TRANSCRIBER'S NOTE

Obvious typographical errors and punctuation errors have been corrected after careful comparison with other occurrences within the text and consultation of external sources.

The cover image was created by the transcriber and is placed in the public domain.

More detail can be found at the [end of the book.]

SCIENCE
FOR THE
SCHOOL AND FAMILY.
PART I.
NATURAL PHILOSOPHY.

BY

WORTHINGTON HOOKER, M.D.,

PROFESSOR OF THE THEORY AND PRACTICE OF MEDICINE IN YALE COLLEGE,
AUTHOR OF "HUMAN PHYSIOLOGY," "CHILD'S BOOK OF NATURE,"
"NATURAL HISTORY," &C.

Illustrated by nearly 300 Engravings.

NEW YORK:

HARPER & BROTHERS, PUBLISHERS,

FRANKLIN SQUARE.

1873.

By Dr. Worthington Hooker.

The Child's Book of Nature. For the Use of Families and Schools; intended to aid Mothers and Teachers in training Children in the Observation of Nature. In three Parts. Illustrated by Engravings. The Three Parts complete in one vol. Small 4to, Cloth, $2 00; Separately, Cloth, 90 cents each.

Part I. PLANTS.

Part II. ANIMALS.

Part III. AIR, WATER, HEAT, LIGHT, &c.

First Book in Chemistry. For the Use of Schools and Families. Illustrated by Engravings. Square 4to, Cloth, 90 cents.

Natural History. For the Use of Schools and Families. Illustrated by nearly 300 Engravings. 12mo, Cloth, $1 50.

Science for the School and Family.

Part I. NATURAL PHILOSOPHY. Illustrated by nearly 300 Engravings. 12mo, Cloth, $1 50.

Part II. CHEMISTRY. Illustrated by numerous Engravings, 12mo, Cloth, $1 50.

Part III. MINERALOGY AND GEOLOGY. Illustrated by numerous Engravings. 12mo. Cloth, $1 50.

Published by HARPER & BROTHERS, Franklin Square, N. Y.

Any of the above Works sent to any part of the United States, postage pre-paid, upon receipt of the Price.

Entered, according to Act of Congress, in the year one thousand eight hundred and sixty-three, by Harper & Brothers, in the Clerk's Office of the District Court of the Southern District Court of New York.

[PREFACE.]

Daniel Webster, in his Autobiography, speaks thus of his entering upon the study of law: "I was put to study in the old way—that is, the hardest books first, and lost much time. I read Coke on Littleton through without understanding a quarter part of it. Happening to take up Espinasse's Law of Nisi Prius, I found I could understand it; and arguing that the object of reading was to understand what was written, I laid down the venerable Coke et alios similes reverendos, and kept company for a time with Mr. Espinasse and others, the most plain, easy, and intelligible writers. A boy of twenty, with no previous knowledge on such subjects, can not understand Coke. It is folly to set him on such an author. There are propositions in Coke so abstract, and distinctions so nice, and doctrines embracing so many conditions and qualifications, that it requires an effort, not only of a mature mind, but of a mind both strong and mature, to understand him. Why disgust and discourage a boy by telling him that he must break into his profession through such a wall as this? I really often despaired. I thought that I never could make myself a lawyer, and was almost going back to the business of schoolkeeping. Mr. Espinasse, however, helped me out of this in the way that I have mentioned, and I have always felt greatly obliged to him."

Here is most graphically depicted a defect which is now, as it was then, very prominent in all departments of education. It is even more so in early education than in that of the college and the professional school. Even in tender childhood pupils are put to studying books of which, as was true of Webster with his Coke on Littleton, they do not understand "a quarter part." If the rule is not "the hardest books first," there are many things in the books that it is not only hard but impossible for them to understand. And the hardest things are often put first. For example, in a very popular primary geography which lies before me the pupil is introduced to the world and its grand divisions at the outset, while he is taught about his own state and country only at the conclusion of the book. And this unnatural mode is the one very commonly pursued. Similar criticism can be passed upon most of the books used in teaching young children. Some of them are wholly useless. This is true of the grammars for primary schools. The formal statements, called the rules of grammar, are beyond the understanding of very young scholars, and therefore are useless burdens upon their memories. They are as useless to them as the three-fourths of Coke which Webster could not understand was to him.

If we follow education, upward from the primary school we find the same defect throughout the whole course. In the books which are used in teaching natural science it is especially prominent. Even in the elementary books, or compendiums, so called, formal propositions and technical terms render the study uninviting, and to a great extent unintelligible. The pupil is apt to be disgusted and discouraged, as Webster was with Coke on Littleton, and for the same reason.

Another defect intimately connected with that of which I have spoken is the very sparing and late introduction of the physical sciences. They are generally postponed to the latter part of the course of education, and then but little time is devoted to them. Generally, when a pupil designs to go through college, the study of these sciences is wholly neglected in his preparation, because a knowledge of them is not required for admission. Then in the college they are not attended to till the latter part of the course, and in the short time allotted to them there is so much to be learned that the teaching of them is a failure. Especially is this true of Chemistry and Geology.

This defect is a radical one. A thorough change should be effected in this respect in the whole course of education. The natural sciences should be made prominent from the beginning to the end, not only because they are of practical value, but also because they are as useful in their way for mental discipline as the study of mathematics and of language. They can be taught to some little extent to the youngest pupils. There are facts about air, water, and the various objects that they see around them, which they can understand if they be presented in the right manner. And the busy inquiries which they make after the reasons of the facts, and their appreciation of them if stated simply and without technical terms, show the appropriateness of such teaching. Children are really very good philosophers in their way. They have great activity not only of their perceptive but of their reasoning faculties also, to which due range should be given in their education.

Beginning thus, not a year should pass during the whole course when the pupil shall not be engaged in studying some one of the physical sciences to some extent. This continued attention to such studies in a reasonable amount, so far from interfering with the due prosecution of the other studies deemed so essential, will so promote the pupil's advance in them as to more than make up for the time that is taken from them. It will do this not only by the genial influence which such studies exert upon the mind, but by the contributions which they make to the knowledge of language and mathematics; for language is largely built up from natural objects and from the acquisitions of science, and there is an abundance of interesting applications of portions of the mathematics in the facts which the physical sciences develop to us.

I have said that the teaching of the natural sciences in our colleges is generally a failure, and it always will be so as long as the present plan is continued. In order to have it successful there must be the same gradation in teaching them that we have in teaching language and the mathematics. The college student needs to be prepared for the lectures which he hears on natural philosophy, chemistry, etc., and for his study of those branches, by previous familiarity with the simpler portions of them acquired in the school-room.

There is another very important reason for the early introduction of the physical sciences into education. By far the larger portion of pupils in our schools stop short of the college, or even the academy and high school. That they should go forth into the world with no knowledge of the principles that lie at the basis of the arts in which so many of them are to engage is a shame and a wrong, if the communication of such knowledge be indeed practicable, as it undoubtedly is. Even those who are not to engage in these arts will be greatly benefited by this knowledge, because in addition to its constant practical applications in the management of life, it will contribute to their mental power, and, what is no small consideration, to their enjoyment; and it is in fact requisite to constitute them well-informed persons.

If the views which I have presented be correct, there should be a series of books on the natural sciences carefully adapted to the different periods of the course of study. Those intended for the young beginner should be exceedingly simple, and should not attempt to present any thing like a full view of the subjects treated. They should deal largely with familiar facts or phenomena. The terminology of science and formal statements of principles, such as we often see in so-called compendiums, should have no place in them, but should be gradually introduced as the series advances, and should be made complete only in the concluding books.

It has been the object of the author to supply a part of such a series. The first book in the series is the "Child's Book of Common Things," intended to teach the observation of familiar facts, or, in other words, the beginnings of philosophy, to children as soon as they have got well started in reading. Next comes the "Child's Book of Nature," which in its three parts (Part I., Plants; Part II., Animals; Part III., Air, Water, Light, Heat, etc.) extends considerably the knowledge of the philosophy of things which the child has obtained from the first book in the series. Then follows the "First Book in Chemistry." On a level with this is my "First Book in Physiology." The next step in the gradation brings us to three books under one title, "Science for the School and the Family;" Part I., Natural Philosophy; Part II., Chemistry; Part III., Mineralogy and Geology. On a level with these is another book, "Natural History," and another still is to be written, an "Introduction to Botany."

The three books, of which the present is one, are intended for the older scholars in what are commonly called grammar-schools. At the same time they are suited to scholars who are advanced to a higher grade who have not gone through the previous books of the series. The preparation of books especially adapted to high schools and colleges I have left to others, except in one branch of science, Physiology, on which I some years ago published a work entitled "Human Physiology."

All of these books are from the press of Harper and Brothers except the two works on Physiology, published by Sheldon and Co., New York, and the "Child's Book of Common Things," published by Peck, White, and Peck, New Haven.

The general plan and style of these books are very different from what we see in most of the books for schools on the same subjects. The order of the subjects and the mode of developing them differ from the stereotype plan which has so generally been adopted. One prominent feature is the free use of illustrations from familiar phenomena. This leads the pupil to reason or philosophize about common things, thus giving an eminently practical character to his knowledge. At the same time it makes the books suitable for use in the family as well as the school, between which there should be more common ground than the present mode of education allows.

The style which I have chosen for all the books I have written for use in teaching is what may be called the lecture style. There are three other kinds of style which are more commonly used in school-books. The most common is what I term the formal statement style. In this principles and rules are stated, and then illustrations are given. This makes a formal and uninviting book. The bare skeleton of the science is generally for the most part presented, and the young pupil is apt to learn the statements by rote without understanding them. It is a style fitted only for books intended for advanced scholars. Another style is the catechetical. This is an unnatural mode of communicating knowledge; and besides, it encourages learning by rote as the formal statement style does. In the third style, the dramatic, conversations are held between the teacher and some learners. The chief objection to this is that it undertakes to put in permanent shape what should be extemporized in the recitation. What is needed in the book is simply clear and concise statement in an interesting style, and the living teacher and his scholars can best furnish the conversational element as the recitation goes on.

In the lecture style there may be and should be as much precision of statement as in the formal statement style, while it is more interesting, because it is the natural mode of communicating knowledge. In this style the facts are ordinarily so stated as to develop principles; while in the other the order is reversed, the principles being first stated and the facts given afterward. One of the most successful books ever used in our colleges—"Paley's Natural Theology"—is in the lecture style, and it is a matter of surprise that this fact has had so little influence with those who have prepared books for instruction.

Whatever may be true of advanced scholars, in teaching the young student in science bare, dry statement should be avoided, and the subjects should be presented in all their attractive features. I would not be understood as advocating the dressing up of science in adventitious charms. This is not necessary. Science possesses in itself an abundance of charms, which need only to be properly developed to attract the young mind; and the lecture style furnishes the best vehicle for such a development.

One grand essential for giving interest to any study is the presentation of the various points in the natural order in which they should enter the mind. They should be so presented that each portion of a book shall make the following portions more interesting and more easily understood. This principle, which is so commonly transgressed, I have endeavored to observe strictly in the preparation of these volumes.

Questions are put at the end of this book for those teachers who desire to use them. There is also an Index.

W. Hooker.

January, 1863.

[CONTENTS.]

NATURAL PHILOSOPHY.

[CHAPTER I.]
MATTER.

1. Matter and Spirit.—The distinction between matter and spirit is almost universally recognized even by those who have given little thought to such subjects. It is a distinction of which we are conscious in our own persons. We know instinctively that there is a something within us that causes the movements of our material bodies, and that something we call spirit.

2. Bishop Berkeley's Ideas.—Some philosophers, in their speculations, have denied that there is any such thing in existence as matter. Bishop Berkeley, for example, taught that the impressions which we suppose that we receive from material objects do not come from real substances, but are the "effects of the immediate agency of an ever-present Deity." It is no wonder that the wisdom and learning of a man who could seriously adopt such a belief could not save him from being the dupe of quackery. He believed that tar-water was a sovereign cure for all diseases; and Dr. Holmes playfully remarks of him, that "he held two very odd opinions: that tar-water was every thing, and that the whole material universe was nothing."

