LIBRARY 
 
 OF THE 
 
 UNIVERSITY OF CALIFORNIA. 
 
 GSFT OF 
 
 Class 
 
r 
 
ELEMENTARY PHYSICS 
 
ELEMEISTTABY 
 PHYSIOS 
 
 BY 
 
 FRANK WILLIAM MILLER, A.M. 
 
 AND 
 
 AUGUST FREDERIC FOERSTE, PH.D. 
 
 INSTRUCTORS IN PHYSICS, STEELE HIGH SCHOOL 
 DAYTON, O. 
 
 
 NEW YORK 
 
 CHARLES SCRIBNER'S SONS 
 1903 
 
N\S' 
 
 COPTBIGHT, 1903 S BY 
 
 FRANK WILLIAM MILLER 
 
 AND 
 
 AUGUST FREDERIC FOERSTB 
 
 , ..,...:- 
 ,. ::,:'; : 
 
 TROW DIRECTORY 
 
 PRINTING AND BOOKBINDING COMPANY 
 NEW YORK 
 
CA" 
 
 PREFACE 
 
 THE purpose of this book is to make the pupil ac- 
 quainted with the more elementary facts of physics 
 and physical chemistry, to give some idea of methods 
 of experimentation, to illustrate the drawing- of conclu- 
 sions from experiments and observations, and to show that 
 theories are merely attempts to explain, by means of cer- 
 tain suppositions, various phenomena whose existence is 
 unquestioned but whose nature can not be more satisfac- 
 torily explained by other means. 
 
 In order to secure these results within the time usually 
 devoted to physics in a high-school course, it is necessary 
 to limit the number of phenomena selected for study. In 
 this selection we have been influenced not only by the 
 desire to choose those phenomena most worthy of study, 
 but also by the necessity of confining the choice to those 
 phenomena which admit of a logical treatment within 
 the limits of this book. It is considered inexpedient 
 to attempt to include all matters of popular interest. If 
 the material here selected does not fully occupy all the 
 time of the pupil, the teacher may with profit add a dis- 
 cussion of various other subjects of direct interest to his 
 immediate community. The arrangement of this material 
 in chapters is due to the following considerations. 
 
 Many of the phenomena shown by gases and liquids 
 
vi PREFACE 
 
 may be illustrated by means of very simple apparatus. 
 Their explanation is usually simple and does not involve 
 the use of theories. For this reason their study is espe- 
 cially adapted to serve as an introduction to methods of 
 experimentation and direct reasoning. 
 
 Next we endeavor to show that various phenomena, 
 otherwise inexplainable, can be understood if a certain 
 theory be accepted. A theory, in order to be of any real 
 service, must rest upon a very good basis of judiciously 
 observed facts. For this reason we believe that, before 
 a theory is advanced, there should be presented a series 
 of phenomena which can be explained only by means of 
 that theory. 
 
 For instance, in the second chapter, in order to make 
 clear the need of the Molecular theory, we discuss cer- 
 tain of the more elementary phenomena of heat, diffu- 
 sion, osmosis, evaporation, solution, and crystallization, 
 and show that a comprehension of these phenomena is 
 materially assisted by the Molecular theory. This is 
 followed in the third chapter by a more extended ac- 
 count of those phenomena of heat which can be ex- 
 plained by the use of the Molecular theory without the 
 addition of the Ether theory. 
 
 The fourth chapter includes the study of the Atomic 
 theory, the foundation of chemistry. This theory de- 
 mands greater powers of imagination. It is not possible 
 in support of it to produce so many simple experiments, 
 easily performed and easily understood, which show the 
 connection between the phenomena considered and the 
 theory by means of which they can be explained. 
 
PKEFACE vii 
 
 The fifth chapter takes up enough of the subject of 
 sound to bring out the idea of vibrations and of waves. 
 
 This idea is utilized in the sixth chapter, which is espe- 
 cially devoted to a consideration of the Ether theory. 
 This is one of the most abstruse theories of science. Spe- 
 cial emphasis should be placed, therefore, upon the rea- 
 sonableness of this theory as a true explanation of the 
 facts. The phenomena studied are those of radiant 
 energy. In the treatment of this subject as little distinc- 
 tion as possible is made between those vibrations of the 
 ether which produce both heat and light and those which 
 produce only heat. 
 
 The Molecular, Atomic, and Ether theories are attempts 
 to get at the facts underlying the phenomena considered. 
 The result is that most physicists believe in the actual 
 existence of molecules, atoms, and the ether, although 
 they may not have clear conceptions of the form in which 
 they exist. 
 
 Lines of Force (seventh chapter), as used to explain 
 the phenomena of Magnetism, are excellent examples of 
 what may be called merely useful conventions. No phys- 
 icist believes in the actual existence of these Lines. To 
 him Lines of Force represent merely the directions in 
 which the magnetic forces act. Nevertheless, these con- 
 ventions have proved very useful in the correlation of 
 widely different phenomena of magnetism and electro- 
 magnetism. 
 
