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GIFT OF
Dr. L. A. Williams

EDUCATION DEPT

COMMON SCIENCE

NEW-WORLD SCIENCE SERIES

Edited by John W. Ritchie

COMMON SCIENCE. By Carleton W. Washburne

GARDENING. By A . B. Stout

GENERAL SCIENCE SYLLABUS. By /. C. Loevenguth

HUMAN PHYSIOLOGY. By John W. Ritchie

LABORATORY MANUAL FOR USE WITH "HUMAN PHYSIOLOGY.'

By Carl Hartman

SANITATION AND PHYSIOLOGY. By John W. Ritchie
SCIENCE FOR BEGINNERS. By Delos Fall
TREES, STARS, AND BIRDS. By Edwin Lincoln Moseley

CHEMICAL CALCULATIONS. By Bernard Ja/e

EXERCISE AND REVIEW BOOK IN BIOLOGY. By /. G. BlaisdeU

HIGH SCHOOL CHEMISTRY. By George Howard Bruce

INTRODUCTORY CHEMISTRY. By Neil E. Gordon

LABORATORY EXERCISES IN ZOOLOGY. By William Morton Barrows

LABORATORY MANUAL OF HIGH SCHOOL CHEMISTRY.

By George Howard Bruce

PERSONAL HYGIENE AND HOME NURSING. By Louisa C. Lippitt
RECORD BOOK FOR INTRODUCTORY CHEMISTRY. By Neil E. Gordon
SCIENCE OF ANIMAL LIFE. By William Morton Barrows
SCIENCE OF PLANT LIFE. By Edgar Nelson Transeau

COLLEGE CHEMISTRY. By Neil E. Gordon
EXPERIMENTAL ORGANIC CHEMISTRY. By Augustus P. West
GENERAL BOTANY. By Edgar Nelson Transeau
INTRODUCTORY COLLEGE CHEMISTRY. By Neil E. Gordon
LABORATORY AND FIELD WORK IN GENERAL BOTANY.

By E. N. Transeau and H. C. Sampson
QUALITATIVE ANALYSIS. By William C. Cooper
ZOOLOGY. By T. D. A. Cockerell

LD SCIENCE SERIES

Edited by John W. Ritchie

COMMON SCIENCE

Carleton W* Washburne

> i

Superintendent of Schools, Winnetkay Illinois

Formerly Super-visor 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

re /?30^

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 de-
struction employed generally and systemat-
ically 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 them-
selves 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, there-
fore, 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 -1 1

PREFACE

A COLLECTION of about 2000 questions asked by chil-
dren 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 educa-
tional 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 indica-
tion of the parts of science in which children are most
interested. Physics, in simple, qualitative form, — not
mathematical physics, of course, — comes first ; astron-
omy 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 ap-
plications. But the examples and descriptions through-
out 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.

M113O52

vi Preface

The best method of presenting the principles to the
children was the next problem. The study of the ques-
tions 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, there-
fore, 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, quan-
titative 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

Preface

VII

effect state the problems which the section helps to
solve.

Following the questions are some introductory para-
graphs 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 conversa-
tional style. The purpose is to awaken the child's im-
agination 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. Con-
trast is essential to consciousness. To contrast a condi-
tion 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 psychologi-
cally sound method of bringing out the essential and
dramatic features of a principle which is in itself interest-
ing, 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 an-
other 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

viii Preface

help him solve his problem. The experiments are made
as simple and interesting as possible. They usually re-
quire very inexpensive apparatus and are chosen or in-
vented both for their interest value and their content
value.

With an explanation of the experiments and the ques-
tions 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 va-
rious 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 in-
dividual 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 re-
cite 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 sec-
tion. If the children are interested, they will remember
what is valuable to them ; if they are not, do not pro-
long the agony. The questions which accompany and
follow the experiments, the applications or required ex-
planations 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 sub-
stituted for gasr but they are less satisfactory.

It is a good plan to have pupils report additional exem-
plifications 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 accord-
ing to the regular class system of instruction, or according

x To the Teacher

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 experi-
ments 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 prin-
ciples, with a blank after each. Whenever a pupil cor-
rectly explains an example, a figure i 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 I'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 re-
duced. This plan would probably have to be altered
in order to adapt it to particular conditions.

The Socratic method can be employed to great advan-
tage in handling difficult inferences. The children dis-
cuss 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 essen-
tial problems into relief.1

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

To the Teacher xi

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 sub-
jects rather than according' to daily lessons. The vary-
ing 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 5o-min-
ute periods a week give a better division of time and also
ought to finish the course in a year. Under the individ-
ual 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 in-
structions as to the organization and equipment of the
laboratory or demonstration desk, complete lists of ap-
paratus and material needed, and directions for the con-
struction 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 astron-
omy 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 classifi-
cation and selection of inferences. Miss Beatrice Harper
assisted in the preparation of the tables of supplies and ap-
paratus, 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 photo-
graphs. The photographs are by Joseph Marron.

xii

CONTENTS

CHAPTER

i. GRAVITATION

1. A real place where things weigh nothing and

where there is no up or down . . . i

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 at-

traction ..... . 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
1 6. Cooling from expansion .... 94
17. Freezing and melting . . . . * 96
1 8. Evaporation ..... g£| 100

19. Boiling and condensing . . . 107

20. Conduction of heat and convection . . 116

xiv Contents

CHAPTER PACE

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 . v . . . . . 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

Contents xv

CHAPTER PAGE

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 I 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

CHAPTER ONE

GRAVITATION

SECTION i. 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 every-
thing 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.

2 Common Science

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 dis-
cover 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

Gravitation 3

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 uni-
verse has on everything else. The more massive a
thing is, the more gravitational pull it has on other

4 Common Science

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

Gravitation 5

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.

6 Common Science

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

Gravitation 7

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 be-
come 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 i. 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
i. 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.

Common Science

FIG. i. 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 reser-
voir, 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.

Gravitation 9

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^ f rom 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 ?

Application 3. A man was about to open a garage in San
Francisco. He had a large oil tank and wanted a simple

FIG. 2. Where is the best location for the tank?

Common Science

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

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, con-
necting 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?

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 pres-
sure 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

Gravitation n

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 :

Common Science

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

Gravitation 13

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 pres-
sure 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

Common Science

FIG. 5. The water is held in the tube by air pressure.

tube, would have to leave a vacuum, which the air out-
side 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 ?

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:

Gravitation

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 hemi-
spheres (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.

Common Science

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 hemi-
spheres 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

Gravitation 17

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 com-
pressed 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 true 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

Common Science

FIG. 8. A siphon. The air pushes the water over the side of the pan.

so strong outside the balloon, and so the gas inside makes
the balloon expand until it bursts.

Experiment 9. Lay a rubber tube flat in the bottom of
a pan of water, so that the tube will be rilled with water.
Let orte 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

Gravitation 19

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

FIG. 9. A glass model suction pump.

20 Common Science

because a perfect vacuum cannot be made. There is
always some water vapor formed by the water evaporat-
ing 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.

If you had a straw over 33 feet long, and if some one
held a glass of lemonade for you down near the side-
walk 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 differ-
ent 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

Gravitation

winds. Naturally, if the air is pressing harder in one
place than in another, the lower air will be pushed

FIG. 10.

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.

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

22 Common Science

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 ex-
plained 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 in-
stance, 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.

Gravitation 23

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 sub-
stances 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

Common Science

FIG. ii. The battleship is made of steel, yet it does not sink.

than the object. It is the water or air being pulled
under the object by gravity, that pushes it up. There-
fore, 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.

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

Gravitation 25

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.

26 Common Science

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 : Any-
thing 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 n. Fill one test tube to the brim with kero-
sene 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 card-
board out from between the two tubes, or have some one
else do this while you hold the two tubes mouth to mouth.

Gravitation

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

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.

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, there-
fore, 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, " be-
cause," 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

28 Common Science

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 be-
ginning, 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.

1 6. 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.

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

Gravitation

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 rilled 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 top-
pling. 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. FIG. 13. The Leaning Tower of Pisa.

30 Common Science

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. l 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 5X9 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
FIG. 14. shown in Figure 15.

f To THE TEACHER. If you have a laboratory, it is well to have this
cylinder already made for the use of all classes.

Gravitation

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.

FIG. 1 6. 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.

Common Science

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, hold-
ing it in place by stuffing
some crumpled paper in

FIG. 17. So even if the cylinder is laid on on . toP of . it:- Wil1 the
its side it immediately comes to an upright Cylinder tip OVCr now ?
position again. Take ^ stone ^ pufc

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. When-
ever 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

Gravitation

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?

Where do you suppose the center of weight of the

FIG. 1 8. Which vase would be the hardest to upset?

34 Common Science

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.

Gravitation 35

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 sheet-
ing. 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

Molecular Attraction 37

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. Every-
thing is withering. The moisture does not move
through the earth to where the roots of the plants
can reach it. Before everything withers completely,

38 Common Science

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.

