FIRST SCIENCE BOOK PHYSICS AND CHEMISTRY BY LOTHROP D. BIGGINS, Pn.B. INSTRUCTOR IN SCIENCE IN THE STATE NORMAL SCHOOL DANBUBY, CONN. GINN & COMPANY BOSTON NEW YOEK CHICAGO LONDON COPYKIGHT, 1905 BY LOTHROP D. HIGGINS ALL BIGHTS RESERVED 514.11 gfte GINN & COMPANY PRO- PRIETORS BOSTON U.S.A. PEEFACE This book is designed to serve as an introduction to scientific study, and at the same time to present a thorough course in the science of common phenomena. Whether the pupil has been prepared by courses in " nature study " or by his independent observation of things about him, he will find many subjects that are already known to him here treated in a manner which should explain the mysteries and clarify his ideas. Finishing this course, the pupil should be well fitted to take up the science studies of preparatory schools, and should have a store of serviceable knowledge. In adapting methods and language to the view point of young pupils, the author has drawn upon an experi- ence of several years with them. While it is one object of the book to teach the terms and expressions of science, care has been used to keep the meaning clear. Methods of treatment that easily convey wrong impressions have been avoided, as well as those which offend pupils of grammar school age. An effort has been made to show the practical bearing of the various subjects upon affairs in our daily experience, such matters being introduced wherever they may serve to illustrate or explain. The experiments are simple and may be performed by the teacher, or by the pupils with his oversight. Briefly, it is believed that the book will present a course which iii 374162 iv PREFACE shall be simple, interesting, and instructive, yet losing nothing of its accuracy. The proof has been read by Professor G. F. Hull of Dartmouth College, for whose critical suggestions the author is very grateful. Several corporations and firms have kindly furnished photographs for the engravings as follows : General Electric Company, the dynamo ; H. K. Porter Company, Pittsburg, Pennsylvania, the air loco- motive ; Baldwin Locomotive Works, the steam locomo- tive ; Illinois Steel Company, the blast furnace ; Cunard Steamship Company, the steamship. LOTHROP D. HIGGINS CLINTON, CONNECTICUT August, 1905 CONTENTS PART I. PHYSICS CHAPTER I MATTER AND ENERGY PAGES Definitions. Scope of Physics. Composition and States of Matter. Properties of Matter. Gravitation. Weight and Specific Gravity 1-22 CHAPTER II FLUID PRESSURE Cause of Fluid Pressure. Pressure in Liquids. Water Supply. Buoyancy and Floating Bodies. Hydraulics. Atmos- pheric Pressure. Vacuum. Barometer. Pumps a"nd Siphon. Air Pump. Pressure in Gases. Compressed Air. Buoyancy in Gases 23-45 CHAPTER III MOTION AND FORCE Newton's Laws of Motion. Inertia. Momentum. Center of Gravity. Stability. Centrifugal Force. Falling Bodies. Pendulum. Work and Power. Machines 46-66 CHAPTER IV HEAT AND ENERGY Sources of Heat. Explanation of Heat. Temperature and Thermometers. Expansion and Contraction. Changes of State due to Heat. Evaporation and Condensation, v vi CONTENTS PAGES Distillation. Latent Heat. Conduction. Convection. Kadiation and the Ether. Cooling of Bodies. Artificial Cold. Transformation of Energy. Heat as a Source of Mechanical Energy. Heat Engines . 67-90 CHAPTER V SOUND Wave Motion. Vibration. Sound explained. Sound Waves. Echoes. Forced and Sympathetic Vibration. Resonance. Tones and Noises. Loudness. Pitch. Quality. Voice . 91-107 CHAPTER VI LIGHT Radiation and Light Waves. Luminous and Illuminated Bodies. The Ether. Transparent, Translucent, and Opaque Substances. Shadows. Reflection; Mirrors. Refraction. Prism and Lenses. Formation of Images. The Eye; Camera; Microscope; Telescope. Color. White Light. Spectrum. Absorption. Color of Objects. 108-128 CHAPTER VII ELECTRICITY Production and Control of Electrical Energy. Electrical Effects. Potential and Electro-Motive Force. Elec- tric Charges. Electrostatic Induction. Discharges. Lightning. Electric Current. Voltaic Cell. Circuit. Resistance. Batteries ; Uses of Current. Electrical Measurements. Magnets. Magnetic Poles. Magnetism of the Earth ; Compass. Induced Currents. Dynamo. Transformer. Induction Coil. Uses of Electrical Energy: Motor ; Cars ; Telephone ; Telegraph ; Electroplating ; Lights ............. 129-169 CONTENTS vii PAKT II. CHEMISTRY CHAPTER VIII OUTLINE OF CHEMICAL STUDY PAGES Scope of Chemistry. Chemical Changes. Composition and Decomposition. Elements, Compounds, and Mixtures. Atoms. Chemical Affinity. Symbols. Classes of Substances : Acids ; Bases ; Metals ; Salts ; Oxides ; Minerals; Ores; Alloys; Solutions 171-191 CHAPTER IX COMMON SUBSTANCES Elements: Oxygen; Hydrogen; Nitrogen; Carbon; Sulphur; Phosphorus ; Chlorine ; Iron ; Other Metals. Com- pounds: Water; Sulphuric Acid; Carbon Dioxide; Ammonia; Cellulose; Starch; Sugars; Alcohol; Fats and Oils ; Soap. Mixtures : Air ; Soil ; Glass ; Wood ; Paper; Coal; Illuminating Gas; Petroleum; Explosives; Foods; Fuels 192-220 CHAPTER X COMMON CHEMICAL PROCESSES Combustion. Explosion. Flame. Oxidation. Oxidation in Animal Bodies. Decay. Fermentation. Bread Making. Disinfection . . 221-230 FIRST SCIENCE BOOK PART I. PHYSICS CHAPTER I MATTER AND ENERGY SECTION I DEFINITIONS 1. Introduction. It is important, in beginning a new study, to have some clear idea of what the study is to be. The word science may perhaps call to mind strange things of which we have heard, so that we think of it as the study of uncommon things. We shall find, how- ever, that science treats of very common matters, many of which we already understand and most of which affect our daily lives. The main difference between scientific study and simply seeing things happen is a difference in the manner of doing it. Scientific study is orderly; each fact is studied in connection with others that are like it, thus making the whole more simple. In science we are not content to find that a thing does happen, but we try to find out how it hap- pens, what causes it, and what is its effect upon other l 2 MATTER AND ENERGY happenings in turn. So then let us enter upon these studies resolved not only to learn all that we can but also to search deeply into each new fact until we fully understand it. 2. Physics. It is easy to see that if science is the study of common things, it must include a great number of subjects that are very different from each other. In order to separate these subjects so that each may be treated more simply, scientific study is divided into many branches, such as physics, chemistry, botany, geology, and others. Still these branches are more or less related to each other, the teachings of one often being applied to several of the others ; in fact, one could hardly know any of the sciences well without knowing something about one or more besides. The teachings of physics are perhaps most generally used, and for this reason it forms a natural starting point for our study. In its broadest meaning, physics is the study of matter and energy. Concerning matter, physics treats of such changes as affect its forms and motions. 3. Matter. To understand this definition it is neces- sary to know first what is matter. No one can really tell what matter is it can only be described ; and it occurs in so many forms and has such various features that no full description would apply to all sorts of mat- ter. Some kinds are of one color and some another, while many show no color at all ; different kinds vary in weight; there are hard substances and soft; and in many other ways we look in vain for features which DEFINITIONS 3 shall apply to all bodies. We find, however, that every- thing which we commonly consider to be matter occu- pies space or takes up room. Therefore, for want of a better definition, we may say that matter is anything which occupies space. Experiment 1. Hold a tumbler bottom upward and push it into water, as in Fig. 1. What do you observe ? What is in the tumbler before it is pushed downward? Does it take up room? Give a rea- p^ l son for your answer. Is air a sort of matter? Is there any form of matter that cannot be seen? 4. Natural Laws. The word law as used in science has a meaning with which we may not be familiar. A natural law is simply a statement of a happening as it occurs in nature. Such laws are not made by man. From time to time men may find out new ones by studying nature, and they may state them for the first time; but no man can make a natural law or destroy one. Doubtless there are many natural laws which man has never discovered, though we can see the results of their operation. 5. Energy. This is a subject of great importance in physics, and should be studied carefully. As with mat- ter, it is hard to say just what energy really is ; but we may say for a definition : Energy is the ability to produce motion. 4 MATTER AND ENERGY Matter cannot of itself cause motion ; when any body of matter seems to cause motion in itself or in some other body, it does so by virtue of the energy which for the time it possesses. A body of matter may gain or lose energy without changing in size or weight, or in any other way that we should commonly notice. Energy must then be something quite different from matter, having no substance and occupying no room, yet very important because without it no motion would be possible. 6. Force. When the energy of a body is used in an effort to cause motion, we generally say that force is exerted. Thus a moving body is said to " exert force " upon anything that it strikes. Similarly in the case of a bat acting upon a ball, a bowstring upon an arrow, a hanging body upon the support from which it is sus- pended, steam in an engine, or an exploding powder charge in a gun, each is an example of some force acting. Notice that the effect of these forces is to cause motion or to make an effort to do so. A moving body gives motion to the body that it strikes, even if it is only the air; the ball, the arrow, and the bullet in the gun move when forces act upon them. But in some cases, as the hanging body or the steam in a boiler, the force may not be seen to cause motion; still the steam pushes hard upon the sides of the boiler, and the hanging body tries to pull its support downward. In these examples force is exerted; but because it is not great enough it may seem to cause no motion, and we say that it tends to produce motion. COMPOSITION OF MATTER 5 From our study we shall become more familiar with the use of this word than any definition can make us, but for the sake of a definition we may say, Force is the direct cause that tends to produce motion or a change of motion. 7. Forms of Energy. Like matter, energy occurs in many different forms. Naturally, too, we apply names to many different forces acting in the world. We shall learn more about these forms of energy and these forces as we study further. QUESTIONS 1. What do you understand the term science to mean ? 2. Why is scientific study divided into many branches ? Name several of these branches. How are they related ? 3. Define physics. What is included in the word matter? Name some forms of matter. Can you name anything which is not*matter ? 4. What is a natural law? How are natural laws discovered? Are there any that are not yet known ? Can natural laws be broken? 5. Define energy. Define force. Carefully show the differ- ence between them. 6. Gwe examples of bodies which have energy but exert no force. Give examples of forces tending to produce motion, without succeeding. SECTION II COMPOSITION OF MATTER 8. Three States of Matter. Matter occurs in three states or conditions as solids, liquids, and gases. Solids are those bodies that keep the same size and shape unless changed "by some outside force. Glass, wood, 6 MATTER AND ENERGY iron, cloth, paper, wax, ice, leather, and rock are ex- amples of solid substances. Liquids keep the same size, but change their shape according to the vessel in which they are. Water is the most common liquid ; others are alcohol, benzine, kero- sene, ether, and mercury. Crases do not keep either their size or shape, but expand without limit. For this reason a gas cannot be kept FIG. 2 pure unless it is in a tightly closed vessel. If a bottle of some gas be uncorked and left so, in a few minutes the gas will escape into the air and the bottle will con- tain only a very little, mixed with a large amount of air. This is because the tiny particles of any gas are always in rapid motion, and so they become separated from each other, mixing with particles of other gases. This process is called diffusion. Gases are much lighter than liquids or solids. Most of them are invisible (cannot COMPOSITION OF MATTER 7 be seen) and only a few have any color. Air is a gas, as is also steam, hydrogen, oxygen, chlorine, and others. Many gases which cannot be seen may be dis- covered by their odor ; as coal gas, illuminating gas, ammonia, etc. Vapor is a name given to such gases as easily change to liquids ; as steam. True steam is invisible, the white cloud that we call steam being made of tiny drops of water. At the spout of a kettle we sometimes notice a seemingly vacant space (Fig. 2) where there is really steam; in a moment this steam has cooled into drops. Smoke is not a gas, but is a mass of solid particles. 9. Changes of State. In general, substances change from solids to liquids and from liquids to gases upon being heated. For some kinds of matter great heat is necessary. Experiment 2. Gently heat small quantities of ice, wax, par- affin, sugar, or butter, and note the changes which take place. Experiment 3. Fit a stopper into a test tube, and through the stopper run a glass tube, as in Fig. 3. Into the test tube put a small quantity of water, alcohol, ether, or benzine. Dip the open end of the glass FlG 3 tube under water and gently heat the liquid in the test tube. Note carefully and explain all that you see. Be sure to apply only slow heat and use caution with volatile liquids. 8 MATTER AND ENERGY In general, gases change to liquids and liquids to solids upon being cooled. Air and other gases are now changed to liquids, and some even to solids, by cooling to a very low degree. Steam is so commonly changed to liquid particles that we carelessly call the liquid " steam." Experiment 4. Boil some water in a test tube. Hold a cool dish in the steam just as it escapes from the tube. (A glass dish, if clean and dry, may best serve the purpose.) Explain what you observe. Experiment 5. Melt some paraffin, wax, or sugar, and let it cool rapidly. What change takes place ? Under what condition does the change occur? Most forms of matter occur commonly in only one state, because the temperatures at which they would change are unusually high or low. Some substances, however, are common in two states: ice, wax, sugar, and vaseline easily change to liquids ; while such liquids as alcohol, ether, naphtha, and chloroform readily change to the gaseous state. Water commonly occurs in all three states ; as ice (a solid), water (a liquid), and steam (a gas). 10. Composition of Matter ; Molecules. All matter is made up of tiny particles called molecules. A mole- cule is the smallest particle of any substance which can exist alone without changing its nature. From this definition it is clear that a molecule is too small to be seen; for the smallest bit of matter which could be seen would be capable of division into other bits much smaller. Still we know that there must finally be particles so small that they can no longer be divided ; and although no one has ever seen them, we can give them a name molecules. COMPOSITION OF MATTER 9 11. Molecular Theory Now because these molecules are too small to be discovered, no one knows exactly how they are put together to form any body of matter. There are several like cases in scientific study, when things cannot be definitely known but are only explained by guess. Such guesses are not rash, however, or hasty; they are made by men who have studied and thought deeply, and may generally be taken as probably true explanations. An explanation of this sort is called a theory. The explanation of the composition of matter is called the molecular theory ; while it cannot of course be proved by ordinary methods of proof, it is generally believed by scientists. The molecular theory states that the molecules in any body are separated from each other by spaces called pores. These pores are larger than the molecules them- selves, and in them the molecules are supposed to move rapidly to^ and fro. The rapid to-and-fro motion is called vibration. Experiment 6. The existence of pores may be shown by dip- ping a piece of gold (one of the densest of solids) into a cup of mercury (quicksilver). The molecules of mercury will fill the pores between the molecules of gold. (The mercury can be removed from the gold by dipping into nitric acid.) Similarly iron will take water into its pores. QUESTIONS 1. Name the three states of matter. Define each. Give exam- ples of each. 2. What features do most gases have in common ? What is meant by diffusion ? How is diffusion caused ? What is a vapor ? What is smoke? 10 MATTER AND ENERGY 3. How, in general, may the state of a substance be changed ? Give any common examples of changes caused by heating ; by cooling. 4. Can steam be seen ? Why does steam always form a white cloud when set free in the air ? Of what is that cloud composed ? 5. Give examples of substances common in two states. Why are not all substances common in more than one state ? 6. What is a molecule ? What are pores ? 7. State the molecular theory in your own words. What does it attempt to explain ? Is it known to be true ? SECTION III PROPERTIES OF MATTER 12. Common Properties. The features which any form of matter possesses are called its properties; such, for example, as its color, density, hardness, and the like. As there are many different substances, there are also many different properties. Very few of these are pos- sessed by all forms of matter, though several are com- mon to many substances. We shall consider only a few of the more common properties. 13. Impenetrability. This long word names a prop- erty which is common to all matter, that of completely occupying the space in which it is. This fact is some- times stated as follows: No two bodies can occupy the same space at the same time. No body of matter can enter a space already filled, without first driving out the substance which fills it. Notice that we say the same space; this does not mean that two things cannot be in the same dish, for example, since they can both be in a dish together without filling the same space. PROPERTIES OF MATTER Experiment 7. Hold a bottle, mouth downward, over watei and push it downward (Fig. 4). Compare the height of the watei in the bottle with that around it outside. Give a reason for this difference. Now tip the bottle side wise, as in Fig. 5, and note all that happens. What takes place now that did not occur when FIG. 4 FIG. 5 the bottle was held in the other position ? How do you explain the difference ? It is hard to fill a small-mouthed bottle with a thick liquid like oil or molasses, because of the bubbles of air which' must escape as the liquid enters. 14. Cohesion. Cohesion is the force which holds the molecules of a body together. This is a property common to solids and liquids ; the molecules of gases, we have learned, do not cohere, and therefore are free to become widely separated. The greater the cohesive force between the molecules of a substance, the more the substance resists being broken or pulled apart. When cohesion is great, a body is said to be tough. Experiment 8. Try to break pieces of different substances, e.g. wood, glass, leather, bone, steel wire (knitting needles), iron wire, copper wire, etc. Make a list of these in the order of their cohesive force. Experiment 9 Hold a drop of water on a glass rod. Is there anything about this to show that the molecules of water cohere ? MATTER AND ENERGY Broken solids cannot be mended by simply pushing the pieces together, because the molecules cannot be forced near enough to each other. Some substances, such as iron, may be heated until soft, however, and then the broken ends may be pounded until they unite. This process is called welding. 15. Adhesion. Adhesion is the force which holds the molecules of one body to those of another. Only a few sub- stances have this property, and even they will not adhere to many others. No paste, glue, or cement will stick to A everything; each is made for cer- tain substances. Experiment 10. Balance a piece of glass, a (Fig. 6), with weights on the pan b. Place a vessel beneath a, and pour water into the vessel till its surface just touches the piece of glass. Add more weights to the pan b until the glass a is lifted from the water. Why do you have to add more weights ? How much more do you add ? What force are you now measuring ? 16. Hardness. A hard substance is one in which the molecules resist any change of position. This prop- erty of course applies only to solids, for the particles of liquids and gases move about freely. Experiment 11. Using pieces of wood, glass, iron, copper, lead, soap, and quartz, try to scratch each with the others. Which scratches all of them, and which scratches none ? Arrange them in a scale according to hardness. FIG. 6 PROPERTIES OF MATTER 13 Articles made of soft iron may be hardened by heat- ing to a high degree and plunging at once into water or oil. Springs are tempered in this way, after being bent. Hardened iron may be made soft by heating and then slowly cooling ; this is called annealing. Try each process. 17. Brittleness. It may be noticed that certain sub- stances, though they are hard, are easily broken by a blow. Glass, for example, is easily broken to bits by a blow from a hammer, by being dropped upon a hard surface, or sometimes by mere pressure of the hand. Some knife blades, though of very hard steel, may be snapped in the fingers. Such substances as are thus easily broken by a blow are said to be brittle. Experiment 12 Test the brittleness of various substances, by hammering lightly, by dropping them upon a hard surface, or by trying to snap them in the fingers. Chalk, pasteboard, glass, iron wire, a steel needle, copper wire, a cracker, a bit of china, a watch spring, or a wafer may serve as examples. 18. Malleability. A solid substance that may be hammered into thin sheets is said to be malleable. Several of the metals are very malleable and may be rolled into sheets thinner than tissue paper. Gold is the most malleable of substances ; it can be hammered into sheets that are only ^nnnnr ^ an ^ nc ^ thick. We are familiar with thin sheets of this metal under the name " gold leaf." . Experiment 13. Hammer some bits of different substances into the thinnest sheets you can make. Try lead, iron, tin, cop- per, aluminium, and others 14 MATTER AND ENERGY 19. Ductility Some solid substances may be drawn out into small wires ; they are said to be ductile. In several cases a ductile substance is also malleable, but this is not always true; a body whose malleability is great may have small ductility, and the reverse is also true. The ductility of platinum is very great, for it may be drawn into wires that are only Q-^-Q-Q of an inch in diameter. Experiment 14. Heat a small glass tube in a flame, holding it at both ends. When the tube is well heated in the middle (i.e. red hot and soft), remove it from the flame and quickly draw the ends outward. Examine the portion that was heated. 20. Tenacity. The property whereby a solid body resists being pulled in pieces is called tenacity. This property is similar to cohesion, though not so broad in its meaning. Steel wire has great tenacity, a very small wire being able to support a heavy mass. Experiment 15. Take a strip of fresh writing paper two inches wide. Fold so as to make it one inch wide and of double thickness. Grasp one end firmly, while some one else grasps the other end. Now both pull steadily till the paper breaks in two pieces. What do you conclude with regard to paper? 21. Porosity. We have learned that all bodies of matter are composed of molecules and pores or spaces between molecules. The size of these pores varies greatly in different substances, but in any body they are much larger than the molecules. Even in dense masses, such as a piece of lead or silver, these pores are present, though the mass appears to have no such spaces and none can be seen through a microscope. PROPERTIES OF MATTER 15 Some substances, though the spaces between their molecules cannot be seen, include in their structure many larger spaces that can be seen. Such substances are usually made of many fibers or cells loosely put together, and the spaces between these small parts are sometimes called pores also. A sponge or a piece of blotting paper clearly shows this structure. A body of this sort is generally said to be porous. 22. Compressibility. Given a bundle of loose cot- ton, we know that it could be crowded into a much smaller bundle ; but in such a case its fibers would be much nearer together. In a similar manner the mol- ecules of some substances may be crowded nearer to- gether, the pores becoming smaller and the whole body losing some of its size. The property whereby a body may thus be crowded into a smaller space is called compressibility. As a general rule, solids and liquids are not very compressible ; great force is required to crowd their molecules nearer together. Gases, however, have great compressibility; their molecules under ordi- nary pressure are widely separated, and when great force is used they may be driven very much nearer together. 23. Elasticity Elasticity is that property by which a body goes back to its former size and shape after the force which changed it has been removed. It is very important to notice that a body is elastic not because it may be bent or stretched, but because it goes back to its former state as soon as the force ceases to act. Ivory is very elastic, as is glass; rubber is not so nearly 16 MATTER AND ENERGY perfect in elasticity. Gases are very elastic and liquids as well. Substances like clay, putty, and butter are said to be inelastic. Experiment 16. Fill a football or bicycle tire with air under pressure. Push upon its surface with the hand, at once removing the hand. Is any dent left in the surface ? Is air elastic ? Measure a coiled spring. Push it at both ends so as to shorten it ; then let go and again measure the spring. Is it any shorter V Is it elastic ? 24. Crystallization. Many substances have the prop- erty of forming themselves into crystals upon chang- ing from a liquid to a solid state. These crystals always have a definite shape, though in different substances the shapes may differ. Quartz crystals are commonly seen in rocks ; also garnets. Diamonds, rubies, emeralds, and other gems are crystals of rock. Sugar is the crystal- lized juice of cane or beet, snowflakes are crystals of water, and many salts are crystals. Experiment 17. Dissolve some alum or sugar in warm water until no more can be made FlG 7 to dissolve. Hang a string in the water (Fig. 7) and let it stand for a day. Examine it from time to time. State what you observe and try to explain it. Experiment 18 Melt some roll sulphur by heating and allow it to cool slowly. When hard, examine the mass and tell what you find. Compare the results of these last two experiments. Compare the methods used, PROPERTIES OF MATTER 17 Some substances do not crystallize; these are called amorphous substances. Crystalline bodies may be rec- ognized by their form, or sometimes by the shining sur- faces that they show when in a mass together. Butter, glass, wood, flour, paper, coal, cloth, and wax are exam- ples of amorphous substances. 25. Capillarity. The adhesive force with which some liquids are attracted to certain solid substances causes a useful and interesting action called capillary action or capillarity. Experiment 19. Put water into a clean glass tumbler and carefully note the surface of the water where it meets the Now put a clean glass tube of very small bore down into the water vertically. (An old thermometer tube, open at both ends, may be used.) Note the height of water in the tube. What force holds it in that position? Water has so great an adhesion for glass .that small amounts of it may be raised by means of this force. The smaller the tube, the higher that small amount of water will rise in it. The oil rises through a lamp wick by capillarity, the wick being a woven mass of tiny fibers. 26. Inertia. One property of matter which, though very passive, is of great importance is that of inertia. To state it briefly, inertia means the complete lack of any ability of matter to cause or to change motion. No body at rest can start itself moving ; some force must be used, and then the body is started gradually. Also no moving body can stop itself or in any way change 18 MATTER AND ENERGY its motion ; again some force must be applied in some way. This is one of the few properties that are common to all matter. It will be treated more fully in 56, 59. QUESTIONS 1. What is meant by the properties of matter? Name as many as you can. Are any properties common to all forms of matter ? 2. What is meant by impenetrability? Name an example of its effect. 3. Define cohesion. Do the molecules of liquids cohere? Do those of gases ? Explain how a blacksmith uses cohesion in welding iron. 4. Define adhesion. Name some substances that adhere to each other. Which have the greater adhesion generally, solids or liquids ? 5. Upon what does hardness depend? How may iron be hardened? How may a body be annealed? 6. Name some substances that are brittle. How would you test the brittleness of a body ? Are brittle substances ever hard? 7. What is meant by malleability? Would you expect a brittle substance to be also malleable? Name some bodies of matter that you think are malleable. 8. Name some substances that you know to be ductile. How do you know that they have this property? Is a malleable body necessarily ductile ? 9. How would you test the tenacity of a body ? 10. What is commonly meant by a porous body ? What sorts of bodies are porous ? 11. What class of substances is most compressible? Explain why bodies of matter may be thus compressed. 12. Define elasticity. Is air elastic ? Is water elastic ? Name some common uses of elasticity. PROPERTIES OF MATTER: GRAVITATION 19 13. Under what conditions may crystals be formed? Name some substances that crystallize. Name some substances that do not form crystals. What are such substances called ? 14. Explain the cause of capillary action. Name some impor- tant use that is made of capillarity. 15. Name the property shown by the substance in each of the following cases : a watch spring in unwinding ; a blotting paper in absorbing ink ; a stick when it is not easily broken ; a bit of steel that can scratch glass; the air forced into a bicycle tire,- vapor in the air when it forms into snowflakes ; a wire when it supports a heavy weight. SECTION IV PROPERTIES OF MATTER: GRAVITATION 27. Gravitation. We already know three facts : that bodies near the earth fall towards it if they are free to fall ; that all bodies on the earth are held down' by some means which we cannot see ; and that the earth, moon, and planets are held in place also .by some invisible means. It is clear that there must be some great force doing these things, and we call this force gravitation. 28. Gravity. Not only do these things occur, but scientists tell us that every body of matter has the power of attracting every other body not only solids but liquids and gases as well. The only reason that they do not succeed in drawing together is that the earth draws each body more strongly, thus holding each in place. The force exerted by the earth in attract- ing and holding bodies is just the same as the common force exerted by all matter, that is, gravitation; but 20 MATTER AND ENERGY for convenience it is called gravity when spoken of in connection with the earth. It is hard to realize the importance of gravity and the part that it plays in our daily lives. Without this force nothing .would stay on earth if it were once moved upward ; a ball thrown into the air would never return ; a locomotive would have no weight upon the rails ; and we could not even walk. 29. Law of Gravitation. Care must be taken to avoid thinking of gravitation as magnetic force. The two are very different; for while magnetism is shown in only a few substances, gravitation is a common prop- erty of all forms of matter equally. One form of mat- ter can exert as much of this force as another form, and the amount which any body can exert depends only upon the quantity of matter that it contains. It is because the earth contains so much matter that the force of gravity is so strong. When two bodies attract each other, the strength of the force depends upon two things the quantity of matter in them and the distance between their centers. The greater the amount of matter, the more they attract each other; the greater the distance, the less they attract. These two facts taken together make up the Laiv of Gravitation. 30. Weight. Keeping this law in mind, a little thought will show us that the force with which two bodies at the same place will be drawn towards the earth depends only upon the quantities of matter in them. That is, the more matter a body contains, the stronger PROPERTIES OF MATTER: GRAVITATION 21 will be the action of gravity upon it. Thus by measur- ing the force with which gravity pulls a body we can judge of the amount of matter in it. The weight of any body is the measure of the force with which gravity pulls it. This may be found by holding the body suspended by some A E known force. Fig. 8 shows a common spring balance ; anything hung upon the hook will be pulled downward until the stretched spring exerts as much force upon it as does gravity; there it will stop, and the pointer will show this force in pounds or ounces, etc. 31. Specific Gravity. Of two pieces of lead, the larger weighs more ; but a piece of lead may weigh more than a much larger piece of wood. That is, some forms of matter are naturally heavier than others. In order to compare the weights of different forms of matter, we must weigh equal amounts of volume (size) of the dif- ferent substances. To express these comparisons easily, the weight of each substance is referred to that of water as a standard. For example, a piece of iron is found to weigh seven times as much as an equal volume of water, a piece of lead eleven times as much, a piece of gold nineteen times as much, and so on. A list is then made, each substance being named and followed by the num- ber showing how many times the substance is heavier than water, and the number is called the specific gravity of the substance. FIG. 8 22 MATTER AND ENERGY Specific gravity is the weight of a substance compared with the weight of an equal volume of water. The specific gravity of water is 1 ; of cork, 0.2 ; of ice, 0.9 ; of iron, 7.2 ; of lead, 11.4 ; of mercury, 13.6 ; of gold, 19.4 ; of glass, 3.4 ; and of platinum, 21.2. QUESTIONS 1. Name some examples of the effect of gravitation. 2. What is meant by gravity? Is it any different from gravitation ? 3. Name some effects of gravity. If there were no such force, would bodies fall to the ground ? Why could we not walk if there were no such force ? 4. Upon what two things does the strength of gravitation depend ? Would the moon attract a body more or less than the earth, that is, would a body weigh more on the moon or on the earth? If we were on the moon, could we jump higher than we can on earth? 5. Define weight. Explain how weight gives us an idea of the amount of matter in a body. 6. What is specific gravity ? Does it depend upon the kind of matter or upon the size of the body? Would a lump of gold weigh more or less than the same volume of lead? CHAPTER II FLUID PRESSURE SECTION I PRESSURE IN LIQUIDS 32. Fluids. Liquids and gases are called fluids because they will flow. This property of liquids and gases is due to the fact that their molecules move freely among each other from place to place. The molecules of a solid, of course, vibrate, each in its position ( 11), but none of them can easily change its position among the others ; hence a solid body preserves its shape. The molecules of fluids, on the other hand, change their posi- tion so easily that the simple force of gravity is enough to move them, pulling each downward as far as it will go. Experiment 20. Pour a tumbler of water into a large flat dish ; it spreads out to the edges of the dish. Pour it upon a larger sur- face (a board or piece of glass) and note what happens. How may this be explained ? What force acts upon the liquid ? Is its action strongly resisted? Thus we learn why it is that liquids flow downward. Gravity acts in the same way upon the molecules of solid bodies also, pulling each of them downward; but in them the force which holds each molecule in its place among the others is greater than the force of gravity upon it, so that it does not move. In liquids and gases the molecules are free to move as gravity pulls them. 23 FLUID PRESSURE 33. Cause of Pressure in Liquids. If a liquid flow or be poured downward until it is stopped, gravity still acts upon it and causes it to push upon whatever stops it. Thus when the flow of a liquid is stopped by the bottom and sides of a dish, a pond, or even the ocean, the liquid exerts force upon those sides and the bottom. Each particle is still acted upon by gravity, and in an effort to go lower it exerts force upon the particles around it in all directions. For this reason water will spurt from a leaky hose equally in any direction. This principle may be stated as follows : At the same depth in a liquid, pressure is equal in all directions. Experiment 21. Cover the end of a lamp chimney with cardboard and push it into water in a glass dish (Fig. 9). Pour water into the chimney till it reaches the same level as the water outside. Add a bit more water and watch the result. Explain what you see. Experiment 22. Bend three tubes, as a, b, and c (Fig. 10), so that the ends may open upward, downward, and sidewise. Put equal quantities of mercury into each, so that it may stand at the same level in all. Now lower the tubes into water till FIG. 10 PRESSURE IN LIQUIDS 25 the openings are all at the same depth. The mercury is thus forced up into the long arm of each tube (which must reach above water) by the force of the water at the end. Compare the height of the three mercury columns. What does this show regarding liquid pressure at a given depth? 34. Pressure depends upon Depth. Now, since pres- sure in liquids is caused by the action of gravity upon their molecules, and gravity acts downward only, it is clear that the pressure upon any point will depend only upon the weight of the molecules above it. It is also clear that the weight will depend upon the number of particles above the point, .and that their number will in turn depend upon the depth of that point below the surface. From this we may state the general principle : In any liquid, pressure upon a point increases with its depth below the surface. Experiment 23. Push an empty can into water slowly, tak- ing care not to get it entirely below the surface. Do you notice any difference in the force that you have to exert as the can goes farther down? 35. Surface Level. Since gravity pulls all particles of a liquid as low as possible, and the particles are all free to move, no part of a liquid surface can be higher than another unless acted upon by some force that is stronger than gravity ; that is, the surface of a liquid at rest is always level. 