SOUTHERN BRANCH, UNIVERSITY OF CALIFORNIA, LIBRARY, JLOS ANGELES, CAUF. FIRST PRINCIPLES NATURAL PHILOSOPHY. A TEXT-BOOK FOR COMMON SCHOOLS ELROY M. AVERY, PH.D., AUTHOR OF A SERIES OF TEXT-BOOKS ON PHYSICAL SCIENCE. NEW YORK . CINCINNATI -.- CHICAGO AMERICAN BOOK COMPANY 67361 DR. AVERY'S PHYSICAL SCIENCE SERIES. I St. FIRST PRINCIPLES OF NATURAL PHILOSOPHY THE ELEMENTS OF NATURAL PHILOSOPHY. 3d- THE ELEMENTS OF CHEMISTRY. 4 th. THE COMPLETE CHEMISTRY. Containing the ELEMENTS OF CHEMISTRY, with an additional chapter or Hydrocarbons in Series or ORGANIC CHEMISTRY. It can be used in the sair clss with THE ELEMENTS OF CHEMISTRY.. CajyH&f, 1884,'"$ 'Sheldon & Co. A. THIS book is the result of an attempt to meet the wants of schools which cannot give the time required for the completion of the author's Elements of Natural Philosophy. No effort has been spared by author or publishers' to make it worthy of the place it is intended to fill. Especial care has been taken to provide simple, teach- ing experiments which do not require expensive ap- paratus. The latest developments of science, such as the intro- duction and use of electrical units, have been freely utilized ; the mere polemics of physics has been ignored. Any teacher or pupil using this book and finding any error therein is requested to communicate the same to the publishers or author. If, in its study, any person encounters difficulties not easily removable by other means, he may feel free to write to the author (in care of the publishers) for fur- ther information. Such letters should be accompanied \ with addressed envelopes for replies. CHAPTER MATTER. SBC. PACK I. MASSES, MOLECULES AND ATOMS.., 1 II. PHYSICAL PROPERTIES OF MATTER 10 III. THREE CONDITIONS OF MATTER... . 20 CHAPTER II. MOTION AND FORCE DYNAMICS GRAVITATION ENERGY. I. MOTION AND FORCE 26 II. GRAVITATION 36 III. FALLING BODIES 45 IV. PENDULUM 51 V. ENERGY... .... 67 CHAPTER III. SIMPLE MACHINES. I. PRINCIPLES OF MACHINERY LEVER 67 II. WHEEL AND AXLE PULLEY 78 III. INCLINED PLANE WEDGE SCREW 80 vi CONTENTS. CHAPTER IV. LIQUIDS. SBC PAGE I. LIQUID PRESSURE 96 II. EQUILIBRIUM BUOYANCY 107 III. SPECIFIC GRAVITY 113 IV. HYDROKINETICS 117 CHAPTER V. PNEUMATICS. I. ATMOSPHERE ATMOSPHERIC PRESSURE 122 II. PUMPS SIPHON 129 CHAPTER VI. ELECTRICITY AND MAGNETISM. I. GENERAL VIEW 140 I!. KRICTIONAL ELECTRICITY 151 III. VOLTAIC AND THERMO-ELECTRICITY 180 V. MAGNETISM 207 V. INDUCED ELECTRICITY 227 CHAPTER VII. SOUND. I. NATURE, REFRACTION AND REFLECTION OF SOUND... 245 ,'l. TELEPHONE COMPOSITION AND ANALYSIS OF SOUNDS 259 CHAPTER VIII. HEAT. I. TEMPERATURE THERMOMETERS EXPANSION 281 II. LIQUEFACTION AND VAPORIZATION .. 289 CONTENTS. Yii SEC. PACK III. LATENT AND SPECIFIC HEAT 397 IV. MODES OF DIFFUSING HEAT 308 V. THERMODYNAMICS 817 CHAPTER IX. LIGHT. I. NATURE, VELOCITY AND INTENSITY OF LIGHT 328 II. REFLECTION OF LIGHT 336 III. REFRACTION OF LIGHT 347 IV. CHROMATICS AND SPECTRA 365 V. OPTICAL INSTRUMENTS 373 CONCLUSION ENERGY 382 APPENDIX 887 INDEX... .. 393 CHAPTER !* MATTER. SECTION I . MASSES, MOLECULES AND ATOMS. " Read Nature in the language of experiment." Experiment I. Place a cork on the surface of water. Invert a glass tumbler or jar over the cork and push the glass down into the water. Notice that the water cannot enter far into the vessel, although it enters far enough to show that its tendency is thus to enter, but that something prevents it. The air in the glass prevents the water from entering. Air and water cannot be in the same place at the same time. Experiment 2. Try to get your two hands in the same place at the same time. Repeat the attempt with two books and other articles, until you are sure that no two things that you can handle can be in the same place at the same time and that every one of them takes up room. I. What is Matter ? Matter is anything that occupies space or "takes up room." Mind, truth and hope do not take up room and so we know that they are not forms of matter. The earth, the air, a table, an orange, a slate or a raindrop does take up room and so we know that each is a kind or form of matter. % NATURAL PHILOSOPHY. 2 2. Divisions of Matter. Matter exists in atoms, molecules and masses. It is very important that we clearly understand what these words mean or we shall have trouble in trying to understand much that is to follow. 3. What is an Atom? An atom is the small- est quantity of matter that can enter into com- bination and thus form molecules and masses. In nearly every case an atom is a part of a molecule. (a.) We may say that atoms are the smallest particles of matter that can exist. They seldom exist alone, but quickly unite with others like themselves to form elementary molecules, or with others unlike themselves to form compound molecules. For example, one atom of oxygen combines with another like itself to form an ele- mentary molecule of oxygen, while one atom of oxygen combines with two of hydrogen to form a compound molecule of water. There are sixty-six kinds of atoms now known. 4. What is a Molecule ? A molecule is a quantity of matter so small that it cannot be divided without changing its nature. The word molecule means "a little mass," but in Natural Philosophy we must be very careful to use the word with accuracy that is, in accordance with the above definition. (a.) A molecule is so very small that a total of 8,000,000,000 molecules of water is barely visible in the best of modern micro- scoi>es. ll a drop of water could be magnified until it appeared to Vie as large as the earth on which we live, each molecule in the drop thus magnified would still look sma'ler than a base-ball. But while the pupil may thus get the idea that a molecu.e is very small, he most not think that, like the fairies, it exists only in 5 MASSES. 3 fancy. Molecules are as real as base-balls and much more numer- ous. If a molecule, by any means, be divided, its nature will be changed. A molecule of water may be changed into two atoms of hydrogen and one of oxygen. (6.) In an elementary molecule the atoms are alike ; in a com- pound molecule the atoms are of two or more kinds. Some com- pound molecules are very complex. The common sugar molecule contains forty-five atoms of three kinds. Elementary molecules make elementary masses or substances. Compound molecules make compound masses or substances. The nature of the mole- cule determines the nature of the substance. 5. What is a Mass ? A mass is any quan- tity of matter that is composed of jnolecules. If a quantity of matter is large enough to be seen, even with a powerful microscope, you may know that it is a mass. If it may be divided without changing the nature of the substance, you may be sure that it is a mass. Masses are elementary or compound. An elementary mass is called an element. There are as many elements as there are kinds of atoms. Compound masses or sub- stances are innumerable. (a.) We may take a lump of salt, which is a mass, and break it into many pieces ; each piece will be a mass. We may take one of these pieces and crush it to finest powder ; each grain will still be a mass. We may imagine one of these grains of powdered salt to be divided into so many parts that any further division will change them from salt to something else. These particles of salt, which are so small that further division would change their nature, are too small to be called masses ; they are molecules. If one of these molecules be divided, it will cease to be salt. Instead of salt, we shall have an atom of chlorine gas and an atom of the metal sodium. Two molecules would make a mass, but the mass would be so small as to be invisible, except in imagination. 4 XATtrRAL PBILOSOP&Y. 6 Experiment 3. Heat the mercury in the bulb of a common thermometer. The bulb remains full, but the liquid rises in the tube. There seems to be more mercury than there was before How can this be ? There must be a greater number of molecules, the molecules must be larger or they must be farther apart. Experiment 4. Heat some water in a glass tube without boiling it. Bubbles arise and attach themselves to the sides of the tube. These are bubbles of air. This air came from the water. The air particles were either in the same place with the water mole- cules, at the same time, or were in otherwise vacant spaces between the water molecules. Can you believe that they were in the same place at the same time ? Experiment 5. Carefully pour alcohol into a test tube about three quarters full of water until the tube is full. Close the mouth of the tube with thumb or finger and shake the liquids until they are mixed. Notice that the tube is not full now. Some of the molecules have been destroyed or dropped into the spaces between other molecules. Can you believe that any of the molecules were destroyed ? Experiment 6. Make a common goose quill pop-gun. Notice that when you use it the air confined between the two wads is compressed or made to occupy about half its original space. The air particles were reduced in size or in number, or were crowded together more closely. Perhaps the matter of which a body is made does not actually fill all the space which the body seems to occupy. 6. Continuity of Matter. When you look at a brick wall from a considerable distance, it has an appar- ent uniformity of structure; you cannot see that it is made of many bricks, separated by mortar-filled spaces. This is the fault of your sense of vision and not the fault of the wall. As yon come nearer, you see whaf 7 FORMS OF ATTRACTION. 5 you did not see before the individual and separated bricks. But such is the structure of the wall whether you can see it or not. Rub the smooth handle of a fine awl over a piece of fine wire gauze and the gauze seems to present a con- tinuous surface. It is the fault of the instrument in your hand and not the fault of the gauze. Rub the point of the awl over the gauze and you soon find open- ings between the metal threads. But the openings are there whether you can feel them or not. Rub the point of a fine sewing needle over the surface of a window pane. The glass seems to be continuous in its structure and the needle cannot get through. It ia the fault of the instrument you are using. Try one more delicate. Let a ray of light fall upon the glass and it easily finds a passage way between the solid mole- cules. Rays of light are often used by scientific men as instruments for their work. There are spaces between the molecules of every form of matter, whether you can detect them by any of your physical senses or not. 7. Forms of Attraction. Each of these three divi- sions of matter has its form of attraction. The attraction of masses is called gravitation. The attraction of molecules is called cohesion or adhesion. The attraction of atoms is called chemical affinity. 6 NATURAL PHILOSOPHY. 8 8. Forms of Motion. It is probable that each of these three divisions of matter has its own form or mode of motion. The motion of a mass is often called mechan- ical motion. The motion of a hammer or of a flying bullet is an example. The motion of the molecules in a mass consti- tutes heat or light or, probably, electricity or magnetism. If a flying bullet strike the target, we may imagine that the shock which destroys its mechanical mo- tion, produces or increases the vibration of the molecules among themselves. We ought to have little difficulty in imagining these little molecules rapidly swinging to and fro within the mass. These molecular vibrations consti- tute heat. We know that when a bullet is stopped by the target, the bullet is heated and the production of heat is to be explained only in this way. The motion of atoms within the molecule is probable, but has not yet been proved. (a.) Fancy a million flies surrounded by an imaginary shell. If each fly be allowed to represent a molecule, the contents of the shell will represent a mass, because it is composed of many molecules. We may imagine this shell to be thrown through the air. The motion of the shell would represent mass or mechanical motion. As the shell is moving through the air, the flies are moving slowly among themselves within the shell. This motion of the flies represents molecular motion and is a very different thing from the motion of the shell. When the shell strikes the ground, the mechanical motion is destroyed but the molecular motion is increased, for the flies are set in much more rapid motion by the shock. This is just about what happens when the bullet is fired against the target. 10 CONSTITUTION OF MATTER. 7 (6.) These motions of atoms, molecules and masses give to mattet the power of doing work, the scientific name of which power is Energy. (c.) Try to get a clear mental picture of theso molecules, each separated from its neighbor by a distance much greater than its own diameter. If a mass could be magnified until its molecules were as large as worlds, the spaces between the molecules would be as great as the spaces between the planets. Try to imagine a creature small enough to live on a molecule as we live on the earth. Imagine this creature looking at the neighboring molecules as we look upon the stars at nigrht. Remember that each molecule is in motion back and forth among its neighbors, sometimes strik- ing them, perhaps, and rebounding from them. When we heat the mass, we make each molecule move faster, strike harder blows and push its neighbors further away, thus producing expansion of the mass. 9. The Constitution of Matter. Every body of matter is made of a vast number of minute par- ticles called molecules, no two of which are in contact. Every molecule is ceaselessly vibrating to and fro among its neighbors, often hitting them and rebounding from them. 10. Physical Science. Physical science com- prises Physics and Chemistry. Physics deals with masses and molecules; chemistry, with atoms and combinations of atoms. Experiment 7. Bring a piece of warm sealing wax near some small bits of paper, but without touching them. Notice that the paper bits do not move. Briskly rub the sealing wax with a piece of warm flannel and quickly bring it near the paper bits as before. Notice that the sealing wax now produces very peculiar motions in 8 NATURAL PHILOSOPHY. II the paper bits. Notice also that you have wrought an important change in the sealing wax, but that still the stick is sealing wax. Experiment 8. Rub a brass button on the floor or carpet until it is uncomfortably warm. Notice that while you have produced a change in the button, it is still brass; you did not change the substance of which it is made. 11. What is a Physical Change?.^ physical change is one that does not change the nature of the molecule. (a.) A piece of marble may be ground to powder, but each grain is marble still. Ice may change to water and water to steam, yet the identity of the substance is unchanged. A piece of glass may be electrified and a piece of iron magnetized, but they still remain glass and iron. These changes alter the distances and arrange- ment of the molecules, but leave the molecules themselves un- changed ; they are physical changes. A change like the rusting of iron or the burning of wood, changes the nature of the molecule itself, and, consequently, of the substance. Stick a change as this is called a chemical change. 12. What is a Property of Matter? Any quality of matter is called a property of matter. Lead is heavy ; heaviness is a property of lead. It may be manifested without changing the lead to any- thing else, and is, therefore, called a physical property. Sulphur or brimstone is brittle. The brimstone may axhibit this property and still remain brimstone. Hence, brittleness is a physical property of brimstone. Sulphur is also combustible it may be burned. But this property of sulphur (combustibility) can not be shown without changing the sulphur to something else. Such are called chemical properties of mqtter* 14 DEFINITION. 9 13. Definition. Physics, or Natural Philosophy, is the branch of science that treats of the laws and physical properties of matter, and of those phenomena that depend upon physical changes. 14. Recapitulation. To be reproduced and ampli- fied by the pupil for review. 2 w S > i 10 NATURAL PHILOSOPHY. 15 SECTION II. THE PHYSICAL PROPERTIES OF MATTER. 15. Division of Physical Properties. The phys- ical properties of matter are divided into two classes, universal and characteristic. 16. What are Universal Properties ? Uni- versal properties of matter are such as belong to matter of every kind. 17. List of Universal Properties. The prin- cipal universal properties of matter are extension, impenetrability, weight, indestructibility, inertia, divisibility, porosity, compressibility, expansi- bility and elasticity. 18. What is Extension ? Extension is that property of matter by virtue of which it takes up room. It is involved in the definition of matter given in 1. (a.) In this country and in England, the foot and yard are units commonly used at the present time. But in most other civilized countries of the world, the international or metric units are used. These units are almost universally adopted hy scientific men even in England and America. In this book, frequent use will be made of the terms meter, liter and gram, their multiples and divisions. The pupil should l8 THE PHYSICAL PROPERTIES OF MATTER. 11 familiarize himself with these units. He will find further informs tion concerning them in Appendix B. at the end of this volume. Experiment 9. Pass a bent tube and a funnel, as shown in Fig. 1, or a funxxel tube as shown in Fig. 2, through the cork of a bottle. Be sure that alt joints are air tight. The delivery tube is best marte of glass which may be bent when headed to redness in an alco- hol or gas flaine. Place the end of the delivery tube in a tumbler of water. Pour water through the funnel. As it runs into the bottle, air will be forced out and may be seen bubbling through the water in the tumbler. A bottle con- venient for this and other experiments may be prepared by per- forating the cover of a glass fruit jar, as shown in Fig. 2. The holes carry cork or caoutchouc stoppers, through which the tubes pass. Full directions for bending glass tubing, boring holes, etc., may be found in Appendix 4, of Chemistry. FIG. 1. JIMi FIG. 2. Experiment 10. Thrust a lamp chimney into water. The water will rise inside the chimney, entering at the lower end and pushing the air out at the top. Repeat the experiment, closing the upper end of the chimney with the hand (or use an inverted tumbler). The water can not rise as before because the vessel is filled with air that can not escape. Experiment II. Drive a nail into a piece of wood ; the particles of wood are either crowded more closely together to give room for the nail, or bume of them are driven 12 NATURAL PHILOSOPHY. 19 oat before it. Clearly, the iron and the wood are not in the same place at the same time. 19. What is Impenetrability ? Impenetra- bility is that property of matter by virtue of which two bodies can not be in the same place at the same time. 20. What is Weight ? Weight is the measure of mass attraction or gravity. (a.) The word gravity generally points out the tendency ot bodies, not supported, to fall to the ground. (6.) A body is heavy or light according to the amount of attrac- tion ; the greater the attraction, the greater the weight. (c.) If an apple be held in the hand, the attraction between the earth and the apple produces pressure upon the hand. This pres- sure is not the attraction, but it is the measure of it and is called weight. (d.) If the same apple were upon the moon, its weight would be the measure of the attraction between the apple and the moon. But as the moon has less matter than the earth, the attraction between the apple and the moon would be less than that between the apple and the earth and the weight would, consequently, be less. 21. What is Indestructibility? Indestructi- bility is that property of matter by virtue of which it can not be destroyed. (a) No human being can create or destroy a single atom of matter. Water evaporates and disappears only to be gathered in clouds and condense and fall as rain. Wood burns, but the ashes and smoke and the invisible gases formed, contain the identical 22 tNERTTA. 13 atoms of which the wood was composed. In a different form, the matter still exists and weighs as much as before it was burned. The experiment is difficult, but has been repeatedly performed. The universe contains the same atoms to-day that it did at th close of the creation not one more, not one less. Experiment 12. Upon the tip of the fore-finger of the left hand, place a common calling-card. Upon this card and directly over the finger, place a cent. With the nail of the middle finger of the right hand, let a sudden blow or " snap " be given to the card. A few trials will enable you to perform the experiment so as to drive tlu card away and leave the coin resting upon the finger. The card flies away on account of the force of the " snap." The cent remains on the finger because the blow is so quick that the card has no time to give any of its motion to the coin. Experiment 13. To n.ake the experiment still more interest ing, use a bullet instead of the cent and the open top of a bottle instead of the finger-tip. Keep trying until you succeed in drop ping the bullet into the bottle. The experiment illustrates the inertia of the bullet, the card being driven away before it lias opportunity to impart its motion to the bullet. 22. What is Inertia? Inertia is that prop- erty of matter by virtue of ichich it has a ten- dency ivhen at rest to remain at rest or when in motion to continue in motion. (a.) A ball cannot put itself in motion. When the ball is thrown through the air, it has no power to stop and it will not stop until Borne external force compels it to do so. This external force may 14 NATURAL PHILOSOPHY. 22 be the bat, the catcher, the resistance of the air or the force of gravity. It must be something outside the ball or the ball will move on forever. (&.) Illustrations of the inertia of matter are so numerous that there should be no difficulty in getting a clear idea of this property. The "running jump" and " dodging " of the playground, the fre- quent falls which result from jumping from cars in motion, the hackward motion of the passengers when a car is suddenly started and their forward motion when a car is suddenly stopped, the diffi- culty in starting a wagon and the comparative ease of keeping it in motion, etc., etc., may be used to illustrate this property of matter. Experiment 14. Strike a piece of loaf sugar or of brick with a hammer. The sugar or brick is separated into many parts. 23. What is Divisibility ? Divisibility is that property of matter by virtue of ivhich a body may be separated or divided into parts. (a) The divisibility of matter may be carried to such an extend as to excite our wonder and test the powers of imagination itself. It is said that the spider's web is made of threads so fine that enough of this thread to go around the earth would weigh but half a pound and that each thread is composed of six thousand filaments. A single inch of this thread with all its filaments ma;r be cut into thousands of distinct pieces and each piece of each filament be yet a mass of matter composed of molecules and atoms We may consider that the atom marks the limit of divisibility. Experiment 15. Fill a test tube with water. Slowly add sugar. A considerable quantity may be added without increasing the bulk of the liquid. Experiment 16. Take a test tube three quarters full of water and carefully add alcohol until it is filled. Close the tube with the thumb and shake. Notice that the tube is no longer full. We know that the water and the alcohol cannot be in the same plac 25 COMPRESSIBILITY, 15 at the same time and are forced to the conclusion that some of the water molecules have been received into the space between the alcohol molecules or vice versa. 24. What is Porosity ? Porosity is that prop- erty of matter by virtue of which spaces exist between the molecules. (a.) When iron is heated, the molecules are pushed further apart, the pores are enlarged and we Bay that the iron has expanded. If a piece of iron or lead be hammered, it will bo made smaller because the molecules are forced nearer together, thus reducing the size of the pores. (6. ) These pores are very large in comparison with the size of the molecules. As was said a few pages back, if a race of persons could be imagined small enough to live on a molecule as we live on our earth, we might fancy them looking across the space around it and seeing the nearest molecule as we look off into the sky and see the moon or stars. (c.) Cavities or cells, like those of bread and sponge, are some- times improperly spoken of as pores. 25. What is Compressibility? Compressibility is that property of mat- ter by virtue of u-hich a body may be reduced in size. Experiment 17. Invert a thin glass bottle over a plate of water, as shown in Fig. 4. The heat of the hand will expand the air in the bottle and some of it will escape in bubbles. If no bubbles appear, pour warm water over the bottle. 16 XATtfoAL PtitLOSOPXT. 26 26. What is Expansibility ? Expansibility is that property of matter by virtue of which a body may be increased in size. (a.) Compressibility and expansibility are the opposites of each other, resulting alike from porosity. Let each pupil prove, by experiment with an ordinary pop-gun or other apparatus, that air Is compressible and expansible. Experiment 18. Provide strips of rubber, whalebone, wood, iron, steel, brass, copper, zinc and lead. Stretch the piece of rubber and notice what takes place when the stretching force ceases to act. Bend each of the other strips and notice what takes place under similar circumstances. 27. What is Elasticity ? Elasticity is that property of matter by virtue of which bodies resume their original form or size ivhen that form or size has been changed by any external force. All bodies possess this property in some degree. Solid bodies have elasticity of form; liquids and gases have not. (a.) Different substances possess this property in different degrees. Solid bodies are not perfectly elastic. They may contract after being stretched or expand after being compressed but they do not return to exactly their former size. Liquids and gases are perfectly elastic. No matter how great the pressure that may be exerted upon them, they return to exactly their former size when the pressure is removed. The ordinary spring-balance owes its value to the elasticity of the spiral steel spring, which is stretched by the weight. 30 COHESION AND ADHEStON. I? 28. What are Characteristic Properties? Characteristic properties of matter are such as belong to matter of certain kinds only. They enable us to distinguish one substance from another. Glass is brittle, and by this single property may be distinguished from India-rubber. 29. List of Characteristic Properties. The characteristic properties of matter are numerous. They depend chiefly upon cohesion and adhesion. The most important are hardness, tenacity, brittleness, mal- leability and ductility. Experiment 19. Take a sheet of gold-leaf in your fingers and try to pick the metal off with the fingers of the other hand. Some of the gold will " stick to your fingers." 30. What are Cohesion and Adhesion? Co- hesion is the force that holds together like mole- cules ; adhesion is the force that holds together unlike molecules. (a.) Cohesion is the form of molecular attraction that holds most substances together and gives them form. If you pull on an iron wire and are unable to break it, you are to understand that the cohesion of the iron particles is stronger than you are. (6.) Adhesion is the form of molecular attraction that causes the penci/ or crayon to leave marks upon the paper or blackboard and makes paste, glue, mortar and cements " stick." (c.) In a brick wall, cohesion binds together the molecules of the mortar layer into a single, hardening mass ; adhesion reaches out and grasps the adjoining bricks and holds them fast a solid wall. Each acts only through distances too small to be measured. 18 NATURAL PHILOSOPHY. 31 Experiment 20. Get pieces of chalk, glass, iron, lead, copper and marble. Try to scratch each one with each of the others. Make a note of the result of each experiment, thus : Glass will scratch copper ; copper will not scratch glass. Deter- mine which is the hardest substance in your collection and which is the softest. 31. What is Hardness? Hardness is that property of matter by virtue of which some substances resist any attempt to force a passage between their particles. It is measured by the degree of difficulty with which one substance is scratched by another. Fluids are not said to have hardness. 32. What is Tenacity ? Tenacity is that prop- erty of matter by virtue of which some sub- stances resist a force tending to pull their par- ticles asunder. (a.) Like hardness and the other characteristic properties of matter, it is a variety of cohesion which is the general term for the force which holds the molecules together and keeps masses from crumbling into dust. 33. What is Brittleness tBrittleness is that property of matter by virtue of which some substances may be easily brolcen, as by a blow. (a.) Glass furnishes a familiar example of this property. The idea that brittlenoss is the opposite of hardness, elasticity or tenacity, should be guarded against Glass is harder than wood, but very brittle : it is very elastic, but very brittle also. Steel is far more tenacious than lead and far more brittle. 36 RECAPITULATION. If 34. What is Malleability ? Malleability it that property of mutter by virtue of which some substances may be rolled or hammered into sheets. (a.) Gold is the most malleable metal. It has been beaten so thin that a pile of 282,000 leaves would be but an inch high. Experiment 21. Heat the middle of a piece of glass tubing about six inches long, in an alcohol flame, until red-hot. Roll the ends of the glass slowly between the fingers and, when the heated part is soft, quickly draw the ends asunder. That the fine glass wire thus produced is still a tube, may be shown by blowing through it into a glass of water and noticing the bubbles that will rise to the surface. 35. What is Ductility ? Ductility is that prop- erty of matter by virtue of which some sub- stances may be drawn into wire. (a.) Platinum wire has been made OTFunds and pulls on a rope held by a man weighing 200 pounds and standing in a boat weighing 2000 pounds. Compare the velocities and momenta of the two boats. 36 NATURAL PHILOSOPHY. 59 SECTION II. GRAVITATION. 59. What is Gravitation ? Every particle of matter in the universe has an attraction for every other particle. This attractive force is called gravitation. 60. Laws of Gravitation. (1.) Gravitation va- ries directly as the product of the masses. For example, doubling either mass doubles the attrac- tion ; trebling either mass will multiply the attraction by three. (2.) Gravitation varies inversely as the square of the distance between the centres of gravity ( 65). For example, doubling the distance quarters the at- traction ; trebling the distance will divide the attraction by nine. Doubling both the product of the masses and the distance, will halve the attraction ; trebling both the product and the distance will divide the attraction by 61. What is Gravity ? Gravity is th attrac- tion between the earth and bodies upon or near its surface. 64 GRAVITATION. 37 It is the kind of gravitation with which we are most familiar. It acts in a vertical di- . rection, as shown by the plumb line, Fig. 11. 62. What is Weight? Weight is the measure of gravity. The attraction between two pounds of iron and the earth is twice as great as the attraction between one pound of iron and the earth. Because its gravity is twice as great, we say that its Fio. 11. Weight is twice as great or that it is twice as heavv. 63. Law of Weight. Bodies weigh most at tne surface of the earth. Below the surface, the weight decreases as the distance to the centre of the earth decreases. Jlbovc the surface, the weight decreases as the square of the distance from the centre of the earth increases. 64. Examples. How much will a pound of iron weigh one thousand miles below the surface of the earth ? Ans. As the iron would then be only three-fourths s far from the earth's centre, it would weigh only three- fourths as much, or twelve ounces. 67361 38 NATURAL PHILOSOPHY. 64 How far below the surface of the earth will a ten-pound ball weigh only four pounds ? Ans. As it is to weigh only f as much, its distance from the earth's centre must be only f of the earth's radius : of 4,000 miles is 1,600 miles. It will be 1,600 miles from the centre or 2,400 miles from the surface. A body at the earth's surface weighs 900 pounds : What would it weigh 8,000 miles above the surface ? Ans. It would then be 12,000 miles from the earth's centre or three times as far. The square of three is nine. According to the law above given, it would weigh only as much, or 100 pounds. Experiment 34. Six inches from the upper end of a string about two feet long, tie a small loop. Fasten any convenient weight, as an apple or large key, to the lower end of the string. Stick two stout pins or tacks into adjacent corners of the frame of your slate. Slip the loop over one pin and support the slate by the short part of the string, allowing the weight to hang free. When the slate and string have come to rest, mark a line on the slate, showing exactly the position of the string. Support the slate in a similar manner from the other pin and draw another line on the slate, again showing exactly the position of the string. Place your finger at the point where these two lines on your slate cross each other and balance the slate horizontally upon your finger tip. 65. Centre of Gravity. The centre of gravity of a body is the point about which all the mat- ter composing the body may be balanced. A body thus balanced is said to be "in equilibrium." The weight of a body may be supposed to be concen- trated at the centre of gravity. 66 GRAVITATION. be Outside 66. The Centre of Gravity may of the Body. The centre of grav- ity may be outside of the matter of which a body consists, as in the case of a ring, hollow sphere, box, or cask. The same fact is illustrated by the "balancer," represented in Fig. 12. The centre of gravity is in the line joining the two heavy balls, MM, and thus under the foot of the waltzing figure. (a.) The "balancer" may be bought at the toy store for a few cents. The pupil may better make one, using a large cork for the body and a smaller one for the head. The neck, arms and legs may be made of Btout pins, parts of hair-pins or other wire. The heavy balls may be two small potatoes. . The uplifted arm may be made of extra length and serve as a staff for a paper flag, neatly colored with red and blue ink or pencil. Instead of using the heavy balls, the prongs of two forks or the open blades of two penknives, as shown in Fig. 13, may be thrust into the cork, the only thing necessary being to bring the centre of gravity lower down than the foot of the figure. It is far better for the pupil to mnk his apparatus, when he can, than to buy it. He will understand more dearly, learn mort rapidly, and soon enjoy the exercise of his in- gem.ity. The remark applies to girls as well as to boys, and in many classes it happens that the girls are more ingenious and enterprising than the bov^ Fio 40 NATURAL PHILOSOPHY. 6"/ 67. Equilibrium. When the centre of gravity is supported, the whole body will rest in a state of equilibrium. The centre of gravity will be supported when it and the point of support are at the same place or in the same vertical line. (a.) A yard-stick may be supported by a needle thrust through its middle ; the point of support and the centre of gravity are together. (6.) It may be balanced with one end resting on the finger ; the point of support and the centre of gravity are in the same vertical line, the latter being directly over the former. (e.) It may be supported by hanging it by a string ; the point of support and the centre of gravity are again in the same vertical line, the latter now being directly under the former. (d.) The yard-stick in these three positions illustrates the three conditions or kinds of equilibrium. 68. Stable Equilibrium. A body supported in such a way that, when slightly displaced from its position of equilibrium, it tends to return to that position, is said to be in stable equilibrium, Such a displacement raises the centre of gravity. Examples : a disc or round flat plate supported above the centre; a semi-spherical oil-can; a pendulum or plumb- line. The cavalry-man represented in Fig. 14 may rock up and down balanced upon his horse's hind-feet, because the heavy ball brings the centre of gravity of the com- bined mass below the points of support. The " balancer" (Fig. 12) affords another example of stable equilibrium. 70 GRAVITATION. 4i (a.) This apparatus is easily made. A piece of board an inch thick may be fashioned into a shape something like the body of a horse. The head and neck may be made of paste- board. A feather makes a good tail and small nails answer admirably for legs. A potato or apple may be used for the heavy ball. The support may be made by plac- ing a lath across th3 backs of two chairs. The " balancer " previously made may ride this bare-backed horse standing wherever he is placed, the ' weight of the apple and the length or curvature of the wire being varied to suit the circumstances. 69. Unstable Equilibrium. A body supported in such a way that, ivhen slightly displaced from its position of equilibrium, it tends to fall further from that position, is said to be in un- stable equilibrium. Such a displacement lowers the centre of gravity. The body will not come to rest until the centre of gravity has reached the lowest possible point, when it will be in stable equilibrium. Examples : A disc sup- ported below its centre ; an egg standing on its end ; a stick balanced upright upon the finger. 70. Neutral Equilibrium. A body supported in such a way that, when displaced from its posi- tion of equilibrium, it tends neither to return tq 42 NATURAL PHILOSOPHY. JQ its former position nor to fall further from it, is said to be in neutral or indifferent equi- librium. Such a displacement neither raises nor lowers the centre of gravity. Examples : A disc supported at its centre ; a sphere resting on a horizontal surface. 71. Line of Direction. *4 vertical line drawn downward from the centre of gravity is called the line of direction. It may be considered as a line connecting the centre of gravity of the given body and the centre of the earth. 72. The Base. The side on which a body rests is called its base. If the body be supported on legs, as a chair or table, the base is the figure formed by joining the points of support. 73. Stability. When the line of direction falls within the base, the body stands; when without the base, the body falls. When the body rests upon a point, as does the sphere., or upon a line, as does the cylinder, a very slight force is sufficient to move it, no elevation of the centre of gravity being necessary. The broader the base and the lower the centre of gravity, the greater the stability. (a.) These facts explain the stability of leaning towers like those of Pisa and Bologna. In some such towers the centre of gravity is lowered by using heavy materials for the lower part and light materials for the upper part of the structure. 74 GRAVITATION. 43 (6.) It is difficult to stand upon one foot or to walk upon a tight rope because of the smallness of the base. PIG. 15. (c.) A porter carrying a pack is obliged to lean forward : a man Carrying a load in one hand is obliged to lean away from the load, to keep the common centre of gravity of man and load over the base formed by joining the extremities of his feet. 74. Recapitulation. To be amplified by the pupil for review f DEFINITION. GRAVITATION. GRAVITY DEFINITION. WEIGHT {S"''"*' CENTRE OF GRAVITY. -j n-L* r e ( Stable. EQUILIBRIUM -(Unstable. ( Neutral. 44 NATURAL PHILOSOPHY. 74 EXERCISES. 1. Why can a child walk more easily witl- ane than without 1 2. Why will a book placed on a desk-lici stay there while a marble would roll off ? 3. Why is a pyramid a very stable form of structure ? 4. Why is a ton of stone on a wagon less likely to upset than a ton of hay similarly placed ? 5. If the weight of a body be doubled, what will be the effect on its attractive force ? 