rinse
    the dish thoroughly. Now put a piece of silver or anything
    partly made of silver into the bottom of the evaporating dish.
    Do not use anything for the appearance of which you care.
    Cover the silver with some of the dilute nitric acid, put the
    dish over the Bunsen burner on a wire gauze, and bring the
    acid to a gentle boil. As soon as it boils, take the dish off,
    pour some clean, cold water into it to stop the action, and
    pour the liquid off into the clean test tube. Add a few drops
    of the dilute hydrochloric acid to the liquid in the test
    tube. What happens? What does this show must have been in the
    liquid?

You can detect very small amounts of silver in a liquid by this test.
It is a regular test in chemical analysis.

THE IODINE TEST FOR STARCH. A very simple test for starch, but one
that is thoroughly reliable, is the following:

    EXPERIMENT 115. Mix a little starch with water. Add a drop of
    iodine. What color does the starch turn? Repeat with sugar.
    You can tell what foods have starch in them by testing them
    with iodine. If they turn black, blue, or purple instead of
    brown, you may be sure there is starch in them. And if they do
    not turn black, blue, or purple, you can be equally sure
    that they have no starch in them. Some baking powders contain
    starch to keep them dry. Test the baking powder in the
    laboratory for starch. Often a little cornstarch is mixed
    with powdered sugar to keep it from lumping. Test the powdered
    sugar in the laboratory to see if it contains starch.

    Test the following or any other ten foods to see if any of
    them are partly made of starch: salt, potatoes, milk, meat,
    sausage, butter, eggs, rice, oatmeal, cornmeal, onions.

[Illustration: FIG. 188. The white powder that is forming is a silver
salt.]

THE LIMEWATER TEST FOR CARBON DIOXID. In crowded and badly ventilated
rooms carbon dioxid in unusual amounts is in the air. It can be
detected by the limewater test.

    EXPERIMENT 116. Pour an inch or two of limewater into a
    glass. Does it turn milky? Pump ordinary air through it with
    a bicycle pump. Now blow air from your lungs through a glass
    tube into some fresh limewater until it turns milky. By this
    test you can always tell if carbon dioxid (CO_2) is present.

[Illustration: FIG. 189. The limewater test shows that there is carbon
dioxid in the air.]

Carbon dioxid turns limewater milky as it combines with the lime in
the limewater to make tiny particles (a precipitate) of limestone. If
you pour seltzer water or soda pop into limewater, you get the same
milkiness, for the bubbles of carbon dioxid in the charged water act
as the carbon dioxid in your breath did. If you pumped enough air
through the limewater you would produce some milkiness in it, for
there is always some carbon dioxid in the air.

The purpose of these experiments is only to give you a general notion
of how a chemist analyzes things,--by putting an unknown substance
through a series of tests he can tell just what that substance
contains; and by accurately weighing and measuring everything he puts
in and everything he gets out, he can determine how much of each thing
is present in the compound or mixture. To learn to do this accurately
takes years of training. But the men who go through this training and
analyze substances for us are among the most useful members of the
human race.


INFERENCE EXERCISE

    Explain the following:

    551. A little soda used in canning an acid fruit will save
    sugar.

    552. The fats you eat are mostly digested in the small
    intestine, where there is a large excess of alkali.

    553. The dissolved food in the liquid part of the blood gets
    out of the blood vessels and in among the cells of the body,
    and it is finally taken into the cells through their walls.

    554. Ammonia takes the color out of delicate fabrics.

    555. Dishes in which cheese has been cooked can be cleaned
    quickly by boiling vinegar in them.

    556. Prepared pancake flour contains baking powder. It keeps
    indefinitely when dry, but if the box gets wet, it spoils.

    557. When water or milk is added to prepared pancake flour to
    make a batter, bubbles appear all through it.

    558. When a roof leaks a _little_, a _large_ spot appears on
    the ceiling.

    559. Gasoline burns quietly enough in a stove, but if a spark
    gets into a can containing gasoline vapor, there is a violent
    explosion.

    560. Turpentine will remove fresh paint.


GENERAL REVIEW INFERENCE EXERCISE

    Explain the following:

    561. We can remove fresh stains by pouring boiling water
    through them.

    562. A ship can be more heavily laden in salt water than in
    fresh water.

    563. Water flies off a wet dog when he shakes himself.

    564. In cooking molasses candy, baking soda is often added to
    make it lighter.

    565. An egg will not stand on end.

    566. Women who carry bundles on their heads stand up very
    straight.

    567. To get all crayon marks off a blackboard, the janitor
    uses _vinegar_ in water.

    568. Sunlight makes your skin darker.

    569. Water puts out a fire.

    570. You get a much worse shock from a live wire when your
    hands are wet than when they are dry.

    571. Stone or brick buildings are cool in summer but warm in
    winter.

    572. If you take the handle off a faucet, it is almost
    impossible to turn the valve with your fingers.

    573. Sparks fly from a grindstone when you are sharpening a
    knife.

    574. Violin strings are spoiled by getting wet.

    575. The oxygen of the air gets into the blood from the lungs,
    although there are no holes from the blood vessels into the
    lungs.

    576. You push a button or turn a key switch and an electric
    lamp lights.

    577. A rubber comb, rubbed on a piece of wool cloth, will
    attract bits of paper to it.

    578. People whose eyes no longer adjust themselves have to
    have "reading glasses" and "distance glasses" to see clearly.

    579. When you look through a triangular glass prism, things
    appear to be where they are not.

    580. Lye and hot water poured down a clogged kitchen drainpipe
    clear out the grease.

    581. You can draw on rough paper with charcoal.

    582. When little children get new shoes, the soles should be
    scratched and made rough.

    583. You can get your face very clean by rubbing cold cream
    into it, then wiping the cold cream off on a towel or cloth.

    584. Soft paper blurs writing when you use ink.

    585. Water will flow over the side of a pan through a siphon,
    if the outer end of the siphon is lower than the surface of
    the water in the pan.