 A very useful theory for which there is almost no 
 foundation is the Theory of Gravitation (eighth chapter). 
 While it has been known in all ages that an apple will 
 
viii PREFACE 
 
 fall to the ground, and that it falls with a certain increase 
 of velocity, it is by no means certain that the earth 
 actually attracts the apple. 
 
 We hope that the book will serve as an introduction to 
 present methods of scientific thought and research. It is 
 descriptive in character. The experiments discussed 
 should all be performed. In most cases it will be suffi- 
 cient if the experiments are performed by the teacher 
 before the class. To these experiments may be added 
 many others which should be performed by the pupil. 
 In order to be of educative value, most of the experi- 
 ments performed should be chiefly quantitative, although 
 readily within the mental grasp of the pupil. Since 
 the nature of these additional experiments must depend 
 very largely upon the equipment at hand in each school, 
 their selection may be left to the individual teacher. The 
 list of laboratory experiments accepted for entrance by 
 the association of colleges will be found very suggestive. 
 
 The first four chapters may be taught with profit in the 
 first half of the first year in high-school. They include 
 most of the facts in physics which teachers in botany, 
 physiology, zoology, physical geography, and chemistry 
 like to have their pupils know before they enter upon 
 these special subjects. Teachers in these various sciences 
 find the time allotted to their subject too brief to enter 
 upon a full discussion of the physical phenomena in- 
 volved, and also are confronted by the necessity of tak- 
 ing up physical phenomena in the order in which they 
 occur in the text-books devoted to their own work. As 
 a result, their treatment of the physical phenomena is 
 
PKEFACE ix 
 
 often very inadequate and even illogical, and in conse- 
 quence the pupil does not get clear conceptions of the 
 points in question. It is, therefore, better to present 
 the physical phenomena which are of use in explaining 
 the facts of other sciences first at greater length, in their 
 proper general relationship with other physical phenom- 
 ena. In this manner their special significance in ex- 
 plaining the various phenomena studied in other sciences 
 will gain in clearness and force. 
 
 In the preparation of this book we have utilized every 
 available source. We wish especially to express our in- 
 debtedness to Miss Elizabeth G. Evans for suggestions 
 in connection with the text and for assistance in reading 
 the proof-sheets. 
 
 STEELE HIGH SCHOOL, DAYTON, OHIO, 
 February, 1903. 
 
CONTENTS 
 
 CHAPTER PAGE 
 
 I. GASES AND LIQUIDS 1 
 
 II. MOLECULAR PHENOMENA 76 
 
 III. HEAT 100 
 
 IV. CHEMISTRY, ATOMS 140 
 
 V. SOUND 214 
 
 VI. RADIANT ENERGY HEAT AND LIGHT . . . 235 
 
 VII. MAGNETISM AND ELECTRICITY 288 
 
 VIII. MECHANICS . . . . . . . .365 
 
 APPENDIX . .399 
 
 INDEX . .... 403 
 
ELEMENTARY PHYSICS 
 
 FIG. l. 
 
 CHAPTEK I 
 
 GASES AND LIQUIDS 
 
 1. Air Fills Many Spaces Apparently Empty. Insert a 
 lamp chimney vertically half way down into water (Fig-. 1). 
 How does the height of the water within 
 the chimney compare with the height of 
 the water on the outside ? 
 
 Hold in your hand an empty tumbler ; 
 that is, a tumbler which does not contain 
 anything that can be seen. Is it really 
 empty, or is it full of some invisible ma- 
 terial ? To determine this, place the tum- 
 bler, mouth downward, half way under the surface of the 
 water in a glass jar (Fig. 2). Something in the tumbler 
 prevents the water from rising as high within the tumbler 
 as the level of the water in the jar. This is the air which 
 is present in the tumbler. The water presses up against 
 the air and slightly compresses it. At 
 the same time the condensed air presses 
 down and prevents the intruding water 
 from rising within the tumbler more 
 "~" than a short distance above the mouth. 
 
 In the case of the lamp chimney, 
 neither end is closed, hence, as the chimney sinks into 
 the water, the air is pushed out at the top. The water 
 
 1 
 
 FIG. 2. 
 
c . ; s ... I:: ELEME^TARY PHYSICS 
 
 therefore rises in the chimney to the level of the water 
 in the jar outside of the chimney. 
 
 2. Gases Have Weight. Beneath one of the pans of a 
 delicate balance attach a hollow brass globe supplied 
 with a stop-cock, by means of which the passage leading 
 to the interior of the globe can be either opened or 
 closed. Upon the other pan place a sufficient number of 
 weights or of shot to make the beam of the balance per- 
 fectly horizontal. In other words, counterpoise the globe. 
 
 Then, without changing 
 the weights, remove the 
 brass globe, pump out the 
 air, close the stop-cock, 
 and return it to its former 
 position beneath the pan. 
 The side of the balance 
 to which the globe is at- 
 tached now rises, showing 
 that there has been a loss 
 in weight on this side of 
 the balance (Fig. 3). Since 
 the only material removed 
 from this side of the bal- 
 ance is the air which was 
 
 pumped out of the globe, the loss in the weight of the 
 globe can be due only to the removal of the air within it. 
 Therefore air has weight. 
 