6. If two bodies attract each other with a force of four units, what will be their attractive force when the distance between them is doubled? 7. How far does the earth's attraction extend? 75 FALLING BODIES. SECTION III. FALLING BODIES. Experiment 35. Drop a feather and a cent at the same tim from the same height and notice that the cent will reach tin ground first. Experiment 36. Take an iron and a wooden ball of the same size, drop them at the same time from an upper window and notice that they will strike the ground at nearly the same time. You will find that it requires a little practice to drop them at exactly the tame time. 75. Velocities of Falling Bodies. In Experiment 35, the cent fell faster than the feather, because it met with less resist- ance from the air than the feather did and not because it was heavier. In Experiment 30, we made this resistance of the air nearly equal and, probably, convinced ourselves that the velocity of a falling body does not depend upon its weight, unless it has to overcome resist- ance, or perform work, while it is falling. FIG. 1G. 46 NATURAL PHILOSOPHY. 75 When the resistance of the air is removed, the feather and the cent will fall with equal velocities. This resist- ance may be avoided by trying the experiment in a glass tube from which the air has been removed. The experi- ment is difficult to perform and requires expensive apparatus. Fig. 16 shows the cent and the feather falling with equal velocities in a long tube from which the air has been removed with an air pump, an instru- ment that we shall soon consider. 187. 76. Reason of this Equality. The cent is heavier than the feather and is, therefore, pulled downward by a greater force. The irovi ball has the greater weight, which shows that it is acted upon by a greater force than the wooden ball. But this greater force has to move a greater load, has to do more work than the lesser force. For the greater force to do the greater work requires as much time as for the lesser force to do the lesser work. A regiment can march no further in an hour than a single soldier can ; a thousand molecules can fall no further in a second than a single molecule can. 77. Gravity is a Constant Force. In these ex- periments, the feather, the cent and the balls could not change from their condition of rest to that of motion by their own power ( 22)*. It was necessary that some force act upon them to produce their motion. The force of gravity is what did the work. During the first second, the force of gravity gave the falling ball a certain velocity ; it gave the ball just as 79 FALLING BODIES. 4? much more velocity during the next second and just aa mucli more during the third second. At the end of the third second, the ball was moving just three times as fast as it was at the end of the first second. r equilibrium. 10. Explain why a "running jump" if onger than a "stand- ing" one. 11. (a.) Which is the more valuable, a lump of gold weighing, on a spring balance, one pound at the surface of the earth or a lump that would similarly weigh as much 1,000 miles above the surface ? (6.) Why would not a lever balance (Fig. 23) answer for the comparison as well as a spring balance ? 12. If the molecules of your body do not touch one another, why is it that the wind does aot blow you away in the form of fine dust ( 13 What force must be overcome in order to scratch a sub- stance? 14. Dip a glass rod into mercury and tell which is the stronger, the adhesion of the liquid for the rod or T,he cohesion of the liquid molecules ? 145 REVIEW QUESTIONS. 9$ 15. Show that in lifting at the end of the plank mentioned in Exercise 1, page 93, the plank represented a lever of the second class, and indicate the positions of P, F and W. 16. (a.) When water is poured from a jar, it often runs down the inclined side of the vessel instead of falling vertically. What force draws the falling water from a vertical course? (b.) Can you suggest a way to prevent such a result ? 17. If a stone weighs 10 pounds at the level of the ocean, how much will its weight measure, by a spring balance, 1,000 miles nearer the centre of the earth ? 18. Why is it easier to roll a sphere than a cube ? 19. What is a foot-pound ? A kilogram-metei ? A horse- power? 20. Find the kinetic energy of a 100-pound bal) owiutf with a velocity of 2,000 feet a second. CHAPTER LIQUIDS. SECTION I. LIQUID PRESSURE. experiment 44. Fill a small bottle with water, hold a Prince Rupert drop in its mouth and break off the tapering end of the " drop." The whole " drop " will be instantly shattered and the force oi the concussion transmitted in every direction to the bottle which will be thus broken. These "drops" are not expensive and may be obtained from JAMES W. QUEEN & Co., Philadelphia. 146. Transmission of Pressure. Fluids transmit pressure in every direction, upward, downward, and sidewise at the same time. (a.) This property of liquids may >o be illustrated by the apparatus repre- sented in Fig. 42. The globe and cylinder being filled with water and the several openings in the globe closed by corks, a piston is pushed down the cylinder. The pressure thus receded and transmitted by the confined watei LIQUID PRESSURE. expels the cork and throws a jet of water from each aperture and not merely from the one opposite the piston. (6.) Figure 43 represents a corked bottle of water. When the cork is forced downward, it exerts pressure upon all the water molecules in contact with it. These transmit the pressure to every part of the inner surface of the glass. Every part of this surface equal in area to that of the nd of the cork will be subjected to a pressure like that exerted by the cork. The riressure acts in a direction perpendicular to the surface of the gla^s, as shown by the arrows in the figure. (c.) It must be remembered that fluids include both aeriform and liquid bodies. Aeriform bodies are largely compressible; liquids are nearly in- compressible. 147. Pascal's Principle. Wlien fluids are sub- jected to pressure, the pressure sustained by any part of the restraining surface is perpendicular to it and proportional to its area. This may be shown by ex- periment as follows : Provide two communicat- ing tubes of unequal sectional area. When water is pourecl into these, it will stand at the same height in both tubes. If the water in the smaller tube be subjected to pressure by means of a 5 FIG. 44 98 NATURAL PHILOSOPHY. 147 piston, the water will be forced back into the larger tube. To prevent this result, a piston must be fitted to the larger tube and held there with a force as many times as great as the force acting upon the other piston as the area of the larger piston is times as great as the area of the smaller one. If, for example, the smaller piston have an area of 1 square inch and the larger piston an area of 16 square inches, a weight of 1 kilogram or ounce may be made to support a weight of 16 kilograms or ounces. Of course, the weight here referred to includes the weight of the piston itself in each case. (a.) It is evident that in this experi- ment it will be difficult to get the pistons to work without considerable friction. For this reason, the experiment is some- times modified and simplified by filling the lower part of the tubes with mer- cury, which will stand at the same level in both arms. If water be poured into the smaller tube it will depress the mercury surface in that tube ; 16 times the weight of water must be poured into the larger tube to restore the two mer- cury surfaces to the same level. 148. Pascal's Experiment. Pascal firmly fixed a very nar- row tube about 30 feet high into the head of a stout cask. He then filled the cask and tube with FIG. 45. water. The weight of tb; small amount of water in the tube actually burst the cask. ISO LIQUID P". 149. The Hydrostatic Bellows. The hydro- static bellows consists of two boards fastened to- gether by a broad band of stout leather and a small vertical tube communicat- ing with the interior. In the figure, the tube that bears the funnel is to be joined to the tube passing upward from b. This vertical tube may be filled with water and the pressure thus exerted will lift a heavy weight (e. g., several boys) placed upon B. If the 'board, B, has a surface of 66 square inches exposed to the upward pressure of the water in the bellows and the tube have a sectional area of \ square FIG. 46. inch, every pound of water in the tube will support 264 pounds at B. 150. The Hydrostatic Press. The hydrostatic press acts upon the same principle. It is represented in perspective by Fig. 47 and in section by Fig. 48. Pres- sure is produced by the" force- pump A. The substance to be pressed is placed between K, the head of the piston, and an immovable plate M N. The reservoir, B, uiid.the cylinder of the pump, are connected by the tube d. By the action of the pump, the water in the cylinder A is subjected to pressure and this pressure is trans- mitted undiminished to the water in B. The piston, ' 100 XATVRAL PHILOSOPHY. 150 is generally worked by a lever of the second class, result- ing in a still further gain of intensity of power. If the power-arm of the lever be ten times as long as the weight-arm, a power of 40 pounds at the end of FIG. 47. the lever will exert a pressure of 400 pounds upon the water in A. If the piston in A have a sectional area of 1 square inch and the piston in B have an area of 500 square LIQUID PRESSURE. 101 inches, then the pressure of 400 pounds exerted by the small piston will produce a pressure of 400 pounds x 500 or 200,000 pounds upon the lower surface of the large piston. Hence the following rule : Multiply the pressure exerted by the piston of the pump by the ratio between the sectional areas of the two pistons. FIG. 48. Experiment 45. Over the opening of a wide mouthed bottle or fruit jar, tie a piece of thin sheet rubber. Hold the bottle in a tub of water with the mouth downward, sidewise and upward. In each case, the pressure of the water will force the rubber inward. Try the experiment at different depths of water and notice that the pressure will vary with, the depth. 151. Liquid Pressure Due to Gravity. The pressure exerted by liquids, on account of their weight, may be downward, upward, or lateral. We shall now briefly consider these three kinds of liquid pressure, 102 NATURAL PHILOSOPHY. 152 Experiment 46. Into a bent glass tube, ABC, pour mercury (quicksilver) until it covers the bend and rises two or three inches above it. The mercury will stand at the same level, a c, in the two branches of the B tube. If pressure of any kind be exerted upon the surface of the mercury at a, it will be promptly shown by the movement of the mercury and a consequent difference in the heights of the two mercury columns. The greater the pressure, the greater will be the elevation of c above the lvel of a. A common glass funnel or a piece of glass tube may be joined to A by a short piece of snugly fitting rubber tubing. Suppose the funnel to be thus connected and the apparatus supported by any convenient means in an upright position. Pour water into the funnel and mark the level of the water by a suspended weight or other means ; mark the level of the mercury in the long arm of i 11 the tube by a string or small elastic band. Remove the funnel and connect the straight FIG 49 glass tube above A. Fill this with water to the same height as before, as indicated by the suspended weight. Although the quantity of water used is much less, the depression of the mercury at a and its elevation at c will be the same as before, showing the downward pressure at a to be the same in both cases Directions for bending glass will be found in Chemistry, Appendix 4. 152. Downward Pressure. The pressure on the bottom of a vessel containing a liquid depends upon the depth and density of the liquid and the area of the bottom. The quantity of the liquid and the shape of the vessel niake no difference, 154 LIQUID PRESSURE. 103 153. Rule for Downward Pressure. To find the downward pressure on a horizontal surface, find the weight of an imaginary column of the given liquid, with a base the same as the given surface and an altitude the same as the depth of the given surface below the surface of the liquid. NOTE. A cubic foot of water weighs about 1,000 ounces, 62 pounds (more exactly 62.42 pounds). 154. Example. A cask has a base of 3 square feet and a height of 2 feet. A tube is fitted into the upper head. The cask is filled with water ; more water is then added until the tube is filled to a height of 3 feet above the upper head. What is the pressure on the lower head of the cask ? Solution. Our "imaginary column" of water has a base of 3 square feet and a height of 5 feet. It therefore has a volume of 15 cubic feet. The weight of the "imaginary column" of water would be 15 times ^2.42 pounds or 936.3 pounds. This is the pressure exerted upon the lower end of the cask. Although the quantity of water actually used is less than half that of our "imaginary column," the pres- sure exerted by it is the same as explained in 152. If a liquid like alcohol had been used, the pressure would have been only about -fa of 936.3 pounds, for alcohol is only about -^ as heavy as water. If the liquid used had been mercury, which is 13.6 times as heavy as water, the pressure would have been 13.6 times 936.3 pounds or 12,733.68 pounds more than six tons. 104 NATURAL PHILOSOPHY. *55 NOTE. In all such cases the pupil may be allowed to use 62| pounds as the weight of a cubic foot of water if he prefers to do BO. That value (1,000 ounces) is more easily remembered. Experiment 47. Make a small hole in the bottom of a tin fruit ean or similar vessel. Push the can downward into water until the open mouth of the can is " near the water's edge." The liquid Will spurt upward through the hole in a little jet. 155. Rule for Upward Pressure. To find the upward pressure on any horizontal surface, find the weight of an imaginary column of the given liquid with a base the same as the given sur- face and an altitude the same as the depth of the given surface ' below the surface of the liquid. 156. Example. What will be the upward pressure on a horizontal plate a foot square when placed 10 feet beneath the surface of water ? Solution. The volume of our "imaginary column" of water is 10 cubic feet. It would weigh 10 times 62.42 pounds or 624.2 pounds. Consequently, the up- ward pressure would be 624.2 pounds. 157. Rule for Lateral Pressure. To find the pressure upon any vertical surface, find the weight of an imaginary column of the liquid with a base equal to the given surface and an altitude the same as the depth of the centre of the given surface below the surface of the liquid. 159 LIQUID PRESSURE. 105 158. Example. A dam is 25 feet high and 30 feet long. Water stands at a height of 20 feet on one side of the dam. What is the liquid pressure on the dam ? Solution. The dam may be vertical or sloping, but in either case, the surface that we have to consider is 20 feet high and 30 feet long. The depth of its centre below the surface of the water is 10 feet. Our "imaginary column " will, therefore, have a volume of 6,000 cubic feet and a weight of 374,520 pounds. The pressure will, therefore, be 374,520 pounds. 159. Recapitulation. To be amplified by the pupil for review. TRANSMITTED EQUALLY IN ALL DIRECTIONS. PASCAL'S EXPERIMENT. EFFECT ON RESTRAINING SURFACE. \ HYDROSTATIC BELLOWS HYDROSTATIC PRESS. f DOWNWARD. PRODUCED By GRAVITY J UPWARD. I LATERAL. 106 NATURAL PHILOSOPHY. 159 EXERCISES. 1. Referring to Fig. 48, suppose the area of the end of piston, a, to be 1 square inch and that of piston, C, to be 10,000 square jaches and the two arms of the lever, 0, to be 1 foot and 10 feet respectively, what pressure at K will be produced by a pressure of 100 pounds by the operator? Am. 10,000,000 Ib. 2. Where will water issue from a water pipe with the greater force, in the basement or near the top of a house ? Why ? 3. Find the pressure on a dam 25 feet long, the water being 10 feet deep ? Am. 78025 Ib. 4. A tank has a base 6 feet by 8 feet. What is the pressure on that base when the water in the tank is 4 feet deep ? Ans. 11984.64 Ib. 5. A tank has a base 2 meters by 3 meters. What is the pres sure on that base when the water in the tank is 1.5 meters deep ? Ant. 9000 Kg. l6l EQUILIBRIUM. 107 SECTION II. EQUILIBRIUM. BUOYANCY. 160. Equilibrium of Liquids. A liquid of small surface is said to be level when all the points of its sur- face are in the same horizontal plane. The central idea is expressed in the familiar saying, water seeks Us level. This is true whether the liquid be placed in a single vessel or in several vessels that communicate with each other. Experiment 48. Incline a tea-pot that is nearly full of water until the liquid begins to run out at the spout. Notice that the end of the spout and the water surface in the vessel are at the same level. 161. Communicating Vessels. When a liquid is placed in one or more of several vessels communicating with each other, it will not come to rest until it stands at the same height in all of the vessels. Experiment 49. From one end of a scale-beam, suspend a cylindrical bucket of metal, b, and below that a solid cylinder, a, winch accurately fits into the bucket. Counteqwise with weights in the opposite scale-pan. Then place a vessel of water, as shown in Fig. 50, so as to immerse and the counterpoise will descend, showing that a has lost some of its weight. Carefully fill 6 with water. It will hold exactly the quantity displaced by a. Equili- brium will be restored. The bucket and cylinder may be had of dealers in philosophical apparatus. 108 NATURAL PHILOSOPHY. i6i Experiment 50. Insert a short spout in the side of a vessel (as tin fruit-can) about an inch below the top. Fill the vessel with water and let all above the level of the spout escape. This is to replace the vesrel of water in which a (Fig. 50) is immersed. Instead of the bucket, ft, use a cup placed on the scale-pan. Instead i>f a, use any convenient solid heavier than water, as the fragment of a stone, which may be suspended by a horse-hair or a fine thread. Counterpoise the cup and stone in the air. Immerse the FIG. 50. stone in the water and catch, in any convenient vessel, every drop of water that overflows. This will be the fluid that the solid displaces. The equilibrium is destroyed, but may be restored by pouring the water just caught into the cup on the scale pan. It may sometimes be necessary to make allowance for the small quantity of water that adheres to the cup in which the overflow was caught, but with a good balance and good work the result will clearly show the truth of Ardiiinetfw? Principle. 163 BUOYANCY. 109 162. Archimedes' Principle. It is a familiar fact ;hat a person may easily raise to the surface of the water a stone which he can not lift any further. When an arm or leg is lifted out of the water of a bath-tub, it suddenly feels heavier. Many such facts are explained by the important principle here given : The loss of weight of a body immersed in a fluid equals the weight of the fluid which it displaces. Experiment 51. Place the tin vessel with a spout, mentioned in Experiment 50, upon one scale-pan, and fill it with water, some of which will overflow through the spout. When the spout has ceased dripping, counterpoise the vessel of water with weights in the other scale-pan. Place a floating body on the water. This will destroy the equilibrium but water will overflow through the epout until the equilibrium is restored. This shows that the floating body has displaced its own weight of water. Experiment 52. Place a fresh egg in a vessel of fresh water; it is a little heavier than the water and will sink. Place it in salt water ; it is a little lighter than the brine and will float. Carefully jiour the fresh water on the salt water in a tall, narrow vessel like that shown in Fig. 162. Place the egg in the water; it will descend until it reaches a layer of the liquid with a density like its own and there it will float. 163. Floating Bodies. A floating body displaces its own weight of the fluid on which U floats. FIG. 61. Th - g may be g ] lown experimentally by filling a vase with water. When a floating body is 110 NATURAL PHILOSOPHY. 1 63 placed on the surface, the water displaced will overflow and may be caught. The water thus caught will weigh the same as the floating body. (a.) If a body placed on the surface of a fluid can not displace its own weight of the fluid, it will sink and not float. The buoy- ant effect of the fluid is then less than the weight of the body. Sometimes a heavy substance has such a shape that it will dis- place enough of a lighter fluid to float thereon. For this reason, an iron kettle or an iron ship will float on water, although iron is several times heavier than water. In all such cases, the buoy- ant effect of the fluid equals the weight of the floating body, (6.) Just as the gravity of a body may be considered as acting upon a single point called the centre of gravity, so the buoyant effort of a fluid may be considered as act ing upon a single point called the centre of buoyancy. The centre of buoyancy is situated at the centre of gravity of the displaced fluid. (c.) The centre of buoyancy may be considered the point of support of the floating body and the principles of 65- FlG - 53 - C8 applied anew. For example, if the boat's centre of buoyancy be at C and its centre of gravity b at b (as it may be if its occupant is sitting down), the boat will be in stable equilibrium, while if its centre of gravity be at B (as it may be if its occupant is standing up), the boat will b in dangerously unstable equilibrium. RECAPTTULA r/O.V. Ill 164. Recapitulation. To be amplified by the pupil for review. (XI g al I I 11 H 11.3 NATURAL PHILOSOPHY. or 39.2 F. For aeriform bodies, the standard is air or hydrogen. 167. Elements of the Problem. For solids or liquids, the dividend is the weight of the given body ; the divisor is the weight of the same bulk of water; the quotient, which is an abstract nuniber, is the specific gravity. The weight of the same bulk of water is found some- times in one way and sometimes in another but, in every case, it is tlie divisor. By grasping and keeping thi? idea, you will avoid much possible confusion. Of course, when any two of these three are given, the third can be found. 114 NATURAL PHILOSOPHY. 168 168. To Find the Specific Gravity of Solids. The most common method of finding the specific gravity of a solid heavier than water, is to find the weight of the body in the air (= W), then suspend the body by a light thread and find its weight in water (= W), and divide the weight of the body in air by the weight of the same bulk of water ( 162, Archimedes' Principle). Sp. Or.^ (a.) The method is illustrated by the following example ; Weight of substance in air = 58 oz. Weight of substance in water = 51 oz. (Fig. 54). Weight of equal bulk of water Specific gravity 58 1 oz. -*- 7| oz. 170 SPECIFIC GRAVITY. 115 169. To Find the Specific Gravity of Liquids. The principle is unchanged. A sirhple method is as follows: Weigh a flask first empty; next, full of water; then, full of the given liquid. Subtract the weight of the empty flask from the other two weights ; the results represent the weights of equal volumes of the given substance and of the standard. Divide as before. (a.) A flask of known weight, graduated to measure 100 or 1,000 grams or grains of water is called a specific gravity fla&k. Its use avoids the first and second weighings above mentioned and simpli- fies the work of division. (6.) The specific gravity of a liquid may he easily determined as follows : Find the loss of weight of any insoluble solid in water and in the given liquid. Remember ( 162) what these two losses repre. sent. Di vide as before. The solid used is called a specific gravity bulb. (c.) The determination of the specific gravity of gases presents many practical difficulties which can not be considered in thla place. 170. Recapitulation. To be amplified by the pupiJ for review. DEFINITION. _ STANDARDS. SPECIFIC DIVIDEND AND DIVISOR. GRAVITY. OF SOLIDS. ( SPECIFIC GRAV OF LIQUIDS \ ( SPECIFIC GRAV vrrv FLASK ITY BULB. 116 NATURAL PHILOSOPHY. 170 EXERCISES. 1. A body weighs 150 pounds in air and 100 pounds in water, What is its specific gravity ? 2. A body weighs 75 ounces in air and 60 ounces in water. What is the weight of an equal bulk of water? 3. Lead has a specific gravity of about 11 J. Mercury is a liquid having a specific gravity of about luf . Will lead sink in mercury? 4. Sulphuric acid has a specific gravity of 1.8. Will a glass ball lose more weight in the acid than it will in water ? 5. In which will a body weigh more, in fresh water having a specific gravity of 1 or in sea water having a specific gravity of 6. Is it, then, easier for a person to float in fresh water than it JB in sea water ? 7. A piece of brass weighing 41.9 ounces was placed in a vessel full of water. The overflowing water was caught and found to weigh 5 ounces. What is the specific gravity of brass ? An*. 8.38 8. A liter bottle holds 1,000 grams of water or 800 grams of alcohol. What is the specific gravity of alcohol ? 9. Let each pupil experimentally determine the specific gravity of some solid that will sink in water. 174 HTDROKINETIC& 117 SECTION IV. HYDROKINETICS. 171. The Flow of Liquids through Horizontal Pipes. When liquids from a reservoir are made to flow through pipes of considerable length, the discharge is far less than that due to the head. The diminution is chiefly owing to friction against the sides of the pipe. The "head" is the vertical distance from the centre of the orifice to the surface of the liquid. 172. The Flow of Rivers. The friction of a stream against its solid bed fortunately retards the ve- locity of the water. Otherwise the velocity of the cur- rent at the mouth of a river, whose head is elevated 1,000 feet above its mouth, would be about 170 miles per hour. Such a current would be disastrous beyond de- scription. The ordinary river current is from three to five miles per hour. 173. Water-power. Water may be used to turn a wheel and thus move machinery by its weight, the force of the current or both. 174. The Turbine Wheel. The turbine wheel, of which there are many varieties, is the most effective water-wheel known. 118 NATURAL PHILOSOPHY. 174 FIG. 54. (a.) Figure 54 represents one form in perspective and in hori- zontal section through the centre of the wheel and case complete. The wheel, B, and the enclosing case, D, are placed on the floor of a penstock wholly submerged in water, under the pressure of a considerable head. The water enters, as shown by the arrows, through openings in D, which are BO constructed that it strikes the buckets of B in the direction of greatest efficiency. (b. After leaving the buckets, the " dead water " escapes from the central part of the wheel, sometimes by a vertical draft tube, best made of boiler-iron. The weight of the water in this tube increases the velocity with which the water strikes the buckets. (c.) A central shaft, A, is carried by the wheel and communi cates its motion to the machinery above. The wheel itself rests upon a central pivot carried by cross-arms from the bottom of the outer case. The case, D, is covered with a top, T, which protects the wheel from the vertical pressure of the water. The axis of the wheel passes through the centre of this cover. 175. The Overshot Wheel. In the overshot wheel, the water falls into buckets at the top and, bv HTDK O KINETICS. 119 its weight aided by the force of the current, turns the wheel. As the buckets are gradually inverted, the water is emptied aud the load thus removed from the other side of the wheel. PIG. 55. Such wheels require not much water but a consider- able fall. It is said that they have been made nearly 100 feet in diameter. The water is led to the top of the wheel by a sluice, 8 V. The power may be com- municated to the machinery by the shaft, A, or by the rim of the wheel, as at c, according to the desire for intensity of power or for velocity. The water supply may be controlled by a gate at F. 176. The Breast Wheel. In the breast wheel, the water acts upon float boards fixed perpendicular to the circumference. The stream being received at or near the level of the axis, both the weight of the water and the force of the current may be turned to account. 120 NATURAL PHILOSOPHY. 177 FIG. 56. 177. The Undershot Wheel. In the undershot wheel, the stream strikes, near the bottom of the wheel, against a few float boards. It is turned by the force of the current. FIG. 57. NOTE. In point of efficiency, these wheels rank in the ordei ibove given, 177 REVIEW QUESTIONS. 121 REVIEW QUESTIONS. 1. In what direction do liquids at rest exert pressure? 2. Upon what three things does the pressure of a liquid on the bottom of the vessel holding it depend ? 3. How may a little water be made to exert a great pressure ? 4. Show two ways in which a small weight may be made to balance a heavy one? 5. State two points of difference between solids and liquids? 6. If a pendulum of given length swings through an arc five inches long in one second, how long will it take the same pendulum to swing through an arc of ten inches ? 7. Show that the appara- tus represented in Fig. 58 is a lever, tall of what class it is and indicate the posi- tions for P, W and F. 8. Define malleability. 9. In what direction do fluids transmit pressure with the great- est facility? 10. State the principle that underlies the action of the hydro- static press ? 11. What was Pascal's experiment? Find out who Pascal was. 12. It is stated that the specific gravity of silver is about ten. This means ten of what f 13. What is the velocity of an ordinary river current ? 14. Describe the hydrostatic bellows. What is meant by the centre of buoyancy ? 15. Describe the use of the specific gravity bulb. 16. Experiment shows that it is difficult to balance a lead pencil on the finger. Why is it e:isier thus to balance it with the additional load of two penknives, as shown in Fig. 59 ? - 58 - FIG. 59. CHAPTER V. PNEUMATICS. SECTION I. THE ATMOSPHERE AND ATMOSPHERIC PRESSURE. 178. What is Pneumatics ? Pneumatics is that branch of Physics which treats of aeri- form bodies, their mechanical properties and the machines by which they are used. 179. Tension of Gases. However small their quantity, gases always fill the vessels in which they are held. If a bladder or India rubber bag, partly filled with air and having the opening well closed, be placed under the receiver of an air-pump ( 187), the bladder or bag will be fully distended, as shown in the figure, when the air surrounding the bladder is pumped out. The flexible walls are pushed out by the air confined within. This tendency is called elastic force or tension,. FIG. 60. l8l ATMOSPHERIC PRESSURE. 123 We may imagine the countless molecules of air in the bladder or bag as being in constant motion and continu- ally striking against the walls that confine them. These molecular blows push the walls outward and their total effect constitutes tension. This conception is generally referred to as the Kinetic Ttieory of Gases ( 43 ; a). 180. The Type. As water was, for obvious reasons, taken as the type of liquids, so atmospheric air will be taken as the type of aeriform bodies. Whatever mechanical properties are shown as be- longing to air may be understood as belonging to all gases. 181. Weight of Air. Being a form of matter, air has weight. This may be shown by experiment. A hollow globe of glass or metal, having a capacity of sev- eral quarts or liters and provided with a stop-cock, is carefully weighed on a delicate balance. The air is then removed from the globe by an air-pump, the stop-cock closed and the empty globe weighed carefully. The second weight will be less than the first, the differ- ence between the two being the weight of the air removed. (a.) Under ordinary conditions, a cubic inch of air weighs about 0.31 grains. A liter of air weighs about 1.293 grams, being thus about T ^5 as heavy as water. (6.) Measure the length, breadth and height of your school- room and find the weight of the air that it contains. 124 NATURAL PHILOSOPHY. l8l Experiment 53. Fill a tumbler with water, place a slip of thick paper over its mouth and hold it there while the tumbler is inverted : the water will be supported when the hand is removed from the card, as is shown in Fig. 61. Experiment 54. Plunge a small tube, or a tube having a small opening at the lower end, into water, cover the upper end with the finger and lift it from its bath. The water is kept in the tube F IO . 61. by the upward pressure of the atmos- phere. Remove the finger and the downward pressure of the at- mosphere, which was previously cut off, will counterbalance the upward pressure and the water will fall by its own weight. Such a tube, called a pipette, is much used for transferring small quan- tities of liquids from one vessel to another. Experiment 55. Make a "sucker" of a circular piece of thick leather and fasten a string to its middle. Soak it thoroughly in water and firmly press it upon a flat stone to drive out all air from between the leather and the stone. Pull the string gently so that a vacuum may be formed, as shown in Fig. 62. If the stone be not too heavy, it may be lifted by the string. We shall soon see that, in reality, the stone is pushed up by the air instead of being pulled up by the string. Experiment 56. Suck the air from the hol- low stem of a common key and quickly press the FIG. 62. OP 6 " end of tne stem against the lip where it will be held by the pressure of the air. Experiment 57. For a few cents you can buy a four inch test tube of a dealer in chemical glass ware (see Chemistry, AfMOSPBERtC PRESSURE. 125 Appendix 7). Holding it by the open end, heat the tube quite hot in the flame of a lamp or candle. When the heat has expanded the air and driven part of it out of the tube, press the mouth of the tube against the fleshy part of the hand or thumb where it will be held by atmospheric pressure. Experiment 58. Vary the last experiment by placing the tube, mouth downward, in a saucer of water. Atmospheric pressure will force water upward into the tube. Experiment 59. Secure an empty tin fruit can with a hole about two inches in diameter in one end. Smoothly stretch a piece of clean mosquito net- ting over this end of the can, as shown in Fig. 63. Fill the can with water, place a piece of writing paper over the mosquito netting and hold it there while you invert the can. Draw the pa per hor- Fio. 63. meshes of the mosquito netting. izontally from the end of the can. Atmospheric pressure will prevent the water from running out through the Experiment 60. With a small nail, punch a hole in the middle of the closed end of the can used in the last experiment. Repeat that experiment, keeping the nail-hole covered by the forefinger, as shown in Fig. 63. Remove the finger for an instant, quickly covering the hole again. When the atmosphere has a chance to press downward through the nail-hole upon the water in the can, the water runs out through the mosquito netting. When this opportunity is removed by closing the nail hole, the water is held in the can by the upward pressure of the at- mosphere. 126 XATUtlAL PHILOSOPHY. l82 182. Atmospheric Pressure. The atmosphere exerts a great pressure upon the surface of the earth and all bodies found there. This atmospheric pressure de- creases as we ascend from the earth's surface. The weight of a column of air one inch square and extending from the sea-level to the upper limit of the at- mosphere is about fifteen pounds; a similar column, a centimeter square, weighs about one kilogram. We express this by saying that the atmospheric pressure at the sea-level is fifteen pounds to the square inch, or one kilogram to the square cen- timeter. (a.) Several illustrations of atmospheric pressure will be given after we have considered the air-pump. 183. Torricelli's Experiment. The intensity of atmospheric pressure may be measured as follows : Take a glass tube a yard long, about a quarter of an inch in internal diameter. Close one end and fill the tube with mercury. Cover the other end with the thumb or finger and invert the tube, placing the open end in a bath of mercury. Upon removing the thumb, the mercury will sink and come to rest at a height of about 30 inches, or 7GO millimeters, above the level of the mercury in the bath. The apparatus used, when properly graduated, becomes ?. barometer. 184. Pascal's Experiments. Pascal repeated Tor- ricelli's experiment on the top of a mountain and found that the mercury column was three inches shorter, show- 186 ATMOSPHERIC PRESSURE. 127 ing that as the weight of the atmospheric column dimin ishes, the supported column of mercury also diminishes. He then took a tube forty feet long, closed at one end. Having filled it with water, he inverted it over a water bath. The water in the tube came to rest at a height of 34 feet. The water column was 13.G times as high as the mercury column, but as the specific gravity of mer- cury is 13.6, the weights of the two columns were equaL. Experiments with still other liquids gave correspond- ing results, all of which strengthened the theory that the supporting force is due to the weight of the atmos- phere and left no doubt as to its correctness. 185. Pressure Measured in Atmospheres. A gas or liquid which exerts a force of 15 pounds upon a square inch or one kilogram upon a square centimeter of the restraining surface is said to exert a pressure of one atmosphere. A pressure of 60 pounds to the square inch, or 4 kilograms to the square centimeter would bo called a pressure of 4 atmospheres. 186. Recapitulation. To be amplified by the pupil for review. DEFINITION. TENSION. TYPE IS AIR. PRESSURE. Per square inch. Per square centimeter. Measured in atntospkeru. Torricelli. Paseat. 128 NATURAL PHILOSOPHY. l86 EXERCISES. 1. The downward pressure of the atmosphere on the hottom of an ordinary wooden pail is about a ton. Why is not the bottom forced out and how can any person carry the pail ? 2. Suppose a bottle to be tightly corked at the top of a high mountain, carried to the sea-level and opened with its mouth under water. Would air bubble out or water rush in ? 3. What is the pressure on one side of a window 2 feet by 6 feet? AM. 25920 Ib. 4. What is the pressure on one side of a door 1 meter by 2 meters ? 5. Why is a barometer tube closed at the top ? 6. Why is a barometer tube not closed at the bottom ? 7. When the barometer stands at 28 inches, at what height would the water in the tube of Pascal's experiment come to rest ? 8. A steam-boiler was tested " at a pressure of 10 atmospheres/ What does this mean? 187 AIR-PUMPS. SECTION II. AIR-PUMPS. LIFTING AND FORCE-PUMPS. SIPHON. 187. The Air-Pump. The air-pump is an in- strument for removing air from a closed vessel. The essential parts are shown in section by Fig. 64. FIG. 64. The vessel, R, is called a receiver. It fits accurately upon a horizontal plate, through the centre of which is an opening communicating with a cylinder, C, by means of a bent tube, t. An accurately fitting piston moves in the cylinder. At the junction of the bent tube with the cylinder and in the piston, are two valves, v and v', 130 NATURAL PHILOSOPHY. 