36. Water Supply. If a vessel a (Fig. 11) be filled with water to a height cc' and then connected with an empty vessel b by a tube at the bottom, the water will flow out of a and rise in b until it stands at the same 26 FLUID PRESSURE level in both vessels. G-ravity makes the liquid flow from a and causes liquid pressure which is great enough to force it upward into b. In the same way these forces are used to give cities a supply of water. Pipes from a pond or " reservoir, located on high land, lead the water down into the city. Gravity of course causes it to flow down- ward, giving it pressure enough to fill the pipes. In these pipes the water may rise to the tops of buildings, provided they are not as high as- the surface in the reservoir (see Fig. 12). These pipes may be tapped at any points by faucets, hydrants, or fountains, out of which the water will run with some force. The force with which the water runs is called its head; the head of water at any point generally increases with the verti- cal distance from the point to the surface in the reservoir, FIG. 12 as If (Fig. 12). Some force is used up by the rubbing of the water on the pipes as it flows, so that its head is less as the distance away from the source increases. 37. Buoyancy. We have doubtless noticed that many bodies seem to weigh less when held in water. PRESSURE IN LIQUIDS 27 In lifting an anchor or a stone from under water, it seems to be heavier the moment it rises above the sur- face. It may be said, in general, that all bodies seem to weigh less when held in a liquid. This is not because the thing really does weigh less, but because it is then acted upon by some other force which acts in the opposite direction to the force of gravity. The force is exerted by the liquid body and is called buoyant force. Experiment 24. Hang several heavy bodies (e.g. a stone or a scrap of iron), one at a time, to a sensitive spring balance. Note the weight of each. Then, without removing it from the balance, lower each over water till it dips wholly below the sur- face, and again note its weight. Does it pull the pointer down more or less when in the water ? How do you account for this ? Does each body really change in weight or only seem to? Can you measure the buoyant force in each case? 38. Buoyant Force explained The molecules of any liquid at rest will, of course, be as low as gravity can pull them. If, now, any body be lowered into that liquid, some of the particles will be displaced (pushed out of their places) and the surface of the liquid will be raised by an amount just equal in volume to the size of the body which displaced it. These particles will, of course, be still pulled downward by gravity, and in tending to return to their places they will exert force upon the body. Since this force will cause greater pressure upon the bottom (see 34), the body will, of course, be pushed upward. It is clear that any body held entirely in a liquid will displace its own volume (size) of the liquid. And as the body is buoyed upward by the force that these displaced FLUID PRESSURE particles exert, it follows that the force tending to hold it up is equal to the weight of the displaced liquid. In other words, any body held in a liquid is buoyed up by a force equal to the weight of the liquid displaced. Experiment 25. Fill a vessel a (Fig. 13) with water, so that no more can be added. Weigh some heavy body in air and again while dipped in the water, as in Fig. 13. Note the loss of weight. a b FIG. 13 Arrange a vessel b so as to catch the water which spills from a as r is lowered into it. Weigh the water caught, comparing with the loss of weight just found. 39. Floating Bodies. The specific gravity (see 31) of many substances is less than that of water; such bodies float upon its surface. If any body floats upon a liquid surface, neither rising nor falling, it is clear that its weight is just balanced by the buoyant force acting upon it. But we have learned that buoyant force is always equal to the weight of liquid displaced. From these two facts we can easily form the Law of Floating Bodies : Floating bodies displace an amount of liquid equal to their own weight. PRESSURE IN LIQUIDS 29 Experiment 26. Using blocks of different sorts of wood, of cork, ice, Ivory soap, etc., float each upon water. In each case compare the amount above the surface with that below. Try to float iron, copper, lead, or rock upon mercury. Many heavy substances may be so shaped as to hold a great amount of air, and then they may float. Many vessels are now made of iron or steel ; they float because they contain so much space filled with air that the vessel as a whole is lighter than the same volume of water. 40. Specific Gravity of Liquids. Like solid sub- stances, liquids vary much in the kind of matter of which they are made, and therefore they differ in weight. Hence it is desirable to know the specific gravities of liquids as well, as solids. In this case, also, water is used as the stand- ard, the weight of the various liquid sub- stances being compared with that of an equal volume of water. To avoid weigh- ing the liquid a simple device is commonly used, called an hydrometer. An hydrome- ter is a hollow tube of glass weighted at one end and having a scale of specific gravities marked on its stem (Fig. 14). Upon being put into a liquid it sinks more or less, according as the substance is light or heavy, and the mark on the scale where the liquid surface rests will show the specific gravity of that substance. Experiment 27. Shake some oil and water together in a test tube. Let it stand some minutes; examine and explain. Put a drop of mercury into a glass of water. What happens ? FIG. 14 30 FLUID PRESSURE FIG. 15 41. Hydraulic Press. The use of liquid bodies to transmit pressure is common and important. The principle may be stated as follows : If a body of liquid be entirely closed up in any vessel, and any part of its surface be put under pressure from without, that pressure will be felt just as greatly upon every equal part of the inside surface of the closed vessel. For example, the inside surface of a bottle is fifty square inches and the lower surface of its stopper is one square inch ; if the bottle is full of water, the stopper fits tightly, and a force of two pounds pushes the stopper down upon the water, and every one of the fifty square inches of inside surface in the bottle feels a pressure of two pounds upon it. The pressure on the whole inside surface of the bottle is (50 x 2)= 100 pounds. Experiment 28. Get a shallow circular pan, make a small hole in its side, and solder into this a short metal tube (Fig. 15). To this tube at- tach a rub- ber tube two or more feet long. Tie a piece of sheet rubber very firmly over the top of the pan. Fill the whole with water, keeping the tube full. Raise the tube as high as possible. See how great a weight FIG. 16 PRESSURE IN LIQUIDS 31 of books you can raise up from the rim of the pan. To what is the pressure due? Fig. 16 shows how this principle is used in the hydraulic press. The piston p is small and the cylinder c is many times larger ; if p is pushed downward, water in the pipe transmits its pressure to the lower end of ~ SJ~ weights on the ruler, as in A Fig. 33. Try to find the Fl(J 33 e.g. of the whole. Where is it ? Compare the matter upon both sides of the point. Where is the center of mass? 62. Position of the Center of Gravity. A body acted upon by gravity behaves as if the force were applied at its e.g. alone. If gravity really did act only upon the e.g., that point would, of course, move toward the e.g. of the earth until stopped by some other force. And we find it to be true that any body on earth that is free to move takes such a position that its center of gravity shall be as low as possible. SOME EFFECTS OF NEWTON'S LAWS 53 Experiment 46. Try to balance an egg on its end. Explain the result (Fig. 34). Do the same with a weighted ball or disk (Fig. 35). Hang a ball by a thread, as in Fig. 36, and move it Fm. 34 FIG. 35 to a position a. Now where is the e.g. of the whole pendulum? Release the ball and note its behavior. When it comes to rest, where is its center of gravity? 63. The Problem of Support. When a body is fall- ing freely its e.g. moves in a straight line towards the center of the earth, nearly. \ This straight line is called the line of direction. Since gravity acts as if the \ force were applied along this line, a body \ will not fall so long as the straight line from its e.g. to that of the earth passes through its base, or point of support. c O Experiment 47. Find the e.g. of your ruler by balancing, and mark the point. Now place ((T) the ruler on a table, push it over the edge little FIG. 36 by little, and note the position of its e.g. just before it falls. The base of a body is the area inclosed by straight lines drawn from one to another of its outer points of 54 MOTION AND FORCE support taken in order. Fig. 37 shows eight points of support; the base is the area bounded by dotted FIG. 37 FIG. 38 lines. In Fig. 38 dotted lines show the base of a person standing. A pencil supported as at c (Fig. 39) would be said to be in a state of equilibrium; that is, the force of gravity acting on ac is just balanced by that acting on be. If Q b c were moved a little, GL^=^^^ aa ^ miSllxailsa ^ ailliiliis _ m -^^^\ ^^ these forces would no FlG - 39 longer balance, and the pencil would fall in the direction of the greater force. 64. Stability. A body which is less easily tipped over than another is said to be more stable. In general, the lower the center of gravity or the broader its base, the more stable a body will be. Experiment 48. Stand your pencil on its end ; then lay it on its side. In which position has it the broader base ? In which is it the more stable ? Experiment 49. Pile up three books and test the stability of the pile. Then add as many more as you can, and test that. Which pile is the more stable? Why? Try to balance your ruler, first on its side and then on its end. Which is easier, and why ? SOME EFFECTS OF NEWTON'S LAWS 55 In loading carts or in building different structures the heavier material is placed near the bottom, so as to make the e.g. as low as possible. Racing vessels balance their enormous spread of sails by a heavy mass of lead on the keel, which car- ries the e.g. far down (Fig. 40). FIG. 40 65. Centrifu- gal Force. - Since moving bodies tend to go in straight lines ( 55), it is clear that whenever a body moves in a curved path force must be constantly applied to pull it out of a straight line. Such a force is called centripetal because c ^~ s,. it acts toward the center of the x \ \ curve. But since every ac- \ tion has its reaction, centrip- \ etal force will be opposed by *h | another force tending to pull / the body away from the center; / this is called centrifugal force. FIG. 41 Experiment 50. Tie a string to a ball and swing it rapidly about the hand in a circle (Fig. 41). Do you have to use force to hold it ? Why ? Suddenly let the ball go free, and note its motion. What direction does it tend to take ? Try the same thing with a very short string and a very long one. Explain any difference. Note that the two forces exactly balance each other ; for while one acts toward and the other away from 56 MOTION AND FORCE the center A, the ball moves no nearer to and no farther from h than the length of the string allows. As soon as you let go, both forces cease to act and the ball obeys the first law of motion ( 55). Effects of centrifugal force are common. A pail of water may be whirled in a circle overhead, centrifugal force holding the water against the bottom of the pail so that none is spilled. The same force may cause a carriage or car to tip over in rounding a sharp curve. The wheels are held in place by the track or road, while the e.g., tending to go on in a straight line (Fig. 42), passes outside the base. In all cases, note that the force is greater if the body moves rapidly or the curve is sharp. Water would spill from the pail which was swung slowly, and freight trains take curves much more easily than expresses. FIG. 42 66. Falling Bodies. Bodies fall be- cause gravity pulls them. Now since the attraction of gravity depends upon the amount of matter contained in any body, it follows that the greater the mass, the more strongly gravity will pull it ; that is, a heavy body will be acted upon greatly and a lighter one less strongly. The result is that all bodies will fall equal distances in equal periods of time, when not hindered by any other force. Most bodies do fall equally fast ; but a few (such as feathers, leaves, and paper) have so large a surface, compared with their weight, that their falling is greatly hindered. In a vacuum a penny and a feather would fall exactly together (Fig. 43). SOME EFFECTS OF NEWTON'S LAWS 57 Experiment 51. Drop pieces of different substances (wood, stone, iron, lead, and others) from the same height exactly together, and note whether or not they strike together. Repeat several times, for accuracy. Compare with these the fall of a leaf or sheet of paper. Note and explain any differences. Falling bodies offer almost the only common example of motion which is not opposed by any considerable force ; for generally only the air hinders their progress, and its force is not great. Thus it is interesting to note this sort of motion carefully. It has been found that a body will fall about sixteen feet in one second. But at the end of that second its momentum alone is great enough to carry it about thirty- two feet in a second. The result is that in the second second the body will travel thirty- two feet because of its momentum (or inertia) and sixteen feet by force of gravity, making a total of forty-eight feet. So as it goes on it loses little or none of its momentum and con- stantly gathers more, as gravity keeps acting upon it ; so that the farther a body falls, the faster it goes. This is why a long fall generally does more damage than a short one. 67. Pendulum. A pendulum is a device so sup- ported that it is free to swing to and fro about a fixed point. Fig. 44 shows a pendulum, a being its point of support (on which it swings) and b the weight or bob. Lift b to the position c and let it go ; gravity acts upon it, pulling the bob downward toward e. At the position 58 MOTION AND FORCE e gravity ceases to pull b downward ; but the bob then has enough momentum so that it rises to d against the opposing force of gravity. At d the bob stops, gravity now pulls it to e, and it then moves on toward c. The path in which b swings (ced) is called the arc of the pendulum. A single complete sweep across this arc is called one vibration. As a pendulum swings to and fro, its arc constantly be- comes smaller, and in time the bob comes to rest at e. The air offers a slight resistance to the moving body, slowly bringing it to rest. Experiment 52. Make two pendulums of exactly equal lengths, by tying string to stones. Make them about two feet long, using stones of very unequal weights. Start them exactly together and compare the rates of their vibrations, that is, the number of swings made by each in a certain period of time. What effect has the weight of the bob upon the vibration rate of the pendulums ? Experiment 53. Swing a pendulum through a small arc and count its vibrations for 15 seconds. Now swing the same pendu- lum through an arc much greater, and count its vibrations for 15 seconds. What effect has the length of arc upon the rate of vibration ? (The length of the arc makes a slight difference in rate if one arc is much greater than the other, and none at all if both arcs are small less than 3.) Experiment 54 . Make a pendulum 9 inches long and another 36 inches long. Carefully count the vibrations of each for 15 seconds and compare results. Now make one 4 inches long and SOME EFFECTS OF NEWTON'S LAWS 59 another 16 inches long and compare their rates of vibration. How much longer is the second than the first ? Which vibrates the faster ? How much faster ? What one thing do you find to make a marked difference in the vibration rate of a pendulum ? Try to make a statement of the effect of length upon the rate of vibration. QUESTIONS 1. What is inertia? State examples. Why can you not start a bicycle at once at your greatest speed ? 2. What is momentum? Upon what two factors does the momentum of a body depend? How is it generally measured? 3. A rifle ball weighing half an ounce moves at the rate of one thousand feet a second, while a forty-pound cannon ball moves at the rate of one foot per second. Which has the greater momen- tum ? By which would you rather be struck ? Why ? 4. Why does a woodcutter sometimes fasten his ax in a stick and then invert it, striking the block with the stick uppermost ? 5. Why can you not stand an egg on its end? If there were a hole straight through the earth's center from surface to surface, how far into it would a falling body go ? 6. Under what conditions will a body be supported from falling? 7. Upon what does the stability of a body depend, and how ? Why is it hard to walk upon stilts ? Why spread your feet apart to receive a blow in boxing ? 8. Explain the cause of centrifugal force. State examples of it. Why do you lean in turning a corner ? Why is the inside rail of a track placed lower ? What conditions increase centrifugal force ? 9. How far will a body fall in one second? in two seconds? Why does a body constantly increase in its speed as it falls? Why is more damage done by a longer fall, as a rule? 10. Describe a pendulum. What force causes it to swing down- ward ? Why does it then swing upward ? If no force but gravity opposed its upward swing, how far would it go as compared with its downward swing ? 60 MOTION AND FORCE 11. Which has the faster vibration rate, a short or a long pendulum? If a clock loses time, would you make its pendulum longer or shorter in regulating it ? 12. Since a pendulum is made to vibrate by the force of grav- ity, would it swing faster or slower on a mountain top than in a valley? (See 29.) SECTION III WORK AND MACHINES 68. Work. Work is said to be done whenever a force causes motion. From this it is clear that work may be measured in terms of the amount of motion caused by a certain force. The amount of work done* by a force is commonly expressed in foot pounds. One foot pound is the amount of work done in raising one pound of matter a distance of one foot against gravity. The rate at which work may be done is sometimes called power. The ability of an engine, for example, to do work is expressed as so many "horse power. One horse power is the ability to do thirty-three thousand foot pounds of work a minute. 69. Machines. A machine is a device which helps man to do work. Note that a machine cannot of itself do work ; it cannot make energy. It can only help in applying force to good advantage ; and as every machine uses up some of the energy in its own motion, none gives us quite as much work as is done upon it. 70. Uses of Machines. In spite of this fact, however, there are several things gained by the use of machines, which more than make up for this loss in work. WORK AND MACHINES 61 1. They help man to apply force in a more convenient direction. The pulley (Fig. 45) and lever (Fig. 46) are common examples of this. A bit of thought will show how handy it may be, at times, to thus change the direction of motion. 2. We may use other forces than our own to run them. Steam engines, windmills, electric mo- tors, water wheels, and tread- mills all serve to call such forces to mind. 3. They help us to store energy to be used at another time. For example, the spring of a watch, in unwinding, does only the work which was done upon it in winding it up. We could not FIG. 45 easily exert force directly upon the wheels all day long. 4. By their use we may exchange strength of force for speed, and speed for strength of force. That is, we may 62 MOTION AND FORCE use great force slowly and cause a small body to move rapidly, or use small force rapidly and move a great I 1 weight slowly. A few ex- S amples of this will be given Weight Force FIG. 46 in 72 and 73. 71. Law of Machines. The fourth of these uses is one of much importance. It will be more easily under- stood when we have clearly in mind the following Law of Machines: The force and the resistance vary inversely as the distances through which each acts. This means that if a certain force causes motion against a resist- ance that is greater or smaller than itself, the distance through which the resistance acts must be just as many times smaller or greater than the distance through which the force is applied. 72. Pulleys and Levers. - A few simple machines will serve as examples of this law. Fig. 47 shows one movable pul- ley B attached to a weight W\ F is the point at which the force is applied. Notice that as F moves, W will move only half as far. From the law, this shows that the force need be only half as great as the weight. With two movable pulleys the force would act through four times the distance and lift a weight four times as heavy. FIG. 47 WORK AND MACHINES 63 A lever is any device having force applied at one point and resistance at another, the whole turning on a point called a fulcrum. A crowbar may be used as a lever (Fig. 48). a , As the force acts ^^\ from a 1 to 6', the \ weight moves only ]&' FIG. 48 from a to b ; hence a small force at a 1 will, in acting through a greater distance, a'b', move a greater weight at a the lesser distance, ah. Also a great force at b could move a lesser weight at b' with greater speed. Fig. 49 shows three classes of levers. In Class I the fulcrum/ is between the points where the force p and the resistance w are applied. In Class II / is at the end, w being ap- plied between it and p ; this lever gives us a gain in force at the expense of speed. In Class III the force p is applied beween the ful- crum and the resistance, so that we gain in speed at the expense of force. Experiment 55. Pulleys and levers are common, and many experiments may be made, according to the time and mate- rial available. A movable pulley is not hard to find ; but for a fl P w III FIG. 49 64 MOTION AND FORCE substitute smooth steel screw eyes may be used with small hard thread. Examples of levers are always at hand. In the following, name the class to which each belongs and state whether we gain in force or in speed by using it : scissors ; a common pump handle ; FIG. 51 pincers; sugar tongs; steelyards; nutcrackers; a crowbar when its fulcrum is on the ground beneath a weight ; ice tongs ; a tin- smith's shears ; a wheelbarrow ; a claw hammer (Fig. 50). 73. Other Simple Machines. The screw is generally used to gain intensity of force. As the force is applied to the circumference of a wheel b (Fig. 51), the sur- face on which the resistance acts will move ahead only the small width of one thread of the screw. Since the force is applied through a far greater space than the resistance, the gain in force is great. Fig. 52 shows a wheel and axle. The axle is much smaller than the wheel and turns with it. A small force F applied on the wheel may move a much greater weight E on the axle; but E moves proportionally slower than F, that is, a great force at E will move WORK AND MACHINES 65 a small weight at F with a gain in speed. In a windlass and a capstan this device is used to gain in force. G-ear wheels (Fig. 53) are used in a similar way. If a large wheel runs in a smaller, the gain is in speed ; but FIG. 52 FIG. 53 if force is applied to the smaller wheel, the larger turns more slowly but exerts greater force. Gear wheels are commonly used in machinery. The inclined plane is used for a gain of force at a loss of speed. A plank inclined from the ground to a wagon floor enables a man to get a heavy body into his cart. The more gradual the slant, the more he gains in force required. A wedge (Fig. 54) has two inclined faces. It also gains for us intensity of force at the expense of speed. FIG. 54 Experiment 56. A vise, copy press, thumb- screw, or bolt and wrench may serve to experiment with the screw. For a wheel and axle, any grooved wheel with its axle 66 MOTION AND FORCE fixed so as to turn with it may serve, or one can easily be made. An old clock will furnish gear wheels. An inclined plane can be made wherever convenient, and a thick knife blade will do for a wedge. QUESTIONS 1. Define work. How is work measured ? What is the unit of work ? 2. What is meant by power ? What is the unit of the rate of doing work ? How much is one foot pound ? one horse power ? 3. What is a machine? Can a machine do work of itself? Does a machine gain or lose work ? 4. What, in general, is the use of machines to man ? Name four special uses of them. Illustrate each. 5. State the Law of Machines. Show how a lever applies this law. Is a movable pulley used to gain force or speed ? 6. Why do tailors' shears have long blades and short handles, while plumbers' shears have short blades and long handles ? 7. Why does a bicycle of high gear run harder than one of low gear? 8. What is the advantage gained in using: a single pulley? a windmill? a coiled spring in a watch? the walking beam on an engine? a wheel and axle? 9. State the advantage given by a second-class lever and by a third-class lever. What can you say of the advantage in levers of the first class ? 10. Name as many examples of levers in use as you can. Name some familiar uses of the screw and the wedge. 11. Explain the use of gear wheels in machinery. CHAPTER IV, HEAT AND ENERGY SECTION I HEAT 74. Sources of Heat. The sun is a most important source of heat on earth, for without its rays the atmos- phere would be intensely cold and we should not have the supplies of wood, coal, and oil which are used as fuels. Other sources of heat are illustrated in the following experiments. Experiment 57. Using a convex lens, focus the sun's rays upon a piece of tissue paper for a moment (139). Note their effect on the paper. Name other examples of the heating effect of the sun's rays. Experiment 58. Friction. File a soft iron nail for a moment and then feel of the filed surface. Saw through a piece of wood and feel of the saw. Rub a metal button on a smooth piece of cloth. Name any examples of bodies heated by friction. The bearings of car wheels often become very hot. Why ? Experiment 59. Percussion. Hammer a small piece of lead for half a minute and feel of it. Repeat this, using a soft iron nail. Did you ever pick up a rifle bullet that had just been flattened by strik- ing an iron target ? Think of other cases of heating by percussion. Experiment 60. Compression. Pump air into a bicycle tire for a few moments and then feel of the pump. Can you discover evidence of heat being developed by compression ? Experiment 61. Chemical Action. Pour a little hydrochloric acid upon bits of zinc in a test tube. Very carefully and slowly 67 68 HEAT AND ENERGY pour a little sulphuric acid upon water in a test tube. In each case feel of the glass around the liquid. What do you discover about chemical action? Heat is very coAimonly caused by combustion or burn- ing. This is a sort of chemical action and is treated in 260. Electricity is also a common source of heat; its heating effects are shown in electric lights and are used in furnaces and heaters. Its action is explained in 192. 75. Theory of Heat. In studying the molecular theory ( 11) we learned that the molecules of all matter are thought to be in a state of constant vibrating motion. Naturally we may suppose that in some bodies the vibra- tion is more rapid than in others ; also that in the same body the motion may be greater or less at different times. The heat of any body is believed to vary with this vibration of its molecules, as stated in the theory of heat as follows : The heat of a body is the energy of vibration of its molecules ; the faster they move, the warmer is the body. With this theory in mind, the results obtained in Experiments 58 and 59 may be easily understood. Rubbing, in the one case, and pounding, in the other, simply caused the motion of the molecules to become more rapid, and the masses became warmer. The theory applies also in the other cases. Within certain limits we can discover differences in the heat of things about us ; we say that a body feels more or less " warm." It must be carefully noted, how- ever, that this is only the effect that heat produces upon HEAT 69 our sense. We must not judge of the nature of heat by this single effect, for it is only one of many different effects. It is important, in order to understand the further study of this chapter, that we fix firmly in mind the idea that heat is a form of energy the energy of molecular motion. 76. Cold. Cold means simply the absence of heat. Since heat is molecular energy, and the molecules of every mass are in motion, it follows that no body has absolutely no heat. Thus complete cold is unknown. We use the word cold to express a condition of less heat than some other substance has. 77. Temperature. Temperature is the condition of a body with regard to the intensity of its heat. If a body is warmer than another, we say that it has a higher temperature; if colder, we say its temperature is lower. Care must be taken to avoid calling temperature the " quantity of heat " of a body. A cupful of water might have a higher temperature than water in a kettle ; but at the same time the kettleful would have a greater quantity of heat, because there is so much more water. Temperature is the average heat of each particle, while quantity of heat is the average of each particle multi- plied by the number of particles. 78. The Thermometer. The thermometer is a device for measuring temperature. It does this by expressing, in degrees, how much warmer or colder a body is than some other substance taken as a standard. Two thermometers are in common use, the Centigrade and the Fahrenheit. 70 HEAT AND ENERGY The only difference is in the marking of the scale of degrees, as shown by the two side by side in Fig. 55. The Centigrade scale is largely used in scientific work. Its standard is freezing water and is marked zero (0). The temperature of boiling water is marked one hundred (100). The space between these marks on the scale is divided into 100 equal parts, each called a degree (). The Fahrenheit scale is most commonly used by us. Its zero is the temperature of a mixture of ice and salt, and the boiling point of water is 212. Water freezes at 32 above zero on this scale. Experiment 62. Carefully test these substances (freezing water, boiling water, and ice and salt) with both thermometers. Compare the tempera- tures of several substances on both scales, and try to discover a rule for changing one reading to the same temperature on the other scale. 79. How a Thermometer is made. A small tube of hairlike bore, having a bulb at one end, is partly filled with mercury. The air is removed, because its pressure would prevent the mercury from rising, and the tube is completely closed. Mercury will expand when heated (see 81) and shrink when cooled, so that as the temperature rises or falls, the mercury moves up or down the fine FIG. 55 tube. Assuming that its expansion is uni- form, we may compare temperature changes by compar- ing the distances that the mercury column rises or falls. EFFECTS OF HEAT 71 To mark the scale, the bulb is put into ice, and the point to which the mercury rises is marked ; the bulb is then put into steam or boiling water, and the point to which the mercury rises is marked 100. The space between is then divided into equal parts, and the marks may be continued above 100 and below 0. This gives a Centigrade scale. How would the marking of a Fahrenheit scale differ from this? QUESTIONS 1. State the theory of heat. Give examples which seem to show the truth of this. 2. What is the great source of heat upon earth? Can you show how the heat from coal once came from the sun ? 3. What is meant by cold? Is any body absolutely cold? If a body were entirely cold, what would be the condition of its molecules ? 4. Define temperature. Carefully explain the difference between temperature and quantity of heat. 5. For what is a thermometer used? Explain how the ther- mometer is made and how it acts. 6. What two thermometers are in common use ? Which one do we use daily? What is the standard in each? On which scale are the degrees the shorter? 7. Name and describe the more common sources of heat. SECTION II EFFECTS OF HEAT 80. The Effects named. In general, the effects of applying heat to bodies are four in number, chemical effects, electrical effects, changes of volume, arid changes of state. The first two of these effects will be treated in later chapters ; we shall now consider only changes of vol- ume and of state which are caused by the action of heat. 72 HEAT AND ENERGY 81. Changes of Volume. When, without more matter being added, a body grows larger, it is said to expand ; when, without losing any of its particles, a body grows smaller, it is said to contract. As a general rule, masses expand when they are heated arid contract when cooled. Experiment 63. Secure a hollow metal ball which exactly fits into a ring (Fig. 56). Heat the ball, and see if it can be forced through the ring. Heat both ring and ball and try them again ; they should fit. Cool the ball and heat the ring. How do they fit now? Carefully measure a long iron nail. Heat it thoroughly and measure again. Is it longer or shorter? Experiment 64. Fill a test tube with water and fit a stopper lightly into its mouth. Heat the water and note the re- sult. Explain this. (Do not crowd the stopper or heat the water too highly.) Fill a long narrow tube with hot water ; FIG 56 let it cool to an ordinary temperature and note any change in volume. Experiment 65 Arrange a tube so as to run through the stopper of a flask or bottle, into a vessel of water, as in Fig. 57. Heat the flask and explain what you observe. What is in the flask? What change does it undergo? Now remove the heat, watching the tube carefully. As the flask cools, what change takes place in its contents? Try to account for what you notice. 82. Uses of Expansion and Contraction. From these experiments we see that liquids and gases may expand and contract as well as solids. The value of this will be seen when we study convection ( 91). In the case of solids this principle is commonly used to good EFFECTS OF HEAT 73 advantage, for the force exerted by a body in expanding or contracting is very great. To make a wagon tire fit tightly, a blacksmith often puts it on after heating it; upon cooling, it contracts and fits the wheel closely. Similarly the parts of boilers, bridges, and other steel FIG. 57 structures are fastened with rivets which are put in while red-hot; these cool and contract, drawing the parts tightly together. 83. Exceptions to the Rule. A few substances do not obey the general rule for expansion and contrac- tion. Of these, water is a common example. We have seen ice floating upon water, which shows that it is lighter than the liquid ; but as ice is only frozen water, we know that it must have expanded upon cooling. 74 HEAT AND ENERGY Experiment 66. Fill two thin bottles with water and cork them tightly. Heat one and allow the other to freeze. The water may be left to freeze over night ; but do not try to heat the tightly closed bottle without help from your instructor. Note and draw conclusions from the results. Careful study has shown that water has its smallest volume at 4- Centigrade. If heated above or cooled below that point, it expands. 84. Changes of State. Early in our study we learned, that solids change to liquids and liquids to gases upon being heated ; also that gases become liquids and liquids change to solids when cooled ( 9). In some substances these changes occur at ordinary temperatures, and are common enough. In other substances, however, the changes would need such a high or low degree of heat that they are seldom or never accomplished. The change from a solid to a liquid is called melting, fusion, or liquefying; the change from a liquid to a solid is called solidifying. The temperature at which a solid substance liquefies is the same as that at which it solidi- fies from a liquid state. Vaporization is the change from a liquid to a gaseous state; the temperature at which a substance vaporizes is called its boiling point. In a large number of substances pressure upon them raises the temperature at which these changes of state occur. Thus water, which can usually be no hotter than 100 C., rises to a much higher degree in a locomotive boiler, where it is under pressure from the steam. With substances such as ice, which contract when they are melting, pressure lowers the melting point. Thus a block of ice receives the imprint of a dish which rests EFFECTS OF HEAT 75 upon it heavily, the ice beneath the dish melting faster than the other parts. 85. Evaporation We know that a moist cloth soon dries if hung in warm air; also that a thin layer of water in a dish or on some hard surface soon disappears. Clearly the liquid must have gone somewhere. But did it pass off in a liquid state? If so, we should probably have seen it go. We have to suppose, then, that it changes into a gas and passes off" into the air, and we say that it has evaporated. Evaporation may be denned as that sort of vaporization which goes on quietly at ordi- nary temperatures. Note that evaporation, not being produced necessarily by boiling, depends partly upon the ability of the atmos- phere to receive the vapor. Of course some substances vaporize more easily than others, but in general the conditions which aid evaporation are conditions in the air surrounding the liquid. Warm air can hold more vapor than cold; dry air can naturally take on more than that which is moist or humid; and evaporation goes on faster when the atmosphere is in motion. Thus the best conditions would be warm, dry, moving air. Experiment 67. Try these different conditions with small amounts of water. Also use such liquids as alcohol, ether, and naphtha. Blow upon them, and see if there is any faster evapora- tion. Why put damp clothes in a warm place to dry? Will clothes dry when frozen ? Do they dry better on windy days ? 86. Condensation. The amount of vapor which air can hold varies with its temperature; other things being equal, the warmer the atmosphere, the more vapor it can 76 HEAT AND ENERGY hold. When air at any temperature holds all that it can, it is said to be saturated. If now it be somewhat cooled, this air can no longer hold all the vapor that is in it, and some will change back to its liquid state. This change is called condensation. The condensed water vapor may then float about as tiny liquid drops; small masses of these drops may pass to another place and there evaporate again, like the cloud from a locomotive ; while large masses would form a fog or cloud. If the drops were large they would fall as rain. Experiment 68. Put ice or snow into a pitcher and take it into a warm room. Watch the outside of the pitcher, and explain. Breathe upon a cold piece of glass. Why does frost form on the inside of a window pane ? Why do we " see our breath " in cold weather ? 