    586. There is a loud noise when a gun is fired.

    587. Colored cloths should be matched in daylight, not in
    artificial light.

    588. Lamp chimneys are made of _thin_ glass.

    589. When you sweep oiled floors, no dust flies around the
    room.

    590. The ocean is salty, while lakes are usually fresh.

    591. A glass gauge on the side of a water tank shows how high
    the Water in the tank is.

    592. You burn your hand when you touch a hot stove.

    593. Pounding a piece of steel held horizontally over the
    earth and pointing north and south will make it become a
    magnet.

    594. When only one side of a sponge is in water, the sponge
    gradually gets soft all over.

    595. If we breathe on a cold mirror, a fine mist collects on
    it.

    596. Butter is kept in cool places.

    597. Water will boil more quickly in a covered pan than in an
    open one.

    598. Mucilage, glue, and paste all become hard and dry after
    being spread out on a surface for a while.

    599. You cannot see things clearly through a dusty window.

    600. In making fire grates it is necessary to have the bars
    free to move a little.




APPENDIX


A. THE ELECTRICAL APPARATUS

For giving children a practical understanding of such laws of
electricity as affect everybody, the following simple apparatus is
invaluable. It is the electrical apparatus referred to several times
in the text. The only part of it that is at all difficult to get is
the nichrome resistance wire. There is a monopoly on this and each
licensee has to agree not to sell it. It can be bought direct from the
manufacturer by the school board if a statement accompanies the order
to the effect that it is not to be used in any commercial devices,
nor to be sold, but is for laboratory experimentation only. The
manufacturers are Hoskins Manufacturing Company, Detroit, Michigan.

The following diagram will make the connections and parts of the
electrical apparatus clear:

[Illustration: FIG. 190. Electrical apparatus: At the right are the
incoming wires. Dotted lines show outlines of fuse block. _A_, 2
cartridge fuses, 15 A; _B_, 2 plug fuses, 10 A; _C_, knife switch;
_D_, fuse gap; _E_, snap switch; _F_, _H_, lamp sockets; _G_, flush
switch; _I_, _J_, _K_, nichrome resistance wire, No. 24 (total length
of loop, 6 feet), passing around porcelain posts at left.]

The flush switch (G) should be open at the bottom for
inspection,--remove the back. The snap switch (E) should have cover
removed so that pupils can see exactly how it works.

The fuse gap (D) consists either of two parts of an old knife switch,
the knife removed, or of two brass binding posts. Across it a piece
of 4-ampere fuse wire is always kept as a protection to the more
expensive plug and cartridge fuses. Between the resistance wire (_I_,
_J_, _K_) and the wall should be either slate or sheet asbestos,
double thickness. Under the fuse gap the table should be protected by
galvanized iron so that the melted bits of fuse wire can set nothing
on fire when the fuse wire burns out.


B. CONSTRUCTION OF THE CIGAR-BOX TELEGRAPH

The "cigar-box telegraph" shown on page 381 is made as follows: An
iron machine bolt (A) is wound with about three layers of No. 24
insulated copper magnet wire, the two ends of the wire (_B_, _B_)
projecting. The threaded end of the bolt (C) is not wound. A nut (D)
is screwed on the bolt as far down as the wire wrapping. The threaded
end is then pushed up through the hole in the top of the cigar box
as that stands on its edge. Another nut (E) is then screwed on to the
bolt, holding it in position. The bolt can now be raised or lowered
and tightened firmly in position by adjusting the two nuts (_D_ and
_E_), one above and one below the wood.

A screw eye (F), large enough to form a rest for the head of another
machine bolt (G), is screwed into the back of the box about three
fourths of an inch below the head of the suspended bolt (A). Two or
three inches away, at a slightly higher level, another screw eye (H)
is screwed into the back of the cigar box. This screw eye must have
an opening large enough to permit an iron machine bolt (G) to pass
through it easily. A nut (I) is screwed down on the threaded end of a
machine bolt until about an inch of the bolt projects beyond the nut.
This projecting part of the bolt is then passed through the screw eye
(H) and another nut (J) screwed on to it to hold it in place. This nut
must not be so tight as to prevent the free play of the bolt as its
head rises and falls under the influence of the vertical bolt.
The head of the horizontal bolt rests upon the screw eye which is
immediately below the head of the suspended bolt. You therefore have
the wrapped bolt hanging vertically from the top of the box, with its
head just over the head of the horizontal bolt. There should be about
one quarter inch of space between the heads of the two bolts. An
electric current passing through the wires of the vertical bolt will
therefore lift the head of the horizontal bolt, which will drop back
on to the screw eye when the circuit is broken.

[Illustration: Fig. 191. The cigar-box telegraph.]




INDEX

An asterisk (*) indicates use of one or more illustrations in
connection with reference to which appended.

  Acetylene, carbon and hydrogen in, 315.

  Acids, 351 ff.;
    action of, on metals, 351-353*;
    action of, on cloth, 354*;
    action of, on nerves of taste, 354-355;
    distinguished from bases by litmus test, 358-359;
    neutralization of, by bases, 360-364.

  Action and reaction, law of, 77-81*.

  Adhesion, 39, 41-44;
    cohesion, capillary attraction, and, 47.

  Air, cooling of, on expanding, 95-96;
    liquid, 97;
    heat carried by, by convection, 118-119;
    absorption of light by, 169;
    sound produced by vibrations of, 174-181*;
    pitch due to rapidity of vibrations of, 186;
    water vapor in, 275-280*;
    a mixture and not a compound, 309;
    part taken by, in making automobile go, 344;
    limewater test for carbon dioxid in, 375.

  Air pressure, 10 ff., 14*;
    height water is forced up by, in vacuum, 19;
    high and low, 20, 282;
    winds caused by, 20-21.

  Air pump, 14*, 15.