187 opening from the receiver but not toward it. When the piston is raised, v' closes and the atmospheric pressure is removed from v. The tension of the air in R opens v. The air which was in R and t expands and fills R, t and C. When the piston is pushed down, v closes, v' opens and the air in C escapes from the apparatus. NOTE. A. person having an air-pump has the means of per- forming almost numl>erless experiments, some amusing and all instructive. A cheap and efficient air-pump may be bought of JAMES W. QUEEN & Co., Philadelphia. Several experiments with the air-pump are given below. Others will be found in The Element* of Natural Philosophy. Experiment 61. The hand-glass is a receiver open at both ends. See that the lower end fits accurately upon the plate of the air-pump. (It is well to smear the plate with tallow in this and similar experiments.) Place the hand over the other end. When the pump is worked, the pressure of the atmosphere is felt, and the hand can be removed only by a consider- able effort. The appearance of the palm of th? hand at the end of this experiment is due to the tension of the air within tie tissues of the hand. Experiment 62. Perform the experi- ment described in 179. Experiment 63. Ov:>r the upper end of a cylindrical receiver, tie tightly a wet bladder and allow it to dry. Then ex- haust the air. The bladder will be forced inward, bursting with a loud noise. It may be necessary to prick a FIG. 65. FIG. 66 187 AtR-P0Mt>S. 131 pin-hole through the bladder after the receiver has been ex hausted. Experiment 64. Replace the bladder with a piece of thin india. rubber cloth. Exhaust the air. The cloth will be forced inward by atmos- pheric pressure and nearly cover the inner surface of the receiver. NOTE. Ths hand-glass, used in Ex- periment 61, will answer for the two experiments last given, by placing the small end upon the pump-plate. Experiment 65. The "fountain in racuo " consists of a glass vessel, through the base of which passes a tube ter- minating in a jet within, and provided with a stop-cock and screw without. By means of the screw, it may be at- -. tached to the air-pump. Remove the air, close the stop-cock, place the lower end of the tube in water open the stop-stock ; a beautiful fountain will be produced (Fig. 67). Experiment 66. The Magdeburg hemispTieres are made of metal (Fig. 68). They are hollow and generally three or four inches in diameter. The edges being greased and placed together, the air is exhausted from the hollow globe through a tube provided with a stop-cock and screw. When the air has been pumped out, close the stop-cock and remove the hemispheres from the pump. It will l>e found that a considerable force is necessary to poll the hemispheres asunder. This force is equal to the atmospheric pressure upon the circular area inclosed by the edges of the hemispheres. U PHILOSOPHY. 188 ihis area be ten square inches, it will require a pull of 150 pounds to separate the hemispheres. Experiment 67. Partly fill two bottles with water. Connect them by a bent tube which fits closely into the mouth of one and loosely into the mouth of the other. Place the bottles under the receiver and exhaust the air. Water will be driven from the closely stoppered bottle into the other. Readmit air to the receiver and the water thus driven over will be forced back. FIG. 69. 188. The Condenser. TJie condenser is an in- strument for compressing a large amount of gas into a closed vessel. The chief difference between it and the air-pump is that its valves open toward the receiver. 189. The Lifting - pump. The lifting-pump consists of a cylinder or barrel, c, a piston, p, two valves, v v, and a suction pipe, s, the lower end of which dips below the surface of the liquid to be raised. The arrange- ment is essentially the same as in the air-pump. As the piston is worked, the air below it is gradually removed. The downward pressure on the liquid in the pipe being thus re- =2 moved, the pressure of the at- 0: mosphere, exerted upon the FIG. 70. 191 LIFTING A\D FORCE-PUMPS. 133 surface of the liquid, pushes the liquid up through the suction pipe and the lower valve into the barrel. When the piston is again pressed down, the lower valve closes, the reaction of the water opens the piston valve and the piston sinks below the surface of the liquid in the barrel. When next the piston is raised, it lifts the water above it toward the spout of the pump. At the same time, atmospheric pressure forces more liquid through the suction pipe into the barrel. 190. Practical Points. The cistern or well con- taining the liquid must not be cut off from atmospheric pressure, i. e., must not be made air-tight. For water pumps, the suction pipe must not be more than 34 feet high. Owing to mechanical imper- fections chiefly, the practical limit of the water pump is about 28 vertical feet. 191. The Force -Pump. In the force-pump, the piston is often made solid, *. e., without any valve. The upper valve is placed in a dis- charge pipe, d, which opens from the barrel at or near its bottom. When the piston is raised, water is forced into the barrel by atmos- pheric pressure. When the piston is forced down, the suction pipe valve is closed, the water being forced FIG. 7t 134 NATURAL PHILOSOPHY. 192 through the other valve into the discharge pipe. When the piston is raised again, the discharge pipe valve is closed, preventing the return of the water above it, while atmospheric pressure forces more water from below into the barrel. Sometimes the upper valve is placed in the piston, as in the ordinary lifting-pump, the discharge pipe opening from the upper part of the cylinder which is closed at the top. 192. Direction of Valve Openings. In the case of the air-pump, the condenser, the lifting and force- pumps, the valves open in the direction in ivhich the fluid is to move. 193. The FIG. 72. Air-Chamber of a Force-Pump. Water will be thrown in spurts from a pump like tli at represented in Fig. 71. A continuous flow is secured by connecting the discharge pipe with an air-chamber. This air-chamber, c, is provided with a delivery pipe, b or s, which reaches below the surface of the wa- ter in the air-chamber. When water is forced into the air- chamber, it covers the mouth of the delivery pipe and compresses the air confined in the chamber. This less- ening of the volume of the air causes an increased ten- 194 THE SIPHON. 135 sion ( 179), which soon becomes sufficient to force the water through the delivery pipe in a continuous stream. A pump may have both delivery pipes but one of them must be closed by a cock, as shown at *, Experiment 68. Set a pail of water on the table. Place one end of a piece of rubber tubing in the water and let the other end hang over the edge of the pail reaching below the top of the table. Suck some water through the tube. Water will continue to flow until the pail is emptied or the water surface falls below the end of the tube in the pail. 194. The Siphon. Tlie siphon consists of a bent tube, open at both ends, having one arm longer than the other. It is used to transfer liquids from a higher to a lower level, especially in cases where they are to be removed without disturbing any sediment they may contain. The siphon may be first filled with the liquid and then placed with the shorter arm in the vessel, care being hud that the liquid does not escape from the tube until the opening, C, is lower than m n, the surface of the liquid ; or it may be first placed in position and the air removed by suction at the lower end. The pressure of the atmosphere will force the liquid up the shorter arm and fill the tube. In either case, a constant stream of the liquid will flow from the vessel 136 NATURAL PHILOSOPHY. 195 until the surface line, m n, is brought as low as the opening in the shorter arm or, if the liquid be received in another vessel, until the level is the same in the two (a.) The action of the siphon is explained in the author's larger work. If the liquid to be transferred is water, the height, a b, must be less than thirty-four feet. 195. Recapitulation. To be amplified by the pupil for review. PUMPS.. DEFINITION. CONSTRUCTION. AIR LIMITATIONS. EXPERIMENTS. CONDENSER. WATER. ! LIFTING. FORCE. Valve Openings. THE SIPHON. 195 EXERCISES. 137 EXERCISES. 1. What is the total atmospheric pressure (in pounds) upon the surface of a wooden cu be one inch on each edge ? 2. If mercury is 13.6 times as heavy as water, and a given pump will lift water to the height of 28 feet, how high will the same pump, other conditions being similar, lift the mercury ? 3. How can you arrange a single suction or lifting-pump to raise water from the bottom of a well 50 feet deep ? 4. Construct the apparatus shown in Fig. 74, filling each of the three bottles half full of water. Blow in the tube,/, until a jet is formed at n. Explain the continued action of the apparatus. Be sure that all joints made by the corks of the three bottles are air-tight. For directions in working glass tubing, see Chemistry, Appendix 4. 5. How many valves are there in a force-pump? Where are they placed and in what direction do they open ? 6. What is the thing sometimes erroneously called "the force of suc- tion"? FIG. 74. 7. The volume of a given quantity of r, gas or vapor will be inversely proportional to the pressure exerted upon it. A cubic foot of steam (measured under a pressure of one atmosphere) will have what volume under a pressure of 8 atmospheres ? An*. 216 cubic inches. 138 NATURAL PHILOSOPHY. 195 REVIEW QUESTIONS. 1. (a.) When an inverted bottle is held under water, why does not the water fill it ? (b.) Why is it that any water enters the bottle ? 2. (a.) When the sails of a ship are taken down, why does not the boat suddenly stop ? (b.) What finally stops the ship ? 3. Why does not a stone move in a straight line when thrown horizontally ? 4. What is the difference between the words " vertical " and perpendicular " ? 5. When a bullet is flattened by the target, what " law " is thereby illustrated ? 6. Why does a freely falling body increase in velocity ? 7. What has become of the energy expended centuries ago in building the still remaining parts of the Egyptian pyra- mids? 8. What kind of a lever is represented in Fig. 75? In- dicate the positions for P, W and F. 9. What is always the dividend in problems in specific gravity ? 10. If a pendulum, having a certain length and a weight of 2 ounces, vibrates once a second, how often will a pendulum having the same length and a weight of 6 ounces vibrate ? 11. State the law of weight. 12. State, in ordinary language, the meaning of the formula, = \gt*. 18. What is the difference between Kinetic and Potential En- ergy? 195 REVIEW QUESTIONS. 139 FIG. 76. 14. A stratum of sand or gravel through which water can easily work its way, comes to the surface of the ground at a, Fig. 76. This stratum is inclosed between two curved strata imper- vious to water. When a hole is bored at c until it strikes the stratum, a, an artesian well is formed. Explain the biiion of the artesian well CHAPTER VI. ELECTRICITY AND MAGNETISM. SECTION I. GENERAL VIEW. 196. Importance of the Subject. We are now to begin the study of a class of phenomena of intense interest and almost unlimited practical importance. Every pupil has seen the lightning flashing in the stormy sky and wondered at the cause of the terrible, yet beauti- ful, display. Later in life, he was told that lightning is electricity. He has seen or heard of trees and houses shattered by the lightning stroke and may now remem- ber that if electricity can do this work, it must be a form of energy. He is told that the telegraph is electricity under con- trol and working at the will of man. When he reads ^he daily paper and learns of events that took place only a few hours before in Europe, Asia or Africa as well as in every part of his own country, he must, it seems, think of the wondrous speed of this form of energy as it courses under the waters of the sea and over the valleys, plains and mountains of the land. With the aid of ttig Ip7 GENERAL VIEW. 141 telephone, he talks with friends and recognizes their voices though they be miles away. He is a fit subject for pity if he has no desire to know how these things are done. He hears of the mysterious power that points the needle of the mariner's compass to the north and guides the ship across the trackless waters. He is told that this power is called magnetism, that it is closely asso- ciated with electricity and is frequently produced by it. He sees the electric light and is surprised to find that, in turn, magnetism produces electricity. He talks with the merchant or manufacturer about these and other things, that were lately wonders and conveniences but have now become common-place necessities of business life, and finds that the so-called "practical " man of the world is forced to acknowledge his great and ever in- creasing obligation to modern science. He probably gets the idea that it iv ill pay for him to learn more about these things. 197. Simple Apparatus. Provide two stout sticks of sealing-wax and one or two pieces of flannel folded into pads about 20 centimeters (8 inches) square ; two glass rods or stout tubes closed at one end, rs Q 30 or 40 centimeters in length and about 2 centimeters in diameter (long IG ' "ignition tubes" will answer) and one or two silk pads about 20 centimeters square, the pads being three or four layers thick ; a few pith balls about 1 centimeter in diameter (whittle them nearly round and finish by roll- ing them between the palms of the hands); a silk ribbon 142 NATURAL PHILOSOPHY. IQ7 about an inch wide and a foot long ; a balanced straw about a foot long, represented in Fig. 77. The ends of the straw carry two small discs of paper (bright colors preferable) fastened on by sealing-wax. The cap at the middle of the straw is a short piece of straw fastened by sealing-wax. This is supported upon the point of a sewing- needle, the other end of which is stuck upright into the cork of a small glass vial. From the ceiling or other convenient support, suspend one of the pith balls by a fine silk thread. (a.) The efficiency of the silk pad above mentioned may be increased by smearing one side with lard and applying an amal gam, made of one weight of tin, two of zinc and six of mercury. The amalgam which may be scraped from bits of a broken looking- glass answers the purpose admirably. A piece of cat-skin or other fur may be used instead of the flannel pads. See that the sealing- wax and glass rods, the flannel and silk pads are perfectly dry. Have them quite warm, that they may not condense moisture from the atmosphere. Experiment 69. Draw the silk ribbon between two layers of the warm flannel pad with considerable friction. Hold it near the wall of the room. The ribbon will be drawn to the wall and held there for some time. Place a sheet of paper on a warm board and briskly rub it with india-rubber. Hold it near the wall as you did the ribbon. Experiment 70. Briskly rub the sealing-wax with the flannel and bring the wax near the suspended pith ball. The ball will be drawn to the wax. Bring the wax near one end of the balanced straw ; it may be made to follow the wax round and round. Bring it near small scraps of paper, shreds of cotton and silk, feathers and gold leaf, bran and sawdust, and other light bodies; they are attracted to the wax. (Fig. 78.) 197 GENERAL 143 Experiment 71. Repeat all of these ex- periments with a glass rod which has been rubbed with the silk pad. Experiment 72. Make a light paper hoop or an empty egg- shell roll after your rod. FIG. 78. Experiment 73. Place an egg hi a wine-glass or an egg-cup. Upon the egg balance a yard-stick or a common lath. The end of the stick may be made to follow the rubbed rod round and round. Place the blackboard pointer or other stick in a wire loop or stiff paper stirrup suspended by a stout silk thread or narrow silk ribbon. It may be made to imitate the actions of the balanced straw or lath. FIG. 79. Experiment 74. Suspend the rubbed sealing-wax or glass rod as you did the blackboard pointer in the last experiment. Hold your hand near the end of the rod. It will turn round and approach your hand. NOTE. The pupil may be ingenious enough to invent new experiments for himself and the class. The ability to invent is often very valuable and may be acquired early in life. Most of the great inventors began making experiments when mere children. 144 NATURAL PHILOSOPHY. 198. Electric Attraction. The attractions man- ifested in these ex- periments were due to electricity that was developed by friction. Experiment 75. Bring the rubbed sealing-wax or glass rod near the pith ball again. It will attract the ball as in Experiment 70. Allow the ball to touch the rod and notice that in a mo- ment the ball is thrown off. If the ball be pursued with the rod, it will be found that the rod which at- tracted it a moment ago, now repels it. Evidently FIG. 80. the ball has acquired a new property. Experiment 76. Touch the ball with the finger. It seeks the rubbed rod, touches the rod, flies from the rod. Repeat the ex- periments with the sealing-wax after it has been rubbed with flannel. Experiment 77. Rub the glass rod with silk and bring it over the small scraps of paper, as in Experiment 71. Notice that after the attraction the paper bits do not merely fall down, they are thrown down. 199. Electric Repulsion. The repulsions mani- fested in these experiments were due to elec- tricity that was developed by friction. Such 199 GENERAL V7EW. 145 electricity is called frictional or static elec- tricity, The glass or wax is said to be electrified by friction. The ball, after obtaining its new property of repulsion by com- ing into contact with the glass or wax is said to be electrified by conduction. The sus- pended pith ball is called an electric pendulum. Experiment 78. Prepare a bat- tery solution according to the recipe given in 258, 6, using only half the quantity of each substance as therein directed. While the solution is cool- ing, provide a piece of sheet copper and one of sheet zinc, each about 10 centimeters (4 inches) long and 4 centimeters (1 inches) wide. To one end of each strip, solder (see Appendix) or otherwise fasten a piece of copper wire about 15 centimeters (6 inches) long and 1 millimeter ( T V to % inch) thick. Place the zinc strip in a common tumbler about three-fourths full of the battery solution. Notice the minute bubbles that break away from the surface of the zinc and rise to the surface of the liquid. These are bubbles of hydrogen, a com- bustible gas. The formation of the gas is due to chemical action between the zinc and the liquid. Experiment 79. Take the zinc from the tumbler and, while it is yet wet, rub a few drops of mercury (quicksilver) over its surface until it has a brilliant, silver-like appearance. Replace the ' zinc, thus amalgamated, in the solution and notice that no bub- bles are given off. FIG. 81. 146 NATURAL PHILOSOPHY. 200 Experiment 80. Place the copper strip in the liquid, taking care that it or its wire does not touch the zinc or its wire. No bubbles appear on either the zinc or the copper. It may he convenient to place a narrow glass strip be- tween the ends of the metal strips in the tum- bler to keep them apart. Experiment 81. Bring the upper ends of the strips together, as shown in Fig. 82, or, still better, join the two wires, as shown in Fig. 102. being sure that the wires are clean and bright where they are united. Notice the formation of bubbles on the surface of the copper, where none previously appeared. ( 248.) FIG. 83. 20O. Suspicion. It certainly seems that the connect- ing wire is an important part of the apparatus as now arranged and we are led to suspect that something un- usual is taking place in the wire itself. It is evident that we have a complete " circuit " through the liquid, the metal strip and the wire. Experiment 82. Untwist the wires or, in other words, "break the circuit." Connect the copper wires with a short piece of very f.ne iron wire. The connections should be made so that the cir- cuit shall include about 2 centimeters (f inch) of iron wire. The ron will become hot enough to burn the fingers or to ignite a small quantity of gun cotton twisted around it. Experiment 83. If one of the copper wires be twisted around one end of a small file and the other wire be drawn along its rough surface, a series of minute sparks will be produced as the circuit is rapidly made and broken. FIG. 201 GENERAL VIEW. 147 Experiment 84. Place the cell so that the joined wires sliaT. run north and south, passing directly over the needle of a small compass ($ 295, ft.) and near to it. The needle will instantly turn as though it were trying to place itself at right angles to the wire Break the circuit and the needle will swing hack to its north and south position. FIG. 84. 201. Certainty. We now feel sure that something unusual is taking place in the wire of our complete cii> cuit for we have seen the wire become hot, explode gun- cotton, yield sparks and exert a very mysterious influence upon the magnetic needle. As a matter of fact, we now have a current of electricity flowing through a galvanic cell. Electricity thus produced by chemical action is called galvanic or voltaic electricity. It is one form of current electricity. Experiment 85. Wrap a piece of writing paper around a large iron nail leaving the ends of the nail bare. Wind fifteen or twenty turns of stout copper wire around this paper wrapper, taking care that the coils of the wire spiral do not touch each other or the Iron, It is well to use cotton covered or " insulated * 148 NATURAL PHILOSOPHY. 202 wire. Connect the two ends of the wire spiral with the two wires of the galvanic cell or, in other words, put the spiral into the cir- cuit. Dip the end of the nail into iron filings. Some of the filings will cling to the nail in a remarkable manner. Upon breaking the circuit, the nail instantly loses its newly acquired power and drops the iron filings. If the experiment does not work satisfactorily, look carefully to all the connections of the circuit, see that the ends of the wires are clean and bright and that they are twisted together firmly. It may even be necessary to wash the plates, rub more mercury on the zinc and provide a fresh battery solution. 202. Temporary Magnets. You ha -e probably satisfied yourself that the nail has the power A attract- ing iron filings while the electric current is flow- ing through the wire. You have made an electro-magnet and its power of attracting iron is called magnetism. Satisfy yourself, by trial, that the nail loses its mag- netism as soon as the circuit is broken or the current ceases to flow around it and remember that your electro- magnet is a temporary magnet. Experiment 86. While the nail is magnetized, draw a sewinjr- needle four or five times from eye to point across one end of the electro-magnet. Dip the needle into iron filings ; some of them will cling to each end of it. 203. Permanent Magnets. When steel is treated as in the last experiment, it becomes permanently mag- netized. Experiment 87. Cut a thin slice from the end of a vial cork and, with its aid, float your magnetized needle upon the surface of 207 GENERAL VIEW. 149 a bowl or saucer of water. The needle comes to rest in a north and south position. Turn it from its chosen position and notice that after each displacement it resumes the same position and that the same end of the needle always points to the north. 204. A Simple Compass. t^ small magnetized steel bar freely suspended, is called a compass. The one that you have made may be less convenient but is as reliable as the compass of the mariner or the surveyor. 205. Artificial Magnets. The electro-nrugnet and the permanent magnet that you made are, of course, artificial magnets. There is a natural magnet known as lodestone. 206. Other Forms of Current Electricity. Electric currents may be generated by the action of other currents of electricity or by the action of magnets. Electricity thus developed is called induced electricity. A current of electricity that is generated by heating the junction of two metals that form part or all of a cir- cuit is called thermo-electricity. 207. The Different Forms of Electricity are Identical. So far as experiment can show, one form of electricity may have a particular property in greater degree than some other form but all are identical, each having all the properties of any of the others. 150 NATURAL PHILOSOPHY. 208 208. Recapitulation. To be amplified by the pupil for review. All Forms of Electricity are Iden- tical in Nature. s W M H P- $210 ELECTRICITY. 151 SECTION II. FRICTIONAL ELECTRICITY OR ELECTRIC CHARGES. 209. The Nature of Electricity. But little is known concerning the real nature of electricity. It is easier to tell what electricity can do than to tell what it is. The majority of modern physicists consider that elec- tricity is a form of energy producing peculiar phenomena; that it may be converted into other forms of energy and that all other forms of energy may be converted into it. Several theories have been advanced to account for electrical phenomena but none of them is satisfactory. 210. Electric Manifestations. Electricity may reveal itself as a charge or as a current. By means of friction, the glass rod or the sealing wax ( 198, 199) acquired an electrical charge and, conse- quently, the power of attracting and repelling light bodies: by means of chemical action, the galvanic cell generated electricity that manifested itself as a current. In this section, ^ve shall consider electricity that appears as a charge, i. e., static electricity. Experiment 88. Prepare two electric pendulums. Bring the electrified glass rod near the pith ball of one , after contact, the NATURAL PHILOSOPHY. 210 ball will be repelled by the glass. Bring the electrified sealing. wax near the second pith ball ; after contact it will be repelled by the wax. Satisfy yourself that the electrified glass will repel the first ; that the electrified sealing-wax will repel the second. Let the glass rod and the sealing wax change hands. The first ball was repelled by the glass ; it will be attracted by the sealing- wax. The second ball was repelled by the sealing-wax ; it will be attracted by the glass. Experiment 89. Suspend two pith balls, as shown in Fig 85, and touch them with a rubbed glass rod. Instead of continuing to hang side by side, they repel each other and fly apart. If the electrified glass rod be held near them, they separate still further. If the electrified sealing- wax, instead of the glass, be held near them they will fall nearer together. If the rubbed glass rod be suspended, as shown ia Fig. 79, it will be repelled by another rubbed glass rod but attracted by rubbed sealing-wax. 211. Two Kinds of Electricity. The electricity developed on glass "is different in kind from that developed on sealing-wax. They exhibit opposite forces to a third electrified body, each attracting what the other repels. The self-repul- sion of the parts of an electrified body may be beautifully illustrated by electrifying a soap-bubble which will thu& be made to expand.. FIG. 85. 215 FBICTIONAL ELECTRICITY. 153 212. Only Two Kinds of Electricity. All elec- trified bodies act like either the glass or the sealing-wax. 213. The Two Electricities Named. The elec- tricity developed on glass by rubbing it with silk is called positive or + . The electricity developed on sealing-wax by rubbing it with flannel is called negative or . The terms vitreous and resinous respectively were formerly used. 214. The Law of Electrostatics. The most im- portant electrostatic law may be stated thus : Electric charges of like signs repel each other; electric charges of opposite signs attract each other. 215. Electroscopes. An instrument used to detect the presence of electricity, or to determine its kind, is called an electroscope. The electric pendulum ( 199) is a common form of the electroscope. Two strips of the thinnest tissue paper hanging side by side constitute a simple electroscope. It is well to prepare the paper beforehand by soaking in a strong solution of salt in water and drying. The balanced straw (Fig. 77) or, better yet, two gilt pith balls connected by a light' needle of glass or sealing- wax balanced horizontally on a vertical pivot or a goose- quill balanced on the point of a sewing-needle, makes $ convenient electroscope. J54 NATURAL PHILOSOPHY. 215 The gold leaf electroscope is represented in Fig. 86. A metallic rod, which passes through the cork of a glass vessel, termi- nates below in two narrow strips of gold leaf and above in a metallic knob or plate. The object of the vessel is to protect the leaves from disturbance by air currents. The upper part of the glass is often coated with a solu- tion of sealing-wax or shellac in alcohol, to lessen the deposition of moisture from the atmosphere. This instrument may be made by the pupil and, when well made, is very delicate. Experiment 90. From a horizontal glass rod or tightly- stretched silk cord, suspend a fine copper wire, a linen thread and two silk threads, each at least a yard long. To the lower end of each, attach a metal weight of any kind. Place the weight sup- ported by the wire upon the plate of the gold leaf electroscope. Bring the electrified glass rod near the upper end of the wire ; the gold leaves instantly diverge. Repeat the experiment with the linen thread : in a little while the leaves diverge. Repeat the experiment with the dry silk -thread ; the leaves do not diverge at all. Rub the rod upon the upper end of the silk thread ; no divergence yet appears. Wet the second silk cord thoroughly and, with it, repeat the experiment ; the leaves then diverge PIG. 86 217 FRICTION AL ELECTRICITY. 155 216. Conductors. Such experiments clearly show that some substances transmit electricity readily and that others do not. Those that offer little resistance to the passage of electricity are called conductors; those that offer great resistance are called non-conductors or insulators. A conductor supported by a non-conductor is said to be insulated. (a.) In the following table, the substances named are arranged in the order of their conductivity : Conductors. 1. Metals. 2. Charcoal. 3. Graphite. 4. Acids. 5. Salt wa'ir. 6. Freshwater. 7. Vegetables. 8. Animals. 9. Linen. 10. Cotton. 11. Dry wood. 12. Paper. 13. Silk. ;14. India rubber. Insulators. 15. Porcelain. 16. Glass. 17. Sealing-wax. 18. Vulcanite. The fact that a conductor in the air may be insulated, shows that air is a non conductor. Dry air is a very good insulator, but moist air is a fairly good conductor for electricity of high potential. All experiments in frictional electricity should, therefore, be performed in clear, cold weather when the atmosphere is dry, for a moist atmosphere renders insulation for a considerable length of time impossible. Experiment 91. Support a yard-stick or common lath upon a glass tumbler. Bring the glass rod, electrified by rubbing it with silk, to one end of the stick and hold some small pieces of paper under the other end of the stick. The paper will be attracted and repelled by the stick as it previously was by the glass itself. The electricity passed along the stick from end to end. 217. Tension. Electricity exists under widely dif- ferent conditions with respect to its ability to force its way through a poor copductor or to leap across a gap. 156 NATURAL PHILOSOPHY. 217 The electricity developed in a galvanic cell will not pass through even a very thin piece of dry wood ; the electricity developed by rubbing the glass rod will pass through several feet of dry wood. It would require a battery of many cells to force a current across a gap of y^ of an inch. It is not diffi- cult to force frictional electricity across a gap of several inches while we all know that, in the case of lightning, electricity leaps across a gap of many hundred feet. In the one case, the electricity is said to be of low potential; in the other case, it is said to be of high po- tential. We are now dealing with electricity of high potential. The terms, "low tension" and "high tension" are often used in the same sense. 218. Potential. The term electrical potential (or simply potential) has reference to the electrical condi- tion of a body or to its degree of electrification. If the potential of A be higher than that of B, and the two bodies be connected by a good conductor, an electric current will flow from A to B until the poten- tials are alike. Difference of potential is somewhat analogous to differ- ence of liquid level and gives rise to electromotive force. 219. Electromotive Force. Electromotive force (often written E. M. F.) is the mysterious power which causes electricity to move from one point to another. It is somewhat analogous to hydrostatic pressure. Wbcr- S 220 FRICTION AL ELECTRICITY. 151 ever there is difference of potential, there is E. M. F. t but the terms are not synonymous. The unit of electromotive force is called a volt. A volt is a little less than the E. M. F. of a Danieli dell ( 260). 220. Resistance. Every electric circuit offers a re- sistance to the passage of the current. This resistance will, of course, depend largely upon the conductivity of the material used for the circuit. With a given material for the conductor, the resistance varies directly as the length and in- versely as the weight of a given length. If one conductor is twice as long as another made of the same kind of wire, the resistance of the longer will be twice as great as that of the shorter. If one conductor be twice the diameter of another made of the same length and material, the weight per foot or yard will be (2 Z =) four times as great and the resistance of the first will be one-fourth as great as that of the second. If they be made of the same material and length, one weighing twice as much per foot as the latter, the resist- ance of the former will be half as great as that of the latter. The unit of resistance is called an ohm. A galvanized iron (telegraph) wire, 4 millimeters in diameter and 100 meters long, or a pure copper wire, 1 millimeter in diameter and 48 meters long, has a re- sistance of about one ohm. A megohm is a million ohms. 158 XAWRAL P&ILOSOPHT. 221 221. Charging by Contact. If an insulated, un- electrified conductor be brought into contact with a similar conductor that is electrified, or near enough to it for the passage of an electric spark, electricity will pass from the latter to the former until the two con- ductors are equally charged with the same kind of elec- tricity. The former is said to be charged by con- duction. 222. Induction. Actual contact with an electrified body is not necessary for the manifestation of electric action in an unelectri- q. - fied body. When an ft B | \\J\c electrified body, C, is oo vb\ brought near an insu- .^ % *^ with electric pendu- lums, as shown in the figure, the latter shows electric action. The electricity of C rebels one kind of electricity in B and attracts the other, thus separating them. The second body, B, is then said to be polarized. The two kinds of electricity in J9, each of which a moment ago rendered the other powerless, are still there but they have been separated and each clothed with its proper power. This effect is due to the action of the electrified body, C, which is said to produce electric ^Pj\ lated unelectrified con- m^\ ductor, B, provided g 223 PRICTIONAL ELECTRICITY. 159 separation by induction. When C is removed, the sep- arated electricities of B again mingle and neutralize each other. (a.) Conductors for the purposes of this and similar experiments may be made of wood covered with tin-foil, gold leaf or Dutch leaf. They may be insulated by fastening them on top of long necked bottles or sticks of sealing-wax or by suspending them by silk threads. (6.) Prick a pin hole in each end of a hen's egg and blow on the white and the yolk. Paste tin-foil smoothly over the whole surface of the egg. Fasten one end of a white silk thread to the egg with a drop of melted sealing wax so that the egg may hang suspended with its greater diameter horizontal. Three or four such insulated conductors will be found convenient. Sometimes it is convenient for each egg to have two thread supports. Place a loop or ring at the free end of each thread. When the loops are placed on a horizontal rod (e. g., a piece of glass tubing), the greater diameters of the suspended eggs should lie in the same straight line. An elongated conductor, like A B of Fig. 88, may be made by hanging two or three egg conductors so that they are in contact 223. A Neutral Line. If an insulated conductor, bearing a number of pith ball (or paper) electroscopes, be brought near an electrified body, C, (Fig. 88), but not near enough for a spark to pass between them, the pith balls near the ends of the conductor will diverge, show- ing the presence of separated or uncombined electricity. The pith balls at the middle of the conductor will not diverge, marking thus a neutral line. This action will take place across a considerable dis- tance even if a large sheet of glass be held between A and C. 160 NATURAL PHILOSOPHY. 223 If C has a positive charge, the charge at A will be negative and that at B will be positive, as may be shown by charging an electric pendulum and testing at A and B, as shown in Fig. 88. FIG. 88. If C be removed or " discharged " by touching it with the hand, all traces of electrical separation in A B will disappear. The charged pith ball will be attracted at every point of A B. 224. Charging a Body by Induction. If the polarized conductor be touched with the band, or other- wise placed in electric communication with the earth, the electricity repelled by C (Fig. 88), will escape, and the pith balls at B will fall together. The electricity at the other end will be held by the mutual attraction between it and its opposite kind at C. The line of communication with the ground being broken and the conductor being removed from the vicinity of C, the former will be found charged with electricity opposite in kind to that of (7. 226 ELECTRIC CHARGES. 161 A body may be thus charged by induction with no loss to the inducing body. 225. Polarization Precedes Attraction. Wforn an electrified glass rod is brought near an insulated uncharged pith ball (electric pendulum), the pith ball is polarized, as shown in the figure. As the of the ball is nearer the -f- of the glass than is the + of the ball, the attraction is greater than the repulsion. If the pith ball be suspended, not p IG g9. by a silk thread but by some good conductor, the attraction will be more marked, for the + of the ball will escape to the earth through the sup- port and, thus, the repelling influence will be removed. 226. Provisional Theory of Electricity. While the real nature of electricity remains unknown, the fol- lowing theory will be found convenient for classifying results already attained and suggesting directions for further inquiry. But we must not let it influence our judgment as to what is the true and full explanation of electrical phenomena, which explanation may be found hereafter : We may assume that a neutral or unelectri- fied body contains equal and equally distributed quantities of positive and of negative electricity. We may assume these electricities to be un- limited in amount. We shall then conceive that a positively elec- NATURAL PHILOSOPHY. 227 trified body has an excess of 4- electricity and that a negatively electrified body has an excess f electricity. fn this light, we shall see that communicating 4- electricity to a body is equivalent to removing an equal amount of electricity from it, and conversely. 227. The Electrophorus. This simple instrument consists generally of a shal- low tinned pan filled with resin, oil Aivhich rests a mov- able metallic cover with a glass or other insulating handle. The resinous plate may be replaced by a piece of vulcanized India-rubber. The metal surface and the resinous surface touch at only a few points; they are PI practically separated by a Fio. 90. thin layer of air. (a.) The resinous plate may be prepared by melting together equal quantities of rosin and Venice turpentine and then adding a like quantity of shellac. The substances should be heated gradu- ally and stirred together so as to prevent the forming of bubbles. Take care that the mixture does not take fire in course of prepara- tion. The Venice turpentine is desirable but not necessary. For a handle, a stout wire may be soldered to the centre of the disc and covered with rubber tubing; or a piece of sealing wax, of convenient size, may be fastened to the disc for the purpose. A till better plan is to make the cover of wood, a little less in ELECTRIC CHARGES. 163 diameter than the resinous plate and with its edges carefully rounded off. For a handle, a glass rod or tube may be tightly thrust or cemented into a hole in the middle of the cover. Paste tin toil all over the cover and smooth down all rough edges of the foil with the finger nail or a paper folder. The wire support for a pith ball or paper electroscope may be thrust into'the wood of the cover, care being taken that it touches the tin -foil. (b.) The plate is rubbed or struck with flannel or catskin, and thus negatively electrified. The cover is then placed upon the resin and thus polarized by induction. Touch tlie cover with the fineer, as shown in Fie:. 90 ; the free electricity escapes and the leaves fall. The cover is now charged positively. The charged cover will give a spark to the knuckle or other unelectrified body presented to it. (fig. 91.) 