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

Molecular Attraction

FIG. 20. The water rises through the lamp wick by capillary attraction.

The space between the threads of the wick, and espe-
cially 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

40 Common Science

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 re-
call 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

o^er.

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.

Molecular Attraction 41

SECTION 7. How things stick to one another: Ad-
hesion.

Why is it that when a thing is broken it will not stay to-
gether without glue ?

Why does chalk stay on the blackboard ?

Now that you have found out something about capil-
lary attraction, suppose that you should go to the imag-
inary 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 per-
spiration. 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

42 Common Science

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.

Which do you think is the stronger, the pull of
gravity which makes some of the water drip off, or the

FIG. 21. As the finger is raised the water is drawn up after it.

Molecular Attraction 43

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 par-
ticles 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

44 Common Science

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 :

Molecular Attraction

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

46 Common Science

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 world-
wide 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

Molecular Attraction 47

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 mole-
cules 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 be-
cause 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

Common Science

FIG. 23. The mercury does not wet the ringer, and as the ringer is lifted the
, mercury does not follow it.

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?

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.

Molecular Attraction 49

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 un-

starched 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 neces-
sary 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

50 Common Science

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

Molecular Attraction 51

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 impos-
sible 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, when-
ever 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

Common Science

FIG. 24. Hockey is a fast game because there is little friction between the skates

and the ice.

friction between the wheels of a car and the track they
are pushing against, or the wheels would whirl around
and around uselessly.

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

Molecular Attraction 53

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 grind-
stone ; 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

Common Science

FIG. 25. The friction of the stone heats the nail and wears it away.

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.

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

Molecular Attraction 55

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.

56 Common Science

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

68. Certain fish can rise in the water by expanding their air blad-

ders, 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 hi a rowboat or canoe.

CHAPTER THREE

CONSERVATION OF ENERGY

SECTION 10. Levers.

How a big weight can be lifted with a little force; haw 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 philos-
opher 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 com-
pletely 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

Common Science

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

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.

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 yard-
stick must be large enough to let the head of this nail through.

Put the middle hole of the yardstick over the nail, as is

Conservation of Energy

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?

FIG. 27. The yardstick is a lever by which he lifts the pail.

Common Science

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 ?

Conservation of Energy

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 to-
gether. Can you pinch the finger this way harder or less
hard than in the way you first tried?

FIG. 29. You cannot pinch hard enough this way to hurt.

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FIG. 30. But this is quite different.

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

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

Conservation of Energy

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?

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?

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

Common Science

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 sys-
tems, cog systems, der-
ricks, crowbars, and every
kind of machine. In
most machines you either
put in more force than
you get out and gain mo-
tion, 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.

FIG. 32.

His hand goes down as far as
the pail goes up.

Conservation of Energy

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 han-
dles 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 ?

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?

66 Common Science

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 n. 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 sud-
denly, 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

Conservation of Energy 67

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 direc-
tion 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 contract-
ing 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

Common Science

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

hand of the one who caught it as when it left your hand
or the bat.
Try these experiments :

Experiment 23. Fill a glass almost to the brim with water.
Lay a smooth piece of writing paper 10 or n 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.

Conservation of Energy

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 FIG 35 when a boy is moving rapidly>

to Stop him in spite of the it takes force to change the direction of his

fact that he is no longer motion-
pushing himself along.

Now let the boy skate toward you again, coasting as
before; but this tune 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.

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

yo Common Science

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 mov-
ing 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 auto-
mobile 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

Conservation of Energy 71

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.

72 Common Science

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 him-
self 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

Conservation of Energy 73

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 be-
ginning 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

Common Science

FIG. 36. Why doesn't the water spill out?

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.

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 centrif-
ugal 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

Conservation of Energy

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.

How cream is separated from milk by centrifugal
force. The heavier things are, the harder they are

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

76 Common Science

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. Ex-
plain 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.

Conservation of Energy ; 77

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 back-
ward 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

Common Science

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

you took forward, just as the big ball that a performing
bear stands on turns backward as the bear tries to walk
forward.

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.

Conservation of Energy

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

FIG. 39. As he starts to toss the bait up, will he weigh more or less?

Common Science

track backward in mak-
ing the car move for-
ward. 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

FIG. 40. Action and reaction are equal; ,

when he pushes forward on the ropes, he backward push to the

pushes backward with equal force on the seat, garth beneath

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 your-
self up in the swing ; that is, make it swing by moving your

Conservation of Energy 81

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 :

1 01. 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 pin wheel, the wheel whirls around as the

powder burns, and the sparks fly off in all directions,
no. You cannot lift yourself by your own boot straps.

82 Common Science

SECTION 14. Elasticity.

What makes a ball bounce ?

How does a springboard help you dive?

Why are automobile and bicycle tires rilled 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.

Conservation of Energy 83

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 somer-
sault, 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 stretch-
ing has to be always on the alert, because the instant he
stops holding himself in some other position, his mouth

84 Common Science

flies open, his arms fly out, and every one thinks he is
bored to death.

You might touch the clay that a sculptor is mold-
ing 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.

Conservation of Energy 85

Elasticity is the tendency of a thing to go back to its orig-
inal 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

86 Common Science

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 posi-
tion ; 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 :

in. 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.

1 20. 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

Conservation of Energy 87

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 oiform. 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 oiform 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 mer-
cury 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 im-
aginary 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,

Heat

FIG. 41. A thermometer.

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-mov-
ing molecules make up everything in the world.

QO Common Science

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 ex-
pands things. When the molecules in an object move
farther apart, naturally the object expands.

Heat is the motion of the molecules. When the mole-
cules move faster (that is, when the iron grows hotter),
they separate farther and the iron swells.

How we can tell the temperature by reading a ther-
mometer. The mercury (quicksilver) in the bulb of

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.

Heat

FIG. 43. Will the hot ball go through the ring?

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.

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 :

Common Science

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.

Heat

FIG. 45. But notice how it sags when it is hot.

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?

Application 25. I used to buy a quart of milk each eve-
ning 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.

94 Common Science

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
ringers.

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 ex-
pand, 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

Heat

FIG. 46. The expansion of the compressed gas freezes the moisture on the tube.

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.

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

96 Common Science

down on it from above, it expands. So the air that rises
high and expands gets very cold. Consequently moun-
tains 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 imag-
inary microscope that shows us the molecules. The

Heat 97

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 be-
cause 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 be-
come 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. V |

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

Common Science

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

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.

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 ther-
mometer, using a flask of water and a glass tube.
But if you should put that flask into a freezing mix-
ture of ice and salt, you would find that when the water

Heat 99

became very cold it would begin to expand a little im-
mediately 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.

loo Common Science

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 in-
visible 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 sud-
denly stop :

You find that your face is perspiring and your
hands as well. You wipe them on your handkerchief,

FIG. 48. An evaporating dish.

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.

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

IQ2 Common Science

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 shoot-
ing 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

Heat 103

evaporation. That is why you are always cold after
you leave the bathtub until you are dry. The water

o
\

\
\

'lim •-; •/-•
\ \ / - ?

\ v / /

N>; \ / /

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

that evaporates from your body uses up a good deal of
your heat.

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

IO4 Common Science

why you hang clothes in the sun or by the stove
to dry.
Try these experiments :

Experiment 33. Read a thermometer that has been ex-
posed to the room air. Now dip it in water that is warmer
than the air, taking it out again at once. Watch the mer-
cury. 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 con-
tinually 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.

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105

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 evap-
orates 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.

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

1 ' 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 out-

let ; yet it is getting more nearly empty all the time.

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

1 54. 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.

Heat 107

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.

1 60. 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 mole-
cules 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

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FIG. 51. In a minute the cork will fly out.

test tube turned to steam, the steam had to get out and
pushed the cork out ahead of it.

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

Heat

109

FIG. 52. A toy balloon has been slipped over the mouth of a flask that is filled

with steam.

FlG. 53. As the steam condenses and leaves a vacuum, the air pressure forces
the balloon into the flask.

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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 ?

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 locomo-
tive 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

Heat

FIG. 54. Will boiling water get hotter if you make it boil harder?

that many people do not know; and their gas bills
would be much smaller if they knew it. Try this ex-
periment :

Experiment 38. Heat some water to boiling. Put the
boiling-point thermometer into the water (the thermom-
eter 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 appre-
ciably 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?

ii2 Common Science

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 1 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.

Distilling. When liquids are mixed together and dis-
solved in each other, it looks as if it would be impos-
sible 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 kero-
sene. And people make a great deal of practical use of

1 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.

Heat

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

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

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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 ?

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, in-
cluding 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,
an(i 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

Heat 115

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.

Common Science

165. Comets swing around close to the sun, then off again into

space ; how do they get away from the sun ?

1 66. 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.

1 68. 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 :

Experiment 41. Take a solid glass rod and a solid iron
rod, each about a quarter inch in diameter and about 6

FIG. 56. The metal balls are fastened to the iron and glass rods with drops

of wax.

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117

FIG. 57. Does the heat travel faster through the iron or through the glass?