 If the globe had been filled with hydrogen or carbon 
 dioxide, instead of air, it would have been found that 
 these gases also have weight, but that hydrogen is lighter 
 and carbon dioxide is heavier than air. 
 
 3 Additional Experiments Proving that Gases have 
 Weight, That air, hydrogen, and carbon dioxide differ 
 
 FIG. 3. 
 
GASES AND LIQUIDS 
 
 3 
 
 in weight can be shown also by a modification of the pre- 
 ceding- experiment. Make an open paper box, about four 
 inches square and four inches deep, and by means of 
 strings attach it, open end upward, beneath one of the 
 pans of the balance. A paper oyster-bucket will be 
 excellent for this purpose. Counterpoise it with weights 
 in the other pan. The bucket, of course, is full of air. 
 
 Pour carbon dioxide ( 139), which is an invisible gas, 
 into the bucket. The carbon dioxide, which is heavier 
 than air, pushes out the 
 air and takes its place. 
 The side of the balance to 
 which the bucket is at- 
 tached at once goes down 
 (Fig. 4). Tip the bucket 
 so that the carbon dioxide 
 can run out and its place 
 is once more taken by air. 
 The beam again becomes 
 horizontal. Carbon diox- 
 ide evidently has a greater 
 weight than air. 
 
 Hang the bucket, with 
 the open end downward, 
 beneath the same pan of the balance. In order that 
 an open vessel may retain hydrogen ( 137) it must be 
 kept mouth downward. Place the mouth of the ves- 
 sel containing the hydrogen beneath one edge of the 
 bucket ; turn the vessel as if attempting to pour some- 
 thing upward (Fig. 5). The lighter hydrogen runs 
 up into the bucket and pushes down the heavier air 
 until it is entirely replaced by hydrogen. The side 
 of the balance to which the bucket is attached rises. 
 
 FIG. 4. 
 
ELEMENTARY PHYSICS 
 
 Therefore the bucket is filled with a gas lighter than 
 
 air. 
 
 4. Principles Involved in Pouring of Gases When water 
 is poured into a tumbler, it sinks to the bottom and 
 pushes out the lighter air. In the same manner, when 
 carbon dioxide is poured into the paper bucket, it sinks 
 to the bottom and pushes out the air. 
 
 When a tumbler containing air is placed in water open 
 end downward the water is not able to displace the air. 
 
 In the same manner, when 
 a vessel containing hydro- 
 gen is held with the open 
 end downward, the heav- 
 ier air surrounding the 
 vessel is not able to push 
 out the lighter hydrogen. 
 If the tumbler contain- 
 ing air had been placed 
 in the water with the open 
 end upward, the water 
 would have rushed into 
 the vessel and crowded 
 out the air. In the same 
 manner, as soon as a ves- 
 sel containing hydrogen is turned until the open end 
 faces upward, the heavier air runs into the vessel and 
 pushes out the lighter hydrogen. 
 
 Place the tumbler in water so as to permit the air to 
 escape. The tumbler is now full of water. Invert the 
 tumbler, and hold it under the water in this inverted 
 position. Plunge a vessel, containing nothing but air, 
 mouth downward into the water and bring its mouth 
 beneath one edge of the tumbler. Turn the vessel so 
 
 FIG. 5. 
 
GASES AND LIQUIDS 
 
 that the air can escape upward into the tumbler and 
 entirely fill it. In the same manner, if the paper bucket 
 be suspended with the open end downward, and a vessel 
 containing hydrogen, mouth end downward, be brought 
 near the bucket, so that the mouth of the vessel be be- 
 neath one edge of the bucket, the vessel can be turned so 
 as to permit the lighter hydrogen to run up into the 
 bucket and push out the heavier air. 
 
 All gases are not invisible. Some of them have very 
 characteristic colors. Chlorine has a yellowish-green tint. 
 Nitrogen tetroxide and bromine have different shades of 
 reddish brown. The methods of obtaining all of these 
 
 FIG. 
 
 FIG. 7. 
 
 gases can be learned easily from any good work on 
 chemistry. 
 
 5. Air Presses in All Directions. Hold a piece of blot- 
 ting-paper or pasteboard over the mouth of a tumbler 
 filled with water and shake the tumbler until the pa- 
 per is thoroughly moistened. Then invert the tumbler 
 (Fig. 6). What holds the blotter in place, so that the water 
 can not run out ? In what direction is the air pressing ? 
 Hold the tumbler so that the mouth will face sidewise. 
 Does the water run out ? In what direction is the air 
 pressing now ? 
 
 Place an argand-lamp chimney under water, press a 
 
6 ELEMENTARY PHYSICS 
 
 piece of blotting-paper tightly against each end, and 
 remove the chimney from the water. Now carefully 
 remove the hands from the blotting-paper at both ends 
 of the chimney and hold the chimney in a vertical posi- 
 tion. Why does the water not run out ? Hold the chim- 
 ney in a horizontal position (Fig. 7). Hold it in other 
 positions. In what direction is the air pressing in these 
 different cases ? Notice that the air is pressing on both 
 ends of the chimney. Indeed, it is pressing also on the 
 walls of the chimney, but the walls are too strong to give 
 evidence of this pressure. 
 