228. The Elec- trophorus charged by Induction. The cover may be thus charged and dis- charged an indefinite number of times, in favorable weather, without a second electrifying of the resinous plate. This could not happen if the electricity of the cover were drawn from the plate. More- over, JT v^ ch'irsre ^ the cover were drawn lie 91 _64 NATURAL PHILOSOPHY. 22Q from the plate, it would be , and not 4* . The cover is charged by induction and not by conduction. 229. Whence this Energy ? At every discharge of the electrophorus, it gives a definite amount of elec- tricity, capable of doing a definite amount of work. As this is obtained not by the expenditure of any part of the original charge, we are led to seek for the source of this apparently unlimited supply of energy. "As a matter of fact, it is a little harder work to lift the cover when it is charged with the + electricity than if it were not charged for, when charged, there is the force of the electric attraction to be overcome as well as ihe force of gravity. Slightly harder work is done at the expense of the muscular energies of the operator and this is the real origin of the energy stored up in the separate charges." 230. A Charge Resides on the Surface. Many experiments have been made showing that when a conductor is electrified, the electricity passes to the surface and escapes if the body be not insu- lated. Experiment 92. Place a carrot horizontally upon an insulating support. Into one end of the carrot stick a sewing-needle. Bring the electrified glass rod near the point of the needle without touching it. The electricity of the carrot quietly escapes front the point to the rod and the carrot is charged with the + elec, tricity that remains. 231. Density. Experiments show that when a spherical conductor is charged, the electricity is evenlt distributed over jtfce. SJK;&@, ggpvided no other electrified 232 ELECTRIC CHARGES. 165 body be near. The electric density is the same at ever^ point. Experiments on an elongated cylinder, like the prime conductor of the electric machine, show that the density is greater at the ends. On an egg-shaped conductor, like that shown in Fig. 92, the density is greatest at the smaller end. In general, the electric density is very great at any pointed part of a charged conductor. FIG. 92. This density at a point may become so great that the electricity will escape rapidly and quietly, the air par- ticles rapidly carrying off the charge by convection. This explains the action of points, which plays so im- portant a part in the action of electric machines. 232. Electric Machines. Machines have been made for developing larger supplies of electricity more easily than can be done with a rod of glass or sealing- wax or with the electrophorus. Each of them consist; of one part for producing the electricity and anoth' part for collecting it. 166 NATURAL PHILOSOPHY. 233. The Plate Electric Machine. This instru- ment is represented in Fig. 93. It consists of an insu- lator (or electric), a rubber, a negative and a positive or prime conductor. The electric is a glass (or ebonite) plate, A. generally one, two or three feet in diameter. FIG. 93. This plate has an axis, B, and handle, C, and is sup- ported upon two upright columns. The rubber, D, 13 made of two cushions of silk or leather, covered with amalgam (see 197, a). They press upon the sides of the plate and are supported from the negative conductor, with which they are in electric connection. The negative conductor, N, is supported upon an insulating column and, when only positive electricity is desired, is placed in electrical connection with the earth by means of a chain or wire, W. The prime conductor, P, is insulated, One end of the prime conductor termi- 234 ELECTRIC CHARGES. 167 nates in two arras, F, which extend one on either side ol the plate. These arms, being studded witli points pro- jecting toward the plate, are called combs. The teeth of the combs do not quite touch the plate. A silk bag, S, is often supported so as to enclose the lower part of the plate. All parts of the instrument except the teeth of the combs are carefully rounded and polished, sharp points and edges being avoided to prevent the escape of electricity as already explained. This avoiding of points and edges is to be regarded in all apparatus for use with electricity of high potential. (a.) The pupil may make a plate machine without much ex- pense. A glazier will cut for him a disc of plate glass, possibly from a fragment on hand. The edges of this disc may be rounded on a wet grindstone. A hole may be bored in the middle with a round file kept moistened with a solution of camphor in turpentine. The conductors, N and P, may be made of wood covered with gold foil or Dutch leaf and supported on pieces of stout glass tub- ing. The prime conductor may well have two such supports. The arms may consist of two stout wires thrust into the end of a prime conductor, their free ends being provided with knobs of lead or other metal. The combs may be made by soldering pin points to one side of each arm. See that the gold foil makes actual contact with the metal arms. See that all metal parts except the pin points are polished smooth. The columns that support the plate may be made of seasoned wood. The part of the handle to which the hand is applied may be made of glass or insulated by covering it with rubber tubing. 234. Operation of the Plate Machine. The plate is turned by the handle. Electric separation is produced by the friction of the rubbers. The -f elec- tricity of the rubber and negative conductor passes tc 168 NATURAL PHILOSOPHY. 234 the plate; the electricity of the plate passes to the rubber and negative conductor. The part of the plate thus positively charged passes to the combs of the prime conductor. The + of the plate acts inductively upon the prime conductor, polarizes it, repels the + and at- tracts the electricities. Some of the electricity thus attracted streams from the points of the combs against the glass, while some of the + electricity of the glass escapes to the prime con- ductor. This neutralizes that part of the plate and leaves the prime conductor positively charged. The rubber and negative conductor are kept in equi- librium by means of their connection with the earth. As the plate revolves, the lower part, passing from N to P, is positively charged ; the upper part, passing from P to N, is neutralized. If negative electricity be desired, the chain or other ground connection is changed from N to P, and the charge taken from N. NOTE. Other forms of electric machines are made. One of the latest of these, known as the Toepler-Holtz, is very compact and efficient and remarkably free from the limitations of atmos- pheric conditions. It may be described as a continuously acting electrophoras ( 227). A very good one may be bought of JAMES W. QUEEN & Co., of Philadelphia, for $25 or more. One should be provided for the school in some way if possible. Any electrical machine should be free from dust and perfectly dry when used. It should be warmer than the atmosphere of the room, that it may not condense moisture from the surrounding air. The drier the atmosphere the better will be the action of the machine. 235- Construction of the Leyden Jar. The Leyden jar consists of a glass jar, coated within and with- 237 ELECTRIC CHARGES. 169 out for about two-thirds its height with tin-foil, and a metallic rod, communicating by means of a small chain with the inner coat and terminat- ing above in. a knob. The upper part oi the jar and the cork which closes the mouth of the jar and supports the rod are generally coated with sealing-wax or shellac varnish to lessen the deposition of moisture from the air. (a.) Select a candy or fruit jar of greenish glass ; paste tin-foil within and without, as above described, p IG> using flour paste ; thrust a wire through a dry cork ; bend the wire so that, when the cork is in its place, the wire shall touch the tin-foil on the side of the bottle without tearing it ; solder the upper end of the wire to a smooth button or thrust it into a lead bullet ; charge your Leyden jar with a few sparks from the electrophorus and take a shock. 236. Charging the Leyden Jar. To charge the jar, hold it in the hand, as shown in Fig. 95, and bring the knob near or into contact with the prime conductor of an electrical machine which is in action. FIG. 95. 237. Discharging the Leyden Jar. The jar might be discharged by touching the knob with the NATURAL PHILOSOPHY. 237 In this case the experimenter will feel a " shock." If the charge be intense, the shock will be painful or even dangerous. It is better to use a "discharger," one form of which is represented in Fig. 96. This consists of two metal arms hinged together, carry- ing knobs at their free ends and carried by insulating handles. The outer coat should FlG - 96 ' be touched first. (a.) A good discharger may be made by passing a piece of stout copper wire, about a foot long, through a piece of rubber tubing and providing a metal knob for each end of the wire. The flexi- bility of the wire avoids the necessity for a hinged joint. 238. Modes of Discharge. An electrified con- ductor may be discharged in at least three ways, viz., by the disruptive discharge, by the conveciive discharge and by the conductive discharge. The discharge in any of these ways is accom- panied by a transformation of energy, sound, light, heat, chemical action and other phenom- ena being produced. Experiment 93. Present a knuckle of the hand or a metal knob to the prime conductor of an electric machine and " draw sparks" therefrom. 239. The Disruptive Discharge. A discharge of electricity taking place suddenly through a non-con- ductor is called a disruptive discharge, e. g., the sparki drawn from an electric machine in action. 242 ELECTRIC CHARGES. 173 Experiment 94. At- tach a pointed wire to the prime conductor of the electric machine. The flame of a candle held near will be blown away, as shown in Fig. 97. If the candle be placed upon the prime conductor and a pointed conductor be held in the hand near the candle, the flame will still be blown away. Fl 97 - 240. The Convective Discharge. When electric- ity of high potential accumulates with so great a density as to electrify the neighboring particles of air which, driven by electric repulsion, fly off carrying part of the charge with them, we have what is called the convective discharge. Such discharges are best manifested in gases at low pressure, in tubes exhausted by an air-pump. 241. The Conductive Discharge. The flow of a continuous current of electricity constitutes the con- ductive discharge. When electricity flows through a wire from the prime conductor of an electric machine to the rubbers or from the positive pole of a voltaic cell or battery to the nega- tive, we have a conductive discharge. It will be con- sidered in the section especially devoted to voltaic electricity. 242. Lightning. When an electrified cloud floats over the earth, separated from it by a layer of insulating 172 NATURAL PHILOSOPHY. 242 air, the inductive influence of the cloud renders the ground beneath oppositely electrified. Then the cloud, ground and insulating air correspond respectively to the inner and outer coatings and the insulating glass of a Leyden jar. As the charge of a Leyden jar may be made so intense that the attraction of the separated electricities will result in their rushing together and thus piercing the jar, so the charge of a cloud may become sufficiently intense to overcome the resistance of the air and a light- ning stroke ensues. Such electric sparks are sometimes more than a mile in length but the duration is not more than 0.00001 of a second. 243. Lightning-Rods. The value of lightning-rods depends upon the tendency of electricity to follow the best conductor, and upon the effect of pointed conductors upon electrical density ( 231). The lightning-rod should be made of a good conduc- tor; copper is better than iron. It should terminate above in one or more points, tipped with some substance that can be corroded or fused only with extreme diffi- culty. Platinum or iridium is a metal which satisfies these conditions very well. The rod should extend above the highest point of the building in order to offer the electricity the shortest path to the ground. It is important to have each pro- jecting part of the building, as chimneys, towers and gables, protected by a separate rod. All metal work about the roofs or chimneys should be connected with the rod. 243 ELECTRIC CHAROES. 173 The rod should afford an unbroken connection ; the joints, if there be any, should be carefully made. The rod should terminate below in water or in earth that is always inoist. It is well to connect the rod with underground water-pipes when possible or with a large metal plate. Personal attention should be given to thia matter when the rod is put up as, being under ground and out of sight, this part of the rod is not easily inspected subsequently. A rod having a blunted tip, a broken joint or terminating in dry earth is more dangerous than no rod at all. Lightning-rod insulators are undesirable. (a.) The greatest value of a lightning-rod is due to its quiet work in the prevention of the lightning stroke. Bring the point of a knife-blade near the conductor of an electric machine in operation and notice the instant cessation of sparks. The quiet passage of electricity from the earth neutralizes the charge of the conductor and restores the electric equilibrium. In the same way, a lightning-rod tends to restore the electric equilibrium of the cloud and prevent the dangerous discharge. For this quiet but very valuable service, few persons ever give the rod any credit. Every leaf of the forest and every b'.ade of grass is a pointed coa iuctor acting in the same way. (231.) 174 NATURAL PHILOSOPHY. 243 NOTE. It is neither necessary nor very desirable that all of the following experiments be performed. Several of them involve the same principle ; but one school may have one piece of apparatus and another, another piece. Additional experiments may be found in The Elements of Natural Philosophy. Experiment 95. Figure 98 represents the " electric bells." The metal frame is hung from the prime con ductor. The right-hand bell is suspended by a wire ; the other bell is suspended by a silk cord and connected with the ground by means of a chain hanging on the floor. Work the machine slowly ; the clapper vibrates and rings the bells. Explain. _ Experiment 96. Place the two ends of a p " pane of window glass upon two books so that the pane shall be about two inches above the surface of a table. Place several pith balls or numerous bits of tissue paper on the table under the glass. Electrify the glass by rubbing its upper surface with silk. Notice the lively motions of the balls or paper bits. Experiment 97. Electrify a glass rod. Toss a small sheet of gold leaf into the air. Bring the rod near the leaf. The leaf is drawn toward the rod and then thrown off. Chase the leaf with the rod without letting it touch the ground. Explain. Experiment 98. If a pupil, standing upon an insulating stool (a board supported by four warm tumblers will answer) and hav- ing one hand upon the prime conductor of an electric machine in action, bring a knuckle of the other hand near one end of the balanced yard-stick Experiment 73, it will follow the knuckle. Explain. Experiment 99. Place a few bits of paper upon the cover of the electro>x>orus When the cover has been touched with the 243 ELECTRIC CBAKGES. 175 finger and lifted by the insulating handle, the paper will be thrown ofE Explain. Experiment 100. Electrify a doll's head covered with long, dry hair. The hairs will stand out, as shown in Fig. 99, producing an exaggerated appear- ance of fright. Experiment 101. Vary Experiment 94 by placing the candle on the prime conductor and holding the point of a needle toward it, as shown in Fig. 100. The Same will be driven away by the convective discharge. FIG Fio. 100. Experiment 102. Fasten one end of a long fine wire to the knob of the electroscope. Charge the disc of the elec- trophorus and touch it to the other end of the wire. Notice the action of the electroscope. Try the experiment with a dry silk thread instead of the wire. Describe the action of the elec- troscope. What does this ex- periment teach about the wire and the thread? Experiment 103. Place an "electric whirl" (which consists of a set of horizontal wire arms radiating from a pivot-supported centre, the pointed ends being all bent in the same direction) upon the prime conductor. Work the machine and the arms will revolve. (See Fig. 101.) Fio. 101 176 NATURAL PHILOSOPHY. 243 Experiment 104. Place a pupil on the insulating stool. Let him hold an electroscope, with his finger on the knob. Let a second pupil strike him on the back with a cat-skin. Notice the leaves of the electroscope at every stroke. Experiment 105. Get a smooth board and a sheet of paper. Heat them both before the fire. Place the paper on the board and rub it vigorously with a piece of india-rubber. Remove the elec- trified paper from the board and hold it near the wall. It will fly to the wall and cling to it for some time. Experiment 106. Electrify the paper as in the last experi ment. Remove it from the board and hold it by the edges. Let a pupil place a pith ball on the paper. Notice and explain the action of the ball. Experiment 107. Electrify the paper as in the last experiment. While it is lying on the board, cut it into narrow strips. Take hold of all the strips at one end and lift them from the board. Notice the repulsion of light bodies similarly charged. Experiment 108. Place a pupil upon an insulating stool (a board supported by four warm tumblers will answer) and charge him by giving him twenty or more sparks from the disc of the electrophorus. Let another pupil, not insulated, bring his knuckle to any part of the body of the first pupil. Let the pupils describe the result. Experiment 109. Cover one knob of the discharger with gun- cotton sprinkled with powdered rosin. When the Leyden jar is discharged with this discharger, the cotton and rosin are ignited. Experiment 110. Let a pupil, standing on an insulating stool, become charged by holding one hand on the prime conductor when the machine is in operation. If he then bring his knuckle to a metal burner from which a jet of gas is issuing, a spark will nass between the knuckle and the burner, igniting the gas. An Argand or Bunsen burner answers well for this experiment. The experiment may be modified by using, instead of the knuckle, an 244 ELECTRIC CHARGdS. 17? Jcicle.held in the hand. The gas burner may be replaced by a pupil (not insulated) holding a spoonful of ether or chloroform which readily gives off an easily combustible vapor. 244. Relation of Electricity to Energy. The work necessarily performed in operating an electric machine is not all expended in overcoming inertia and friction. Much of it is employed in producing electric separation. It matters not whether this separation be the separation of two fluids or of something else. Wliatever be the nature of the realities sepa- rated, mechanical kinetic energy is employed in the separation and converted into the potential variety ( 100). An electrified pith ball or a charged Leyden jar is simply an electrostatical reservoir of potential energy. In the discharging of such a body, the passage of the current is accompanied by a loss of potential energy. What becomes of this energy ? This leads us to look for effects due to it, to work done by it. Many illustrations of work thus done have been fur- nished in the experiments just described. In every case of electric attraction or repulsion, we have an evident reconversion of this potential energy into mechanical kinetic energy. We shall soon see that the sound, heat and light accompanying electric discharges are forms of energy due to the conversion of the potential energy of electric separation. We shall see other effects, more or less powerful, when we come to study voltaic and other forms of current electricity 178 NATURAL PHILOSOPHY. 245 245. Recapitulation. To be amplified by the pupil for review. f KINDS AND NAMES. ELECTROSTATIC LAWS. ELECTROSCOPES. ( CONDUCTORS. INSULATORS. L RKSISTANCB. 3 - POTENTIAL AND E. M. F. CAPACITY. BY CONTACT. ELECTRIFICATION. { BY INDUCTION. | " Electrophoru* DISTRIBUTION OF CHARGE. DENSITY. LEYDEN JAR. O DisRUPTive. DISCHARGE \ CONVECTIVE. CONDUCTIVB. ( LIGHTNING. THUNDER-STORMS \ \ LIGHTNING-ROD*, EXPERIMENTS. RELATION TO ENERGY 245 EXERCISES. 279 EXERCISES. 1. Why do we regard the two electric charges produced simul- taneously by rubbing together two bodies as being of opposite kinds? 2. Quickly pass a rubber comb through, the hair and determine whether the electricity of the comb is positive or negative. 3. Twist some tissue paper into a loose roll about six inches long. Stick a pin through the middle of the roll into a vertical 3upport. Present an electrified rod to one end of the roll and thus cause the paper to turn about the pin as an axis. Give this piece of scientific apparatus an appropriate name. 4. (a.) Prepare two wire stirrups, A and B, like those shown in Fig. 79, and suspend them by threads. Electrify two glass rods by rubbing them with silk and j-lace them in the stirrups. Bring A near B. Notice the repulsion. (6.) Repeat the experiment with two sticks of sealing-wax that have been electrified by rubbing with flannel. Notice the repulsion, (c.) Place an electrified glass rod in A and an electrified stick of sealing-wax in B. Notice the attraction. Give the law illustrated by these experiments. 5. Why is it desirable that a glass rod used for electrification be warmer than the atmosphere of the room where it is used ? 6. Electrify one insulated egg-shell conductor ( 222, 6). Bring it near a second conductor but not into contact with it. Touch the second egg-shell with the finger, (a.) Experimentally, determine whether the second egg-shell is electrified or not. (6.) If you find that it is, what word explains the method of charging? (e.) If the second egg-shell is charged, will its potential and the potential of the first be of the same or of opposite signs 1 180 NATURAL PHILOSOPHY. 246 SECTION III. VOLTAIC AND THERMO-ELECTRICITY. 246, Chemical Action. All chemical changes ( 11, a,) are accompanied by electric separation. The chemical action between liquids and metals gives results the most satisfactory. Electricity thus developed is called voltaic or galvanic electricity. 247. Current Electricity. The principal classes of electric currents are as follows : ( 1.) Currents produced by chemical action, i. e., voltaic currents. (2.) Currents produced by heat, i. e., thermo- electric currents. (3.) Currents produced by other electric cur- rents or by magnets, i. e., induced cur- rents. (a.) We have seen that, when a body having an electrical charge is properly connected with another of lower potential, there is a transfer of electricity from the former to the latter. This implies that there is an electric current. But this current is only mo- mentary and of little importance in comparison with the currents that we are about to consider. (J>.) Current electricity may differ from static electricity in quau tity, electromotive force, etc., but not in its nature. 249 VOLTAIC ELECTRICITY. 181 248. The Voltaic Current. copper and one of zinc are placed in dilute sulphuric acid or in a battery solution like the one already used, the two strips being connected above the acid by a wire conductor, a current of electricity is produced. The apparatus here described is called a voltaic or galvanic element or cell. When a strip of FIG. 102. (a.) For voltaic purposes, the sul- phuric acid should be diluted by slowly pouring the acid into ten or twelve times its bulk of soft water. Do not pour the water into the acid. 249. Direction of the Current. The metal most vigorously acted upon by the liquid constitutes the gen- erating or positive plate ; the other, the collecting or negative plate. When the wires from the two plates are in contact, it is said that the circuit is closed; when the plates are not thus in electric connection, it is said that the circuit is broken. When the circuit is broken, the ends of the wires are called poles or electrodes. The negative pole is at- tached to the positive plate and vice versa. Strips of platinum are often fastened to the ends of the wires; these platinum strips then constitute the electrodes. In the liquid, the current is from the + to the plate. In the wire, the current is from the 182 NATURAL PHILOSOPHY. 2$Q + to the electrode. In each case, the current passes from -f to . The direction of the current is indicated by the arrows in Fig. 102. 250. Internal Resistance. We may imagine that the two plates of a voltaic cell are connected by a liquid prism. The greater the distance between the plates, the longer this prism and the greater its resistance. The larger the plates, the larger the prism and the less its resistance. Gases are poor conductors. Hence, the hydrogen bubbles that often adhere to the negative plate increase the internal resistance of the cell by lessening the effec- tive surface of the plate. ( 272.) 251. The Ampere. The strength of current or its rate of flow will depend upon electromotive force and resistance, increasing with the former and decreasing with the latter. T/ie unit of current is called an ampere. (a.) At any given instant, the current is the same at every part of the circuit. 252. Ohm's Law. The following important for- mula is known as Ohm's law : r Us = Amperes, or 2=0. Ohms~ R (a.) If we have a difference of potential that secures an E. M. F. of 18 volls and if the total resistance of the circuit be 3 ohms, the strength of the current will be 6 amperes. 18 + 3 = 6. 2 55 VOLTAIC ELECTRICITY. 183 253. The Coulomb. The unit of quantity is called the coulomb. It is the quantity of elec- tricity given by a one ampere current in one .econa. (d.) A 10 ampere current will give 30 coulombs in 3 seconds. 254. Amalgamating the Zinc. Ordinary com- mercial zinc is far from being pure. The chemically pure metal is expensive. When impure zinc is used, small closed circuits are formed between the particles of foreign matter and the particles of zinc. This local action, which takes place even when the circuit of the cell or battery is broken, rapidly destroys the zinc plate and contributes nothing to the general current. This waste is pre- vented by frequently amalgamating the zinc. This ia done by cleaning the plate in dilute acid and then rub- bing it with mercury. See Elements of Natural Phi- losophy, 386, a. 255. Polarization. It was stated in 250 that the accumulation of hydrogen bubbles at the negative plate increases the internal resistance of the cell. But the hydrogen affects the current in another way. It acts like a positive plate (being almost as oxidizable as the zinc) and sets up an opposing electromotive force which tends to set a current in the opposite direction. A cell or battery in this condition is said to be polarized. Sometimes, as a result of polarization, the strength of the current falls off very greatly within a few minutes after closing the circuit 184 NATURAL PHILOSOPHY. 256 256. Varieties of Voltaic Cells. All voltaic cellg belong to one of two classes : (1.) Those using only one liquid. (2.) Those using two liquids. All of the earlier batteries were composed of one-fluid cells. * 257. Smee's Cell. A Smee's cell is represented by Fig. 103. It consists of a platinized silver plate placed between two zinc plates hung in dilate sulphuric acid. The hydrogen bubbles accumulate at the points of the rough platinum sur- face and are more quickly carried up to the surface of the liquid and thus gotten rid of. The cell has an electro- motive force of about 0.65 volts. 258. Potassium Di-chromate Cell. The potassium di-chromate cell has a zinc plate hung between two carbon plates. lution of potassium di-chromate in dilute sulphuric acid is the liquid used. The hydrogen is given an opportunity for chemical union as fast as it is liberated. The E. M. F. of this cell is great to start with (from 1.8 to 2.3 volts) but it falls very quickly when the external resistance is small. It quickly recovers and may be used with advantage where powerful currents of short duration are wanted. It is the only single fluid cell that is free from polarization. 259 VOLTAIC ELECTRICITY. 185 (a.) The bottle form of this cell, represented in Fig. 104, is the most convenient for the laboratory or lecture table. By means of the sliding rod, the zinc plate may be raised out of the solution when not in use. Thus adjusted, the cell may remain for months without any action, if desired, and be ready at a moment's notice. (&.) One of the best proportions for the solution is as follows : One gallon of water, one pound of di-chromate of potash, and from a half pint to a pint of sulphuric acid, according to the energy of action desired. A small quantity of nitric acid added to the solution increases the constancy of the bat- tery. 259. The Leclanche Cell.- FlG " 104 This cell, shown in Fig. 105, contains a zinc plate or rod and a porous earthenware cup filled with carbon and peroxide of manganese. This cup replaces the other metal plate. The liquid used is a solution of ammonium chloride (sal ammoniac) in water. This cell is tolerably constant if it be not used to pro- duce very strong currents, but its great merib is that it is very permanent. It will keep in good condition for months with very little attention, furnishing a current for a short time whenever wanted. It is much used for working telephones, electric bells (see H in Fig. 105) and clocks, railway signals, etc. The manganese oxide prevents polarization by destroy- ing the hydrogen bubbles. If the cell be used continuously for some time, the power of the cell weakens owing to the accumulation of hydrogen, but if left to itself it 186 NATURAL PHILOSOPHY. 259 gradually recovers as the hydrogen is oxidized. Some- times the manganese oxide is applied to the face of the carbon and the porous cup dispensed with. This cell has an E. M. F. of about 1.5 volts. It should be left on open circuit when not in use. FIG. 105- 261 VOLTAIC ELECTRICITY. 187 FIG. 106. 260. Daniell's Cell. This cell consists of a copper plate immersed in a saturated solution of copper sulphate (blue vitriol) and a zinc plate immersed in dilute sul- phuric acid or a solution of zinc sulphate (white vitriol). The two liquids are separated ; usually one liquid is contained in a porous cup placed in the other liquid. Large crystals of copper sulphate are placed on a perforated shelf in the solution of copper sul- phate to keep the latter saturated. Such a cell will furnish a nearly constant current, with an E. M. F. of 1.079 volts and keep in order for a long time. // should be kepi on closed circuit when not in use. The outer cell is sometimes made of copper and serves as the copper plate, as is shown in Fig. 106. The hydrogen passes through the porous cell and acts upon the solution of copper sulphate. Copper, instead of hydrogen, is deposited upon the copper plate. Polarization is thus avoided. 261. The Gravity Cell. This is a modification of the Daniell's cell, no porous cup being used. The copper plate is placed at the bottom of the cell and the zinc plate near the FIG. 107. 188 NATURAL PHILOSOPHY. 26l top. Crystals of copper sulphate are piled upon the cop- per plate and covered with a saturated solution of copper sulphate. Water or, preferably, a weak solution of zinc sulphate rests upon the blue solution below and covers the zinc plate. The two solutions are of different specific gravities and remain clearly separated if the cell be kept on closed circuit when not in use. (Fig. 107.) This cell is very largely used in working telegraph lines. It is sometimes called the Callaud cell. 262. Grove's Cell. The outer vessel of a Grove's ceil contains dilute sulphuric acid. In this is placed a hollow cylinder of zinc. Within the zinc cylinder is placed a porous cup containing strong nitric acid. The negative plate is a strip of platinum placed in the nitric acid. The hydrogen passes through the porous cup and reduces the nitric acid to nitrogen peroxide, which escapes as brownish-red fumes. These nitrogen fumes are disagreeable and injurious; it is well, therefore, to place the battery in a ventilating chamber or outside the experimenting room. The E. M. F. of the Grove cell, under favorable con- ditions, is nearly two volts, while its internal resistance is small, being about one-fifth that of a Daniell's cell. It is much used for working induction coils ( 306), for generating the electric light, etc. It is, however, troublesome to fit up and should have its liquids renewed every day that it is used. Fig. 109 represents a, Grove's battery with cells joined in series. 263. Bunsen's Cell. Bunsen's cell (Fig. 108) differs 26 4 VOLTAIC ELECTRICITY. 189 from Grove's in the use of carbon instead of expensive platinum for the negative plate, thus reducing the cost. The plates are made larger than for Grove's battery. Its E. M. F. is about the same as that of the Grove cell but its internal resist- ance is greater. Fig. 110 represents a battery of Bun- sen's cells joined in multiple FIG. 108. 264. A Voltaic Bat- tery. A number of vol- taic elements connected in such a manner that the current has the same direction in all, constitutes a voltaic battery. The usual method is to connect the positive plate of one clement with the negative plate of the next, as shown in Fig. 109. When thus connected, they are said to be coupled "tandem" or "in series." Sometimes all of the positive plates are connected by a wire and all of the negative plates by another wire. The cells are then said to be joined "parallel," "abreast" or "in multiple arc." (See Fig. 110.) (a.) When two or more cella are joined together, the points of contact should be as large as is convenient and kept perfectly clean The connecting wire should be of good size and, for the sake ol pliability, a part of it may well be given a spiral form by winding it upon a pencil or other small rod. 100 NAtURAL PHILOSOPHY 265 265. Batteries of High Internal Resistance. Each kind of galvanic cell has an internal resistance, aa explained in 250. A battery of cells joined in series is called a "battery of high internal resistance." (Fig. 109.) This method of joining the cells increases the length of the liquid conductor through which the cur- FIG. 109. (a.) In a battery of cells joined in series, the E. M. F. and the internal resistance are those of a single cell multiplied by the number of cells. 266. Batteries of Low Internal Resistance. A battery of cells joined parallel is called a "battery of low internal resistance." (Fig. 110.) This method of joining the cells does not increase the length of the liquid conductor traversed by the current but is equivalent to increasing its diameter or area of cross section. For a circuit of great external resistance, a battery of high internal resistance is needed. For a circuit of 267 VOLTAIC ELECTRICITY. 191 small external resistance, large cells, or several cells joined parallel are preferable. (a.) In a battery of cells joined parallel, the E. M. F. is that of a single cell but the internal resistance is that of a single cell divided by the number of cells. FIG. 110. (6.) A battery of high internal resistance was formerly called an intensity battery, while a battery of low internal resistance was called a quantity battery. 267. The Best Arrangement of Cells. The best method of coupling cells depends on the work to be done by the battery. The maximum effect is at- tained when the internal resistance of the bat- tery is equal to the resistance of the external circuit. For example, suppose that in a given battery of eight cells : ( 1.) Each cell has an E. M. F. of two volts. (2.) Each cell has the very high internal resistance of eight ohms. (3.) The battery is to work through a wire that has a resistance of sixteen ohms. 192 NATURAL PHILOSOPHY. 267 (.) First, couple the cells parallel. The E. M. F. of the battery is that of a single cell, 2 volts. The internal resistance is 8 ohms -5-8 = 1 ohm. Adding the external resistance, we have a total resistance of 17 ohms. (See 252.) This arrangement gives us a current of 0.1176+ amperes. (&.) Next, couple the cells in series. The E. M. F. of the battery is 8 times 2 volts, or 16 volts. The internal resistance is 8 times 8 ohms or 64 ohms. Adding the external resistance, we have a total resistance of 80 ohms. C - E - 16 -02 = = This arrangement gives us a current of 0.2 amperes. (e.) Finally, join the ceils in two rows of four cells each in series and join the rows parallel. The E. M. F. of the battery will be 4 times 2 volts or 8 volts. The internal resistance will be 4 times 8 ohms or 32 ohms for each row, but only half that, or 16 ohms, for the whole battery. Adding the external resistance, we have a total resistance of 32 ohms. C = 1 = 16^16 ^ This arrangement, in which the internal and the external resist- ances are equal, gives us a current of 0.25 amperes, the greatest possible under the given conditions. (d.) A similar application of Ohm's law shows that when the external resistance is large, there is little gain from joining cells parallel and that when the external resistance is very small, there it little gain in joining cells in series. Experiment Ml. From the poles of a potassium di-chromate battery, lead two stout copper wires and connect their free ends by 268 VOLTAIC ELECTKTCirr. 195 two or three inches of very fine iron icire. Coil the iron wire around a lead pencil and thrust a small quantity of gun-cotton into the loop thus formed. Plunge the zinc plate of the battery into the liquid and the iron wire will be heated enough to explode the gun-cotton ; it may be heated to redness or even fusion. The resistance of iron wire is about seven times as great as that of a similar copper wire ; in other words, its conducting power is only about one-seventh as great. The decrease in the size of tha wire also adds to its resistance. 268. Thermal Effects of the Electric Current. Whenever an electric current flows through a con- ductor, part of the electricity is changed into heat. Electric energy is changed into heat energy. T7ie amount of electricity thus changed inh> heat u-iU depend upon the amount of resistance- offered by the conductor. In the last experiment, the stout copper wires wero good conductors, offered but little resistance and con verted but little of the electrical energy into heat energy. The change of material from copper to iron increased that resistance. This increased resistance was again in- creased by reducing the size of the conductor. For this double reason, the fine wire offered so much resistance that a considerable of the current energy was trans- formed into heat. Resistance in an electric circuit always pro^ duces heat at the expense of the electric current Thus, electricity is often used in firing mines in milt tary operations and in blasting. All known metals have been melted in this way. 194 NATURAL PHILOSOPHY. 269 269. Luminous Effects of the Electric Current. When an electric circuit is closed or broken, there is a spark at the point of contact, due to the heating of a part of the conductor to incandescence. We have seen lumi- nous effects produced by winding the wire from one plate of a voltaic cell around one end of a file and drawing the other electrode along the side of the file, thus rapidly closing and breaking the circuit. If the iron wire used in the last experiment was heated sufficiently, it also gave a luminous effect and illustrated the fundamental principle of the incandescent electric lamp. (a.) The most important luminous effects of electricity will be considered in connection with dynamo electric machines ( 311). It will be noticed that all of these are secondary thermal effects. 270. Physiological Effects of the Electric Cur- rent. An electric current may produce muscular con- vulsions in a recently killed animal. Experiments with the Leyden jar and the induction coil ( 30G) show that similar effects may be produced upon the living animal. Electricity is largely used as an agent for the cure of disease; experiments of this kind may do injury and would better be left to the educated physician. The discharge of a large battery may be fatal and a number of persons have lost their lives within the last few years by coming, accidentally or otherwise, into the circuit of a dynamo-electric machine. 271. Chemical Effects of the Electric Current. Many chemical compounds in solution may be decom- 272 T8ERMO- ELECTRICITY. 