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.

What really happens down among the molecules
when the heat travels along the rods is that the mole-
cules 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

n8 Common Science

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 cook-
ing 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

Heat

119

FIG. 58. Convection currents carrying the heat of the stove about the room.

work the same way — carry heat from one place to
another like this, we say that the heat has traveled
by convection.

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 con-
ductor 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.

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Common Science

Hot

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 be-
come warm. A blanket
is just as good for keep-
ing ice from melting, by
shutting the heat out, as
it is for keeping you
warm, by holding heat in.

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

Select from

the following
.! Flame .. A .

list the good

FIG. 59. Diagram of a hot-water heater. What makes Conductors of
. the water circulate? heat from the

poor conductors (insulators) : glass, silver, iron, wood, straw,
excelsior, copper, asbestos, steel, nickel, cloth, leather.

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.

Heat 121

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.

1 80. 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 dark-
ness 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.

Radiant Heat and Light

123

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 radia-
tion stopped, — we should be so completely shut off

124 Common Science Y

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 yi times be-
fore 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

Radiant Heat and Light 125

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 mole-
cules 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 fila-
ment — in the lamp get so hot when the electricity
flows through them that they glow or become incandes-
cent, 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.)

126 Common Science

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

Radiant Heat and Light

127

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

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.

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 for-
ever 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 day-
time radiates away at night. And since it keeps on

128 Common Science

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 Febru-
ary 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 con-
ducted heat in each of the following examples :

(a) The sun warms a room through the window. (£) 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. (/) 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 be-
comes 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.

Radiant Heat and Light 129

185. Clothes get hot when you iron them.

1 86. 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.

1 88. 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 grad-

ually 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 any-

130 Common Science

thing 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 in-
visible 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 invis-
ible 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

Radiant Heat and Light 131

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 per-
son 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.

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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 sur-

Radiant Heat and Light

133

face at a slant, it reflects at the same slant in the oppo-
site direction.

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 re-
flect 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 re-
flection. But if the pane of glass is silvered so that

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.

134 Common Science

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 sur-
face 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. Any-
thing 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.

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."

Radiant Heat and Light

135

FIG. 64. How should the mirror be placed?

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 mir-
ror 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 arrange-
ment 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.

136 Common Science

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 after-

ward.

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 dif-
ferent 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 some-
where else.

As you already know, when you look at a chair you
see the light that reflects from it. You judge where

Radiant Heat and Light 137

the chair is by the direction from which the light is
coming when it reaches your eye. But if the light is

^""" FIG. 65. In passing through the prism
^~"" the light is bent so that an object at b

^~~~ appears to be at c.

^^^

f~**** bent on its way, so that it

i c comes to your eye as it ordi-

narily 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.

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?

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Common Science

FIG. 66. The pencil is not bent, but the light that conies 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 re-
fraction, or bending of light. The light from the part

Radiant Heat and Light

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

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.

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

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Common Science

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

FIG. 68. The light is bent when it enters a

window pane and is bent again in the op- makes anything W6 look

at through a poor win-

dow 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.

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

Radiant Heat and Light 141

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 un-
even 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 per-

forated cylinders.

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

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Common Science

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 ouddle.

Source

Focus

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 flat-
tened ball, as clear as glass, called the lens. If the
lens were left out of your eye, you never could see any-
thing 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.

Radiant Heat and Light

143

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 noth-
ing more.

In front of a
moving-picture
machine there is a
large lens, a piece
of glass rounded
out toward the
middle and thin- FIG. 70.
ner 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 nicker 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 each point of the candle
flame goes out in all directions.

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Common Science

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

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.

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 canolle and the paper, so that the light
will shine through the lens on to the paper. (The magnify-
ing 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

Radiant Heat and Light 145

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

Focus

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

FIG. 73. And the light from the base of the flame is focused at another point.

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.

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Common Science

But of course the light from the base of the flame is
focused at the same time as the light from the point;

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.

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 ?

Radiant Heat and Light

147

The explanation is this: After the light comes to-
gether into a point, it spreads out again beyond the

FIG. 75. The light spreads out again beyond the focus.

- - - ~ _ _ Focus

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

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.

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

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Common Science

the paper, naturally you again have a blur of light instead
of a point, and the image is not sharp and definite (Fig. 77).

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 like-
wise 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 natter and
rounder, so that it can make a clear image on the sensi-
tive 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

Radiant Heat and Light

149

out toward the middle instead of bulging, and so on.
But the only lens that most people make much use of

7 \~7

A Li

FIG. 78. Lenses of different kinds.

(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.

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.

150 Common Science

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 neces-
sary 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

Radiant Heat and Light 151

Experiment 41, but this time move the lens from near
the candle toward the paper, past the point where it

sclerotic coat

muscles

FIG. 79. A section of the eye.

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?

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.

1 Your eye is a small camera. It has a lens in the
front ; it is lined with black ; and at the back there is

1 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.

152

Common Science

FIG. 80. How an image is formed on the retina of the eye.

FIG. 81. A simpler diagram showing how an image is formed in the eye.

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

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

Radiant Heat and Light 153

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.

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

Common Science

bend the lines together, and that the lens in the eye
bends them farther together ; so they cross sooner, and

FIG. 84. Diagram of a microscope.

the image is larger. Figure 83 shows more of the lines
drawn in.

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 magnify-
ing 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.

FIG. 85. This is the way a concave mirror forms a magnified image.

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155

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 dia-
gram shows one.) What actually happens is that the

Common Science

FIG. 87. The great telescope of the Yerkes Observatory at Lake Geneva,

Wisconsin.

first lens makes an image many times as big as the
object; then you look at this image through a mag-
nifying 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.

Radiant Heat and Light 157

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 sur-

face 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.

158 Common Science

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

Radiant Heat and Light

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

these droplets, it is reflected at all angles according to
the way it strikes, as shown in Figure 89.

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 to-
gether 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

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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.

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

FIG. 89. How the droplets in a cloud scatter the rays of light.

Radiant Heat and Light 161

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?

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FIG. go. 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.

Experiment 50. Hold a prism in the sunlight by the
window and make a " rainbow " on the wall. The diagram

VIOLET

WHITE

FIG. 91. The prism separates the white light into the rainbow colors.

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163

FIG. 92. When the wheel is rapidly whirled the colors blend to make white.

here shown illustrates how the prism breaks up the single
beam of white light into different-colored beams of light.

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

164 Common Science

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 " any thing black,
you really see the things around it and the parts of it
that are not perfectly black. A pair of shoes, for in-
stance, 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.

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165

FIG. 93. Which color is warmest in the sunlight?

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?

(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 ?

1 66 Common Science

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 ex-
plained 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 throw-
ing 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 to-
gether 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 "

Radiant Heat and Light 167

to throw back the red lights and keep all the other kinds
(absorb them and change them to heat), we can under-
stand 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

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FIG. 94. A mercury-vapor lamp.

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.

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

Radiant Heat and Light 169

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 simi-
lar 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

170 Common Science

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 imita-
tion 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,

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171

FIG. 95. Explain why the breakers are white and the sea green or blue.

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.

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 (blue-
stone) in the flame of a Bunsen burner. What color does
it make the flame ?

172 Common Science

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 spectro-
scope 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.

Radiant Heat and Light 173

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 under-
stand 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

174

Soimd 175

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

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FIG. 96. An interesting experiment in sound.

them vibrate? Tap it again, and hold the fork close to
your ear. Can you hear the 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

177

FIG. 97. When the air is pumped out of the jar, you cannot hear the bell ring.

sound caused by the jarring of the air pump when the alarm
is on, and the actual ringing sound of the bell.

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 dicta-
phone. 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.

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FIG. 98. Making a phonograph record on
an old-fashioned phonograph.

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 dia-
phragm, a flat disk which
is elastic and vibrates
back and forth very
easily. The air waves
from your voice would
not vibrate the needle it-
self enough to make any
record ; but they vibrate
the diaphragm, and the
needle, being fastened rigidly to it, vibrates with it.

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 dia-
phragm 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.

Sound

179

FIG. 99. A modern dictaphone.

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.

Making a record in this way is, on a large scale, al-
most exactly like the making of a phonograph record.
The smoked paper on which a tracing can easily be made

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Common Science

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.

In all ordinary circumstances, sound is the vibration
of air. But in swimming we can hear with our ears

FIG. 100. How the apparatus is set up.

Sound

181

FIG. 101. When the tuning fork vibrates, the glass needle makes a wavy line
on the smoked paper on the drum.

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. Vi-
brations 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.

Why we see steam rise before we hear a whistle blow.
But even through steel, sound does not travel with

1 82 Common Science

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.

Sound 183

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

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Common Science

FIG. 102. When the wave reaches the end of the sink, it is reflected back.
Sound waves are reflected in the same way.

to the other so rapidly that no distinct echo is heard,
and there is merely a confusion of sound.