  Alcohol, boiling of, 112;
    distilling, 113*-114.

  Alkali, 356;
    in soap, 357-358.

  Alloys, definition of, 310.

  Alternating current, defined, 211-212.

  Alum crystals, experiment with, 265-266*.

  Aluminum, an element, 299.

  Alum in water, testing with litmus paper, 359.

  Amber, electricity produced by rubbing with silk, 196.

  Ammonia, example of a common base, 356;
    action of, in cleaning cloth, 358;
    litmus test of, 359;
    neutralization of acid by, 363.

  Ampere, defined, 246.

  Analysis, chemical, 370-376.

  Aneroid barometer, 285*.

  Arc, the electric, 233-240*.

  Atoms, description of, 196;
    electrons and, 197;
    everything in the world made of, 310-311;
    in molecules, 311.

  Aurora Borealis, cause of, 193.

  Automobile, reason for cranking, 210;
    how made to go, 344-345.

  Automobile races, overcoming of centrifugal force in, 75*.

  Automobile tires, reason for wearing of, 80;
    blow-outs of, 348.


  Baking powder, chemical change by solution shown by, 349-350;
    elements of which made, 367-368.

  Baking soda, a common base, 356;
    testing with litmus paper, 359;
    neutralization of sour milk by, in cooking, 362;
    carbon dioxid in, 366-367.

  Ball bearings, used to diminish friction, 54-55.

  Balloon, expansion of, 17-18, 109*;
    reason for rising of, 26;
    filling of, with hydrogen, 301-304*.

  Barometer, use of, 280-285*.

  Bases, substances called, 355-358;
    litmus test for distinguishing from acids, 358-359;
    neutralization of, by acids, 360-364.

  Batteries, electric, 203-205*;
    different kinds of, 205*-207*;
    general principle of all, 206.

  Bell, electric battery for ringing, 204-205*;
    working of electric, 255*.

  Bending of light (refraction), 136-141*.

  Black, the absence of light, 164.

  Bleaching, process of, 326-327.

  Blow-out of tire, a real explosion, 348.

  Blue-flame heaters, 319.

  Blueness of sky, reason for, 169.

  Blueprints, making of, 330-331.

  Boiling and condensing, 107-115*.

  Borax bead test, 371*-372*.

  Brass, an alloy, 310.

  Bread making, chemical action in, 365.

  Breath, cause of visibility of, on cold days, 288, 289*.

  Bronze, an alloy, 310.

  Burning, explanation of, 308, 312-313.


  Calcium chlorid, 114.

  Camera, lens of, 143, 148;
    human eye as a small, 151*-153;
    explanation of, 327-332.

  Capillary attraction, 36*-40;
    difference between adhesion, cohesion, and, 47.

  Carbon, in electric battery, 203-206;
    resistance of, to electric current, 231;
    an element, 293, 299;
    one of chief elements in fuel, 315-316.

  Carbon dioxid, in seltzer siphon, 17;
    produced by joining of carbon with oxygen, 315;
    combining of water and, by plants, 332-333;
    releasing of, in baking powder, 349-350;
    bubbling of, in effervescence, 365-366;
    in soda water, springs, and baking soda, 366-367;
    limewater test for, 375-376.

  Carbonic acid gas, commercial name for pure carbon dioxid, 365-366.

  Cat's hairs, static electricity in, 201.

  Caustic soda, a common base, 356.

  Center of weight, 30-33*.

  Centrifugal force, 5, 72-74;
    law of, 74-75.

  Charcoal, production of, 316.

  Charging water with carbon dioxid, 366.

  Chemical analysis, 370-376.

  Chemical change, and energy, 293 ff.;
    burning (oxidation), 312-322;
    caused by heat, 323-325;
    caused by light, 326-335;
    caused by electricity, 335-339;
    energy released by, 340-341;
    helped by solution, 349-351.

  Chemical equations, 297-299.

  Chlorine, an element, 299.

  Chlorophyll in plants, work of, 332.

  Cigar-box telegraph, construction of, 249*, 380-381*.

  Circuits, electric, 219-220*;
    breaking and making, 220-221;
    connecting in parallel, 221-223*;
    grounded, 225-229*;
    short, 240-245*.

  Cloth, action of acids on, 354*;
    action of an alkali on, 356, 357*.

  Clouds, how formed, 277-278.

  Coal, carbon and hydrogen in, 315.

  Cohesion, 39, 44*-49.

  Cold, caused by expansion, 94;
    is the absence of heat, 95, 120.

  Color, 161-172*.

  Comb, electricity produced by rubbing, 197-198.

  Compass, use of, 190-195*.

  Complete electric circuits, 219-224*.

  Compounds, how elements hide in, 300;
    definition of, 308-309;
    mixtures distinguished from, 309-310.

  Concave mirrors, 154*, 155*, 157;
    magnification by, 157;
    in reflecting telescopes, 157.

  Conduction, of heat, 116-118;
    of electricity, 213-218.

  Conductors of electricity, good and poor, 213.

  Conduits for electric wires, 237.

  Conservation of energy, 57 ff.

  Convection, carrying of heat by, 118-119.

  Convex lens, 148-149*;
    in microscope, 155-157*;
    in telescope, 157.

  Cooling from expansion, 94-96.

  Coolness at night and in winter, 127-128.

  Copper, a good conductor of electricity, 215;
    an element, 299;
    nickel plating of, 336-339*;
    etching of, with acid, 352*-353.

  Copper nitrate, salt called, 353.

  Cream, separating from milk, by centrifugal force, 75-76.

  Crystals, formation of, 265-268.

  Cylinder of engine, 344*.


  Dead Sea, reason for salt in, 104-105*.

  Decay, a kind of oxidation, 313.

  Dew, 275;
    how formed, 287.

  Dictaphone, working of, 175, 178, 179*.

  Diffusion, 268-274;
    of light, 158-161.

  Direct-current electricity, 211-212.