87. Distillation. Important use is made of these principles ( 84-86) in separating substances from each other or from impurities. Since different sorts of mat- ter vaporize at different temperatures, a mixture may be heated to the low boiling point of one substance without vaporizing the others ; the gas from this one may then be cooled, giving us the desired liquid or solid, free from the others. The process is called distillation. Experiment 69. A device for distillation may be arranged as in Fig. 58. Instead of the condenser c, a long tube of glass or metal may be run through a trough in which cold water is flowing. Fig. 58 shows the condenser as generally used in distilling. Muddy water may be boiled in a closed flask /; the steam runs through a tube t which carries it to the coiled tube e in the con- denser. Cold water running into c from a pipe p surrounds the coiled tube e and runs out at o. The steam in e is cooled and EFFECTS OF HEAT 77 FIG. 58 condensed by the cold water around it, running out at n as water. Since only steam passed from /to e, the water should come from the tube clear and pure. It is called , t distilled water. Distillation is used on ocean steamers to sup- ply water for the boilers; salt sea water would rust them badly, but distilling removes the salt. The pro- cess is also used in making alcohol and liquors, flavoring extracts, per- fumes, and many other compounds. 88. Latent Heat. --The temperature at which ice melts (32 F.) is the same as that at which water freezes. If a piece of ice is put into a vessel and slow heat applied, the ice changes to water; but so long as any ice remains, the temperature of the water remains the same as that of the ice. In other words, though much heat is applied to the mass its temperature does not rise. This fact was early discovered, and the name latent (i.e. hidden) was given to the heat which thus seemed to disappear. The same thing occurs when other substances are changed from solids to liquids, or from liquids to gases. It is also true that the heat thus taken into a body is 78 HEAT AND ENERGY given out again whenever the mass changes back to its solid state, or from a gas to a liquid. Farmers often protect their supplies by taking tubs of water into their cellars ; when it is cold enough to freeze the water, the heat given off by the freezing liquid keeps the cellar warm enough so that the vegetables do not freeze. Experiment 70. Into a glass dish put several pieces of ice ; carefully note the temperature of the ice. Now apply heat slowly, testing the temperature of the mixture from time to time as the ice melts. Just before the last bit melts, remove the source of heat, and when all is liquid test the temperature again. Was heat given to the mass in the dish ? How do you know this ? Did this heat raise the temperature of the whole ? What did it do QUESTIONS 1. What are the common effects of heat? Does heating always produce all of these effects in a body at the same time ? 2. Define expansion and contraction. How in general is the volume of a body affected by heat ? Do liquids and gases expand and contract like solids ? Name any uses of expansion and con- traction that you have seen. 3. Why does ice float ? Show how water freezing in a crevice may break a rock. What is the rule for expansion in water ? 4. Give the general rule for changes of state due to heat alone. What is meant by the words liquefying, solidifying, and vaporization? What effect has pressure upon changes of state? Compare the temperature of water boiling in a locomotive with that of water boiling in an open dish. 5. Define evaporation. What conditions assist evaporation? Why put your clothing in a warm place to dry? 6. When is air said to be saturated ? If it is then cooled, what happens? How are clouds formed? What is a cloud made up of ? Show how rain is formed in the cloud masses. 7. Explain distillation. To what important uses is it put ? TRANSFER OF HEAT 79 8. What is meant by latent heat ? What work is done by this heat? Since the temperature of ice is 32 F., and ice melts at 32 F., why does a block of ice in an ice house remain solid through the summer ? Why does not ice on a pond melt at once when the sun strikes it ? Why does a snowstorm often end in a storm of rain ? SECTION III TRANSFER OF HEAT 89. Methods of Transfer. We know that water standing in a room becomes the same in temperature as the air around it ; that if a warm body be placed near a colder one, it loses some of its heat, while the other becomes warmer; that the earth is warmed from the sun ; and that a room may be warmed from a stove or radiator, or a whole house even may be heated from a furnace in the cellar. These things show that heat must be able to travel from one place to another. Heat may be transferred (carried from place to place) in three different ways, by conduction, radiation, and convection. 90. Conduction. Conduction is the transfer of heat from one particle to another which touches it, without change of relative position of the particles. Heat may flow from place to place in the same mass, or from one body to another which touches it, by conduction. Each vibrating molecule is supposed to increase the energy of vibration of those which it touches; they in turn give greater energy to those that they touch ; and so on. But each molecule remains in its place ; though it may vibrate faster, its position among the others is not changed. 80 HEAT AND ENERGY Experiment 71. Put an iron rod or wire in a hot fire. After a few minutes, try its temperature at different points, beginning with the end that is farther from the fire. Let it remain and see if it grows hotter throughout. Substances which allow heat to pass through them easily in this way are called conductors of heat. In gen- A eral, solids and liquids are good con- ductors as compared with gases, which are very poor. Metals are usually very good conductors, while wood and cloth conduct heat but poorly. Stove lifters and pokers often have wooden handles, FIG. 59 for this reason ; felt also is used around steam and water pipes to keep the heat in. Experiment 72. Arrange four metal wires (e.g. iron, copper, brass, and German silver), as in Fig. 59. Apply heat at A, and note the order in which the other ends become hot. Compare the conducting power of the different metals. Experiment 73. Find the temperature of the air in the room, and of water which has been in the room a long time ; they should be the same. Now put your hand into the water. How does it feel ? "*?^ Which takes heat out of your hand faster, air or water? Which is the better conductor? Experiment 74. Boil the top of water in a test tube, as in Fig. 60. Note how long it is before the bot- tom becomes hot, and compare with a similar length of iron. Fl - 6 91. Convection. Convection is the transfer of heat from place to place by the change of position of heated particles. Since the, molecules of solids are not free to TRANSFER OF HEAT 81 move about, convection is limited to liquids and gases. The direction of movement in convection is upward and downward, warmer particles rising and cooler ones falling. As any portion of a fluid body becomes heated it expands, that is, its particles are farther apart ; thus the heated portion becomes lighter than the cooler parts of the fluid around it. Of course gravity will then pull the heavier parts downward, and the lighter heated portion will be forced upward. Experiment 75. Heat a can of water. Before it is entirely warmed through test its temperature at different depths. 92. Uses of Convection. Fluids, especially gases, are such poor conductors that they can only be heated very slowly by conduction. In fact, dry air is so poor a conductor that it would hardly carry heat at all ; we should have to live constantly in very cold air, if it were not for convection. The rise of heated air from a lamp chimney (Fig. 61), which may be easily noted, shows us how readily air may be set in motion ; and in just the same way the warm air above a stove or other heater rises and is spread about. Similarly the water in a kettle is quickly heated by convection ; the warmer parts, constantly rising to the top as they be- come heated, allow the colder portions to receive heat at the bottom. Fig. 62 shows how the rise of warm air above a fire keeps it supplied with a good draught of fresh air from below. FIG. 61 82 HEAT AND ENERGY Without convection, stoves and lamps would need to be blown from beneath all the time. Practically all winds are started by convection, heated air somewhere being set in motion by cold air pressing upon it. 93. Radiation. Ra- diation is the transfer of heat by vibrations of the ether. This statement may perhaps mean very little as it stands, be- cause it brings up an idea which may be new to most of us. The transfer of energy by as. radiation is so very im- portant, however, that a great effort should be made in trying to understand it. First of all, an example of the transfer of heat by radiation may be helpful. If a fire be kindled in a cold room, objects in the room may become warm some time before the air between them and the stove is equally heated ; clearly the heat is not conducted by the air to the objects, nor does it travel to them by convection. Similarly the sun's heat warms the earth ; yet we know that the space between the sun and the earth contains no matter that is dense enough to carry heat by conduc- tion or convection. FIG. TRANSFER OF HEAT 83 In order to explain how heat can be thus transferred, scientists assume that all space is filled with some medium which is very elastic, and they call it the ether. Through this very elastic medium energy may travel at an exceedingly great speed. The energy is supposed to be transferred by the vibrating of the ether. From its source, then, energy may travel through the ether in straight lines ; and this traveling energy is called radiant energy or radiation. It does not heat the ether through which it travels, but upon reaching certain bodies of mat- ter the radiant energy may stop and become changed into heat in those bodies, the degree of heat being greater or less according to the nature of the substance. We must understand, of course, that this is only sup- posed to be the method by which heat is radiated. The ether is not a substance that can be seen, felt, or weighed, and it does not conform to our usual ideas of matter. Still, scientists are so very sure that the ether really exists that we have come to accept it as a fact and to discuss its behavior without any doubts or hesi- tation. The source of heat radiation must of course be a heated mass, and in giving off the radiant energy it loses some of its own energy or heat. QUESTIONS 1. In what ways may heat be transferred? 2. Define conduction. Explain how heat is conducted through a body. What is a conductor? Are solids, liquids, and gases equally good conductors ? Why are wooden handles better than iron for stove lifters ? 3. Explain the cause of convection. How is it different from conduction ? 84 HEAT AND ENERGY 4. Name some uses of convection. In what sorts of matter is convection possible V Could a kettle of water be heated if placed beneath a fire? Show how convection is useful in stoves and lamps. 5 . How does radiation differ from conduction and convection ? Are all substances heated with equal ease by radiation ? Which is warmer in summer, a tar walk or a grass lawn side by side ? Why ? 6. Carefully explain radiant energy and the ether. SECTION IV ARTIFICIAL COLD 94. How Masses are cooled. Since cold means sim- ply absence of heat, it follows that cold cannot be put into a body, but the only way to cool any mass is to take away some of its heat. This is commonly done by put- ting the thing near some cold substance, when its heat will gradually flow into the other ( 89). In this way, things put into an ice box give up some of their heat to the ice ; thus the ice is slowly melted and the sub- stances become cool. We feel cold on a wintry day, because heat is rapidly taken from our bodies by the cold air about us ; and we wear clothing not to keep out the cold but to keep the heat in. 95. Artificial Cold. The common method of cooling does not give us very low temperatures, for no sub- stance is naturally colder than the lowest degrees which climate allows. Very low temperatures are obtained by performing some process which requires heat, near the body from which we wish the heat to be taken. The processes generally used are melting and vaporization. ARTIFICIAL COLD 85 96. Cold by Melting. The change from a solid to a liquid state requires heat. If it can be performed by some means other than directly applying heat, the sub- stance will take in the necessary heat from wherever it can be had. For example, salt causes ice to melt, but the melting ice must have heat in order to liquefy; thus, if the ice and salt be put into an ice-cream freezer, the heat will be taken from the cream, causing it to become solid. Experiment 76. Put a tablespoonful each of sal ammoniac and ammonium nitrate (solid salts) into a tumbler of water. At once stir the whole with a small test tube containing water (Fig. 63). The solid salts dissolve (becoming liquid) very fast. Does this process require heat? From what is this heat taken? With what result? 97. Cold by Vaporizing. In the same way, liquids in turning to gases take heat from substances around them. Experiment 77. Pour a small amount of alcohol on the hand, allowing it to evaporate. Does it feel cold ? Blow it, to make it vaporize faster. Does it feel colder ? Try naphtha, ether, or chloroform in the same way. Do bottles of these liquids seem cool to the touch ? The making of artificial ice depends upon this princi- ple. Liquid ammonia can be kept in its liquid state only under great pressure ; as soon as the pressure is removed, the ammonia vaporizes rapidly, requiring much heat. This is done near boxes of water, so that the heat is taken from the water, freezing it. 86 HEAT AND ENERGY Very low temperatures are reached by similar means ; even air and other gases have been liquefied when put under great pressure and cooled by other gases expand- ing around them. The temperature of liquefied air is about 191 C. below zero. It has been computed that absolute cold (i.e. a condition of no heat at all) would be reached at 273 C. below zero, or - 459.4 F. The low- est degree that has been reached is about 250 C. QUESTIONS 1. Can cold be put into a body? How may a substance be cooled ? In what way is this commonly done ? 2. Do any substances naturally have very low temperatures? Why? What must be done in order to get very low degrees? What two processes are commonly used? 3. Explain how cold is produced by melting. Show how this method is used in freezing cream. 4. State examples of cold produced by vaporization. How, in genera], is artificial ice made? 5. How are gases liquefied at low degrees? What is the condition of absolute cold ? At what degree would it be reached ? SECTION V ENERGY 98. Transformation of Energy. We have learned that energy is the ability to cause motion (5), and we know that this ability may be given from one body to another. For example, a coiled spring may lie at full length on a table with no ability to cause motion ; but press its coils together (Fig. 64) and it is then able to exert force to get back to its former length. Energy is put into the ENERGY 87 spring from the muscular energy of the arm in pushing. Not only may energy be transferred from one body to another, but one kind of energy may be changed to a dif- ferent kind either in the same body or in passing from one to another. Thus, muscular energy in the arm became elasticity in the spring. Experiment 78. Into a strong test tube put an inch or two of water. Find a stop- per which exactly fits, so that it may move up and down easily within the tube. Push it down upon the water lightly. Now heat the water slowly and with caution. As the heating continues, what do you notice ? What sort of energy is being used? What is its effect upon the water? Is energy imparted to the water ? What is the proof of this ? Experiment 79. Put a piece of zinc into a test tube with hydrochloric (muriatic) acid. Chemical action at once begins, the zinc acting upon the acid. Feel of the tube from time to time. Into what form of energy is the energy of chemical action being transformed ? Many other experiments and common happenings show that one form of energy may be changed into a different form in the same or in another body. This change of energy from one form to another is called transformation of energy. The following principle is generally believed by scientists : All forms of energy are so related that any kind may be transformed into any other kind. The study of these changes is an interest- ing and important part of physics. 99. Heat as a Source of Energy. The use of heat, as a source of mechanical motion has become very common. 88 HEAT AND ENERGY Various engines, run by steam, hot air, gas explosions, and naphtha, which are now widely used for many pur- poses, get their energy from heat. In all these engines the force which finally causes the motion is the expansive force of some gas ; but the energy which causes the gas to expand is supplied by heat. 100. The Steam Engine. It is of course well known that if a certain amount of a liquid be changed to a gas, the volume of the gas will be far greater than that of the liquid. But if this change is made in a closed vessel, / P FIG. 65 the gas will exert great force in trying to expand to its larger volume. A steam engine makes use of the force exerted by steam when thus trying to expand. Heat is applied to water in a boiler, changing it to steam. This steam is at once led to a cylinder where it is allowed to expand, first on one side and then on the other of a piston, p (Fig. 65). The figure (65) should be carefully studied until the action of the engine is plain. Steam comes from the boiler to the steam chest d through a pipe t. A valve v moves to and fro in d, allowing the steam to pass to the cylinder c, first to one end and then the other through ports a and b. The arrows show ENERGY 89 the direction of flow when a is open ; the piston p is forced toward , driving out the steam in that side through an opening e to the air outside. Follow the motion of p as it moves the rod r and turns the fly wheel f ; notice how this causes the valve v to move. When this valve has moved to the posi- Tri-p />/> tion shown in Fig. 66, steam goes through b to the cylinder, moving the piston the other way and driving out the used steam through a. 101. Other Heat Engines. The locomotive is a steam engine which moves itself on a track ; it carries a boiler and two engines (one on each side) all on one frame. The steam turbine contains a set of blades similar to a water wheel; these blades are fastened to a shaft and are made to turn around by jets of steam which strike them. Grasoline engines explode a mixture of gasoline and air; the energy of the explosion moves a piston which is joined to a fly wheel. Naphtha engines burn naphtha, using that heat to vaporize other naphtha in a coiled tube. This vapor is allowed to expand in cyl- inders, so that the action is somewhat like that of steam engines. QUESTIONS 1. Can energy be given from one body to another? If so given, would the body which gave it still contain as much as it had before ? 2. What is meant by transformation of energy ? From what source do we get our muscular energy ? Can this energy be used so that we feel its loss ? 90 HEAT AND ENERGY 3. State examples of transformations of energy. What form of energy is now commonly used in producing motion ? 4. State the principle upon which heat engines are based. 5. Describe the steam engine, explaining its manner of working. 6. Name other heat engines, briefly telling how they apply heat in causing motion. 7. What fuels are commonly used in these engines as sources of energy ? How were these fuels made ? Where did the neces- sary energy come from ? What, then, is the great original source of the energy now commonly used on earth to cause motion? CHAPTER V SOUND SECTION I EXPLANATION OF SOUND 102. Wave Motion. There are two sorts of motion, the movement of a body from one place to another, and motion from particle to particle through a body. The latter is called wave motion or a wave. The motion of any single particle in a wave is called vibration or vibra- tory motion. As a wave reaches any particle in a body, c d e FIG. 67 that particle is moved from its place, giving its motion to the next one and returning again to its position. Experiment 80. Fasten a coiled spring (Fig. 67) by both ends, a and b. Pick up the first few coils and crowd them together (see J) at b ; the part just ahead of these condensed coils is spread apart, as in c. Now let go the coils. The condensed part d travels quickly toward a, the separated coils going ahead (c) and the coils behind d being left as they were at the start, e. Each particle, as the wave goes along, first moves toward b, then toward a, and finally returns to its place, completing one vibration. The body through which a wave passes is called the medium of the wave. The vibrations producing a wave 91 92 SOUND may be of different sorts ; in the spring (Fig. 67) the vibration of each particle is parallel to the direction of the wave motion itself, while Fig. 68 shows each particle FIG. 68 vibrating (ab) at right angles to the direction of the wave, like ripples on water. A wave length is the dis- tance from one particle to the next one which is in the same state of vibration, as bd (Fig. 69). The rate of vibration is the number of vibrations which pass a given point in one second. 103. Definition of Sound. We are familiar with wave motion in water ; any disturbance the wind, a pebble thrown into it, a moving boat or animal is enough to cause ripples, even if slight, so that a body of water is rarely free from waves. In just the same way the air is constantly vibrating. Any slight disturbance sets up wave motion in the elastic atmosphere, and because there are so many more disturbances in air than in water, there are also many more sorts of waves all the time. Of course we cannot see these waves, and we can b d FIG. 69 feel only the greater ones. How then can they be dis covered? Nature has given us an ear for that purpose ; we hear these waves in the air, and we call the sensation EXPLANATION OF SOUND 93 sound. In other words, sound is the sensation made by waves in the air striking the ear. In order to get used to this idea, let us study it a bit further. 104. Sound Waves. Once more let us consider how small a motion in still water will cause ripples to spread far away over its surface. Now, recalling the fact that air is perfectly elastic, it should not be hard to see that waves may likewise be caused in the atmosphere by the many motions which are always disturbing it ; and, as in water these waves may be large or small, so in the air there are long ones and short ones, according to the motion which caused them. Not all of these waves can affect the ear to produce sound, some ^ being too long and some too short. Those a <~ waves which can produce sound in the ear ' I are called sound waves. \ 105. The Cause of Sound Waves. A tuning fork (Fig. 70) may be used in show- ing how sound waves are started, for its vibrations can be easily seen. Experiment 81. Strike a tuning fork sharply on a desk and at once look for any vibrating (buzzing) of its prongs. Again strike the fork ; then hold its prongs downward so that they lightly dip into water. Do you see anything to show that they vibrate? Let us see how this motion causes sound waves in the air. The prong / (Fig. 70), in vibrating, moves rapidly to and fro between a and b. As it moves toward a the air in front of it is condensed, but is quickly rarefied as the prong flies toward b. Thus the prong, FIG. 70 94 SOUND rapidly moving to and fro, causes many condensations and rarefactions to follow each other away from the prong. These pulses of air are sound waves. In Fig. 71 the condensations (A) and rarefactions (B) are supposed to be moving away from a vibrating bell. 106. Vibrating Bodies. It will perhaps be hard to grasp at once the idea of sound as merely a sensation, FIG. 71 which reports to the brain the vibrations in matter about us. One difficulty is that whereas there is no end to the sounds we hear, it is not often we can discover any vibration in the body which caused a sound. Tuning forks, violin strings, piano wires, bells, and a few others show vibration plainly, but in many cases there is none to be seen. Sometimes these vibrations are EXPLANATION OF SOUND 95 largely caused in the air itself, as when a gun is fired or we clap our hands ; often the motion is so great that we lose sight of the little vibrations, as when a load of rocks is dumped. In some cases the motion may be felt even if we cannot see it. Experiment 82. Hold a pan in the hand and strike it. Can the vibration be felt ? Strike several sharp blows on an iron bar held in the hand. Do you feel a tingling sensation ? Blow a horn and touch it lightly with the finger. Sometimes the sound waves from a distant blast or a heavy cannon will shake the windows which they strike. At least we must understand that sound occurs only in the ear. However a vibrating body may arouse sound waves in the air, it is still nothing but a vibrating body. There is no noise in a gun which is fired, and no sound in a piano nothing but motion. To be sure, we are made aware of the motion because of the sound that it produces in the ear ; but the source of this sensation is still only a body in vibrating motion. QUESTIONS 1. What two sorts of motion are there ? What is a wave? 2. What is a vibration? Explain how each particle moves during one complete vibration. 3. Define a medium. Define wave length and rate of vibration. 4. Define sound. Of what use is this sensation to us? 5. What are sound waves ? What is the first cause of waves in the air? Are all of these waves alike? Are they all sound waves ? 6. Show how a vibrating fork may cause waves in the atmos- phere. What property of air makes it a good medium to carry sound waves? 96 SOUND 7. Name some bodies whose vibrations can be seen. Name some whose vibrations can be felt. Why can we not see the vibrations in all masses ? 8. Where is sound located ? Is there sound in a vibrating body or in the air ? SECTION II TRANSMISSION OF SOUND WAVES 107. Different Media Sound waves may travel through other substances than air, though they usually pass through the air a short distance anyway, before reaching the ear. In general, solids carry sound waves FIG. 72 better than liquids, and liquids better than gases. That sounds are sharper under water is known to every boy who swims, and we know that sound waves come through the iron rails of a track much faster than through the air. Experiment 83. Listen at one end of a log or an iron rail while some one scratches the other end with a pin. Do the same with several solids. TRANSMISSION OF SOUND WAVES 97 Experiment 84. Punch holes in the bottom of two clean tin cans, and to each tie one end of a stout string about one hundred feet long. Each taking a can, let two pupils separate until the string is pulled tight (Fig. 72). Can you talk in lower tones through the can than through the air ? What passes along the string ? 108. Speed of Sound Waves. We have, perhaps, seen a man strike a blow at a distance and waited some time before hearing the sound. This is because time is needed for the sound waves to travel through the air. Just as the ripples can be seen to move away from the spot where a pebble is dropped in still water, so the sound waves in air move away from a vibrating body at a speed which can be measured. This speed is a little greater in a warm than in a cold atmosphere. Through air at ordinary temperatures sound waves travel about 1125 feet per second. A mile would be covered in about five seconds. Experiment 85. Stand at some known distance (e.g. half a mile or more) from a steam whistle which is soon to be blown. Note the time when the " steam " appears (using a stop watch if possible), and see how many seconds pass before you hear the sound. Reduce the result to feet per second and compare with the rule. Note other similar things whistles on distant trains or boats, guns fired, blows struck, or engines puffing. Thunder is caused by lightning and both occur at the same instant. Could you tell how distant is the lightning by hearing its thunder ? How would you do this ? 109. Reflection ; Echoes. Roll a ball against a board ; it bounds off at once. The ball is said to be reflected, and the angle at which it leaves the board is the same as c 98 SOUND that at which it struck. Now roll it so as to strike exactly at right angles (as cd in Fig. 73 strikes ab), and the ball will be reflected so as to come straight back to your hand. In just the same way sound waves are reflected from any building, hill, rock, or bank of woods which they strike. Usually these reflected & waves pass off in a different di- rection; but when they strike squarely at right angles, they come back to their starting point and there produce a faint sound. This sound is called an echo. Because the waves have lost some energy in traveling, the echo is generally weak; if the air is too full of other sound waves, these weak echoes may not be heard. 110. Reverberation. In a large empty hall sound waves may be reflected from wall to wall in many directions at the same time. The effect of this confu- sion of waves upon the ear is not a distinct echo, but rather a roar. Such an effect is called reverberation. It may be noticed in caves, wells, and other inclosed empty spaces. 111. Forced and Sympathetic Vibrations. When a vibrating body touches another, its motion may start vibration in that other. In some cases this may be done also ly sound waves, the waves in the air having enough energy to arouse vibration in certain bodies that they strike. This sort of vibration is of two kinds, forced TRANSMISSION OF SOUND WAVES FIG. 74 and sympathetic. To understand the difference, it must be known that every body lias its own natural rate of vibration, at which it vibrates when free to do so. When the motion of one body causes another to vi- brate at a rate which is not its own, these vibrations are said to be forced. When the natu- ral rate of the second is the same as that at which the first is vibrating, its vibration is said to be sympathetic. Experiment 86. Cause a tuning fork to vibrate. Can you easily hear its tone ? Now strike it, and at once hold it to a table, as in Fig. 74. Is the sound any louder? The table is forced to vibrate by the motion of the fork. Experiment 87. Put water in a tall jar and hold a vibrating fork over it, as in Fig. 75. Vary the amount of water till the sound is the loudest. What body adds its vibrations to those of the fork ? If the air column were in forced vibration, would its length make the difference that you now find ? Is its vibra- tion forced or sympathetic ? 112. Resonance. In these experiments the sound seemed louder. The vibrations of the larger body were added to those of the fork, increasing the energy of the sound waves. In such cases the waves are said to be re'en- forced. The ability of a body to reenforce sound waves is called resonance, and the body itself is a resonator. Bodies of this sort are very useful and are employed particularly in musical instruments. . FIG. 75 100 SOUND 113. Resonators. Thin boards, metal tubes, and columns of air are very commonly used as resonators. Fig. 76 shows how organ pipes make use of air columns as resonators. Air ent'ering an opening at the bottom strikes a reed at r, making it vibrate ; this causes air in the pipe to vibrate, giving a loud tone. In the same way all horns, cornets, and other wind instruments are only tubes or pipes full of air; vibration is caused by the lips and a mouthpiece, but most of the sound waves come from the tube and the air PIG. 76 w ithin, which act as resonators. A violin, guitar, or mandolin would be useless without the thin wood body and the air it incloses, both of which reenforce the vibrations of the strings. Pianos have large sound- ing boards of thin wood which serve as resonators. FIG. 77 Megaphones (Fig. 77) or speaking trumpets partly reflect the sound waves which would otherwise escape sidewise, and partly serve as resonators to increase the energy of the waves. By their use sounds may be heard at much greater distances from their sources than usual ; sailors, firemen, and others find them very necessary. DIFFERENT KINDS OF SOUNDS /If QUESTIONS 1. Compare solids, liquids, and gases as media for trans- mitting sound waves. State any common examples which you have noticed, showing how some substances are better media than others. 2. How fast does a wave travel through air ? How far would a wave go in one minute ? 3. Explain the conditions necessary to produce an echo. Why is an echo generally heard better at night? Why better on water? 4. What is a reverberation ? Why do we not hear much rever- beration in a hall full of people ? 5. How are forced and sympathetic vibrations caused? Explain the difference between them. 6. What is a resonator? How do resonators serve to make sounds seem louder? 7. Name different bodies which may act as resonators. Show the value of resonators in musical instruments, naming several instruments and the sort of resonator they use. 8. Explain the action of a megaphone. For what is it used ? SECTION III DIFFERENT KINDS OF SOUNDS 114. Tones. It must be kept in mind that the vibra- tions which cause sound waves are not only very small but usually very rapid in some cases there are several thousand every second. Each of these vibrations causes one wave in the air, which moves away so fast as to be 1125 feet distant at the end of one second. But as the body keeps vibrating, and as each to-and-fro motion causes one wave, it is clear that at the end of one second the air from the body to a point 1125 feet away 102 SOUND in every direction will be full of sound waves (see Fig. 71). Others will follow these so long as the body vibrates. Now when the vibration of the body is simple, every sound wave will be just like every other in length and form. The effect of these regular waves upon the ear is a pleasing sound called a tone. A tone may then be denned as the effect upon the ear of a regular succession of like waves. 115. Noises. Almost no body does really vibrate in this regular manner, however. While a mass may move as a whole, many of its parts vibrate at a rate of their own. Thus, though each single part causes vibrations that are regular, many different sorts of sound waves may be caused by the different vibrating parts at the same time some long and some short. The effect of these many different kinds of waves striking the ear together is a sound which we may call a noise. Note that the difference between tones and noises is not great. Almost no tones are strictly pure, but are mixed with a few weak waves that are not great enough to affect the sound seriously. A noise may be considered as simply a mixture of many tones. 116. Differences in Tones. Tones may differ from each other in three ways in loudness, pitch, and quality. Since noises are merely mixtures of many tones, the same fact is true of all sounds. 117. Loudness. The loudness of a sound means the greatness of the sensation. This may depend upon sev- eral things. The greatness of the original vibration may DIFFERENT KINDS OF SOUNDS 103 affect loudness, and this may depend upon the size of the vibrating body or the energy with which it moves. If the ear is at a greater distance from the source of the waves, the sound is less loud ; the kind of medium through which the waves travel and the direction of the wind both have an effect upon the loudness of sounds. The size of the receiver is also an important factor; speaking tubes and ear trumpets (Fig. 78) serve to make sounds louder by collecting many waves. __ < b On a similar principle is the holding of the hand to the ear, as the aged often do. 118. Pitch. The pitch of a tone is commonly described by the words high or low, shrill or deep. We say that the pitch of a woman's voice is higher than a man's, or that a certain bell has a lower tone than another ; and we know that whatever the pitch of a tone, it is quite different from loudness. We have learned that sound waves vary greatly in length, some being long and others short ; the vibrations of a simple pure tone, however, all have the same wave length. Now since all sound waves travel through the air at the same speed, the shorter the wave length, the more vibrations will pass a given point in a second. The pitch of a tone depends upon how many vibrations reach the ear in a second; the greater the number, the higher the pitch. Of course the length of a wave depends upon the vibrations in the body that caused it ; so we may get some idea of the 104 SOUND source of a sound by noting its pitch. The vibrations of a small body are generally more rapid than in a larger body of the same sort, and the sound produced has a higher pitch. 119. Limiting Pitch. Some waves are too long and some too short to affect the ear ( 104). The limits within which waves may cause sound vary in different persons. Few can hear sounds lower than twenty vibrations per second, or higher than thirty thousand per second. In music, middle C (C natural) has 264 vibrations per second; the octave above (high C) has twice as many (528), and the octave below has one half the number (132). A man's voice can rarely make a tone lower than 150 waves per second; while children may, in screaming, reach a pitch of several thousand vibrations. 120. Quality. The first two features of sounds, pitch and loudness, are common in our experience and not hard to understand ; the third, quality, may need a bit of thought. A piano and a violin may sound the same tone ; it has the same pitch in each case, and may be sounded with equal loudness, yet we should have no trouble at all in telling the sound of a violin from that of a piano. The same would be true of tones made by a cornet, banjo, harmonica, harp, flute, or other instrument. Clearly there is some feature of tones, other than loud- ness or pitch, which seems to depend upon the instru- ment that produces them ; this feature is called quality. 121. Quality explained. Very few sounds are pure tones ( 114, 115); even those which are pleasing enough to be called musical contain a few weaker tones DIFFERENT KINDS OF SOUNDS 105 besides the chief or fundamental tone. These weaker ones are called overtones; their effect is not great enough to make the sound unpleasant or to alter its pitch, but still their presence in the sound can be noticed by the ear. It is the effect of these overtones which gives to a sound its quality. And since the overtones may differ in different instruments, it is plain that one tone may be like another in pitch and loudness and still have a different quality. 