195 posed by forcing the current to traverse the solution. Substances which are thus decomposed are called elec- troll/tea ; the process is called electrolysis ; the com- pound is said to be electrolyzed. The electrolysis of acidulated water is easily accom- plished with a current from two Grove's or Bunsen's cells. (See Chemistry, Experiment 12.) The water is decomposed into oxygen and hydrogen. The apparatus, shown in Fig. Ill, may be called a water-voltameter. ^ FIG. Ill 272. Ions. The products of electrolysis, like the oxygen and hydrogen, are called ions; the one that goes to the + electrode (or anode) is called the aniort ; the one that goes to the electrode (kathode or cathode) is culled the kathioii or cathioti. 196 NATURAL PHILOSOPHY. 272 Experiment 112. From the + pole of a voltaic battery or dynamo-electric machine, suspend a plate of copper ; from the pole, suspend a silver coin. Place the copper and silver electrodes in a strong solution of copper sulphate (blue vitriol). When the circuit is closed, the salt of copper is electrolyzed, the copper from the salt being deposited upon the silver coin and the sulphuric acid going to the copper or + electrode. The silver is thus electro-plated. (Fig. 112.) The countless applications of this process of depositing a me- tallic coat on a body prepared for its reception, constitute the important art of electro-metallurgy. FIG. 112. 273. The E. M. F. of Polarization. The pro- ducts of electrolysis have a tendency to reunite by virtue of their chemical affinity. (Chemistry, 8.) For ex- ample, the electrolysis of zinc sulphate gives zinc and sulphuric acid. But we now well know that the chemical action of these two substances has an electromotive force of its own. This E. M. F. of the ions acts in opposition to that of the electrolyzing current. In some cases, it rises higher than the E. M. F. of the original current and reverses the direction of the current. 274 THERMO-ELECTRICITY. 197 The oxygen and hydrogen, yielded by the electrolysis of water ( 271), tend to reunite and set up an opposing E. M. F. of about 1.45 volts. Thus we see that it re- quires a battery or cell with an E. M. F. of more than 1.45 volts to decompose water. TJiis electromotive force of the ions is called the E. M. F. of Polarization. It may be observed by putting a galvanometer in the place of the battery of the water-voltameter (Fig. 111). The polarization in a voltaic cell acts in the same way. (a.) There is no opposing E. M. F. of polarization when the kathion and the anode are of the same metal. For example, the feeblest current will deposit copper from a solution of copper sul- phate, when ike anode is a copper plate. 274. Secondary Batteries. When a voltameter of an electro-plating bath is supplying a current of elec- tricity, as mentioned in the last paragraph, it constitutes a secondary battery. As the ions do not reunite when the circuit is open, the energy of the decomposing cur^ rent may be stored up as energy of chemical affinity. Wlien ft current is again wanted, the circuit may be closed and the energy of chemical affinity at once appears as energy of electric current. Secondary batteries are, consequently, often called storage batteries. (a.) The Faure battery consists of two plates of sheet lead coated with red lead (lead oxide). These plates are separated by a layer of paper or cloth, rolled up in a loose coil like a roll of carpet and immersed in dilute sulphuric acid. 198 NATURAL PHILOSOPHY. 274 (6.) When a current from a dynamo-electric machine or a vol- taic battery is sent through such a cell, chemical action is pro- duced. Oxygen acts on the coating of the anode plate and converts it into a higher oxide of lead. Hydrogen unites with the coating of the kathode plate and reduces it to metallic lead. When these changes have gone as far as possible, the battery is said to be " charged." The charged plates will remain in this condition for days if the circuit be left open. (e.) By closing the circuit, the plates will, at any time, furnish a current until they are changed to their original chemical condi- tion. As the lead plates and the acid are not rapidly destroyed, the battery may be charged and discharged many times. (d.) Many serious defects in the Faure battery have been ob- viated in the Brush battery, which is the only one yet used to any considerable extent in this country. A Brush dynamo-electric machine ( 311) is operated in the daytime for charging the bat- teries. At night, the same dynamo may be used for operating arc electric lights ( 313), while the charged secondary battery is furnishing the current for the incandescent electric lighta The E. M. F. of each Brush cell is about two volts. For electric light- ing, they are generally prepared in batteries of twenty or more cells. 275. Magnetic Effects of the Electric Current. Any conductor is rendered magnetic by passing a cur- rent of electricity through it. We have already seen that a bar of soft iron may be temporarily magnetized by the influence of the voltaic current. It may be further shown by the action of the bar and helix. (a.) The bar may be a straight piece of stout iron wire ; the helix may be made by winding cotton covered copper wire upon a piece of glass tubing large enough to admit the wire and not quite as long as the iron, 276 THERMO-ELECTEICITT. 199 (6.) A good bclix, convenient for many purposes, may be mada upon an ordinary wooden spool. With a sharp knife, make the shank of the spol as thin as possible and then wind the spool full of insulated copper wire about as large as ordinary broom or stove-pipe wire. The iron bar must be small enough to pass easily through the hole in the spool and long enough to project a little way beyond each end. (c.) Either of these helices may be placed in the circuit of a cell and held in a vertical position, when it will act as a " sucking " magnet. The movable iron core will be held in mid-air " without any visible means of support." (d.) The " helix and ring armature," is shown in Fig. 113. The armature is of soft iron divided into two semi- circles with brass handles. When the helix is placed in a closed circuit, the semicircles resist a considerable force tending to draw them apart ; when the circuit is broken they fall asunder of their own weight The iron ring may be made without handles by any blacksmith. Stout cords will answer for handles. The helix may be made by winding insulated wire upon a pasteboard cylin- der an inch or an inch and a half long. There should be four or five layers of the wire which may be tied together with strings passing through the hole in the helix. (e.) Such temporary magnets as these are called electro-mag- nets. The subject of electro-magnets will be further considered in 298-300. 276. The Electric Telegraph. The electric tele- graph consists essentially of an electro-magnet and a " key " placed in the circuit of a battery. The key is an instrument by which the circuit may be easily broken or closed at will. The armature, A, of the magnet, M, is 2CO NATURAL PHILOSOPHY. 2 7 6 FIG. 114. supported by a spring, S, which lifts it when the circuit is broken. When the circuit is closed, the armature is drawn down by the attrac- tion of the magnet. Thus the armature may be made to vi- brate up and down at the will of the person at the key. The armature may -ct upon one arm of a lever, the other end of which, being provided with a style or pencil, P, may be pressed against a paper ribbon, R, drawn along by clock-work. Thus the pencil may be made to record, upon the moving paper, a series of dots and lines at the pleasure of the operator at the key perhaps hundreds of miles away. When the two stations are several miles apart, one of the wires is dispensed with, the circuit being completed by connecting each station with the earth. The inventor of the practical electric telegraph was an American, S. F. B. Morse. The system of signals devised by him is given in the Elements of Natural Philosophy, 445. To prevent confusion, a small space is left between successive letters, a longer one between words, and a still longer one between sentences. Telegraph operators soon oecome so familiar with this alphabet that they under- stand a message from the mere clicks of the lever and 277 HERMO-EL ECTR1CITY. 201 do not use any recording apparatus. Such an operatol is said to "read by sound"; his instrument is called a "sounder." The same principle of communicating signals by making and breaking an electric circuit is used in fire and burglar alarms, hotel-annunciators, etc. 277. The Galvanometer. We have already seen that the vol- taic current has a marked effect in turning the magnetic needle from its north and south position, tend- ing to place the needle at right angles to the direction of the cur- rent. The galvanometer is a very delicate instru- ment for detecting the presence of an electric current and determining its direction and strength. The magnetic needle is very light and suspended so as to turn easily. The wire conductor is insulated and coiled many times about the needle ; the effect is thus multiplied. A glass cover protects the apparatus from dust and disturbance by air currents. The instru- ment is largely used. One form is represented in Fig. 115. If the instrument shows the presence and direction of the current without measuring its strength, it is a gal' vanoscope rather than a galvanometer. FIG. 115. 202 NATURAL PHILOSOPHY. 277 Experiment 113. Connect an iron and a German silver wire to the binding posts of a delicate galvanometer. Twist the free ends of the wires together and heat the junction in the flame of an alcohol lamp. The deflection of the galvanometer-needle will show that an electric current is traversing the circuit. Cool the junction with a piece of ice. The galvanometer will show that a second current is flowing in the opposite direction. 278. Thermo-Electricity. // a circuit be made of two metals and one of the junctions be heated or chilled, a current of electricity is pro- duced. A thermo-electric pair may be made by soldering together a bar of antimony, A, and one of bis- muth, B, and joining their free ends by a wire. Several such pairs may be joined to form a thermo-electric series, as shown in Fig. 116. Sev- eral such series may be joined to form a thermo-electric pile, the bars being separated by strips of varnished paper and compactly set in a metal frame so that only the sold- ered ends are open to view. The free end of the antimony bar, representing the -f elec- trode, and the free end of the bismuth bar, representing the electrode, are connected with binding screws, which FIG. 278 THERMO-ELECTRICITY, 203 may be connected with a sensitive, short coil galvano- meter (Fig. 115). The thermo-electric pile, with the addition of conical reflectors, is shown in Fig. 117. A change of temperature at either exposed face of the pile produces a feeble current of electricity which is manifested by the movement of the needle of the gal- vanometer. The instrument is much used in scientific work for detecting differences in temperature, being much more sensitive than the mercury thermometer. 204 NATURAL PHILOSOPHY. 279 279. Recapitulation. To be ampli6ed by the pupil for review. (Smee's. Potassium Di-chromat Leclanche. Ctll. H f Daniell's. Two Liquids, -j g^ve'! 8 ' [ Bunsen's. VOLTAIC. ( Tandem. Joined \ Abreast. \ Best Method Battery. . High Internal Resistance. Low Internal Resistance. Current.. Direction. Strength. CHEMICAL ACTION. SIGN OF . Plate. Pole. Electrode. Anode. KathodQ RESISTANCE External. Internal. QUANTITY. ELECTRICITY LOCAL ACTION . POLARIZATION . . THERMAL LUMINOUS. i Cause \ Remedy. ( Cause. I Remedy. \ . M.F. ..Relation to Resistance' CURRENT EFFECTS.. . PHYSIOLGICAL. CHEMICAL - Electro-Metallurgy. E. M. F. of Polariza- tion. ( Fauro's. Secondary Batteries.. \ Uses^ 8 ' / Advantages MAGNETIC. ( Electro-Magnets. 1 Electric Telegraph. ( Galvanometer. THERMO-ELECTRJCITY. 279 EXERCISES. 205 EXERCISES. 1. Does dilute acid or a battery solution act upon zinc mow vigorously than it does upon copper, or otherwise ? 2. State three ways in which the internal resistance of a voltaic cell may be diminished. State two ways in which the strength of current of a voltaic cell or battery may be increased. 3. What is " local action " and how may it be prevented? 4. If the re.-istance of 18.12 yards of No. 30 copper wire be 3.03 ohms, what length of the same wire is there in a coil, the resist- ance of which is 22.65 ohms? 5. Given a battery of five Daniell's cells coupled in series. Each cell has an E. M. F. of 1.1 volts and an internal resistance of 2.2 ohms. The wire of the circuit has a resistance of 44 ohms. What is the strength of the current? Ans. -fa ampere. 6. It is found that one Daniell's cell, however large, will not decompose acidulated water. It is also found that two Daniell's cells, however small, is sufficient for continuous electrolysis. Explain. 7. If the E. M. F. of a Daniell's cell be 1.08 volts and that of a Grove's cell 1.73 volts, the internal resistance of the former being five times as great as that of the latter and the external circuit being a stout, short copper wire, the resistance of which is so small that it may be neglected, show that the Grove's cell will give about eight times as strong a current as the Daniell's. 8. Twenty-four similar cells are arrange:! in four batteries of six cells, each coupled in series. These batteries are joined abreast. (a.) How will the E. M. F. of this battery compare with that of a single cell? (6.) How will Us internal resistance compare with, that of a single cell ? 9. Given a battery of 100 Grove cells, each having an E. M. F. of 2 volts and an internal resistance of 0.25 ohms. The wire of the circuit has a resistance ^f 1000 ohms. Determine the strength 206 NATURAL PHILOSOPHY. 279 of the current (a.) when the cells are joined in series. (6.) When the cells are joined abreast. Ans. (a.) .195 + amperes. 10. Given the same battery as in the last exercise, the external circuit now being a short wire of only 0.001 ohni resistance. De- termine the strength of the current (a.) when the cells are joined abreast, (b.) When the cells are joined in series. Ans. (a.) 571.4 amperes; (6.) 7.99 amperes. 11. Given a single cell like those mentioned in the last two exercises. Join its poles with a short, stout copper wire, which has a resistance of 0.001 ohm. Determine the current that it will give and see how it compares with the current of 50 such cells joined in series, as mentioned in the last exercise. 12. Short circuit the cell mentioned in the last exercise, i. e., make the circuit with a wire of so little resistance that it may be dropped out of the account. Determine the strength of current. Will joining any number of cells joined in series increase this effect? MAGNETISM. 207 SECTION IV. MAGNETISM. 280. Natural Magnets. One of the most valuable iron ores is called magnetite (Fe 3 4 ). Occasional speci- mens of magnetite will attract filings and other small pieces of iron. Such a specimen is called a, load- stone. It is a natural magnet. 281. Artificial Magnets. Artificial magnets are either temporary or permanent. A temporary magnet is usually made of soft iron and is called an electro- .nagnet. A permanent magnet is usually made of steel. Artificial magnets have all the properties of natural mag- nets and are more powerful and convenient. They are, there- fore, preferable for general use. The most common forms are . , , , FIG. 118. the straight or bar magnet and the horseshoe magnet. The first of these is a straight bar of iron or steel ; the second is shaped like a letter U, the ends being thus brought near together, as shown in Fig. 118. A piece of iron placed across the two poles of a horse- shoe magnet is called an armature. We have already learned how to make artificial magnets. 282. Retentivity. It is more difficult to get the magnetism into steel than into iron. It is also more difficult to get the magnetism out of steel than out of 208 NATURAL PHTLOSOPHY. 282 iron. This power of resisting magnetization or demag- netization is called coercive force or retentivity. The harder the steel, the greater its retentivity. Soft wrought iron has but little retentivity. Experiment 1 14. Roll a bar magnet in iron filings. Withdraw the magnet; the filings cling to the ends of the bar but not to the middle. 283. Magnetic Poles. Magnetic attraction is not evenly distributed throughout the bar. It is greatest at or near the ends. These points of greatest attraction are called the poles of thfi magnet. 284 MAGNETISM. 209 It is impossible, by any known means, to develop one magnetic pole without simultaneously developing anothei pole of opposite sign. The middle of the magnet doet; not attract iron and is called the equator or neutral point. Experiment 115. Bring either end of a bar magnet near the end of a piece of iron, A B ; the iron is attracted. Bring the same end of the magnet near the middle of the iron ; the iron is attracted. Bring tin: the same end of the magnet near the other end of the iron ; the iron is at- tracted. Repeat the experiments with ~ ~~ ~~ the other end of the magnet : in each case the iron is attracted. 284. Attraction between a Magnet and Iron. Either pole, of a magnet mill attract ordinary Experiment 116. Freely suspend threa bar magnets, A, B and C, at some distance from each other. This may be done by placing- each magnet in a stout paper stirrup supported by a cord or upon a board or cork floating on water. See Fig. 120. When they have come to rest, each will lie in a north and south line. Magnets for this experiment may be made by magnetizing ( 300) three stout knitting-needles. If there is any electric light, apparatus in your neighborhood in charge of a good natured man, he will probably magnetize the needles for you. Each needle may be suspended by means of a triangular piece of stiff writing-paper. Pass the needle through the paper near the lower corners ; at the other corner affix by wax the end of a horse-hair. The poles may be indicated by little bits of red and of white paper, fastened by means of wax to the ends of the needlea Mark the north-seeking poles, and the south-seeking poles, +. 210 XATtTttAL PHILOSOPHY. 28$ 285. Characteristics of Magnets. Magnets are chiefly characterized by the property of attract- ing iron and by a tendency to assume a particu- lar direction of position when freely suspended. Experiment 117. (a.) Take magnet A of Experiment 116 from its support, and bring its + end near the end of B or 0. Notice the attraction. (6.) Bring the + end of A near the + end of B or C. Notice the repulsion. (c.) Bring the end of A near the end of B or C. Notice the repulsion. (d.) Bring the end of A near the + end of B or C. Notice the attraction. (e.) From experiment (a) we learned that the ends of B and C were each attracted by the + end of A. Bring the end of B near the end of 0. Notice that they now repel. (/.) From experiment (b) we learned that the + ends of B and C were each repelled by the + end of A. Bring the + end of B near the + end of C. Notice that they now repel. (gr.) In similar manner show that the + end of B will attract the end of 0; that the end of B will attract the + end of C. Record the results of your experiments in tabular form, thus : (a.) + attracts . (d.) attracts +. etc. (6.) + repels +. (c.) repels . etc. Experiment I! 8. Var? *, but that the man and the wornajj are attracted toward each o" er. 286 MAGNETISM. 211 Experiment 119. Magnetize a number of fine sewing-needles by drawing the + end of a bar magnet three or four times from the eye to the point of each. Cut several small corks into slices about an eighth of an inch thick. Through each cork disc, ouob FIG. 121. a needle up to its eye and place them in a round dish of water. These little magnets have their like poles presented to each othef and they mutually repel. Bring the bar magnet, with its + end downwards, over the needles; they will be driven to the sides. Similarly, bring the end over them ; they will be attracted toward the centre. 286. Laws of Magnets. (1.) Every magnet has two similar poles; like poles repel each other ; unlike poles attract each other. (2.) Magnetic force, like other forms of attrac- tion and repulsion, varies inversely as the square of the distance. Experiment 120. Dip one of the magnetized knitting-needles Into iron filings, as in Experiment 114. Notice that filings cling to the ends, near the paper discs but that none cling to the middle. Now break the needle in the middle and dip each piece 212 NATURAL PHILOSOPHY. 28P. into iron filings. Notice that the unmarked ends, which were a* the middle of the unbroken magnet, now attract iron filings as well as do the marked ends. Poles have been developed in parts of the needle that previously showed no magnetic at- traction. 287. Effect of Breaking a Magnet. If a mag- net be broken, each piece becomes a magnet with two poles and an equator of its own. These pieces may lie repeatedly subdivided and each fragment will be a per- fect magnet. It is probable that every molecule has its jtoJrs or is polarized and that, could one be isolated, it would be a perfect magnet. 288. Magnetized, Magnetic and Diamagnetic Substances. A magnetized body is one that can bo made to repel a pole of a freely suspended magnet. Substances that are attracted by a magnet are called magnetic ; e. g., iron, steel and nickel. Substances that are repelled by a magnet are called diamagnetio ; e. g., bismuth, antimony and arsenic. Of these, iron is by far the most magnetic, while bis- muth is the most diamagnetic. Experiment 121. Wrap a bar magnet in a piece of cloth With it, attract and repel the poles of a suspended magnet. 2QO MAGNETISM. 213 Experiment 122. Repeat the last experiment, holding a slata or sheet of zinc between the two magnets. Experiment 123. Put one piece of the broken magnet into a bottle ; cork the bottle tightly. With it, attract and repel the poles of a suspended magnet. 289. Magnetic Screens. Nothing but a mag- netic body can cut off the action of a magnet. Experiment 124. Place a piece of card-board or rough draw ing paper over a good bar magnet. Sprinkle iron filings upon the card-board and tap it lightly. The iron particles will move and arrange themselves in well defined curved lines. See Fig. 123. 290. Magnetic Field. A magnet seems to be sur- rounded by an atmosphere of magnetic influence called the magnetic field. The magnetic curves, shown in the above experiments, show the direction of the lines FIG. 123. of magnetic force. If a small magnetic needle be suspended over the card-board, its length will tend to lie in the direction of the lines of magnetic force as mapped out by the iron filings. 214 NATURAL PHILOSOPHY. 2QI The " magnetic curves," formed in the last experiment are very interesting and instructive. The filings in any one of th.ese curves are temporary magnets with adjoin- ing poles opposite and therefore attracting. By using two bar magnets placed side by side, first, with like pules near each other, and, secondly, with unlike poles near each other, their combined effect on the iron filings may be easily observed. Experiment 125. Rub one end of a steel pen against the end of a magnet. Dip the pen into iron filings and notice that the newly made magnet has a pole at each end. Determine the sign of each of these poles, as indicated in Experiment 116. 291. Magnetization by Contact..^ bar of iron or steel may be magnetized by rubbing it against a magnet. Pure or soft iron is easily magnetized but quickly ioses its magnetism when the magnetizing influence is removed. Hardened steel is magnetized with more diffi- culty but retains its magnetism after the removal of the magnetizing influence. Experiment 126. Move the point of an unmagnetized steel pen to and fro very near one end of a magnet but without touch- ing it to the magnet. Dip the pen into iron filings and determine FIG. 124. whether or not it has been magnetized. If it has, determine the sign of each pole, as in the last experiment and notice whether 2Q2 MAGNETISM. 215 the point of the pen is of the same polarity as the end of the magnet near which it was moved. Experiment 127. Bring a short bar of soft iron, /, very near a strong bar magnet, M, end to end, as shown in the figure. Sprinkle iron filings over the end of the iron bar and they will cling as they would to a magnet. The iron bar is a magnet, while it remains in this position. 292. Magnetic Induction. If the end of a bar of soft iron be brought near one of the poles of a strong magnet, the iron becomes, for the time 'being, a magnet. ' The poles of the temporary magnet will be opposite to those of the permanent magnet, t. p blow upon the table opposite the opening at A. The flame will be blown out. Something went from A to B. Did it go through the tube? Experiment 142, iet us ask this question of Nature, speakir g 248 NATURAL PHILOSOPHY. 320 cease while the tube is thus pinched and reappear as soon as the tube is released. 320. Propagation of Sound. Sound is ordinarily propagated through the air. The first layer of air is by the vibrating body, The particles of this 321 WAVE LENGTH. 249 layer give their motion to the particles of the next layer, and so on until the particles of the last layer strike upon the drum of the ear. (a.) See Elements of Natural Philosophy, % 484, a. If the teacher or pupil has a copy of Alfred M. Mayer's little book on " Sound," 1 he may well read the explanation of the propagation of sound given on p. 89. He will also do well to make and use Crova's Disk, as described in Experiment 58 of that book. If necessary, the pupils should "club together" and buy the book /or the class. 321. Wave Length. In such a series of similar waves, measuring in the direction in which the waves are traveling, the distance from any vibrating par- ticle to the next particle that is in the same relative position or " phase " is called a wave length. In the case of water waves, for example, the horizontal distance from one crest to the next crest would be a wave length. The wave length may be found by dividing the velocity by the number of vibrations. (a). Every one knows how to produce a series of waves in g rope as shown in Fig. 148, each curved line of which we may imagine to be an instantaneous photograph of a rope thus shaken, The distance ub or cd, is a half wave length, 250 NATURAL PHILOSOPHY. $2$ 222. Amplitude. Amplitude means the distance between the extreme positions of the vibrating particle, or the length of its journey. Referring to Fig. 148, the distance de or cf is the amplitude of the Experiment 145. Hold one end of a straight spring, as a hick- ory etick, in a vise, pull the free end to one sido and let it go. Elasticity will return it to ita position of rest, kinetic energy will carry it beyond and so on, a vibratory motion toeing thus pro- duced. (Fig. 149.) When the spring is long, the vibrations may be seen. By lowering the spring in the vise, the vibrating part is shortened, the vibrations reduced in amplitude and in- creased in rapidity. As the spring is shortened, the vibra- tions become invisible but audi- ble, showing that a sufficiently rapid vibratory motion may pro- duce a sound. FIG. 149. 323. Sound Waves. The layers of air are crowded more closely together by each outward vibration of the sounding body ; a condensation of the air is thus pro- duced. As the sonorous body vibrates in the opposite direction, the nearest layer of air particles follow it; a rarefaction of the air is thus produced. A sound wave, therefore, consists of two part& tt> condensation and a rarefaction/. 323 SOUND WAVES. 251 --u-v.-tuv.--' : - .- ..-' ':/.::'' - :'.;.//. . Fw. 150. The motion of any air particle is backward and for- ward in the line of propagation, and not " up and down " across that line, as in the case of water waves. Experiment 146. Provide a wooden rod about half an inch square (or 1 inch by } inch) and five or six feet long. Place one end of this rod (preferably made of light, dry pine) against the panel of a door, hold the rod horizontal and place the handle of a vibrating tuning-fork against the other end. Notice the sound given out by the panel. Experiment 147. With the help of an assistant, vary the last experiment by placing one end of the rod against the ear or be- tween the teeth instead of against the panel. See if the sound of the fork at the same distance is perceptible without the help of the rod. Experiment 148. Make a "string telephone" as follows: melt the bottoms from two small, round, tin boxes about an inch or an inch and a half in diameter. Ground spices are often sold in boxes of the kind desired. The bottom may be removed by set- ting it for a moment on a hot stove. Over one end of each tubo thus provided, firmly tie a piece of well-soaked bladder, which NATURAL PHILOSOPHY. $324 may be had of a butcher or apothecary. When the bladder is dry, pass one end of a long, fine string through the middle of the head of each tin box and tie a knot in each end of the string to prevent it from drawing back. The knot should be on the inaide of the box. Let one pupil hold the open end of one box to his ear, the other box being held by another pupil at such a distance that the FIG. 151. string shall be drawn tight. If this pupil bring the foot of a vibrating tuning-fork to the tin, the vibration will travel along the string and the sound be heard by the first pupil. If the string be a hundred feet long or more and tightly stretched, conversa- tion may be carried on through the apparatus. It is desirable that no solid touch the string between the tin boxes. 324. Sound Media. Any elastic substance may become the medium for the transmission of sound, bat such a medium is necessary. Sound cannot be transmitted through a vacuum. 327 VELOCITY OF SOUND. 263 (a.) Soldiers and Indians sometimes detect the approach of the enemy at a great distance by putting their ears to the ground. 325. Velocity of Sound in Air. The transmission of sound is not instantaneous. The blow of a hammer i. often seen several seconds before the sound is heard ; steam escaping from the whistle of a distant locomotive becomes visible before the shrill scream is audible ; the lightning precedes the thunder. 17z, which corresponds to the disc, a, of Fig 144. In front of the diaphragm is a wooden mouth-piece with a hole, about the size of a dime, at the middle of the diaphragm and opposite the end of the magnet. The outer case is made of wood or of hard rubber. The external appearance of the complete instrument is repre- sented by Fig. 155. The binding posts of one instru- 300 NATURAL PHILOSOPHY. 337 toent being connected by wires with the binding posta rf another at a distance, conversation may be carried on between them. Carefully review 314 and 315. 336. Action of the Tele- phone. When the mouth-piece is brought before the lips of a person who is talking, air waves beat upon the diaphragm and cause it to vi- brate. Eacli vibration of the dia- phragm induces an electric current in the wire of B. These curreu ts are transmitted to the coil of the con- nected telephone, at a distance of, perhaps, several miles, and there produce vibrations exactly like the original vibrations produced by the voice of the speaker. These vibra- tions of the second diaphragm send out new air waves. The two sets of air waves being alike, the resulting sen- sations produced in the hearers are alike. Not only dif- ferent words but also different voices may be recognized. Remember that an electric current, and not sound waves, passes along the line wire. The arrangement being the same at both stations, the apparatus works in either di- rection. No battery is necessary with this arrangement 337. The Transmitter. In practice, a transmitter, shown at C in Fig. 156 is generally used. The vibrations of the diaphragm of C, when acted upon by sound waves, . 155. 337 THE TELEPHONE. produce a varying pressure upon a carbon button placed in the circuit of a galvanic battery, D. This varying pnr > ure results in a varying resistance to the passage of the Fio. 156. current through the button and, consequently, in varia tions in the current itself. This varying current, passing through the primary circuit of a small induction coil ir. the box, C, induces a current in the secondary IMOUII thereof. This current, thus induced, flows over the L .e phone wires and, at the other station, passes thro igii a telephone like that shown at B 3 which is held close If the ear of the listener. 262 NATURAL PHILOSOPHY. 337 The message is transmitted by C at one station and received by B, of a similar instrument at the other station. At each station is placed an electric bell, A, which may be rung from the other station, for the purpose of at- tracting attention. When the stations are a considerable distance, apart, one binding post of each instrument may be connected with the earth, as in the case of the telegraph. See Fig. 15G. (a.) In most of our cities, the telephones are connected by wire with a central station, called a telephone exchange. The " Ex- change" may thus be connected with the houses of hundreds of patrons in all parts of the city or even in different cities. Upon re- quest by telephone, the attendant at the central station connects the line from any instrument with that running to any other in- strument. Thus, each subscriber may communicate directly with any other subscriber to the exchange. Experiment 151. The effect of repeated impulses, each feeble but acting at the right instant, may be forcibly illustrated as follows: Support a heavy weight, as a bucket of coal, by a long string or wire. To the handle of a bucket, fasten a fine cotton thread. By repeated pulls upon the thread, each pull after the first one being given just as the pendulum is beginning to swing toward yo* from the effect of the previous pull, the weight may be made to swing through a large arc, while a single pull out of time will snap the thread. A little practice will enable you to perform the experiment neatly. Experiment 152. Vary the last experiment by setting the pendulum in motion by well-timed puffs of air from the mouth or from a hand bellows. The same principle is illustrated in the action of the spring board, familiar to most boys, who know that the desired effect san be secured only by "keeping time." Soldiers are often or 337 SYMPATHETIC VIBRATIONS. 263 deb dered to " break step " in crossing a bridge, lest the accumulated energy of many footfalls in unison break the bridge. Experiment 153. Suspend several pendulums from a frame as shown in Fig. 157. Make two of equal length so that they will vi- brate at the same rate. Be sure that tliey will thus vibrate. The other pendulums are to be of different lengths. Set a in vibration. The swinging of a will produce slight vibrations in the frame which will, in turn, transmit them to the other pendulums. As the successive impulses thus imparted by a keep time with the vibrations of b, this energy accumulates in b, which is soon set in perceptible vibration. As these impulses do not keep time with the vibrations of the other pendulums, there can be no such accumulation of energy in them, for many of the impulses will act in opposition to the motions produced by previous impulses and lend to destroy them. Experiment 154. Place two mounted tun- ing forks (Fig. 159) several feet apart. The forks must be exactly in unison. Sound one j fork by rapidly separating its prongs with a wooden rod or by drawing a resined bass-viol bow across their ends. After a few seconds, stop the vibrations of this fork with the fin- gers ; it will be found that the other fork has been put into sympathetic vibration and is giving forth a souno. Weight one of the prongs of the second fork with wax ; an at- '.empt to repeat the experiment will fail. Experiment 155. Tune, to unison, two strings upon the same eonometer (Fig. 158). Upon one string, place two or three paper riders. With a violin bow, set the other string in vibration. The sympathetic vibrations thus produced will be shown by the 264 NATURAL PHILOSOPHY. 338 dismounting of the riders, whether the vibrations be audible or not Change the tension of one of the strings, thus destroying the unison. Repeat the experiment and notice that the sympathetic vibrations are not produced. 338. Sympathetic Vibrations. The string of a violin may be made to vibrate audibly by sounding near it a tuning-fork of tbe same tone. By prolonging a vocal tone near a piano, one of the wires seems to take up the note and give it back of its own accord. If the tone be changed, another wire will give it back. Thus the vibrations of the strings may produce sonorous waves and the waves, in turn, may produce vibrations in another string. The string absorbs only the particular kind of vibration that it is capable of producing. PIG. 158. (a.) The sonometer box may be made by any carpenter. It is about fifty-nine inches long, 4f inches wide and 4f inches deep. The ends are made of inch oak boards, the sides of ^ inch oak boards and the top of inch pine board. The top should be glued 338 SYMPATHETIC VIBRATIONS. 265 on ; no bottom is needed ; the box may sit directly on the table Three or four one-inch holes may wel) be bored in each side-piece The two bridges, shown at A and B (Fig. 158) should be of very hard wood and glued to the cover just 47f inchec (120 centimeters) apart, measured from centre to ceutre. The strings may be such as are used on bass-viols ; they should be alike. Two similar pieces of piano-forte wire (large size) may be used. The strings may be stretched by weights as shown in the figure or by two piano string pegs turned with a wrench or a piano tuner's key. The familiar screw arrangement of the bass-viol may be used for the purpose. If piano wires are used for strings, the ends must be annealed by heating them red hot and cooling them slowly, BO that they may remain fixed when wound around their fasten ings. Lines should be drawn across the top of the box, exactly dividing the distance between the middle of the bridges (at which points the strings are supported) into halves, thirds and quarters. Provide a block of wood, about two inches wide, 4 inches long and just thick enough to slip between the strings and the top of the box. (6.) When the two strings are in unison, they will vibrate at exactly the same rate. The second and subsequent pulses sent out by the first string strike the second string, already vibrating from the effect of the first pulse, in the same phase of vibration, and thus each adds its effect to that of all its predecessors. If the strings be not in unison, they will vibrate at different rates and but few of the successive pulses can strike the second string in the same phase of vibration ; the greater number will strike it at the wrong instant. Experiment 156. Strike a tuning-fork held in the hand. Notice that the sound heard is feeble. Strike the fork again and place the end of the handle upon a table. The loudness of the sound heard is remarkably increased. Repeat Experiment 146. Experiment 157. Strike the fork and hold it near the eat NATURAL PHILOSOPHY. 339 counting the number of seconds that you can hear it: Strike the fork again with equal force, place the end of the handle on the table and count the number of seconds that you can hear it. 339. Sounding-boards. In the case of the sonom- eter, piano, violin, guitar, etc., the sound is due more to the vibrations of the resonant bodies that carry the strings than to the vibrations of the strings themselves. The strings are too thin to impart enough motion to the air to be sensible at any considerable distance ; but as they vibrate, their tremors are carried by the bridges to the material of the sounding apparatus with which they are connected. These larger surfaces throw larger masses of air into vibration and thus greatly intensify their sound. It necessarily follows that the energy of the vibrating body is sooner ex- hausted ; the sounds are of shorter * duration. FIG. 159. (<*) For class or lecture experiments, tuning forks should be mounted as shown in Fig. 159. Experiment 158. Support horizontally, between two fixed sup- ports, a soft cotton rope a few yards in length. With a stick, Strike the rope near one end a blow from below and a crest will be FIG. 160. 