  Distilling of liquids, 112-115*.

  Doorbell, electric battery for ringing, 204-205.

  "Down," meaning of word, 4.

  Drainpipe, cleaning of, with lye, 358.

  Dry-cell battery, 206*.

  Dust, reason for clinging to walls, 43-44.

  Dynamite, 343*;
    making of, 347.

  Dynamo, how electric current is made to flow by, 207*-210*.


  Earth, magnetism of, 190-195.

  Easy circuit, a short circuit an, 244-245.

  Echoes, explanation of, 183-185.

  Effervescence, process of, 365;
    generally due to freeing of carbon dioxid, 367*-368;
    helped by stirring or shaking, 368.

  Elasticity, 82-86;
    of form distinguished from elasticity of volume, 86-87.

  Electrical apparatus, 216-217*, 222-223*;
    description of, 379-380.

  Electric arc, the, 233*-240.

  Electric battery, the, 203-206*.

  Electricity, magnetism and, 190 ff.;
    static, 196-202;
    negative and positive charges of, 198*-200;
    action of, in thunderstorms, 200-201;
    flowing, 203 ff.;
    flowing of, in dynamo, 207-210;
    alternating and direct-current, 211-212;
    conduction of, 213*-218;
    chemical change caused by, 335-339.

  Electric lamps, vacuums in, 12*, 317;
    incandescent, 125;
    gas-filled, 317.

  Electric motors, 256*-257*.

  Electrolysis apparatus, 294-295*.

  Electromagnets, 247-257*.

  Electrons, 193;
    description of, 197;
    number of, in negative and in positive charges, 198-200.

  Elements, defined, 293;
    chemists' abbreviations of, 297-299;
    list of common, 299-300;
    hiding of, in compounds, 300-301.

  Emulsion, defined, 261;
    difference between solution and, 263.

  Energy released by chemical change, 340-341.

  Engine, working of cylinder and piston of, 344*.

  Ether, carrying of heat and light by, 124-125;
    light as waves of, 163-164.

  Ether waves, 124-125, 163-164.

  Evaporating dish, 101*.

  Evaporation, 100-106*;
    part taken by, in formation of clouds, rain, and dew, 277.

  Expansion, caused by heat, 88-93;
    cooling from, 94-96*.

  Expansion ball and ring experiment, 91*-92.

  Explosions, use of, 342* ff.;
    automobiles made to go by succession of, 344-345;
    cause of, 345;
    shooting of guns caused by, 345-346.

  Explosives, manufacture of, 347.

  Extension lights, 238.

  Eye, lens of, 142;
    section of, 151*;
    working of, 151*-153.


  Fading, process of, 326-327.

  Filament of incandescent lamp, 125.

  Fire engines, need of, to force water high, 9.

  Fire extinguishers, action of, 317.

  Fires, caused by electric arcs, 236;
    putting out of, by water, 317.
    _See_ Burning.

  Flames, formation of, 318.

  Floating, sinking and, 23-28.

  Focus of light, 142*-149*.

  Fogs, cause of, 288.

  Food, light necessary to production of, 332-333.

  Force, overcoming of extra motion by, in lever, 63-64*;
    reason for, of steam, 110.

  Forecasters, weather, 282-285.

  Form, elasticity of, 86-87.

  Freckles, cause of, 327.

  Freezing and melting, 96-99.

  Friction, 49-55*;
    electricity produced by, 197-198*.

  Frost, 97, 275;
    explanation of, 287.

  Fuel, chief elements in, 315-316.

  Fulcrum of lever, 59-60*.

  Fuse gap, the, 241*, 379*.

  Fuses, short circuits and, 240-245.


  Gas, cooling of, on expanding, 94-95;
    carbon and hydrogen in, 315;
    used for filling electric lamps, 317-318;
    will not burn until hot enough, 323-324;
    an explosion the sudden release of a confined, 348.

  Gases, diffusion of, 269-271;
    as elements, 293-294.

  Gas heaters, action of, 319, 321*, 322*.

  Gasoline, evaporation of, 103;
    boiling of, 112;
    distilled from petroleum, 114;
    elements of, 315;
    action of, in making automobiles go, 344-345.

  Geysers, cause of, 110.

  Glass, a poor conductor of heat, 118;
    used as insulator of electricity, 215.

  Glowworms, reason for glowing of, 341-342.

  Gold, an element, 293, 299;
    plating of, 339.

  Gravitation, defined, 3.

  Gravity, 1;
    pull of, opposed to pull of adhesion, 42-43.

  Grease, friction diminished by, 53-54;
    combined with lye to form soap, 357.

  Great Salt Lake, reason for salt in, 104-105.

  Greeks, early knowledge of electricity possessed by, 196.

  Green color of water, reason for, 169-171*.

  Grounded circuits, 225-229*.

  Gun, shooting of, caused by explosion, 345-346.

  Gunpowder, action of, in shooting of a gun, 345-346;
    how made, 347.


  Hail, explanation of, 286.

  Heat, a result of friction, 53;
    is the motion of molecules, 90;
    not caused by expansion, 94-95;
    cold is absence of, 95, 120;
    required to evaporate liquids, 102-103;
    conduction of, 116-118;
    carried by air, by convection, 118-119;
    radiation of, 122-128;
    of incandescent lamp, 125-126;
    brought to focus by convex lens, 149;
    chemical change caused by, 323-325.

  Heaters, hot-water, 120*;
    electric, 230, 232;
    gas, 319, 321*, 322*.

  Heat waves, cause of, 141.

  Hydrochloric acid, getting hydrogen from, 301-304;
    testing for silver with, 373.

  Hydrofluoric acid, 351.

  Hydrogen, an element, 294, 299;
    in water, 295-296;
    experiments with, 301-304*;
    one of chief elements in fuel, 315-316;
    part taken by, in burning, 312-319.


  Ice, slight friction of, 52*;
    action of molecules in, on freezing and melting, 96-97;
    reason for floating of, 98-99.

