Common Science
by Carleton W. Washburne
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Transcriber's Note:

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


This is a work of historical interest only and much of the scientific content has been superseded.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

Do not try this at home!

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NEW-WORLD SCIENCE SERIES Edited by John W. Ritchie

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TREES, STARS, AND BIRDS By Edwin Lincoln Moseley

COMMON SCIENCE By Carleton W. Washburne




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SCIENCE OF PLANT LIFE By Edgar Nelson Transeau

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ZOOeLOGY By T. D. A. Cockerell


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NEW-WORLD SCIENCE SERIES Edited by John W. Ritchie


by Carleton W. Washburne

Superintendent of Schools, Winnetka, Illinois Formerly Supervisor in Physical Sciences and Instructor in Educational Psychology State Normal School San Francisco, California


Yonkers-on-Hudson, New York WORLD BOOK COMPANY 1921



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

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


Copyright, 1920, by World Book Company

Copyright in Great Britain

All rights reserved


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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

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

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

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


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




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

2. "Water seeks its own level" 6

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

4. Sinking and floating: Displacement 23

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


6. How liquids are absorbed: Capillary attraction 36

7. How things stick to one another: Adhesion 41

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

9. Friction 49


10. Levers 57

11. Inertia 66

12. Centrifugal force 72

13. Action and reaction 77

14. Elasticity 82

4. HEAT 88

15. Heat makes things expand 88

16. Cooling from expansion 94

17. Freezing and melting 96

18. Evaporation 100

19. Boiling and condensing 107

20. Conduction of heat and convection 116


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


31. Magnets; the compass 190

32. Static electricity 196


33. Making electricity flow 203

34. Conduction of electricity 213

35. Complete circuits 219

36. Grounded circuits 225

37. Resistance 229

38. The electric arc 233

39. Short circuits and fuses 240

40. Electromagnets 247


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


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


53. Chemical change helped by solution 349

54. Acids 351

55. Bases 355

56. Neutralization 360

57. Effervescence 365

12. ANALYSIS 370

58. Analysis 370


A. The Electrical Apparatus 379

B. Construction of the Cigar-box Telegraph 381





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

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

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

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

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

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

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

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

It is literally a topsy-turvy place.

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

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

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

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

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

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

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

SECTION 2. "Water seeks its own level."

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

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

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

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

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

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

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

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

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

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

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

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

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

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

What is suction?

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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

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

1. Rain falls from the clouds.

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

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

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

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

6. You can drink lemonade through a straw.

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

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

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

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

SECTION 4. Sinking and floating: Displacement.

What keeps a balloon up?

What makes an iceberg float?

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

If iron sinks, why do iron ships not sink?

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

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

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

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

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

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

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

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

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

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

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

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

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

Which way would the floats have worked best?

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

Who was right?


Explain the following:

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

* * * * *

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

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

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

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

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

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

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


Explain the following:

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

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

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

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

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

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

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

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

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

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



SECTION 6. How liquids are absorbed: Capillary attraction.

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

How does a towel dry your face?

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

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

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

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

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

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

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

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

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

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

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

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

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

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


Explain the following:

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

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

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

34. Cream comes to the top of milk.

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

36. Oiled shoes are almost waterproof.

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

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

39. The oceans do not flow off the world.

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

SECTION 7. How things stick to one another: Adhesion.

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

Why does chalk stay on the blackboard?

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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:

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

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

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

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

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

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

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

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

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

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

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


Explain the following:

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

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

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

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

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

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

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

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

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

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

SECTION 9. Friction.

What makes ice slippery?

How does a brake stop a car?

Why do things wear out?

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


Explain the following:

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

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

63. Brushing your teeth with tooth powder cleans them.

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

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

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

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

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

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

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



SECTION 10. Levers.

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

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

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

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

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

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

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

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