34O COINCIDENT WA VES. 267 formed as shown in Fig. 160. Vary the tension of the rope, if necessary, until the crest is easily seen. Notice that the crest, c, travels from A to B where it is reflected back to A as a trough,*. By striking the rope from above, a trough may be started which will be reflected as a crest. See Mayer's "Sound," Experiment 57, in which tho wire spring shows waves of condensation and rarefaction. Tie a piece of string to one turn of the wire and no- tice the motion of the string as the wave passes. Experiment 159. Start from A a trough. At the moment of its reflection as a crest at B, start a crest at A as shown in Fig. 161. The two crests will meet near the middle of the rope. The crest at the point and moment of meeting results from two forces acting FIG. 161. in the same direction consequently, it will be grea^r than either of the component crests. 340. Coincident Waves. In the case of water waves, when crest coincides with crest, the water reaches a greater height. So with sound waves, when, conden- sation coincides with condensation, this part of the wave will be more condensed; when rarefac- tion coincides with rarefaction, this part of the wave will be more r art fled. This increased difference of density in the two parts of the wave means increased loudness of sound, because there is an increased amplitude of vibration for the par- ticles constituting the wave. 268 NATURAL PHILOSOPHY. 341 341. Reinforcement of Sound. This increased intensity may result from the blending of two or more series of similar waves in like phases, or from the union of direct or reflected waves in like phases. Under such circumstances, one set of waves is said to reinforce the other. The pJienomenon is spoken of as the reinforcement of sound. Experiment 160. Hold a sounding tuning-fork over the mouth of a glass jar, 18 or 20 inches deep ; a feeble sound is heard. Care- fully pour in water until the liquid reaches such a level that the sound suddenly becomes much louder. The water has shortened the air col- umn until it is able to vibrate in unison with the fork. The length of the air column is one- fourth the length of the wave produced by the fork. 342. Resonance. Resonance is a variety of the re- inforcement of sound due to sympathetic vibrations. The resonant effects of solids were shown in 339. The resonance of an ail g 343 RESONANCE. 269 column was shown in the last experiment. The loudness of sound of wind instruments, like the pipes of a church organ, is largely due to the resonance of the air enclosed in them. 343. Helmholtz's Resonators. Helmholtz, the German physicist, constructed a series of resonators, each one of which resounds powerfully to a single tone of certain pitch or wave length. They are metallic vessels, nearly spherical, hav- ing a large opening, as at A in Fig. 163, for the admission of the sound waves. The funnel-shaped projec- tion at B has a small opening and is inserted in the outer ear of the observer. Fm Experiment 161. Using the rope as described in Experiment 158, start a crest at A. At the moment of its reflection at B as a trough, start a second crest at A. The trough and crest will meet near tlie middle of the rope. The rope at this time and place will be urged upward by the crest and downward by the trough. The FIG. 164. resultan* effect of these opposing forces will, of course, be equal to their difference. If crest and trough exert equal forces, the difference will be zero. Consequently the motion of the rope at 270 NATURAL PHILOSOPHY. 345 the meeting of crest and trough will be little or nothing. Thu one wave motion may be made to destroy the effect of another wave motion. Experiment 162. Hold a vibrating tuning-fork near the eat and slowly turn it between the fingers. During a single complete rotation, four positions of full sourd and four positions of perfect silence will be found. When a side of the fork is parallel FIG. 165. to the ear, the sound is plainly audible , when a comer of the prong is turned toward the ear, the waves from one prong com- pletely destroy the waves started by the other. Experiment 163. Over a resonant jar, as shown in Pig. 164, slowly turn a vibrating tuning-fork. In four positions of the fork we have loud, resonant tones ; in four other positions we have complete interference. While the fork is in one of these positions 344 INTERFERENCE OF SOUND. 271 of interference, place a pastoboard tube around one of the vibrating prongs, Fig. 167; a resonant tone is instantly heard; the cause of the interference has been removed. (See Mayer's "Sound,' Experiments 60 to 67.) 344. Interference of Sound. If, while a tuning, fork is vibrating, a second fork be set in vibration, the waves from the second must traverse the air set in mo- tion by the former. If the waves from the two forks be of equal length, as will be the case when the two forks have the same pitch, and the forks be any number of whole wave lengths apart, the two sets of waves will unite in like phases (Fig. 1G6), condensation with con- densation, etc., and a reinforcement of sound will ensue. PIG. 166. But if the second fork be placed an odd number of half wave lengths behind the other, the two series of waves will meet in opposite phases ; where the first fork requires a condensation, the second will require a rarefac- tion. The two sets of waves will interfere, the one with the other. If the waves be of equal intensity, the air particles, thus acted upon, will remain at rest; this means silence. In Fig. 167, an attempt is made to represent this effect to the eye, the uniformity of tint indicating the absence of condensations and rarefactions. 272 NATURAL PffTLOSOPffT. 345 FlG. 167. By adding sound to sound, both may be de- stroyed. This is the leading, characteristic prop- erty of wave rnotion. The phenomenon here de- scribed is called interference of sound. (a.) The sound of a vibrating tuning-fork held in the hand is almost inaudible. The feebleness results largely from inter- ference. As the prougs always vibrate in opposite directions at Ihe same time, one demands a rarefaction where the other de- mands a condensation. By covering one vi orating prong with a pasteboard tube, the sound is more easily heard. Experiment 164. Use the two forks mentioned in Experiment 154, one of them being loaded with wax. Set both forks in vibra- tion and notice the palpitating effect. Experiment 165. In a quiet room, strike simultaneously one of the lower white keys of a piano and the adjoining black key. The palpitating effect will be heard. 345. Beats. If two tuning-forks, A and B, vibrating respectively 255 and 256 times a second, be set in vibration at the same time, their first waves will meet in like phases and the result will be an intensity of sound greater than that of either. After half a second, B having gained half a vibration upon A, the waves will meet in opposite phases and the sound will bo weakened 347 VIBRATIONS OF STRINGS. 273 DF destroyed. At the end of the second, we shall have another reinforcement ; at the middle of the next second, another interference. This peculiar palpitating effect is due to a suc- cession of reinforcements and interferences, and is called a beat. The number of beats per second equals the differ- ence of the two numbers of vibrations. (See Mayer's " Sound," Experiments 71 and 72. 346. Vibrations of Strings. The laws of musical tones are best studied with the help of stringed instru- ments. The Laws of the Vibrations of Strings is treated in 455 of the Elements of Natural Philosophy. Experiment 166. Give a fixed support to one end of a rubber tube about two yards long and hold the other end in the hand. Move the hand up and down so as to send a series of waves along the tube. By varying the tension of the tube and the rate of vi- bration of the hand, you will soon be able to produce an appear- ance of gauzy spindles much like those shown in Figs. 148 and 170. This gauzy appearance is due to an optical effect known as " the persistence of vision." The motion of the hand produces a regular succession of equal crests and troughs, which are reflected and neutralize each other as explained in Experiment 161. 347. Fundamental Tones and Overtones. A string may vibrate transversely as a whole, or as inde- pendent segments. Such segments will be aliquot parts of the whole string and separated from each other by points of no motion called nodes or nodal points. (See a and b, Fig. 148.) The part of the string between tw adjacent nodes is called a ventral segment. 2?4 NATURAL PHILOSOPHY. 347 The tone produced by the vibrations of ili6 whole length of a string is called its fundamental tone. The tones produced by the vibrations of the segments of a string are called its overtones or harmonics. 348. Fundamental Tones. When a string vibrates so as to produce its fundamental tone, its extreme po- sitions may be represented by the continuous and the M8- dotted lines of Fig. 1C8. This effect is obtained by leaving the string free and bowing it near one of its ends. If a number of little strips of paper, doubled in the middle, be placed like riders upon the string and the string bowed as just described, all of the riders will be thrown up and most of them off. This shows that the whole string vibrates as one string ; that there is no part of it between the fixed ends that is not in vibration. 349. The First Overtone. If the string of the sonometer be touched exactly at its middle with a feather, a higher tone is pro- duced when che string is ^--- IZx^ HI = howod This tone is an ^ ^ ; octave higher than the fundamental. The string now vibrates in such a way that the point touched remains at rest; it is a node. The tone is due to the vibrations of the tuo Halves of the string, which thus give the octave instead of the fundamental. The existence of the 350 orsnTONES. 275 node and segments will continue for some time after the feather is removed. If riders be placed at C, JVand D, the one at N will remain at rest while those at and D will probably be dismounted. (a.) Instead of the sonometer string, a piece of rubber tubing filled with sand and hung in a vertical position with both ends fastened may be used. Hold the middle of the tube to form a node and pluck at the middle of the lower half. FIG. 170. 350. Higher Overtones. In like manner, if the vibrating string be touched at exactly one-third (see L'ig. J 70), one-fourth or one-fifth of its length from one end, it will divide into three, four or live segments, with 276 NATURAL PHILOSOPHY. 350 vil: rations three, four or five times as rapid as the fun- damental vibrations. 351. Quality or Timbre. As a sounding body vi- brates as a whole and in segments at the same timCj the fundamental and the harmonics blend. The result- ant effect of this blending of fundamentals and harmonics constitutes what we call the quality or timbre of the sound. We recognize the voice of a friend not by its loudness not by its pitch, but by its quality. When a piano and violin sound the same tone, we easily distinguish the Bouivd N f one from that of the other because, while the fundamentals are alike, the harmonics are different. Experiment 167. Take your seat before the key-board of a piacd. Press and hold down the key of "middle C," marked 1 in Fi^ 171 which represents part of the key -board. This will lift the FIG. 171. damper from the corresponding piano wire and leave it free to vi- brato. Strongly strike the key of 0", an octave below. Held this key down for a few seconds and then remove the finger The damper will fall upon the vibrating wire and bring it to rest. Whoa the sound of 0' has died away, a sound of higher pitch is heard. The tone corresponds to the wire of 1, which wire is now vibrating These vibrations are sympathetic with those that produced the first overtones of the wire that was struck. These vibrations in 35 2 COMPOUND TOXES. 277 the wire of 1 prove the presence of the first overtone In the vibrating wire of C'. (See 338.) In similar manner, successively raise the dampers from the wires of 2, 3, 4, 5, 6 and 7, striking C" each time. These wires will ac cumulate the energy of the waves that correspond to the respec tive overtones of the wire of G* and give forth each its proper tone Thus we analyze the sound of the wire of C' and prove that at least seven overtones are blended with its fundamental. Some of these tones of higher pitch, thus produced by VIM aliens sympathetic with the vi'srcUions of the segments of the wire o)' 7 ., r- feebler than others. This shows that the quality of a f on vie pends upon the relative intensities as well as the numbei Hut. overtones that blend with the fundamental. 352. Simple and Compound Tones. The weP. trained ear can detect several sounds of different pitch when a single key of a piano is struck. In other wwiiia. the sound of a vibrating piano wire is a compound sound. The sound of a timing fork is a fairly good example of a simple sound. Simple sounds all have the same quality, differing only in loudness and pitch. (a.) A series of Helmholtz's resonators enables the student of acoustics to analyze any compound sound. Each component tone may be reproduced by a tuning fork of appropriate pitch. By sounding simultaneously the necessary number of forks, each of proper pitch and with appropriate relative intensity, Hehnholtz showed that the sounds of musical instruments, including evei the most wonderful one of all (the human voice), may be produced synthetically. NOTE. The author takes this last opportunity to advise the pupil to procure and study Mayer's " Sound." See Chapter 5VH 278 NATURAL PHILOSOPHY. 353 353. Recapitulation. To be amplified by the pupil for review. 1 SYMPATHETIC VIBRATIONS. ANALYSIS OF f CONSTRUCTION. Resonance. SONOMETER MOUNTED TUNING-FORKS. . . SOUNDING BOARDS AIR COLUMNS. HELMHOLTZ'S RESONATORS.. REINFORCEMENT f Characteristic of wave motion INTERFERENCE... \ [Seats. FUNDAMENTAL TONES. OVERTONES. 353 EXERCISES. 279 EXERCISES. 1. Two forks vibrate 256 and 264 times a second respectively. (.) Which yields the longer soundwaves? Which produces tones of the higher pitch ? 2. What is the length of waves produced by a fork vibrating 280 times a second, when a centigrade thermometer shows a tem- perature of 15 degrees ? Ana. 4 feet. 3. A resonant air column (Fig. 162) is one foot long. What is the wave length of the tone to which it will respond most satis- factorily ? Ana. 4 feet. 4. One tuning-fork vibrates 256 times a second and makes two beats per second with a second fork. What is the rate of vibra- tion of the second fork ? 5. Is the motion of a sound wave vibratory or progressive? 6. What is the difference between a longitudinal and a traaa verse vibration ? Illustrate each. 7. What is the velocity of sound in a vacuum? 8. What determines the pitch of a musical tone* NATURAL PHILOSOPHY. 353 REVIEW QUESTIONS. 1. (a.) How may one vary the pitch of a violin string? (6.) Describe the electrical condition of a polarized body. 2. When a door is swung open or shut by pressing with the hand near the hinges, what class of levers is illustrated ? 3. Fig. 172 represents apparatus made of glass tubing and a bottle. There are openings at a, b and c. Tell how the apparatus a may be used to transfer a liquid that is dangerous to handle, e. g. sulphuric acid. 4. Describe the waves produced by throwing a stone into quiet water. 5. Describe the waves produced by striking a bell in the air. 6. What is the difference between kinetic and po- tential energy. 7. Define physical change and give an illustration thereof. 8. Describe the barometet 9. What is always the divisor in problems in spe- cific gravity? 10. WhatisaKathion? 11. What is meant by the length of a sound wave ? On a certain dav at the freezin & temperature, I saw the flash of a gun. Ten seconds later, I heard the report, (a.) What was the distance of the gun? (b.) Why do I state the temperature ? FIG 172 ' FIG. 173. 13. Fig. 173 represents an electric current flowing through an insulated wire wound around a piece of soft iron. What is the name of this combination? CHAPTEB UI* HEAT. SECTION I. TEMPERATURE, THERMOMETERS, EXPANSION. Experiment 168. Rub a brass button on the floor or carpet it soon becomes uncomfortably warm. Experiment 169. Place a nail or coin on an anvil or stone and hammer it vigorously ; it soon becomes too hot to handle. In this way, blacksmiths sometimes heat iron rods to redness. Experiment 170. Hold a piece of iron or steel against a dry grindstone in rapid revolution ; the shower of sparks noticeable ia due to the fact that the small particles of metal torn off by the grindstone are heated to incandescence. 354. Heat Produced by Mechanical Energy. Tlie arresting of mechanical motion transforms visible energy into heat. (See 99.) 355. A Day Dream. As our young philosopher notices tha quickly falling blows of the blacksmith's hammer on the nail and anvil, he begins to think very intently. He knows that the descend/ 282 NATURAL PHILOSOPHY. 355 ing hammer nas energy for it is able to do work. He understands that this is so because the hammer has weight and motion. After the last blow has been struck and the hammer lies at rest on the anvil, he begins to wonder what has become of all the energy that he knows was in the hammer when it was in motion a moment ago. He knows that the visible, kinetic energy has disappeared, for the hammer has no motion. He cannot see that it has been transformed into potential energy. Yet he remembers that he has been told that energy, like matter, is indestructible. Here is a prob- lem ; shall he give it up? No, for he is our " young philosopher." The puzzling problem perplexes him so that be falls into a day- dream. His " scientific imagination " begins to work. He again sees the falling hammer; he hears and, with quickened percep- tion, almost feels the shock. The motion of the hammer, as a ham- mer, ceases ; the energy of the hammer, as a hammer, is destroyed. But his mind's eye sees the myriad molecules of the hammer, nail and anvil, each for itself, pick up a portion of the motion lost by the hammer as though the shock had given to each a shiver. He sees that, as these molecules now have weight and motion, they necessarily have kinetic energy. He sees that, at least, part of these molecular motions were produced by the mass motion of the hammer and that the total quantity of energy thus gained oy the molecules must be just equal to the quantity of energy lost by the hammer. He then awakens ; he sees the hammer, nail and anvil but he cannot see the molecular motions that were so vivid in his day- dream. He places his hand upon the iron masses and finds that they were heated by the blows. Like Archimedes, he shouts "Eureka! I cannot see these molecular motions but I can feel them. The visible energy of the moving mass would have been jevealed to my hand as a crushing blow ; the invisible energy of the moving molecules is revealed to my hand as heat." Experiment 171. Pass a bent glass tube through the air-tight cork of a flask half full of water, and let it dip beneath the sur- the water. Heat the flask. The heat will raise some 358 TEMPERATURE. 283 FIG. 174. of the water to the end of the tube where it may be caught as shown in Fig. 174. 356. Mechanical En- ergy Produced by Heat. Previous experiments showed us that mechanical energy can produce heat. The last experiment shows that heat can perform me- chanical work: it lifted water against the force of gravity There certainly seems to be a close con nee tion between heat and the ordinary forms of energy. 357. What is Heat? Heat is a form of en- ergy. It consists of the ceaseless, vibratory motions of the molecules of matter or results from such motions and gives rise to the sensations of warmth and cold. 358. What is Temperature ? The temperature of a body is its condition considered with refer- ence to its ability to communicate heat to other bodies. Temperature is a very different thing from quantity jf heat. A cup of water dipped from a lake will have the same temperature as the lake, but the water in the lake will have incomparably more heat than the water ua the cup. NATURAL PHILOSOPHY. 359 359. Thermometers. An instrument for meas- uring temperature is called a thermometer. The mercury thermometer is the most common. 360. Thermometric Scales. There are two scales used in this country, the centigrade and Fahrenheit's. For these scales, the fixed points are marked as follows : Centigrade. Fahrenheit. Freezing point, 32 Boiling point, 100 212 The tube between these two points is divided into 100 equal parts for the centigrade scale and into 180 for Fahrenheit's. Hence a change of temperature of 5 C. is equal to a change of 9 F., or an interval of one centigrade degree is equal to an interval of -| of a Fahrenheit degree. Fiu. 175. Experiment 172. Provide a metal ball which, at ordinary temperatures, will easily pass through a certain ring ; heat the ball and it will no longer pass through the ring. If the ball be cooled by plunging it into cold water, it will again pass through the ring. (Fig. 176.) 361 Expansion. In- crease of volume is the first visible effect of heat upon bodies. FIG. 17ft, 364 H&PAiratoX. 285 Whatever raises the temperature of a body increases :he energy with which the molecules of that body swing to and fro. Molecules thus vibrating must push each other further apart, and thus cause the body which they constitute to expand. Experiment 173. Saw a piece from one side of a large iron link and force the ends of the opened link slightly together so that, the small piece may be pressed hard enough to hold it in place. When the opposite side of the link is heated, it will expand and the piece will fall out of its place. 362. Expansion of Solids. The energy of ex- pansion and contraction of solids, whan heating and cool- ing, is remarkable. This expansion of metals by heat is utilized by coopers in setting hoops, by wheelwrights in setting tires, by builders in straightening bulging walls, fe 363. Anomalous Expansion of Water. Water presents a remarkable exception to the general rule. If icatcr at 0. be heated, it will contract until it readies 4 C., its temperature of greatest density. Heated tiborc fit is point, it expands. 364. Results of this Exception. This property of water is of great importance. Were it otherwise, the ice would sink and destroy everything living in the water. The entire body of water would soon becoma a solid mass which the heat of summer could not wholly meh, for, ns we shall soon see. wafer has little power to carry heat downward. 286 KAtVRAL PHILOSOPHY. 365 Experiment 174. Partly fill a bladder with cold air, tie up the opening and place the bladder near the fire. The air will ex- pand and fill the bladder. Experiment 175. Heat a closed flask having a delivery tube terminating under water. Some of the expanded air will be forced to escape, and may be seen bubbling through the water. By " collecting over water " the air thus driven out, it may be accurately measured. (Fig. 177.) Experiment 176. Cut a flat spiral with two or three turns from a piece of stiff writing paper about three inches in diameter thrust a pin through the centre of the paper, take the pin by the point and FIG. 177. bold the spiral over a hot stove or lighted lamp. As- cending currents of heated air will cause the spiral to turn alxmt the pin as an axis. 365. Expansion of Gases. The ascension of "fire balloons" and the draft of chimneys are due to the ex- pansion of gases by heat. When the air in the chimney of a stove or lamp is heated, it is rendered lighter than the same bulk of surrounding air and, therefore, rises. The cooler air conies in to take its place. 367 R EGA PITULA TION. 287 366. Absolute Zero of Temperature. Tlie tern perature at which the molecular motions constitut- ing heat wholly cease is called the absolute zero. It has never been reached, and has been only approxi. mately determined. (a.) Temperature, when reckoned from the absolute zero, ia called absolute temperature. Absolute temperatures are obtained by adding 460 to the reading of a Fahrenheit thermometer, or 273 to the reading of a centigrade thermometer. 367. Recapitulation. To be amplified by the pupil for review. DEFINITION. PRODUCED BY FRICTION. INVISIBLE MOLECULAR ENERGY. DEFINITION. TEMPERATURE. EXPANSION.,.. THERMOMETERS ABSOLUTE. CAUSE. Souus. LIQUIDS. (Centigrade. 1 L I Fahrenheit. I NATURAL PHILOSOPHY. 367 EXERCISES. 1. If a centigrade thermometer records 15, what will be th reading of a Fahrenheit thermometer by its side ? 2. Express a temperature of 59 F. in degrees centigrade. 8. What is the absolute temperature of 15 C. ? What does your answer mean ? 4. How can you easily show that mechanical energy may be converted into heat energy ? 5. Why does not Lake Erie freeze solid to the bottom ? 6. Show how the draft of a chimney depends upon SECTION II. LIQUEFACTION AND VAPORIZATION. Experiment 177. Melt some ice in any convenient dish When it is partly melted, stir the liquid part with a chemical thermometer and notice carefully the temperature at which ice melts. Frequently deter- mine the temperature until the ice is all melted and notice that the tem- perature is constant until the last solid particle disappears. p IG Experiment 178. Repeat the ex- periment with tallow, beeswax and Bulphur in succession. It may be ob- served, in each case, that the hotter the fire, the more rapidly the solid will melt but that there will be no rise of temperature from the time the solid begins to melt until it is all melted. Heat the sulphur somewhat above the melting poi it and allow it to cool. Notice the temperature when it begins to solidify and while it is solidifying 368. Laws of Fusion. It has been found by ex- periment that the following statements are true : (1.) Evefy solid begins to melt at a certain tern* perature wliicli, for a given substance, is invariable if the pressure be constant. When cooling, the sub- stance will solidify at the temperature of fusion. 13 290 NATURAL PHILOSOPHY. 368 (2.) TJie temperature of the solid or liquid re- mains at the melting point from the moment that fusion or solidification begins until it is complete. 369. Vaporization. If, after liquefaction, further additions of heat be made, a point will be reached at which the liquid will pass into the aeriform condition. This change of form is called vaporization. Vaporization is of two kinds evaporation and ebullition. 370. Evaporation. Evaporation signifies the quiet formation of vapor at the surface of a liquid. 371. Ebullition. Ebullition or boiling, signifies the rapid formation of vapor bubbles in the mass of a liquid. (See Elements of Natural Philosophy, 501.) Experiment 179. Tn a beaker half full of water, place a ther- mometer and a test tube half filled with ether. (Fig. 179.) Heat the water. When the thermometer shows a temper- ature of about 00 C., the ether will begin to boil. The water will not boil until the temperature rises to 100 0. The temperature will not rise beyond this point. Experiment 180. Half fill a Florence flask with water. Boil the water until the steam drives the Jj'ia. 179. air from the upper part of the flask. Cork tightly, remove the lamp and invert the flask. (Fig. 180.) The exclusion of the air may be made more certain by immers- ing the corked neck of the tiask in water that has been boiled. 372 BOILING. 291 When the lamp was removed, the temperature was not abov 100 ( '. By the time that the flask is inverted and the boiling ha3 ceased, the temperature will have fallen below 100 C. When the boiling stops, pour cold water upon the flask ; directly the boil ing begins again. The cold water poured upon the flask lowers the temperature oi the water in the flask still further, but it also condenses some or the steam in the flask 01 reduces its tension ( 179). This reduc- tion of the tension lessens the work necessary to boiling. There being enough heat iu the water to do this lessened amount of work, the water again boiteand increases the pressure until the boiling point is raised above the temperature of the water. The flask may be drenched and the water made to boil a dozen times in succession with a single heating. The experiment may be made more striking by plunging the whole flask under cool water. While water boils at 212 P. under a pressure of one atmosphere, it must be heated to about 2oO F. to boil under a pressure of two atmospheres or to more than 350 F., under a pres- sure of ten atmosj v hfn\s. rio. 372. Laws of Ebullition. (1.) Even/ li(/nieen formed by allowing condensed air charged with watery vapor to escape suddenly from the confining vessel. The rapid expan- sion of carbon dioxide (" carl onic acid gas") when a bottle of beet or champagne is opened, often produces a fog in the neck of the bottle. (See C/temintry, % 197.) 302 NATURAL PHILOSOPHY. 384 (6.) Carbon dioxide may be liquefied by great pressure. When this liquid escapes through a small orifice into the air, evapora- tion is so rapid that some of the dioxide is frozen in the form of a fine snow. This carbon dioxide snow dissolves in sulphuric ether and with it forms one of the most intense freezing mixtures kmown. By aiding the evaporation of this mixture with an air pump, Faraday obtained a temperature of 110" C. Experiment 191. Mix a pound of ice-cold water (0 C.) with a pound of water at 80 C. Note the temperature of the mixture. We have two pounds of water at 40 C. The heat lost by the hot water in cooling 40 degrees was equal to the heat gained by an equal weight of water in warming 40 degrees in fact it was identical with it. Experiment 192. Place a pound of melting ice (0 C.) in a pound of water at 80 C. When the ice is all melted, note the temperature of the water. We have two pounds of water at C. 384. The Latent Heat of Water In Experi- ments 192 and 193, the heat which warmed one pound of water from to 80 C. was used to melt a like weight of ice. Hence, by definition, the latent heat of water is 80 C. (or 144 F.). TJie amount of heat required to melt a quan- tity of ice without changing its temperature is eighty times as great as the heat required to warm the same quantity of water one centigrade degree. (a.) Because of this great latent heat of water, the processes of melting ice and freezing water are necessarily slow. 386 LATENT HEAT. 303 385. The Latent Heat of Steam. The amount of heat necessary to evaporate one gram of water would suffice to raise the temperature of 537 grams of water 1 C. Hence, the latent heat of steam is 537 C. (or 967 F.). TJie amount of heat required to evaporate a quantity of water without changing its tempera- ture is 537 times as great as the heat required to warm the same quantity of water one centi- grade degree. (a.) When a gram of steam is condensed, 537 heat units (centi- grade-gram-water) are liberated. In this, we see an explanation of the familiar fact that scalding by steam is so painfully severe. (6.) Were it not for the latent heat of steam, when water reached its boiling point it would instantly flash into steam with tremendous explosion. 386. Problems and Solutions. (1.) How many grams of ice at C. can be melted by one gram of steam at 100 C. ? One gram of steam at 100 C., in condensing to water at the same tempera- ture, parts with all its latent beat, or 537 heat units. The gram of water thus formed can give out 100 more heat units. Hence, the whole number of beat units given out by the steam in changing to water at C., the temperature at which it could no longer melt ice, is 537 + 100=637. Since it requires 80 heat units to melt one gram of ice, 637 heat units will melt as many grams as 80 is contained times in 637, which is 7.96. Therefore, the steam will melt 7.96 grams of ice. (2.) How many pounds of steam at 100 C. will just melt 100 pounds of ice at C. ? To melt 100 pounds of ice, (80 x 100=), 8,000 heat units will be required. Each i>ound of steam can fur- 304 NATURAL PHILOSOPHY. 386 nish 63? heat units for the work required. 8,000 -H 637 = 12.55. the number of pounds of steam. (3.) What weight of steam at 100 C. would be required to raise 500 grams of water from D C. to 10 3 C. ? Each gram o* water will require 10 heat units ; 500 grams of water will require 5000 heat units. Each gram of steam can furnish (537 + 90 ) 627 heat units for the work required. 5000 -*- 627 = 7.97, the number of grams of steam. Experiment 193. Mix any convenient quantity of ice-cold water (0 C.) with the same quantity of water at a temperature of 30 C. Ascertain the temperature of the mixture; jt will be about 15 C. If you used a kilogram of water in each case, the 15000 heat units lost by the warm water and the 15000 heat units gained by the cold water were identical. 387. Equality of Loss and Gain. A substance loses as much heat in cooling a given number of degrees as would be gained by a like quantity when warmed an equal number of degrees. Experiment 194. Suspend a vessel containing 3 kilograms of mercury in boiling water until you are sure that it has the tem- perature of the water (100 C.) Quickly pour the mercury into one kilogram (or liter) of ice cold water, C. The temperature of the mixture will be about 9 C. The kilogram of water, in being warmed 9 C., gained 9000 heat units ; consequently, the 3 kilograms of mercury in cooling 91 C., must have lost 9000 heat units, for our experiment has neither created nor destroyed any heat and, if it was neatly per- formed, but little was lost. For each degree of change of temperature, the kilogram of water gained 1000 heat units ; for each degree of change of temperature 9000 one kilogram of mercury lost ^ -= = 33 heat units. In other yi x o words, it takes about 30 times as much heat to warm a giveq 39 SPECIFIC HEAT. 305 yeight of water one degree as it does thus to warm the same weight of mercury. 388. Definition of Specific Heat. The specific heat of a body is the ratio between the quantity of heat required to warm it one degree and the quantity of heat required to warm an equal weight of water one degree. (.) It is very important to bear in mind that specific heat, like specific gravity, is a ratio ; nothing more nor less. The 8|>ecific heat of water, the standard, is unity. The specific heat of iron is 0.1138; of copper, 0.0902; of tin, 0.0563 ; of lead, 0.0314 ; of bis- muth, 0.0308 ; of ice, 0.5 and of steam, 0.48. These ratios will be the same for any given substance, whatever the thermal unit or ihermometric scale adopted. 389. Heated Balls Melting Wax. The difference between bodies in respect to specific heat may be roughly illustrated as follows : small balls of equal weight, mad severally of iron, copper, tin, lead and bismuth are heated to a temperature of 180 or 200 C. by immersing them in hot oil until they all acquire the temperature of the qil. They are then placed on a cake of beeswax about 306 NATURAL PHILOSOPHY. 389 half an inch thick. The iron and copper will melt their way through the wax, the tin will nearly do so, while the lead and bismuth sink not more than half way through the wax. This shows that the iron or copper had more heat than an equal weight of lead or bismuth at the same temperature. 390. Specific Heat of Water. Water in its liquid form, has a higher specific heat than any other substance except hydrogen. For this reason, the ocean and lakes are cooled and heated more slowly than the land and atmosphere. They thus modify sud- den changes of temperature, and give rise to land and sea breezes and to the well known fact that the climate of the sea-coast is warmer in winter and cooler in summer than that of inland places of the same latitude. 391. Recapitulation. To be amplified by the pupil for review. UNITS. HEAT. ENERGY.. KINETIC=SENSIBLE HEAT. Convertibility on ' Critical occasions. 1 I. POTENTIAL = LATENT HEAT Definition. Of Fusion. Of Solution. Of Water. Of Steam. Freezing Mixture* Solidification. DEFINITION. SPECIFIC. I ILLUSTRATION. I OF WATER. 8 39* EXERCISES. 307 EXERCISES. 1. Why does sprinkling the floor of a room on a warm day add to the con: fort of the occupants of the room ? 2. Does a dog's lolling his tongue on a summer day render him less warm ? Explain. 3. Why do we often bathe a fevered brow with water or with a mixture of alcohol and water ? 4. How many heat units are required to melt three pounds of ice ? Am. 432 units (pound-Fahrenheit). 5. How much heat is required to raise three pounds of ice-cold water to the boiling temperature ? Ann. 540 units. 6. How much heat is required to vaporize 3 grams of boiling water? Aw. 1611 lesser calories. 7. How much heat is needed to melt and evaporate 8 pounds of ice ? AM. 3873 units (pound-Fahrenheit). 8. How much heat is needed to change a kilogram of ice at -10 C. to water at 15 C. ? (Take note of the specific heat of ice.) Ans. 100 calories, or 100,000 lesser calories. SECTION IV. MODES OF DIFFUSING HEAT. Experiment 195. Place one end of an iron poker into the firfe The other end soon becomes too warm to handle. 392. Conduction. TJie process by which heat is transferred from, the hotter to the cold v parts of a body, passing from one molecule to tlie next nioteviUe, is called conduction of heat. The propa- gation is very gradual and as rapid through a crooked as through a straight bar. Experiment 196. Instead of the iron poker of Experiment 195, use a glass rod or wooden stick. The end of the rod may be melted or the end of the stick burned without rendering the other end uncomfortably warm. Experiment 197. Thrust a silver and a German-silver spoon into the same vessel of hot water; the handle of the former will become much hotter than that of the latter. Experiment 198. Place a bar of iron and one of copper end to end so as to be heated equally by the flame of the lamp. Fasten small balls (or nails) by wax to the under surfaces of the bars ?t 393 MODES OF DIFFUSING HEAT. 309 equal distances apart. More balls may be melted from the cop- per than from the iron. 393. Difference in Conductivity. These experi- ments show that some substances are good conductors of heat and that some are not. (a.) Relative thermal conductivity of some metals : Silver 100 Lead 9 Copper 74 Platinum 8 Brass 24 GernuJi silver 6 Iron 12 Bismuth 2 The above-named metals arrange themselves in the same order with reference to the conduction of electricity, silver being the best and bismuth the poorest. This relation suggests a similarity of nature between these two forms of energy. Experiment 199. Pass the tube of an air thermometer or of an inverted mercury thermometer through a cork in the neck of a funnel. Cover the thermometer bulb to the depth of about half an inch with water. Upon the water, pour a little sul- phuric ether and ignite it. The heat of the flame will be intense enough to boil a small quantity of water held over it, but the thermometer below will be scarcely affected. Experiment 200. Fasten a piece of ice at the bottom of a glass test or igni- tion tube and cover it to the depth of several inches with water. Hold the tube obliquely and apply the flame of a lamp be- low the upper part of the water. The water there may be made to boil without melting the ice beiow. Instead of using ice and water, pack the tube full of moist snow if you can get it. 310 NATURAL PHILOSOPHY. 394 394. Conductivity of Fluids. Liquids and aeri- form bodies are poor conductors of heat. The sui> face of a liquid may be intensely heated without sensibly affecting the temperature an inch below. The conduc- tivity of iron is about 80 times that of water ; that of copper is about 500 times that of water, 'f he conduc- tivity of gases is probably less than that of liquids. Experiment 201. Drop a small quantity of cochineal or oak sawdust into a glass vessel containing water. Heat the water by a lamp placed below. Notice the currents indicated by the motion of the solid particles. 395. Convection. Fluids (with the exception of mercury which is a metal) being poor conductors, they cannot be heated as solids generally are. Water, e.g., must be heated from beloiv ; the heated mole- cules expand and rise while the FIG 188 cooler ones descend to take their place at the source of heat. TJiis method of diffusing heat, by actual motion of heated fluid masses, is called convection. 