  Incandescence, defined, 125.

  Incandescent lamps, 125-126;
    number of electrons in, 197;
    working of, 229-232.

  Inertia, 66-71;
    definition of, 70.

  Insulators, of heat, 118;
    of electricity, 213;
    substances used as, 215.

  Iodine, an element, 299;
    testing with, for starch, 373-374.

  Iron, a good conductor of heat, 118;
    an element, 299.

  Irons, electric, 229*, 230, 232.

  Iron salt, formed by lemon juice on steel, 353.

  Iron ships, reason for floating, 24*-26.


  Kerosene, boiling of, 112;
    distilled from petroleum, 114;
    carbon and hydrogen in, 315.


  Laughing gas, 309.

  Lava in volcanoes, 110.

  Lead, an element, 299.

  Lead pencils, arc light from, 233*-234*.

  Leaning Tower of Pisa, 29*-30.

  Lemon juice, action of, on silver and on steel, 353;
    litmus test of, 359.

  Lens, of eye, 142, 151*-153;
    of camera, 143, 149*, 328;
    convex, 148-149;
    concave, 149*;
    in telescope, 157.

  Levers, 57-65*.

  Light, radiation of, 122, 123*-128;
    reflection of, 129-135*;
    refraction of, 136*-141;
    focus of, 142-149*;
    brought to focus by convex lens, 149;
    diffusion of, 158-161*;
    color a kind of, 162;
    speed of, 182;
    chemical change caused by, 326-335;
    and manufacture of food in plants, 332-333.

  Lightning, cause of, 200-201.

  Limewater test for carbon dioxid, 375*-376.

  Liquid air, 97, 112.

  Liquids, absorption of, 36-40;
    diffusion in, 272.

  Litmus paper, experiments with, 358-359.

  Litmus test, the, 358-359.

  Lye, a common base, 356;
    experiment with, 356;
    soap made from, 357;
    used for clearing out drainpipe, 358;
    neutralization of, by acids, 363.


  Machinery, oiling of, to decrease friction, 53-54.

  Magdeburg hemispheres, 15, 16*-17.

  Magnetism, 190 ff.

  Magneto, of automobile, 210, 211*;
    of old-fashioned telephone, 210-211.

  Magnets, 190-195*.

  Magnification, 150-157;
    by concave mirror, 157.

  Magnifying glass, convex lens in, 149;
    operation of, 150-156*.

  Manganese dioxid, an essential in explosives, 347.

  Megaphone, working of, 184.

  Melting, freezing and, 96-99.

  Membrane, diffusion through a, 272.

  Mercury, cohesion of, 47-48*;
    use of, in thermometer, 89*, 90-91;
    an element, 299.

  Mercury-vapor lamps, 167-168*, 172.

  Metals, good conductors of heat, 118;
    good conductors of electricity, 215;
    as elements, 310;
    plating of, 336-339*;
    action of acids on, 351-353.

  Microscope, 88;
    working of, 155-157*.

  Mirrors, concave, 154*, 155*, 157.

  Mixtures, distinguished from compounds, 309-310.

  Molecular attraction, 36 ff.

  Molecules, pull of, on each other, 46-47;
    explanation of, 88-89;
    heat defined as the motion of, 90;
    action of, in evaporation, 102-103*;
    action of, in boiling water, 107;
    action of, in conduction of heat, 117;
    action of, in radiation of heat and light, 125;
    action of, in magnetizing, 194*-195;
    made up of atoms, 196, 310;
    mingling of, 259 ff.;
    action of, in formation of clouds, rain, and dew, 277.

  Moon, cause of ring around, 131.

  Morse telegraph code, 253.

  Motion-picture machines, lenses of, 143, 148.

  Motor, the electric, 255-257*.

  Mountains, rainfall on, 286-287.

  Musical instruments, pitch of, 185-187*, 188;
    vibrating devices of, 188.


  Nail plug, the, 241*, 379*.

  Needle, magnetizing of, 192*, 193*-195.

  Negative charges of electricity, 198-200.

  Neutralization of acids and bases, 360-364.

  Niagara Falls, electricity generated by, 210.

  Nickel, an element, 299.

  Nickel-plating copper, process of, 336-339*.

  Night, reason for coolness at, 127-128.

  Nitric-acid, etching copper with, 352*-353;
    action of, on cloth, 354*.

  Nitrogen, an element, 299;
    a non-burning gas, 308;
    used in electric lamps, 317.

  Northern Lights, cause of, 193.


  Ocean, why salt, 104-105.

  Oil, reason for floating of, 26-27;
    decreasing of friction by, 53-54;
    softening due to, 290-292;
    carbon and hydrogen in crude, 315;
    why water will not put out burning, 317.

  Oil heaters, action of, 319.

  Orange, litmus test of, 359.

  Osmosis, process called, 272-274.

  Osmotic pressure, 272-273*.

  Oxidation, 312-322.

  Oxygen, an element, 293, 299;
    an element of water, 295-296;
    experiments in getting, from two solids, 305-308*;
    function of, in burning, 308;
    part taken by, in burning (oxidation), 312-313;
    released in manufacture of plant food, 333-335;
    a compound of, an essential in explosives, 347.


  Pancakes, made from sour milk, 362.

  Paper, carbon and hydrogen in, 315.

  Paraffine, production of, 114.

  Parallel circuits, 221-223*.

  Peat, carbon and hydrogen in, 315.

  Pencils, making arc light with, 233*-234*.

  Periscope experiment, 134-135*.

  Petroleum, gasoline and kerosene distilled from, 114.

  Phonograph, working of, 177-178*.

  Phosphorescence, cause of, 341-342.

  Phosphorus, an element, 300;
    meaning of name, 341.

  Photographs, process of making, 327-332*.

  Pitch of sound, explanation of, 185-188*.

  Plants, light and the manufacture of food in, 332-333;
    how oxygen is supplied by, 333-335.