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 direc-
tions. But when you. shout through a megaphone the
air waves are all concentrated in one direction and conse-
quently they are much stronger in that direction. How-
ever, 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

Sound 185

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

1 86 Common Science

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 vibra-
tions 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

Sound

187

strike it. Now pour half
tlie 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 tight-
ness? 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 ?

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.

FIG. 103. When the prongs of the tuning
fork are made longer or shorter, the pitch
of the sound is changed.

1 88 Common Science

In general, the longer or larger the object is that
vibrates, the slower the rate of vibration and conse-
quently 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 in-
creased.

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 Christ-
mas. 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.

Sound 189

273. A rather slight contraction of a muscle lifts your arm a con-

siderable 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 ex-
planation in the fact that the earth is a great big magnet.
The experiment which follows will help you to under-
stand 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

190

Magnetism and Electricity

191

FIG. 104. The compass needle follows the magnet.

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?

Common Science

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 be-
came magnetized when it was rubbed on the small
magnet.

FIG. 105. Magnetizing a needle.

Magnetism and Electricity

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

194

Common Science

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 un-
magnetized piece of iron or steel) these molecules would
be facing every way, as shown in Figure 107.

But when a piece of steel or iron that is already mag-
netized is brought near the unmagnetized needle, all
the north poles of the molecules of the needle are pulled

FIG. 107. Diagram of molecules in
unmagnetized iron. The north and
south poles of the molecules are sup-
posed to be pointing in all directions.

FIG. 1 08. Diagram of magnetized
iron. The north and south poles of
the molecules are all supposed to point
in the same direction.

Magnetism and Electricity 195

in the same direction — it is almost like combing tangled
hair to stroke a needle over a magnet. Then the mole-
cules 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 elec-
tricity 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 hunt-
ing 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 mer-

cury-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.

196 Common Science

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 rever-

berating 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 mole-
cules, 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.

Magnetism and Electricity 197

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 saw-
dust. What does it do to them? Hold it over some one's
hair. The rest of this experiment will work well only on

198

Common Science

cool, clear days. Rub the
comb again, a dozen or
more times in quick suc-
cession. 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

FIG. zoo. When the comb is rubbed on the end of the hair clings to
coat, it becomes charged with electricity, 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.

Objects negatively and positively charged with elec-
tricity. 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 elec-
trons than usual in an object, we say that it has a positive
charge of electricity on it, or that it is positively charged.

Magnetism and Electricity

199

You might expect a
" negative charge " to
indicate fewer electrons
than usual, not more.
But people called the
charge " negative " long
before they knew any-
thing 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 FIG
call it " positive " when

The charged comb picks -up
pieces of paper.

there are unusually few.

A negative charge means that more electrons are present
than usual. A positive charge means that fewer elec-
trons are present than usual.

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

200 Common Science

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 num-
ber 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 thunder-
storms 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 num-
ber, or it leaps from the heavily charged cloud down to
a tree or house or. the ground. You see the electricity

Magnetism and Electricity 201

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, com-
ing 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 elec-
tricity. " 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.

2O2 Common Science

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 elec-
tricity 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 am-
moniac 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

203

204 Common Science

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 witli 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,

Electricity

205

FIG. in. A wet battery of three cells connected to ring a bell.

fasten another piece of wire from the carbon of the second
cell to the zinc of the third, as shown in Figure in.

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.

Different kinds of batteries. There are many dif-
ferent 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

2o6 Common Science

FIG. 112. A battery of three dry cells.

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.

But all the different batteries work on the same gen-
eral principle : A chemical solution divides into two parts,
one with many electrons and the other with a less num-
ber. 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

Electricity

207

the zinc the negative.
Although we now know
that the electrons flow
from the zinc to the car-
bon, it is much more con-
venient 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 elec-
tricity 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 posi-
tive 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.

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 under-

FIG. 113. A storage battery.

208 Common Science

stand 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
FIG. 114. Spinning loops of can be done by spinning a loop of

between the poles of a mag-

electricity to flow through net. Whenever a loop of wire is
turned between the two poles of a

magnet, the magnetism pushes the electrons that are al-
ready in the wire around and around the loop. As long as
we keep the loop spinning, a current of electricity flows.
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

FIG. 115. The more loops there are, the stronger the current.

around the loops — a powerful current of electricity
flows through the wire.

Now suppose you bring one loop of the long wire out,
as shown in Figure 115, and suppose you spin the rest

Electricity

209

of the loops between the poles of the magnet. Then,
to flow through the loops by the magnet the electricity

FIG. 1 1 6.

If the electricity passes through a lamp on its way around the circuit
the filament of the lamp glows.

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

FIG. 117. A dynamo in an electric light plant.

2io Common Science

spinning coil of wire, as shown in Figure 116. Such
an arrangement as this is really an extremely simple
dynamo.

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 com-
plicated 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 elec-
tricity 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.

Electricity

211

FIG. 1 1 8. The magneto in an automobile is a small dynamo.

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.

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 volt-
age (pressure) of the current if instead of being direct
it is alternating; that is, if for one instant the electricity

212 Common Science

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 com-
ing 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 some-

times stands up and flies apart more and more as you con-
tinue to brush it.

305. You cannot see a person clearly through a ground-glass

window, although it lets most of the light through.

Electricity 213

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 6s.1 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?

1 Read footnote, page 226, before doing this experiment.

214

Common Science

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 ordi-
nary current of electricity, no 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

Electricity

215

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 conduc-
tors, and they are therefore used as insulators, — to
keep the electricity from going where you do not want it
to go.

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

FIG. 120. Will electricity go through the glass?

2l6

Common Science

FIG. 121. Electrical apparatus : A, plug fuse; B, cartridge fuse; C, knife

switch; H, lamp socket;

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.

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 any-
thing wet, as a pretty good conductor of electricity.
Absolutely pure, distilled water is an extremely poor
conductor; but most water has enough minerals dis-
solved 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.

switch ; D, snap switch ; E, socket with nail plug in it ; F, fuse gap ; G, flush
/, /, K, resistance wire.

Application 50. Robbers had cut the telegraph line be-
tween 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 con-
necting the broken ends of the telegraph wire ?

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 1 2-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

218

Common Science

FIG. 122. Which should he choose to connect the broken wires?

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.

Electricity 219

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 cir-
cuit. 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 How of the v/ater
in the tank we have been imagining. The long loop
of wire extending out from the dynamo to your house

22O

Common Science

FIG. 123.

Electricity flows around a completed circuit somewhat as water might
be made to flow around this trough.

and back again corresponds to the tank. The elec-
tricity 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 elec-
tricity 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.

Breaking and making the circuit. The most con-
venient way to put an air partition into an electric cir-
cuit 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

Electricity 221

froni 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 conies 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 build-
ing. 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

222 Common Science

FIG. 124. Diagram of the complete

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.

Figure 125 illustrates the principle just explained.
As there diagramed, 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 cir-
cuit 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 cir-
cuits 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

Electricity

223

circuit through the laboratory switches.

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?

Circuit A Circuit B
FIG. 125. Parallel circuits.

Circuit G

224

Common Science

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 elec-
trician 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.

Electricity 225

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 moun-

tains 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 tele-
graph 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

226 Common Science

packed with earth ; but since water pipes go down deep
and the earth is already packed around them, the most

FIG. 127. The ground can be used in place of a wire to complete the circuit.

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.

Experiment 68. Caution: Keep the switches turned off
throughout this experiment.1

(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, in-
stead of into the other wire. The ground carries the elec-
tricity back to the dynamo just as a wire would.

(b) Put a new piece of fuse wire across the gap. Keep
switches turned of. 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.

1 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.

Electricity

227

FIG. 128.

Grounding the circuit. The faucet and water pipe lead the electricity
to the ground.

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.

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.

228

Common Science

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 in-
sulation 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.

Electricity

229

FlG. 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 hair-

pin 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 ?

230 Common Science

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 some-
what ; 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 elec-
tric 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

Electricity

231

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?

copper but of substances which do not conduct elec-
tricity 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 notice-
ably.

Experiment 70. Turn on the switch that lets the elec-
tricity 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 mix-
ture of metals that will resist electricity much more
than ordinary metals will. This is the same kind of

232 Common Science

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 regu-
lar 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.

Electricity 233

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 hi 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

234

Common Science

FIG. 133. The pencil points are touched together and immediately drawn apart.

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.

Electricity

235

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.

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

FIG. 134. A brilliant arc light is the result.

236 Common Science

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 impor-
tant point about this experiment with arcs : most fires
that result from defective wiring are caused by the form-
ing 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 in-
tensely hot such an arc is. Two wires rubbing against
each other, or a wire not screwed tightly to its connec-
tion, 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 some-
times 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.

Electricity

237

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 cur-
rent 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

238 Common Science

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 danger-
ous 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

Electricity 239

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 under-
standing 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 screw-
ing it in tightly ?

240 Common Science

1 1 . 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 trum-

pets.

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

Electricity

241

FIG. 136. A, the "fuse gap" and B, the "nail plug."

in a new fuse. The next three experiments will help
you to understand fuses.