3. Hume's Ideas.—The infidel Hume went beyond Bishop Berkeley, denying even the existence of the soul as an individual and responsible agent. He made every thing to consist of ideas and impressions, and said that these have no necessary connection, but are "a bundle of perceptions that succeed each other with inconceivable rapidity, and that therefore I myself of to-day am no more the I myself of yesterday or to-morrow than I am Nebuchadnezzar or Cleopatra." A wag proposed the following epitaph for his tomb-stone as a suitable illustration of his theory:

"Under this circular idea, vulgarly called tomb,

Impressions and ideas rest which constituted Hume."

4. Origin of the word Spirit.—The name spirit came originally from an attenuated form of matter, the air or breath, because the air, like spiritual existence, is invisible. The formation of language is to a large extent thus based on analogies.

5. Spirit not an Object of Sense.—None of the senses can perceive spirit itself, though they perceive the effects which spirit produces on material substances. If, for example, you move your arm, it is the spirit within you, acting upon the muscles through the nerves, that causes it to move; and you see here the effects produced by spirit upon matter, but you do not see the spirit itself.

6. Effects of Matter on the Senses.—Some forms of matter can be perceived by all the senses; others can be perceived by only a part of them; some by only one. Air you can not see, nor smell, nor taste; but you can feel it, and hear the sound of its motion. Sometimes matter affects only the sense of smell, or that with the sense of taste. Sea-air smells salt; but the salt in the air is so finely divided that we can not see it. And yet it is the salt, entering the nostrils and coming in contact with the extreme fibres of the nerve of smell, that produces the effect. So when we smell a flower, matter comes from it in particles so fine that no microscope can detect them, but they produce sensation when they strike upon the nerve.

7. Forms of Matter.—Matter appears in three forms: solid, liquid, and gaseous or aeriform—that is, like air. Sometimes matter is spoken of as having only two forms—solid and fluid. In this case fluids are divided into two classes, the elastic and non-elastic. The air and the various gases and vapors are the elastic fluids; while those which are called liquids are the non-elastic fluids. A foot-ball bounds because the air in it is an elastic fluid. If it were filled with a non-elastic fluid; as water, it would not bound. When water takes the form of steam it is an elastic fluid. Though it is very common to use the expression elastic fluids, the division of matter into three forms is the one usually recognized.

8. Solids.—In solid matter the particles can not be moved about among each other; but each particle generally retains the same position in relation to those particles which are around it—in other words, it does not change its neighborhood. This is more true of some solids than of others. It is absolutely true of such hard solids as granite and the diamond. In these the particles are always in the same relative position. But it is not so with gold or lead. By hammering these you can change greatly the relative position of their particles. India-rubber is a solid, but the relative position of its particles can be much altered in various ways.

9. Liquids.—It is the grand characteristic of a liquid that its particles change their relative position from the slightest causes. It is in strong contrast with solids in this respect. When you move any portion of a solid body you move all the other portions of it, and generally in the same direction. But a body of liquid can not be moved all together as one body except by confining it; as, for example, in the case of a water-pipe or a syringe. And then, the moment that the water can escape, the particles use their liberty of altering their relative position. As wind and other agents act continually upon water, no particle stays for any length of time in the neighborhood of the same particles. "Unstable as water" is, then, an exceedingly significant expression. Water is never at rest. A particle of it may at one time be floating on the surface of the ocean, and at another be in depths beyond the sounding of man. It flies on the wings of the wind, falls in the rain, runs in the stream, is exhaled from a leaf, trembles in the dew-drop, flows in the blood of an animal or in the sap of a plant, and is always ready to be jostled along in its ever-changing course.

10. Gases.—The particles of gaseous or aeriform substances move among each other even more freely than those of a liquid. Air, therefore, is more unstable and restless than water. Even when the air seems to be perfectly still its particles are moving about among each other. You can see this to be true if you darken a room, leaving a single shutter a little open. Where the light enters you will see motes flying about in every direction, which would not be the case if the air were really at rest. The particles of air have a greater range of travel than those of water; for the sea of atmosphere which envelops the earth rises to the height of about fifty miles. How far water rises in its evaporation we know not; but it is not at all probable that it rises to the uppermost regions of the atmosphere.

11. Filling of Spaces by Liquids and Gases.—It is the freeness with which the particles of liquids and gases move among each other that enables them to insinuate themselves into spaces every where. They are ever ready to enter into any substances which have interstices or pores of such size as will admit them. There are mingled with the grains of the soil not only water, but air and gases. These are present also in all living substances, both vegetable and animal. Water is the chief part of sap and of blood, and air and gases always go with water. Part of the air that we breathe in enters the blood in the lungs, and courses with it through the system. The fishes could not live in water if there were not air mingled with it. This can be proved by experiment. If you put a fish into a close vessel it will soon die, because it uses up all the air that is in the water. In an open vessel the fish is kept alive by the constant accessions of fresh air to the water.

12. Solution.—In solutions of solid substances in water it is the freedom with which the particles of water move about among each other that enables them to take in among them the minute particles of the solid. And when water ascends into air by evaporation it may be said to be a real solution of water in the air; for the particles of water mingle with those of the air, just as the particles of a solid mingle with those of water in a solution.

13. Relation of Heat to the Forms of Matter.—Some kinds of matter are seen in all the three forms. Whether these shall assume one form or another depends on the amount of heat present. Thus when water is solid, ice, it is because a part of its heat is gone. Apply heat, and it becomes a liquid, water. Increase the heat to the boiling point, and it becomes steam, or an aeriform substance. Alcohol has only two forms—liquid and aeriform. It has never been known to be frozen. Iron is usually solid; but in the foundry, by the application of great heat, it is made liquid. Mercury is liquid in all ordinary temperatures; but it often becomes solid in the extreme cold of arctic winters. A mercurial thermometer is of course useless under such circumstances, and the alcoholic thermometer is relied upon to denote the degree of cold. The difference between mercury, water, and iron in regard to the liquid state is this: It takes but little heat, comparatively, to make mercury liquid, while more is required for this condition in water, and much more for it in the case of iron.

14. The Nature of Matter Unknown.—What now, let us inquire, do we know of the nature of matter? Can we say that we know any thing of it? We may observe its phenomena, and learn its properties; but with our most searching analyses of it we can no more determine what matter is than we can what spirit is. Newton supposed "that God in the beginning formed matter in solid, massy, hard, impenetrable particles." This he believed to be true of liquids, and even of gases, as well as solids. In the gas these hard particles are much farther apart than in the solid. The supposition is a very probable one; but if it be true it does not let us know what matter is, for it leaves us in the dark as to the nature of the particles. Newton farther supposed that these particles have always remained unaltered amidst all the changes that are taking place; these changes being occasioned by "the various separations and new associations and motions of these permanent particles." When, for example, any thing is burned up, as it is expressed, not one of these particles is either destroyed or altered, but they merely take on new arrangements. Though most of the substance has flown off in the form of gas, the ultimate particles composing the gas are the same now that they were when making a part of the solid substance; and they may soon again become a part of some new solids. Such changes in the forms of matter are every where going on; and when you become acquainted with Chemistry, in the Second Part, you will be familiar with them.

15. Atomic Theory.—These ultimate particles of matter are so minute that they have never been seen by man. The smallest particle that can be seen with the most powerful microscope is probably made up of very many of them united together. These ultimate particles we term atoms; and the theory in regard to the composition by them of different substances is called the atomic theory. The atoms of different substances are not supposed to be alike, but to differ in both size and weight. This theory will be more particularly noticed in the Second Part.

16. Imponderable Agents.—There are certain agents—light, heat, electricity, etc.—which are supposed by some to be forms of matter. If they are, they are exceedingly attenuated; for their presence, as has been proved by many experiments, never adds in the least to the weight of any substance. They have therefore been styled imponderable agents. Their agency is of great importance and very active, producing every where constant changes. Two of them—heat and light—are obviously and immediately essential to life. What their real nature is remains as yet an entire mystery.

[CHAPTER II.]
PROPERTIES OF MATTER.

17. Variety in the Properties of Matter.—All matter has properties or qualities. Some of these are different in the different kinds of matter. Thus its three forms have different properties, as you saw in Chapter I. There is variety also in the properties of substances of the same class. Thus liquids are unlike each other in some respects. Some, for example, are lighter than others. Oil is lighter than water. Gaseous substances also differ in this and in other respects. But the variety in the properties of solids is greater than in those of gases or liquids. This will appear as I proceed.

18. Divisibility of Matter.—Any visible portion of matter can be divided into parts. Even if it be so small that you can see it only with a powerful microscope, it could still be divided if you could have an instrument sufficiently fine for the purpose. Divisibility, then, is said to be a general property of matter; that is, a property belonging to all kinds of matter.

19. Examples of Minute Division of Matter.—There are numerous examples in which the division of matter is carried far beyond that which can be effected by any cutting instrument. Some of these I will notice:

A gold-beater can hammer a grain of gold into a leaf covering a space of fifty square inches. So thin is it that it would take 282,000 of such leaves, laid upon each other, to make the thickness of an inch. And yet so even and perfect is this thin layer of gold, that when it is laid upon any surface in gilding it has the appearance of solid gold. A fifty millionth part of this grain of gold thus hammered out can be seen by the aid of a microscope which magnifies the diameter of an object ten times. But the division of gold is made even more minute than this in the manufacture of the wire of gold-lace. It is done in this way: A bar of silver weighing 180 ounces is covered with a layer of gold weighing an ounce. It is then drawn through a series of holes in a steel plate, diminishing in diameter, till it at length comes out a very fine wire 4000 feet long. Each foot of it then has only the one 4000th part of the ounce of gold, and yet the silver is well covered.

A soap-bubble is a beautiful example of the minute division of matter. That thin wall which incloses the air which you have blown into it is composed of particles of the soap and of the water mingled together. It is supposed to be less than one millionth of an inch in thickness.

The thread of the silk-worm is so minute that the finest sewing-silk is formed of many of these threads twisted together. But the spider spins much more finely than this. The thread by which you see him letting himself down from any height is made up of about 6000 threads or filaments, each coming from a separate hole in his spinning machine. A quarter of an ounce of the thread of a spider's web would extend 400 miles.

A grain of blue vitriol, dissolved in a gallon of water, will make the whole blue. Such a diffusion could not be without an exceedingly minute division of the particles.

Perhaps the most minute division of matter is exemplified in odors. A grain of musk will scent a room for years, and yet have no perceptible loss of weight. But all this time the air is filled with fine particles coming from the musk.

The microscope reveals to us many wonderful examples of the minuteness of the particles of matter, both in the vegetable and the animal world.

If you press a common puff-ball a dust flies off like smoke. Examined with a microscope, each particle of this dust, which is the seed of the plant, is a perfectly round orange-colored ball. This ball is of course made up of very many particles, arranged in this regular form. Beautiful examples of various arrangements of the minute particles of matter we have in the pollen of different plants, as seen with the microscope.

Each particle of the dust which adheres to your fingers as you catch a moth is a scale with fine lines upon it regularly arranged. And if you look through the microscope at the wing of the moth, you will see, where the dust is rubbed off, the attachments by which the scales were held standing up from the surface of the wing, like nail-heads on a roof where the shingles have been torn off.

The organization of exceedingly small animals, as revealed by the microscope, furnishes us with wonderful examples of the minute division of matter. A little of the dust of guano, examined through a powerful microscope, is seen to contain multitudes of shells of various shapes. These shells are the remains of animalcules that lived in the water, their destiny seeming to be in part to furnish food to other animals larger than themselves. In the chalk formations of the earth are seen multitudes of such shells. They have been discovered even in the glazing of a visiting-card; for they are so small that the fine grinding up of the chalk does not wholly destroy them. There are animals, both in the air and in the water, so small that it would take millions of them to equal in bulk a gram of sand, and a thousand of them could swim side by side through the eye of a common-sized needle. Now in all these animals there are organs, constructed of particles of matter, which are arranged in them with as much order and symmetry as in the organs of our bodies. How minute then must these particles be!