  Plating of metals, 336-339*.

  Platinum, an element, 300.

  Poles, positive and negative, 206-207.

  Porcelain, used as insulator, 215.

  Positive charges of electricity, 198-200.

  Potassium, experiment with, 304.

  Potassium chlorate, an essential in
  explosives, 347.

  Precious stones, formation of, 263-264.

  Prism, refraction of light by, 136-140*;
    separation of light into rainbow colors by, 162-163*.


  Quicksilver. _See_ Mercury.


  Radiation of heat and light, 122*-128.

  Radium, an element, 300.

  Rain, 275;
    cause of, 278-280.

  Rainbow, making a, on wall, 162*-163;
    how formed, 170-171.

  Reading glasses, 144*;
    convex lens in, 150.

  Red color of sky at sunset, reason for, 170.

  Reflecting telescopes, 157.

  Reflection of light, 129-135*.

  Refraction of light, 136-141*.

  Resistance, electrical, 229-232.

  Retina of eye, 151*, 153.

  Reverberation of sound, 183-185.

  Ring around moon, cause of, 131.

  Rock candy, how made, 267.

  Rubber, used as insulator, 215.

  Rusting of iron, 349.


  Safety valves on steam boilers, 347.

  Salt, reason for, in sea, 104-105*;
    a compound, 308;
    elements of, 310-311;
    formed by hydrochloric acid and zinc, 351;
    iron, formed by lemon juice on steel, 353;
    acids and bases turned to water and, by combining, 361-362.

  Salt water, litmus test of, 359.

  Samson cells, 204.

  Scattering of light (diffusion), 158-161*.

  Seesaw, example of a lever, 57-58*.

  Seltzer siphon, working of, 17.

  Ships, reason for floating, 24*-26.

  Shock, electrical, 214-215.

  Short circuits and fuses, 240-245.

  Silver, an element, 300;
    plating of, 339;
    hydrochloric acid test for, 373.

  Silver chlorid, formation of, 327.

  Sinking and floating, 23-28*.

  Siphon, 18*.

  Sky, reason why blue, 169;
    why red at sunset, 170.

  Smoke, consistency of, 318-319.

  Snow, 275;
    formation of, 285-286.

  Snowflakes, 97, 286*.

  Soap, how made, 357-358.

  Soda water, how made, 365-366.

  Sodium, experiment with, 304.

  Softening due to oil or water, 290-292.

  Soil, litmus test of, 359.

  Solution, defined, 261;
    difference between emulsion and, 263;
    a mixture and not a compound, 309;
    chemical change helped by, 349.

  Sound, cause of, 174;
    rate of speed, 181-182;
    action of, in echoes, 183-185*;
    pitch of, 185-188.

  Sour milk, litmus test of, 359;
    neutralization of, by baking soda, 362.

  Sourness, taste of, caused by acids, 353, 354-355.

  Spectroscope, use of the, 172.

  Spectrum, the, 172.

  Spring water, carbon dioxid in, 366.

  Stability, 29-34.

  Starch, iodine test for, 373-374.

  Stars, twinkling of, 141;
    how to tell of what made, 171-172.

  Static electricity, 196-202*.

  Steam, reason for force exerted by, 110;
    geysers and volcanoes caused by, 110;
    real, not visible, 112 n.

  Steel, generally an alloy, 310.

  Stereopticons, lenses of, 148.

  Storage battery, 206, 207*;
    action of electricity in, 339.

  Stoves, electric, 230, 232.

  Street car, electric motor of, 255-257.

  Suction pump, 19*.

  Sugar, making of, by plants, 332-333;
    litmus test of, 359.

  Sulfur, an element, 300.

  Sulfuric acid, action of, on cloth, 354;
    litmus test of, 359.

  Sun, radiation of heat and light from the, 122-128;
    how to tell of what made, 171-172.

  Sunbeams, explanation of, 131.

  Sweat glands, function of, 291.


  Tanning, process of, 327.

  Telegraph apparatus, 247-252*, 380-381*.

  Telegraph code, 253.

  Telephone, working of, 253-255.

  Telescopes, 156*, 157;
    how made, 157;
    reflecting, 157.

  Temperature, finding the, by reading a thermometer, 90-91.

  Thermometer, the, 89*-91*.

  Thermos bottle, how made, 126-127*.

  Thunder, cause of, 200-201.

  Tin, an element, 300.

  Tin salt, poisonous, 353.

  Toasters, electric, 230, 232.

  Tomatoes, use of soda to neutralize acid of, 362-363.

  Tungsten, in incandescent lamps, 231.

  Tuning-fork experiments, 181*, 186-187*.

  Twinkling of stars, cause of, 141.


  "Up," meaning of word, 4.


  Vacuum, defined, 11;
    reason for, in electric lamp, 12*, 317;
    use of, in manufacture of thermos bottles, 126-127*;
    impossibility of producing sound in, 176-177.

  Valves, safety, on boilers, 347.

  Vaseline, production of, 114.

  Vibrations, of air, 174-181*;
    pitch due to rapidity of, 186.

  Vinegar, litmus test of, 359;
    neutralization of lye by, 363.

  Violin, tuning of, 187.

  Volcanoes, cause of, 110;
    explosions and, 346*.

  Volume, elasticity of, 86-87.


  Washing soda, a common base, 356;
    litmus test of, 359.

  Water, seeks its own level, 6-10;
    gurgling of, when poured from bottle, 13;
    experiment with, to show centrifugal force, 73-74;
    used for making thermometer, 90*-92;
    expansion of, when frozen, 98;
    evaporation of, 100-106;
    action of, in geysers and volcanoes, 110;
    absorption of light by, 169-170;
    as conductor of electricity, 216;
    use of, for generating electricity, 256-257;
    softening due to, 290-292;
    elements of, 294-297;
    a compound and not a mixture, 308;
    formed by burning fuel, 316;
    why fire is put out by, but not burning oil, 317;
    combining of carbon dioxid and, by plants, 332-333;
    rusting of iron by, 349;
    acids and bases turned to salt and, by combining, 361-362.