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 elec-
tricity 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 to-
gether, or connect them with a piece of any metal, and turn

242 Common Science

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 imme-
diately if you hold a match to it; try this and see.
Consequently, long before the fuse wire becomes red

Electricity

243

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 cir-
cuit, of course, so that no more electricity can flow.
In this way the fuse protects houses from catching fire
through short circuits.

Unfortunately, however, the fuse is almost no pro-
tection 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

FIG. 137. What will happen when the pin is thrust through the cords and the
electricity turned on?

244 Common Science

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 any-
thing 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 some-
times cause fuses to blow out by pinning pictures to elec-
tric 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 elec-
tricity.

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

Electricity 245

it is the path of least resistance. Wherever the elec-
tricity 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 every-
thing 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

246 Common Science

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 re-
sistance 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 shav-
ing 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, ex-
amine the fuse and see how many amperes (how much cur-
rent) it will allow to flow through it. It will say 6 A if it
allows 6 amperes to pass through it; 25 A 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 loA.

Was the boy or was the father right? If the father was
right, should they have got a fuse marked 6 A, one marked
io;l, or one marked 15 A ?

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-

Electricity 247

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 loampere 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 ce-
ment, 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 tele-

248

Common Science

FIG. 138. The magnetized bolt picks up the iron filings.

phone receiver, and so on. The following experiments
will show you how electricity moves things :

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

Electricity

249

FIG. 139. Sending a message with a cigar-box telegraph.

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.

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 dis-
connecting 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.

250

Common Science

FIG. 140. Connecting up a real 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.

Experiment 77. Connect two regular telegraph instru-
ments, leaving one at each end of the long laboratory table.
Make the connections as follows :

Electricity

251

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. Be-
fore 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 Ground

Fii. 141. Diagram showing how to connect up two telegraph instruments.
The circles on the tables represent the binding posts of the instruments.

252

Common Science

FIG. 142. Telegraphing across the room.

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. Tele-
graph your own name ; your address.

Electricity 253

Here is the Morse telegraph code in dots and dashes :

LETTERS
A B C D E F G

• Ha ••••• •• 9 mmmm • •••• mm mm 4

H I J K L M N

OP Q R S T U

• • ••••• ••••• • •• ••• mm *mmm

V W X Y Z &

• •••• vmmmm ••••• •• •• ••• m • •••

NUMERALS
i 2 3 4 5

67890

By using the Morse code, telegraph and cable mes-
sages 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 vibra-
tions started by your voice press the "key," and in which
the sounder can vibrate swiftly in response to the elec-
tric 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

254 Common Science

the current flowing in the circuit. And each time the
current in the circuit is increased, the disk in the re-
ceiver is pulled down, just as the sounder of a tele-
graph 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 pass-
ing 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 instru-
ment. 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.

Electricity 255

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 prin-
ciple. In the next experiment you will see the work-
ing 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, elec-
tric sewing machines, electric vacuum cleaners, and
electric player-pianos, are moved by magnetizing a

256

Common Science

FIG. 144. A toy electric motor that goes.

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.

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 mil-
lions of coils of wire between the poles of powerful mag-
nets. 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

Electricity

257

FIG. 145. An electric motor of commercial
size.

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. Tremen-
dous 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.

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.

258 Common Science

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 amaze-
ment when, bringing it to his lips, he finds himself drink-
ing 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 any-
thing 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

259

260 Common Science

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 float-
ing 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 experi-
ments 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 pre-
pared cleanser into the test tube, adding a little more water.
Shake the test tube thoroughly and rinse. Put it away
clean.

Mingling of Molecules 261

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 FIG. 146. Will heating the water make

scattered all through the more *** dissolve?

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.

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.

262

Common Science

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 i 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

Mingling of Molecules 263

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 dif-
ference. 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. There-
fore 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 ame-
thysts were colored in the same way, but by nature.

264 Common Science

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 dis-
solved. 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 emulsify-
ing the dirt you want to get rid of, and washing it away
with the liquid. Do not forget that heat helps to dis-
solve 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.

Mingling of Molecules 265

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 black-

board.

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 experi-
ment:

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, un-
covered. 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.

266

Common Science

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 mole-
cules 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

Mingling of Molecules 267

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, form-
ing 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 al-
ways 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.

268 Common Science

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 evapo-
rated.

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 run 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

Mingling of Molecules 269

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, moun-
tains, 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
ho!4 your thumb or a piece of cardboard over its mouth
while you place it upside down in a pan of water. Take

270

Common Science

FIG. 149. Filling a test tube with gas.

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

Mingling of Molecules

271

FIG. 150. The lower test tube is full of air; the upper, of gas. What will
happen when the cardboard is withdrawn?

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 ?

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 expla-
nation is that the molecules of gas and those of air were
flying around at such a rate that many of the gas mole-
cules 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.

272 Common - Science

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 i 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.

Mingling of Molecules

273

FIG. 151. Pouring the syrup into the "osmosis tube."

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 nourish-
ment, the dissolved food diffuses out through the walls

274 Common Science

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.

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 ?

Mingling of Molecules 275

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.

276 Common Science

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.

Mingling of Molecules 277

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 mole-
cules 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:

278 Common Science

When air or any gas expands, it cools. Do you remem-
ber 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 drop-
lets of water, and these form a cloud.

Each droplet forms a gathering place for more con-
densing 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

Mingling of Molecules 279

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 under-
ground 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

280 Common Science

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 barom-
eter is lower than in clear weather. You will under-
stand 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.1 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

1 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.

Mingling of Molecules

281

FIG. 152. Filling the barometer tube with mercury.

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. Meas-
ure 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.

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.

Of course you understand what holds the mercury
up in the tube. If you could put the cup of mercury

282

Common Science

into a vacuum, the mer-
cury 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 flow-
ing out of the tube and
so leaving a vacuum in
there. If the air pushes
down hard on the mer-
cury in the cup, the
mercury will stand high
in the tube. This is
called high pressure. If
the air does not press
FIG. 153. inverting the filled tube in the hard on the mercury in

cup of mercury. ^ ^ ^ mercury

stands low in the tube. This is called low pressure.

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 ba-
rometer is low.

Mingling of Molecules

283

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.

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 tele-
graph. 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

FIG. 154. Finding the pressure of the air
by measuring the height of the mercury in
the tube.

284

Common Science

way. From several such
reports the weather men
to the east can tell how
fast the storm is travel-
ing and exactly which
way it is going. Then
they can tell when it
will reach their station
and can make the correct
prediction.

Weather men do not
have to wait for an ac-
tual storm to be re-
ported. 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 approach-
ing 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

FIG. 155. The kind of mercury barometer
that you buy.

Mingling of Molecules

285

before it gets to him. Then there may be a shower of
rain when he has predicted fair weather. Or some-
times 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 some-
times 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 mis-
taken and to forget the many, many times when it is
right.

How snow is formed. The difference between the
ways in which snow and rain are formed is very slight.

FIG. 156. An aneroid barometer is more convenient than one made with mer-
cury. 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.

Common Science

FIG. 157. Different forms of snowflakes. Each snowflake is a collection of
small ice crystals.

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 snow-
flakes. 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 evapo-
rated once more.

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 rain-
fall. When air that is moving along reaches a mountain

Mingling of Molecules 287

range, it either would have to stop, or rise and go over
the mountain. The pressure of the air behind it, mov-
ing 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 cool-
ness 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.

288 Common Science

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 navi-
gation, 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 mois-
ture 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 con-
denses 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

Mingling of Molecules

289

pressed on so hard by
the air above it. And as
in the case of the ris-
ing 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 FIG. 158. If you blow gently over ice, you

cover the glass. Be sure can see your breath'

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 ?

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.

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.

290

Common Science

FIG. 159. The glass does not leak; the moisture on it comes from the air.

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.

Mingling of Molecules 291

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.

292 Common Science

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 oil-
ing. 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 water-
proof 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 prepara-

tions 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

293

2 94 Common Science

gas which is often used to fill balloons that are to go very
high ; it is the lightest in the world and is called hydro-
gen. 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 (f " X 3") they will fill more quickly. Dissolve
a little lye (about J 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 f" pieces of platinum wire (#30),

Chemical Change and Energy 295

FIG. 1 60. The electrodes are made of loops of platinum wire sealed in glass

tubes.

which are soldered to two pieces of insulated copper wire,
each about 2 feet long.1 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.

Bend the platinum electrodes up so that they will stick
up into the test tubes from below. Bubbles should im-
mediately begin to gather on the platinum wire and to rise
in the test tubes. As the test tubes fill with gas, the water

1 If the copper wire is drawn through a piece of ^-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 satis-
factory. The pieces of glass tubing should be about 6 inches long (see
Fig. 160).

296 Common Science

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 i 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 char-
coal into the test tube full of oxygen.

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.

Chemical Change and Energy 297

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 for-
tunate, because S already stands for the element sulfur.
Fe means iron (Latin, ferrum). But I stands for the

298 Common Science

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 : H2O (pronounced " H two O ").
" H2O " 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 :

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 :

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 :

HC2H3O2 +NaHC03-^H20 + CO2 +NaC2H3O2.