How do such facts extend our views of the power of the Deity! The same power that moulded the earth, sun, moon, and the whole "host of heaven," gave form, and life, and motion to the millions which sport in every sunbeam; the same eye that watches the immense heavenly bodies as they move on in their course, looks upon one and all of these legions of animals in earth, air, and water, though they are unseen by human eyes, seeing that every particle shall take its right position, so that this part of creation may with all the rest be pronounced very good; and the same bountiful hand that dispenses the means of life and enjoyment to the millions of the human race, forgets not to minister to the brief life and enjoyment of each one of these myriads of animalcules, though they seem to be almost nothingness itself.

20. Pores and Spaces in Matter.—In all matter there are spaces about the particles. Those bodies which are called porous have quite large spaces in them. But even in those which are not commonly considered porous the particles are by no means close together. A celebrated experiment tried in Florence a long time ago showed that there are spaces among the particles of so dense a substance as gold sufficiently large to let water through them. A hollow golden globe containing water was subjected to great pressure, and its surface was bedewed with the water that came out through the pores of the gold. In all substances in which there are pores visible to the naked eye, or by the aid of the microscope, there are other spaces or interstices among the particles around the pores. Indeed, it is supposed that there is space around every ultimate particle or atom, and that no two of these atoms are in actual contact. The fact that substances which have no pores can be compressed into a smaller space than they usually occupy shows that there are spaces or interstices in them. Solids can be thus compressed, some more than others. But the most compressible substances are the gases and vapors. The amount of space between their particles must be very large to allow of so great compression.

Fig. 1.

21. Space in Gaseous Substances.—We can have some idea of the great amount of space in a gaseous or aeriform substance by observing the difference between water in its liquid and in its aeriform state. A cubic inch of water, when it becomes steam, occupies 1696 times as much room as it did when it was water. The difference in proportion is exhibited in Fig. 1, the inner circle representing the water, and the outer the steam into which it is converted. Now the water is not altered at all in its nature by being changed into steam. The particles are simply put farther apart by the heat, and as soon as the heat is withdrawn they come together again to form water, or, in other words, the steam is condensed into water. It is plain, therefore, that the space between the particles is 1696 times as great in steam as it is in the water from which the steam is made.

22. Solutions.—When any substance, as sugar or salt, is dissolved in water, its particles are diffused through the spaces that exist between the particles of the water. So also when water evaporates (§ 12), the particles of water are diffused through the spaces between the particles of the air. In like manner are the particles from an odorous substance diffused in these spaces, and thus mingled with the particles of the air they are carried into the nostrils, and strike upon the minute extremities of the nerve of smell.

23. Relation of Heat to the Spaces of Matter.—The variation in the amount of space between the particles of matter in any substance generally depends on the variation of the amount of heat present. Thus heat expands iron; that is, it increases the spaces between the particles of the iron. So also heat increases the spaces between the particles of mercury, and thus makes it occupy more room in the thermometer. This effect of heat will be considered more fully hereafter.

The general views which I have given of the constitution of matter will throw light upon the different qualities of different substances, some of which I will notice.

24. Density and Rarity.—The density of a substance depends upon the quantity of matter it contains in a given space. The more dense, therefore, a substance is the greater is its weight. A piece of lead is forty times heavier than a piece of cork of the same size. Mercury is nearly fourteen times heavier than an equal bulk of water. You see, then, that density must depend on the nearness of the atoms to each other. In so dense a substance as gold the atoms are all very close together; in wood there are spaces, some of which are so large that you can see them; and in air, steam, and the gases there is a great deal of space among the particles (§ 21), so that we speak of their rarity instead of their density.

25. Tenacity.—The power of holding together, termed tenacity, depends on the degree of attraction between the particles. By attraction I mean a disposition in particles to come together, this disposition being manifested in opposition to any force tending to draw them apart. I shall soon speak of this more particularly. Tenacity does not exist at all in gaseous substances. The particles of air and of steam, for example, show no disposition to cling together; that is, have no tenacity. This property is weak in liquids. It is only strong enough in water to enable its particles to hang together in the shape of a drop. It is strong in solids, enabling their particles not only to hold together in large quantities, but to hold up also heavy weights suspended to them. It is stronger in iron than in any other solid. It is stronger in wrought iron than in cast iron; and strongest of all in steel.

26. Comparative Tenacity of Substances.—Various metals and other substances have been tested in reference to their comparative tenacity. It was done in this way: Wires were made of the metals, all of the same size. Weights were suspended to them, and additions were made to the weights by little and little till the wires broke. The table underneath was made by placing against each metal the greatest weight that its wire would hold:

Oak wood, tried in the same way, was found to hold up 12 pounds, one more pound than silver. Some animal substances have great tenacity, as the thread of the silk-worm, hair, wool, and the ligaments and tendons of our bodies and of other animals.

27. Value of Tenacious Substances.—"The gradual discovery," says Dr. Arnot, "of substances possessed of strong tenacity, and which man could yet easily mould and apply to his purposes, has been of great importance to his progress in the arts of life. The place of the hempen cordage of European navies is still held in China by twisted canes and strips of bamboo; and even the hempen cable of Europe, so great an improvement on former usage, is now rapidly giving way to the more complete and commodious security of the iron chain—of which the material to our remote ancestors existed only as useless stone or earth. And what a magnificent spectacle is it, at the present day, to behold chains of tenacious iron stretched high across a channel of the ocean, as at the Menai Strait between Anglesea and England, and supporting an admirable bridge-road of safety, along which crowded processions may pour, regardless of the deep below, or of the storm; while ships there, with sails full-spread, pursue their course unmolesting and unmolested."

28. Hardness.—This property seems to depend upon some peculiar arrangement of the particles of matter. We should suppose that the densest substances would be the hardest. But it is not so. Iron is the hardest of the metals, but its particles are not so close together as those of gold, which is quite a soft metal. And gold is five times as heavy as the diamond, which is so hard as to cut glass easily. Common flint is hard enough to scratch glass, but will not cut it like the diamond.

Fig. 2.

29. Flexibility and Brittleness.—If you bend a flexible body as a piece of wood, as represented in Fig. 2, it is obvious that the particles on the upper or convex side must be put a little farther apart, while those on the under or concave side are brought a little nearer together. But the wood does not break, because the particles that are thus moved a little apart still retain their hold upon each other. This is the explanation of what we call flexibility. On the other hand, the particles in a rod of glass can not be put farther apart in this way. They are not actually in contact any more than the particles of the wood are (§ 20), but they are in a fixed relative position; that is, a position which can not be disturbed without a permanent separation of particles. If you attempt to bend the rod there is no slight separation of many particles, as in the bent wood, but a full and permanent separation in some one part of the rod. We call the property on which this result depends brittleness. Brittle substances are generally hard. Glass, while the most brittle of all substances, is hard enough to scratch iron. Brittle substances also have much tenacity. A rod of glass can hold up a heavy weight, although a slight blow suddenly given would break it.

30. Flexible and Brittle Steel.—There are two kinds of steel, flexible and brittle. The steel of most cutting instruments is brittle. The steel of a sword-blade is quite flexible, and that of a watch-spring is so much so that we can wind it up in a coil. This difference is owing to a difference in the mode of cooling the steel. If it be cooled suddenly, it is brittle; if slowly, it is flexible. The process by which it is cooled slowly is called annealing. The explanation of all this is quite plain. The steel being expanded by heat—that is, its particles being put farther apart than they usually are—when they are suddenly brought together again they have not time to arrange their relative position properly. Brittleness is therefore the result. But, on the other hand, when the cooling is effected gradually, time is given for the arrangement.

31. Tempering of Steel.—Steel suddenly hardened is too brittle for common use. A process called tempering is therefore resorted to for diminishing the brittleness. The steel is reheated after the hardening, and is then allowed to cool slowly. The degree in which the brittleness is lessened depends on the degree of heat to which the steel is subjected. It can be entirely removed by a red heat, for then the particles have a full opportunity to readjust themselves; and the more the heat comes short of this point the less thorough will be the adjustment, because the less perfectly are the particles released from their suddenly-taken position. In lessening the brittleness we lessen hardness also, and therefore the tempering is varied in different cases according to the degree of hardness which is desired.

32. Annealing of Glass.—Glass is always annealed. If this were not done our glass vessels and windows would be exceedingly brittle, and would therefore be constantly breaking. Articles made of glass are annealed by being passed very slowly indeed through a long oven which is very hot at one end, the heat gradually lessening toward the other end.

Fig. 3

33. Prince Rupert's Drops.—We have a striking example of brittleness induced by sudden cooling in what are called Prince Rupert's drops. These are made by dropping melted green glass into cold water, and they are of the shape represented in Fig. 3. If you break off ever so small a bit of the point of one of these drops, the whole will at once shiver to pieces. That is, the sudden arrangement of the particles is so slight and unnatural that the disturbance of the arrangement in a small part suffices to destroy the arrangement of the whole, very much as a row of bricks falls over from the fall of the first in the row. Mr. Farraday says that these drops were not, as is commonly supposed, invented by Prince Rupert, but were first brought to England by him in 1660. They excited much curiosity at that time, and were considered "a kind of miracle in nature." But you see that this, like many other wonders, receives with a little thought an easy explanation.

34. Malleability and Ductility.—Those metals which can be hammered into thin plates are called malleable. Gold furnishes us with the best illustration of this property. Silver, copper, and tin are quite malleable. Most of the other metals are very little so, and some of them are not at all, breaking at the first blow. A substance is said to be ductile when it can be drawn out into wire. The principal metals that have this quality are platinum, silver, iron, copper, and gold, and in the order in which I have named them. Melted glass is very ductile. It can be drawn out in a very fine thread, and when this thread is cut and arranged in branches it resembles beautiful white hair. In hammering metals into plates, or drawing them into wire, there is a considerable change of relative position in the particles, similar to that which we have in fluids, though nothing like as free. In this change of position those particles that do remain in close neighborhood have a remarkable tenacity or attraction, preventing their separation. In welding two pieces of iron, which is done by the blacksmith by hammering them together when red-hot, there must be enough movement among the particles to have those of one piece mingle somewhat with those of the other.

35. Compressibility.—Porous substances can be considerably compressed. Force applied to them can bring their particles nearer together, making them to fill up in part their pores. The most familiar example you have of this is in sponge. The more porous wood is the more can it be compressed. But even such dense substances as the metals can be compressed in some degree; that is, the interstices between their particles can be made smaller. Medals and coins have their figures and letters stamped upon them by pressure, just as impressions are made upon melted sealing-wax. The heavy and quick pressure required to do this actually compresses the whole piece of the hard metal, putting all the particles nearer together, so that it occupies less space than it did before it was stamped.

36. Incompressibility of Liquids.—We should suppose, from the freeness with which the particles of liquids move among each other, and from the spaces (§ 22) which exist among them, that these substances could be easily compressed. But it is not so. The heaviest pressure is required to compress them even in a slight degree. Water can be compressed so very little that practically it is regarded as incompressible.

37. Influence of Heat on the Bulk of Liquids.—Although the interstices between the particles of liquids can not be varied by mechanical pressure, they can be by variations of temperature. Liquids are dilated or expanded by heat; that is, their particles are put farther apart. They are contracted or compressed by cold; that is, their particles are brought nearer together by the abstraction of heat. The most familiar example that we have is in the thermometer. The mercury rises in the tube when the heat increases the interstices between its particles; and it falls when the loss of heat allows the particles to come near together. The same effects are seen when alcohol is used in the thermometer, as is done in the arctic regions, because mercury may freeze there. A thermometer with water in it would answer if we wished only to measure temperatures between the freezing point and the boiling point of water. The expansive influence of heat will be particularly treated of hereafter.

38. Compressibility of Aeriform Substances.—Aeriform bodies are more compressible than any other substances, showing that in their ordinary condition there is a great deal of space among their particles. While they are thus unlike liquids in compressibility, they are affected by heat in the same way that liquids are.

Fig. 4. Fig. 5.