  Wear, a result of friction, 53.

  Weather, forecasting of, 282-285.

  Weight, center of, 30-33*.

  Wet battery, 204-205*.

  White, a combination of all colors, 162.

  Winds, cause of, 20-21.

  Winter, reason for cold in, 127-128.

  Wiring for arc lamps, 236-239.

  Wood, poor conductor of heat, 118;
    carbon and hydrogen in, 315.


  Yardstick, experiment with, to show leverage, 59*-60.

  Yeast, action of, in bread making, 365.

  Yellow, in flames, 318.

  Yerkes Observatory, telescope of, 156*.


  Zinc, in electric battery, 203-206;
    an element, 300;
    used for driving hydrogen out of acid, 301, 304.

       *       *       *       *       *


_CONSERVATION SERIES_

[Illustration]

Conservation Reader

_By_ HAROLD W. FAIRBANKS, _Ph. D._

_Lecturer, University of California; Geography Supervisor Berkeley
Public Schools_

A small book bringing out in a simple and interesting manner the
principles of conservation of natural resources has long been wanted,
or there has been little on the subject that could be placed in
the hands of pupils. It is to answer this need that Fairbanks'
CONSERVATION READER has been prepared.

The book touches upon every phase of conservation, but it deals at
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is one of the author's main purposes to arouse a stronger sentiment
for preserving what remains of the forests as well as for extending
their areas. This is because proper forestation will lessen the danger
of floods and of erosion of the soil, and it will encourage the return
of the wild creatures that are of so much economic importance and add
so much to the joy of life.

The matter is presented in an easy narrative style that is calculated
to arouse the intelligent interest of children. The text is
illustrated with photographs of wild animals, trees, landscapes, and
rarely beautiful birds, printed in colors. The subject is timely and
the treatment is happy throughout.

CONSERVATION READER should be used as a reader or as a book for
regular study in every elementary school in the country.

  _Cloth_, vi + 216 _pages_.
  _Price $1.40_.


WORLD BOOK COMPANY

YONKERS-ON-HUDSON, NEW YORK
2126 PRAIRIE AVENUE, CHICAGO

[Illustration]

       *       *       *       *       *


_INDIAN LIFE AND INDIAN LORE_

INDIAN DAYS OF THE LONG AGO

IN THE LAND OF THE HEAD-HUNTERS

TWO BOOKS FOR YOUNG PEOPLE BY

EDWARD S. CURTIS

_Author of "The North American Indian"_


In _Indian Days of The Long Ago_ the author gives an intimate view
of Indian life in the olden days, reveals the great diversity of
language, dress, and habits among them, and shows how every important
act of their lives was influenced by spiritual beliefs and practices.

The book tells the story of Kukúsim, an Indian lad who is eagerly
awaiting the time when he shall be a warrior. It is full of mythical
lore and thrilling adventures, culminating in the mountain vigil,
when Kukúsim hears the spirit voices which mark the passing of his
childhood. _Illustrated with photographs by the author and drawings by
F. N. Wilson._

       *       *       *       *       *

Theodore Roosevelt once said that Mr. Curtis has caught glimpses, such
as few white men ever catch, into the strange spiritual and mental
life of the Indians. In _In the Land of the Head-Hunters_ these
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The story centers about Motana, the son of the great War Chief. The
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and his warriors, the rescue of the captured Naida, and the final
victory, celebrated by ceremonial dances, are all described. The
action is rapid and the story is told in the direct, simple style of
the true epic. _Illustrated with thirty full-page photographs by the
author._

_Price $1.60 each._

WORLD BOOK COMPANY

YONKERS-ON-HUDSON, NEW YORK
2126 PRAIRIE AVENUE, CHICAGO

[Illustration]

       *       *       *       *       *




INSECT ADVENTURES

_By_ J. HENRI FABRE

_Selected and Arranged for Young People by Louis Seymour Hasbrouck_

[Illustration]


A new supplementary reader in nature study for the intermediate
grades. A book containing a vast amount of information relating to
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This is the first time that Fabre's writings have been made available
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The many quaint sketches with which the book has been illustrated by
Elias Goldberg complete its charm.

A useful index is included.

_Cloth. 300 pages. Price $1.48._


WORLD BOOK COMPANY

YONKERS-ON-HUDSON, NEW YORK 2126 PRAIRIE AVENUE, CHICAGO

       *       *       *       *       *




_NEW-WORLD SCIENCE SERIES_

_Edited by John W. Ritchie_

TREES, STARS _and_ BIRDS

A BOOK OF OUTDOOR SCIENCE

By EDWIN LINCOLN MOSELEY

_Head of the Science Department, State Normal College of Northwestern
Ohio_


The usefulness of nature study in the schools has been seriously
limited by the lack of a suitable textbook. It is to meet this need
that _Trees, Stars, and Birds_ is issued. The author is one of the
most successful teachers of outdoor science in this country. He
believes in field excursions, and his text is designed to help
teachers and pupils in the inquiries that they will make for
themselves.

The text deals with three phases of outdoor science that have a
perennial interest, and it will make the benefit of the author's long
and successful experience available to younger teachers.

The first section deals with trees, and the discussion of maples is
typical: the student is reminded that he has eaten maple sugar; there
is an interesting account of its production; the fact is brought out
that the sugar is really made in the leaves. The stars and planets
that all should know are told about simply and clearly. The birds
commonly met with are considered, and their habits of feeding and
nesting are described. Pertinent questions are scattered throughout
each section.

The book is illustrated with 167 photographs, 69 drawings, 9 star
maps, and with 16 color plates of 58 birds, from paintings by Louis
Agassiz Fuertes.

It is well adapted for use in junior high schools, yet the
presentation is simple enough for pupils in the sixth grade.