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,

Chemical Change and Energy

299

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 ac-
quainted 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)

Chlorine (Cl)

Copper (Cu)

Gold (Au)

Hydrogen (H)

Iodine

Iron

Lead

Mercury

Nickel

Nitrogen

Oxygen

(I)

(Fe)
(Pb)
(Hg)

(Ni)
(N)
(O)

Charcoal, diamonds, graphite (the lead in a
pencil is graphite), hard coal, and soot are
all made of carbon.

A poison gas that was used in the war.

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.

It is a solid ; what you use is iodine dissolved
in alcohol.

This is another name for quicksilver.

About four fifths of the air is pure nitrogen.

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 experi-
ment.

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Common Science

Phosphorus (P)

Platinum
Radium
Silver
Sodium

Sulfur

Tin

Zinc

(Pt)
(Ra)
(Ag)
(Na)

(S)

(Sn)

(Zn)

Phosphorus makes matches glow in the dark,
and it makes them strike easily.

You are not acquainted with sodium by it-
self, but when it is combined with the poison
gas, chlorine, it makes ordinary table salt.

For the rest of the elements you can refer to any text-
book 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 combi-
nation apart. Take the black element carbon, for in-
stance. 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

Chemical Change and Energy 301

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 (HC1) , 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 inciden-
tally 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 con-
nections 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 (HC1) diluted with three

302

Common Science

FIG. 162. Filling a balloon with hydrogen.

parts of water. Find the bottle marked "HC1, 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 (HC1) 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 attach-
ments into the flask, so that the gas that bubbles up will
blow up the balloon.

Chemical Change and Energy 303

FIG. 163. ' Adding more acid without losing the gas.

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

304 Common Science

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.

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 (HC1) 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 driv-
ing 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 potas-
sium (K) which are as soft as stiff putty and as shiny as
silver; if you put a tiny piece of sodium (Na) or potas-
sium (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.

Chemical Change and Energy 305

FIG. 164. Trying to se.e if hydrogen will burn.

Getting oxygen, a gas, from two solids. Oxygen (0)
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 tea-
spoonful 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

306

Common Science

FIG. 165. Filling a bottle with oxygen.

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

Chemical Change and Energy

307

FIG. 166. The iron really burns in the jar of oxygen.

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 out-
side 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

308 Common Science

sulfur and light it, and then plunge it into the bottle, you
will see iron burn.

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 bril-
liantly, 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 hydro-
gen. Water is H2O, you remember.

What compounds are. When elements are combined
with other elements, the new substances that are formed
are called compounds. Water (H2O) 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

Chemical Change and Energy 309

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 (0) and nitrogen
(N). If they were combined, instead of mixed, they
might form laughing gas, — the gas dentists use in put-
ting 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. Like-
wise 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

3io Common Science

solutions are only mixtures and are not chemical com-
pounds.

Everything in the world is made of atoms. Every-
thing 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 com-
pounds. 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

Chemical Change and Energy 311

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 mole-
cule.

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.

312 Common Science

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 com-
bining 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 pre-
served 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

Chemical Change and Energy 313

which you move come entirely from the burning (oxida-
tion) 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 burn-
ing (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 (com-
bining 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 can-
not make things burn unless you have oxygen to com-
bine with them :

314 Common Science

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 hydro-
gen 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.

Chemical Change and Energy 315

FIG. 167. The water rises in the bottle after the burning candle uses up the oxygen.

Carbon and hydrogen the chief elements in fuel. Car-
bon 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 car-
bon dioxid (CO2) . When the hydrogen in the fuel joins
oxygen, what must we get?

316 Common Science

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 out-
side 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 ;

Chemical Change and Energy 317

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 remem-
ber, 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
(CO2) which covers the burning material and keeps the
free oxygen in the air from coming near enough to com-
bine 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 fila-
ment 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

318 Common Science

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 parti-
cles 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 com-
bining 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

Chemical Change and Energy 319

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 be-
cause 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 labora-
tory. 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

320

Common Science

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 open-
ing) when the air opening
is too wide.

Application 69. An oil
lamp tipped over and the
burning oil spread over the
floor. Near by were a pail

FIG. 168. The Bunsen burner smokes when r watpr „ nqn nr acV,Pc
the air holes are closed. ot Water> a Pan ot 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 com-
pletely covered by it, or they would have decayed," said
one.

Chemical Change and Energy 321

FIG. 169. Regulating the air opening in a gas stove.

" 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 ?

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

322 Common Science

FIG. 170. The air openings in the front of a gas stove.

kind of radiant heat? Of course the yellow flame is the
hottest. Leave the stove alone."
Who was right?

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 mois-

ture wheri 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.

Chemical Change and Energy 323

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

324

Common Science

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 :

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

FIG. 171. Why doesn't the flame above the wire gauze set fire to the gas below ?

Chemical Change and Energy 325

FIG. 172. The part of the match in the middle of the flame does not burn.

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 ?

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.

326 Common Science

455. Striking a match makes it burn.

456. When you have something hard to cut, you put it hi 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 cur-
tains 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

Chemical Change and Energy 327

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 sun-
light 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
(AgNOa) 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 (HC1). The white
powder formed is a silver salt, called silver chlorid (AgCl) ;
the rest of the liquid is now a diluted nitric acid (HNOs).

Pour the suspension of silver chlorid (AgCl) on a piece

328 Common Science

of blotting paper or on a paper towej, 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 de-
pends on this action of light. The plates or films
are coated with a silver salt, — usually a more sensi-
tive 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

Chemical Change and Energy

329

FIG. 173. The silver salt on the paper remains white where it was shaded by

the key.

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.

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, how-
ever, 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

33° Common Science

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
L 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 i or 2 minutes according to the bright-
ness 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 blue-
print paper (not the negative) into a basin of water, face

Chemical Change and Energy

331

FIGS. 174 and 175. Where the negative is dark, the print is light.

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.

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 some-
times 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.

332 Common Science

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 mo'st 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 (H20) from the earth and carbon
dioxid (C02) from the air. When the sun shines on
chlorophyll, the green substance in plants, the chloro-
phyll 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 op-
posite sides of a leaf that is exposed to the sun. The next

Chemical Change and Energy 333

day take jJiis 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. Be-
sides 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 com-
bining oxygen with the carbon in our food (forming

334 Common Science

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 ex-
changed 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*). 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 (C02) and water (H2O) 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 com-
bine it with hydrogen to make water and with carbon
to make carbon dioxid. Since the water and carbon

Chemical Change and Energy 335

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 ex-
change 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 photog-
rapher 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 photo-
graphed.

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?

336

Common Science

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

FIG. 176. The copper and the nickel cube ready to hang in the cleansing solution.

Chemical Change and Energy 337

FIG. 177. Cleaning the copper in acids.

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.

Pour over the piece of copper enough of the cleansing
solution to cover it.1 The cleansing solution contains
strong acids. If you get any on your skin or clothes, wash
it of 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

1 The formula for making the cleansing solution is as follows :

i cup water.

i cup concentrated sulfuric acid.

i cup concentrated nitric acid.

i 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.

338

Common Science

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.

Turn on the electricity. If the copper becomes black
instead of silvery, clean it again in the cleansing solu-
tion, and move the two bare wires much farther apart,
— practically the full width of the bowl. If the copper

FIG. 178. Plating the copper by electricity.

Chemical Change and Energy 339

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 elec-
tricity 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.
47 2. 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.

340 Common Science

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.

47 7 . 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 be-

comes 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

Chemical Change and Energy 341

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 door-
bells 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 auto-
mobiles, flashlights, and doorbells. Since burning (oxida-
tion) 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 phos-
phorescence. The name " phosphorus " means light-
bearing, and anything like the element phosphorus,
that glows without actively burning, is said to be phos-
phorescent. 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 phos-
phorescent. 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

342 Common Science

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 glowworm shines 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 re-

sulting 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

Chemical Change and Energy

343

FIG. 179. The explosion of 75 pounds of dynamite. A "still" from a motion-
picture film.

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.

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Common Science

What makes an auto-
mobile go. In all the
above cases the explo-
sions 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 gaso-
line turns to gases such
as steam and carbon
dioxid, the gases take

much more room. But FIG. 180. Diagram of the cylinder of an
ji . • 11 j/i A. r engine. The piston is driven forward by

that IS not all that hap- thfexploSion of the gasoline in the cylinder

pens. Much heat is re-
leased by the burning of the gasoline spray, and heat
causes expansion. So the gases formed by the burn-
ing 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. 1 80), and the suddenly expanding gases shove this
piston down and escape. The piston is attached to the

Chemical Change and Energy 345

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 con-
fined 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 ar-
ranged 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

346

Common Science

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.

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.

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.

Chemical Change and Energy 347

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 ex-
pands 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 explo-
sives 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 card-
board 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

348 Common Science

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 sun-

light.