39. Elasticity.—Closely allied with the compressibility of matter is its elasticity. We see this property strikingly exemplified in India-rubber. It occasions the rebounding of a ball of this substance when thrown down. Observe flow exactly what occurs in this case. The ball as it meets the resistance of the floor is flattened, as represented in Fig. 4. Then, as it assumes the round shape, as seen in Fig. 5, it pushes downward upon the floor. It is this sudden pushing downward that makes it rebound. It is as if there were a compressed spring between the ball and floor. It may be likened also to jumping. When one jumps he bends his limbs at the thigh and knee joints, and then, in straightening himself up, gives a sudden push, like that given by the ball as it assumes its round shape, and so is thrown forward or upward, according to the direction in which the pushing force is made. The same flattening occurs in an ivory ball, though not to the same degree. You can prove that it does occur by experiment. Let a marble slab be wet and drop the ball upon it. Quite a spot will be made dry by the blow of the ball, showing that it touched more of the marble than it does when it is merely placed upon it.

40. Elasticity Shown in Other Ways.—If a stick be bent, as in Fig. 2, as soon as the bending force is withdrawn the stick becomes straight again from its elasticity. It is this elastic force of the bow, straightening it, that speeds the arrow. Observe in this case that while the particles on the concave side of the bent bow are brought nearer together or compressed, those on the convex side are moved apart. This moving apart of the particles is often shown in India-rubber. You can see how very far apart particles that are in near neighborhood may be carried, if you will stick two pins close together in a strip of India-rubber before you stretch it.

41. Degrees of Elasticity in Different Substances.—Some substances have so very little elasticity that they are practically considered as having none. Lead is one of these. A rod of lead when bent remains so, and a leaden ball does not rebound. While aeriform substances are the most compressible of all, they are also the most elastic. Air compressed returns to its usual condition the moment that it is relieved from the pressure, and with a force proportioned to the amount of the pressure. So it is with steam and the gases. The varied results of this quality of aeriform substances will claim our attention more particularly in some other parts of this book.

42. Definition of Elasticity.—You see from the illustrations that have been given that elasticity is that property of matter by which its particles, when brought nearer together or carried farther apart by any force, return to their usual condition when the force is withdrawn.

43. Usefulness of Variety in Properties of Matter.—The various properties of matter brought to view in this chapter are providential adaptations to the necessities of man. Each substance has those properties which best fit it for his use. Iron, for example, designed by the Creator to be both the strongest and most extensively useful servant of man among the metals, is therefore provided in great abundance, and has those strong, decided, and various qualities which fit it for the services it is to perform. Gold and silver, on the other hand, designed for services less extensive, lighter, and in a great measure ornamental, are provided in very much less quantity, and have properties admirably adapting them to the services for which they are so manifestly intended. The same can be substantially said of all other substances, and especially of those very abundant ones, air and water. And it may be remarked also that the ingenuity of man is continually discovering new modes of bringing the various properties of matter into his service. I will give but a single illustration—the tempering of steel. "This discovery," says Dr. Arnot, "is perhaps second in importance to few discoveries which man has made; for it has given him all the edge-tools and cutting-instruments by which he now moulds every other substance to his wishes. A savage will work for twelve months with fire and sharp stones to fell a great tree and to give it the shape of a canoe, where a modern carpenter, with his tools, could accomplish the object in a day or two."

[CHAPTER III.]
THE ESSENTIAL PROPERTIES OF MATTER.

44. Extension.—You can not conceive of any portion of matter, however small it may be, that has not shape or figure. It may be so small as to appear only as a point to the naked eye, but seen through the microscope its shape becomes obvious. Even an atom must have length, breadth, and thickness, though it be so small that we can not measure it, nor see its shape with the most powerful microscopes. Extension, which is the term commonly used to express this idea, is then an essential property of matter; that is, it is a property of which no form or kind of matter can be destitute. The distinction, in this respect, between this property and those which I have before noticed may be made obvious to you by an example. Hardness is not an essential quality of matter, for some kinds of matter are destitute of it; but no portion of matter, hard or soft, can be destitute of extension or shape. The air is sometimes spoken of in common language as being shapeless. This is partly because it is invisible, and partly because no portion or body of air assumes any definite shape. But air is continually forced into definite shapes by confinement in rooms, boxes, etc.; and then its extension in different directions can be measured as accurately as the extension of a solid can be. And besides, the atoms of which air is composed are undoubtedly solid, and we can not conceive of their existence without attaching to them the idea of figure or extension.

45. Impenetrability.—In common language one substance is said to penetrate another. Thus a needle penetrates cloth, a nail penetrates wood, etc. But this is not strictly true. The needle does not go into the cloth, but goes between the fibres of it, pushing them to the one side and the other. So the nail goes between the fibres of the wood, and not into them. It does not occupy the same room that the fibres do at the same time. So, also, no atom of matter can penetrate or go into any other atom. It can only push it out of the way, and then occupy its place. Impenetrability is therefore said to be one of the essential qualities of matter. This means simply that no portion of matter can occupy the same space with another portion of matter at the same time.

Fig. 6.

Fig. 8.

Fig. 7.

46. Illustrations.—Many illustrations might be given of this property. I will give a few. If you press a tumbler into water with its open end downward, you can not fill it with water, for the air confined in the tumbler prevents it from rising. It can not occupy the same space with the air. It fills, indeed, a portion of the tumbler, but this is because the air is compressible. If you introduce a glass funnel, a, Fig. 6, into a jar of water, b, with your thumb on its mouth, c, the water will not rise to fill it. But if you take your thumb off, the water will rise to the level of the water outside of the funnel, pushing up the air before it. If you have not a funnel, a vial or bottle with its bottom broken off will answer for the experiment. The following is a very pretty experiment, illustrating the same point. Place a lighted taper, a, Fig. 7, on a large flat cork in a jar of water. Put over it an open jar or receiver, b, having a stop-cock, c. Closing the stop-cock, press the receiver down into the water, and you will see the taper sink with it, as represented in the figure, the air preventing the water from entering the receiver. If now you open the stop-cock the water rushes in, thrusting the air upward, and making the taper to appear as if rising out of the water. The diving-bell offers a good illustration. It consists of a vessel, a a, Fig. 8, shaped like a bell, made sufficiently heavy to sink in water. It is let down by a chain and cable, as seen in the figure. The water does not enter the bell any farther than the compressibility of the air permits it. In order that the men in it may remain under water for some time fresh air is supplied by the tube b, it being forced down by a forcing-pump. At the same time the vitiated air can be let off by a valve provided for that purpose. There are windows in the top of the bell to give the requisite light for work on the sea's bottom. Treasures are often recovered by this means which would otherwise be lost. You see a resemblance between the diving-bell and the arrangement in Fig. 7, the receiver representing the bell and the lighted taper the men in it.

47. Other Illustrations.—If you drop a bullet into a tumbler of water it pushes the particles to the one side and the other, and occupies the room thus made. If you drop in several there is a manifest rise of the water, and you may drop in enough to make it overflow. The same thing is true of the finest needle dropped in the water—it does not penetrate it, but like the bullet displaces some of its particles and occupies their room; and you can make the water overflow by dropping in many needles. We can truly say, then, that water can not be penetrated even by a needle. When any substance, as sugar, is dissolved in water, its particles do not penetrate the water, but go into the spaces between its particles. So, also, when particles from odorous substances are diffused in the air, they are not really in the air, but are between its particles.

48. Inertia.—Matter has no power to put itself in motion. When it is moved it is moved by some force which is either outside of the matter or is communicated to it in some way. When your arm is moved it is not the matter in your arm that is the cause of its motion. It is caused by a force in you which I will not dwell upon here, because the subject belongs to Physiology. When air moves it is set in motion by some force acting upon it, as when you blow it from your lungs or move it with a fan. When the wind blows, the air is set in motion by heat and the attraction of the earth, as will be explained to you in another part of this book. I might multiply examples to any extent, showing that matter of itself can not move. This property of matter is termed inertia.

49. Inertia Shown in the Inability of Matter to Stop its Motion.—Matter, when once set in motion, has no power to stop itself. If it could stop itself it could not be said to be inert. And as it is inert, it would, when once set in motion, keep on moving forever unless stopped by some force. When a stone falls to the ground, it stops simply because the earth stops it. If the earth were not in the way, the stone would move straight on until it were stopped by something else. So, also, a stone that is thrown up in the air would keep on, and soon be out of sight, and never return to the earth, if it were not made to come down by forces acting upon it. One of these forces is the resistance of the air, which, from the moment the stone starts, is destroying its motion. Another force as constantly operating to retard the stone is the attraction, or drawing force, exerted by the earth upon it. This powerful though unseen force will be treated of fully in the next chapter.

50. Matter Equally Inclined to Rest and Motion.—It was formerly taught by philosophers that matter is more inclined to rest than to motion; and this is the popular notion now. This is because the chief causes that stop the motions that we see from day to day—viz., the air and the attraction of the earth—are not visible. For this reason, until we investigate the subject, it seems to us that motion has a natural tendency to stop, or is spent, as it is expressed. When friction has an agency in arresting motion we see it plainly; but the common idea is, that in this case the motion is in part spent and in part destroyed. But in no case is motion spent, but it is always destroyed by obstacles. The more thoroughly these are removed, the longer will the motion continue; and if they were wholly removed, the motion would never cease. Perpetual motion, then, is not in itself impossible; for matter has no more tendency to stop moving when once put in motion, than it has to begin motion when it is at rest. All motion would be perpetual if there were not forces opposing it. If there were only one body in the universe, and that were set in motion, it would move forever through empty space in a straight line; for there is no matter any where to resist its motion or to attract it away from its onward course.

51. Divisibility.—Though divisibility is a general quality of matter (§ 18), it is not an essential one. For if it be true that every portion of matter is composed of atoms that remain whole (§ 14), this property can not be said to belong to these atoms. It is only the bodies made up of these atoms that can be divided. When we come to the atoms themselves the division must stop.

52. Weight.—In speaking of the properties of matter I have said nothing about weight, although in the popular mind this is thought of as being one of the most prominent of the properties of some kinds of matter. This will be appropriately treated of when I come to speak of attraction, for it is a mere result of attraction. Suffice it to state here, that the weight of a body is the pressure occasioned by the attraction existing between it and another body. If when a stone is raised from the ground the attraction between it and the earth could be destroyed, the stone would remain there. It would not press down, and so would have no weight. It is plain, therefore, that weight, so far from being an essential property of matter, is not really a property at all. It is only an effect of a property—attraction. If there were only one body in the universe, it would have no weight, for it would press in no direction because there is nothing to attract it. But as it is, all matter has weight, for there is other matter to attract it.

[CHAPTER IV.]
ATTRACTION.

53. Nature of Attraction.—If you attempt to break a very tenacious solid substance, why do you not succeed? It is because the particles are so strongly fastened together. But how? By some kind of cement or glue, or by some mechanical contrivances as nails or hooks? No. They are fastened together by some unseen force. We know nothing of the nature of this force. We know only that it exists, and we call it attraction. The name is a proper one, for it simply expresses the fact that one particle attracts or draws another particle toward itself.

54. Newton's Idea of Attraction.—It was stated in § 20 that the particles of matter, even in the densest substances, are not in actual contact, but have spaces around them. Now it was supposed by Newton that there is some kind of ethereal substance pervading all these spaces, which causes this attraction between the particles. He supposed also that this ether was every where in space, causing attraction between masses of matter. But all this is mere supposition, and we know not whether there is this sort of ethereal glue keeping the universe together, or whether it is some property in the particles themselves that makes them thus attract each other. But the fact of the attraction we know, and we can observe the phenomena which it produces, and discover the laws or rules by which this force is regulated in its action.

55. Attraction in Solids.—Attraction is stronger in some solids than in others. The mason with his trowel easily divides a brick; but he can not do this with a piece of granite, for its particles have a greater attraction for each other than those of the brick. So a rap which would break a glass dish would not injure a copper one of the same thickness. A weight that would hang securely from an iron wire would break a lead wire of the same size; that is, it would tear the particles apart, because they are not strongly attracted to each other. Attraction has different modes of action in different solids. It therefore fastens their particles together in different ways, and thus produces all the various qualities, already noticed, which are so useful to us—tenacity, hardness, softness, ductility, flexibility, etc.