 6. Effect of Inequality of Air-Pressure. The pressure of 
 the air can be further illustrated by the following experi- 
 ment. Tie a small piece of sheet 
 rubber over the top of a bladder- 
 glass and place the glass on the 
 plate of an air-pump. As soon as 
 part of the air is pumped out of 
 the glass, the pressure of the air 
 beneath the rubber is less than 
 the pressure of the air above, and 
 the rubber is forced down into the 
 glass (Fig. 8). Kemove the rub- 
 ber and close the opening at the 
 
 top of the bladder-glass firmly with the palm of the hand. 
 Pump out all or nearly all of the air. The hand now is 
 forced down so strongly against the glass, that it cannot 
 be easily withdrawn. What is forcing it down ? 
 
 At the beginning of the experiment the pressure of the 
 air within the vessel and the pressure of the air surround- 
 ing it is the same. As soon as a part of the air is pumped 
 out of the vessel, the pressure of the air remaining within 
 it becomes less than the pressure of the whole quantity 
 
GASES AND LIQUIDS 7 
 
 of air which was originally present. In consequence it 
 becomes less also than the pressure of the air surround- 
 ing the vessel. The hand is forced down with a force 
 equal to the difference between the pressures exerted by 
 the air within and without the vessel. When the air is 
 once more admitted to the bladder-glass the pressures 
 are equalized and the hand may be readily removed. 
 
 Instead of a bladder-glass, a small lantern globe may 
 be used. In this case the ends of the globe should be 
 ground smooth by rubbing them over a piece of plate- 
 glass covered with a thin layer of fine emery powder 
 moistened with water. 
 
 Press most of the air out of the thin rubber bag at- 
 tached to a toy whistle ; close it tightly, and place it 
 beneath a bell-jar (receiver) on the plate of an air-pump. 
 The base of the bell -jar, where it rests upon the pump 
 plate, should be rubbed with tallow to make it air-tight. 
 Pump out of the bell-jar the air which surrounds the 
 rubber bag. Why does the rubber bag dilate as the air 
 around it is gradually removed ? 
 
 7. Vacuum Fountain. The vacuum fountain apparatus 
 consists of a tall glass vessel open only at the base. 
 From this base a narrow, tapering brass tube extends 
 several inches up into the interior of the vessel, and ends 
 with a small opening. The base of the apparatus is pro- 
 vided with a screw thread, so that the vacuum fountain 
 vessel can be fastened, in an upright position, on the 
 plate of an air-pump. The entrance to the tube through 
 this base can be opened or closed by means of a stop- 
 cock. 
 
 Fasten the vacuum fountain vessel on the plate of an 
 air-pump. Remove nearly all of the air. Close the stop- 
 cock. Place the lower end of the vessel beneath the sur- 
 
8 
 
 ELEMENTAEY PHYSICS 
 
 face of the water in a glass jar. Open the stop-cock. 
 
 The water is forced up into the vessel in the form of a 
 
 tiny spray like a fountain (Fig. 9). 
 
 Explain. In the figure the vessel 
 
 is represented as if screwed into a 
 
 support which permits the ready 
 
 entrance of water from beneath. 
 
 This makes it unnecessary to hold 
 
 the apparatus during the latter 
 
 part of the experiment. 
 
 Fill a quart bottle about one- 
 
 FlQ. 10. 
 
 third full of water. Rotate in the flame of a Bunsen 
 burner one end of a piece of glass tubing, about ten 
 inches long, until the opening at this end becomes very 
 small. Thrust the other end through the hole in a rubber 
 stopper placed in the mouth of a quart bottle, so that the 
 lower end of the glass tube is within a short distance of 
 
GASES AND LIQUIDS 9 
 
 the bottom of the bottle. Through the glass tube blow 
 vigorously into the bottle for a short time, and then re- 
 move your mouth quickly. What causes the fountain 
 (Fig. 10) ? In the drawing the glass tube is replaced by 
 a brass tube supplied with a stop-cock. The stop-cock is 
 closed before the mouth is taken awaj 7 . 
 
 Compare in these two experiments the relative press- 
 ures of the air within the vessel and the air without the 
 vessel. 
 
 8. Water Held Up in a Tube by Air- Pressure. By means 
 of the thumb close a test-tube filled with water, invert it, 
 and place the mouth of the test-tube a short 
 distance beneath the surface of the water in 
 a glass jar. Remove the thumb. Why does 
 the water not run out of the test-tube (Fig. 
 11) ? Because the pressure of the air on the 
 surface of the water in the glass jar is suf- 
 
 ficient to prevent this. 
 
 Is there any limit to the amount of pressure which the 
 air exerts? If so, there must be a limit to the weight 
 of water which the pressure of the air can hold up. 
 
 If, instead of the short test-tube, a stout tube 35 feet 
 or more in length and closed at the top had been used, 
 would the pressure of the air on the surface of the water 
 in the vessel have been sufficient to hold up all of the 
 water in the tube ? 
 