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 (combin-
ing 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.1 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.

Another chemical change, greatly helped by solution,
is the combining of the two things that baking powder

1 The rusting of iron is not quite as simple as this, as it probably under-
goes two or three changes before finally combining with oxygen. But
the solution helps all these changes.

349

35° Common Science

is made of, and the setting free of the carbon dioxid
(CO2) 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 hap-
pens?

The chemical action which takes place in the -baking
powder and releases the gas in bubbles — the gas is
carbon dioxid (CC^) — 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 dis-
solved 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.

Solution and Chemical Action 351

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 hydro-
chloric 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 im-
mediately with ammonia or a strong soda solution.

FIG. 182. Etching copper with acid.

Drop a little candle grease on a piece of copper about
f 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.

The nitric acid did to the copper in this experiment
exactly what the hydrochloric acid did to the zinc shav-

Solution and Chemical Action 353

ings 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.

354

Common Science

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?

Action of acids on the nerves of taste. Acids act on
the taste nerves in the tongue and give the taste of

FIG. 183. Strong acids will eat holes like this in cloth.

Solution and Chemical Action 355

sourness; everything sour is an acid. Lemon juice,
sour milk, and sour fruits are all too weak acids to in-
jure 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^ % pure," — such phrases as

356 Common Science

these are used in advertising soaps. What is meant by
99TTo % Pure ? What is free alkali ? Why should any
soap injure fabrics ? WThat 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.

Solution and Chemical Action 357

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 99r¥o% 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 un-

358 Common Science

combined 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. // 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 am-
monia 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

Solution and Chemical Action 359

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 no. 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:
i, sugar; 2, orange; 3 , dilute sulf uric 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;
n, washing powder in water; 12, sour milk; 13, corn-
starch 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.

360 Common Science

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 (CO2), 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 con-

tact with aluminum.

524. When you stub your toe while walking, you tend to fall for-

ward.

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 im-

mediately 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?

Solution and Chemical Action 361

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 (HC1), 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 Experi-
ment 1 08. Yet if you put these two strongly poisonous
chemicals together, they promptly turn to ordinary
table salt (NaCl) and water (H20). Or, as the chemists
write it :

NaOH+HCl->-NaCl+H2O.

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 in. 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 (HC1)
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

362 Common Science

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 ; re-
move 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 (HC1) 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 (HC1) combines with the
oxygen and hydrogen (OH) of the caustic soda (NaOH)
to form water (H2O) . Chemists write this as follows :

NaOH+HCl->-NaCl+H2O.

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

Solution and Chemical Action 363

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 neu-
tralize 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 neu-
tralize a strong acid ; you would have to use a large
amount of baking soda or ammonia to neutralize con-
centrated 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

364 Common Science

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, kero-
sene, 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 hang-
ing the curtains out tp 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 : am-
monia, 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.
$36. A piece of soot blown against them will stick.

537. A drop of oil that may spatter against them will spread.

Solution and Chemical Action 365

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, i% teaspoonf uls 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 (CO2), 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 (CO2) 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

366 Common Science

gas) and compress it in iron tanks. These iron tanks
of carbon dioxid (CC^) are shipped to soda-water foun-
tains 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 ele-
ments as caustic soda (NaOH), with carbon dioxid
(CO2) combined with them. The formula for baking
soda could be written NaOHC02, but usually chemists
put all of the O's together at the end and write it
NaHC03. 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
(CO2) 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 pan-
cakes are light as well as not sour. The effervescing
carbon dioxid makes bubbles all through the batter,

Solution and Chemical Action

367

FIG. 185. Making a glass of soda lemonade.

while the caustic soda (NaOH) in the baking soda
neutralizes the acid of the sour milk.

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

368 Common Science

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 (HC1) 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

Solution and Chemical Action 369

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 to-

matoes 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 agri-
cultural 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 im-
portant. But in this course it is impossible and un-
necessary for you to learri 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.

370

FIG. 1 86. 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: man-
ganese 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

372

Common Science

FIG. 187. Making the test.

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.

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 co-
balt combined with oxygen, you could really find out by
taking a grain on a borax bead and seeing if it turned
blue.

Analysis 373

The hydrochloric acid test for silver. The experiment
in which you tested the action of light in effecting chem-
ical change, and in which you made a white powder or
precipitate in a silver nitrate solution by adding hydro-
chloric 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 i to 4. Heat
this to boiling, then add a few drops of hydrochloric acid
diluted i 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 appear-
ance 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 follow-
ing:

Experiment 115. Mix a little starch with water. Add a
drop of iodine. What color does the starch turn? Repeat

374

Common Science

FIG. 1 88. The white powder that is forming is a silver salt.

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.

Analysis

375

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 lime-
water 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
(CO2) is present.

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

FIG. 189. The limewater test shows that there is carbon dioxid in the air.

376 Common Science

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.

Analysis 377

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.

378 Common Science

579. When you look through a triangular glass prism, things ap-

pear to be where they are not.

580. Lye and hot water poured down a clogged kitchen drain-

pipe 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 grad-

ually 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 Chromel 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 manu-
facturer by the school board if a statement accompanies the
order to the effect that it is not to be used in any com-
mercial devices, nor to be sold, but is for laboratory ex-
perimentation only. The manufacturers are Hoskins Manu-
facturing Company, Detroit, Michigan.

The following diagram will make the connections and
parts of the electrical apparatus clear :

FIG. 1 90. 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; /, /, K, Chromel C resistance wire, No. 22 (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.

379

380 Appendk

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 cart-
ridge fuses. Between the resistance wire (7, /, K) and the
wall should be either slate or sheet asbestos, double thick-
ness. 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 £), one above
and one below the wood.

A screw eye (F), large enough to form a rest for the head
of another machine bolt (<7), is screwed into the back of the
box about three fourths of an inch below the head of the
suspended bolt (^4). 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 (/) is screwed down on the threaded
end of a machine bolt until about an inch of the bolt pro-
jects beyond the nut. This projecting part of the bolt is

Appendix

381

then passed through the screw eye (H) and another nut
(/) screwed on to it to hold it in place. This nut must

FIG. 191. The cigar-box telegraph.

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.

INDEX

An asterisk (*) indicates use of one or more illustrations in connection with refer-
ence to which appended.

Acetylene, carbon and hydrogen in,

315-

Acids, 351 ff. ; action of, on metals,
351-353*; action of, on cloth, 354*;
action pf, 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, capil-
lary attraction, and, 47.

Air, cooling of, on expanding, 95-96;
liquid, 97 ; heat carried by, by con-
vection, 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 mix-
ture 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,
ii3*-H4.

Alkali, 356; in soap, 357~358.

Alloys, definition of, 310.

Alternating current, denned, 211-212.

Alum crystals, experiment with, 265-
266*.

Aluminum, an element, 299.

Alum in water, testing with litmus
paper, 359.

Amber, electricity produced by rub-
bing with silk, 196.

Ammonia, example of a common base,
356; action of, in cleaning cloth,
358; litmus test of, 359; neutrali-
zation of acid by, 363.

Ampere, denned, 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
3ii-

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 fric-
tion, 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, 2O5*-207*; general prin-
ciple 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, 37i*~372*.

Brass, an alloy, 310.

383

Index

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*~4o; differ-
ence 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 elec-
tricity, 335-339; energy released
by, 340-34 1 ; helped by solution,
3497351-

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; mix-
tures 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.

Index

385

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, 2o7*-2io*.

Earth, magnetism of, 190-195.

Easy circuit, a short circuit an, 244—
245-

Echoes, explanation of, 183-185.

Effervescence, process of, 365; gener-
ally due to freeing of carbon dioxid,
367*-368; helped by stirring or
shaking, 368.

Elasticity, 82-86; of form distin-
guished from elasticity of volume,
"86-87.

Electrical apparatus, 216-217*, 222-
223*; description of, 370-380.

Electric arc, the, 233*-24o.

Electric battery, the, 203-206*.

Electricity, magnetism and, 1 90 ff. ;
static, 196-202 ; negative and posi-
tive charges of, i98*-2oo; action
of, in thunderstorms, 200-201 ;
flowing, 203 ff . ; flowing of, in dy-
namo, 207-210; alternating and
direct-current, 211-212; conduc-
tion of, 2i3*-2i8; 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 posi-
tive 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 pis-
ton 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 j
cooling from, 94—96*.

Expansion ball and ring experiment,
9i*-92.

Explosions, use of, 342* ff. ; automo-
biles 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*-! 53.

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, i42*-i49*.
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, no.

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.

336

Index

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 ele-
ments, 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, no.
Glass, a poor conductor of heat, 1 1 8 ;

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, i ; 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 con-
vection, 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 ele-
ments in fuel, 315-316; part taken
by, in burning, 312-319.

Ice, slight friction of, 52*; action of
molecules in, on freezing and melt-
ing, 96-97; reason for floating of,
98-99.

Incandescence, defined, 125.

Incandescent lamps, 125-126; num-
ber of electrons in, 197; working
of, 229-232.