56. Attraction in Liquids.—In a liquid the attraction between the particles is very feeble compared with that in solids. The attraction of the particles of steel is in strength about three million times that of the particles of water. We make the estimate in this way: We find that a steel wire will sustain a weight equal to 39,000 feet of the wire. But a drop of water hanging to the end of a stick can not be more than one-sixth of an inch in length; that is, water will hold together by the attraction of its particles only to this extent, which is a little less than the three millionth part of the length of steel wire which could hang without breaking.

57. Freeness of Movement of the Particles of Liquids.—There is one prominent characteristic of liquids which is probably not entirely owing to the feeble attraction of their particles—I mean the freeness with which these particles are moved among each other. This is owing probably in part to some peculiar arrangement of the atoms in making the particles of a liquid. I will illustrate this in a coarse way. If the atoms of lead in shot were so arranged as to make irregular jagged forms, they could not readily be moved among each other. We suppose the ultimate atoms of a liquid to be so arranged in the formation of particles as to make them not only round but very smooth. Hence comes the great ease with which they circulate among each other.

Fig. 9.

Fig. 10.

58. Globular Shape of Drops of Liquids.—As the particles of a liquid move thus freely among each other, their attraction disposes them to assume a globular or round shape. The reason of this can be made plain by Figs. 9 and 10. The outside of a perfect sphere is all at the same distance from the centre. So all the circumference of a circle is at the same distance from the centre, as represented in Fig. 9. But this is not true of all parts of the surface of a cube or of a square: a, for example, is farther from the centre than b is. Now in a drop of liquid all the particles are attracted toward the centre, for in that line from each particle lies the largest number of particles to attract it. This can be made obvious by taking some point in the drop, as represented in Fig. 10, and drawing lines from it through the centre and in other directions. If a be the point in the drop, it is plain that the line from it through the centre is longer than a b or a c. Therefore a particle, a, will be attracted toward the centre rather than in the direction a b or a c, because there are more particles in the direction of the centre, and the more particles there are the stronger is the attraction. But this is not all. The particles in the line a c, tending to make a go toward c, are balanced by the particles in the line a e, tending to make it go toward e. The two lines of particles therefore together tend to make it go in a middle line between them, that is, toward the centre, just as two strings pulling equally, the one to c and the other to e, would make a body, a, move in a middle line between these two directions. The same can be shown of the two lines of particles a b and a d, and so of any other two alike in situation on each side of the line through the centre. The tendency of every particle is, then, to go toward the centre, and it would go there if there were not particles between to prevent it. You see how this would operate in the case of the particles on the surface of the drop. As these are all striving, as we may say, in obedience to attraction, to get to the centre, none of them will be raised up into an angle or a point, as would be the case if the drop were in the shape of a cube. If this should be done it would show that some of the particles were not as strongly attracted toward the centre as others are, which is an impossibility.

59. The Globular Form in Different Liquids.—The disposition to form a sphere is seen more distinctly in mercury than in any other liquid. If you drop a little of it upon a plate it separates into globules, which roll about like shot. Why can not the same thing be done with water? Why do the drops of water hang upon the window-pane, showing only in an imperfect way their disposition to the globular arrangement? It is because the particles of water have a greater attraction for other substances, and less attraction for each other, than the particles of the quicksilver have. Water sometimes exhibits its disposition to the globular form in full on the leaves of some plants, and rolls about in balls like mercury. This is because there is something on the surface of the leaf which repels rather than attracts the water. If you put your finger, however, on one of these drops, it will spoil it, and your finger will be moistened, because there is an attraction between the particles of your skin and of the water. Take another illustration of this difference in attraction. If you drop a little oil upon the surface of water it will float about in round drops. This is because the water repels the oil, as the surface of some kinds of leaves does water. But when oil is spilled upon wood or cloth its particles have so strong an attraction for their particles that they unite with them, instead of gathering up into little round companies as they do on the surface of water.

60. Manufacture of Shot.—We have a beautiful example of the tendency of fluids to the globular arrangement in the manufacture of shot. The melted lead is poured into a large vessel in the top of the shot-tower. This vessel has holes in its bottom, from which the metal falls in drops. Each drop, as it whirls round and round in its fall, takes the globular form. By the time that it reaches the end of its journey, about two hundred feet, it becomes so far cooled as to be solid, and as it is received in a reservoir of water, its globular form is retained. Bullets can not be made in this way, because a quantity of melted lead sufficient to make a bullet will not hold together in a globular form.

61. Globular Form of the Earth and the Heavenly Bodies.—It is supposed that the sun, moon, earth, and all the heavenly bodies were once in a liquid state, and that they owe their globular shape to this fact. As they whirled on in this condition in their course, the different solids were gradually formed, and at length they acquired their present state. How all the mighty changes could be effected in our earth, converging it from a liquid into a body with a solid crust, having such various substances in it, and so variously arranged, with its depressions containing water, and the whole covered with its robe of air fifty miles in thickness, we can not understand. And yet there are some portions of the process which chemistry and geology have revealed to us, giving us some glimpses of the wonders which, during the lapse of ages, God wrought in our earth in preparing it for the habitation of man.

Fig. 11. Fig. 12.

62. Crystallization.—The arrangement of the particles of solid substances is different from that of liquids. The tendency here is to straight lines and angles; that is, to crystalline forms. Alum or common salt, when it becomes solid from a solution, forms crystals. So also does sugar. The crystals of different substances are different. In Fig. 11 you have the crystal of common salt, and in Fig. 12 that of alum. We see this crystalline tendency every where, even in the rude rocks and common stones. The rocks are disposed to exhibit regular layers, or columns, or battlements, and always do so except when interfering circumstances prevent. And when you examine their composition, or that of the stone under your feet, you see the same crystalline disposition in detail that you see in the mass.

63. Crystallization of Water.—Water, when it changes into a solid, shows the same disposition, of which the crystals of the snow and the frost-work on our windows are familiar examples. When snow forms, the water of the clouds is suddenly crystallized by the cold air, the particles taking their regular places more readily and certainly than if they were guided by intelligence, because in obedience to an unerring law established by the Creator. We sometimes have an example of this sudden crystallization of water under our eye. The water in a pitcher may remain fluid, although it is cooled down to the freezing point, and even below it, if it be kept perfectly still. But on taking up the pitcher the water at once becomes filled with a net-work of ice-crystals. The explanation is this: The stillness of the water has prevented its particles from taking on the new arrangement needed for the formation of ice; but the jostling of them in taking up the pitcher has served to make them do it thus suddenly.

Fig. 13.

64. Frost and Snow.—The frost-work on our windows is a wonderful exhibition of the variety of forms that crystallization can produce. It sometimes presents figures like leaves and flowers, such as we see chased on vessels of silver, but much more delicate and beautiful. So varied and fantastic are the forms in which these water-crystals are arranged, that it is very natural to ascribe them, as is done universally in the dialect of the nursery, to the ingenuity of a strange and tricksy spirit. Every snow-flake is a bundle of little crystals as regular and beautiful as the crystals which you so much admire in a mineralogical cabinet. And there is great variety in the grouping of these crystals. You have some specimens of these groups in Fig. 13 as they appear on examining them with the microscope. Over six hundred different forms have been enumerated, and a hundred have been delineated. It is a very quick operation by which the particles of water in the clouds thus marshal themselves, as if by magic, in these regular forms. But a quicker operation is that by which hail is formed—so quick that the particles have not time to set themselves in the crystalline arrangement, but are huddled together without order. The brilliant and glistening whiteness of the snow is owing to the reflection of light from its minute crystals. In the arctic regions the beauty of the snow is often much greater than with us. "The snow crystals of last night," says Captain M'Clintock in his "Discovery of the Fate of Sir John Franklin," "were extremely beautiful. The largest kind is an inch in length; its form exactly resembles the end of a pointed feather. Stellar crystals two-tenths of an inch in diameter have also fallen; these have six points, and are the most exquisite things when seen under a microscope. In the sun, or even in moonlight, all these crystals glisten most brilliantly; and as our masts and rigging are abundantly covered with them, the Fox never was so gorgeously arrayed as she now appears."

65. Order in Nature.—We see in this general tendency to crystallization a striking illustration of the fact that God is a God of order. Disorderly arrangement is never seen except where there is an obvious necessity for it. And even when there is apparent disorder, a little examination generally shows that essentially there is order. The rocks that give so much variety to scenery are not piled up in confusion, and order has evidently reigned in their construction. Pick up a common stone, and on breaking it you will see the crystalline arrangement in its interior. Nay, more, much of the very soil is made up of separated and broken crystals.

Fig. 14.

66. Particles Must be very Near to Each Other to Adhere.—Why is it that when you have broken any thing made of glass, however accurately you may bring the two parts together, you can not make them unite in one again? It is simply because the particles of substances will not attract each other enough to be united unless they are brought very near together. Now it is impossible to bring the particles on the two surfaces of a broken piece of glass as near together as they were before it was broken. If you could do so, no crack would be visible. You can join them by some kind of cement. This is because the particles of the cement, while it is soft, can be insinuated among the particles of the glass; and thus, when it dries, it becomes a bond of union between the particles on each side of the breach. For the same reason you can make the cut surfaces of some yielding substances adhere. If you divide a piece of India-rubber with a clean cut, you can make the two surfaces adhere by pressing them together firmly. The particles in this case are not unyielding, as those of the glass are, and some of them are therefore brought into such near neighborhood as to attract each other sufficiently to unite together. So, too, if you cut two bullets so as to have a very smooth flat surface on each, you can make them adhere quite strongly by pressing them together, especially if you give a little turning motion at the same time that you press, for this will cause the particles on the two surfaces to be somewhat mingled together. If you have quite large balls of lead with handles, as represented in Fig. 14, it will require considerable force to separate them when they have been thus pressed together.

67. Other Illustrations.—Silver and gold may be made to adhere to iron by a very great and sudden pressure. The iron must be made very smooth, and the silver or gold plate very thin. A powerful blow brings the particles of the thin plate into such nearness to those of the iron that union is effected, or, in other words, that they attract each other sufficiently to be united. So, also, a sheet of tin and one of lead can be made to adhere so as to form one sheet by the pressure of the rollers of a flatting-mill. Two very smooth panes of glass laid one upon another may have so many particles brought into great nearness as to occasion some adhesion. It will be slight, however, few comparatively of all the particles coming near enough to adhere, for the smoothest glass is full of inequalities, as may be seen by the microscope.

68. Strength of Adhesion.—In no case do particles come in actual contact (§ 20), and their adhesion depends on the nearness of their neighborhood to each other. The strength of union, then, between two surfaces depends on the number of particles brought near enough to adhere together. In the case of the two bullets or the lead balls, if all the particles of the two surfaces were near enough to adhere, the lead would be just as strong at the junction as any where else. The reason that so strong adhesion takes place between portions of some substances when we soften them by heat is that the particles of the two softened ends are all brought near enough together for adhesion. Thus the two ends of a broken stick of sealing-wax may be firmly united by heating them and then pressing them together. The same thing can be done with glass. When iron is welded, as it is termed, some hammering is required to make the particles of the two softened ends of the iron unite.

Fig. 15.

69. Attraction Between Solids and Liquids.—The attraction which solids and liquids have for each other furnishes us with many interesting phenomena. The adhesion of drops of water to glass and other solids is a familiar example of this attraction. If you dip your hand into water, it is wet on taking it out, because your skin has sufficient attraction for the water to retain some of it. A towel will retain more of it for two reasons—with the interstices between its fibres it presents much more surface to the water, and it has none of the oily substance which on your skin, though being in small quantity, serves somewhat to repel the water. The attraction of solids and fluids for each other is shown very prettily in the experiment represented in Fig. 15 (p. 47). A piece of wood is attached by the string, a, to one end of a balance, and weights just sufficient to balance it are placed in the opposite scale. If now the wood is brought in contact with the water in the vessel, b, it will require additional weight in the scale to separate the wood from the water.

Fig. 16. Fig. 17.

Fig. 18.

Fig. 19.