122. Musical Sounds. The sounds in music are usu- ally tones that are nearly pure. They are made by dif- ferent bodies in a state of nearly simple vibration; strings, air columns, metal plates and tubes, sheets of wood and skins, wires, and other devices are used. Sometimes the tones are made singly and often in groups, several being sounded at once. In such cases the tones are generally of such pitch that their waves cause a regular movement upon the 'ear, making a pleasing sound called a chord. A careless ar- rangement of tones may pro- duce a jarring sound called a discord. A succession of chords is called harmony. 123. The Voice. The voice is caused by the vibration of the vocal cords. These are narrow strips or folds of membrane (aa) on either side of an opening, b (Fig. 79), leading to the lungs. Air passing through 6 causes the cords to vibrate. FIG. 79 106 SOUND The pitch of the voice is raised or lowered by drawing the cords more or less tightly ; loudness depends upon the energy with which the air is driven out, and quality upon the shape and movements of the throat and mouth. /Speech is made by movements of the lips, tongue, palate, teeth, and other parts of the air passages. We form words mainly by varying the quality of the voice ; but among some nations, like the Chinese, pitch also is of importance in talking. QUESTIONS 1. Define a tone. Explain the difference between a pure tone and a mixed noise. 2. In what ways may tones differ from each other ? 3. What is meant by loudness? Upon what different con- ditions may it depend? Show how ear trumpets help to make sounds louder. 4. Name some familiar sounds that have a high pitch, and some having a low pitch. Upon what doe's pitch depend? How? Do short waves or long waves have the higher rate of vibration? 5. What is the lowest, and what the highest, number of vibra- tions per second that the ear generally can hear? How many vibrations per second has middle C? 6. Give an example of two tones differing only in quality. What determines the quality of a tone or sound? What is an overtone ? 7. In music, what is a chord ? 8. Explain how the voice is produced. How is the pitch of the voice varied ? How do we vary its loudness and quality ? 9. How is speech effected ? Do dumb persons have a voice ? Why can they not talk? 10. Name some different musical instruments. Classify each as a stringed instrument or a wind instrument, etc. DIFFERENT KINDS OF SOUNDS 107 11. In stringed instruments what is the original vibrating body ? Does this have to be reenf orced in any way ? Explain the use of the head of a banjo, the sounding board of a piano, or the body of a guitar. Why does a piano give louder sounds than a harp? 12. What effect upon the pitch of a tone do you produce by tightening the string that caused it ? How does the length of a string affect its tone? Does a heavy or a light string generally give the deeper tone ? 13. How does a violin player vary the pitch of his tones as he plays ? How is a piano tuned ? 14. What usually causes the vibrations in a wind instrument ? How are the tones usually made louder ? What is the use of so much tubing in a horn ? CHAPTER VI LIGHT SECTION I NATURE OF LIGHT 124. What is Light ? We already know a few things about light, that it may come from objects which are hot, that it travels through air and also through some solids and liquids, that it may travel very long dis- tances, that it affects the eye so as to produce sight, and other facts. To the questions, What is light? and How does it travel ? we can give only a partial answer. 125. Light Waves. In the study of heat we learned that the sun and other heated bodies give off radiations, which may travel to a distance and there cause heat in some substances but not in some others. We also know that the sun may shine upon different bodies equally, making some appear light and others dark. And again, we have noticed that the sun may affect our skin to color it in summer, may cause cloth to fade and paper to become yellow, and may bring about other changes that are chemical in their nature. So we see that the sun's radiations may produce three different effects, heat, light, and chemical changes. Scientists think that these radiations (or rays) may be all of the same sort, differing only in wave length. 108 NATURE OF LIGHT 109 Different effects produced by them vary with their wave length and also with the substance upon which the rays fall. For convenience, however, those which produce heat in a body are called heat radiations ; those which cause chemical changes, actinic rays; and those which affect the eye to produce sight are called light waves. 126. Luminous and Illuminated Bodies. All objects are seen by means of the light waves that pass from them to the eye. These light waves may have their origin (starting point) in the body itself, or they may fall upon it from some outside source and then be directed to the eye from that object. Bodies which give out light waves from themselves are called luminous ; those which give off only waves which have fallen upon them from some other source are said to be illuminated. The sun, lamp flames, glowing coals, the electric arc, and very hot iron are examples of luminous bodies they are sources of light waves. Such bodies may be seen when no other source of light is present ; whereas illuminated objects chairs, tables, books, flowers, clothing, the earth, plants, animals, the moon, and many others disappear from sight as soon as all sources of light waves are taken away. 127. Rays. Light waves start from a luminous point, as o (Fig. 80), md extend in all directions in straight lines. The straight line marking the direction of any one wave is called a ray, oa (Fig. 80). Note that no wave ever goes in a curved path, FIG. 110 LIGHT so long as the medium is constant ; whenever the direc- tion of a ray is changed, it is sharply broken at a point and passes on in a straight line until again changed. 128. The Ether. Light waves travel long distances, as from the sun and far more distant stars, through space which we know to be rarer than any vacuum that man can make. Clearly no air is needed to carry these waves. Yet we must suppose that some medium is neces- sary, even though it may be very rare; therefore we speak of this medium just as if it were known to exist, and call it the ether ( 93). The ether is supposed to fill all space, even entering the pores of solid matter. Light waves are then assumed to be vibrations of the ether, as sound waves are vibrations of the air. 129. Speed of Light Waves. Through space, light waves travel about 186,000 miles per second. This speed is so great that for all distances through which we can see on earth, the waves travel instantly. A ray of light would pass entirely around the earth seven times in one second; and rays from the sun, 93,000,000 miles away, reach the earth in a little over eight minutes. As with sound waves, the speed of light waves varies in different media. In general, rays travel faster through a rare than through a dense medium. 130. The Passage of Light Waves Some substances allow light waves to pass freely through them; glass, air, and water are examples. We can see through them clearly, and they are said to be transparent. Other substances allow light waves to pass through, but scatter them in different directions ; ground glass NATURE OF LIGHT 111 and oiled paper are examples. Such bodies are called translucent ; they let light through easily, but we can- not see objects through them. Opaque bodies are those through which light waves will not pass at all. Wood, granite, iron, and brick are opaque. Since rays will not pass x through an opaque substance, it is clear that those which fall upon it must either be taken into the body and stopped there, or be turned off from its surface. Waves taken in by a body are said to be absorbed; when they are turned off from its surface, they are said to be reflected ( 131. Shadows. When an opaque body is placed so as to stop the waves that stream from a luminous source, FIG. 81 it is said to cast a shadow. In other words, a shadow is a space from which light waves are excluded by some opaque mass. Whenever the luminous body is of suffi- cient size, there will be a lighter edge about a shadow, 112 LIGHT in which the rays from some part of this luminous body may fall, as the portions ace 1 and bdd' in Fig. 81. A dark central portion, as ac'd'b in Fig. 81, receives no rays at all ; this is called the umbra of the shadow. The lighter portion, which receives some (but not all) of the rays from fe, is called the penumbra. Darkness, on earth, is always due to shadows. Even the darkness of night is caused by our passing into the shadow of the earth. In daytime we do not find it dark within shadows of trees, buildings, or other objects ; this is because sunlight is reflected into these spaces from air particles and other bodies all around them. 132. Reflection. When a light wave strikes any sur- face and is turned off from it, the wave is said to be reflected. This is important. It is due to reflection that objects can be illuminated ; that is, objects that are 6 , not luminous can be " seen by means of the waves which fall upon them and are reflected to the eye. Experiment 88. Stand before a mirror, as at a (Fig. 82), hold- ing a lighted candle in front of y.ou. Rays from the candle strike the mirror at right angles, at &, and come straight back to you. Now move the flame toy. Find a point e from which your eye sees the reflection of the flame at b. Draw lines fb and eb ; measure the angles dbf and cbe. How do they compare ? Take a new pointy and repeat. FIG. 82 NATURE OF LIGHT 113 From this experiment we may learn the Law of Reflection: The angle at which rays leave a surface is equal to that at which they strike it. Experiment 89. Hold a small mirror before you, just below the eyes, about ten inches away, with its glass side facing away from you. In the other hand hold another mirror a few inches farther away, an inch or two higher than the first, and facing you. Look over the first into the second ; with a little care these may be so placed that you will see several reflections of your eyes. This shows that reflected waves may be again reflected many times. 133. Reflection from Different Surfaces. Light waves from the same source may fall upon different objects and there be so treated ^ that the objects will present a variety of appearances to the eye. For example, rays from the sun may FlG - fall upon several bodies : one of these may appear to be green, another red, a third may seem dark, and still another very bright. Yet they are all seen by means of rays which come from one source. These differences are due to the different behavior of substances toward light waves ; many give off only a part of the waves that fall upon them, and their appearance to the eye depends upon what waves they give off. But among surfaces which reflect nearly all of the rays that strike them, there is still a difference ; some reflect regularly and others reflect only scattered light waves. When parallel rays from an object cd (Fig. 83) strike 114 LIGHT a smooth surface ab, such as glass, still water, polished metals, etc., each ray is reflected at the same angle as all the others, and thus their positions among each other are unchanged. These reflected rays would form an image of the object, as ef. Fig. 84 may serve to show how such rough surfaces as snow, plaster walls, white cloth, etc., may reflect light waves which do not make images. Rays from the ob- ject c'd' strike the rough surface a'b f ; each ray is of course reflected from the point where it falls, so as to obey the law of reflection ; but as these points lie in short lines of many directions, the rays will leave the surface a f b r in various directions, forming no image. 134. Mirrors and Reflectors Mirrors are smooth surfaces which reflect nearly all of the light waves which fall upon them. Plane mirrors generally form images which are erect and like the object. Standing before a mirror, our image seems to be twice as far away as the distance to the mirror; in other words, the image appears just the same size as the object would appear if it were twice that distance away. Rays striking a curved surface are reflected as if they struck a plane which touched the curve at that point only. Thus, in Fig. 85, parallel rays moving as shown by arrows are turned off from the curved surface just as if they had struck the straight lines back of the NATURE OF LIGHT 115 curve. All rays parallel to these three would be simi- larly reflected to the point/; this is called the principal focus of the mirror. Experiment 90. Using a concave (in-curving) mirror, reflect the sun's rays to a focus. Hold a narrow strip of paper in front of the center of the mirror, and try to find this focus by moving the paper back and forth. When found, the focus will appear as a small spot of light, but very bright. Why is this spot so much brighter than is common in sunlight ? If now any luminous source be placed at the focus of a concave surface, those of its waves which strike the FIG. 85 FIG. 86 surface will be reflected away in parallel lines, as in Fig. 86. Smooth curved surfaces used in this way are called reflectors; they are much used in locomotive headlights, search lights', and signal lights. They catch many rays that would be lost sidewise and back of a light, bending them all in the direction where they are needed. 135. Intensity of Illumination. From a luminous source light waves move in straight lines in every direction. But it is evident that the rays, moving in straight lines away from a point, must be spread farther apart as the distance from the point increases. That is, 116 LIGHT the total area covered by the radiations at any given distance from the luminous point is greater at a greater distance from the point. And as the same rays have to illuminate a greater area, it is clear that the intensity of the illumination cannot be as great. In other words, the farther a surface is removed from a luminous source, the less brightly it is illuminated. Experiment 91. Place before a lamp a large screen of wood or cardboard so that its surface will be 5 inches from the flame. At a point nearest the flame cut a hole one inch square in the screen. Now place another screen 5 inches beyond the first, so that the rays streaming through the hole will all strike this screen. Measure the illuminated spot and compare with the size of the opening. The second screen is now twice as far from the source of the rays as is the first. Move it to a point three times as far (i.e. 15 inches from the flame), and again four times as far. In each case measure the illuminated spot, compare with the opening, and note the brightness of the illumination. Make a general rule to apply. QUESTIONS 1. What three different effects are produced by the radiations from the sun? What do we call these radiations when they affect the eye, causing sight ? 2. Do any other bodies besides the sun give off light waves ? What are such bodies called ? Do other bodies than the sun give off heat radiations and actinic rays ? State any examples to prove this. 3. How are we able to see such bodies as do not give out light waves of their own ? What are such bodies called ? 4. What is a light ray ? What sort of a line do rays usually take? Can a wave travel in a curved path? How then can the sun's rays light a room into which they do not stream directly? 5. What is meant by the ether? Is it known to exist? How does it compare in density with any matter that we know ? REFRACTION 117 6. How fast do light waves travel ? How long time are the sun's rays in coming to earth? How does the speed of light waves vary with the density of the medium ? 7. Define a transparent substance ; a translucent substance ; an opaque body. State examples of each. What may become of the waves that fall upon an opaque body ? 8. What is a shadow? How is a shadow caused ? Name the two portions of a shadow, explaining the difference. 9. Why is it dark at night? Why is it not entirely dark in the shade in daytime ? Why is it not dark on cloudy days ? 10. What is meant by reflection? State the Law of Reflec- tion. Why do light waves from the same source make different impressions upon the eye when reflected from different things? 11. Explain the difference between reflection from smooth and from rough surfaces. What sort of surfaces reflect rays so as to form images ? Give examples. 12. How are waves reflected from curved surfaces? What is the focus of a curved reflector ? Explain the use of reflectors in obtaining a powerful illumination. SECTION II REFRACTION 136. Examples of Refraction. "We have learned that light waves travel usually in straight lines ( 127), and also that the direction of these lines may be changed whenever a ray strikes a point and is reflected (132). In another way the direction of a wave may be changed, when it passes from one medium to another that is denser or rarer. Such a change of direction is called refraction, and the wave is said to be refracted. FIG. 87 118 LIGHT Examples of refraction are common. A stick thrust into clear water often appears to be broken at the point where it enters the water. Objects often seem irregu- lar when viewed through window glass, and the size of bodies may seem greater or less than real when seen through a lens. In such cases the changes are due only to the change in direction of the light rays in passing to the eye through different media. FIG. 88 Experiment 92. Hold a piece of thick glass over a pencil so that a line from your eye to the glass meets its surface at an acute angle (less than a right angle). Does the pencil appear broken (Fig. 87)? Now hold the glass so that the line from your eye would meet its surface at right angles (Fig. 88). Does the pencil now appear to be broken ? Experiment 93. Place a coin in the bottom of a dish of water (Fig. 89), looking at it as from e. The rays are refracted at c, so that the coin appears to be at p. Now look straight down FIG. 89 FIG. 90 upon it (Fig. 90), so that the rays pe meet the water surface at right angles. Are the rays refracted? Explain the reason for these results. 137. Refraction defined From these experiments we see that rays travel in straight lines through each REFRACTION 119 medium, and that they are bent only at the point where they pass from one to the other. Also we find that when the rays meet the surface between the media at right angles, they are not bent at all ; the angle must be acute to cause refraction. Putting these facts together, we may say : Refraction is the bending of light rays when they pass from one medium to another of different density, at an acute angle to the surface between the media. 138. Cause of Refraction. To understand refraction, it must be kept in mind that light waves travel faster in a rare than in a dense medium. In Fig. 91 let abc be the cross section of a prism of glass. Rays from an object (the arrow) move through the FIG. 91 air with equal speed, so that when the first one reaches the glass at e' the others will all have reached the line e'e. Now since glass is denser than air, the first ray e 1 will move only to /', while e (still in air) moves to /; thus when e gets to / all the rays will have come to the line ff. Through the glass they now move with equal speed, but in a changed direction. The first ray to leave the glass at m will now travel faster than those still within, so that when the last one leaves the glass at c, the direc- tion of the waves will again be changed. Note that the waves are refracted on entering and on leaving the prism, and both times toward its base. 120 LIGHT FIG. 92 139. Lenses. A lens is generally a piece of glass hav- ing one or both of its faces curved ;. its use is to refract light waves for different purposes. When the faces curve inward the lens is called concave; when they curve outward the lens is convex. Fig. 92 shows both shapes (in dark lines), and it also shows how each is like two prisms taken together. Look at each carefully until it is plain that parallel rays would spread apart after /\ leaving a concave lens, and would come together after passing through a convex lens (Fig. 93). Convex lenses are the more widely used. The general effect of convex lenses is to converge (bring together) the rays that pass through them. The point where such rays meet is called tine focus, /(Fig. 93). When the rays entering the lens areparallel they all meet at one point, which is commonly called the principal focus. FIG. 94 ^ Experiment 94. Using a convex lens, hold it so as to converge the sun's rays on a piece of paper (Fig. 94). Move the paper nearer or farther from the REFRACTION 121 glass till the rays all fall upon one small spot. What do you notice regarding this spot? If the lens were not in the way, how great an area would be covered by the rays that now fall here? Cause the spot to fall on tissue paper or a bit of gunpowder. Account for the intensity of the light and heat at this point. 140. How Images are formed. When light waves are sent off from an object, each point sends off a separate set of rays which will be collected by a lens and refracted to a separate focus. In Fig. 95 let ab be an object and I the lens. Now every ray sent out from a point a and passing through PIG. 95 I will be refracted to a focus a' ; and every ray from b which is refracted, will come to a focus at b'. In the same way rays from all points in ab will be refracted to points between a' and b r . Each focus will appear the same, and will have the same position among the others, as the point in the object from which its rays came. Thus, if a screen be placed so that these foci (plural of focus) may be formed upon it, the group of foci together will form an image just like the object. 141. The Eye. Just behind the dark opening in the eye is a lens, the crystalline lens. This forms images on a screen called the retina, in the back part 122 LIGHT of the eyeball. This retina is made of nerve fibers and endings, through which the image is reported to the brain. Near sight is caused by too long an eyeball; the image is formed in front of the retina. Concave glasses may correct this fault; they spread the rays, making them come to a focus farther back. Far sight is due to an eyeball that is too short ; the image is formed behind the retina. Convex glasses will bring the image forward and correct the trouble. 142. The Photographic Camera. The camera is a box into which no light can enter except through a lens c * 3 - This lens is AAAA/WWj P just far enough from I c ^ a sensitive plate a so e that the rays from an outside object will be brought to a focus on "Tr^AAAAAA /] a, forming an image there. The plate is FlG - 96 covered with a sub- stance upon which the light acts, affecting each point according to the amount of light focused upon it. Later the plate is developed and fixed in a dark room, after which the image appears plainly. 143. The Microscope. When an object is placed nearer to a lens than its principal focus, the rays that come from the object will spread apart after passing through the lens. Such rays then entering the eye make the object seem larger, or magnified. REFRACTION 123 A single convex lens used to magnify small bodies is called a simple microscope. A compound microscope is a group of several convex lenses in a tube, so placed that each magnifies the image formed by the one before it. With such an instrument objects may seem to be hun- dreds of times their real size. 144. The Telescope. A telescope is a device for view- ing distant bodies. It generally contains two convex lenses, an objective o (Fig. 97), and an eyepiece e. The objective serves to collect as many rays as possible, and FIG. 97 the eyepiece magnifies the image formed by the objec- tive. A few telescopes are enormous. One in the Yerkes Observatory, in Wisconsin, has an objective forty inches in diameter. Some of these can magnify images over five thousand times. o QUESTIONS 1. What is meant by refraction ? State any examples of refrac- tion that are familiar to you. 2. Name the conditions that are necessary in order that a wave may be refracted. At what point does refraction occur ? 3. Explain the cause of refraction. Show why a wave is not refracted when it meets a surface at right angles. 4. What is a lens? How \s a lens related to a prism in its effect upon light rays? Name the two general shapes of lenses. 5. What, in general, is the effect of a concave lens upon parallel rays? of a convex lens? 124 LIGHT 6. What is a focus? What is the principal focus? Can there be more than one focus formed through a lens? Can more than one principal focus be formed ? 7. Explain, by a diagram, how images are formed through a convex lens. 8. Briefly show how images are formed in the eye. What is the cause of near sight and of far sight ? How may each be remedied ? 9. Describe the use of the photographic camera. How is it usually focused ? , 10. What is a microscope ? How does a convex lens magnify images ? What is a compound microscope ? 1 1. State the use of a telescope. For what purpose are the larger ones made ? Are spyglasses, field glasses, opera glasses, etc., micro- scopes or telescopes ? SECTION III COLOR 145. What is Color? We are so used to thinking of color as a part or property of any substance, that we may at first find it hard to understand that color is a property of light waves. Experiment 95. Examine colored pieces of paper in day- light ; then in red, blue, or other colors of light. (This may be done by cutting a small opening in a wooden box and putting a lamp inside ; red, blue, or other colored glass may be held over the opening. This should be done in a darkened room.) Now look at white paper in daylight, red light, etc. Does it always seem to be the same color ? Examine pieces of dark cloth (dark blue, green, gray, etc.) in lamplight ; mark each, and later look at them in daylight. Can you guess the color of each piece cor- rectly by lamplight ? From these experiments we learn that the color of a body seems to change when differently colored waves COLOR 125 fall upon it. Also we know that any illuminated body- is seen by means of such waves as fall upon it and are reflected to the eye. Thus it is clear that the color of any object depends upon what waves it sends to the eye, and that color itself is a property of the light waves. * Sound waves, we recall, vary greatly in their rate of vibration, and the resulting difference in sounds is called pitch. In the same way light waves differ greatly in rate of vibration; the effect of these differ- ent vibration rates upon the eye is that the waves have different colors. In other words, the color of a light wave depends upon its rate of vibration. For example, red waves have a low vibration rate, green a higher rate, while violet waves vibrate about twice as fast as red ones. 146. White Light. Just as sound waves of many different vibration rates may travel together from a vibrating body to the ear, so light waves of many colors may mix together in one beam of light. The color of the learn as a whole might be very different from that of any single wave, just as the pitch of a noise is unlike that of any one tone in the noise. Experiment 96 Paste pieces of col- ored paper (violet, green, and red) on a * circular cardboard, as in Fig. 98. Loop a stout string through the two holes near the center ; twist this string so as to rotate the card rapidly (any boy knows how). What color does it seem to be while rotating? Do the same with other colors two or more at a time. 126 LIGHT The sun sends out waves of many different colors, so many that we do not know the number. These many-colored waves unite to form sunlight, which is commonly said to be white light. Thus white light is a mixture of many colors. 147. The Spectrum. When light waves are refracted, those that have the faster vibration rate are bent more than those having the slower rate. Thus if a beam of light ac (Fig. 99), containing three colors, red, green, and violet, is refracted by a glass prism, the violet waves will be bent most, the green next, and the red least ( 145). In this way g the single beam ac is split up, its three n colors of waves are separated, and each falls on a different spot on the screen mn. The appear- ance of the separated colors on mn is called a spectrum. Experiment 97. Hold a glass prism (a cut-glass pendant or stopper) so as to refract sunlight. A spectrum will be formed, which may be seen somewhere about the room. Turn the prism till the spectrum falls upon a wall or any surface where it may be easily seen. This is the spectrum of white light (sunlight). How many colors can you count? Name them in order. Does each stop sharply, or gradually shade into those next to it ? The spectrum of white light shows seven distinct shades; these are called the prismatic colors. In order, they are red, orange, yellow, green, greenish blue, blue, COLOR 127 and violet. Note that these are not all of the many colors in white light, but only the more important groups. 148. The Rainbow. After a shower, when the. air still contains many heavy clouds, drops of water in these clouds may serve to refract the sunlight passing through them, and thus separate its several colors, like the prism in 147. Thus there will appear reflected to the eye of an observer a spectrum which will contain the seven prismatic colors. This spectrum usually ap- pears as an arc of a circle low down in the sky; it is called a rainbow. 149. Color of Light Waves. Waves from a luminous source sometimes appear to change color after passing through a substance ; a chimney of red glass seems to give a red color to the rays from a lamp, or sunlight seems blue after passing through blue glass. Now the fact is not that color is given to the waves, but rather that some of the waves are taken from the learn of light. Red glass, for example, contains a substance which absorbs (takes into itself) many of the waves that enter it, allowing the red rays to pass on through ; in the same way, blue glass allows mostly the blue waves to pass through it. The sun often seems red at sunrise or in setting. This is because its waves have to pass such a long distance through the denser and dusty air near the earth that many of the shorter waves are absorbed, leaving mostly the red ones to pass entirely through. 150. Colors of Objects. The color of luminous objects depends directly upon the color of the waves they send out. 128 LIGHT Since illuminated objects are seen only by the waves that they reflect to the eye ( 126), clearly the color of such bodies will depend upon (1) the colors of waves that fait upon them, and (2) which of these waves they reflect. When the light upon an illuminated object is white light, the object will appear white if all the waves are reflected; if none of the waves are reflected from the object, but all are absorbed, it is said to be black ; if all the waves pass through a body, being neither reflected nor absorbed, the substance is called colorless. In the great number of colored objects, however, part of the waves are absorbed while others are reflected; the color of such substances depends upon the color of the waves that are reflected. f QUESTIONS 1. Is color a property of objects or of light waves? Why do different objects seem to be of different colors ? 2. Upon what does the color of a light wave depend? Do waves of different colors ever unite in one beam? What is white light? 3. What is a spectrum? Explain how a spectrum is formed when light waves pass through a prism. 4. How many distinct colors in the spectrum of white light ? Name them in order. Which has the fastest and which the slowest vibration rate ? 5. What is a rainbow ? How is a rainbow formed ? When and where do rainbows generally appear? 6. Explain why light waves seem to change color on passing through certain substances. Why does the sun often appear red at sunset? Why is this more often true in hot, dry weather? 7. Upon what two factors may the color of an object depend ? In sunlight, what objects appear white? black? colorless? What objects seem red ? green ? blue ? 8. Is black a color ? How are black objects perceived ? CHAPTER VII ELECTRICITY SECTION I THE NATURE OF ELECTRICITY 151. Electrical Energy. The question, What is elec- tricity? has never been fully answered. Many things have been learned about its behavior, but with all their study men have never been able to discover what it is. For convenience we often speak of currents of " electricity," charges of " electricity," etc., as though it were a form of matter ; but though we may do this, it must be borne in mind that we do not know anything which proves it beyond a doubt. While men have been seeking to learn the nature of electricity, however, they have discovered many things which enable us to make it very useful. Particularly important is the fact that it possesses energy which may be made to do work for us when it is properly controlled. In this study we shall consider not so much the nature of electricity as the means by which electrical energy may be made useful to man. We shall seek to learn some- thing about electrical energy along three general lines, how it can be produced, how it may be controlled, and what are its effects. The study of these matters will doubtless show us much that is of interest as well as of importance. 129 130 ELECTRICITY 152. How Electrical Energy is produced. We have learned that in order to produce any sort of energy we must transform some other kind of energy into the de- sired sort ( 98). In general we may say that the kinds of energy commonly transformed into electrical energy are heat, chemical energy, and mechanical energy. Elec- trical energy seems also to be developed upon certain bodies by friction, by simple contact, and by dipping into certain liquids. For common uses, the devices for producing electricity are usually the voltaic cell or the dynamo. These will be explained in a later section. 153. How Electrical Energy is controlled. Electrical energy is easily controlled because electricity passes readily through some substances and not through others. A body through which electricity passes easily is called a conductor; one through which it passes with great diffi- culty, or not at all, is called an insulator. It must be clear that if a conductor is surrounded by an insu- lator, electricity may be kept within the conductor and made to travel long distances sometimes. This gives a partial idea of how electrical energy is controlled; a fuller explanation of conductors and insulators is made in 157. Among the better conductors are metals (copper, zinc, iron, etc.), water, animal bodies, the earth, and others. Of insulators, dry air is one of the best; more than any other substance, perhaps, it keeps electricity from spreading about where it would only be lost and might do injury. Other insulators are w*ood, cloth, rubber, and glass. THE NATURE OF ELECTRICITY 131 154. Electrical Effects. The effects produced by electrical energy may be divided into four classes, electrolytic, physiological, thermal, and magnetic effects. Briefly, the electrolytic effect is that, when an elec- tric current is passed through certain compound sub- stances, it will break up the compound into the simpler substances that compose it. This process is useful in chemistry ; it is used also in electroplating and elec- trotyping. The physiological effects are those produced by elec- tric action upon living bodies usually animals. Heavy currents of electricity may kill animal life ; weaker cur- rents are sometimes used in treating certain diseases. Thermal effects are those in which electricity causes heat. The wire through which a strong current is pass- ing may become very hot, as seen in small electric lamps. Electric heaters and furnaces also depend upon this effect. Perhaps most important of all is the magnetic effect. Wherever electrical energy is used to cause motion in motors, cars, elevators, call bells, telegraph and tele- phone systems, signals, etc. force is applied by means of magnets ; and these, of course, make use of the magnetic effect of electricity. 155. Potential. In describing the electrical condi- tion of a body the word potential is used in somewhat the same way as the word temperature is used to de- scribe its condition of heat. As a body may have a high or low temperature, so the potential of a body is said to be high or low ; and as heat passes from a body of 132 ELECTRICITY high temperature to one whose temperature is lower, so electricity may pass from a point having high potential to a point of lower potential. The potential of two points is described as high and low, or positive (+) and negative ( ) ; and this means that their electrical condition is such that electricity would tend to pass from one to the other. The one from which electricity would pass is called positive and the one to which it would pass is said to be negative. This means only that the potential of the first is higher than that of the second, without regard to any fixed standard. 156. Electro-Motive Force (E.M.F.) Different sub- stances vary in the ease with which they carry elec- tricity, but even the best of conductors offer some resistance to the electric current passing through them. To overcome this resistance a certain sort of electri- cal " pressure " is required ; and this is furnished by a difference in potential between two points in the con- ductor. If one point has a high potential and another point has a lower potential, a current may be caused to flow from the first point toward the second. This may perhaps be better understood if we compare it with the flow of heat from a body of high temperature to another of lower temperature. The difference in potential between two points in a conductor, which causes the current to flow and to overcome resistance, is called the electro-motive force of the current. The greater the difference between the potentials of two points, the greater is the electro-motive STATIC ELECTRICITY 133 force of the current in the conductor that connects them, and the greater is the ability of this current to flow against resistance. QUESTIONS 1. Why is electricity important to man? What, in general, may be studied regarding electrical energy? 2. How is electrical energy made? What forms of energy are commonly used for the purpose ? 3. What is a conductor? What is an insulator ? Name exam- ples of conductors and of insulators. Why should some insulators (as wood or cloth) become good conductors when wet ? 4. Show the use of conductors and insulators in controlling electrical energy. What might happen if air were a good con- ductor ? 5. Name the four general electric effects. Name uses to which each of these is put. 6. What is meant by potential ? What is high potential and low potential ? To what is potential somewhat similar. ? 