 This has been tried, and it has been found that the 
 pressure of the air, at sea level, is usually sufficient to 
 hold up about 34 feet of water, but no more. It can, of 
 course, hold up any height less than 34 feet. The water 
 in a tube longer than 34 feet, therefore, falls until the 
 upper surface of the water in the tube is about 34 feet 
 above the surface of the water in the vessel (Fig. 12). If 
 
10 ELEMENTARY PHYSICS 
 
 the pressure of the air on the surface of the water in the 
 vessel were greater, it could hold up a column of water 
 having a greater vertical height. If the pressure of the 
 air were less, the height of the column of water held up 
 would be less ( 29). 
 
 It is evident that the greatest height of water which 
 can be held up in a long tube closed at- the top can be 
 used to determine the strength of pressure of the air on 
 the water in the vessel. If we know the force with which 
 the water in the tube is pressing downward, we also know 
 the force with which the air must be pressing on the 
 water in the vessel in order to prevent the water in the 
 tube from escaping. 
 
 9. Amount of Pressure Exerted by Air. If the area of the 
 opening at the base of the tube is one square inch, and if 
 the height of the water in the tube is 34 feet, or 408 
 inches, then the quantity of water in the tube is 408 
 cubic inches. It has been determined by actual weighing 
 that the weight of one cubic inch of water is .036 + 
 pounds. Then the weight of the total quantity of water 
 present in the tube is .036 x 408 = 14.688 (14.7) pounds. 
 Therefore the water in the tube presses downward with 
 the force of 14.7 pounds on the square inch of water at 
 the base of the tube, and the air evidently presses down- 
 ward on the surface of the water in the vessel with the 
 same force ( 12). 
 
 10. Height of Column Dependent upon Density of Liquid. 
 In the case of liquids heavier than water, the greatest 
 height of the liquid which can be held up by this air 
 pressure of 14.7 pounds on each square inch of surface 
 must necessarily be less than 34 feet. Since mercury 
 weighs 13.6 times as much as an equal volume of water, 
 the highest column of m,ercury which can be held up by 
 
GASES AND LIQUIDS 
 
 11 
 
 the air-pressure of 147 pounds, is only T J : ^ or T 1 ^ F as high 
 as the highest column of water held up by the same air 
 pressure. This height is T W of 408 = 30 inches (Fig. 12). 
 
 In order to show the height of mer- 
 cury which can be held up by air-press- 
 ure, take a stout glass tube more than 
 30 inches long and closed at one end ; 
 fill it with mercury, close the open end 
 with the finger, and dip this end under 
 the surface of some mercury in a small 
 vessel. A broad salt-cellar is very con- 
 venient. Eemove the fin- 
 ger. If the tube be held 
 vertically, mercury will 
 run out until the upper 
 surface of the mercury in 
 the tube does not rise over 
 30 inches above the level 
 
 n 
 
 the 
 
 Uft. 
 
 Mercury 
 
 Water 
 
 FIG. 12. 
 
 of the mercury 
 vessel. 
 
 11. Height of Column 
 
 Measured Vertically. If the tube be inclined, mercury 
 runs toward the closed end of the tube, so that a greater 
 quantity of mercury is held up within the tube. If, how- 
 ever, the vertical distance from the surface of the mer- 
 cury within the tube to the surface of the mercury in the 
 vessel be measured, it will be found to be still 30 inches 
 (Fig. 13). In other words, an air-pressure of 14.7 pounds 
 per square inch can hold up mercury to the vertical height 
 of 30 inches, irrespective of the variation in the quantity 
 of mercury held up. If tubes of very different diameters 
 be taken, it will be found that mercury can be held up to 
 the same vertical height of 30 inches in all of these tubes; 
 
ELEMENTARY PHYSICS 
 
 although, of course, the quantity of mercury in the wider 
 tubes greatly exceeds that in the narrow ones. 
 
 If now tubes, variously bent and varying in diameter, 
 be dipped in vessels exposing different amounts of surface 
 of mercury, it will be found that the same air-pressure 
 can always hold up the same vertical height of mercury, 
 irrespective not only of the form of the tube and its di- 
 ameter, but also of the area of the mercury exposed in the 
 vessel into which the base of the tube is dipped. There- 
 fore, in determining the pressure of the air by means of 
 
 the column of mercury held up 
 in a barometer, it is customary 
 to state that the pressure of the 
 air is equivalent to the pressure 
 of a column of mercury a cer- 
 tain number of inches or cen- 
 timeters in height, and no men- 
 tion is made of the diameter of 
 the tube, the weight of the mer- 
 cury which it contains, or of the 
 area of the mercury exposed in 
 the vessel. In other words, in- 
 stead of stating that the pressure of the air is 14.7 pounds 
 per square inch it is often stated that the pressure of the 
 air is equal to 30 inches. 
 
 12. Amount of Air-Pressure on One Square Inch at Sea 
 Level, In the experiments just described, the pressure 
 of the air is not 14.7 pounds on the entire surface of the 
 mercury in the vessel, but 14.7 pounds on each square 
 inch of that surface. An explanation of the reasons for 
 this fact involves a knowledge of Pascal's Law (53), and 
 had best be deferred until that law has been studied. 
 The fact that the pressure of air is 14.7 pounds on each 
 
 FIG. 13. 
 