70. Farther Illustrations.—When you see stems of plants rising above the surface of stagnant water you will observe that the water is considerably raised about them. This is from the attraction between them and the water. For the same reason water is not as high in the middle of a tumbler as it is at the sides. If you immerse a piece of glass in water, the water will rise at its sides as represented in Fig. 16. If you immerse two together, as in Fig. 17, the water will rise higher between than outside of them, because the particles between are attracted by two surfaces, while those outside are attracted only by one. It is for the same reason that two men can raise a weight higher than one of them can alone. And if the pieces of glass be brought quite near together, as in Fig. 18, the water will be raised higher still, because there is less to be raised by the two surfaces. It is just as two men can raise a small weight higher than they can a large one. The same thing may be beautifully illustrated in this way: Let two pieces of glass, as represented in Fig. 19, be immersed in colored water, with two of their edges joined together at A B, the opposite edges at E D C being separated. The height to which the fluid rises will make a curved line, A F C, it being lowest at the edges which are separated, and highest at the edges which are joined together.

Fig. 20.

Fig. 21.

71. Rise of Liquids in Tubes.—For the same reason that water rises higher between plates of glass than outside, so it will rise higher in a tube than it will outside of it. The diagram in Fig. 20 will make this clear. I represent in this a transverse section of a tube, enlarged so that the demonstration may be plain. We will take a particle on the inside and the outside at equal distances from the glass. It is clear that the particle a is not as near to as many particles of the glass as the particle b is. The lines drawn show this. The longest lines extending from the particles a and b to the glass are equal in length; that is, a e and a f are equal to b g and b h. It is clear, therefore, that all the glass between the lines at c and d is as near to the particle b as the glass between the lines at e and f is to the particle a. But this is not all. The particle b is near enough to all the inside of the tube to be attracted by it, while very little attraction is exerted upon a by any part of the glass beyond that which is included between e and f. The same difference can be shown with regard to all the particles on the inside of the tube compared with those outside. The former are nearer to more particles of the glass than the latter, and therefore are more strongly attracted. Again, as the nearer the plates of glass are (§ 70) the higher the water rises between them, so the smaller the tube is the higher will the water rise in it. You can try the experiment as represented in Fig. 21. It is obvious that the particle b (Fig. 20) would not be very strongly attracted by the part of the tube opposite if the tube were a large one; but it would be if the tube were very small, for then it would be quite near to that part.

72. Capillary Attraction.—The term capillary (derived from the Latin word capilla, hair) has been commonly applied to the attraction exhibited under the circumstances just noticed, because it is most obvious and was first observed in tubes of very fine bore. The same term is used when the attraction is seen in the rising or spreading of a liquid in interstices as well as in tubes. Thus capillary attraction causes the rising of oil or burning fluid in the wicks of lamps. The liquid goes up in the interstices or spaces between the fibres, as it does in the spaces of tubes. I will give some other examples. If you let one end of a towel be in a bowl of water, the other end lying over upon the table, the whole towel will become wet from the spreading of the water among the fibres in obedience to capillary attraction. If you suspend a piece of sponge so that it merely touch the surface of some water, or if you lay it in a plate with water in it, the whole sponge will become wet. So, too, if you dip the end of a lump of sugar in your tea, and hold it there a little time, the whole will be moistened. In very damp weather the wood-work in our houses swells from the spreading of water in the pores of the wood in obedience to capillary attraction. Especially will this be so in basement rooms, where the water can go up from the ground in the pores of the walls, as well as from the damp air. In watering plants in pots, if the water be poured into the saucers, it will pass up through the earth by capillary attraction. For the same reason plants and trees near streams grow luxuriantly, being abundantly supplied with water, which rises to their roots through the pores of the soil. The disposition of wood to imbibe moisture in its pores has sometimes been made use of very effectually in getting out millstones. First a large block of stone is hewn into a cylindrical shape. Then grooves are cut into it all around where a separation is desired, and wooden wedges are driven tightly into them. These absorb moisture from the dews and rain, and therefore swell so much as to split the stone in the direction of the grooves. The blotter which you use furnishes an illustration of capillary attraction, the ink being taken up among the fibres of the paper. Ordinary writing paper will not answer as a blotter, because the sizing fills up the interstices between the fibres.

[CHAPTER V.]
GRAVITATION.

73. Attraction Between Masses.—I have thus far treated of attraction as existing between the atoms or particles of matter when they are brought very near together, which is called the attraction of cohesion. But it exists also between any portions of matter that are separate from each other. Thus if two cork balls be placed on the surface of water near to each other, their attraction will soon bring them together. To have the experiment striking, the balls must be varnished, that they may glide easily over the water. Bubbles of glass will exhibit the same attraction. So, also, floating pieces of wood are apt to be found together; and when a ship is wrecked, as soon as the sea becomes calm the parts of the wreck are in collections here and there. Now when a stone falls to the ground, it does so for precisely the same reason that the two cork balls come together in the water. The idea of all who have not been informed on such subjects is, that the stone comes to the ground because the ground is down and the stone is up, and there is nothing to support the stone in the air. They have no idea that some power makes the stone come down. There is such a power, and it is the attraction which the earth and the stone have for each other. If you hold the stone in your hand, and thus prevent its falling, you simply resist a power which is pulling it down. If you could in any way suspend the attraction of the earth and the stone for each other, you could let go of the stone, and it would remain just there in the air, and would not come down until the attraction is restored.

74. Attraction Mutual.—The cork balls move toward each other because their attraction is mutual. So do the earth and stone really move toward each other for the same reason. As the stone is drawn toward the earth, so is the earth drawn toward the stone. But the earth is so large a thing to be drawn that its motion is exceedingly small—so small that practically it may be considered as nothing.

75. Illustration.—This may be clearly illustrated if we compare the force of attraction to the force of muscular action. Suppose a man in a boat pulls on a rope which is made fast to a ship lying loose at the wharf, and in this way draws his boat toward it. He does not dream that he moves the ship at all; but he in reality does, for if instead of one boat a hundred or more pull upon the ship, they will move it so much as to make the motion apparent. In the case of the single boat, the ship as really moves as when a hundred boats are pulling on it, but it is only the one hundredth part as much. Now let the ship represent the earth, and the little boat some body, as a stone, attracted by it. The earth and the stone move toward each other, just as the ship and the boat do. And if, as we multiplied the number of boats, we should multiply the bulk of the stone till it is of an immense size, it would have by its attraction a perceptible influence upon the earth's motion.

Observe in regard to the illustration, that it makes no difference whether the man be in the boat or in the ship as he pulls. In either case he exerts an equal force on the ship and the boat, making them to approach each other. So it is with the attraction between the earth and the stone. It is a force exerted equally upon both. Its effect on the earth is not manifest, because it is so much larger than the stone; just as the effect of the man's pulling is not manifest upon the ship, because it is so much larger than the boat.

76. Proportion of the Mutual Motions of Attraction.—Let us pursue the illustration a little farther. If a man stand in a boat, and pull a rope made fast to another boat of the same size and weight, both boats, in coming together, will move over the same space. Just so it is with the attraction between two bodies having the same quantities of matter or equal weights—they attract each other equally, and therefore meet each other half way. Let now one boat be ten times as great and as heavy as the other. The small boat would move ten times as much as the large one when the man brings them together by pulling the rope. In like manner, if a body one-tenth as large as the earth should approach it, they would attract each other, but in coming together the body would move ten times as far as the earth would. In the case of falling bodies, even though they may be of great size, the earth moves so slightly to meet them that its motion is wholly imperceptible. It has been calculated that if a ball of earth the tenth part of a mile in diameter were placed at the distance of a tenth part of a mile from the earth, as the earth and this body would be moved by their attraction to meet each other, the motion of the earth would be only the eighty thousandth of a millionth ( 1 80,000,000,000 ) of an inch.

77. Attraction Universal.—The attraction of which I have been speaking exists between all bodies, however distant they may be from each other. Sun, earth, moon, and stars attract each other; and in obedience to this attraction they have a tendency to come together in one great mass, and would do so if another force acting in opposition to this did not prevent it. This force will be treated of in another part of this book.

78. The Tides.—One effect of the attraction between the earth and the moon is quite familiar. I refer to the tides. When the tide rises it is because the water of the ocean feels the attracting force of the moon. The moon actually lifts the water toward itself. The attraction of the sun sometimes increases and sometimes diminishes the tides, according to its position in relation to the moon and the earth. If the land were as movable as the water, or, in other words, if its particles were held together by no stronger attraction than those of water, there would be the same motion that there is in the ocean over the surface of the earth, as in its revolution successive portions of it present themselves toward the moon.

79. Meaning of the Word Gravitation.—The attraction thus existing between different bodies of matter separated from each other is called the attraction of gravitation or gravity, in distinction from the attraction of cohesion treated of in the previous chapter. This name was given to it because we have such common examples of its influence in the fall of bodies to the earth. They are said to gravitate toward the earth. And they are said to do so by the force or attraction of gravitation or gravity. The term terrestrial gravitation is sometimes used in speaking of the earth's attraction, in distinction from the same thing in operation in other planets.

Fig. 22.

Fig. 23.

80. Attraction Toward the Earth's Centre.—All bodies are attracted toward the centre of the earth. This is because the earth is globular, as may be made clear by Fig. 22. Let the circle represent the earth, and a a body attracted by it. The lines drawn from the body to the earth represent the attractive force exerted by the earth upon the body. It is obvious from these that there is as much attraction on the one side of the line drawn from the body to the earth's centre as there is on the other. The attractive force, then, of the earth as a whole is exerted upon the body in the direction of this middle line. It tends to draw it, therefore, toward the centre. If, therefore, a weight be suspended by a string, the line of the string continued would go to the centre of the earth. This being so, it is clear that two weights suspended by two strings do not hang perfectly parallel to each other. The difference is so slight in an ordinary pair of scales that it can not in any way be perceived. But if it were possible to suspend in the heavens a beam so long as to stretch over a large extent of the earth's circumference, as represented in Fig. 23, the scales attached to it would be very far from hanging parallel to each other. Substances suspended in different parts of the globe are hanging in different directions, and those which are hung up by our fellow-men on the opposite side of the earth, are hanging directly toward us.

Fig. 24.

81. Up and Down.—All falling bodies fall toward the centre of the earth, and the same remarks can be made on this point that I have made in relation to suspended weights. Up and down are merely relative terms—up being from the centre of the earth, and down toward it. As the earth moves round on its axis, the same line of direction which we call upward at one time is downward at another. This may be illustrated on Fig. 24. Let the circle represent the circumference of the earth. In the daily revolution we pass over this whole circle. If we are at D at noon, we are at E at six o'clock, and at F at midnight. If, therefore, the ball A be dropped from some height at noon, the line in which it falls will be at right angles to a line in which it will fall if you drop it from the same height at six o'clock; for this height will have moved in this time from A to B. If it be dropped from the same height at midnight its line of direction will be directly opposite to what it was twelve hours before; for the height will have moved in that time to C.

It is not always true that falling bodies tend exactly toward the centre of the earth. It is nothing in the centre that attracts them, but it is the substance of the whole earth; and as this is irregular in its density and form, the attraction will be irregular also. Thus it is found by accurate experimenting that a plumb line suspended in the neighborhood of a mountain is so attracted by it that it will not hang exactly parallel with another suspended at some distance from the mountain. The difference is not, however, enough in any case to have any practical bearing.

82. Weight.—I have said before (§ 52) that what we call weight is not a property of matter, but merely the result of a property, the attraction of gravity. This I will now illustrate. If two bodies are falling to the earth, and one of them contains ten times as many particles of matter as the other, ten times as much force of gravity is required, and is actually exerted, to bring it to the ground. This will appear plain to you if you bear in mind that a body does not come to the ground because there is nothing to keep it there, but because it is drawn down by the force of attraction, and then compare this force to any other force, as, for example, that of muscular action. If you draw toward you two weights, one of which is twenty times as heavy as the other, or, in other words, has twenty times the quantity of matter that the other has, you must exert twenty times as much strength on the former as you do on the latter. Just so it is with the force of attraction. The earth attracts or draws toward itself a body having twenty times the quantity of matter that another has with twenty times the amount of force. And the first body will have twenty times the weight of the other, for it will make twenty times the pressure upon any thing that resists the force with which the earth draws it toward itself. Weight, then, is the amount of the pressure occasioned by the attraction existing between the earth and the body weighed. If you place a substance in one side of a pair of scales, it goes down because of the attraction between it and the earth. By placing weights in the other side until the scales are balanced, you find how much is needed to counteract the downward pressure caused by the attraction of the substance and the earth for each other; or, in other words, you find out how much it weighs. In doing this you use certain standard weights; that is, certain quantities of matter which have been agreed upon by mankind, and are called by certain names, as pounds, ounces, etc. When a spring is used in weighing, the spring has been tried by these standard weights, and its scale has been marked accordingly.