_Cloth. viii + 404 + xvi pages. Price $1.80._


WORLD BOOK COMPANY

YONKERS-ON-HUDSON, NEW YORK 2126 PRAIRIE AVENUE, CHICAGO

       *       *       *       *       *




_NEW-WORLD SCIENCE SERIES_

_Edited by John W. Ritchie_

SCIENCE _for_ BEGINNERS

_By_ DELOS FALL

_Professor of Chemistry, Albion College_


To supply the need for a course that will give the preparatory
training which any scientific study demands, SCIENCE FOR BEGINNERS
by Professor Delos Fall was made. The aim in this text is to win
the interest of pupils, to give them conceptions of nature that are
fundamental, and above all to ground them in the method of science.

The subject matter has to do with the earth sciences, and principally
with physics and chemistry. In the development of each topic, every
advantage that the pupils' experience and interest may afford is
utilized. Exercises or experiments are interspersed throughout the
work, and for these only the simplest materials are required. The
studies are carried to those connecting principles which permit the
organization of knowledge. The book is illustrated with a number of
excellent photographs and over 200 drawings of more than usual merit.

The text is adapted _for use in grades seven, eight, and nine_, or in
any classes that are about to take up their first work in science. It
will prove helpful to the teachers and pupils who use it directly,
and its influence will continue with classes as they advance. It will
thoroughly ground pupils in those ideas that are prerequisite to any
right work in science.

_xi + 388 pages. Price $1.68_


WORLD BOOK COMPANY

YONKERS-ON-HUDSON, NEW YORK 2126 PRAIRIE AVENUE, CHICAGO

       *       *       *       *       *




NEW-WORLD SCIENCE SERIES

_Edited by_ JOHN W. RITCHIE


The publication of books that "apply the world's knowledge to the
world's needs" is the ideal of this house and it is intended that the
different volumes of this series shall express this ideal in a very
concrete way.


_Completed_

HUMAN PHYSIOLOGY. By _John W. Ritchie_, Professor of Biology, College
of William and Mary. A text on physiology, hygiene, and sanitation for
upper grammar or junior high schools. _$1.60._

LABORATORY MANUAL FOR HUMAN PHYSIOLOGY. By _Carl Hartman_, University
of Texas. A manual to accompany Ritchie's Human Physiology. BOUND IN
PAPER AND CLOTH. _60 cents and $1.00._

SCIENCE FOR BEGINNERS. By _Delos Fall_, Albion College, Michigan. A
beginning text in general science for intermediate schools and junior
high schools. _$1.68._

EXERCISE AND REVIEW BOOK IN BIOLOGY. By _J. G. Blaisdell_, Yonkers, N.
Y., High School. A combined laboratory guide, notebook and review
book for students' use. Written from the standpoint of efficiency
and furnishing material for a year's work and to accompany any one of
several high-school texts in general biology. BOUND IN STRONG PAPER.
_$1.20._

TREES, STARS, AND BIRDS. By _E. L. Moseley_, Ohio State Normal
College, Bowling Green. A book of outdoor science for junior high
schools and the upper grammar grades. _$1.80._

PERSONAL HYGIENE AND HOME NURSING. By _Louisa C. Lippitt_, University
of Wisconsin. A practical text for use with classes of young women in
vocational and industrial high schools, colleges, and normal schools.
_$1.68._

SCIENCE OF PLANT LIFE. By _E. N. Transeau_, Ohio State University. A
scientific and very practical text for high schools. _$1.88._

ZOÖLOGY. By _T. D. A. Cockerell_, University of Colorado. A text for
college use. _$3.60._

EXPERIMENTAL ORGANIC CHEMISTRY. By _A. P. West_, University of the
Philippines. A text for college use. _$3.20._

COMMON SCIENCE. By _Carleton W. Washburne_, Superintendent of Schools,
Winnetka, Illinois. Especially made for junior high schools. _$1.68._
Also _Manual_ for above, _20 cents_.

_Other volumes are also in preparation._


WORLD BOOK COMPANY

YONKERS-ON-HUDSON, NEW YORK 2126 PRAIRIE AVENUE, CHICAGO

       *       *       *       *       *




INDIAN LIFE AND INDIAN LORE

THE HERO OF THE LONGHOUSE

By MARY E. LAING

_Illustrations from 27 paintings by David C. Lithgow_


This story gives a portrayal of the noblest of Indians--Hiawatha. It
follows established facts, and bares to the reader the heart of his
race. It is a convincing tale.

The training of the Indian youth is shown; the career of the hero as
a warrior is told; his great work for peace with the Five Tribes is
described.

Besides the story, there is an account of the historical Hiawatha;
also a complete Glossary giving definitions as well as pronunciations
of the new Indian words. A map of the country of the Longhouse will
enable the reader to follow the journeys of the Indian people.

The book is intended as a supplementary reader in schools, being
adapted to the sixth grade or above. It will also be valuable in
groups of the Wood-craft League, Camp-fire Girls, and Boy Scouts.

_Cloth, xxvi + 329 pages. Price $1.60._


WORLD BOOK COMPANY

YONKERS-ON-HUDSON, NEW YORK 2126 PRAIRIE AVENUE, CHICAGO

       *       *       *       *       *

[Illustration]


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[Illustration]




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the largest, most modern and sanitary plants of their kind on earth.
More pounds of Calumet are sold than of any other brand of baking
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That is because Calumet is the best purest, most dependable baking
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[Illustration]

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Transcriber's Notes:

page 238: changed "diagramed" to "diagrammed" (As there diagrammed,
the electricity passes out...)

page 253: the Morse telegraph code is as in the original; this is not
the modern International Morse code

Page 412, changed "conrcete" to "concrete" (... shall express this
ideal in a very concrete way.)

General: variable spelling of iodin/iodine in the original has been
preserved

General: spelling of dioxid and chlorid in the original has been
preserved