7. By what means is a current kept up? Explain fully the meaning of electro-motive force. How can the electro-motive force of a current be increased and decreased? SECTION II STATIC ELECTRICITY 157. Electric Charges. Some substances, such as glass, resin, silk and woolen cloth, when rubbed with other bodies of a similar nature, behave in a peculiar manner, as we may have noted. For example : a glass rod rubbed with silk cloth will pick up bits of paper; the dry hair sometimes seems drawn toward a rubber comb that is run through it, or seems inclined to " stand 134 ELECTRICITY on end " after being thus combed ; sparks are sometimes seen to pass from fur to the hand that is rubbing it. In these cases both the body that is rubbed and the one that does the rubbing are said to be electrified, or charged with electricity, or to have upon them a charge. Experiment 98. Rub a glass rod with a silk cloth; at once bring the rod near small bits of paper. Quickly do the same thing with the cloth. Repeat the experiment, using a stick of sealing wax and a woolen cloth. Do you see evidence that any of these bodies are electrified ? In these cases the charge appears to be on the sur- face of the charged body, and only at the parts which were touched when the rubbing was done. This would be true when the substances used were glass, rubber, dry wood, silk, paper, sealing wax, sulphur, porcelain, or cloth. If certain other substances were used, such as the metals, the charge would appear not only at the part touched, but all over the surface of the body. Moreover, if this body, while still charged, were brought to touch another body which was like it in this respect, its charge would at once spread all over the other body as well. Thus it is clear that, in order to charge such a body, we should first separate it from other bodies by a substance of the sort first described, cloth, glass, etc. Substances of the first sort, over which the charge does not spread, are called insulators ( 153). As has been said, dry air is one of the most important of these. Substances of the second sort are called conductors ; all the metals, the earth, animal bodies, and water contain- ing acids or salts are conductors. We have said ( 153) STATIC ELECTRICITY 135 that electricity passes through a conductor ; this way of speaking is in common use, but it should be noted that the charge is on the surface of the body and not actually within it. We learn, then, that the electric charge upon a con- ductor moves rapidly and covers its whole surface, while that upon a " nonconductor " (insulator) remains at rest upon the part where it was developed. In this section we shall study the case of charged insulators, where the charge is at rest, and to this study we give the name electrostatics. 158. Positive and Negative Charges. When bodies are electrified we find that their charges may be one or the other of two sorts, which seem to have certain different effects. Experiment 99. Charge a glass rod by rub- bing with silk, and hang it by a silk thread, as in Fig. 100. Using that part of the silk cloth which touched the rod, bring it near to one end of the latter as it hangs free to turn. Carefully note what happens. Now charge another glass rod in the same way, and bring its charged portion near that of the suspended rod. Note the result in this case. Is there anything in this experiment that would seem to show a dif- ference between the electrification of the body that is rubbed and that of the one that does the rubbing? If you conclude that the glass and the silk are differently electrified, what would you say about the behavior of two unlike charges toward each other? Supposing the two rods to bear like charges, what do you learn about the behavior of two like charges toward each other? FIG. 100 136 ELECTRICITY These experiments show that when glass is charged by contact with silk, both the glass and the silk are electrified, but their charges are opposite in effect. It has been agreed to call the charge upon the glass posi- tive, and the charge upon the silk negative. The effect of these opposite charges upon each other is such that when the bodies bearing them are light and easily moved, these bodies will move toward each other ; for example, the glass and the silk. But when two bodies bearing like charges are brought near each other, these charges act so as to push the bodies apart ; for example, the two rods of glass. Many experiments may be made with small electrified bodies to prove the general law that Like charges repel, and unlike charges attract, each other. The sort of electrification that any body receives varies according to its own nature and that of the sub- stance with which it is rubbed. Thus, while glass may be positively electrified by rubbing with silk, it is nega- tively charged by flannel. Other conditions may also affect the sort of charge received. 159. Electrostatic Induction. If a body that is not charged is brought near a charged body, and both are separated from the earth by insula- tors, the first shows signs of being electrified. In Fig. 101, let a be an electrified body and b an uncharged FIG. 101 k 0( jy . SUS p enc i them by silk strings and bring b near to a. The side of b that is nearer a will receive a charge of the opposite kind to that of a, while STATIC ELECTRICITY 137 the side farther from a will receive a charge of the same kind. The body b is said to be charged by induction, and the charges which it receives are called induced charges. If a ball of pith be suspended by a silk thread, as in Fig. 102, it may be used in several experiments with electrified bodies. Charge a glass rod with silk and bring it near the pith ball ; note how far the rod can be removed from the ball before the force ceases to make it move. Note also how readily the ball moves when the rod is near it. We may get from this an idea of the extent of the field of force about a body bearing even a small charge. The medium between the rod and the ball must be the seat of energy, and the ball moves so as to make this energy less. When the uncharged ball becomes charged by induction, the medium between the rod and the ball must be considered as in a state of strain. 160. Discharges. Now if the body b is charged by induction from the electrified body a (Fig. 101), it is clear that these two charges bear some relation to each other. In other words, if there is no change in the positions of the two bodies, then a change in the potential of the FIG. 102 138 ELECTRICITY charge in a will cause a corresponding change in the charge of b. But since a induces a negative charge in the nearer portion of 6, any change that makes the potential of a more positive will result in charging that portion of b still more negatively, that is, even more unlike the charge in a. Thus we see that a difference in potential ( 156) exists between an inducing charge and the nearer por- tion of the charge that it induces ; also that as the inducing charge becomes more intense this difference becomes greater. Now difference in potential gives rise to electro-motive force, which can overcome resistance; so when the difference between the potentials of the inducing charge and the induced charge is great enough to overcome the resistance of the insulator that separates these two charges, a spark passes between them. The path of this spark through air is a good conductor at that instant, and electricity passes from one charge to the other, making them of equal potential. This sudden and rapid flow of electricity is called a discharge. The sparks seen when a cat's fur is rubbed are caused by discharges between the fur and the hand. 161. Lightning. Lightning is an electric discharge similar to that just explained, but on a far greater scale. The positively and negatively charged bodies are generally clouds, though in many cases the nega- tive charge is on the earth. In some instances the positive charge is on the earth, the negative charge being in the clouds. The clouds are more commonly charged during the very rapid and heavy condensation STATIC ELECTRICITY 139 of vapor that causes sudden showers. These heavy rain- falls occur more often in the summer, so that lightning is more common at that season. Lightning may be explained as follows. Suppose a positively charged cloud passes near the earth: build- ings or the earth itself may become charged by induc- tion, receiving a charge of the opposite kind from that of the cloud. If now the positive charge in the cloud increases in potential, the induced charge in the earth becomes more negative. This may go on until the dif- ference in potential between the two charges is great enough to overcome the resistance of the air between them, and then a discharge takes place from one to the other. The passage of this discharge through the air pro- duces electrical changes which cause the spark of flash. Discharges may pass from clouds to the earth, or from the earth to a cloud ; more often they go from one cloud to another. Objects through which the discharge passes are commonly said to be " struck." Tall buildings, towers, spires, trees, and the like are more in danger of being struck, because a charge induced on them is nearer the cloud. " Heat lightning " is the reflection of distant lightning. Thunder is the sound caused by waves of air set in vibration by the discharge. QUESTIONS 1 . How may a body become charged with electricity ? Name some substances that can be charged in this way. Do they, in general, belong to the class of conductors or insulators ? 2. What name is commonly given to the electricity in a charge of this sort? Why are not conductors easily charged in this way? 140 ELECTRICITY 3. Name any cases of electrically charged bodies that are known to you. Experiment with a rubber comb and report the results. 4. What two sorts of charges are named? How do two bodies behave toward each other when similarly charged ? How do they act when their charges are unlike ? 5. What is meant by an induced charge? How is a charge, induced from one body to another ? Does the inducing charge lose some of its electricity in the process ? 6. How does the induced charge compare with the inducing charge ? State the condition of a charge induced in a body. 7. Explain the nature of an electric discharge. What must be the relation of two charges before a discharge can occur ? 8. Compare the potentials of the two charges after the dis- charge. What generally attends a discharge ? State any experi- ence that you have had with discharges ; any that you have seen or felt. 9. What is lightning? Between what bodies does it usually take place ? When, in the year, is lightning most common ? Why ? 10. Fully explain the cause of the discharge. How long does it last ? What objects are in most danger of being struck ? 11. Explain the flash of lightning. What is thunder? Why is it not heard as soon as the flash is seen ? SECTION III THE ELECTRIC CURRENT 162. The Voltaic Cell Most of the devices by which man employs electrical energy make use of what is called a current, that is, a stream of electricity passing along a conductor. This electric current is commonly pro- duced in one of two ways, by a dynamo or by a voltaic cell. In this section we shall consider the cell, which generally produces weaker currents than the dynamo. THE ELECTRIC CURRENT 141 Experiment 100. Attach a piece of copper wire to a strip of zinc, and another piece to a strip of copper. Nearly fill a tumbler with water and pour into it a little sulphuric acid. Put the two metal strips into the water, being very careful that they do not touch each other at any point (Fig. 103). Now touch the ends of the wires together, as in the figure, hold the wire over a compass needle, and note any movement of the needle. This device is a simple cell, which may produce a weak current of electricity. A cell consists of two different solid conductors placed in any liquid con- ductor (except in a fused metal). It is found that when two such solid con- ductors are placed in such a liquid, they will be charged, but with a differ- ence in the potential of each. If the solid bodies are of zinc and copper, as in Experiment 100, and the fluid is sulphuric acid in water, the potential of the copper strip will be higher than that of the zinc. Now we have learned that the charge upon a con- ductor covers its whole surface, and spreads at once to any other conducting surface which touches it ( 157). Thus if the two strips are not allowed to touch each other, and a wire of some conductor (copper) joins them outside the liquid, the charge on the copper strip (having the higher potential) will at once discharge along the conducting wire to the zinc strip, which has the lower FIG. 103 142 ELECTRICITY potential. This discharge along the wire is the electric- current. The discharge would, of course, bring the two charges to the same potential ; but the action of the liquid upon the two strips is such that it renews the difference in potential just as rapidly as the discharges are made. Thus the difference in potential is kept up and the cur- rent continues to pass along the wire. The two metal strips are called plates or poles ; the copper plate, having the higher potential, is said to be positive, while the zinc plate is called negative. Note that the plates must not be connected within the cell by any conductor except the liquid, nor outside the cell by any conductor but that through which it is intended the current shall flow. Other- wise all or a part of the current would be lost to useful work. 163. Kinds of Cells. Different substances may be used in cells, for poles or for liquids. For the poles, copper and zinc, or carbon and zinc are most com- monly used. The liquid is generally water, in which some acid or salt has been dissolved. Sometimes the liquid is a weak solution of sulphuric acid; but as this destroys the zinc plate rapidly it is not often used in practice. More commonly the liquid is a solution of sal ammoniac ; cells of this sort are used for ringing call bells and for light work; the THE ELECTRIC CURRENT 143 current is not very strong, but the cells serve for months sometimes. On telegraphs and signal work, gravity cells are used (Fig. 104). The plates are zinc and copper, and the liquid is a solution of copper sulphate in water. Many cells have to be used in order to give a current of any strength, but they need little care except to be filled with water now and then. Many so-called " dry cells '' are in use; these contain a liquid, but the outside covering (which usually forms the zinc plate) is sealed so that the liquid cannot get out. 164. The Circuit. In order that a current may flow from the positive to the negative plate these must not Bell } Push Button only be connected by a conductor but there must be another conducting path back again to the starting point. In other words, there must be a complete path of conductors from the positive plate through wires or instruments to the negative, then through the liquid to 144 ELECTRICITY the positive again (Fig. 105). This complete conducting path is called the circuit. If there is any break in a circuit at any point from beginning to end, no current will flow through any part of the circuit. This matter is of great importance to man in controlling electrical energy. For example, note in Fig. 105 that the circuit is not complete, being broken at the push button for the weak current will not go through the air space. The circuit being broken at this one point, no current will go through any part of it, and the bell will not ring. But close the circuit by press- ing the button, and the current travels at once through every part of the conducting path and rings the bell. A completed circuit is said to be closed, or made; when interrupted at any point by an insulator, it is said to be open, or broken. 165. Resistance of the Circuit. We have learned that any conductor offers some resistance to the pas- sage of a current ( 156). The resistance of a circuit is divided into two classes, internal, or that offered by the cell ; and external, or that of the wires, instruments, or other outside conductors. External resistance depends upon three things : the kind of substance, the length of conductor, and its area of cross section. Other things being equal, the greater the length of a conductor, or the smaller its area of cross section, the more resistance it offers to the current. Naturally a current traveling through a greater length of conductor would meet more resistance ; and, lengths being equal, passage along a small conductor would, of course, be more difficult than over a larger. THE ELECTRIC CURRENT 145 166. Divided Circuits. A main circuit may be tapped at different points by short branches (called shunts) which take the current to separate instruments. Each of these branches must of course join the main circuit at some point farther on, in order that the cur- rent may pass through it. When a current is divided in this way, the amount of current that each branch gets depends upon its resistance : the more resistance each branch offers, the less current flows through it. This principle also is used in controlling electrical energy. For example, suppose it is desired to run a certain motor at different speeds: a device called a rheostat is put into the same shunt which runs to the motor. The rheostat contains several coils of wire, of different resistances, and a switch for making the current pass through one or more of these coils at will. The more coils the current is made to pass through, the greater the resistance offered by that shunt, and consequently the less current passes through it to the motor. Motormen on electric cars move a switch and control the speed of the car in this way. 167. Batteries. A group of cells arranged on one circuit is called a battery. One cell does not furnish enough current to do very much work, so that the com- bined strength of two or more is generally used. Cells may be combined in two ways, in series or in parallel. In parallel arrangement all the positive plates are joined together and all the negatives likewise ; the two sets are then connected by a wire. This arrange- ment decreases internal resistance, but gives no gain in 146 ELECTRICITY electro-motive force. When cells are arranged in series, the negative plate of each cell is joined to the positive plate of 'another; between any two cells the wires may lead away to some in- strument (Fig. 106). Series arrangement is the more com- FIG.IOG mon; each cell added to a battery gives a gain of electro-motive force. 168. Uses of Battery Currents. Battery currents are not powerful enough to do heavy work. They are used for call bells, door openers, spark coils for firing explo- sives, electric signals, medical batteries, telegraph and telephone systems, and like purposes. 169. Electrical Measurements. Quantity of electricity is expressed in terms of coulombs. Current strength^ or greatness of current, is measured by amperes. The electro-motive force of a current is expressed in volts, while the ohm is the unit of resistance. Electrical power, or the rate of doing work, is measured by watts. One watt is the rate at which work is done when a current of one ampere flows between two points under a pressure of one volt. The value of a watt is about y^g of a horse power ( 68). QUESTIONS 1. What is a cell? Tell how a cell may be made. Fully explain its action. What causes a current to flow from it ?. 2. What different elements are sometimes used in cells? What liquids? What sort of cell is used in telegraph systems, and why ? MAGNETISM 147 3. What is a circuit? If a circuit is broken at any point, what is the effect upon the current ? Show the use of this in control- ling electrical energy. 4. Distinguish internal and external resistance. Upon what does the resistance of a conductor depend, and how? 5. What is a shunt? When a current is divided, how much does each branch receive ? Show how the supply of current to a motor may be controlled by using resistance. 6. What is a battery? In what two ways may cells be ar- ranged ? What is gained in each case ? 7. Name some- common uses of battery currents. 8. What is measured in units of coulombs ? of amperes ? of watts? of ohms? of volts? 9. Compare the value of one watt with one horse power. SECTION IV MAGNETISM 170. Magnets. We have learned that one of the most important electrical effects is the magnetic effect, and that it is owing largely to this that electricity can be so generally used to cause motion ( 154). When it is used to cause motion, the magnetic force is com- monly applied by means of a magnet. A magnet may be described as a body which can attract iron. In other words, if a magnet is brought near a bit of iron, a force will act between them and cause them to move toward each other, if they are free to move. Certain other substances may be attracted by magnets. A kind of iron ore found in the earth is one of these ; also steel, cobalt, and nickel. Some substances are repelled (pushed away) by a magnet ; for example, zinc and bismuth. It has been found that the nature of this 148 ELECTRICITY action (that is, whether the magnet shall attract or repel a body) varies according to the medium in which the magnet is held ; a body which is attracted by a magnet, while both are in the air, may be repelled by it when they are in some other medium. 171. Magnetic Field. The fact last stated (170) shows that the magnetic force acts in the medium around the magnet. Simple experiments with a magnet and a compass needle will show that this force can act through considerable distances from the magnet. The whole space in which the magnetic force may be felt is called the magnetic field. Any magnet, then, is surrounded by a magnetic field, the different parts of which have vary- ing intensities of force. In describing this field we commonly speak of it as containing lines of magnetic force, or simply lines of force. In that portion of the field where the force is most intense the lines of force are most numerous; that is, there the lines are most densely crowded together. To get an idea of the arrangement of these so-called " lines of force," lay a piece of cardboard upon a magnet and sprinkle iron filings over the card. Try several dif- ferent positions of the card upon the magnet. 172. How Magnets are made. It is an easy matter to make a magnet of a piece of iron or steel. A body so treated is said to be magnetized. Two methods are generally used. The first method is simply to place the piece of iron or steel near a magnet, better in the more intense part of its field. (Compare this, but do not con- fuse it, with 159.) MAGNETISM 149 The other method depends upon an effect of electric currents, which may be shown by an experiment. Experiment 101. Pass a wire ab through a card, as in Fig. 107, and sprinkle iron filings on the card. Now send a strong current through the wire. Note any change in the filings; without disturbing the card, study their positions closely; stop the cur- rent, watching the filings carefully. Place a small com- pass c on the card and note any sign of a force acting. i I FIG. 107 This experi- ment shows that while a current is passing through a wire, the latter is surrounded by a field of force. If the wire is covered by an insulator and is coiled around a bar of iron or steel, this bar will be magnetized when a current is passed through the wire coil. As a general rule, a piece of soft iron remains magnet- ized only so long as it is being acted upon by the force in the magnetic field ; a piece of steel, however, remains a magnet for a long time after it is removed from the field. 173. Electro-Magnets. A piece of soft iron wound about with a coil of insulated wire is called an electro- magnet. When a current flows through the wire it affects the particles of the iron in such a way as to make the whole bar a magnet. The intensity of such a magnet 150 ELECTRICITY may be greatly increased by winding several layers of the wire around the iron, like thread on a spool. Of course the wire must be insulated, for if it were bare the current would be conducted straight across the coil without going through its many turns. Experiment 102. Wind a piece of soft iron (e.g. a cut nail) with insulated wire, as in Fig. 108. Join one end of the wire to a battery, holding the other end in the hand so as to close and open the circuit at will. With the circuit open, touch one end of the nail to a common tack. Can you lift the tack in this way ? Now close the circuit and try again. This device is a small electro-magnet. What is needed in order that it may exert mag- netic force ? Again opening the circuit, bring the end of the nail down to within one sixteenth of an inch from the tack lying on a desk ; close tiie circuit, watching the tack. Lift the magnet and tack a few inches, and open the circuit. How long before the tack drops from the nail ? How long a time is required for the soft iron nail to become magnetized, and to lose its magnetism? Electro-magnets may be made very FIG. 108 r T i . powerful by increasing the number of turns in the wire coil and the strength of the current. Magnets of this sort are used in dynamos and electric motors. The use of soft iron for the core of an electro- magnet allows it to become magnetized almost instantly, and it is demagnetized (loses its magnetism) when the current ceases to flow ( 172). For this reason electro- magnets are very useful in all electrical devices where motion is to be produced at will ; for example, call bells, motors, signals, telegraph systems, etc. MAGNETISM 151 174. Permanent Magnets. We have learned that when a piece of steel is magnetized it remains a magnet after it is removed from the magnetic field. For this reason a magnet made of steel is called permanent. Of course there are many kinds of steel, and these vary greatly in their value as perma- nent magnets. Experiment 103. Make an electro-magnet as in Experiment 102. Across one end of it draw a small piece of steel (a needle, knife blade, or steel pen) several times, always in the same direction. Try to pick up small tacks with this. Two forms of permanent magnets are common, the horseshoe (Fig. 109) and the Fm - 109 bar magnet (a straight bar of steel). They are not made nearly as powerful as some electro-magnets are. Their use in small dynamos and in telephones is most important. 175. Magnetic Poles. In using magnets we have perhaps noticed that the force seems to be greatest at the ends, while at the center none at all is felt. For this reason many magnets, both permanent and electro- magnets, are made in a horseshoe form, so as to bring the ends near together and exert greatest force FIG. no at that point. Experiment 104. Lay a bar magnet down upon a mass of iron filings ; lift it carefully by the center. Notice the arrange- ment of the filings that cling to i (Fig. 110). Where are they most numerous? Where are they least in number? Does the number change gradually or sharply? 152 ELECTRICITY The ends of a magnet, or the points where its mag- netic force seems greatest, are called its poles. In any magnet the two poles act differently toward other mag- netized bodies, so they are separately named: one is called the positive (-[-) or north pole, and the other the negative () or south pole. Permanent magnets are usually marked by a line or groove across the positive pole ; or the positive pole may be marked N and the negative S. It must be noted that the steel or iron body is mag- netized not only at its poles but throughout the body ; the poles are simply the parts where the magnetic force acts with the greatest intensity. To show this more clearly, magnetize a steel needle, dip it in iron filings, and note its middle part. Now break it in the middle, dip one part in the filings, and note the end that was a portion of the middle before you broke it. 176. Law of Magnets. Both poles of a magnet will attract a piece of iron that is not magnetized. Toward a magnetized body, however, the two poles act in an opposite manner. Experiment 105. The positive pole of a compass needle is the one that points northward. Secure a bar magnet whose poles are marked. Bring the positive pole of the bar near the + pole of the needle ; note what happens. Now bring the same ( + ) pole of the bar near the negative end of the needle, while it is at rest, and make a note of the result here. Again, present the negative pole of the magnet to the negative end of the needle, noting this result. Finally bring the negative end of the bar to the positive pole of the compass needle, and observe. What poles seem to attract each other,,and what poles repel? Sum up your results in a statement of how the poles act. MAGNETISM 153 Similar experiments with magnets give the same results. The facts may be stated briefly in a Law of Magnets as follows : Like poles repel, and unlike poles attract, each other. 177. Magnetism of the Earth. A magnetized needle suspended so as to swing in a vertical plane (" up and down ") is called a dipping needle. Fig. Ill shows five different positions of a dipping needle placed on a bar magnet. Note that at the nega- tive pole of the bar the positive end of the needle FlG ' m is down ; note its other positions with care, and if pos- sible perform the experiment for yourself. Now it has been found that a dipping needle carried north or south along any meridian of the earth behaves in much the same way. This and other occurrences show that the earth is a great magnet, having its positive and negative poles like any magnetized body. These magnetic poles are two points on its surface, toward which the compass needle points. The negative mag- netic pole is northwest of Hudson Bay, about 20 south of the north pole. Straight through the earth from this point, at a spot in the Antarctic Ocean about 20 north of the south pole, is the positive magnetic pole. 178. The Compass. A magnetized strip of steel, finely balanced on a point so that it turns freely, will be so acted upon by the earth's magnetism that its positive pole will point toward the negative (northerly) 154 ELECTRICITY FIG. 112 magnetic pole of the earth, and its negative end of course toward the positive magnetic pole. Such a mag- netized bit of steel may be used as a compass needle. This needle is finely balanced, and usually swings over a card on which the "points of the com- pass" are marked (Fig. 112). The compass has long been a valuable aid to sailors and trav- elers, who often have little else to guide them. But owing to the position of the magnetic poles and variations in the earth's magnetism at different points, the needle points to the true north at only a few places on earth. To find the true north, the user of the compass has to know how far wrong the compass direction is at that point, and allow for it. QUESTIONS 1. What is magnetic force ? Define magnetism. 2. What is a magnet ? Name two kinds of magnets. 3. How is an electro-magnet made? How is it magnetized? Why should the wire be insulated ? 4. Name various uses of electro-magnets, showing why this form of magnet is particularly useful in each case. How may the power of an electro-magnet be increased? 5. Of what are permanent magnets made? How are they magnetized ? Show any points of difference between permanent magnets and electro-magnets. Which form is most used in elec- trical devices, and why ? 6. What are the poles of a magnet ? How are they named ? Why are magnets often made in horseshoe form ? INDUCED CURRENTS 155 7. State the Law of Magnets. How do both poles act toward unmagnetized iron ? 8. Why does the compass needle point steadily in one direc- tion? Where is the north magnetic pole? Is it positive or negative ? SECTION V INDUCED CURRENTS 179. Induced Electro-Motive Force. It has been found that currents can be produced by means of a magnet, and the electric currents so made are called induced currents. The very powerful currents now in common use are induced currents, and for this reason it is interesting to learn how they are produced. Experiment 106. Balance a thin card upon the two ends of a horseshoe magnet, and sprinkle iron filings evenly over the card. Note the positions taken by the filings. The arrangement of the filings on the card shows the position of " lines of force " in the field between the poles of the magnet (Fig. 113). Now, if a circuit of wire be moved in such a magnetic field, a current will flow in this circuit while there is a change in the number PIG. us 156 ELECTRICITY of lines of force cut by the . circuit. In other words, so long as there is any change in the intensity of that part of the field which is within the circuit, an electro-motive force ( 156) will be set up in the circuit ; and this E. M. F. will vary according to the rate of change in the number of lines of force which fall within the area of the circuit. Of course it is- at once clear that such induced currents will last but a moment, unless we can arrange to have the number of lines that fall within the area of the circuit continually changing. This can be done either by varying the intensity of the magnetic field, or moving the magnet, or by moving the wire cir- cuit in the field. In practice the wire circuit is com- monly made to rotate in the field, thus moving through a changing number of lines of force. 180. The Dynamo. - - The dynamo is a device for producing induced currents. From what we have just learned it is evident that such a device must consist of a magnet and a wire circuit, together with some arrange- ment for varying the number of lines within the area of the circuit. The method in common use is to arrange the wire circuit on an axle so that it can be rotated within the magnetic field. In the section of a dynamo, Fig. 114, m is the mag- net; the space between its poles, p and p\ is crossed by lines of magnetic force. The coil of wire a (called the armature] is made to turn about an axle e, and in turning it cuts the lines of force. This sets up a cur- rent in the coil, which passes out through one brush b to the outside circuit c, supplying instruments on INDUCED CURRENTS 157 the circuit, and back to the armature a through the other brush. The energy by which the dynamo is run is usually furnished by a steam engine or water power. It is applied at the axle, being used to turn the armature. This turns at a high speed, the strength of the current generally increasing as its speed is raised. 181. Direction of the Current. The whole circuit, outside and within the dynamo, is one continuous path, FIG. 114 the outside conducting wire being joined to that of the armature through the brushes 5, b (Fig. 114). Now the direction of the current through this circuit depends upon the direction in which the armature a moves across the lines of force ; and since each part of a cuts these lines in one direction during one half of a turn, and in the opposite direction the other half, the current will travel through the circuit first in one direction and 158 ELECTRICITY then in the other. Such a current is called an alternat- ing current. The alternations, or changes of direction, occur with great rapidity. Alternating currents may be used for some purposes, and their use is coming to be more important. For some uses, however, the current must be made to flow always in one direction. This is done by a simple device on the armature called a commutator. The cur- rent flowing steadily in one direction is called a direct current. 182. Kinds of Dynamos. Electro-magnets are used in dynamos that are meant to furnish strong currents, because they can be made more powerful than per- manent magnets. The cur- rent to supply the coils of these electro-magnets may be taken from the dynamo itself, or from a separate PIG. 115 ni -, generator called an exciter. An exciter is a small dynamo, the magnets of which are permanent. Fig. 115 shows a permanent-magnet dynamo whose armature is turned by hand. Such devices are used in telephones to ring the call bell, and in a few other ways. Their current is not great, though stronger than the usual battery currents. 183. Uses of the Current. Dynamo currents are used in motors, electric lights, furnaces, electric cars, electroplating and electrotyping, and for other purposes where powerful currents are needed. For lighting and PLATE IV. AN ALTERNATING CURRENT DYNAMO INDUCED CURRENTS 159 for the motors of some electric car systems alternatr ing currents are employed. Direct currents are used in electrolysis, electroplating and electro typing, for many motors, etc. 184. The Transformer The current that supplies small electric lamps in many houses and other buildings has to be of high potential (great electro-motive force) in order to travel through the long circuit. But such a current might prove danger- ous if used freely in houses. To lessen the danger and still keep up the flow, a transformer is employed. A transformer consists of a coil of long, fine insulated wire, surrounded by a coil of short, coarse insulated wire (Fig. 116). The high-potential alternating current from the dynamo being passed through , , . FIG. no the long, fine wire coil, an alternating current is set up in the coarser wire by induction. This current has greater strength than that from the dynamo, but it has a lower potential. From the coarse wire coil, then, this low-potential current is led to the building where it is used. Because of its lower potential it is less dangerous. For some purposes it may be desired to change a cur- rent of great strength but low potential into a current of less strength and high potential. To do this the 160 ELECTRICITY alternating current is sent through the coil of coarse wire ; the current induced in the fine wire coil will have less strength but higher potential than the other. 185. The Induction Coil. By means of this last- named principle, currents from batteries are often changed into currents of high potential. For this pur- pose an induction coil is used. The induction coil has two coils of wire, a fine and a coarse one as has the transformer, the battery current generally passing through the coarse wire a (Fig. 117). This coil a, through which the battery current passes, is called the primary, and the other (in which a current is induced) is called the secondary coil. Of course the wire of the secondary coil cuts many of the lines in the magnetic field around the primary, and a current is induced in the secondary whenever there is any change in the number of lines of force that it cuts ( 179). In the transformer this change is secured by using an alter- nating current ; but since a battery gives a direct cur- rent, the induction coil must be so arranged as to produce the necessary change in some way. This is usually done by a make-and-break piece c (Fig. 117), which opens and closes the circuit very rapidly. The current thus rapidly flowing and stopping causes a change in the number of lines in the field surrounding FIG. 117 INDUCED CURRENTS 161 the coarse wire coil; a current is induced in the fine wire coil during each separate change. This induced current has high potential, and because the changes are so rapid the effect is nearly that of a constant (though not direct) flow. Coils of this sort are used in " medical batteries," Rontgen ray apparatus, wireless telegraphy, etc. QUESTIONS 1. How, in general, are induced currents produced? How does the electro-motive force of these currents vary? 2. What is a dynamo ? What are its important parts ? 