GASES AND LIQUIDS 
 
 13 
 
 FIG. 14 A. 
 
 square inch can be demonstrated without a knowledge of 
 Pascal's Law by the following experiment. 
 
 Suspend from a strong support, seven or more feet above 
 the floor of the room, a short but broad brass cylinder, 
 closed at the top, with the open end 
 facing downward (Fig. 14 A). A mov- 
 able brass plate, having an area of 
 26 square inches, fits air-tight at all 
 levels within this cylinder. A small 
 brass tube at the top of the cylinder 
 permits the escape of most of the air 
 when the plate is 
 thrust to its highest 
 position within the cylinder. A brass 
 handle is attached to the lower surface 
 of the movable plate. From the handle 
 may be suspended, by means of a rope, 
 a board such as is used for swings. 
 Thrust the brass plate as far up in the 
 brass cylinder as possible and permit 
 the air to escape. With a pinch-cock 
 close the rubber tube slipped over the 
 top of the brass exit tube by means of 
 which the air escapes, and, using boys, 
 place as great a weight upon the swing- 
 board as the apparatus will safely hold 
 (Fig. 14 B). This total weight will be 
 found to equal or exceed 260 pounds. 
 The boys are evidently held up by the 
 excess of pressure of the air against the lower surface of 
 the plate, as compared with the pressure of the small 
 quantity of air remaining in the cylinder above this 
 plate. This demonstrates that the pressure of the air on 
 
 FIG. 14 B. 
 
14 ELEMENTAKY PHYSICS 
 
 26 square inches either equals or exceeds 260 pounds. 
 Therefore the pressure of air on each square inch equals 
 or exceeds 10 pounds. 
 
 If a cylinder of larger diameter and a plate of larger 
 area had been used, it would have been found that a cor- 
 respondingly greater weight could have been held up 
 by the air. By this means it can be demonstrated that 
 there is a definite pressure of the air on every square inch, 
 and that the total pressure of the air depends upon the 
 total area of the surface exposed. If all the air above 
 the plate in the cylinder had been removed by an air- 
 pump, the total pressure of the air on the lower surface 
 of the plate would, at sea level, have been found to be 
 more nearly 14.7 pounds on each square inch. 
 
 13. Variation in Air-Pressure Due to Elevation A small 
 quantity of air has only a slight weight, but, since the 
 atmosphere extends for a distance of more than 100 miles 
 above the earth, the total quantity of air overlying any 
 square inch of the earth's surface is very great, and the 
 pressure produced by that quantity of air on one square 
 inch at the surface of the earth must be considerable. 
 The pressure of the atmosphere upon any square inch 
 of surface depends upon the total quantity of air directly 
 overlying that surface. This quantity will be greater 
 at the level of the sea than on the summit of a high 
 mountain, in the bottom of a valley than at the top of 
 a hill, at the base of a building than in one of its upper 
 stories. 
 
 If the amount of pressure of the atmosphere on any 
 surface be proportional to its height above sea level, it is 
 evident that the pressure of the atmosphere upon that 
 surface can be used to determine its elevation above the 
 sea. This is so nearly true, when corrections are made 
 
GASES AND LIQUIDS 15 
 
 for temperature, especially when within moderate dis- 
 tances from the surface of the earth, that the degree of 
 air-pressure on a square inch of surface at any locality 
 has often been used as a means of determining 
 its distance above sea level. The height of most 
 mountains has been determined in this manner 
 ( 100). This method is especially useful where it 
 is either too difficult or too expensive to deter- 
 mine the elevation by means of the more accu- 
 rate methods used by the surveyor. 
 
 14. The Barometer. The barometer is the in- 
 strument used in measuring atmospheric press- 
 ure. There are two kinds of barometers in ordi- 
 nary use, the mercurial and the aneroid. 
 
 A glass tube usually 34 inches long, with a 
 small bore (internal diameter) and very thick 
 walls, and closed at one end, is filled with mer- 
 cury. The open end is held shut by means of 
 the thumb, and in this condition is inserted be- 
 neath the surface of some mercury placed in a 
 small sized cup. The air-pressure on the mer- 
 cury in the cup is not sufficient to hold up or 
 balance the entire vertical height of mercury in 
 the tube. It will be remembered ( 10) that the 
 height of mercury held up by the average air- 
 pressure at sea-level, is only 30 inches (Fig. 15). 
 Therefore the mercury in a barometer-tube 34 
 inches long will fall at least 4 inches at sea-level. 
 
 If the barometer be carried to elevations above M|I< 
 
 sea-level, the column of mercury in the tube 
 gradually falls below 30 inches at the rate of FlG - n - 
 1 inch for a rise in elevation of about 910 feet. The 
 height of the column of mercury in a barometer-tube 
 
16 
 
 ELEMENTARY PHYSICS 
 
 at any locality may therefore be utilized to determine its 
 elevation above sea-level. 
 