83. Weight not Fixed, but Variable.—Weight does not depend alone upon the density of the body weighed, but also upon the density of the earth. For the attraction causing the pressure which we call weight is a mutual attraction, and is in proportion to the quantities of matter in both the body and the earth. If, therefore, the density of the earth were increased twice, three times, or four times, the weights of all bodies would be increased in the same proportion; that is, the force with which the earth would attract them would be twice, three times, or four times as great as now. This would not be perceived by any effect on balances, for the weights and the articles weighed would be alike increased in weight. But it would be perceived in instruments that indicate the weights of bodies by their influence on a spring. These would disagree with scales and steelyards just in proportion to the increase of the earth's density. It would be perceived also in the application of muscular and other forces in raising and sustaining weights. Every stone would require twice, three times, or four times the muscular effort to raise it that it does now.

84. Weight Varies with Distance.—The nearer two bodies are to each other the greater is their attraction. The nearer a body is to the earth the greater is the attraction that presses it toward the earth; in other words, the greater is its weight. The force of gravity, or weight, is greatest, therefore, just at the surface of the earth, and it diminishes as we go up from the earth. As we go from the earth, the force of gravity lessens in such a proportion that it is always inversely as the square of the distance from the centre of the earth. I will explain. If the distance from the centre of the earth to its surface, which is 4000 miles, be called 1, then 4000 miles from the earth would be called 2, or twice as far from the centre, and 8000 miles from the earth would be 3, and so on. The squares of these numbers would be 1, 4, 9, 16, etc. Now as weight lessens so as to be inversely as the square of the distance, a body weighing a pound on the surface of the earth would weigh but a quarter of a pound at the distance of 4000 miles, and but the ninth part of a pound at 8000 miles. A body weighs less on the summit of a high mountain than it would in the valley below, because it is farther away from the great bulk of the earth, and therefore is not so strongly attracted. The difference, however, is but small. A man weighing two hundred and fifty pounds in the valley would weigh but half a pound less if on the summit of a mountain four miles high.

85. Weight Every Where.—I have spoken of weight only in relation to the earth. But there is weight in bodies every where, for there is attraction wherever there is matter. The weight of substances on the surface of different heavenly bodies varies according to the quantities of matter in those bodies. As the moon is much smaller than the earth, what weighs a pound with us would weigh much less than a pound in the moon. And as the sun is much larger than the earth, what is a pound with us would be much more than a pound there. If we knew the exact densities of the sun and the moon and the earth, as well as their size, we could estimate exactly the difference in the weights which any body would have in them; for the attraction which causes the pressure that we call weight is as the quantity of matter, and the quantity of matter depends on both density and size.

86. Cohesion, Capillary Attraction, and Gravitation the Same.—The attraction of cohesion, capillary attraction, and gravitation are only different modes of action of the same power; viz., the attraction which matter every where has for matter. At first thought it would appear that there is something peculiar in the attraction of particles when they are brought together so as to adhere. For if we take any substance, a piece of glass, for example, its particles seem to be held together by an attraction vastly stronger than that attraction which inclines different bodies to move toward each other. If you break the glass, however closely you may press the two pieces together, they will not unite again. It would seem, at first view, that there must be some peculiar arrangement of the particles which is destroyed by breaking the glass. But we can readily account for the facts in another way. The attraction between bodies of matter is the greater the nearer we bring them together. The nearer, for example, is the moon to any portion of the earth, the greater is the attraction which it exerts, as seen in the tides; and if it were much nearer to the earth than it is, our tides would prove awfully destructive. What is true of masses is also true of the particles of which they are composed. Though their attraction is comparatively feeble when at a distance from each other, it increases, not in the arithmetical but the geometrical ratio (§ 84), as they come nearer together; so that when they are exceedingly near together the attraction is very powerful. It must be remembered in regard to the pieces of broken glass that you can not bring the particles on their surfaces as near as they were before the glass was broken, for the crack does not disappear. And as the attraction is inversely as the square of the distance, a little distance must make a great difference. The particles of some substances you can bring so near together as to cause adhesion, as you saw in the case of the two bullets (§ 66). That their adhering together depends merely upon their particles being brought near to each other appears from the fact, that the smoother you make the surfaces the more strongly will they adhere. And the reason that liquids and semi-liquids adhere so readily to solid substances is, that their particles, moving freely among each other, have thus the power of arranging themselves very near to the particles of the solid. Thus, when a drop of water hangs to glass, all the particles of water in that part of the drop next to the glass touch, or rather are exceedingly near to, the particles of the glass.

87. Variety in the Results of Attraction.—It is one and the same force, then, which binds the particles of a pebble together, and makes it fall to the ground—which "moulds the tear" and "bids it trickle from its source"—which gives the earth and all the heavenly bodies their globular shape, and, in connection with another power hereafter to be noticed, makes them revolve in their orbits. How sublime the thought that this one simple principle that gives form to a drop extends its influence through the immensity of space, and so marshals "the host of heaven" that, without the least interruption or discord, they all hold on their course from year to year and from age to age! It is thus that Omnipotence makes the simplest means to produce the grandest and most multiform results.

88. Opposition Between the Modes of Attraction.—Although cohesion and gravitation are essentially the same thing, we see them continually acting in opposition to each other. Abundant illustrations might be given, but, I will cite only a few.

Fig. 25. Fig. 26.

89. Why Pitchers have Lips.—If you pour water out of a tumbler there is a struggle between the attraction of cohesion and gravitation for the mastery—the attraction of cohesion tending to make the water adhere to the tumbler, and run down its side, as in Fig. 25, and gravitation tending to make it fall straight down. But when water is poured out of a pitcher, as in Fig. 26, the lip of the pitcher acts in favor of the attraction of gravity; for the water would have to turn a very sharp corner to run down the outside of the pitcher in obedience to the attraction of cohesion. In pouring water from a tumbler, we can often, by a quick movement, throw the water, as we may say, into the hands of gravity before the attraction of cohesion can get a chance to turn it down the tumbler's side. If you can only make the water begin to run from the tumbler without going down its side there will be no difficulty; for there is an attraction of cohesion between the particles of the water, tending to make them keep together, which in this case acts against the cohesion between the water and the glass, and therefore acts in favor of gravitation. It is cohesion that forms the drop on the lip of a vial as we drop medicine—cohesion between the particles of the liquid, and cohesion between these particles and those of the glass. It is gravitation, on the other hand, that makes the drop fall, it becoming so large that the force of gravity overcomes the cohesion between the drop and the vial.

90. Size Limited by Gravity.—Were it not for the attraction of gravity there would be no limit to the size of drops of any liquid. When the drop reaches a certain size, it falls because it is so heavy; or, in other words, because with its slight cohesion the attraction of the earth brings it down. Now if this attraction could be suspended, and the attraction of cohesion left to act alone, particles of water might be added to the drop to any extent, and they would cling there. You can see the struggling between cohesion and gravitation very prettily illustrated if you watch the drops of rain on a window-pane. If two drops happen to be quite near together they unite by attraction, and then, being too large to allow of its being retained there by cohesion in opposition to gravitation, the united drop runs down. If it meet with no other drop it soon stops, because by cohesion some portion of it clings to the glass all along its track, and so at length lessens it sufficiently to allow it to remain suspended again. It is from the influence of the attraction of gravitation that different kinds of liquids furnish drops of different sizes, the heavier giving small, and the lighter large ones. Thus you can drop from a vial a larger drop of alcohol than of water, and a larger one of water than of nitric acid. You have another illustration of a similar character in the adhesion of chalk to a black-board or any surface. The chalk crayon itself can not adhere, for the attraction of the earth does not permit it. But small quantities of it can adhere for the same reason that water adheres to surfaces in small quantities. So also dust clings to sides of furniture, though a lump of dirt would not.

Fig. 28.

Fig. 27.

91. Illustrated in Solid Bodies.—We can illustrate the limitation of size in solid masses by Figs. 27 and 28. Suppose that a and b, Fig. 27, are two projections of timber from a post, b being twice as large as a. It is evident that b can not support twice as much weight as a, for gravitation is dragging it downward from its attachment to the upright post with twice the force that it does a. The case is still stronger when, as represented in Fig. 28 (p. 64), the larger timber is twice as long as the smaller. Here d has four times the bulk of c. But it can not support four times as much weight at its end, not only because its own weight presses it downward, but because half of its weight is at a greater distance from the place of attachment than the smaller beam is. Gravitation here operates in opposition to cohesion in such a way that the projecting timber, if carried to a certain size, will fall by its own weight, either breaking in two, or tearing away from its attachment. This tendency is very commonly resisted in buildings and other structures by braces, as represented in Fig. 29. Here the weight of the horizontal timber at some distance on each side of a, is made to press upon the upright post instead of directly downward.

Fig. 29.

92. Farther Illustrations.—The size of bodies, both animate and inanimate, is limited by the Creator in obedience to the principles above developed. This is seen in the fact that there are no animals on the land to compare in size with the monsters of the deep. A whale does very well in the water, because he is buoyed up by that element; but an animal as large as a whale could not well exist on land, because gravitation would act so strongly in opposition to cohesion. At least it would be necessary, in order that he might walk about, or even hold together, that his great bulk should be made up of very firm and tenacious materials. Whenever any thing very large or tall is to be supported, its support is always broad and composed of very cohesive materials. We see this exemplified in the massive trunks of full-grown trees, as compared with the slender trunks of trees of the same kinds in the nursery. We see it exemplified in the fact that the highest mountains are built of the hardest rocks, while the soft chalk formations are confined to those of small size. There is a limit to the height even of the granite mountains in the influence of gravity. If carried much higher than they are, the attraction of the earth, in its opposition to cohesion, would tear them apart in their fissures, or cause the immense weight to crush their foundations. In the moon, where gravitation is less than on the earth (§ 85), mountains can be much higher without these results, and accordingly the telescope shows them to be so. In Jupiter, on the other hand, which is much larger than the earth, the mountains, if there be any, can not rise to any great height, and if there be any living beings as large as we are in that planet they must be made of vastly firmer materials to prevent their being crushed by their own weight.

93. The Above Principles Transgressed by Man.—Man often transgresses these principles in his structures. For example, a building settles because the foundation is not strong enough to bear the superincumbent weight; in other words, the force of gravitation is not sufficiently taken into the account. When a very tall building is erected, the lower portions ought to be made of very cohesive substances. Firm granite is therefore an appropriate material for the lower story of tall brick buildings. At least should the walls of the lower stories of such buildings be made thicker than they ordinarily are, to resist properly the force of gravitation in the weight above. Stores intended to bear much weight on their floors are often built without due regard to the cohesive force required to sustain the weight. Long timbers are sometimes supported only at the ends, when their own weight, to say nothing of what may be brought to press upon them, requires that they should be supported at other points. While in modern buildings the timbers are often too small, in some old buildings the upper timbers are so heavy as to lessen rather than increase the strength of the structure. Especially is this true of the unsightly beams which in some ancient houses we see extending along the ceilings above. Many other examples could be given, but these will suffice.

The practiced eye, in looking at a building, instinctively requires that every part should be seen to be suitably supported. A gallery in a church, therefore, if without pillars or braces from the wall, is displeasing to such an eye, even though there may be really sufficient support provided in the mode of structure. The same can be said of galleries supported by slender iron pillars, especially if they be painted of some light color so as to look as if they were wood rather than iron. For the same reason porticoes without pillars are unsightly. So, too, the eye instinctively looks for a sufficient base to every pillar and pilaster. The concealment of the base in any way, or the substitution of any thing else for it, is an unpleasant anomaly, and yet such anomalies are sometimes seen even in expensive buildings.