3. How is the energy for running a dynamo applied? In what part of the dynamo is the current produced ? How is the current taken from the armature to the outside part of the circuit ? 4. What is an alternating current? a direct current? Why should a dynamo current alternate ? Can such a current be used ? How is it changed to a direct current ? 5. What advantage have electro-magnet dynamos over those using permanent magnets ? What advantage have the latter over the former ? 6. Name uses of currents from electro-magnet dynamos. For what are permanent-magnet dynamos used ? How do the coils of the electro-magnets in a dynamo receive their current? 7. What is a transformer ? Describe its structure. For what purposes are transformers used ? 8. Through which coil in a transformer would you pass a cur- rent that was to be changed to a higher potential? to a lower potential ? What sort of a current is used in either case ? 9. Describe the induction coil. For what purpose is it gener- ally used ? Show how an induction coil may use a direct current, as the transformer uses an alternating current. 10. In which of the two coils is the current induced ? State carefully the condition under which a current will be induced in this coil. 162 ELECTRICITY SECTION VI USES OF ELECTRICAL ENERGY 186. Electric Motor. The parts of an electric motor are the same as those of the dynamo ( 180) ; in fact, an ordinary dynamo could be made to serve as a motor. The difference is that whereas the armature of a dynamo is turned by some outside means and a current is gen- erated in it, the armature of a motor receives a cur- rent from some outside source and, being turned by its action, is able to impart motion to other bodies. Fig. 118 may serve to show the action of a motor. The armature ab is to turn about its axle, between the poles (+ and ) of the magnet. A current from a dynamo or battery enters through the brush m, travels around the arma- ture coil ab, and out through n, as shown by arrows. Passing through it in this way, the current makes an electro-magnet of the armature, its negative pole being above and its positive pole below the horizontal position (dotted lines). That is, the part a becomes a pole and the part b a 4- pole. Now since like poles repel and unlike attract each other, a is repelled by the pole of the magnet and attracted by the -f pole ; also b is repelled by + and attracted by . All of these forces tend to make the armature turn about its axle in the direction of the curved arrow. When it has turned so FIG. 118 USES OF ELECTRICAL ENERGY 163 that a is below and b above the horizontal position, a commutator c changes the direction of the current in the coil, so that b becomes the negative and a the positive pole of the armature. Thus the motion goes on always in the same direction. Such a motor would use a direct current. Motors are now commonly made without commutators, to use an alternating current. These are often built upon the same frame as the dynamo which furnishes the current. 187. Electric Cars. An electric car is driven by means of a motor. This is on the under side rf the car FIG. 119 m (Fig. 119) ; as its armature turns, the motion is trans- mitted to the wheels by a set of gear wheels ( 72). The current is supplied from a wire w, or from a " third rail" beneath the car. After passing through the con- troller s to the motor, the current leaves through the wheels, traveling through the rails back to the dynamo. 164 ELECTRICITY The speed of the motor and car is governed by adding more or less resistance to the shunt at the controller * ( 166). 188. The Telephone. Only a very general explana- tion of the telephone can be given here. In Fig. 120, suppose some one talking at A to a person at B. The two instruments, at A and B, may be alike, though they usually differ in appearance. In each, m is a permanent c m FIG. 120 magnet and c a coil of wire which is continuous with the circuit ; d is a disk of thin iron. As you talk before 0, the sound waves cause the disk d to vibrate. The disk vibrating near the magnet in- duces alternate currents in the coil c in one direc- tion when d approaches c and in the opposite direction when d moves away. These currents go through the circuit to the coil c at B. Their effect is to strengthen and weaken the magnetic field acting upon d (at j5), attracting and repelling d. In this way the disk at B is made to vibrate just like that at A. Its vibrations cause weak sound waves which may be heard by the ear at B. Note that the sound waves are produced at B ; alter- nate currents pass through the wire not sound waves. USES OF ELECTRICAL ENERGY 165 The circuit is completed by allowing the current to return through the earth. A battery is commonly used on the circuit to overcome the resistance of the wire. 189. The Telegraph. The sender of a telegraph message uses a key k (Fig. 121), that simply closes and opens a circuit. In the distant office is a sounder s, by which the message is received. Pressing on k closes the circuit; the current then magnetizes the electro- magnet m ; this draws the armature a so that it strikes feu el FIG. 121 the frame f, making a clicking sound. The key being lifted, m is demagnetized, and a spring pulls a till it strikes the frame above, making a different sound. If these two sounds are separated by only an instant, a dot is said to be made; when a longer bit of time comes between them, the report is a dash. Every letter is represented by a different group of dots and dashes ; the sender can make each at will, by pressing his key for a shorter or longer time. Any telegraph operator must learn to know each letter instantly by the sound of the dots and dashes which represent it. The current, as in the telephone, goes through one wire and returns to the battery through the earth (Fig. 121). As a rule, the batteries are composed of a few gravity cells placed at intervals along the circuit. 166 ELECTRICITY 190. Electric Bells. A common call bell is shown in Fig. 122. The hammer or striker is attached to a spring s; m is an electro-magnet. Follow the course of the current carefully. When the circuit is closed, m is mag- netized and attracts the ham- mer; this moving quickly toward m strikes the bell once. But in so moving, a is also moved away from c, breaking the circuit at that point. At once m loses its magnetism and the spring s causes the hammer to move back again; but this also brings a again in contact with c. Thus again the circuit is closed, m is mag- netized, the hammer hits the bell, and all is repeated. This happens very rapidly, producing the familiar buzzing sound of electric bells. 191. Electroplating, Articles covered with a thin layer of metal (gold, nickel, silver, etc.) are said to be . plated. Plating is commonly done by use of the electro- lytic effect of electrical energy ( 154). The articles to be plated are hung in water that contains a salt ( 213) of the metal to be used; a plate of the metal is also hung in the liquid. This plate and the articles are connected by separate wires with a dynamo or battery, in such a way that the current has to pass through the liquid (Fig. 123) from the plate to the articles. The FIG. 122 USES OF ELECTRICAL ENERGY 167 current, in passing through the water, acts upon the salt so as to set free the metal that it contains. This metal, FIG. 123 in tiny particles, gathers about the solid bodies through which the current leaves the liquid, thus covering those articles with a metal coating. The current is commonly furnished by a dynamo, though a battery of cells may be used in experiments. 192. Electric Lights Electric lights depend upon the thermal effect of elec- trical energy. A current is made to pass through a poor conductor against great resistance ; in doing this, it heats the con- ductor until it is luminous. Two sorts of lamps are common, incandescent and arc lamps. Fig. 124 shows the familiar bulb of an incandescent light. The fine thread of carbon inside the bulb offers great resist- ance to the current that is sent through it. Thus the carbon thread becomes very hot and luminous. The space within the glass bulb is a nearly perfect FIG. 124 168 ELECTRICITY vacuum; if air were admitted, the hot carbon would quickly burn up. In the arc lamp a current having very great electro- motive force is made to pass through two carbon pencils placed end to end. Because these pencils just loosely touch each other, great resist- ance is offered to the current at that point. The current flowing against this resistance heats the ends of the pencils to white heat. Tiny bits of the carbon are detached from one pencil and pass, in a glowing condition, to the other pencil. Now the two points are drawn apart to a distance of about a quarter of an inch, the space between being filled with these glowing particles (Fig. 125). This space filled with the particles is called an arc, and the current is conducted through it. But the resistance is now even higher than at first; thus the two carbon points and the arc between are all very highly heated, so that they glow and give a bright light. Arc lamps are used in lighting streets, halls, and large rooms. They are arranged in series on the circuit FIG. 125 IL_H_J FIG. 126 (Fig. 126), and need a current of high potential. Each lamp contains a device for keeping the carbon points the proper distance apart as they are consumed. USES OF ELECTRICAL ENERGY 169 193. Wireless Telegraphy. Messages are now trans- mitted without wires over long distances by means of waves in the ether. These waves are caused when elec- trical discharges are produced. They travel with great speed in all directions from their source, growing weaker as they advance. Of course these waves can do no great work at a distance ; but delicate instruments are made, by means of which the weak waves serve to close the circuit of a local battery and telegraph receiver, so that this receiver shall be operated by them. QUESTIONS 1. Explain the action of the electric motor. What sort of cur- rent can be used ? Name uses of electric motors. 2. How are electric cars driven ? How is the speed of the motor and car controlled ? How is the main circuit completed ? 3. Explain the telephone. What travels over the wires? How are the sound waves that the listener receives produced ? 4. How is a telegraph message sent by an operator? Describe the sounder on which it is received. Explain the action of the sounder. 5. Carefully explain the common electric bell. 6. What would you do with an article that you were going to plate ? What is put into the water ? What is the action of the current upon this substance ? 7. Of what electrical effect do electric lights make use? Explain how a current is used to make a body luminous. What two sorts of electric lamps are common ? 8. Explain the incandescent light. Why is the carbon thread placed in a vacuum ? 9. Explain the arc light. What sort of a current is needed, and why ? From what are the light waves sent out ; that is, what parts of the lamp are luminous? PAET II. CHEMISTEY CHAPTER VIII OUTLINE OF CHEMICAL STUDY SECTION I GENERAL INTRODUCTION 194. Chemistry. Chemistry, like physics, treats of matter, but in a different way. Physics is the science of matter with regard to its motions, etc., whereas chemistry is the study of substances with regard to the kind of matter that is in them. For example, in physics we have found and studied several forces and their action, without much regard to the sort of matter acted upon, while in chemistry we shall be constantly dealing with kinds of matter, seeking to know what this or that substance is made of, how it may be made, how it may be destroyed, what can be made from it, and the like. 195. Chemical Changes. Matter may of course be changed in many ways. The size, shape, or state of a body may be altered; it may be hardened, heated, or crystallized, etc. All such changes, which do not affect the substance or kind of matter of the body, are called physical changes. But when any substance is so acted 171 172 OUTLINE OF CHEMICAL STUDY upon that there is some change in the kind of matter, a chemical change is said to take place. Experiment 107. Dissolve some common salt in water until no more can be taken up. Has any change occurred ? Now boil the salt water to dry ness. Does anything remain ? Was this a physical or a chemical change ? Experiment 108. Place a bit of sulphur in an old spoon and heat it over an alcohol lamp or gas burner. The sulphur melts and, if still heated, vaporizes. Hold a saucer just above the spoon ; sulphur collects on the saucer in tiny particles. What sort of change ? Now burn a bit of sulphur in the spoon, holding the saucer above it as before. No. sulphur collects on the dish. In burning, the sulphur unites with oxygen from the air, forming a different substance, that passes off as a gas. Is this a physical or a chemi- cal change ? 196. Composition and Decomposition. In a general way, any chemical change falls under one of two classes, composition and decomposition. Composition is the process of uniting two or more substances to form an- other. When a substance is broken up into the two or more substances that compose it, the process is called decomposition, and the body is said to be decomposed. 197. Kinds of Substances We do not need to be told that there is an almost countless number of differ- ent substances on earth. Many of these we know can be made from simple substances, by processes which man has devised. Others are found in the rock and soil of the earth, having been made by natural processes long ages ago. And a still larger number, perhaps, are made by the growth and action of living matter, plant GENERAL INTRODUCTION 173 and animal. These many kinds of substances may be considered in three different classes, elements, com- pounds, and mixtures. 198. Elements. Of this great number of substances there will of course be some that are composed of sev- eral simpler ones. These simpler ones may, in turn, be made of others that are still more simple. But clearly we cannot go on, without limit, breaking up each of these simple substances into simpler ones; that is, we must soon reach substances that are perfectly simple that cannot be broken up into anything else. Such sub- stances, that cannot be divided into anything else, are called elements. They are absolutely pure, each composed of only the one kind ; the smallest particle of an element would be of just the same nature as a large mass of it. Now with these facts clearly in mind, we shall easily see that elements cannot be made, as some substances are, by composition, since each is composed of itself only. The elements only occur ; that is, they are found on earth, sometimes in a pure state but more often united with other elements. They may be separated from these other elements by different methods, called analysis. Every substance, then, is made of elements ; either of one alone or of two or more together. Nearly eighty elements have been discovered and named. Most of these are uncommon. Hardly a dozen occur in very large quantities. Of the common elements, oxygen, hydrogen, nitrogen, and chlorine are gases ; mercury is the only familiar liquid ; of solids, there are carbon, sulphur, phosphorus, and some metals ( 212). 174 OUTLINE OF CHEMICAL STUDY 199. Compounds. When two or more elements unite with each other in a definite proportion, the new sub- stance formed is called a chemical compound. This will be explained more fully later ( 202, 203). Note, however, that the ele- ments must be united, that is, not simply lying side by side in the same mass, but the smallest particles of each actually combined with those of the others. Also note that the result is a new substance, unlike either of the elements which compose it; even to its molecules, the compound is differ- ent from either of the elements. Water is a chemical compound ; its elements are hydrogen and oxygen both gases. Other common compounds are starch, sugar, alcohol, quartz, and many acids, bases, and salts. Experiment 109. Put a small piece of zinc into a little hydro- chloric acid (an inch in a test tube) ; note all that happens. If the zinc does not finally disappear, add more acid. When the zinc can no longer be seen, boil the liquid in an evaporating dish (Fig. 127) till dry. Examine the substance that remains. Do you think this a compound ? Why ? 200. Mixtures When two or more substances, with- out uniting chemically, together form another substance, that mass is called a mixture. A mixture may differ in some ways from each of the substances that compose it, but no new substance is formed ; that is, the mixture has no molecule of its own, being composed of molecules of each substance lying side by side but not combined. FIG. 127 GENERAL INTRODUCTION 175 Mixtures are very common ; many are made in nature and many are made by the work of man. Wood and coal are mixtures, also the air. Various sorts of vege- table and animal products cloth, paper, leather, and many other mixtures are common. The difference^ between chemical compounds and mix- tures is important. In compounds the elements unite to form a new substance, which has a molecule of its own ; that is, all its molecules are like each other and like the mass. In a mixture the molecules are those of the ele- ments or compounds that compose it ; thus they are of different kinds, and the mixture, as a substance, has no molecule of its own. For further explanation see 207. Experiment 110. Grind small quantities of sulphur and iron filings together in a mortar till well mixed. Draw a magnet through the mass. Is it a compound or a mixture ? QUESTIONS 1. Of what, in general, does the science of chemistry treat? 2. Show the difference between physical changes and chemical changes. Give examples of each. Does wood suffer a physical or a chemical change when burned? 3. What is meant by chemical composition? Define decom- position. Try to think of examples of each process. 4. What is meant by an element ? How are elements gener- ally obtained ? How many have been discovered ? 5. Are masses ever formed of one element alone ? Name some common elements. With how many are you familiar? 6. What is a chemical compound? How does a compound mass differ from an elementary mass ? Is the molecule of a com- pound the same in its nature as the mass ? 7. What is a mixture ? How do compounds differ from mix- tures ? Name some common substances that are mixtures. 176 OUTLINE OF CHEMICAL STUDY SECTION II CHEMICAL ACTION 201. The Atomic Theory. The name molecule has been given to " the smallest particle of any substance that can exist alone " ( 10). Now we have learned that ele- ments combine to form compounds, and that each mole- cule of a compound is just like all the others. This would seem to show that each molecule must contain in itself a small portion of each of the elements in the compound. But this at once raises the question, How can a molecule (the smallest particle that can exist alone) be made up of smaller parts ? Scientists have answered the question by formulating an atomic theory. They say that there may be parti- cles smaller than molecules, but that these can never exist alone, that is, they must always be united with at least one other. These smaller particles are called atoms. An atom may unite with others of its kind or of dif- ferent kinds, but it must always be in a union. A molecule, then, is said to be composed of atoms. Therefore we see that each molecule of a substance may be just like the others, and yet every one may be made up of atoms of different elements. In other words, when two or more elements unite to form a compound, the mole- cules of each element break up and the atoms of the dif- ferent kinds unite with each other, forming molecules that will be all alike. 202. Chemical Affinity. The atomic theory allows us still to say that the molecule is the smallest particle CHEMICAL ACTION 177 of a substance that can exist alone. Clearly, a com- pound will no longer exist if its molecules are divided again, for each molecule is made of atoms of different elements. The molecules of an element are also the smallest bits that can exist alone, each molecule being made of atoms that cannot be separated unless by unit- ing with other different atoms. Between elements and compounds, however, there is this difference : the atoms of an element are just alike, and the same in substance as the molecule, whereas the molecule of a compound is made of different kinds of atoms. In any molecule, the force that binds the atoms together is called chemical affinity. Its action in holding atoms together in molecules is somewhat similar to that of cohesion, which binds molecules together in masses. Without cohesion we should have no masses of definite form ; without chemical affinity, no substances of definite composition. 203. Chemical Combination. When two or more ele- ments unite to form a compound they are said to com- bine ; the process is called chemical combination. The number of atoms which may combine to form a mole- cule varies according to the substance formed ; some molecules contain only two atoms, while others contain nearly one hundred. In any one substance, however, the molecule must always contain the same number of atoms; moreover, these atoms must always be those of the same elements, and each element must always be present with the same number of atoms. For example : water is a compound ; a molecule of water must always 178 OUTLINE OF CHEMICAL STUDY contain three atoms ; two of these atoms must be those of the element hydrogen, and one atom must be of oxygen. If the two elements combined in any other proportion (say one atom of each), or if they combined with any other element, the molecule formed would not be that of water. Thus we may say that chemical com- bination takes place only between definite proportions of certain elements. Every element does not by any means combine with every other element. Some elements may combine with several different ones, while others can unite directly with only two or three. The study of what elements combine with certain others is of course an important part of the chemist's work. Sometimes the same elements may combine in more than one proportion. In such cases the resulting com- pounds would of course be different. For example, hydrogen and oxygen combine to form water (two atoms of hydrogen and one of oxygen), while if two atoms of oxygen unite with the two of hydrogen, a very different substance is formed. The three elements, carbon, hydro- gen, and oxygen, combine in a great many different proportions, forming as many different compounds. 204. Decomposition. When a compound is broken up into its elements it is said to be decomposed. Naturally each element has a stronger affinity for some of the elements with which it may combine, than for others. Thus in some compounds the elements will be more strongly united than in others. Some compounds are so weak that they slowly decompose if simply left CHEMICAL ACTION 179 to stand in air or in sunlight. Such compounds are called unstable. Strong compounds, which do not easily decompose, are said to be stable. Often when two or more compounds are mixed together, they so act as to decompose each other ; the atoms then unite with others for which they have greater affinity, and form new substances. 205. Heat assists Chemical Action. Heat is a very important aid to chemical action, both composition and decomposition. Many changes which will not take place at ordinary temperatures easily occur if the sub- stances are heated. Experiment 111. Into a clean, dry test tube put a little sugar ; heat gently. Notice what occurs, and when the mass becomes solid examine it. Has a chem- ical change occurred ? In a similar way, treat some small bits of wood in a test tube, and examine. Can you discover whether heat has here caused composition or decomposition ? Experiment 112. Put a small piece of lead (a BB shot) into a test tube and . FlG - 128 add a little cold sulphuric acid (concentrated). Look for any action. Carefully heat the acid and look again for signs of action (Fig. 128). In this case the change includes decomposition and composition. Combustion, or burning, is a very common sort of chemical action; and we know that to burn any com- mon substance it must first be heated. Gunpowder and other explosives suffer rapid chemical change when heat is applied ; and in many other sorts of chemical action we find that heat plays an important part. 180 OUTLINE OF CHEMICAL STUDY 206. Heat from Chemical Action. Not only does heat aid chemical action, but it is also given off during such activity. Some of the chemical energy set free during a chemical change is transformed into heat, and this is one of our important sources of heat ( 74). Experiment 113. Put a small piece of zinc into a test tube with hydrochloric acid. Do you see any sign of chemical action? Grasp that part of the tube where the acid is. What further evi- dence of chemical action do you discover? Experiment 114. Cut a bit of metallic potassium the size of a small pea. Throw it upon water and stand away. The potas- sium decomposes the water, setting- free its hydrogen and oxygen. Is this a chemical action? Do you note any sign that heat is given off during this action ? In burning a substance we have first to apply heat to it. The chemical action that is caused by this gives off enough energy itself to heat more of the mass ; and so the combustion keeps on by its own heat, until stopped by some means. 207. Compounds and Mixtures. We have seen that in a compound the molecules are all alike, and every one contains the same number of atoms of each element in the substance. That is, the molecules of each ele- ment have been broken up, their atoms combining with other atoms to form a new sort of molecule. In a mix- ture no chemical combination takes place. The molecules of each element or compound lie side by side in the mixed mass ; each is unchanged and no new molecule is formed. The proportion of substances in a mixture is not definitely fixed, as in a compound, but the same ones may be mixed in any proportion whatever. CHEMICAL ACTION 181 Experiment 115. Grind a mixture of one half ounce of iron filings and one ounce of sulphur in a mortar. Examine carefully and draw a magnet through the mass. Is it a compound or a mixture still? Into an old test tube put a little of the mass, and heat it slowly but well. When solid, allow the mass to cool. Break the tube and examine the substance. Is it iron? Is it sulphur? Is it a compound or a mixture ? 208. Symbols. For convenience, a system has been devised so that names of elements and compounds, and even chemical changes, may be expressed by symbols. The names of elements are generally expressed by their first letter, or two letters : hydrogen, H ; oxygen, O ; carbon, C ; calcium, Ca ; zinc, Zn, etc. Moreover, the symbol for any element (e.g. C, O, or H) means also one atom of that element. To express more than one atom, a small figure is placed after the letter; thus H 2 means " two atoms of hydrogen"; O 3 means " three atoms of oxygen." The symbol for a compound is made by writing the symbols of its elements in order, each showing the num- ber of its atoms in the substance. Thus HC1 means that in the compound hydrochloric acid one atom of hydro- gen is combined with one of chlorine ; HNO 3 (nitric acid) is a compound in which one atom of hydrogen, one of nitrogen, and three atoms of oxygen are combined. The symbol of a compound (HC1, HNO 3 , etc.) of course repre- sents one molecule of the substance. Two molecules would be thus written, 2 HC1, 2 HNO 3 , etc.; three molecules, 3 HC1, etc. The number (2, 3, etc.) so written belongs to the whole group of ele- ments, and means that in the whole quantity represented the quantity of each element is taken just that number of times. For 182 OUTLINE OF CHEMICAL STUDY example, 2HNO 3 means "two molecules of nitric acid, each containing one atom of H, one of N, and three of O " ; and it further means that in the whole quantity (two molecules) there are (2x1) two atoms of H, (2 x 1) two atoms of N, and (2x3) six atoms of O. A list of the more common elements and their sym- bols follows. In cases where the symbol is quite unlike the word, it has generally been obtained from the Latin name of the element. Aluminium . . Al. Antimony . . . Sb. Bismuth .... Bi. Boron B. Bromine .... Br. Calcium .... Ca. Carbon C. Chlorine .... Cl. Copper Cu. Fluorine . . F. Gold Au. Hydrogen . . . H. Iodine I. Iron Fe. Lead Pb. Magnesium . . Mg. Manganese . . Mn. Mercury .... Hg. Nickel Ni. Nitrogen . . . . N. Oxygen . . . . O. Phosphorus . . P. Platinum . . . Pt. Potassium . . . K. Silicon Si. Silver Ag. Sodium .... Na. Sulphur . . . . S. Tin Sn. Zinc . . Zn. QUESTIONS 1. Of what is a molecule composed? Can these particles exist alone ? If a molecule is broken up, what becomes of its particles ? 2. In what way do the molecules of an element differ from those of a compound ? What is chemical affinity ? 3. How many atoms may a molecule contain ? Can two mole- cules of the same substance contain different numbers of atoms? Can the atoms in them be arranged in different proportions? 4. What is meant by chemical combination ? When elements combine, can each then be seen in the compound? Can any ele- ment combine with every other element ? 5. What is meant by decomposition of a compound? 6. State examples to show that heat assists chemical action. CLASSES OF SUBSTANCES 183 7. Explain how heat may be given off during chemical changes. Show how the heat set free during combustion serves to keep up the burning. 8. Explain the difference between a compound and a mixture. 9. Tell everything about these substances that you can learn from their symbols : H 2 O (water) ; H 2 SO 4 (sulphuric acid) ; NaCl (common salt) ; C 12 H 22 O 11 (sugar) ; C 2 H 5 OH (alcohol). SECTION III CLASSES OF SUBSTANCES 209. Acid-Forming and Base-Forming Elements. Two important classes of substances are adds and bases. Some elements have the power to unite with others to form acids, while different elements in a similar way usually form bases. Elements that commonly form acids are called acid-forming, or negative, elements; those that form bases are called base-forming, or positive, elements. The common negative (acid-forming) elements are bro- mine, carbon, chlorine, fluorine, iodine, nitrogen, oxygen, phosphorus, silicon, and sulphur. Of positive (base-form- ing) elements there are aluminium, calcium, copper, gold, iron, lead, mercury, nickel, platinum, potassium, radium, silver, sodium, tin, and zinc. The element hydrogen seems to hold a neutral place, being found in both acids and bases. Sometimes a group of elements acts like a single atom in combining with other elements ; for example, NO 8 in nitric acid, or SO 4 in sulphuric acid. Such a group of elements is called a radical. Like elements, radicals are either positive or negative. 184 OUTLINE OF CHEMICAL STUDY 210. Acids. An acid is a compound made up of hydrogen and a negative element or radical. Note these symbols of acids : HC1, hydrochloric; HBr, hydrobro- mic; HNO 3 , nitric; H 2 SO 4 , sulphuric. Acids generally have a sharp or rather sour taste ; they often act upon other compounds, causing chemical changes; some acids act strongly upon animal matter, and some are poisonous. The sharp taste of many fruits is due to acids. Lem- ons, raspberries, and currants contain citric acid ; grapes contain tartaric acid; apples and cherries, malic acid. Vinegar owes its sour taste to acetic acid, and sour milk contains lactic acid. 211. Bases. A base is composed of OH in combina- tion with a positive element or radical. OH is a neg- ative radical ; it is sometimes called hydroxyl. Four bases are common: NaOH, sodium hydrate; KOH, potassium hydrate; Ca(OH) 2 , calcium hydrate; NH 4 OH, ammonium hydrate. The last of these, NH 4 OH, is diluted with water and used for household purposes under the name ammonia. NaOH and KOH are used in making soap. Ca(OH) 2 is sometimes used in making other bases. An alkali is a base that is soluble (can be dissolved) in water. The strongly basic compounds NH 4 OH, NaOH, and KOH are alkalis. 212. Metals. The positive or base-forming ele- ments are commonly called metals. We usually think of a metal as a solid, heavy, and rather hard substance. These properties are true of some metals, but not of all ; CLASSES OF SUBSTANCES 185 for example, mercury is a liquid ; sodium and potassium float upon water and are also soft. Thus it is difficult to find any common property by which to define a metal, and in this study we must be content to learn some of the important metallic elements, together with their general behavior. By far the greater number of elements are metals. Some of these are very common on earth, while others are very rare. A few metals (e.g. iron, copper, and zinc) are of much importance in the life of man; but there are also several whose existence is never realized by us, and whose very names are never heard except among scientists. A few metals are sometimes found free in the earth, though most of them occur only in compounds with other elements. Pure metals are obtained by breaking up the salts or the ores in which they occur. The fol- lowing metallic elements are familiar: Al, Bi, Ca, Cu, Au, Fe, Pb, Hg, Ni, Pt, K, Ag, Na, Sn, Zn ( 208). Of these, Ca, Na, and K are not common in a free (not combined) condition. 213. Salts. The salts form a large and important group of substances. Many different salts may be formed by the action of metals upon acids, or of bases upon acids. In either case, a salt is formed when the hydrogen of an acid is set free and some metal taken on in its place. Experiment 116. To a little hydrochloric acid (HC1) in a test tube add a piece of zinc (Zn). Note the action. Bubbles show that a gas is given off ; this is hydrogen (H). When the action ceases, boil the liquid to dryness. Describe the substance 186 OUTLINE OF CHEMICAL STUDY that is left. Is it zinc ? Is it hydrochloric acid ? Is it a new sub- stance ? Name the three elements that you had in the test tube at first. Which one of these escaped ? What ones have united ? The substance is a salt zinc chloride. Repeat the experiment, using HNO 3 and Hg ; again, using H 2 SO 4 and Cu. What elements combine in each case ? Salts are named usually from the metals and the acids that compose them. For example, salts of H 2 SO 4 (sulphuric acid) are called sulphates: Cu and H 2 SO 4 form CuSO 4 , copper sulphate ; Fe and H 2 SO 4 form FeSO 4 , iron sulphate; Zn and H 2 SO 4 form ZnSO 4 , zinc sulphate ; etc. Salts of HNO 3 (nitric acid) are called nitrates: NaNO 3 , sodium nitrate; KNO 3 , potas- sium nitrate ; AgNO 3 , silver nitrate ; etc. Salts of HC1 are called chlorides : NaCl, sodium chloride ; KC1, potas- sium chloride ; CaCl 2 , calcium chloride ; HgCl 2 , mercuric chloride ; etc. Not only metals but positive radicals may unite with acids to form salts. The positive radical NH 4 (ammonium) forms two common salts, NH 4 C1, ammo- nium chloride (sal ammoniac), and NH 4 NO 3 , ammonium nitrate. Since there are many different acids and metals, the number of different metallic salts is great. Some of these occur in the earth ; NaCl (common salt) is very abundant, also KNO 3 (saltpeter) and NaNO 3 . Many salts can be prepared by man, and in some cases they are prepared by him in great quantities. The uses of salts are also numerous. Some are useful as foods, notably chlorides and phosphates ; very many are used in medicine, chlorides, bromides, phosphates, sulphates, nitrates, carbonates, etc. ; others are used as CLASSES OF SUBSTANCES 187 sources from which to obtain acids or metals. Salts are used in many mechanic arts, in photography, in elec- troplating, electrotyping, and batteries; in plaster, in fertilizers, in explosives, and in other ways. 214. Oxides. Nearly all elements combine directly with oxygen (O) ; that is, each forms with O a com- pound in which itself and oxygen are the only ele- ments. The compound formed by the direct union of an element with oxygen is commonly called an oxide. Some oxides are solids and are very hard, some are gases, and still others are liquids. They occur very often as powders, that is, masses of small particles. Iron rust is an oxide of iron ; lead scraped bright and then exposed to the air becomes covered with a thin, dull coating of lead oxide. An oxide of carbon, CO 2 , is a gas; it is found mixed with the air, and is formed whenever C burns in O. Similarly, sulphur (S) combines directly with some elements to form sulphides. Of these, iron sulphide (FeS 2 ) is very common ; Cu, Pb, Sn, and Ag also form common sulphides. 215. Minerals. The earth, so far as we can discover, is composed largely of rock masses (and soil on the sur- face) which are either pure minerals or mixtures of min- erals. Mineral substances are compounds, commonly oxides, carbonates, or sulphates. The oxide of silicon, SiO 2 , is very common we call it quartz ; other oxides are those of Al, Ca, Mg, K, Na, and Fe. The important carbonate is that of calcium, CaCO 3 , called limestone; and the sulphate of calcium, CaSO 4 (gypsum), is also common. 188 OUTLINE OF CHEMICAL STUDY 216. Ores. Most of the metals are found in the earth in the form of ores. An ore is a mineral substance containing a metal that may be removed from it for man's use. The mineral substance may be any sort of rock mass. The metal itself is mixed with the rock, sometimes in its free (or uncombined) state, but more often as an oxide, a sulphide, or a salt. That is, if we were to see an ore of some metal, we should see a rock in which were scattered masses of possibly the pure metal itself, but more likely of some salt, oxide, or sulphide of the metal. Iron, copper, tin, lead, silver, gold, zinc, and a few other metals are taken from ores. 217. Alloys. An alloy is a mixture of two or more metals, made by melting them together. Many alloys may at first thought seem to be metals ; they are not elements, however, but are made by man's work. Brass is an alloy of copper and zinc ; bronze is made of tin and copper ; solder contains tin and lead ; gun metal and bell metal contain copper and tin in different pro- portions. G-erman silver is an alloy of copper, zinc, and nickel; type metal contains lead and antimony; and pew- ter is an alloy of lead and tin. 218. Hydrocarbons. An hydrocarbon is a compound of hydrogen and carbon. There are many hydrocarbons, for these elements unite in various ratios. Two common hydro- carbons may serve as examples : acetylene (C 2 H 2 ), a com- mon illuminant used in automobile headlights ; and marsh gas (CH 4 ), the explosive " fire damp " of coal mines. Kero- sene and other petroleum products contain hydrocarbons. CLASSES OF SUBSTANCES 189 219. Carbohydrates. A very important group of com- pounds can be made from the elements carbon, hydrogen, and oxygen. They are made in nature, chiefly by the activity of plants. Because of their composition (G and H 2 O) they are called carbohydrates. Starch, sugars, and cellulose are common carbohydrates ; they occur in seeds, all parts of living plants, and fruits. The carbohydrates form a very important part of the food of most animals. 