396. Luminiferous Ether. In the case ot kinetic, mechanical energy, the rapid motion of bodies, e. g., a vibrating guitar string, is partly carried off by the air in lb. shnpe ut SQUUCJ., 397 MODES OF DIFFUSING HEAT. 311 There is sufficient reason for believing that thert is a medium pervading all space which carrier off part of the invisible motions of molecules, just as the air carries off a portion of the motion of moving masses. This medium is called the -lu~ jniniferous ether. It is supposed to occupy intermolecular as well as inter- planetary space, and to pass as freely between the particles of ordinary matter as the winds do between the trees of the forest. Experiment 202. Take a white-hot poker into a dark room. It emits heat and a white light. The light gradually becomes reddish and less bright, and finally fades from view as a dull red glow. Long after it has ceased to be visible, the poker continues to give heat to surrounding objects, as may be shown by holding the hand or face near it on any side, above or below. There has been a continuous change from the emission of white light and much heat to that of no light and less heat. 397. Radiant Heat. The molecules of a heated body are in a state of active vibration. The motion of these vibrating molecules is communicated to the ether and transmitted by it, as waves, with wonderful velocity. Thus, when you hold your hand before a fire, the warmth that you feel is due to the striking of these ether-waves upon your skin; they throw the nerves into motion just as sound-waves excite the auditory nerve and the consciousness corresponding to this motion ia what we call warmth. Heat thus propagated by the ether, instead of by ordinary forms of matter, is called radiant heat. 312 NATURAL PHILOSOPHY. 397 (a.) From the last experiment, we naturally conclude that radiant heat and light are identical. Thorough experimental in- vestigation has confirmed this conclusion. When the energy of the ether waves produces a cer!ain effect upon the retina of the eye, we call it light ; when it produces another effect upon the nerves of touch, we call it radiant heat. " Thus radiant heat is brought under the undulatory theory of light, which in its turn becomes an- nexed as a magnificent outlying province of the kinetic theory of teat." ( 420.) 398. Radiation of Heat. The transfej-rence of heat from one body to another at a distance ir- respective of the temperature of the intervening medium, is called the radiation of heat. 399. Incident Rays. When radiant heat falls upon a surface, it may be transmitted, reflected or absorbed. If transmitted, it may be refracted. Eock-salt crystal transmits nearly all, reflects yery little, and absorbs nardly any. Polished silver reflects nearly all, absorbs a little, and transmits none. Lampblack absorbs nearly all, reflects very little, and transmits none. Experiment 203. Hold a pane of glass between the face and a hot stove. Notice that the glass shields the face from the heat of the stove. Next, hold the glass between the face and the sun. Notice that the glass does not shield the face from the heat of the sun. 400. Diathermancy. -Bodies that transmit ra- diant heat freely are called diathermanous ; those that do not, are called athermanous. These terms are to heat, what transparent and opaque are to light. Bock salt Is the most diathermous substance known. 402 MODES OF KTFFUSING HEAT. 313 401. Obscure and Luminous Heat. Heat that is radiated from a non-luminous source, as from a ball heated below redness, is called obscure heat ; while part of that radiated from a luminous source, as from the sun or from a ball heated to incandescence, is called lumi* nous heat. Heat from a luminous source is generally composed of both luminous and obscure rays. Glass, water or a solution of alum allows luminous heat rays to pass but absorbs nearly all of the heat rays from a vessel filled with boiling water. In other words, these substances are diathermanous for luminous rays but athermanous for obscure rays. A solution of iodine in carbon di-sulphide transmits obscure rays but absorbs luminous rays. By means of these substances, luminous and obscure rays may be sifted or separated from each other. Experiment 204. Hold a spectacle-glass or a larger lens (g 450) from an opera glass in the sunlight, perpendicular to the sun's rays. A point may easily be found below the lens where it is unusually warm. At this point, called the focus, hold the tip of a common friction match or a hit of gun cotton that has been blackened with lamp black or soot. The concentrated rays of the sun will set fire to the easily combustible substance. GTIH cotton may be ignited at the focus of a lens made of ice. Experiment 205. Stand beside an open fire and let your as- sistant stand in front of the fire. Let him use a piece of bright tin with which to throw back the light of 'the fire into your face. Notice that heat as well as light is thrown back, your face feel ing warmer. 402. Refraction and Reflection of Heat. Heat rays may be bent from a straight line on entering and 314 NATURAL PHILOSOPHY. 402 leaving a body, as shown in Experiment 204. This bending of the ray is called refraction. The refraction of obscure rays cannot be shown by a glass lens, since glass is athermanous for such rays. A rock-salt lens and a thermopile may be used for such an experiment. (See 278.) Kadiant heat may be reflected like light. (See 430.) 403. Change of Radiant into Sensible Heat Of all the rays falling upon any substance, only those that are absorbed are of effect in heating the body upon which they fall. The motion of the ether waves may be changed into vibrations of , Dlecules of ordinary matter, and thus produce sensible heat but the same energy can- not exist in waves of ether and in ordinary molecular vibrations at the same time. Lamp-black is a remark- ably good absorbent. Experiment 206. Provide two small tin cans of the same size and shape. You can get them for the asking. In the cover oi each, make a hole through which you may pass the bulb of a chemical thermometer. Blacken the outside of one can with paint or candle soot. Fill both cans with hot water from the same vessel and, consequently, of the same temperature. At the end oi half an hour, pass the bulb of the thermometer through the holes in the covers and ascertain the temperature of the water in each jan. It will be found that the blackened can has radiated its heat (or cooled) more rapidly than the other. 404. Relations of Absorption, Reflection and Radiation.-Cr00$ absorbents are good radiators and poor reflectors, and vice versa. The powers of ab- sorption and radiation go hand in hand. ( 338.) 405 RECAPITULATION. 315 The radiating power of a body depends largely upon the nature of its surface ; smoothing and polishing the surface increases reflecting power and diminishes absorb- ing and radiating powers; roughening and tarnishing the surf^e increase the absorbing and radiating powers and diminish the reflecting power. 405. Recapitulation. To be amplified by the pupil for review. DIFFUSION OF HEAT. 316 NATURAL PHILOSOPHY. 405 EXERCISES. 1. Why is a moist, cold atmosphere more severe than a dry atmosphere of the same temperature ? 2. Why does fanning one's self on a warm day increase one's personal comfort? 3. Why is it that, in a cold room, some things seem colder than others ? 4 (a.) Is a good conductor or a non-conductor better for keeping a warm body warm ? (6.) For keeping a cold body cool ? 5. How is heat transmitted through a crooked iron rod ? 6. Is a good absorbent of heat better for radiation or for reflec tion of heat ? SECTION V. THERMODYNAMICS. 406. Definition of Thermodynamics. Thermo- dynamics is the branch of science that considers the connection between heat and mechanical work. It has especial reference to the numerical rela- tion between the quantity of heat used and the quantity of work done. 407. Correlation of Heat and Mechanical En- ergy. We know that heat is not a form of matter be- cause it can be created in any desired quantity. We must continually remember that it is a form of energy. When heat is produced, some other kind of energy must be transformed. Conversely, when heat disappears, some other form of energy appears. 408. Heat from Percussion. A small iron rod placed upon an anvil may be heated to redness by re- peated blows of a hammer. (Experiment 169.) The energy of the moving mass is broken up, so to speak, and distributed among the molecules, producing the form of molecular motion that we call heat The same trans- formation was illustrated in the kindling of a fire by the " flint and steel " of a century ago. The bullet is heated by its blow against the target and the water is wanner at the foot of Niagara than in the rapids above. 318 NATURAL PHILOSOPHY. 41 1 409. Heat from Friction. Common matches are ignited and cold hands warmed by the heat developed by friction. It is said that some savages kindle fires by skillfully rubbing together well-chosen pieces of wood. A railway train is really stopped by the conversion of its motion into heat by means of the brakes. Examples of this change are matters of every-day experience. (Ex- periment 168.) 410. Heat from Chemical Action. When coal is burned, the carbon and oxygen particles rush together with tremendous violence, energy of position being con- verted into energy of motion. The molecular motions produced by this clash- ing of particles constitute heat and have a me- chanical value. 411. Heating Powers. If a given weight of carbon be burned, the heat of the combustion would warm about 8000 times that weight of water 1 C. In like manner, the combustion of a gram of hydrogen would yield more than 34000 heat units (gram-centigrade). (a.) The following table shows the heating powers of several substances when burned in oxygen : Hydrogen 34,462 I Carbon Petroleum 12,300 Alcohol (C 8 H S O). (6 ) The heating powers mentioned above may be adapted to Fahrenheit degrees by multiplying them respectively by . As they stand, the numbers represent the number of times its own weight of water could be wanned 1 C. by burning the substance in oxygen. 414 THERMODYNAMICS. 3l9 412. First Law of Thermodynamics. When heat is transformed into mechanical energy or mechanical energy into heat, the quantity of heat equals the quantity of inechanical energy, 4 I 3- Joule's Equivalent. It is a matter of great importance to determine the numerical relation between heat and mechanical energy; to find the equivalent of a heat unit in units of work. This equivalent was first as- certained by Dr. Joule, of Manchester, England. His experiments were equal in number and variety to the importance of the subject. He showed that the mechan- ical value of the heat required to warm a given weight of water I" C., would lift the water J424 meters against gravity. \ 1390 feet 1 F., would lift the water 772 Any weight unit may be used without changing the above values which should be remembered. Eeferring to the first iinit mentioned in 376, we say that the mechanical value of a heat unit is 424 gram- meters or 1390 foot-pounds. Referring to the second unit there mentioned, we say that the mechanical value of the heat unit is 772 foot- pounds. 414. The Steam-Engine. The steam-engine is a machine for utilizing the tension of steam. Its essential parts are a boiler for the generation of steam and a cylinder for the application of the tension to a piston. 320 NATURAL PHILOSOPHY. 4*5 415. Double-Acting Engine. In a double-acting steam-engine, the steam is admitted to the cylinder alternately above and below the piston. This alternate admission of the steam is accomplished by means of a sliding-valve. The sliding-valve is placed in a steam- chest, 8, which is fastened to the side of the cylinder, G. FIG. 189. (?,.) In the figure, the steam-chest is represented as being placed at a distance from the cylinder; this is merely for the purpose of making plain the communicating passages to and from the chest. Steam from the boiler enters at M, passes through A to the cylin- der, where it pushes down the piston as indicated by the arrows in Fig. 189. The steam below the piston escapes by B and F. (&.) As the piston nears the opening of B in the cylinder, tho sliding-valve is raised, by means of the rod, R, to the position indicated in Pig. ISO. Steam now enters the cylinder by B aud 416 THERMODYNAMICS. 321 pushes up the piston. The steam above the piston escapes by A and N. As the piston neare the opening of A in the cylinder, the sliding- valve is pushed down by R and the process is thus repeated. FIG. 190. (c.) The piston-rod and the sliding-valve rod work through s: earn-tight packing-boxes. The piston is connected with a crank on the shaft of the engine, so that the to-and-fro motion of the piston produces a rotary motion of the shaft. Smoothness of motion is secured by attaching a heavy fly-wheel to the shaft of the engine. A Uttle reflection will show that the fly-wheel also acts as an accumulator of energy. 416. Non-Condensing Engines. When the steam is forced out at N (Fig. 190), it has to overcome an at- mospheric pressure of 15 pounds to the square inch. Such an engine is known as a non-condensing engine. It may be recognized by the escape of steam in puffs. PHiLOSOPBT. 416 It is generally a high-pressure engine. The railway locomotive is a high-pressure, non-condensing engine. 417. Condensing Engines. The steam may be conducted from the exhaust pipe, N (Fig. 190). to a cham- ber called a condenser. Steam from the cylinder and a spray of cold water being admitted at the same time, a vacuum is formed and the loss of energy due to atmos- pheric pressure is avoided. Such an engine is known as a condensing, or low-pressure engine. Steamboat en- gines are generally condensing engines. (a.) Low-pressure engines are always condensing engines. A low-pressure engine will do more work with a given amount o* fuel than a high-pressure, non-condensing engine will, is less liable to explosion and causes less wear and tear to the machinery. But it must be larger, more complicated, more costly and less easily portable. 418. Heat and Work of Steam-Engines. More heat is carried to the cylinder of a steam-engine than ia carried from it. Tlie piston does work at every stroke and this work comes from the heat that disappears. Every stroke of the piston transforms heat energy into mechanical energy. Careful experiments show that the heat destroyed and the work performed are in strict agreement with Joule's equivalent. With a given supply of fuel, the engine wih give out less heat when it is made to work haid than when it runs without doing much work. 419 RECAPITULA TtON. 419. Recapitulation. To be amplified by the pupil for review. f DEFINITION. MECHANICAL POWER CHANGED TO HEAT BY (PERCUSSION, FRICTION. ATOMIC ATTRACTION. JOULE'S EQUIVALENT. STEAM-ENGINES A SOURCE OK HEAT. IEATING POWERS OF SEVERAL SUBSTANCES ESSBNTIAI PASTB. DOUBLE ACTING. CONDENSING. NON-CONI R3LATION BKTWKBN HEAT AMD W OWt 824 NATURAL PHILOSOPHY. 419 EXERCISES. 1. If it were possible to convert heat into mechanical power without lobs, how far would the heat giren out by 1 gram of water In cooling 1 C. lift another gram of water against the force o? gravity ? Give your answer in meters and in feet. 2. How higli would it lift 2 grams of water? Ans. 212 m., or 695 feet. 3. How high would it lift 2 grams of lead ? 4. How high would the heat thus given out by a pound of water lift a pound weight ? 5. How high would the heat thus given out by 10 pounds of water lift a 5 pound weight ? Ans. 848 m., or 2780 feet. 6. How high would the heat thus given out by 5 pounds of water lift a 10 pound weight? 7. If a pound of water falls 772 feet, how much will it be heated by the s'opping of its motion ? 8. If a 2 pound weight falls 772 feet, how much heat will be developed by the percussion when it strikes the ground ? 9. How many heat units (gram-centigrade) must be transformed In order to raise 1000 grams to a height of 424 meters ? 10. How many, thus to raise 2 kilograms? Ans. 2000. 11. How many grams of water can be heated from the freezing to the boiling temperature by the heat produced by burning 1 gram of hydrogen ? 12. How many weights of water may be thus heated by burn- ing 1 weight of pure coal (carbon)? Ans. 80.8. 13. If a good steam-engine utilizes only about 10 per cent, of the heat energy of its fuel, how many foot pounds of work can it do with a ton of coal that is assumed to be pure carbon ? Ans. 2000 x 8080 x 1390 x T V 14 Would it do more work or less with the burning of a like weight of petroleum ? 15. Can heat be destroyed? Can energy? 419 REVIEW QUESTIONS. 325 REVIEW QUESTIONS. 1. What three things determine the rapidity of vibration of a string ? 2. (a.) Will sound waves pass through water? (b.) Through a vacuum ? 8. What is in the upper part of a thermometer tube ? 4. Upon what does the loudness of sound depend ? 5. Why are spaces left between the ends of the rails in laying railway tracks? 6. Why is a gallon of alcohol worth more in cold than in warm weather ? 7. Will a siphon work in a vacuum ? Wliy ? 8. Can you pump any water out of an air-tight cistern full of water? Explain. 9. Can you pump any water out of an air-tight cistern half full of water? Explain. 10. Construct a simple piece of apparatus to illustrate your answers to the last two questions. 11. Which seems colder, a wiudy or a still day, the temperature being the same 1 Why ? 12. If there were no water on the earth, would the difference* in temperature between day and night and between summer and winter be greater or less than they now are? Why ? 13. Does heat always expand water? 14. How can you heat water above 212 F. ? 15. Do all solids and gasea expand equally ? 16. Show that the appa. ratus represented in Fig. 191 is a lever and tell of what PlO. 191. class. Indicate the positions for P f W and F. 17. How does perspiration increase one's personal comfort iu warm weather? 326 NATURAL PHILOSOPHY. 419 18. If a pound of water at 100 C. be added to a pound of watei at C., what will be the resulting temperature? 19. Fig. 192 represents a " dropping bottle." (a.) Why does ail bubble up from the lower end of tube a when liquid drops from the lower end of c t (6.) Why does the liquid cease to drop when the finger closes the tube at a ? 20. The " return ball " is a common toy made by fastening a wooden ball to the end of an elastic cord. When thrown from the hand, the cord being held by the free end, the bail returns and is caught in the hand. Does the motion of the ball resemble water waves or sound waves the more closely? 21. A centigrade thermometer records 33. What will be the reading of a Fahrenheit thermometer under similar circum- FIG. 192. 22. Show haw the apparatus represented in Fig. 193 acts to obviate the trouble arising from the " boiling away " of the water in the basin. 23. Describe the suction pump. 24 What is the cause of dif- ference of pitch ? 25. What is meant by the temperature of the maximum density of water? 26. What is meant by the ab- solute zero of temperature ? 27. What is the ordinary me- dium for the transmission (a.) of sound ? (6.) Of radiant heat ? 28. Give the rule for finding the downward pressure of FIG. 193. liquids caused by gravity. 419 REVIEW QUESTIONS. 327 29. What are ions? 30. A bullet is thrown downward with a velocity of 20 meters per second. What will be its velocity at the end of 4 seconds ? 31. Fill with water a cup sus- pended by three cords. Turn the cup round and rouud, ti^us twisting together the cords. Leave the cup free to untwist the cords and water will fly from the edges of the cup as shown in Fig. 194. What physical law is thereby illustrated ? 32. Note the temperature by the school-room thermometer. What is the velocity of sound now and here? 33. Define chemical change and give an illustration thereof. FIG. 194 CHAPTEB IX. LIGHT. SECTION I. THE NATURE, VELOCITY AND INTENSITY OF LIGHT. 420. What is Light ? Light is the mode of motion that is capable of affecting the optic nerve. It is physically identical with radiant heat, the only difference, in any case, being one of wave length. (See 397.) (a.) We have seen that the vibrations of air particles in a sound wave are to and fro in the line of propagation. In the case of ra- diant heat and light, the ether particles vibrate to and fro across the line of propagation. Vibrations in a sound wave are longitudinal; those of a heat or light wave are transversal. 421. Luminous and Non-Luminous Bodies. Bodies that emit light of their own generating, as the sun or a candle, are called luminous. Bodies that merely diffuse the light that they receive from other bodies are said to be non-luminous or illuminated. Trees and plants are non-luminous. 422. Transparent, Translucent and Opaque Bodies. Transparent bodies allow objects to be seen distinctly through them, e. q., air, glass and water. 424 MATURE OF LIGHT. 329 Translucent bodies transmit light but do not allow bodies to be seen distinctly through them, e. g., ground glass and oiled paper. Opaque bodies cut off the light entirely and prevent objects from being seen through them at all. 423. Luminous Rays. A single line of light is called a ray. Tlie ray may, without considerable error, be deemed the path of the wave. 424. Luminous Beams and Pencils. A collec- tion of parallel rays constitutes a beam; a cone of rays constitutes a pencil. The pencil may be converging or diverging. FIG. 195. Experiment 207. Provide two or three perforated screens and arrange them as shown in Fig. 195, so that the holes and a candle game shall be in the saine straight line. When the eye is placed 330 NATURAL PXILOSOPHT. 425 in this line behind the screens, light passes from the flame to the eye ; the flame is visible. A slight displacement upward, down- ward or sidewise of the eye, the flame or any screen, cuts off the light and renders the flame invisible. Make the screen as follows: Prepare a piece of wood, Ix2| x 18 inches, taking care that the edges are square. Saw it into six pieces, each three inches long. Prepare three pieces of wood, 3 x 4 x inches. Place three postal cards one over the other on a board, and pierce them with a fine awl or stout needle, ^ inch from the end and 1 inch from either side of the card. With a sharp knife pare off the rough edges of the holes, and pass the needle through each hole to make the edges smooth and even. Over the ^ x 3 inch surface of one of the blocks, place the unperforated end of one of the postal cards and over this place one of the 3x4 inch pieces, so that their lower edges shall be even. Tack them in this position. Make thus two similar screens. The three screens, with a bit of candle three inches long, piaced upon one of the remaining blocks, furnishes the material for the experiment above. Save the screens and three blocks for future use. (See Pig. 200.) 425. Rectilinear Motion of Light. A medium is homogeneous when it has uniform composition and density. In a homogeneous medium, light travels in straight lines. (a) The familiar experiment of "taking sight" depends upon this fact, for we see objects by the light which they send to ifie eye. We cannot see around a corner or through a crooked tube. Experiment 208. Place a lighted candle about a yard from a white screen iu a darkened room. (The wall of the room may answer for the screen.) Pierce a large pin-hole in a card and hold it between the flame and the screen. An inverted image of the flame will be found upon the screen, 426 NATURE OF LIGHT. 331 Experiment 209. Cover one end of a tube, 10 or 12 cm. long, with tinfoil ; the other end with oiled paper. Prick a pin-hole in the tinfoil and turn it toward a candle flame. An inverted image may be seen upon the oiled paper. The size of the image will depend upon the distance of the flame from the pin-hole. This apparatus rudely represents the eye ( 473), the pin-hole corresponding to the pupil and the oiled paper to the retina. Al- most any housekeeper will give you an empty tin can. Place it upon a hot stove just long enough to melt off one end, thrust a stout nail through the centre of the other end, cover the nail-hole with tinfoil and you will have the greater part of the apparatus. 426. Inverted Images. If light from a highly- illuminated body be admitted to a darkened room through a small hole in the shutter and there received upon a white screen, it will form an inverted image of the ob- ject upon the screen. Every visible point of the illumi- nated object sends a ray of light to the screen. Each ray brings the color of the point which sends it and yrints the color upon the screen. As the rays are NATURAL PHILOSOPHY. 425 straight lines, they cross at the aperture; hence, the in- version of the image. The image will be distorted un- less the screen be perpendicular to the rays. The dark- ened room constitutes a camera obscura. The image of the school playground at recess is very interesting and easily produced. 427. Velocity of Light. Light moves with a velocity of about 186,000 miles per second. (a.) It would require more than 17 years for a rannon-ball to pass over the distance between the sun and the earth ; light makes the journey in 8 min. 18 sec. For the swiftest bird to pass around the earth would require three weeks of continual flight ; light goes as far in less than one seventh of a second. For terrestrial distances, the passage of light is practically instantaneous (% 325). 36' FIG. 197. Experiment 210. Let a candle or lamp at L, (Fig. 197) be the source of light ; 8, a cardboard screen with a small perforation at the level of the flame ; A, a screen one inch square and a foot from S; B, a screen two inches square, two feet from 8; C. a screen three inches square, three feet from S. It will easily be seen thai A will cut off all the light from B and C. If A be removed, ti,e quantity of light which it received, no more and no less, will fall upon B. If now B be removed, the quantity of light which pre riously illuminated A and B wjlj fall upon Q 428 NATURti OF LIGHT, 333 We thus see the same quantity of light successively illuminating one, four and nine square inches. * One square inch at B will re- -eive one-fourth, and one square inch at C will receive one-ninth as much light as one square inch at A. The light being spread over * greater surface is correspondingly diminished in intensity. 428. Effect of Distance upon Intensity. The intensity of light received bij an illuminated body caries inversely as the square of its distance from the source of light. FIG. 198. (a.) This principle is applied in photometry or the measurement of light. A simple photometer is represented in Fig. 198. S is a screen of white paper or cardboard ; R is a small rod placed up- right a few inches from S (a cheap j en and pen-holder or a lead penc 1 held by a bit of wax on the table will answer). The flame of the candle and that of the lamp should be at the same level ; the flat lamp-wick should stand diagonally to the screen. Place the candle about 20 inches from 8 and move L about until the two shadows upon S just touch and are of equal darkness. The candle and the lamp are now throwing equal amounts of light upon H. PHILOSOPHY. 429 If the distance from 8 to L be twice that from 8 to C, then S is four times as powerful a light as'C; if the distance be three times as far, L is nine times as powerful. If C be 20 inches from PIQ. 199. polished plane surface, as at B, it will be continued in a different direction, as toward C. AB is called the incident beam and BC, the reflected beam. The 43^ REFLECTION OF LIGHT. 337 incident and the reflected beams are in the same modium, the air. A change in the direction of light without a, cJtange in its medium is called reflection of light. Experiment 211. Place two of thesceens and the three extra blocks mentioned in Experiment 297 in position, as shown in Fig. 200. At the middle of the middle block, place a bit of win-low glass, painted on the underside with black varnish. Ou the blocks that carry the screens, place bits of glass, n and o, of the same thickness as the black mirror on the middle block. FIG. 200. Place a candle flame near the hole in one of the screens, as shown in the figure. Light from the candle will pass through A, be reflected at m and pass through B. Place the eye in such a position that the spot of light in the mirror may be seen through Ji. Mark the exact spot in the mirror with a needle held in place by a bit of wax. Phice a piece of stiff writing paper upright upon m and n. mark the position of B and of m, and draw on the paper a straight line joining these two points. The angle between this line and the lower edge of the paper coincides with the angle Bmn. Reverse *}ie paper, placing it upon m and o. It will be found that the same angle coincides with Amo. Amo and Bmn being thus equal, the angle of incidence equals the angle of reflection. ( 57.) 15 338 frATfrSAL ffflLOSOPHf. 43! Experiment 212. Get a piece of looking glass, about an inch square. On the back of this little mirror, at the edges and in a triangular position, place three bits of soft wax each the size of a pea. Place the mirror on the wrist with one of the wax supports on- the pulse. Let this mirror replace the mirror shown in Fig. 199, hold the arm steady and watch the motions of the sun spot on the wall or ceiling. They are like the beatings of the pulse which they make visible to the whole class. The reflected beam moves through an angle twice as great as does the mirror. If the pupil trying the experiment laughs, or becomes excited in any way, the change in the movement of the pulse becomes evident to all in the room. 431. Law of Reflection. The reflection of light from a polished surf ace obeys the now familiar law : The angle of incidence is equal to the angle of reflec- tion. Experiment 213. Let a beam of light fall upon a sheet of drawing paper in a darkened room ; it will be scattered and illu- minate the room. Let it fall upon a mirror ; nearly all of it will be reflected in a definite direction and intensely illuminate only a part of the room. Experiment 214. Place, side by side upon a board, a piece of black cloth (not glossy), a piece of drawing paper and a piece of looking-glass. Allow a beam of sunlight to fall upon the cloth and notice the absorption. Let it fall upon therpaper, and notice the diffusion of the light and its effects. Let it fall upon the look- ing-glass, and notice the regular reflection and its effects. Move the board so that the cloth, paper and glass shall pass through the beam in quick succession and notice the effects. Experiment 215. In the darkened room, place a tumbler of water upon a table ; with a hand mirr.,r, reflect me sunbeam down 433 SEfLECTlOtf OP LIGffT. $3$ into the water ; the tumbler will be visible. Stir a teaspoonful of milk into the water and again reflect the sunbeam into the liquid ; the whole room will be illuminated by the diffused light, the tumbler of milky water acting like a luminous body. 432. Diffused Light. Light falling upon an opaque body is generally divided into three parts : the first is regularly reflected in obedience to the law above given ; the second is irregularly reflected or diffused ; the third is absorbed. The irregular reflection is due to the fact that the bodies are not perfectly smooth, but present little pro- tuberances that scatter the light in all directions and thus render them visible from any position. (Fig. 239. ) Light regularly reflected gives an image of the body from which it came before reflection ; light irregularly reflected gives an image of the body that diffuses it. A perfect mirror would be invisible. Luminous bodies are visible on account of the light that they emit; non-luminous bodies are visible on account of the light that they diffuse. 433- Apparent Direction of Bodies. Every point of a visible object sends a cone of rays to the eye. The pupil of the eye is the base of the cone. TJie point al- ways appears at the place where these rays seem to intersect (i. e., at the real or apparent apex of the cone). If the rays pass in straight lines from the point to the eye, the apparent position of the point is its real poi- 340 NATUItAL PHILOSOPHY. 433 tion. If these rays be bent by reflection or in any other manner, the point will appear to be in the direc- tion of the rays as they enter the eye. No matter how devious the path of the rays in coming from the point to the eye, this rule holds good. Experiment 216. Hold a printed page before a common mirror. Notice that each printed line is reversed ; each letter is turned side for side. "Look at yourself " in the mirror. The right hand of the image is opposite your left hand just as it would be if the image were another person facing you. This reversing effect is called lateral inversion. Experiment 217. Place a jar of water 10 or 15 cm. back of a pane of glass placed upright on a table in a dark room. Hold a lighted candle at the same distance in front of the glass. The jar will be seen by light transmitted through the glass. An imago of the candle will be formed by light reflected by the glass. The image of the candle will be seen in the jar, giving the ap- pearance of a candle burning in water. The same effect may be produced in the evening by partly raising a window and holding the jar on the outside and the candle ou the inside. 434. Plane Mirrors ; Virtual Images. If an ob- ject be placed before a mirror, an image of it appears be- hind the mirror. This is called a virtual image. All virtual images are optical illusions and are to be clearly distinguished from the real images to be studied soon. Each point of the image will seem to be as far behind the mirror as the corresponding point of the object is in front of the mirror, llence, images seen in still, clear water are inverted. 435 REFLECTION OF LIGHT. 341 (a.) In Fig. 201, rays of light from A are reflected at B and B' and enter the eye fit 7). These r;i ys HUH ply 1 1 />/'(!> 1o converge at A', \vhidi is, therefore, called the rirtiiiil image of A. 435- Concave Mirrors. A spheri- cal, concave mirror may be considered as a small part of a spher- ical shell with its inner surface highly polished. Let MN (Fig. 202) represent the section of such a concave, spheri- cal mirror and C, the centre of the corresponding sphere. C is called the centre of curvature ; A is the centre ot FIG. 201. FIG. 202. the mirror. A straight line drawn from A through C, as A CX, is called the principal axis of the mirror. A straight line drawn from any other point of the mirror through C, as JCd, is called a secondary axis. The point, F, midway between A and C, is called the princi- pal focus. The distance, AF, is the focal distance of the 342 NATURAL PHILOSOPHY. 435 mirror; the focal distance is, therefore, one-half the radius of curvature. The angle, MCN, is called the aperture of the mirror. (.) It should be borne in mind that radii drawn from G to points in the mirror, as / and J, are perpendicular to the mirror at these points. Thus, the angles of incidence aud of reflection for any ray may be easily determined. 436. Effect of Concave Mirrors. The tendency of a concave mirror is to make incident rays converge more or diverge less. 437. The Principal Focus. A focus is the point toward which rays converge. All incident rays parallel to the principal axis of a concave mirror will, after re- flection, converge at the principal focus. The princi- pal focus is the focus of rays parallel to the prin- cipal axis. The rays will be practically parallel when their source is at a very great distance, e.g., the sun's rays. Solar rays coming to the earth do not diverge a thousandth of in inch in a thousand miles. 438. Conjugate Foci. Rays diverging from a lumi- nous point in front of a concave, spherical mirror and at a distance from the mirror greater than its focal distance, will converge, after reflection, at another point. Kays diverging from B will form a focus at b. (Fig. 203.) Kays diverging from b would form a focus at B. Two such points a.re called conjugate foci. 439 REFLECTION OF LIGHT. 343 Conjugate foci are two points so related that each forms the ima&e of the other. FIG. 203. Experiment 218. In a dark room, hold a candle between the eye and the concave side of a bright silver spoon held a little ways in front of the face. Notice that the inverted image of the flame is in front of the spoon. Place the spoon between the flame and your face but so as to allow the face to be illuminated by the candle. Notice the image of the observer. 439. Projection of Real Images by Concave Mirrors. The real image formed by a concave mirror may be rendered visible by projecting it upon a screen. In a darkened room, let a candle flame be placed in front of a concave mirror, at a distance from it greater than the focal distance and less than the radius of curvature of the mirror. Incline the mirror so that the flame shall not be on the principal axis. Place a paper screen at the conjugate focus of any point in the luminous ob- ject. The proper position for the screen may easily be found by trial. Shield the screen from the direct, rays of the flame by a card painted black. The inverted image may be seen by a large class, (Fig. 204.) 344 XA Tl'L'A L PHIL OSOPHY. 439 FIG. 204 If the image fall between the mirror and the candle, as it will if the candle be at a distance from the mirror greater than the radius of curvature, the screen should be quite small. The image of any powerfully illuminated object may thus be produced, as shown in Fig. 205. In both of these cases, the reflected rays actually con- verge on the screen ; these images are, therefore, called real images to distinguish them from virtual images. Experiment 219. Hold the convex side of a bright silver spoon toward you and bring the spoon and a candle into the positions described in Experiment 218. Notice that the erect image of the flame is back of the spoon. Place the spoon between the flame and your face but so as to allow the face to be illuminated by the candle. Notice the image of the observer, 441 REFLECTION OF LIGHT. 345 FIG. 20.'). 440. Convex Mirrors. In convex mirrors, the images are virtual, erect and smaller than their objects. 441. Recapitulation. To be amplified by the pupil for review. DEFINITION. f Plane. MIRRORS. -I Co LAW. osition represented at c. Conse- quently, the line of march was bent or refracted at the point where the platoon passed from hard to soft ground. The velocity being lessened, the line was refracted toward tlie perpendicular. 444 JtEFRACTIOy OP LtGHT. 351 In similar manner, at m, the left of the platoon first emerges from the morass and again gains upon the right flank. This changes the platoon front to the position represented at n and re- fracts the line of advance from the perpendicular at the point where the platoon emerges from the morass and is able to increase- its velocity. 444. Laws of Refraction of Light. (1.) When light passes perpendicularly froin one medium to another it is not refracted. (2.) When light passes obliquely from a rarer to a denser medium it is refracted toward the perpendicular, or toward a line drawn, at the point of incidence, perpendicular to the refracting surface. (3.) Wlien light passes obliquely from a denser to a rarer medium, it is refracted from the perpendicular. Experiment 224. Place the bottle shown in Fig. 206 upon several books resting upon a table and invert the card so that a beam of light reflected obliquely upward from a mirror on the table may enter through the slit near the bottom of the bottle, taking a direction through the water similar to the line IA of Fig. 212. Notice that the sunbeam is turned downward at the upper surface of the water. Experiment 225. Look into an aquarium in a direction similar to that represented by the line IA of Fig. 212, and you may often see images of the fish or turtles near the surface of the water. Experiment 226. Place a strip of printed paper in a test tube ; hold it obliquely in a tumbler of water and look downward at the printing, which will be plainly visible. Change the tube gradually toward a vertical position, and soon the part of the tube in the water takes a silvered appearance and the printing becomes invisible. The disappearance of the reading is due to "total re NATURAL PHILOSOPHY. 445 flection." By dissolving a small bit of potassium dichromate in the water, the tube will have a golden instead of a silver-like ap- pearance. Fill the test tube with water and notice that the read- ing is visible, the total reflection at the surface of the air in the tube being destroyed. Experiment 227. Place a bright spoon in a tumbler of water with the handle leaning from you. Hold the tumbler considerably above the level of the eye. Notice that you see not only the lower part of the spoon in the water but also an image of the shank of the spoon above the upper surface of the water as represented in Fig. 209. The free liquid surface glistens and reflects as does a mirror. 