 In order to determine approximately the height of the 
 top of a mountain above its base (not above sea-level), 
 ascertain the height of the column of mercury in a 
 barometer-tube both at the top of the mountain and at its 
 base and multiply by 910 the difference between the two 
 heights expressed in inches. The product will be the 
 height of the mountain expressed in feet. The heights 
 of mountains are, however, given in geographies as so 
 many feet above sea-level. 
 
 15. The Barometer as an Indicator of Changes in Weather. 
 Variations in the height of the barometer are also due 
 
 to atmospheric conditions which cause changes in the 
 weather. Such variations are especially noticeable at 
 
 times preceding violent 
 storms. In general it may 
 be said that a rapid rise of 
 the mercury in the barom- 
 eter indicates fair weather 
 and a rapid fall indicates 
 stormy weather, including 
 probably rain, sleet, or snow. 
 A steady barometer indi- 
 cates a continuance of exist- 
 ing conditions. The use of 
 the barometer in determin- 
 ing variations in the weather 
 can be studied with profit in 
 some book on meteorology.* 
 
 16. Aneroid Barometer. The aneroid barometer 
 (Fig. 16) consists of a round flat box from which most of the 
 
 * Davis, Elementary Meteorology. Ginn & Co. 
 
 FIG. 16. 
 
GASES AND LIQUIDS 17 
 
 3 
 
 air has been pumped. It is made of metal so thin that 
 any increase in the pressure of the air on the exterior can 
 slightly press down its upper surface. When the press- 
 ure of the air is decreased, the top of the box rises again. 
 The amount of motion of the top of the box is so slight 
 that it cannot be readily noticed. It is, however, made 
 evident by a little mechanism attached to the top of the 
 box, which causes the end of a small pointer to move a 
 considerable distance around the face of a dial covering 
 the box, even when the motion of the thin top of the box 
 is but slight. The dial resembles very much the face of 
 a watch, but is supplied with two sets of figures. The 
 inner one of these indicates the height in inches at which 
 a mercurial barometer would stand under similar con- 
 ditions of air-pressure. The outer set, often placed on a 
 movable ring, is used to indicate approximately in feet 
 the level of the observer above the sea (Fig. 16). 
 
 If there has been a change of weather between the 
 observations at two localities, the barometer cannot be 
 used to determine with any reasonable degree of accu- 
 racy their difference in elevation. 
 
 17. Variation of Density of Air with Altitude. If the 
 opening of a pocket bicycle-pump is closed by means of 
 a finger and the plunger is then pushed inward, the air 
 within the pump is forced by the plunger into a smaller 
 space, or, is compressed- The greater the pressure ex- 
 erted by the plunger upon the air, the smaller is the 
 volume, or the amount of space occupied. The air which 
 has been crowded together into smaller space is said to 
 be more dense. The density of air within any space varies 
 with the amount of air crowded into that space. 
 
 The upper part of the atmosphere rests upon that 
 which is below. It possesses weight, and, because of 
 
18 
 
 ELEMENTARY PHYSICS 
 
 this weight, it pushes down upon and compresses the 
 air beneath it. On this account the lower parts of the 
 atmosphere are more compressed than the upper parts. 
 
 FIG. 17. 
 
 Consequently, the greater quantity of the air is found 
 near the surface of the earth. At an elevation of 1J 
 miles above the earth, the density of the air is less than 
 f of the density at the level of the sea, at 3J miles it is 
 
GASES AND LIQUIDS 19 
 
 about of the density &t sea level, and at 6 miles it 
 is less than J as great (Fig. 17). Half of the entire atmos- 
 phere is found crowded together within 3J miles of the 
 sea level, and about f of the atmosphere is found within 
 6 miles of sea level. It is evident that 100 miles above 
 the earth's surface the air must be exceedingly thin or 
 rare. 
 
 18. Total Pressure Exerted by Atmosphere on Human 
 Body. The pressure of the air at sea level is about 14.7 
 pounds per square inch ( 12). Every square inch of the 
 surface of the human body is subjected to a pressure of 
 14.7 pounds. Since the total surface of the body is equal 
 to hundreds of square inches, the total atmospheric press- 
 ure upon the body amounts to a number of tons. It has 
 been calculated to exceed 15 tons, or 30,000 pounds, for 
 an average-sized person. If the body were not saturated 
 with blood, lymph, and other liquids, it could not with- 
 stand this enormous pressure. But our bodies are con- 
 structed so as to be most healthy when under this press- 
 ure. Without it we feel uncomfortable. On the tops 
 of high mountains, where the pressure is much reduced, 
 breathing becomes more rapid and more laborious, the 
 pulse beats more quickly,, painful headaches and vomiting 
 often result, and any physical exercise causes great fatigue. 
 
 Fish living at the bottom of the sea are subjected to 
 enormous pressure. Nevertheless, fish adapted to those 
 great depths live there comfortably. In the fish here 
 figured (Fig. 18) the eyes are very large in proportion to 
 the size of the fish, and the lower half of the body is cov- 
 ered with phosphorescent spots. This enables the fish 
 to see at great depths. "Were the great pressure to which 
 they are accustomed diminished to any very consider- 
 able extent, they would suffer great pain, possibly death. 
 