220. Proteids. Another group of substances that are necessary to the life of higher animals is called proteids. These contain the elements carbon, hydrogen, oxygen, and nitrogen; sometimes sulphur or phosphorus also. Proteids occur in the white of eggs, in lean meat, cheese, wheat flour (gluten), gelatin, etc. 221. Solutions. When a substance is dissolved in a liquid it is said to be in solution. The liquid in which a substance is dissolved is called a solvent. Certain solids, liquids, or gases may be thus put in solution; their molecules are separated and they mix with those of the solvent. This mixture of a substance in a solvent is called a solution. There is, of course, a limit to the amount of any given substance that a liquid can dis- solve. When the solvent holds in solution all that it can dissolve of any substance, the solution is said to be saturated. In the case of solids, dissolving is hastened if the solvent be heated. It is well known that some substances dissolve better in hot water than in cold. Stirring or shaking assists solution by mixing the parti- cles more rapidly. A substance that can be dissolved in a liquid is said to be soluble. 190 OUTLINE OF CHEMICAL STUDY Experiment 117. Dissolve the following substance^ in equal volumes of water : common salt, sugar, sal ammoniac, ammonium nitrate, magnesium sulphate, and calcium sulphate. Note how much of each can be dissolved in the water. Which are the more soluble ? Which of these substances are soluble in water: HC1, oil, alcohol, kerosene, molasses, mercury, and NH 4 OH ? Experiment 118. Into two equal volumes of water put equal quantities of sugar. Stir one and allow the other to stand quietly. Which dissolves more rapidly ? Again try to dissolve two equal quantities of sugar in equal volumes of water, one cold and the other heated. Try to dissolve some lead chloride (PbCl 2 ) in cold water in a test tube ; now heat the water (Fig. 129) and note the result. Does heating help in dissolving solids ? Some substances are much more soluble than others, when put into the FIG. 129 ^ /|f/ same liquid ; and many that will not mix with one solvent will dissolve in another. Of the solvents, water is the most common and important. Many salts, acids, and bases are soluble in it, besides some other substances. For this reason water is widely used as a cleansing agent. Alcohol is also a common solvent ; tinctures and essences are solutions of different things in alcohol, and its use in medicines is important. Many of the fats and oils are soluble in the alkalis, such as NH 4 OH, NaOH, and KOH. Mercury dissolves several of the metals, forming amalgams. Ether, turpentine, and carbon disulphide are also used as solvents for certain substances. CLASSES OF SUBSTANCES 191 QUESTIONS 1. What class of elements are commonly called negative? What are positive elements ? Name some elements in each class. What is a radical ? Name two radicals. 2. Define an acid. What properties do acids generally pos- sess? Name any acids that you can. Name any substances that you think may contain an acid. 3. What is a base ? Name four common bases. For what are these sometimes used ? What is an alkali ? 4. Name several metals. Are they elements, compounds, or mixtures ? How are the metals generally obtained ? 5. Under what conditions is a salt formed? Name any salts that you can. Are salts very numerous ? Are they important ? 6. In what ways are salts obtained ? Name uses of salts. 7. What is an oxide? Name any oxides that you know of. 8. What sort of substances compose minerals? Are minerals compounds or mixtures ? 9. What is an ore ? Does the metal in an ore occur in a free state or combined with other elements? Name metals that are obtained from ores. 10. What is an alloy? Name some common alloys. 11. Name the elements that compose hydrocarbons. Name any common hydrocarbons. 12. What elements combine to form carbohydrates? Name any such compounds, telling where they commonly occur. 13. What is a solution? Is it a confound or a mixture? Name some common solvents. What is a soluble substance ? 14. When is a solution saturated ? What condition in the sol- vent may assist the dissolving ? Of what use are proteids ? 15. What sort of substance is a tincture ? What is an essence ? What is an amalgam ? 16. Name the class of substances to which each of the follow- ing belongs : iron ; copper sulphate ; sugar ; mercury ; kerosene ; common salt ; gelatin ; household ammonia ; starch ; iron rust ; lead; brass; gasoline. CHAPTER IX COMMON SUBSTANCES SECTION I ELEMENTS 222. Oxygen. Oxygen is a gas without color or odor ; it occurs most widely of all the elements. Many salts and acids, and all bases, carbohydrates, and oxides contain O. It is also a very important element in water, air, and the solid earth. In the air oxygen is free (not combined with other elements), and it serves two great purposes it supports combus- tion (burning) and helps to support animal life. We see its importance FIG 130 i at once when we know that without O in the air animals could not live and common fires would not burn. Experiment 119. Fit a stopper to a large test tube. Perfo- rate the stopper with a round file and push through this hole one end of a glass tube, bent as in Fig. 130. Hang the whole on a 192 ELEMENTS 193 ring stand, so that the other end of the tube shall dip below the surface of water in a large vessel (see Fig. 130). Into the tube put 5 grams of potassium chlorate (KC1O 3 ) mixed with 5 grams of manganese dioxide (MnO 2 ). Stop the tube tightly and heat it. Bubbles of gas soon appear in the water. Now fill two or three pint jars with water; tip one bottom upward under water and hold it over the tube so that the gas shall go up into it. Be sure that the jar is full of water at the start, and allow no air to enter it. As the gas flows, the water in the jar is pushed down and out. When the jar is full of gas (i.e. the water is all out of it), cover it with a piece of stiff cardboard or glass and lift it from the water. In the same way fill two other jars, and keep each covered (Fig. 131). The gas is O. By heat- ing, KC1O 3 is decomposed into KC1 and 3 O. Experiment 120. Into one jar of O put a glowing splinter of wood. In another hold a bit of burning sulphur (Fig. 132). In the third place a lighted bit of candle. Be careful to keep the jars covered as much of the time as possible. In each case what do you notice when the burning substance is first put into the jar ? What do you notice after it has burned a few moments ? Try to explain this. FIG. 131 FIG. 132 These substances seem to burn better in the jar of pure O than in the air. In either case the burning is a process of chemical combination, the substance com- bining with oxygen ; in the jar the O is nearly pure, while in the air it is mixed with a much larger quantity of another gas (N), so that the bodies burn better in the jar. Note carefully that when sub- stances burn in air (e.g. wood, kerosene, paper, etc.), it 194 COMMON SUBSTANCES is because some of their elements are uniting with the O of the air. It is for this reason that a draught of air is necessary in stoves, lamps, and various fires. 223. Hydrogen. Hydrogen also is a colorless and odorless gas. It is an important element, forming a part of all acids and of water. Animal and vegetable substances contain large quantities of H, but it is not common in its free state. The element may be sepa- rated from acids by the action of metals upon them ( 213). Hydrogen is the lightest substance known, being 14 J- times lighter than air. Balloons are usually filled with it, so as to make them rise in the air. Experiment 121. Arrange the apparatus the same as for mak- ing O (Experiment 119), filling jars w.th water. Into the test tube put 5 grams of zinc with 5 cubic centimeters (cc.) each of water and HC1, but do not heat. After the gas has flowed a few seconds, collect some in jars by the same method as in Experiment 119. This gas is H. 2 HC1 + Zn = ZnCl 2 + 2 H. Experiment 122. Uncover one jar of H, at once holding a lighted match near the opening. Always be careful with H, for it burns in air and explodes if mixed with it. Using a small jar, partly uncover it for an instant; then cover it again and shake it once. Now apply a match to the opened jar, being careful not to get the face too near. If the air and H are mixed, a slight explo- sion may occur. Does H burn in the air? Does it kindle easily? With what element does it combine ? Hydrogen combines with very easily if it be heated to its kindling temperature; once lighted, it burns readily. Pure H burning in pure O makes a very hot flame. It is the H in many substances, such as kero- sene, paraffin (candles), wood, paper, etc., that makes them kindle easily. ELEMENTS 195 224. Nitrogen. Like H and O, nitrogen is a colorless and odorless gas. It occurs free in the air, nearly four fifths of the air being N. In combination with O (i.e. NO 3 ) it forms a part of those salts that are called nitrates, and it is a factor in the proteids, which occur mostly in ani- mal matter. N is not an active element, and it does not support combustion. Owing to this last fact, N in the air serves a very great use by checking fires; that is, if a larger portion of the air were O, fires would burn more fiercely and they could not be controlled so easily. 225. Carbon. The element carbon is a solid. Several substances are nearly pure C ; for example, charcoal, coke, lampblack, boneblack, and gas carbon. Coal also contains a large amount of carbon. Notice that each of these substances is one that remains after some com- pound has been broken up; for example, charcoal is left when wood is burned imperfectly, lampblack when oils are burned without a good supply of air, etc. This shows that C occurs in compounds which may be broken up by heat. The gases in the compounds are first driven off, leaving the C. If plenty of air be supplied and the heat be great enough, C will combine with O (i.e. will burn) and pass off as a gas, CO 2 (carbon dioxide). Experiment 123. Burn a match (wooden) in air, allowing it to burn completely. How much ash remains ? Now break up the wood of a match or a splinter into bits, place these in a test tube, cover with a little dry sand, and heat over a flame. Do you see any evidence of decomposition? What remains in the tube? Explain the difference between this result and that from the burning in air. Similarly, heat some sugar in a test tube till it is solid. Note and explain the result. 196 COMMON SUBSTANCES Carbon occurs very commonly in living matter, partic- ularly in vegetable substances. In these cases it is nearly always combined with other elements, usually O and H. The element occurs free in two forms, diamond and graphite. Diamond is the hardest of minerals, and graphite one of the softest ; both are crystalline, and each is nearly pure C. Graphite mixed with clay is used as " black lead " in pencils. 226. Sulphur. Sulphur is a solid element, brittle, and of a yellow color. It occurs free in the earth, especially near volcanoes ; it also occurs combined with metals in sulphides and sulphates. It burns easily, forming with O a gas, sulphur dioxide (SO 2 ). The com- pounds of sulphur (e.g. FeS 2 , H 2 SO 4 , H 2 S, etc.) are of great importance to man. In its free state, S is used in preparing matches, gunpowder, and rubber goods ; also in medicine. Sulphuric acid (H 2 SO 4 ) is one of the most important of chemical compounds. 227. Phosphorus. Phosphorus is a solid element, slightly yellow in color, and of a waxy nature at usual temperatures. It is an acid-forming element and occurs largely in phosphates. P is very active, combining with several elements directly and at low degrees of heat. It should always be kept and cut under water. Experiment 124. Cut a piece of P no larger than half a small pea. Dry this on blotting paper and place it in an evaporating dish. Place a bit of iodine so as to touch the P. Do you notice anything that is unusual ? Caution. Do not touch P with the hands, and do not breathe the fumes from burning P. The substance is very poisonous. Also be careful never to leave the least bit lying around. ELEMENTS 197 Combination of P with O also takes place very easily. Sometimes P will burn as soon as it is placed in the air, especially if it be cut or rubbed a little. Owing to the ease with which it kindles, P is commonly used in making matches. The red tip contains some P mixed with other substances. Simple rubbing heats this tip enough to make the P burn, and this kindles the wood. P gives out a faint glow in the dark; hence it is used in luminous paint, etc. 228. Chlorine. Chlorine is a greenish-yellow gas hav- ing a disagreeable odor. It is not common in a free state, but occurs in a group of salts called chlorides. With hydrogen Cl forms hydrochloric acid, HC1. The pure ele- ment acts strongly upon the throat and lungs if inhaled. It is used as a disinfectant and as a bleaching agent. Experiment 125. Arrange apparatus as in Fig. 133, passing the tube to the bottom of a jar through a loosely fitting cover of cardboard. Into the test tube put 5 g. of MnO 2 and 10 cc. of 198 COMMON SUBSTANCES HC1. Heat the mixture. Cl gas is set free and flows into the jar, driving out the lighter air. Do not breathe any of this gas. Note the color of Cl. This is one of the few gases that have color and can be seen. (If Cl is accidentally inhaled, pour alcohol on a cloth and breathe through the cloth for a few moments.) Experiment 126. When the gas in the jar is very yellow, remove the flame, wait a half minute, then remove the glass tube from the jar, keeping the jar covered. Now moisten a small piece of colored calico, drop it into the Cl, and quickly cover the jar again. If no change is noticed soon, try another piece of a different color. A substance called bleaching powder is much used in bleaching cloth and paper, because it contains Cl. 229. Iron. Iron is the most important of metallic elements in man's work. Its uses are too common to need mention here. The element occurs in several ores, usually combined with O or S. The sulphide, FeS 2 , is commonly called pyrite. Iron is obtained from its ores by heating them in a blast furnace. In this big furnace coke or coal is mixed with the ore (usually an oxide of iron) and burned. A blast of air is forced into the furnace, and the fire (which burns all the time) gives a very great degree of heat. In this heat the ore is decomposed; its O unites with the C of the coke, and the iron in a melted state collects at the bottom of the furnace. From here it is drawn off into molds, and is called pig iron or cast iron. It is very impure. Steel is a better grade of iron, which contains a fixed amount of carbon. It is commonly made by blowing air through a mass of highly heated pig iron. The impuri- ties in the iron unite with the O of the air and are thus burned off, and then a known amount of carbon is mixed with the heated mass. ELEMENTS 199 230. Sodium and Potassium. The solid metallic ele- ments Na (sodium) and K (potassium) are not found free in nature. Their salts, however, are very common and important. The elements may be separated from some of their salts. Neither is common outside of labora- tories, and no great use is made of them. They are soft, waxy metals, lighter than water. Na acts upon water to decompose it, and K does the same, but more strongly. Experiment 127. Cut a small piece each of Na and K (the size of a small pea). Throw the Na on some water in a dish, being careful then to keep away from it. Next do the same with the K. What difference in these two cases ? Try to explain how this difference proves that K acts upon water more strongly than Na (see 206). Of the salts of Na, NaCl is common and important ; also Na 2 SO 4 (sodium sulphate) and NaNO 3 . Potassium carbonate, K 2 CO 3 , occurs in the earth, is absorbed by plants, and forms a part of wood ashes ; KNO 3 (salt- peter) is an important salt of K. Na and K form strong bases or alkalis, NaOH and KOH. 231. Calcium. Calcium (Ca) is a solid metallic ele- ment ; like Na and K, it is common only in compounds with other elements. Some of its compounds, however, are important and are found in large quantities. CaCO 3 , calcium carbonate, occurs widely in the earth ; in differ- ent forms it is called limestone, marble, or chalk. CaSO 4 , sometimes called gypsum, also occurs in the earth; when heated it forms a white powder, plaster of Paris. The oxide of calcium, CaO, is called lime; it is used 200 COMMON SUBSTANCES in making mortar and plaster. Ca is a base-forming element, the base being calcium hydrate, Ca(OH) 2 . 232. Mercury. The element mercury (Hg) is a metal, which is a liquid at ordinary temperatures. It is some- times called quicksilver. It occurs free in rocks, and also as the sulphide (HgS). The metallic Hg is a heavy liquid, 13.6 times heavier than water. It is used in ther- mometers and barometers ; also in forming the reflecting surface of mirrors. It dissolves several metals, and for this reason it is used in separating silver and gold from their ores. Hg forms several salts, among them two chlorides, HgCl and HgCl 2 . HgCl, mercurous chloride, is called calomel and is used in medicine. Mercuric chlo- ride, HgCl 2 , is called corrosive sublimate; it is used as a disinfectant to kill bacteria. Hg is a poison, as are some of its compounds. 233. Other Metallic Elements. The metals copper (Cu), lead (Pb), tin (Sn), zinc (Zn), silver (Ag), and gold (Au) are familiarly known to us. Cu, Ag, and Au occur free in nature ; Cu and Sn occur as oxides ; Cu, Pb, Zn, and Ag occur as sulphides, and Zn as a carbonate. These metals are in common use, and we can easily think of the uses of each. Cu and Zn form sulphates that are common, and Pb forms lead carbonate, ^PbCO 3 , known as white lead and used in making paint. Silver easily forms the sulphide Ag 2 S ; as there is nearly always a little S in the air, and always some given off in the per- spiration from the body, silver articles become coated with Ag 2 S and are said to " tarnish." ELEMENTS 201 The metal aluminium (Al) is a very important ele- ment; it occurs as a part of some of the most abundant kinds of rocks. As a metal, Al is silver-white, strong, but very light in weight. The oxide of Al, A1 2 O 3 , occurs in nature as sapphires and rubies ; it is also powdered and used for polishing, being called emery. Common alum is a sulphate of Al and K, A1K(SO 4 ) 2 . 234. Silicon. The element silicon (Si) never occurs free in nature, though in compounds it is very abundant in the earth. The most common compound, SiO 2 , is called silica or quartz ; it is a white or colorless rock, and in a finely broken form it is white sand. Silicates are salts of silicic acid, H 2 SiO 3 . QUESTIONS 1. What sort of a substance is O? In what does it occur? How common is O ? What two important uses does it serve ? 2. Describe the element H. Why is H used in balloons ? What chemical property makes it a very important element ? In what substances does it occur? 3. In what does N occur? Is it an active element? What purpose does it serve in the air? 4. Name substances that are nearly pure carbon. How are these substances made ? In what sorts of matter is C very impor- tant? Name two different crystalline forms of C. 5. Describe the element S. Where does it occur in nature, and in what form ? What important compounds contain S ? For what is free S used? 6. For what chemical property is P a valuable element? Ex- plain its use in matches. Why cut P under water? Why not breathe its fumes? 7. Describe chlorine. What important compounds does it form ? What uses are made of the element ? 202 COMMON SUBSTANCES 8. In what form does iron occur in the earth ? What is the symbol for iron ? How is iron obtained from its ores ? What is pig iron ? How is steel made ? 9. Name salts of Na that are important ; also salts of K. Explain the action of these elements upon water. 10. Name important salts of Ca that occur in the earth, and give common names for each. What is lime ? For what is lime used ? 11. Describe the element Hg. Name some of its uses. Name two of its common salts, stating the use of each. 12. In what form does each of these metals occur : Cu, Ag, Au, Pb, Sn, Zn ? Name uses of each. 13. Describe the element Al. In what forms does it occur? What familiar compounds does it form? 14. In what form does Si largely occur? What is its most common compound? What is a silicate? SECTION II COMPOUNDS 235. Water. Water is a compound of hydrogen and oxygen; its molecule contains two atoms of H and one atom of O, its symbol therefore being H 2 O. Water is a most important compound, occurring not only in rivers, lakes, and oceans, but in the earth and the atmosphere (as vapor). Water is evaporated from the ocean, etc., and taken into the air as vapor; here it is condensed and falls to earth as rain. Some of this water sinks into the earth, flows along on hard rock as underground streams, and later comes to the surface again through springs or wells. Mineral waters are found in streams that dissolve some of the rock through which they flow. Rivers carry much material from the surface of the land to the ocean ; this has been going on for so long that COMPOUNDS 203 the ocean water contains nearly 3^ of mineral salts in solution. A large part of this dissolved matter is com- mon salt (NaCl) ; the limestone (CaCO 3 ) in ocean water furnishes material for the shells of many small sea animals. In chemistry H 2 O is of great value. It dissolves more different things than any other liquid, and as it is a neutral compound (not acting chemically upon the substance dis- solved in it), water is a very useful solvent. To plant life H 2 O is of first importance. Plants absorb large quantities of water through their roots ; in their leaves and bark some of it is FlG - 134 combined with carbon, making the great bulk of the solid matter. Water is hardly less important in animal life. It is present in nearly all foods, and all parts of the animal body contain a great deal of it. Experiment 128. Arrange apparatus for making H, as in Experiment 121. Heat the glass tube and draw it out to a small opening (Fig. 134). Into the test tube put Zn and HC1 to make H. When the gas has flowed a few seconds, col- lect some in a small test tube (by hold- ing the tube mouth downward over the end of the glass de- livery tube); touch a lighted match to the FIG. 135 , . ... gas collected in this small test tube. If a slight explosion occurs, wait a moment and then repeat ; if the gas only burns quietly, then light the gas escaping from the delivery tube. This gas is H, now burning in air (containing O). 204 COMMON SUBSTANCES At once hold a cool glass beaker or tumbler over the flame (Fig. 135) and note the condensing of water vapor upon it. This water vapor is given off when the H unites with the O, H 2 + = H 2 0. 236. Sulphuric Acid. Sulphuric acid, H 2 SO 4 , is made in great quantities by the union of the elements of SO 2 , H 2 O, and O. Being a very strong acid, it is used to break up the salts of many other acids, setting those acids free. In this way HC1 is made from NaCl, HNO 3 from NaNO 3 , etc. H 2 SO 4 forms with different metals an important group of salts called sulphates. 237. Carbon Dioxide. Whenever carbon is burned in a good supply of air, a gas called carbon dioxide (CO 2 ) is formed. CO 2 is a colorless, odorless gas ; it is heavier than air, and is sometimes called carbonic acid gas. In the earth CO 2 occurs widely in carbonates, chiefly as CaCOg (limestone, marble, etc.). It is given off, in its free gaseous state, from burning wood, coal, kerosene, illuminating gas, etc. ; also from the lungs of animals, mixed with the air breathed out. Carbon dioxide occurs (in a very small quantity) in the atmosphere, where it forms an important part of the food of plants. The gas which causes the " lightness " of bread and cakes is gen- erally CO 2 , and the same gas causes the effervescence of soda water and bottled tonics. Experiment 129. Into a large test tube put a few bits of mar- ble, and add HC1. Stop the test tube, running a delivery tube to the bottom of a loosely covered jar, as in Experiment 125. When the gas has flowed freely for two or three minutes remove the tube from the jar, carefully covering the latter. In this way fill two jars. The gas is CO 2 . CaCO 3 + 2 HC1 = CaCl 2 + H 2 O + CO 2 . COMPOUNDS 205 Experiment 130. Carefully balance a thin glass beaker on a delicate set of scales. The beaker of course contains air. Now pour the CO 2 from one jar into the beaker, as in Fig. 136. If the balance is not changed, repeat the experiment carefully. Compare the weights of CO 2 and air. Experiment 131. Into the other jar of CO 2 thrust a lighted stick or taper, and note what happens. What does this show with regard to CO 2 ? Try to explain why the gas should behave in this way. Carbon dioxide is not a direct poison to animals, but because it does not supply the free O that they must breathe, animals cannot live in it. For the same reason it is injurious to man. Oil and gas heaters give out large quantities of CO 2 , using up the O from the air; they should not be used in rooms unless a constant supply of fresh air is possible. CO 2 neither burns nor supports combustion. When C is burned with a poor supply of air, another gas is formed, called carbon monoxide (CO). This gas is very poisonous to man, even in small amounts. It is often formed in coal fires, from which it may be given off ; hence the danger of sleeping in rooms with a coal fire. 238. Ammonia. Ammonia is a compound of N and H, its symbol being NH 3 . It is formed when certain animal matter decomposes in air, though it is generally FIG. 136 206 COMMON SUBSTANCES formed when coal is distilled in making illuminating gas. NH 3 is a gas, but it dissolves very easily in water. A solution of NH 3 in water forms NH 4 OH, ammonium hydrate. This is a strong alkali; somewhat weakened with more water, it is used as "household ammonia." Two salts of this base are common, NH 4 C1, famil- iarly called sal ammoniac, and NH 4 NO 3 , ammonium nitrate. A solution of sal ammoniac in water is used in many kinds of voltaic cells. 239. Cellulose. Cellulose may be described as the chief substance which makes up the structure of plants. It is found in many different forms, though its chemical composition does not change. All wood fiber, trunks of trees, their branches, roots, stems, veins of leaves, and parts of fruits are composed largely of cellulose ; also such fibers as cotton, flax, and hemp. The substance cellulose is a carbohydrate ( 219) having the symbol C 6 H 10 O 6 . It is formed by the activity of the plants, largely from the H 2 O and CO 2 that the soil and the atmosphere furnish. 240. Starch. Starch is a carbohydrate having the symbol C 6 H 10 O 5 . It is made by the action of plants, and is found throughout the vegetable world ; seeds of all sorts contain starch, and some plants store up large masses of it, as sago and tapioca. Starch is prepared in large quantities from corn and from potatoes. It forms an important food for man, both in its prepared state and as cereals, barley, oats, wheat, rye, rice, etc. In cold water, starch is usually not soluble ; but in hot water it partly dissolves, forming a paste. COMPOUNDS 207 Notice that starch has the same chemical composition as cellulose (C 6 H 10 O 5 ). The chief difference between the two compounds is that cellulose is the actual sub- stance of which the plant is composed, while starch is food stored by the plant for some future use. Thus seeds sprout, and the young plant grows for a short time by using the starch stored in the seed. The starch in a potato serves the same purpose. 241. Cane Sugar. Common sugar occurs in several vegetable substances. It is generally obtained from sugar cane or beets. The cane or beet is usually cut up and bruised under water, the sugar being dissolved out ; the solution of water and sugar is drawn off and boiled to a syrup. As this syrup cools some of the sugar forms in crystals ; these are dried and crushed to make granulated sugar. The liquid that remains is boiled over, and again cooled; the crystals that now form are called brown sugar. After boiling the liquid two or three times more, no crystals will form and the syrup is then called molasses. Cane sugar is a carbohydrate, its symbol being C 12 H 22 O n . Its uses are too well known. 242. Dextrose. Many fruits, such as grapes, plums, peaches, etc., owe their sweetness to another carbohy- drate, dextrose (C 6 II 12 O 6 ). This substance is sometimes called glucose, grape sugar, etc. It can be made from cane sugar and is made in large quantities from starch. Dextrose is about three fifths as sweet as cane sugar. In fruits it forms an easily digested food. Confectioners use a great deal of the dextrose that is made from starch. 208 COMMON SUBSTANCES 243. Alcohol. Common alcohol (C 2 H 5 OH) is formed when dextrose or grape sugar ferments ( 267). Hence it often appears when fruit juices are allowed to stand for some time. As a solvent, alcohol is much used in making varnishes, 'tinctures, perfumes, and drugs. It is useful in medicine because it stimulates the action of certain parts of the body; but a continual use of alcohol in any quantity is injurious. It burns with a hot, smokeless flame, being thus useful in several of the arts. 244. Fats and Oils. Fats and oils are salts formed by the union of glycerin with different acids. Fats are solid substances, and occur usually in animal matter. Oils are liquids ; they occur in both plant and animal matter. The acids that may unite with glycerin to form fats or oils are called fatty acids. 245. Soap. A soap is an alkali salt of a fatty acid. Soap may be made by boiling fats together with an alkali (NaOH or KOH). The fats break up into their acids and glyceryl, the metal of the alkali uniting with the acid to form soap, and glycerin being given off. Thus soap is a salt of the metal Na or K with an acid obtained from a fat. Soap acts upon the oily matter that is mixed with dirt on the skin or on fabrics ; the oil is broken up into tiny particles that may be washed away by water, carry- ing the dirt with them. Water alone could not do this, because fats and oils do not dissolve in it. Many kinds of soap contain much free alkali ; this renders them effect- ive for cleansing, but often injurious to certain fabrics. COMPOUNDS 209 QUESTIONS 1. Of what is water composed? What is its symbol? 2. What are mineral waters ? Why is ocean water salt ? 3. Of what use is water in chemistry? Why is water a valuable solvent? State what you can of the importance of water in living bodies. 4. What is the symbol for sulphuric acid? How and why is sulphuric acid used in obtaining other acids? What is a sulphate ? 5. How is carbon dioxide formed? Describe the compound. Name any common occurrence of carbon dioxide. Is CO 2 useful to plants ? Is it useful to animals ? 6. What is the objection to oil stoves and gas heaters in a room? Does CO 2 support combustion? 7. Under what conditions is carbon monoxide formed ? Why is it more dangerous than CO 2 ? State the danger from coal stoves. 8. Of what is ammonia composed? What is the substance that is commonly called " household ammonia " ? Name two salts of NH 4 OH. 9. What is cellulose? State its symbol. Name parts of plants that are composed largely of cellulose. 10. Of what is starch composed ? How is it made? Where is it found ? Of what use is starch in seeds ? Of what use in other parts of plants ? What is the use of starch to man ? 11. From what is cane sugar obtained and by what means? What is its symbol? W r hat is molasses? 12. Where does dextrose occur? What other names are given to it ? From what other compounds can it be made ? 13. State uses of common alcohol. How is it formed? 14. What are fats and oils ? Where is each found ? What is a fatty acid ? 15. What is a soap? Explain how soap is formed. Show the action of soap in cleansing things. Why does not water alone serve as well ? 210 COMMON SUBSTANCES SECTION III MIXTURES 246. Air. The air is a mixture of gases, the quan- tities of which may vary somewhat. Pure air usually contains about four fifths nitrogen and one fifth oxygen, besides a very small amount of carbon dioxide. In the atmosphere there is also more or less water vapor all the time, the quan- tity varying greatly at different times and places. These four substances, N, O, CO 2 , and H 2 O, are each of some important use in the air; but there are also other gases, such as H, Cl, H 2 S, NH 3 , etc., which mix with the atmosphere in very small amounts now and then. The quantity of such im- purities in the air is generally greater in cities, near fac- tories or chemical works, near marshes and swamps, in mines, etc. Air in the country or over the sea is usually more nearly pure, though no strict rule can be stated. Experiment 132 Float a cork on water in a large vessel. Place a bit of P on the cork and light it ; at once cover the cork with a large jar, as in Fig. 137, holding the mouth of the jar under water all the time. Allow the P to burn as long as it will, FIG. 137 MIXTURES 211 carefully holding the jar. As the P burns (combining with O), the oxygen that was in the jar is used up, leaving the nitrogen nearly pure. The O combines with the P, the compound then being dissolved in the water. Thus the air in the jar loses from its total volume the volume of the O that was in it at first, and the water rises into the jar to take its place. Compare the volume of water that so rises with that of the gas still left in the jar. Of what is this gas largely composed now ? Experiment 133. Carefully cover the jar under water ; then lift it out and set it right side up on the table. Carefully uncover, at once thrusting a lighted splinter into the jar which is now filled with N. Does N support combustion ? The use of O in the atmosphere is to help support animal life, and to support combustion. Nitrogen, being very inactive, serves to check too strong an action of O ; in an atmosphere of pure O fires would burn beyond control and animals could not live. The use of CO 2 in the air is to supply to plants the C that they need in making starch and cellulose. Water vapor in the air serves to temper the climate of some places, and is very important in furnishing rain. Without evaporation and rainfall the soil would everywhere become dry and the water would slowly drain from the land into the ocean. 247. Soil. The earth is thought to be composed of solid rock. A great deal of its surface is covered with a layer of loose earthy matter called soil, which varies in thickness from an inch or two to several hundred feet in some places. On the average the soil is but a few feet deep. Soil is made of tiny particles of rock that have been worn off from the solid rock mass in different ways. As the kinds of rock that have been thus worn are many, so we find many different kinds of 212 COMMON SUBSTANCES soils. In many places decayed plant and animal sub- stances have mixed with the soil, making more or less change in its composition. The soil is very necessary to plant life. It allows the roots a good support ; it holds moisture which the plant may slowly absorb ; and it supplies small amounts of mineral matter, which is dissolved by the water and so taken into the plant structure. 248. Earthenware and Porcelain. Clay is a kind of soil that contains a large quantity of aluminium silicate ( 234). Clay may be moistened slightly and then molded into different shapes. If it is then baked in a furnace for some time the silicate becomes hard so that the vessel will keep its shape. In this way brick and vessels of earthenware (Fig. 138) are made. Sometimes the aluminium silicate is found pure. If this be treated similarly to the clay, a finer grade of ware will be made ; this is called porcelain. 249. Glass. Grlass is a mixture of silicates of two or more metals, usually Ca, Na, K, Pb, Al, or Fe. It is MIXTURES 213 formed by heating white sand (silica, SiO 2 ) together with a compound of each of the metals to be used. Sand alone will not melt, but when heated with these other compounds they all melt and unite together, forming the silicates. This heated mass must then be cooled slowly, to make the glass as tough as possible. The kind of glass formed, depends upon what metals are used. Window glass is a silicate of Ca and Na; bottle glass is a silicate of Ca, Na, Al, and Fe ; glass used for lenses is a silicate of K and Pb. Very few sub- stances act upon glass ; air does not affect it, and liquids do not generally pass through it. For these reasons it is very useful as material from which to make bottles, jars, and other vessels. It is also one of the very few transparent solids that are common. Its importance is great. 250. Wood. In general, wood is made up mostly of cellulose ; the chief elements that it contains are therefore C, H, and O. Mixed with the cellulose are usually small quanti- ties of mineral salts ; these are left as ashes after wood is burned. Living wood always contains some water ; and many kinds of FlG- 139 wood contain other substances, such as oils, acids, pitch, resin, balsams, etc. The cellulose in wood usually occurs in the form of fibers. In a cornstalk the fibers may be seen singly (Fig. 139), but they are more commonly grouped together in masses. The sap flows in spaces between the masses of fibers. 214 COMMON SUBSTANCES Experiment 134. Secure a stick of oak wood, cut for a stove. Split it lengthwise ; examine the freshly opened surface, using a microscope if possible. Can you see fibers? Do you see groups of fibers ? Examine the end of the stick, looking for masses of fibers. Do you see the openings '' ~^ ; > for the sap ? Experiment 135 Put some bits of dry wood into a test tube and apply heat (Fig. 140). When the wood is thoroughly black, cease to heat it. Examine the remains. What is left in the tube and what has gone from the wood ? FIG. 140 251. Cotton Cloth. Sev- eral plants cotton, flax, and hemp produce growths of fine fibers that may be easily separated. These fibers are twisted together to form rope, twine, and small threads. Threads of cotton or linen are woven together to form different sorts of cotton and linen fabrics. Plant fibers of this sort are the same in chemical structure as the fibers of wood ; they are therefore cellulose. 252. Paper. Paper is a mass of fibers very firmly pressed together. The fibers are obtained from cotton or linen rags and from wood pulp (a mass of short separated wood fibers). To make paper, the rags are cleansed and then torn and shredded into tiny fibers by machines. Water is then added to the mass of fibers, so that the mixture easily flows. This mixture is poured out upon a flat surface, made so that the water may partly drain out, leaving MIXTURES 215 the fibrous mass spread evenly. An endless strip of cloth now picks up this fibrous mass, carrying it through several rollers. After passing through these, the paper is strong enough to go on through several more rollers without the cloth. The rollers are heated, and they press the fiber so firmly together that the mass becomes paper. Wood pulp is often mixed with the fibers from rags ; some cheaper papers are made largely of wood pulp. Since it is made from wood and cloth fibers, paper also must be mostly cellulose. Experiment 136 . Into one crucible put a mass of cotton threads, and into another some bits of paper. Cover each sub- stance with a little dry sand, heat for a few minutes, and examine. 