445. Total Reflection. When a ray of light attempts to pass from a denser into a rarer medium, there are conditions under which the angle of refraction cannot be greater than FIG 209. the angle of incidence. Under such circumstances, the ray cannot pas* out from, the denser medium but will be wholly reflected at the point of incidence. Fig. 210 represents luminous rays emitted from A, under water, and seeking a passage into air. Passing from the perpendicular, the angle of refraction increases more rapidly than th'e angle of incidence until one ray is found FIG. 210, 446 REFRACTION Of LIGHT. 353 that emerges and grazes the surface of the water. Rays beyond this eaunot emerge at all. (a.) Fig. 211 represents a glass vessel partly filled with water. Mirrors are placed at m and n. In this way, a ray may be reflected at m, n and o, and refracted at i. 446. The Critical Angle. Imagine a spherical flask (Fig, 212) half filled with water. A ray of light from L will be refracted at A FIG. ail. in the direction of R. If the angle of incidence, CAL, be gradually increased, the angle of refraction will be gradually increased until it becomes 90, when the ray will graze the surface of the water, AM, If the source of light be still further removed from C, as to I, the ray will be reflected to r. For cM m-edia, there is an incident angle of this kind, called the crit- ical or limiting angle, be- yond which total internal reflection will take the place of refraction. (a.) The reflection is called "total" because all of the inci dent light is reflected, which is never the case in ordinary reflec- tion. Hence, a surface at which total reflection takes place con- stitutes the most perfect mirror possible. PIG. 212. 354 NATURAL PHILOSOPHY. 447 447. Three Kinds of Refractors. When a ray of light passes through a refracting medium, three cases may arise : (1.) When the refractor is bounded by planes, the re- fracting surfaces being parallel. The refractor is then called a plate. (2.) When the refractor is bounded by planes, the re- fracting surfaces being not parallel. The refractor is then called a prism. (3.) When the refrac- tor is bounded by two surfaces of which at least one is curved. The refractor is then called a lens. 448. Plates. ^^ When a ray passes through a plate, the refractions at the two surfaces are equal and contrary in direction. The direction of the ray after passing through the plate is parallel to its direction before entering. Objoctsseen obliquely through such plates appear slightly displaced from their true position. An object at 8 would appear to be at S' (Fig. 213). 449. Prisms. A prism produces two simultaneous effects upon light passing through it ; a change of di- rection and decomposition. The second of thes. effects will be considered under the head of dispersion ( 4G3). 450 ItEFRACTTON Of LTGffiT. 355 (a.) Let mno represent the section of a prism. A ray of ligh*. from L being refracted at a and 6 enters the eye in the direction FIG. 214 be. The object being seen in the direction of the ray as it enters the eye ( 433), appears to be at r. (6.) An object seen through a prism seems to be moved in the direction of the edge that separates the refracting surfaces. (c,) The refracted rays are bent toward the side that separates the refracting surfaces, or toward the thickest part of the prism. 450. Lenses. The curved surfaces of lenses are generally spherical. With respect to their shape, lenses are of six kinds. FIG. 215. 356 NATURAL PHILOSOPHY. 450 (1.) Double-convex, "1 (2.) Plano-convex, ' Thl< * er at the middle tha " ;"' ' at the edges, (d.) Concavo-convex, or meniscus, J The double-convex may be taken as the type of these. (4.) Double concave, | (5.) Plano-concave, I Thinner at the middle than (6.) Convex-concave, or diverging | at the edges, meniscus, The double -con cave may be taken as the type of these. (a.) The effect of convex lenses may be considered as produced by two prisms with their bases in contact ; that of concave lenses, by two prisms with their edges in contact. (6.) Tlie effects of lenses may be illustrated with spectacles or eye-glasses. The common magnifying: glasses used in botanical study and for other purposes will answer. The larger lenses which may easily be removed from an opera-glass or a magic lan- tern mny be made to furnish the apparatus needed for our present purposes. See description of "The Water Lens" and "The Fountain of Fire," in the little book of " Light " mentioned at the top of page 336. 451. Centre of Curvature ; Principal Axis ; Optical Centre. A double convex lens, ab, (Fig. 216), may be described as the part common to two spheres whicl; PIG. 216. 452 REFRACTION OF LIGHT. 357 intersect each other. The centres of these spheres, c and C, are the centres of curvature of the lens. The straight line, XY, passing through the centres of curvature is the principal axis of the lens. In every lens there is a point, o, on the principal axis called the optical centre. It is generally at equal distances from the two faces of the lens. Any straight line, other than the principal axis, passing through the optical centre is a secondary axis, as II K. Experiment 228. Hold one of the large lenses of an opera glass in the sun's rays. Notice the converging pencil formed by the rays (after passing through the lens) as they pass through air made dusty by striking together two blackboard erasers. T1i'> focus and its distance from the lens maybe seen. Measure ti.Y distance. Hold a similar lens by the other, face to face. Noli j that the rays after passing through both lenses converge r..- . quickly, lessening the distance of the focus from the lens. 452. Principal Focus.-/// incident rays pnr' li'l to tJte principal axis of a convex lens if/- 1 (tj'trr tiro refractions, converge at a point P-<('!I // the principal focus. This point may lie on either side of the lens, ncconl ing to the direction in which light moves ; it is a rc;;l focus. Its distance from the lens is called thefwal dis- tance- FIG. 217. 358 NATURAL PHILOSOPHY. 452 (a.) The position of the principal focus of a lens is easily deter- mined. Hold the lens facing the sun. The parallel solar rays in. cident upon the lens will converge at the principal focus, F (Fig. 217). Find this point by moving a sheet of paper back and forth behind the lens until the sunny spot formed upon the paper is as bright and small as you can make it. Owing to the identity between heat rays and luminous rays, a convex lens is also a " burning glass." (6.) Kays diverging from/, a point at twice the principal focal distance from the lens, will converge at f at twice the focal dis- tance on the other side of the lens. This may be shown by expert menting with a lens and candle-flame until the flame and its image upon a movable screen are at equal distances from the lens. (c.) In such experiments, it is well to fit neatly the lens into a pasteboard or other opaque screen, to cut off from the screen all luminous rays other than those passing through the lens. (d.) The foci situated at twice the principal focal distance are called secondary foci. They are conjugate foci. Experiment 229. Hold a magnifying glass so as to see an ob- ject distinctly. Now move the object from the lens. The eye must be placed closer to the lens to secure distinct vision. 453. Conjugate Foci of Convex Lenses. Kays . 21$. 453 REFRACTION OF LIGHT. 359 diverging from a luminous point in the principal axis at a small distance beyond the principal focus on either side of the lens will form a focus on the principal axis beyond the other principal focus. Thus, rays from L will converge at I (Fig. 218) ; conversely, rays from I will converge at L. The luminous point and the focus of its rays will lie in the same primary or secondary axis. Two points thus related to each other are called conjugate foci ; the line joining them always passes through, the optical centre. (a.) When the luminous point is at the focal distance, the re- fracted rays will be parallel (Fig. 219 [;] ) and no focus will be formed. FIG. 219. (6.) When the luminous point, L, is at less than the focal dis- tance, the refracted rays will still diverge as if from a point, I, on the same side of the lens, more distant than the principal focus. (Fig. 219 [2 ] ). This focus will be virtual. (c.) Conversely, converging rays falling upon a convex lens will form a focus nearer the lens than the principal focus. 360 NATURAL PHILOSOPHY. 45/ 1 . 454. Images Formed by Convex Lenses. The analogies between the convex lens and the concave mirror cannot have escaped the notice of the thoughtful pupil. Others will appear. If secondary axes be nearly parallel to the principal axis, well-defined foci may be formed upon them, as well as upon the principal axis. A number of these foci may determine the position of an image formed by a lens. 455. Diminished Real Image.// the Abject. A B, be more than twice the focal distance from the convex lens, its image will be real, smaller than the object and inverted. (Fig. 220.) FIG. 220. Kays proceeding from A are focused at a upon the secondary axis, A0a\ similarly with rays from B and from all intermediate points. Hence the image, ab. 456. Magnified Real Image.--// the object be further from the lensfwn the principal focus, but 457 REFRACTION OF LIGHT. 361 at a distance less than twice the focal distance, the image will be real, magnified and inverted. (Fig. 221.) FIG. 221. 457. Virtual Image. If the object, AB, be placed nearer the lens than the principal focus, the FIG. 232. image will be virtual, magnified and erect. from .1 appear to come from a; rays from B appear to come from b. Experiment 230. Repeat Experiment 228, using the eyt* glasses or small leases of an opera glass. Notice th.at the re- fracted rays form a diverging pencil. 362 NATURAL PHILOSOPHY. 458 458. Conjugate Foci of Concave Lens. Rays from a luminous point at any distance whatever will be made more divergent by passing through a concave lens. Rays parallel to the principal axis will diverge after refraction as if they proceeded from the principal focus. In any case, the focus will be virtual and nearer the lens than the luminous point. In Fig. 223, the vir- tual image of L is at I, from which point the refracted rays appear to proceed. 459. Image formed by Concave Lenses. Images formed by a concave lens are virtual, tmaller than the object and erect. 460 RECA P ITU LA TION. 363 460. Recapitulation. To be amplified by tbe pupil for veview. REFRACTION OF LIGHT. I ** W D * 2 I I i 8 r > z 1 ^ e r P* * 1 ^ ? ? F ! 1 j a ? i PJ ^j^_ ^ 3 0, ^3 ? ^ ^ M 3 1 a 1 It ? f ri] > 8 g i r | f 1 1 I 3 5 g> $' 9 b ^ M I i 1 1 ? ft 1 ! 1 I! 564 NATURAL PHILOSOPHY. 460 EXERCISES. 1. (a.) What is refraction of light? (6.) State the laws govern- ing the same, (c.) Give an illustrative diagram. 2. (a.) Name and illustrate by diagram the different classes of lenses. (6.) Explain, with diagram, the action of the burning- glass. 3. Explain total reflection. 4. Show, with diagram, how the secondary axes of a lens mark the limits of the image. 5. Using a convex lens, what must be the position of an object in order that its image shall be real, magnified and inverted ? 6. Show, by a diagram, the course of a ray of light passing through the centre of a glass sphere. 7. Show, by a diagram, the course of a ray of light passing through a glass sphere at one side of the centre. 8. Draw the section of a prism and draw lines showing the path of a ray of light through it. 9. A cathetal prism (one whose section is an isosceles, right- angled triangle) may be used as a mirror. Draw a diagram show- ing how this may be done. 10. Draw diagrams showing the direction of rays of light before and after refraction by a double convex lens, the rays starting from a luminous point (a.) at the principal focus. (6.) On the principal axis between the principal and secondary foci, (c.) On the prin- cipal axis beyond the secondary focus, (d.) On the principal axis, lietween the lens and the principal focus. SECTION IV. CHROMATICS. SPECTRA. 461. Other Results of Refraction. Most lumi- nous objects emit light of several kinds blended together. We must learn to sift these varieties one from the other and to deal with any one kind by itself. F'TG. 224. Experiment 231. Admit a sunbeam through a very small opening in the shutter of a darkened room The opening may be prepared by cutting a slit an inch (25 mm.) long and % of an inch (1 mm.} wide in a card. See that the edges of the slit are smooth. Tack the slit over a larger opening in the shutter. If we look at the aperture from E (Fig. 224), we shall see the pun beyond. The path of the beam from S to E is made visible by the floating dust. If a prism be placed in the path of the beam , as shown in the figure, the sides of the slit and edges of the prism Wing horizontal, the beam will be refracted upward. If the refracted beam be 366 NATURAL PHILOSOPHY. 462 caught upon a screen, it will appear as a band of differently colored light, passing very gradually from red at the bottom, through orange, yellow, green, blue and indigo to violet at the upper end of the beautifully colored band. By placing the slit in a vertical position and standing the prism on its end so that its edges will be parallel with the sides of the slit, the spectrum m:iy be projected as a horizontal band. The rays thus separated by the prism are identical with each other and with radiant heat except in the matter of wave length or (what amounts to the same thing) rapidity of vibration. 462. The Solar Spectrum. The colored land produced in Experiment 231 is called the solar -pectruin. The initials of the names of these different colors f, r - the meaningless word, VIBGYOR, which may aid the ] pil in remembering these prismatic colors in their pro , r order. 463. Dispersion. By looking at Fig. 224, it will l.o seen that the red rays have been refracted the least and the violet the most of all the luminous rays. Tfiis sejxt- ration of the differently colored rays by the prism is called the dispersion of light ; it depends upon the fact that rays of different colors arc refracted in different degrees. Experiment 232. Let the rays that have been dispersed by a prism fall upon a convex lens as shown in Fig. 225. They will be refracted to a focus and recombined to form white light. A toncave mirror may be used to reflect the rays to a focus instead of using the lens as above described. 463 CUE OMA TICS SPECTRA . 367 PIG. 225. Experiment 233. Make a "Newton's disc" of cardboard painted with the prismatic colors in proper proportion as indicated by Fig. 226. It is better to divide the surface given to each color into smaller sectors arranged alternately as shown in Fig. 227. You FIG. 226. FIG. 227. may paste sectors of properly colored paper upon the card-board instead of painting them. Cause this disc to revolve rapidly by means of the whirling table or by fastening it to a large top. Notice that the colors are blended and that the disc appears grayish white. Experiment 234. Hold a second prism near one that is used to produce a solar spectrum, the position of the second being inverted with reference to the first. If the dispersing prism be held as shown in Fig. 224 or 225, the second should be held with the refracting edge uppermost, the facing surfaces being parallel. 36$ NATURAL PfflLdSdP&f. 464 The dispersed rays emerging from the first prism will pass through the second. The rays separated by the first will be again blended by the second and appear as white light. Experiment 235. Hold a hand mirror near the dispersing prism so as to reflect the refracted rays to a distant wall or ceiling. Give to the mirror a rapid, angular motion so that the spectrum is made to move to and fro very quickly in the direction of its length. The spectrum changes to a band of white light with a colored spot at each end. The effect is due to what is known as the "Persistence of Vision," familiarly illustrated by the experi- ment of producing a ring of light by whirling a firebrand around a circle. 464. The Composition of White Light. We have now shown, by both the processes of analysis and synthesis, that white light is composed of the seven prismatic colors. We have decomposed white light into its seven constituents and recombined these constit- uents into white light. Experiment 236. Paint three narrow strips of cardboard, one vermilion red, one emerald green, and the other aniline violet. Be sure that the coats are thick enough thoroughly to hide the cardboard. When dry, hold the red strip in the red of the solar spectrum ; it appears red. Move it slowly through the orange and yellow ; it grows gradually darker. In the green and colors be- yond, it appears black. Repeat the experiment with the other two strips and carefully notice the effects. Experiment 237. Make a loosely wound ball of candle wick ; soak it in a strong solution of common salt in water ; squeeze most of the brine out of the ball ; place the ball in a plate and pour al- cohol over it. Take it into a dark room and ignite it. Examine objects of different colors, as strips of ribbon or cloth, by this yellow light. Only yellow objects will have their usual appearance 4^7 CHROMATICS SPECTRA. $6$ 465. Color of Bodies. The color of a body is its property of reflecting or transmitting to the eye rays of & particular kind, the other rays being generally absorbed. (a.) Properly speaking, color is not a property of matter, but of .iglit. A ribbon is called red, but the redness belongs to the light, not to the ribbon. There would be more propriety in saying that the ribbon has ail the other colors of the rainbow, because it keeps the others and reflects the red. If the red ribbon be placed in the green or blue of the spectrum, it will appear black because it receives no red rays to reflect. 466. The Rainbow. The rainbow is due to re- fraction, reflection and dispersion of sunlight by water- drops. The necessary conditions are : (1.) A shower during sunshine. (2.) That the observer shall stand with his back to the sun, between the falling drops and the sun. (See Elements of Natural Philosophy, 706-710.) 467. The Luminous Spectrum. The color of light depends only upon wave length or rapidity of vibration. Color is to optics what pitch is to acous- tics. "Red light has a wave length o.f ST JoiF nicn 5 violet light lias a wave length of -g-r&w inch. The wave lengths that correspond to the other colors are intermediate be- tween these in value. The shorter the wave, the more it is refracted (See Fig. 224). Of course, the wave length depends upon the rapidity of vibration of the molecule that produced the wave in the ether. 370 NATURAL PHILOSOPHY. 468 468. Other Properties of the Sunbeam. We have decomposed a sunbeam, and thus produced the seven prismatic colors. But we must go still further. Beyond the limits of the visible spectrum, in, both directions, there are rays that do not excite the op- tic nerve, the existence of which, however, may be easily proved. These visible and invisible rays differ only in respect to wave length. Some of these waves are so short and fall so rapidly that the optic nerve cannot respond to their vibrations. OtLers are so long and fall so slowly that the optic nerve cannot respond to their vibrations either. In either case, the rays are incapable of exciting vision. 469. Actinic Spectrum. The ac'inic or chemi- cal effects of sunlight are familiar to all. The sensitive paper of the photographer will remain unchanged in the dark ; it will be quickly blackened in the light. If a piece of paper, freshly washed in a solution of sul- phate of quinine, be held successively in the different parts of the visible spectrum, it will be affected least in the red and most in the violet. In this way actinic effects may be found at a point beyond tJie violet^ wholly outside the visible spectrum. We thus detect ultra-violet rays con- stituting an actinic spectrum. A quartz prism is desirable for this experiment, as glass quenches most of the actinic rays. 470. Thermal Spectrum. If a very delicate ther- 471 RECA PITVLA TIOX. 371 mometer or thermopile be successively placed in various parts of the spectrum, it will be found that the tempera- ture is scarcely affected in the violet, but that there is a continual increase in temperature as the thermometer is moved toward the other end of the spectrum, it being quite marked in the red. A very marked rise of temperature takes place beyond the red, wholly outside the visible spec- trum. We thus detect ultra-red rays constitut- ing a heat spectrum. A rock-salt prism is desirable for this experiment, as glass absorbs most of the ultra-red rays. Radiant heat differs from light only in wave length. All luminous rays have heating power ; heat rays are luminous if their vibrations come at such a rate that the organ of sight can absorb their energy and respond with sympathetic vibrations ( 338). 471. Recapitulation. To be amplified by the pupil for review. C By Prisms. (_ By Water Drops. f Luminous. Actinic. COMPLEXITY OF SUNBEAM.... COLOR OF BODIES. ANALYSIS. Spectra Thermal. By Lenses. By Mirrors. By Prisms. By Persistence of Vision. NATURAL PHILOSOPHY. 471 EXERCISES. 1. What is the difference between the " diffusion of light " and " the dispersion of light " ? 2. Why does a red body appear red ? 3. Name the colors of the solar spectrum in their proper order, beginning with the one of least wave length. 4. To what quality of sound does color correspond ? 5. Why does the moon look green when viewed through a piece of green glass ? 6. You hold in your hand a red rose. You carry it into a room that is wholly dark. Has the rose then any color? Explain your answer. 7. Imagine a line 186,000 miles long extending from your eye toward the sun. Radiant energy (heat or light) will traverse such si line in a second ( 427). Suppose that light from the sun is com- ing toward you. (a.) After the first wave reaches the far end of this line, how long will it take the same wave to reach you V (b.) If the light is red, how many such waves can lie on this line r.t once ? (c.) How many such waves will enter your eye in a second ? 8. You see a red ribbon, (a.) Does the ribbon reflect any light ? (It.) How do you know ? (c.) If it does, what is the color of the re fleeted light ? (d.) Some of the sun's rays enter the ribbon ; what is their effect ? 9. " Converging " and " diverging " lenses are sometimes men- tioned. What are they? 10. You see a rainbow. State the threefold work done upon a sunbeam by each raindrop that contributes to the success of the exhibition. SECTION V. A FEW OPTICAL INSTRUMENTS. 472. Photographer's Camera. The photogra- pher's camera is nearly the same as the camera-obscura described in 426. Instead of the darkened room we have a darkened box ; instead of the simple hole in the shutter we have a convex lens, placed in a tube at A. (Fig. 228.) (a.) A ground-glass plate is placed in the frame at E, which is adjusted so that a well- defined, inverted image of the object in front of A is projected upon the glass plate. This adjustment or "focusing" is com- pleted by moving the lens and it 3 tube by the toothed wheel at D until the ob- ject in front of A and the plate at E are at the con- nigate foci of the lens at A. When the " focus- ing" is satisfactory, A is covered with a cloth, the ground-glass plate is replaced by a chemically-prepared sensitive plate, the cloth removed and the image projected on the plate. The light works certain chemical changes where it falls upon this plate and thus a more lasting image is produced. The many other processes in- volved in photography cannot be considered here. 473- The Human Eye. This most admirable of all optical instruments is a nearly spherical ball, capable FIG. 228. 374 NATURAL PHILOSOPHY. 473 of being turned considerably in its socket. The outer coat, S, is called the white of the eye. Its transparent part in front, C, is called the cornea. The cornea is more convex than the rest of the eyeball. The cornea fits into the coat, S, as a watch crystal does into its case. Behind the coat, S, is a dark, opaque coat, N. Behind the cornea -is a curtain, /, called the iris. It is colored and opaque ; the circular window in its centre is called the pupil. The color of the iris constitutes the color of the eye. Back of the pupil is the crystalline lens, L, built of concentric shells (layer on layer, as in an onion) of varying den- sity. Its shape is shown in the figure. This lens di- vides the eye into two chambers. The anterior chamber contains a limpid liquid called the aqueous humor ; the posterior chamber contains a transparent jelly, F. called the vitreous humor. The cornea, aqueous humor, crystalline lens and vitreous humor are refracting media. Back of the membrane, H, which incloses the vitreous humor, is the retina, R, an expansion of the optic nerve. The eye, optically considered, is simply art ar- rangement for projecting inverted, real images of visible objects upon a screen, li, made of nerve filaments. If the images thus formed be well defined and sufficiently luminous, the vision is distinct. FIG. 475 OPTICAL INSTRUMENTS. 375 (a.) Objects are not seen distinctly unless the image falls di- rectly upon the retina. If the image of a distant object falls in front of the retina, the person is said to be near-sighted. Near- sightedness may be relieved by the use of concave glasses. If the image of a near object tends to fall back of the retina, the person is said to be far-sighted. Far-sightedness may be re- lieved by the use of convex glasses. (b.) The eye of the common house-fly is said to have 4,000 lenses ; that of some kinds of beetles is said to have 25,000, each with its own cornea and retina. " If each of these lenses forms a separate picture of each object, what an awful army of cruel giants must the beetle behold when he is captured by a schoolboy I" 474. Magnifying Glasses. A magnifying glass, or simple microscope, is a con- vex lens, generally double- convex. The object is placed between the lens and its principal focus. The image is virtual, erect and magnified. 475. Compound Mi- croscope. The compound microscope consists of two or more convex lenses placed iu a tube. One of these, o, called the object-glass or objective, is of short focus. The object, ab, being placed slightly beyond the principal focus, a real image, cd, mag- nified and inverted, is formed within the tube ( 456), Fio. 380. 376 NATURAL PHILOSOPHY. 475 The other lens, E, called the eye-glass, is so placed that the image formed by the objective lies between the eye-glass and its focus. AB (Fig. 230) is a magnified, virtual image of the real image, formed by the eye-glass and seen by the observer. 476. Galilean Telescope ; Opera Glass. In the telescope attributed to Galileo, the objective, 0, is a double convex and the eye-piece, C } is a double con- cave lens. The concave lens intercepts the rays before they have reached "the focuc of the objective. Fio. 231. The rays from A, converging after refraction by 0, are rendered diverging by C '; they seem to diverge from a. In like manner, the image of B is formed at b. The image, ab, is erect and very near. An opera glass consists of two Galilean telescopes placed side by side. 477. Astronomical Telescope ; Refractor. As- tronomical telescopes are of two kinds refractors and reflectors. Fig. 232 represents the arrangement of the lenses and the direction of the rays in the refracting telescope. The object glass is of a large diameter that it may collect many rays for the better illumination qi the image. 478 OPTICAL INSTRUMENTS. 377 FIG. 232. The inverted, real image, ab, formed by the objective, 0, is magnified by the eye-piece, as in the case of the compound microscope. The visible image, cd, is a vir- tual image of ab, which is the real image of AB. The lenses are enclosed in a tube. (a.) The largest refracting telescope (1884) is at the Pulkowa Observatory. Its object glass is 30 inches in diameter. A 36 inch object glass is making for the telescope of the Lick Obser- vatory in California 478. Reflecting Telescopes. A reflecting tele- scope consists of a tube closed at one end by a concav.e FIG. 233. mirror, M, so placed that the image formed by it maj be magnified by a convex lens used as an eye-piece. The rays from the mirror are reflected at mn, andareaJJ 78 NATURAL PHILOSOPHY. 478 image formed at db. This image is magnified by the glasses of the eye-piece and a virtual image formed &ted. The Earl of Rosse built a telescope with a mirror six feet in diameter and having a focal distance of fifty-four feet. Experiment 238. Reflect a horizontal beam of sunlight into a darkened room. In its path, place a piece of smoked glass on which you have traced the representation of an arrow, AS, (Fig. 234) or written your autograph. Be sure that every stroke of Fro. 234. the pencil has cut through the lamp black and exposed the glass beneath. Place a convex lens beyond the pane of glass, as at L, so that rays that pass through the transparent tracings may be refracted by it as shown in the figure. It is evident that an image will be formed at the foci of the lens. If a screen, SS, be held at the positions of these foci, a and 6, the image will appear clearly cut and bright. If the screen be held nearer the lens or further from it, as at ff or 8", the picture will be blurred. 479. Magic Lantern. In the magic lantern (Fig. 235), a lamp is placed at the common focus of a convex lens (called the "condenser") in front of it and of a con- cave mirror behind it. The light is thus concentrated upon ab, a transparent picture, called the "slide." A of lenses, m, is placed at a little more than its 479 OPTICAL INSTRUMENTS. 379 FIG. 235. focal distance beyond the slide. A real, inverted, mag- nified image of the picture is thus projected upon the screen, 8. The tube carrying in is adjustable, so that the foci may be made to fall upon the screen and thus render the image distinct. By inverting the slide, the image is produced right side up. (a.) Directions for making a simple magic lantern may be found on page 84 of Mayer and Barnard's little book on Light. Fig. 23B FIG. 336. NATURAL PBILOSOPHT. /" 1 if represents a very compact and efficient lantern, known as Marcy's Sciopticon and furnished by James W. Queen & Co., of Philadelphia. Experiment 239. Close the left eye and hold the right hand so that the forefinger shall hide the other three fingers. Without changing the position of the hand, open the left and close the right eye. The hidden fingers become visible in part. Experiment 240. Place a die on the table directly in front of you. Looking at it with only the left eye, three faces are visible, as shown at A, Fig. 337. Looking at it with only the right eye, it appears us shown at B. 480. Stereoscopic Effects. Jrom the last two ex- periments, we see that when we look aS a solid, the images upon the retinas of the two eyeo are dif- ferent. If, in any way, we combine two drawings, so a? to produce images upon the retinas of the two eyes like those produced by the solid object, we obtain the idea of solidity. 481. The Stereoscope. To blend these two pictures is the office of the stereoscope. Its action will be readily understood from Fig. 238. The diaphragm, D, prevents either eye from seeing both pictures at the same time. Kays of light from B are refracted by the half-lens, E', so that they seem to come from C. In the same way, rays from A are re- Fio. m fracted by E so that they also seem o A.o2 JlECA FiTULA TlOffl* 381 to come from C. The two slightly different pictures, thus seeming to be in the same place at the same time, are successfully blended and the picture "stands out" or has the appearance of solidity. If the two pictures of a stereoscopic view were exactly alike, this impression of solidity would not be produced. 482. Recapitulation. To be amplified by the pupil for review. OBSCURA. CAMERA... . HUMAN EYE. ( OBSCURA. PHOTOGRAPHER'S. I SIMPLE. MICROSCOPES. COMPOUND. { REFLECTORS. REFRACTORS . MAGIC LANTERN. STEREOSCOPE. Galilean. Opera Glats. Astronomical. CONCLUSION. ENERGY. 483. Varieties of Energy. Like matter, energy is indestructible. We have already seen that energy may be visible or invisible (i. e., mechanical or molecular), kinetic or potential. We have at our control at least eight varieties of energy. (a.) Mechanical energy of position (visible, potential). (6.) Mechanical energy of motion (visible, kinetic). (e.) Latent heat (molecular, potential). (d.) Sensible heat (molecular, kinetic). (e. ) Chemical separation (molecular or atomic ; potential). (/.) Electric charges (probably molecular, potential). (g. ) Electric currents (probably molecular, kinetic). (h.) Radiant energy, thermal, luminous or actinic (molecuiar, kinetic). 484. Conservation of Energy. The doctrine that, considering the universe as a whole, the sum of all the forms of energy is a constant quantity, is known as the Conservation of Energy. a+b+c+d+e +f+g + h=& constant quantity. This does not mean that the value of a is invariable ; we have seen it changed to other varieties as b or d. We have seen heat changed to electricity and vice versa, and either or both changed to mechanical energy. It does not mean that the sum of these eight variable quantities in the earth is constant, for we have seen that energy 486 ENERGY. 383 may pass from sun to earth, from star to star. But it does mean that the sum of all these energies in all the worlds that constitute Uie universe is a quantity fixed, invariable. 485. Correlation of Energy. The expression Cor- relation of Energy refers to the convertibility of one form of energy into another. Our ideas ought, by this time, to be clear in regard to this convertibility. One important feature remains to be noticed. Radiant energy can be converted into other forms, or other forms into radiant energy only through the in- termediate state of absorbed heat. 486. Recapitulation. To be amplified by the pupil for review. VISIBLE OR MECHANICAL. OfPotition Potential. Of Motion Kinetic. e.g., Hanging Apple, Head of Water. e.g., Falling Apple, Flowing Water. INVISIBLE OR MOLECULAR. {Of Position, e. g.. Latent Heat. Potential. Of Motion, e.g.. Sensible Heat. Kinetic: Of Motion, ELECTRICITY . Of Position, e. g., Charged Leyden Potential. jar, Battery with cir- cuit broken. Of Motion, e. g., Leyden jar dis- Kinttic. charging ; Battery with circuit closed. GENERAL 1. (a.) Define science, matter, mass, molecule and atom. (6.) Define physics. 2. (a.) What are physical properties of matter? (6.) Define and illustrate two properties of matter. 3. Define solid, liquid and gas ? 4. (a.) Give Newton's laws of motion. (6.) Give the law of re- flected motion. 5. How may we measure distances by sound ? 6. Upon what does the pitch of a sound depend ? 7. Why do the rays of the evening sun come to us in curved lines ? 8. How does distance effect the intensity of light ? 9. What is an angle of reflection ? 10. (a.) What is the general effect of a concave mirror ? (5.) Of a concave lens ? 11. Fig. 339 represents reflection of luminous rays from a rough surface. Draw a similar figure illustrat- ing reflection by a plane mirror. 12. Is light always re- fracted in passing from one medium to another ? 13. How does a straight stick appear when partly immersed in water ? Why? 14. Under what cir- cumstances will the object and the image be on the same side of a convex lens ? 15. Why will an iron gate that opens easily in winter often ttick in summer ? 486 GENERAL REVIEW. 385 16. What is meant by intermolecular spaces and how do they compare with molecular diameters? 17. How are centigrade and Fahrenheit thermometers graded ? 18. Why are inland cities subject to greater extremes of tem- perature than seaside resorts ? 19. Can rays of light cross one another without interfering with the effect of the rays ? 20. The two sides of the greased paper photometer ( 438,6.) are equally illuminated by a candle flame on one side, distant 18 inches, and by a lamp flame on the other side, distant 6 feet. How does the illuminating power of the lamp compare with that of the candle ? 21. Are all the luminous rays of the same color? 22. Which is the more bulky, a pound of ice or a pound of water ? 23. How can you make water boil at a lower temperature than 212 F. ? 24. What is an anode? A node? 25. Does the vaporization of water change the size of the mole- cules or the distances between them ? 26. How do you change centigrade into Fahrenheit readings ? 27. What piece or pieces of physical apparatus have you made since you began studying this book ? 28. What is meant by ineitia ? 29. How far will a freely falling body move in 10 seconds ? 30. Which is the more general term, fluid or liquid? 31. Give a general law of machines. 32. How can energy be destroyed. 33. Define force and energy. 34. State Archimedes' principle. 35. Define specific gravity. 36. Explain gaseous tension. 37. Describe and explain the experiment with the Magdeburg hemispheres. 38. What is a dynamo ? 39. Explain the action of intermittent springs. 40. Give Ohm's law. 386 NATURAL PHILOSOPHY. 486 41. What is the difference between noise and music ? 42. Why does not Hudson Bay freeze solid to the bottom? 43. What is an ampere ? 44. Explain interference of sound. 45. What is meant by E. M. F. ? 46. What is meant by the " mechanical equivalent of heat " ? 47. How should the cells of a voltaic battery be joined to give the best effect ? 48. On what optical fact does the success of a sharpshooter (Fig. 340) depend ? FIG. 240. 49. Do you think that you know all about Natural Philosophy* APPENDIX A. Mathematical Formulas. 7r=3.14159. D=diameter. R radius. Circumference of circle=7r D. Area of a circle=7r R*. Surface of a sphere=4 IT R*=7r D*. Volume of a sphere f n W=$ n D. APPENDIX B. International or Metric Measures. This sys- tem bids fair to come into general use in this country. For this reason, as well as for its greater convenience, an acquaintance with it is now desirable and may soon be necessary. It has been already legalized by act of Con- gress. The meter is defined as the forty-millionth of the earth's meridian which passes through Pans. It is equal to 39.37 inches. Like the Arabic system of notation and the table of U. S. Money, its divisions and multiples vary in a tenfold ratio. 8 NATURAL PHILOSOP&Y. Metric Measures of Length. Ratio = 10. (Millimeter (mm.)= .001 m.= 0.03937 in. T " \Centimeter (cm.) = Loi w.= 0.3937 " [Decimeter (dm.) = .1 w.= 3.937 UNIT. Meter (m.) 1. m.= 39.37 " fDekameter (Dm.)= 10. m. =393.7 g MTOTI-! Hectometer (JZ TO.) = 100. m. =328 ft. 1 in. 1 PLES. I Kilometer (Km.) = 1000. i.= 0.62137 mi. lMyriameter(J/>.)= 10000. m.= 65137 " 3 -ZVote. The table may be read : 10 millimeters make g one centimeter ; 10 centimeters make one decimeter, etc. | (Fig. 241.) The denominations most used in practice are v printed in italics. The system of nomenclature is very M simple. The Latin prefixes, mitti-, centi- and deci-, sig- 2 nifying respectively ^Vs* -j-^, and -j^, and already fa- miliar in the mill, cent and dime of U. S. Money, are s used for the divisions, while the Greek prefixes deka-, " hecko-, kilo- and myria-, signifying respectively 10, 100, ^ 1000 and 10000, are used for the multiples of the unit. Each name is accented on the first syllable. It may be | noticed that the meter corresponds somewhat closely to S the yard, which it will replace. Kilometers will be used o instead of miles. - 1 inch= 25.4000 TOOT. 0.0254 m. or about 2| cm. lfoot= 30.4800 cm. =0.3048 m. " 30 " 1 yard= 0.9144m. " {? of a meter. 1 mile=1609.0000 m. =1.6090 Km. " 1 T % Km. Metric Measures of Surface. Ratio =io 2 FIG. 241. =100. f Square millimeter (sq. 7i.)=0.000001 sq. m. DIVISIONS. -| Square centimeter (q. cm.) =0.0001 " [ Square decimeter (sq. dm.) =0.01 UNIT. Square meter (sq. m.) =1. " etc. , etc. APPENDIX. 389 Note. The table may be read: 100 sq. mm.=l sq. em. ; 100 iq. cm. 1 sq. dm. , etc. The reason for the change of ratio from 10 to 100 may be clearly shown by representing 1 sq. dm., and dividing it into sq. cm. by lines, which shall divide each side of the sq. dm. into 10 equal parts or centimeters. Metric Measures of Volume. Ratio =io 3 = 1000. f Cubic millimeter (cu. mm.)=0.000000001 cu. m, DIVISIONS. \ Cubic centimeter (cu. cm.) =0.000001 " [Cubic decimeter (cu. dm.) =0.001 " UNIT. Cubic meter (cu. m.) =1.308 cu. yds. etc. etc. Metric Measures of Capacity. Ratio =10. For many purposes, such as the measurement of articles usually sold by dry and liquid measures, a smaller unit than the cubic meter is desirable. For such purposes the cubic decimeter has been selected as the standard, and when thus used is called a liter (pronouncecj leeter). fMilliliter(mZ.) = 1 cu.cm. =0.061022 cu. in. DIVISIONS. S Centiliter (cl.) = 10 " =0.338 fld. oz. 1 Deciliter (