Inertia, 66—7 1 ; 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, 1 10.

Lead, an element, 299.

Lead pencils, arc light from, 233*-

234*.
Leaning Tower of Pisa, 29*~3O.

Index

387

Lemon juice, action of, on silver and
on steel, 353 ; litmus test of, 359.

Lens, of eye, 142, i5i*-iS3 ; of camera,

* 143, 149*, 328; convex, 148-149;
concave, 149*; in telescope, 157.

Levers, 57-65*.

Light, radiation of, 122, 123*-! 28;
reflection of, 129-135*; refraction
of, i36*-i4i; focus of, 142-149*;
brought to focus by convex lens,
149; diffusion of, 158-161*; color
a kind of, 162 ; speed of, 182 ; chem-
ical 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; dif-
fusion 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, i6*-i7-
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 com-
pounds, 300-310.

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*, 1 88; 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.

388

Index

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; experi-
ments 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 ex-
plosives, 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 rain-
bow 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 ol
339 5 hydrochloric acid test for, 37^.

Silver chlorid, formation of, 327.

Sinking and floating, 23-28*.

Siphon, 1 8*.

Index

389

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, denned, 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; neu-
tralization 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,

no; geysers and volcanoes caused

by, no; 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*~9i*.
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, denned, 1 1 ; reason for, in
electric lamp, 12*, 317; use of, in
manufacture of thermos bottles,
1 26-1 2 7* ; 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; neu-
tralization of lye by, 363.

Violin, tuning of, 187.

Volcanoes, cause of, no; 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 bot-
tle, 13 ; experiment with, to show
centrifugal force, 73~745 used for
making thermometer, 9o*-92 ; ex-
pansion of, when frozen, 98 ; evapo-
ration of, 100-106; action of, in

390

Index

geysers and volcanoes, no; absorp-
tion of light by, 169-170; as con-
ductor 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*-6o.

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.

NEW -WORLD SCIENCE SERIES
Edited by John W. Ritchie

TREES, STARS and BIRDS I

A BOOK OF OUTDOOR SCIENCE
By EDWIN LINCOLN MOSELEY

Head of the Science Department, State Normal College of
Northwestern Ohio

^TpHE usefulness of nature study in the schools has been |
JL 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 teach- |
ers 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 them-
selves.

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
I commonly met with are considered, and their habits of
1 feeding and nesting are described. Pertinent questions
| are scattered throughout each section.

| The book is illustrated with 167 photographs, 69 draw-
| ings, 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. -f- xvi pages. Price $1.80.

WORLD BOOK COMPANY

YONKERS-ON-HUDSON, NEW YORK
2126 PRAIRIE AVENUE, CHICAGO

NEW-WORLD AGRICULTURE SERIES

NATURE-STUDY
AGRICULTURE

A Textbook for Beginners
By WILLIAM T. SKILLING

Super-visor of Nature Study and Agriculture
State Normal School, <San Diego

HERE is a book written in a style so simple that it can
be used in the seventh grade. Yet it covers the essen-
tials so well that it may be used in any first course in this
subject.

Practically every paragraph has a marginal note which the
student will find helpful in review and which the teacher
can easily use for questions to pupils.

Every chapter has a list of Experiments to be performed and
a list of Observations to be made. The list of References
is valuable because the bulletins named are easily available.

The book is especially adapted to the project method of
teaching agriculture to young people. The procedure is to
present principles in the classroom, demonstrating them by
simple experiments where possible, and also have each
pupil do work in the school or home garden.

The book meets the needs and interest of the pupil. It can
be used in any part of the country; It is largely self-teaching.
The illustrations are unusually clear and appropriate. There
are 266 of them.

Cloth, •viii -^-322 pages

WORLD BOOK COMPANY

YONKERS-ON-HUDSON, NEW YORK

2126 PRAIRIE AVENUE, CHICAGO

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1 GENERAL SCIENCE I
SYLLABUS I

BY J. C. LOEVENGUTH

Principal, James Allison Junior High School
Wichita, Kansas

THE teacher who wishes to follow wholly or in
part, his awn general science course or supple-
ment the textbook by references to other books
will find this Syllabus of definite value. It offers ma-
terial for classes of every description so that a selection
of subject matter can be made to suit the particular
needs of any junior high school or high school.

The Syllabus covers all subjects required for a com-
plete course by giving exact page references to text-
books in which the topics are treated. In all, thirteen
books are referred to including nine most commonly
used general science textbooks.

In order to aid the teacher in selection of material, the
Syllabus is printed in two sizes of type. The part in
large type represents a full course that may be followed
in schools where time is limited c>r classes are of not
more than average strength. The portion in smaller
type may be omitted entirely or used to supplement the
course with material from the various fields of science.

General Science Syllabus is not intended to be followed
slavishly. It simply outlines, with many references,
the whole subject matter of general science so that the
teacher may choose the content of his own course and
the students may have a guide to the study of any
topic assigned.

Cloth, viii + 64 pages. Price 80 cents.

WORLD BOOK COMPANY

YONKERS-ON-HUDSON, NEW YORK
2126 PRAIRIE AVENUE, CHICAGO

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NEW-WORLD SCIENCE SERIES

Edited by John W. Ritchie

SCIENCE OF I
ANIMAL LIFE [

By WILLIAM M. BARROWS

Ohio State University

THIS textbook in high school zoology gives the student I
a broad understanding of the principles of the science;
it is not limited to immediate utilitarian phases, but is |
concerned also with the more far reaching and significant
relationships of biology to human society.

The content of the book is sane and balanced; there is |
inclusion of much material of a general biological nature. 1
The life processes and reactions of living animals are
emphasized while description has been subordinated. The
study of ecology is given meaning and unity by basing
the treatment of animal associations upon behavior.

The book covers in a thorough way the prescribed work |
for the high school and will make the teaching of a half
year course practical and effective.

Science of Animal Life is written simply and with swift |

movement and stimulation. There is abundant illustra- |

tion. In every way this is the type of textbook which |

lifts zoology and biology to their rightful place in the |

curriculum by including material that is of lasting value. |
It meets the individual and social needs of our time.

Cloth, xii+1%6 pages. Illustrated. Price $1.76

WORLD BOOK COMPANY

YONKERS-ON-HUDSON, NEW YoRK

2126 PRAIRIE AVENUE. CHICAGO

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THE EARTH AND
ITS LIFE

By A. WADDINGHAM SEERS

THIS book contains a clear account of the origin of
our planet in the light of modern science. It recounts
the story of evolution, culminating in the origin of man,
and relates man's struggles against the animal world
with his eventual triumph, and his conquest of the earth
through the discovery of the means of locomotion.

Many facts and hypotheses in the fields of geology,
paleontology, botany, and ethnology are presented in a
clear, vivid, instructive way. The book covers the history
of the earth from the earliest days to the dawn of our
present civilization, and forms a useful introduction to
biology and anthropology.

The story is told simply and fascinatingly, and will appeal
strongly to old and young readers alike. It is as engross-
ing as any fairy tale, and at the same time makes a strong
appeal to the scientific spirit.

The subjects considered are not often dealt with in
elementary books, but are of great value from a cul-
tural as well as a scientific point of view. Children above
twelve years of age can not fail to derive from this vol-
ume a keen sense of the mystery and wonder of the world.

Cloth. Illustrated. Price $1.20

WORLD BOOK COMPANY

YONKERS-ON-HUDSON, NEW YORK
2126 PRAIRIE AVENUE, CHICAGO

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SCIENCE SERIES

Edited by JOHN W. RITCHIE

GARDENING

AN ELEMENTARY SCHOOL TEXT

TREATING OF THE SCIENCE AND ART

OF VEGETABLE GROWING

BY A. B. STOUT
Director of the Laboratories
New York Botanical Garden

THIS text emphasizes the educational as well as the
practical aspects of gardening and was prepared as
a guide for both teacher and student.

It furnishes a broad scientific background for an ap-
preciation of the plant as a living thing, and with this
working knowledge of plants as a basis for practical
gardening gives exact and detailed instructions for the
actual growing of the various garden crops. Theory
and practice are presented together, and the art of
gardening is made an intelligent application of principles
to methods.

The book has been made as complete as possible and
every device is employed to make the content applicable
to all sections of the country. The garden steps de-
scribed are based on practical experience and a thorough
knowledge of plant growing.

The well-ordered, concise, practical course outlined in
Gardening will be of great aid in giving form and value
to school-garden work. To the inexperienced teacher
of the subject the book will prove of especial value.

Cloth, xvi + 354 pages. Illustrated.

WORLD BOOK COMPANY

YONKERS-ON-HUDSON, NEW YORK
2126 PRAIRIE AVENUE, CHICAGO

UNIVERSITY OF CALIFORNIA LIBRARY
BERKELEY

Return to desk from which borrowed.
This book is DUE on the last date stamped below.

LD 21-957N^ll,'50(2877sl6)476

H7H77

THE UNIVERSITY OF CALIFORNIA LIBRARY

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