94. Attraction of Natural Philosophy and of Chemistry.—The attraction of which I have treated in this and the previous chapters is that which belongs to Natural Philosophy, in distinction from that of Chemistry. Its effects are only mechanical, while the attraction of Chemistry goes beyond this, and affects the composition of substances. For example, the attraction between the two gases, oxygen and hydrogen, which makes them unite to form water, belongs to Chemistry; while that which makes the particles of water cohere is in the province of Natural Philosophy.

[CHAPTER VI.]
CENTRE OF GRAVITY.

Fig. 30.

Fig. 31. Fig. 32.

95. Centre of Gravity Illustrated.—If you balance a ruler on your finger, as in Fig. 30, it is balanced because there is just as much weight on one side as on the other. Now just over your finger, in the middle of the ruler, there is a point that we call the centre of gravity; or, in other words, the centre of the weight of the ruler. This point is indicated in the figure. There is as much of the weight of the ruler on the one side of this point as on the other, and also as much above it as below it. If your finger should be a little to the one side or the other of this point, the ruler would not be balanced, and would fall. When balanced it does not fall, simply because this central point is supported by being directly over the end of the finger. The weight of the ruler, then, may be considered practically as all being in that point, for it is there that is exerted all the downward pressure of the ruler as it is balanced. So, also, when the ruler is balanced on the finger, as represented in Fig. 31 (p. 68), this same centre of gravity is directly over the point of the finger, and is therefore supported. If it be to the one side or the other, as in Fig. 32, it is not supported, and the ruler therefore falls. You see, then, that if a body be balanced, the centre of gravity is directly over the point of support. If, on the other hand, a body is suspended, the centre of gravity is directly under the point of support.

Fig. 33.

96. Definition.—If a plumb-line from the centre of gravity could be extended into the earth it would go directly to its centre. The body may be considered as making all its pressure from its centre of gravity in that direction, in obedience to the attraction of gravitation. The best definition, then, that we can give of the centre of gravity is, that it is that point in a body from which its pressure as a whole toward the centre of the earth proceeds. It is that point, therefore, the support of which insures the support of the whole body. And in speaking of the weight of a body, or its downward pressure, we may consider all the matter composing it as collected or concentrated in that point. The body, therefore, can be balanced in any position in which this point is supported, as shown in Figs. 30 and 31. And when a body is suspended, it is at rest only when the centre of gravity is directly under the point of support. Thus, if you have a circular plate suspended at E, Fig. 33, it will not be at rest when it is moved to the one side or the other, as represented by the dotted lines, but only when the centre of gravity, c, is directly under the point E.

Fig. 34. Fig. 35.

97. How to Find the Centre of Gravity of a Body.—If you take a piece of board, and suspend it at A, Fig. 34, and suspend a plumb-line from the same point, the centre must be somewhere in that line. But exactly at what point it is you do not know. How will you ascertain this? Mark the line A B on the board, and suspend the board by another point, as in Fig. 35. As the centre of gravity must be somewhere in the plumb-line as it now hangs, of course it is at O, where the two lines cross.

Fig. 36. Fig. 37.

98. Scales and Steelyards.—When two bodies are connected by a rod or bar, the centre of gravity of the whole is somewhere in the connection. If the two bodies be equal in weight, as in Fig. 36, the centre of gravity is exactly in the middle of the rod, as marked. But if the bodies are unequal, as in Fig. 37, the centre of gravity is nearer to the larger body than to the smaller. In balancing a body in one scale with weights in another, you have a case parallel to that of Fig. 36. The centre of gravity of the body weighed, the weights, and the scales, as a whole, is midway between the scales, at the point of support. In the steelyard you have the heavy body to be weighed nearer the centre of gravity than the small weight is on the long arm, and so the case is parallel with that of Fig. 37.

Fig. 38.

99. The Centre of Gravity of a Body not Always in the Body Itself.—The centre of gravity of a hollow ball of uniform thickness is not in the substance of the ball, but it is in the centre of the space in the ball, for the line of the ball's downward pressure would be from that point. If the ball had a frame-work in it, as represented in Fig. 38, the centre of gravity would obviously be at A, the centre of this frame-work. But if there were no frame-work, and perpendicular lines were supposed to be drawn from different points of suspension, C, B, D, and E, these would intersect at the point A, showing that this is the centre of gravity, according to the rule for finding it given in § 97. So, also, the centre of gravity of an empty box, or an empty ship, would be an imaginary point in the space inside. In a hoop it is the centre of the hoop's circle.

Fig. 39.

100. The Centre of Gravity Seeks the Lowest Point.—The centre of gravity always takes the lowest place which the support of the body will allow. In a suspended body, therefore, it is always directly under the point of suspension. To get to the one side or the other of this position it must rise. This the attraction of gravity forbids, and if by any force it is made to rise, this attraction at once brings it back. This is manifest in the case of a suspended ball, Fig. 39. If the ball be moved to b, it will, on being let go, return to its first position, simply because its centre of gravity, in obedience to the earth's attraction, seeks the lowest place possible. From inertia (§ 49) it moves beyond this point, and continues to vibrate back and forth for some time; but when its motion is stopped, it hangs perpendicularly; that is, in such a way that its centre of gravity shall have the lowest possible position. I add a few other illustrations of the same point. When a rocking-horse is at rest, its centre of gravity is directly over the point at which it touches the floor, for thus it has its lowest possible place. If it be rocked, the centre of gravity is moved to a higher point, and for this reason it rocks back again. The same is seen in the swing, the cradle, the rocking-chair, etc. Most interesting illustrations of the same thing are found in the Laggan, or Loggan Stones as they are called, several of which are seen on the rugged parts of the British coast. An immense rock, which has been loosened by some convulsion, rests with a slightly-rounded base on another rock which is flat, and it is so nicely balanced that one person alone can produce a perceptible rocking motion in it. I saw, many years ago, a large rock near Salem, Massachusetts, thus situated. There is one also in Great Barrington, Massachusetts.

Fig. 40. Fig. 41.

Fig. 42.

Fig. 43.

101. Farther Illustrations.—It is because the centre of gravity always seeks the lowest place that an egg lies upon its side. When on its side, the centre of gravity is at its lowest point, as is manifest from comparing Fig. 40 with Fig. 41 (p. 71). Children often have a toy, called a witch, which illustrates the same thing in another way. It is a piece of light substance, as pith, with a shot fastened in one end. It always stands up on its loaded end, and can not be made to lie down on its side, because the centre of gravity would not then be at the lowest point. There is an amusing Chinese toy of the same kind. It is a figure of a fat old woman, Fig. 42, loaded with lead at the bottom, so that its centre of gravity is at a. If the figure be thrust over to one side, as shown by the dotted lines, the centre of gravity is raised, and the upright position is at once resumed. If the toy were not loaded, it would lie in the position represented in Fig. 43, just as the egg lies on its side.

Fig. 44.

Fig. 47.

102. Curious Experiments.—You can not hang a pail of water on a stick laid upon a table, as represented in Fig. 44, for the centre of gravity is not supported. But if you place another stick a as a brace, in the manner represented in Fig. 45 (p. 73), so as to push the pail under the table, it will hang securely, because the centre of gravity is now under the point of suspension. The explanation of the following experiment is the same: Run a large needle through a cork; fasten to the cork a fork, and you can suspend the whole on the edge of a table, as seen in Fig. 46. Here the centre of gravity is directly under the point of suspension, which is at the point of the needle. The same can be said of the very common toy represented in Fig. 47. The horse, made of very light material, stands securely, because the centre of gravity of the whole is in the heavy ball, which is under the point of suspension. If the horse be made to rock back and forth, the centre of gravity in the ball moves in a curved line, as in the case of a ball suspended by a string (Fig. 39). It is at its lowest place only when the horse is at rest. The hanging of a cane with a hook-shaped handle on the edge of a table is to be explained in the same way.

Fig. 45. Fig. 46.

Fig. 48. Fig. 49. Fig. 50.

Fig. 51.

Fig. 52.

103. Stability of Bodies.—The firmness with which a body stands depends upon two circumstances—the height of its centre of gravity, and the extent of its base. The lower the centre of gravity, and the broader the base, the firmer does the body stand. A cube, represented in Fig. 48, is more stable, that is, less easily turned over, than a body shaped as Fig. 49, because it has a larger base. The contrast is still greater between Figs. 48 and 50. The reason of the stability of a body with a broad base is found in the fact, that in turning it over the centre of gravity must be raised more than in turning over one of a narrower base. The curved lines indicate the paths of the centres of gravity as the bodies are turned over. In the case of a perfectly round ball, the base is a mere point, and therefore the least touch turns it over. Its centre of gravity does not rise at all, but moves in a horizontal line, as shown in Fig. 51. The pyramid is the firmest structure in the world, because it possesses in the highest degree the two elements—a broad base, and a low position of the centre of gravity. On both these accounts the centre of gravity must ascend considerably when the body is turned over, as seen in Fig. 52.

Fig. 53.

Fig. 54.

104. Bodies not Upright Unstable.—When a body does not stand upright, its stability is diminished simply because only a portion of the base is concerned in its support. In Fig. 53 the base is broad, but the body is so far from being upright that the centre of gravity bears upon the very extremity of the base on one side, as the perpendicular line from it indicates. The least jostle will turn it over, because the centre of gravity need not ascend the least when this is done. You see, then, that the less upright a body is, the less of the base is of service in its support, because the farther is the line of direction of the downward pressure of the centre of gravity from the centre of the base. The famous tower of Pisa, Fig. 54, one hundred and thirty feet high, overhangs its base fifteen feet. It was undoubtedly built intentionally in this way to excite wonder and surprise, for what would otherwise have been a very unsafe structure is rendered stable and safe by the arrangement of its materials. Its lower portion is built of very dense rock, the middle of brick, and the upper of a very light porous stone. In this way the centre of gravity of the whole structure is made to have a very low position.

Fig. 55.

105. Familiar Illustrations.—You see now the explanation of the fact which common experience teaches every one, that the taller is a body, and the narrower its base, the more easily is it overturned. This is exemplified in the two loads, Fig. 55. The base is the space included by the wheels. The centre of gravity is so high in the tall load that a perpendicular line drawn from it falls outside of the base if the cart come upon a considerable lateral inclination of the road. But the smaller load, under the same circumstances, is perfectly secure from overturning. A high carriage is more easily overturned than a low one, for the same reason. A stage, if loaded on its top, is very unsafe on a rough road. Stability is given to articles of furniture by making their bases broad and heavy, as you see in tables supported by a central pillar, candlesticks, lamps, etc. The tall chairs in which children sit at table would be very insecure if their legs were not widely separated at the bottom, thus widening the base of support. In the ladder, so commonly used now in picking fruit, a broad base is furnished between the foot of the ladder and the two standards which are spread out to sustain its top.

Fig. 56. Fig. 57.

106. Support of the Centre of Gravity in Animals.—The base of support which quadrupeds have, viz., the space included between their four feet, is quite large; and this is one reason that they walk so soon after birth. A child does well that can walk at the end of ten or twelve months, for the supporting base is quite small compared with that of a quadruped. It consists of the feet and the space between them. It requires skill, therefore, in the child to manage the centre of gravity in standing and walking, and this is gradually acquired. If one should grow up without ever standing on his feet, he would find, as the infant does, that some training is necessary to enable him to do it. It is on account of the smallness of the base furnished by the feet that the statue of a man is always made with a large base or pedestal. Although we exert considerable skill in walking, by no means so much is requisite as the Chinese ladies must put in exercise with their small feet. Still more skill is exercised by one who has two wooden legs, or one who walks on stilts. The base made by the feet can be varied much by their position. If the toes be turned out and the heels brought near to each other, the base will not be as large as when the feet are straight forward and far apart, as is manifest in Figs. 56 and 57. It is for this reason that the child, in his first essays at standing and walking, instinctively manages his feet as in Fig. 56.

Fig. 58.

Fig. 59.