253. Coal. Millions of years before man appeared on earth plants grew upon its surface. In some places large masses of trees, leaves, ferns, and other plant forms were piled up as they died, and were later cov- ered by layers of soil and rock. These slowly decomposed, much of the gaseous part of the wood passing off, but the carbon remain- ing. In time these masses became hard, and to-day we find them in the earth as coal. Thus we see FIG. 141 that coal contains the same elements that are found in wood, but the gaseous elements are much less in quantity and the C largely remains. Anthracite contains more carbon and less of other elements than soft coal. Some anthracite is over nine tenths carbon ; it is the hard kind, such as is burned in 216 COMMON SUBSTANCES stoves (see Fig. 141). Bituminous coal is a softer vari- ety. It contains more gases, burns at a lower tempera- ture, and shows much more flame while burning than does hard coal. 254. Illuminating Gas. If soft coal (bituminous) be heated to a high degree without any supply of air, the coal will be decomposed, its elements combining and mixing with each other to form new substances. The solid substance that remains IB is nearly pure C ; it is called coke. The liq- uids unite in a mixed mass called coal tar. The gases that are given off ar e first passed through water, which dissolves the ammonia (NH 3 ) and thus removes it. The gases which remain in the mixture form illuminating gas. This contains some free hydrogen and some compounds of hydrogen and carbon. So we see that illuminating gas contains largely the elements C and H, both of which burn in air. The process is carried on in gas works. Coal is put into large iron retorts (a, Fig. 142) and heated by a FIG ' 142 MIXTURES 217 fire, b, placed underneath. Note that the coal itself is not burned but only heated without air until it is decomposed. 255. Petroleum. Petroleum is an oily liquid found in the earth in some places. Pennsylvania and Texas have large oil fields. Petroleum contains many hydro- carbons ( 218). Among the useful mixtures obtained from it are kerosene, benzine, gasoline, naphtha, and paraffin. Candles are commonly made from paraffin. Note that each of these substances contains largely the elements C and H. 256. Coal Tar. In the process of making illuminat- ing gas from coal, a thick black liquid called coal tar is formed ( 254). This liquid has been found to contain a great number of compounds, so that coal tar is now the source of many common and important substances. Among these we may mention phenol, or carbolic acid ; saccharine, a substance that is far sweeter than sugar ; aniline dyes of many shades ; and various essences and perfumes. The compounds found in coal tar contain chiefly the elements C, H, 0, and N. 257. Explosives. Gunpowder and other explosives are mixtures of such substances as may easily and quickly act upon each other so as to produce a large volume of gas. Explosives are generally either solids or liquids. Under a slight impulse (a spark or a sudden jarring) they quickly form into gases. These gases nat- urally take up far more room than the solid or liquid mass, and in expanding to their natural volume they exert great force. 218 COMMON SUBSTANCES G-unpowder is a mixture of saltpeter (KNO 3 ), carbon, and sulphur. Upon exploding, the gases N and CO 2 are set free. Gf-un cotton is a nitrate of cellulose chiefly. Glycerin also unites with HNO 3 , forming a nitrate known as nitroglycerin. Nitroglycerin is a pale yellow liquid, highly explosive. It is used in making dynamite and some other explo- sives. Note that these substances all contain some nitrogen; because N is so inactive it forms compounds that easily break up and set the N (gas) free. FIG. 143 Experiment 137. In a mortar mix 12 grams KNO 3 , 2 g. of C (charcoal), and \% g. of S. When thoroughly mixed, put a small amount on a piece of metal and touch it with a lighted match. Notice how it burns. The mixture is gunpowder. Why does it not explode with a loud report ? Try to burn some gunpowder by con- verging the sun's rays upon some one spot, as in Experiment 94 (see Fig. 143). 258. Foods. Animal bodies are made up of the same elements that compose matter in general, and only a few of these elements are present in any quantity. Since animals grow by taking in food, we can get a good idea of what elements are most needed by study- ing the foods used. Man is supplied with food that is largely of either animal or plant growth ; but since the animals eat either plants or other animals that may live MIXTURES 219 upon plant growths, we see that nearly all of our food comes from the soil in the first place. Man's foods may be divided into five general classes. First of all is water, which is needed in all parts of the body, and of which man uses a large amount. Next in quantity are the carbohydrates ( 219) composed of C, H, and O ; these supply energy and heat to the body. Proteids ( 220) contain C, H, O, and N ; they serve to build up muscle and other parts. Small quantities of fats serve to give energy to the system. Last of all are the salts, of which many occur in small quan- tities in other foods. The elements P, Cl, S, Ca, Na, K, Fe, etc., are taken on in slight amounts as salts. 259. Fuels. The substances commonly used as fuels have already come to our attention ; among them we recall wood, coal, illuminating gas, kerosene, gaso- line, naphtha, benzine, and alcohol. Other things less commonly used as fuels are paper, rags, straw, and peat (partly decomposed vegetable matter). In all these sub- stances note that the elements hydrogen and carbon are present ; both of these burn in air (i.e. combine with O). QUESTIONS 1. What four substances does air usually contain ? State the uses of each of these. In what proportion does the N and the O occur ? 2. Of what is the soil generally composed ? By what different means may it have been formed? Name any common uses of the soil. 3. How are earthenware vessels made ? Of what is porcelain made? 220 COMMON SUBSTANCES 4. What is glass? Of what substances is glass made? How are these compounds treated to form glass? Name as many uses of glass as you can. What two properties make it valuable? 5. Of what substance is wood largely composed ? What ele- ments are present in this substance? Name other things that occur in some woods. Of what are ashes formed ? 6. From what is cotton cloth made ? What then is the chemi- cal composition of cotton cloth, i.e. what elements are present? 7. Of what is paper made ? Describe the process of making paper. What elements does it contain ? 8. Of what is coal formed? How was it formed? What element constitutes the larger part of hard coal ? What other ele- ments are present? Distinguish anthracite and bituminous coal. 9. Describe the making of illuminating gas. What other substances are formed at the same time ? Of what elements is illuminating gas largely composed? 10. What is petroleum? What substances are obtained from it? 11. Name some important substances that are formed from coal tar. 12. What sort of a mixture is an explosive? Show how explosive mixtures may be used to exert great force. State the composition of gunpowder ; of gun cotton ; of nitroglycerin. 13. What are the five classes of foods used by man? Name some elements that are common in the body. Of what use are carbohydrates in the system ? 14. Name some common fuels. What two elements do they all contain ? CHAPTER X COMMON CHEMICAL PROCESSES 260. Combustion. Combustion is a chemical union which takes place rapidly, giving off light and heat. The word fire is commonly used instead of combustion. Two things are necessary in order that combustion may take place a substance to burn (called a combustible) and a substance with which it may unite. The latter sub- stance is said to support the combustion. We have learned that the things commonly burned as fuels con- tain the elements C and H ( 259) ; also that the great supporter of combustion in the air is O (222). With these facts in mind, it will be seen that the most com- mon fires are simply the rapid union of carbon and hydrogen with oxygen. The compounds formed by this union will be carbon dioxide (CO 2 ) and water vapor (H,0). Now it is well known that in order to make any substance burn, heat must be applied. In other words, oxygen will not easily unite with other substances unless their temperature be raised. The temperature at which different substances will burn in air varies greatly ; carbon, for example, needs a greater degree of heat than hydrogen, while matches containing phos- phorus may be sufficiently heated by simply rubbing them. When we wish to start a fire, however, we do not heat the whole mass that is to be burned, but only 221 222 COMMON CHEMICAL PROCESSES a small portion of it. This part burns, giving off heat; thus the parts right around it become heated until they also burn; and in this way the whole mass is finally heated and burned. We have seen that in order for any substance to burn in air, it must be heated and constantly supplied with oxygen. Clearly, then, a fire may be stopped by cooling the burning mass or by cutting off the supply of oxygen (or air). Water is commonly applied, and it serves both purposes ; but water is not always the best thing to use. Chemical fire extinguishers are of value when the fire is small ; they are usu- ally devices for mak- ing a large amount of CO 2 on the spot and CO 2 does not sup- port combustion. The FIG. 144 ,J[1L^ mos t effective way to stop a fire when first started, is to cover it closely with rugs, clothing, earth, flour, or any solid which does not easily burn; in this way the air is kept away and the fire is " smothered." Experiment 138. Using a long or circular oil burner, turn the wick up just above the metal and light it at one point. Note the creeping of the flame along the wick as each part is heated from the burning portions. Experiment 139. Try to set fire to small quantities each of wood, alcohol, charcoal, sulphur, kerosene, phosphorus, hydrogen, soft coal, etc. (To use alcohol and kerosene, pour a few drops on a flat piece of wick.) Roughly compare the temperatures at which these substances burn. Other things could be used. Be careful with phosphorus, alcohol, kerosene, etc. EXPLOSION 223 FIG. 145 Experiment 140. Into a large test tube fit a bent delivery tube (of small size), as in Fig. 144. Put bits of marble into the test tube and pour upon them strong HC1, so as to make a good flow of CO 2 (Experiment 129). Direct this stream of CO 2 gas upon a candle flame or a small fire made of chips. Note the effect, and explain. Experiment 141. Cut holes in a piece of cardboard and fit it into a small glass chimney. Stick a lighted candle on the card (Fig. 145). Now cover the chimney tightly at the top for a moment. Light the candle and set the chimney upon some flat surface that will close it at the bottom. Explain. 261. Explosion. An explosion is a sort of combustion that takes place very rap- idly in a confined space. Two or more substances that may easily unite are mixed together ; a mere spark at some point in the mixture will often cause action throughout the whole mass in a moment. If the mixture is confined in a small space, the gases that are formed by the chemical action will have so much larger natural volume that they will expand and burst the walls that confined them. 262. Flames. A burning gas gives rise to a flame; burning solids usually glow and are luminous ( 126), but without flame. When a solid substance, such as wood, burns with a flame, it is because the substance is being decomposed by the heat, and the gases that are given off cause the flames. Experiment 142. Make some H as in Experiment 121, using care in lighting the gas. The flame is usually somewhat colored by solid particles from the heated glass tube ; but if the end be 224 COMMON CHEMICAL PROCESSES fairly large or protected by a piece of platinum (Fig. 146), it may be possible to show that H burns with a colorless flame. Experiment 143. Pour a little alcohol over a few bits of charcoal (carbon) about the size of marbles. Pile these A up on a glass or metal plate and light the mass. When I I the charcoal is well kindled, JJJ note that it glows and gives <.! _.N.ig-. f: ^^^ off light but no flame. What is being formed? Why is there no flame? Do not try this without the teacher's help. The substances commonly burned to furnish light illuminating gas, kerosene, gasoline, and paraffin can- dles are made up mostly of the gas hydrogen and the solid carbon. Upon being lighted, the H burns and furnishes the flame, while the C in tiny particles becomes heated in this flame and glows, so that the whole gives off light. When a lamp "smokes " it is because the oil is being decomposed and the carbon particles given off faster than they can be heated and burned in the flame. In any case, smoke is made up of particles of matter that were not consumed in the flame. Experiment 144. Light a candle and trim it to give a good flame. Hold a piece of earth- FlG - 147 enware in this flame as in Fig. 147 ; note the de- posit of soot. Of what is it composed? The solid object cools the flame so that it does not consume all of the C that is given off from the wick. 263. Fire. As commonly used, the word fire may easily give us a wrong idea of its meaning. A fire is OXIDATION 225 only a process of combustion, and the word fire means all that combustion means, the chemical union of dif- ferent elements, together with the heat, flame, light, etc., that may occur in the process. A little thought will make the matter clear. The chemical action in com- mon fires is between the elements H or C and O ; the heat is given off as from any chemical union ( 206) ; flames show that a gas (usually H) is burning ; light is given off from glowing solid particles (commonly of C) ; smoke is a mass of solid particles that were not entirely burned; and ashes are made up of mineral matter that could not burn. 264. Oxidation. We have learned that oxygen com- bines directly with many elements ( 222), and that it does this rapidly if they be heated to a high enough degree ( 260). Now it also happens that several ele- ments will combine with oxygen even at the ordinary temperature of the air, but they do this very slowly. The process is called oxidation; the compound formed is called an oxide. Experiment 145. File a piece of iron till bright ; dip it in water, remove it, and without even shaking off the drops of water, set it aside. In two or three days examine it, and tell what has happened. Experiment 146. Scrape a piece of lead till its surface is bright and clean, then set it aside. In a few days examine the lead, note its surface, and explain the change. Most of the metals will combine directly with O ; gold does not, and silver forms a sulphide rather than an oxide in air. The presence of water usually assists 226 COMMON CHEMICAL PROCESSES oxidation. Iron rust, the most common of metallic oxides, is formed by the union of iron with oxygen, but this is always greatly helped by water, even if only the moisture in the air. 265. Oxidation in Animal Bodies We breathe air into the lungs for the oxygen that it contains. Carbon is taken into the body in the food that is eaten (258), and is found all over the system. In the lungs O is separated from the air and is carried by the blood to all parts of the body. There it unites with the C which is already in those parts, and forms CO 2 . This chemical union of the C with O gives off energy, as does any chemical union ; the energy is used by the body, partly as heat to keep us warm, and partly as muscular energy so that all parts may move and do their work. The CO 2 that is formed is carried to the lungs, where it leaves the body in the air that is breathed out. Plants take air into their leaves, separate the C from the O of the CO 2 , use the carbon in making starch, and give out pure oxygen to the air again. 266. Decay. Many substances, particularly of plant and animal matter, will decay after a time unless cared for in some special way. The signs of decay are many : the body is usually soft and easily crumbles ;, it is gen- erally much smaller in size than before ; and often an odor is given off. The smaller size is due to the fact that a large proportion of any animal or plant matter is of gaseous elements ; these of course pass off when they are set free by decay. The odor is caused by gases that are formed ; one of the most common of these gases is FERMENTATION 227 hydrogen sulphide (H 2 S), a compound that is found in large amounts in eggs that have lost their usefulness. Decay is a process of decomposition which goes on slowly and quietly. Its causes are not well understood in all cases, but it is thought to be sometimes due to very tiny vegetable forms called bac- teria. These tiny bodies are too small to be seen without a powerful micro- scope ; they are common in the air, the soil, and in water, as well as in various other substances. Heat gen- erally kills them, and most kinds seem to work best in a good supply of oxygen. Fruits are often put up in jars while hot and at once covered tightly; in this way they may be kept for a long time without decaying. Fig. 148 shows several bacteria, greatly magnified. There are of course other causes of decay. 267. Fermentation. If apple juice is allowed to stand for a time, we know that alcohol may form in it and the juice becomes cider; similarly, grape juice may become wine, containing alcohol. Clearly a chem- ical change goes on in the liquid, and this change is called fermentation. It is caused by something that is present in the fruit juice, or that gets into it from the air. These things that may cause fermentation are called ferments. Many different ferments are known, and they act upon many substances. One of the most common of ferments is yeast; it acts upon dextrose or grape sugar 228 COMMON CHEMICAL PROCESSES ( 242), breaking up the dextrose into carbon dioxide and alcohol. Since dextrose occurs widely in fruits, this sort of fermentation is very common. All sorts of alcoholic stimulants wines, whisky, etc. are made by allowing something to become fermented. Yeast is a very low form of plant ; in its nature it is somewhat like bac- teria. Fig. 149 shows a few bits of yeast, greatly magnified. It is found frequently in the air, from which it easily gets into many substances. Other sorts of fermentation are common. Apple juice partly ferments and becomes cider; then another fer- ment acts upon the alcohol in the cider, changing that to an acid, so that the liquid becomes vinegar. The sour- ing of milk is a process of fermentation. 268. Bread Making. Yeast may be easily made to grow till it forms a large mass. This is commonly done by many cooks, who make what they call potato yeast ; in this case the potato serves as a substance in which the yeast plant may grow. Cakes of compressed yeast may be bought of the grocer ; these are masses which have been grown for the purpose in large quantities. Bread is made of flour mixed with milk or water to form dough ; yeast is added to " raise " the dough. Flour is largely starch. When the mass is put in a warm place the yeast acts upon the starch, changing it to dextrose; this is further acted upon, so that it ferments, forming alcohol and CO 2 . The CO 2 cannot DISINFECTION 229 escape through the dough, so it simply forms in bub- bles, making the mass " light." In baking, the alcohol is mostly driven out and the heat stops any further action of the yeast. 269. Disinfection. The bacteria of which we have studied are very numerous ; there are also many differ- ent kinds. They are too small to be seen without a strong microscope, except in masses composed of many. Some kinds of bacteria are harmless and some are even useful, but a few kinds are known to be the cause of certain diseases in animals and man. These kinds are usually given off in some quantity from persons who are ill with such diseases ; and as they may be taken into the bodies of other persons and there cause illness, it is important to try to kill the "germs." The killing of these bacteria is called disinfection. Many methods are used. Heat is of great use, as a tem- perature of 100 C (boiling water) will destroy all com- mon forms in a short time. All dishes and cloths used by the patient should be carefully boiled in water, and papers should be burned. Fresh air in the sick room is important, and sunlight kills many bacteria. For a liquid disinfectant, weak solutions of carbolic acid or of some chlorides are good, but nothing seems to equal a weak solution of corrosive sublimate in water (1 part in 1000). After all, the best way to avoid diseases is to keep ourselves clean and keep our general health at the highest. Many disease germs doubtless enter the body of a well person and do no harm because of his strong, healthy condition. 230 COMMON CHEMICAL PROCESSES QUESTIONS 1. Define combustion. What is a combustible? In common fuels, what elements usually burn ? What substance commonly supports combustion ? Name the compounds generally formed by combustion of fuels in air. 2. How, in general, may we start combustion? How, after being started, does the process keep itself going on ? By what means may combustion be stopped ? 3. What is an explosion ? Why is an exploding mixture able to exert so great force ? 4. What sort of substances burn with a flame ? How do solids burn ? Show why flames are seen when some solids (e.g. wood) are burned. Explain the burning of such substances (e.g. kero- sene, candles, etc.) as furnish light. What is smoke ? 5. What is meant by the word fire ? In common fires explain each of these : the heat, light, flame, smoke, and ashes. 6. Explain the meaning of oxidation. What substances com- monly form oxides in this way? What is iron rust? Under what conditions is it usually formed? 7. What element in the air is needed by animals ? What becomes of this element when it is taken into the lungs ? With what does it unite ? Where ? What does this process supply to the body ? 8. What is meant by decay ? What sorts of substances usually suffer decay ? 9. What is a ferment ? Name a common ferment. When a ferment acts upon dextrose what is formed? How is vinegar made? 10. What sort of a substance is yeast ? Explain the action of yeast in bread making. What gas is formed, and what is its use ? 11. Explain how disease may be given from one person to another. What is meant by disinfection? What methods are useful ? INDEX [The references are to pages.] Absolute cold, 86. Absorption of light waves, 111, 127, 128. Acids, 183, 184 ; fatty, 208. Adhesion, 12. Air, composition of, 210 ; com- pressed, 43; dome, 39; lique- fied, 86 ; pump, 42. Alcohol, 190, 208. Alkalis, 184, 190, 208. Alloys, 188. Alternating current, 158. Alum, 201. Aluminium, 201. Amalgam, 190. Ammonia, 85, 205. Ampere, 146. Aniline dyes, 217. Annealing, 13. Anthracite, 215. Arc, electric, 168 ; lamp, 168 ; of pendulum, 58. Armature, of dynamo, 156 ; of motor, 162. Artificial, cold, 84 ; ice, 85. Atmospheric pressure, 32, 33 ; effects of, 34, 36-39. Atom, 176. Atomic theory, 176. Bacteria, 227, 229. Balloons, 44, 194. Barometer, 36. Bases, 183, 184. Battery, 145 ; uses of current, 146. Bell, electric, 166. Bell, metal, 188. Brass, 188. Bread making, 228. Brick, 212. Brittleness, 13. Bronze, 188. Buoyancy, 26, 27 ; in gases, 44. Calcium, 199; compounds, 199. Camera, 122. Cane sugar, 207. Capillarity, 17. Carbohydrates, 189, 219. Carbon, 195, 215, 226; com- pounds, 189, 204 ; dioxide, 195, 204, 210 ; monoxide, 205. Cars, electric, 163. Cells, dry, 143; gravity, 143; . kinds of, 142 ; voltaic, 140, 141. Cellulose, 189, 206, 207, 213. Center, of gravity, 61, 52; of mass, 52. Centigrade thermometer, 70. Centrifugal force, 55. Centripetal force, 55. Charges, electric, 133-137. 231 232 INDEX Chemical, action, 67, 177, 179; affinity, 176, 177 ; changes, 108, 171. Chemistry, scope of, 171. Chlorides, 186, 197. Chlorine, 197. Circuit, 143; divided, 145. Cloth, 214. Clouds, 76. Coal, 195, 215, 216; tar, 216, 217. Cohesion, 11. Coke, 216. Cold, 69; absolute, 86; by vap- orizing, 85 ; by melting, 85. Color, explanation of, 124 ; of light waves, 127; of objects, 127. Combination, chemical, 177, 178, 180. Combustion, 68, 179, 193, 195, 221. Commutator, 158, 163. Compass, 153, 154. Composition, chemical, 172; of matter, 5, 8, 9. Compounds, 173, 174, 180, 202. Compressed air, 43 ; engine, 43. Compressibility, 15. Compression, as a source of heat, 67 ; of gases, 43. Condensation, 75. Condenser, 76. Conduction of heat, 79, 80. Conductors, of electricity, 130, 134; of heat, 80. Contraction, 72, 73. Convection, 79, 80, 81. Copper, 200. Cotton, 214. Coulomb, 146. Crystallization, 16. Current, alternating, 158 ; direct, 158; electric, 140; induced, 155; strength, 146 ; uses of, 146. Darkness, 112. Decay, 226. Decomposition, 172, 178, 227. Dextrose, 207, 227. Diffusion, 6. Dipping needle, 153. Discharge, 137. Disinfection, 229. Distillation, 76. Divided circuit, 145. Ductility, 14. Dynamite, 218. Dynamo, 130, 140, 156 ; currents, 157, 158; kinds of, 158. Ear, 93, 95 ; trumpets, 103. Earthenware, 212. Echoes, 97, 98. Elasticity, 15. Electric, cars, 163 ; current, 140 ; discharge, 137 ; lights, 167 ; mo- tors, 162, 163. Electrical, effects, 131 ; energy, 129, 130, 162; potential, 131, 132, 138. Electricity, 129 ; charges of, 133, 134, 135, 136; generation of, 140, 155 ; static, 135. Electrolysis, 131. Electrolytic effect, 131, 166. Electro-magnet, 149, 158. Electro-motive force, 132; in- duced, 155; unit of, 146. Electroplating, 166. Electrostatics, 133, 135. INDEX 233 Elements, chemical, 173, 183; symbols of, 182. Emery, 201. Energy, 68, 69; definition of, 3; forms of, 5 ; from heat, 89 ; ra- diant, 83; transformation of, 86, 87. Engine, compressed-air, 43 ; gaso- line, 89; naphtha, 89; steam, 88. Equilibrium, 54. Essences, 190, 217. Ether, 82, 83, 110, 169. Evaporation, 75. Exciter for dynamo, 158. Expansion, 72, 73; of gases, 41. Explosion, 223. Explosives, 217. Eye, 121. Eyeglasses, 122. Fahrenheit thermometer, 70. Falling bodies, 56, 57. Fats, 208, 219. Fatty acids, 208. Fermentation, 227. Fire, 221, 224; engine, 39; ex- tinguisher, 222. Flames, 223. Floating bodies, 28 ; law of, 28. Fluids, 23; pressure in, 24, 25, 30, 32. Focus, of lenses, 120, 122 ; of mir- rors, 115. Fog, 76. Foods, 218. Foot pound, 60. Force, 4 ; pump, 38. Forced vibration, 98, 99. Friction as a source of heat, 67. Fuels, 219. Fulcrum, 63. Fusion, 74. Gas, 6, 8; compression of, 15; expansion of, 41 ; illuminating, 216. Gasoline engine, 89. Gear wheels, 65. German silver, 188. Glass, 212. Glucose, 207. Gold, 9, 13, 200. Gravitation, 19 ; law of, 20. Gravity, 19, 20; cell, 143; center of, 52 ; specific, 21, 29. Gun cotton, 218. Gun metal, 188. Gunpowder, 217, 218. Gypsum, 199. Hardness, 12, 13. Harmony, 105. . . Heat, 67; effects of, 71; energy, 69; latent, 77; mechanical energy from, 87 ; sources of, 67 ; theory of, 68 ; transfer of, 79. Horse power, 146. Hydraulic, jack, 31 ; press, 30. Hydrocarbons, 188, 217. Hydrochloric acid, 197. Hydrogen, 44, 183, 188, 189, 194, 217, 219, 224. Hydrometer, 29. Ice, artificial, 85. Illuminating gas, 216. Illumination, 109; intensity of, 115. Images, formed by a convex lens, 121, 234 INDEX Impenetrability, 10. Incandescent lamp, 167. Inclined plane, 65. Induction, coil, 160 ; coil, use of, 161; electrostatic, 136; mag- netic, 160. Inertia, 17, 47, 50. Insulators, 130, 134. Iron, 198 ; rust, 226. Kerosene, 188, 194, 217. Lamp, arc, 168; incandescent, 167. Latent heat, 77. Law, natural, 3 ; of floating bod- ies, 28 ; of gravitation, 20 ; of machines, 62 ; of magnets, 152, 153 ; of motion, 46 ; of reflec- tion, 113. Lead, 200. Lens, 120, 121, 122 ; effects of, 121, 122 ; focus of, 120 ; glass, 213. Lever, 61, 63. Lifting pump, 37. Light, 108 ; rays, 109. Lights, electric, 167. Light waves, 108, 109, 110; color of, 124 ; reflection of, 112, 114 ; refraction of, 117, 119 ; velocity of, 110. Lightning, 138. Lime, 199. Line of direction, 53. Lines of magnetic force, 148, 155, 160. Liquefied air, 86. Liquefying, 74. Liquid, 6, 7 ; level, 25 ; pressure, 24, 25, 30. Locomotive, air, 43; steam, 89. Loudness, 102. Luminosity, 109. Machines, 60, 61, 64; law of, 62. Magnet, 147 ; electro-, 149 ; law of, 152 ; permanent, 151 ; poles of, 151. Magnetic, action, 147, 152 ; effect, 131, 147 ; field, 148 ; force, 147 ; induction, 160 ; lines of force, 148 ; poles, 151 ; poles of the earth, 153. Magnetism of the earth, 153. Malleability, 13. Mass, 51 ; center of, 52. Matches, 197. Matter, 2, 4 ; composition of, 8 ; properties of, 10 ; states of, 5, 6,7. Measurements, electrical, 146. Mechanical uses of heat energy, 89. Megaphone, 100. Melting, 74. Mercury, 9, 200 ; compounds, 200. Mercury air pump, 42. Metals, 80, 130, 184, 185, 188, 200, 225. Microscope, 122. Mirror, 114. Mixtures, 173, 174, 180, 210. Molasses, 207. Molecular theory, 9. Molecule, 8, 176, 180; vibration of, 68. Momentum, 50. Motion, laws of, 46, 47, 48 ; wave, 91. INDEX 235 Motor, electric, 162, 163. Musical, instruments, 100 ; tones, 104, 105. Naphtha engine, 89. Newton's laws of motion, 46. Nitrates, 186, 195. Nitric acid, 186. Nitrogen, 195, 210, 218. Nitroglycerin, 218. Noise, 102. Ohm, 146. Oils, 208. Opaque bodies, 111. Ores, 185, 188. Overtones, 105. Oxidation, 225, 226. Oxides, 187, 188, 225. Oxygen, 189, 192, 210, 225, 226. Paper, 214. Parallel arrangement, 145. Pendulum, 57, 58. Penumbra, 112. Percussion, 67. Permanent magnet, 151, 158. Petroleum, 188, 217. Pewter, 188. Phosphorus, 196. Photographic camera, 122. Physical changes, 171. Physics, scope of, 2. Physiological effects of electricity, 131. Pitch, 103 ; limiting, 104. Platinum, 14. Poles, in a cell, 142 ; magnetic, 151; of the earth, 153. Porcelain, 212. Pores, 9, 14. Porosity, 14. Potassium, 199; compounds, 199. Potential, electrical, 130, 132, 138. Power, 60; electrical, 146 ; horse, 60. Pressure, atmospheric, 32-39 ; ef- fect of, on boiling point, 74 ; fluid, 24 ; liquid, 24 ; transmis- sion of, 30. Prism, 119, 126. Prismatic colors, 126. Propeller, screw, 49. Proteids, 189, 219. Pulley, 61, 62. Pump, air, 42 ; force, 38 ; lifting, 37 ; steam, 39. Push button, 144. Quality of tones, 104, 106. Quartz, 201. Radiant energy, 83. Radiation, heat, 82, 83, 108 ; light, 108-110. Radicals, 183, 186. Rain, 76, 202. Rainbow, 127. Rays, light, 109. Reaction, 48, 55. Reflection, of light waves, 112, 113 ; of sound waves, 97, 98. Reflector, 114. Refraction, 117, 118; cause of, 119; use of, 120, 121. Reservoir, 26. Resistance, to electric current, 132, 144 ; unit of, 146 ; uses of, 145. Resonance, 99. Resonators, 99, 100. 236 INDEX Retina, 121. Reverberation, 98. Salts, 185, 186, 188, 219. Saturation, 76, 189. Screw, 49, 64. Series arrangement, 146. Shadows, 111. Shunts, 145. Sight, far, 122 ; near, 122. Silicon, 201. Silver, 200. Siphon, 39. Smoke, 7, 224. Soap, 208. Sodium, 199; compounds, 199. Soil, 211. Solidifying, 74. Solids, 5, 7. Solutions, 189. Solvents, 189, 190. Sound, definition of, 92, 93 ; loud- ness of, 102 ; musical, 105 ; ori- gin of, 94. Sound waves, 93, 164 ; reflection of, 97; transmission of, 97; velocity of, 97. Specific gravity, 21, 22, 29. Spectrum, 126, 127. Speech, 106. Stability, 54. - Starch, 189, 206, 207. Static electricity, 135. Steam, 7, 8; engine, 88; loco- motive, 89; turbine, 89. Steel, 198. Substances, kinds of, 172. Sugar, 16, 189 ; cane, 207 ; grape, 207. Sulphates, 186, 204. Sulphides, 187, 188. Sulphur, 187, 196. Sulphuric acid, 195, 204. Surface level, 25. Symbols, 181. Sympathetic vibration, 99. Telegraph, 165 ; wireless, 169. Telephone, 164. Telescope, 123. Temperature, 69. Tenacity, 14. Theory, atomic, 176; molecular, 9 ; of heat, 68. Thermal effect, 131, 167. Thermometer, 69, 70. Tin, 200. Tinctures, 190. Tones, 101 ; differences in, 102 ; loudness of, 102 ; musical, 102, 104, 105 ; pitch of, 103 ; quality of, 104. Transformation of energy, 86, 87. Transformer, 159. Translucent bodies, 111. Transmission, of fluid pressure, 30; of sound waves, 96. Transparency, 110. Turbine, steam, 89. Type metal, 188. Umbra, 112. Units of electrical measurement, 146. Vacuum, 33, 34. Vapor, 7. INDEX 237 Vaporization, 74. Velocity, 51 ; of light waves, 110 ; of sound waves, 97. Vibrating bodies, 94, 95. Vibration, 9, 91, 92, 94; forced, 98, 99; of the ether, 110; rate of, 92; sympathetic, 98, 99. Voice, 105. Volt, 146. Voltaic cell, 130, 140. Volume, 27 ; changes of, 72, 73. Water, composition of, 202 ; ex- pansion of, 73; mineral, 202; use of, 190; vapor, 210. Watt, 146. Wave, 91 ; length, 92, 103 ; light, 108, 109, 110; motion, 91; sound, 93. Weather changes, 36. Wedge, 65. Weight, 20. Welding, 12. Wheel and axle, 64. White light, 125, 126. Winds, 82. Wireless telegraphy, 169. Wood, 213. Work, 60 ; electrical, 146. Yeast, 227. Zinc, 147, 200 ; plate, 142. ANNOUNCEMENTS LESSONS IN PHYSICS By LOTHROP D. HIGGINS lamo, cloth, 379 pages, illustrated, 90 cents HIGGINS'S "Lessons in Physics" provides a thorough course in physics for schools which offer little or no laboratory work. Principles are explained by references to common or familiar phe- nomena rather than to set laboratory experiments. Commercial and industrial uses of the various principles are discussed in con- nection with the principles themselves. The author has taken great pains to secure simplicity and clear- ness of expression. The material is arranged to develop a logical succession of ideas, and technical words are in most cases defined when they first appear. For words which are not thus explained a glossary is appended to the text. SIMPLE EXPERIMENTS IN PHYSICS By LOTHROP D. HIGGINS lamo, cloth, 114 pages, illustrated, 35 cents /'"YVER three hundred simple exercises designed to provide a ^-J laboratory course for classes in elementary physics. By careful arrangement and by illustrating each successive step with a separate exercise, the manual is made to serve as an efficient guide to the study of principles by observation and thought. It may be used either for a course of study by experiment or to supplement a class-room course. 1 66 GINN AND COMPANY PUBLISHERS ELEMENTS OF GENERAL SCIENCE By OTIS W. CALDWELL, Head of the Department of Natural Science, and WILLIAM LEWIS EIKENBERRY, Instructor in the University High School, School of Education, The University of Chicago 8vo, cloth, xiv -f- 308 pages, illustrated, $1.00 THIS book presents a logically arranged and teachable first-year high-school course in general science, dealing with concrete scientific facts of everyday interest and worth-while significance. The material is drawn from the home, school, and community environment, and all the sciences contribute to the survey, each being used as needed. Unity is secured through the logical and progressive arrangement of the topics which make up the course. These fall under five main heads: I. The Air, II. Water and its Uses, III. Work and Energy, IV. The Earth's Crust, V. Life upon the Earth. Each topic is connected with that which follows, the last in one main division leading naturally to the first in the next division. The material has been chosen for its value in developing a scientific method of thinking, and in giving the young pupil a much- needed basis for later science work in high school. Laboratory work is intended to accompany the text, which suggests many simple but valuable experiments. The book is the outcome of six years' experiment with general science in the University High School, The University of Chicago. For the past four years the course has been given essentially as now published. Its plan and materials have in addition been submitted to many high-school teachers, and a variety of helpful suggestions thus secured. In short, the book has been made in the laboratory and the classroom and is admirably adapted to meet actual class needs. GINN AND COMPANY PUBLISHERS A STUDY OF FOODS By RUTH A. WARDALL, Head of Home Economics Department, State University of Iowa, and EDNA N. WHITE, Head of Home Economics Department, Ohio State University I2mo, cloth, 174 pages, illustrated, 70 cents THIS book, prepared to meet the needs for a textbook in domestic science, offers a teachable and worth-while treatment of the fundamentals of the subject. It discusses food materials and their values, and outlines laboratory exercises in cooking and in other forms of food preparation to follow each discussion. It lays much stress on the money value of food, as well as its nutritive value, and gives exercises in calculating the cost of raw materials and prepared dishes. Each subject is developed as a unit, with no effort to divide it into separate lessons, the nutrients, or food principles, form- ing the basis of the work. This treatment makes the book suitable for very general use. The course is flexible and equally well adapted to eighth- grade and beginning high-school classes, and to more advanced classes in the high school. Bibliographies at the end of each chapter permit of the expansion of the course to any desired length. The book has a further value in correlation with courses in chemistry, physiology, and physics, and will prove useful also for study clubs, as a guide to club work. '67 a GINN AND COMPANY PUBLISHERS ELEMENTARY APPLIED CHEMISTRY By LEWIS B. ALLYN, State Normal School, Westfield, Mass. , cloth, 127 pages, illustrated, 60 cents THIS book offers practical applications of chemistry to pres- ent-day civic and industrial problems. The course is essentially that conducted by the author in the Westfield State Normal School, where a pure-food campaign has been made one of the objects of the course. The widespread results shown in the almost complete elimination of the sale of impure foods in the city of Westfield are matters of national knowledge. Besides the work with food products, the book includes exercises with water, textile fabrics, drugs, soils, and similar materials. It gives a particularly complete analysis of each subject and brings out clearly the practical relation which chemistry bears to everyday matters. The order of presen- tation differs radically from that of the usual textbook in chemistry, the common elements, bases, and radicals being taken up as they naturally occur. For supplementary use with any regular textbook " Elemen- tary Applied Chemistry " is invaluable. Teachers of domestic science will find the chapter on food values most important, while for- boards of health, inspectors of milk, and all those inter- ested in the pure-food problem, the book has a special function in its valuable information and tests relating to their work. GINN AND COMPANY PUBLISHERS UNIVERSITY OF CALIFORNIA LIBRARY BERKELEY Return to desk from which borrowed. This book is DUE on the last date stamped below. MAR 24 1948 MAR 25 1948 80ct'48AP ^ **$& SEP 2 4 1953 UK LD 21-100m-9,'47(A5702sl6)476 REC'D it rx* YB 17564 3741 62 UNIVERSITY OF CALIFORNIA LIBRARY