20 
 
 ELEMENTARY PHYSICS 
 
 Alexander Agassiz says, " In fish brought up from deep 
 water, the swimming-bladder often protrudes from the 
 mouth, the eyes are forced out of their sockets, the scales 
 
 FIG. is. 
 
 have fallen off, and they present a most disreputable ap- 
 pearance." 
 
 Just as deep-sea fish need the greater pressures of the 
 deeper water, so we need the greater pressures of the air 
 found at lower altitudes on the earth's surface. 
 
 19. Balancing of Liquids in U-tubes, Pour mercury 
 into a U-tube of uniform bore with arms at least 20 
 inches long, until the mercury rises to a 
 height of about 4 inches in one arm. What 
 is the height of the mercury in the other 
 arm ? Pour additional mercury into the 
 same arm of the tube until the mercury on 
 this side rises to a height of about 6 inches. 
 The mercury rises to the same height in the 
 other arm, and the two columns of mercury 
 exactly balance each other. 
 
 Now pour 3 ounces of water on top of the 
 mercury in the right arm (Fig. 19). What effect is pro- 
 duced upon the level of the mercury in the other arm ? 
 
 FIG. 19. 
 
GASES AND LIQUIDS 
 
 21 
 
 Although the liquids in the two arms do not rise to the 
 same level, the w r ater and the mercury remaining in the 
 right arm of the tube together bal- 
 ance the total quantity of mercury 
 which is now present in the left 
 arm. The mercury remaining in 
 the right arm balances only that 
 part of the mercury in the other 
 arm which does not rise above the 
 level of the mercury in the right 
 arm. That part of the mercury in 
 the left arm which rises above this 
 level, is balanced by the water 
 which has been poured into the 
 right arm. If the pressure exerted 
 by the water is 3 ounces, then the |f 
 pressure exerted by that part of || 
 the mercury in the left arm which s| 
 is balanced by the water is also 3 f 5 
 ounces. ** 
 
 20. Boyle's Law : Apparatus, 
 Pressure affects the volume of 
 gases. To illustrate the relation 
 between pressure and volume, the 
 following apparatus has been de- 
 vised (Fig. 20). A long glass tube 
 of uniform bore is bent at the mid- 
 dle, so as to resemble a very much 
 elongated letter U. The Jube is 
 mounted vertically, with the open 
 ends upward. The parallel arms 
 
 should have a length of 180 centimeters, or about 71 
 inches. A glass stop-cock is inserted 100 centimeters, or 
 
 FIG. 20. 
 
22 ELEMENTARY PHYSICS 
 
 about 40 inches, above the base of the left arm, in order 
 to close this arm whenever necessary. A short tube with 
 a second stop-cock is attached to the base of the U-tube 
 for convenience in emptying the apparatus at the close 
 of the experiment. 
 
 Close the stop-cock at the base of the tube and open the 
 stop-cock in the left arm. Pour in mercury until it rises 
 slightly above the curved part at the base of the two arms 
 of the tube. The mercury rises to the same level on both 
 sides. Since the air in the two arms is subjected to the 
 same atmospheric pressure, the density of the air in both 
 is the same. Close the stop-cock in the left arm. This 
 neither increases nor diminishes either the density or the 
 pressure of the air in that arm. It merely confines the 
 air already present. Since the pressure of the confined 
 air remains the same, the surface of the mercury remains 
 at the same level in the two arms. In other words, while 
 the total atmosphere is pressing with a certain force 
 upon the mercury at the base of the right arm, the small 
 quantity of air confined in the left arm is pressing with 
 equal force on the mercury there. 
 
 21. Boyle's Law: Experiment Determine the height 
 of the mercury in^the barometer at the time of the experi- 
 ment. 
 
 Pour mercury into the right arm of the apparatus 
 used in the preceding experiment until the difference be- 
 tween the levels of the mercury in the two arms is equal 
 to the height of mercury in the barometer (Fig. 20). The 
 mercury now present in the left arm balances only an 
 equal height of the mercury in the right arm. But the 
 mercury in the right arm above this level is balanced by 
 the increased pressure now exerted by the air confined 
 in the left arm. This increased pressure is caused by 
 
GASES AND LIQUIDS 23 
 
 the fact that the air confined in the left arm has been con- 
 densed into a smaller space and therefore presses back 
 with greater force. Since that part of the mercury in the 
 right arm which rises above the level of the mercury in 
 the left has the same height as the mercury held up in 
 the barometer, it exerts the same relative pressure as the 
 atmosphere at the time of the experiment. 
 
 At the beginning of the experiment the pressure of 
 the air confined in the left arm balanced the pressure of 
 the total atmosphere pressing down the open arm. The 
 air confined in the left arm therefore also exerted a 
 pressure of one atmosphere. Now the pressure of the 
 air in the closed arm balances both the pressure of the 
 air in the open arm and the excess of mercury in that 
 arm, a total pressure of two atmospheres. Therefore, 
 when the difference between the levels of the mercury in 
 the two arms is equal to the height of the mercury in the 
 barometer, the pressure of the air confined in the left arm 
 is equal to two atmospheres. 
 
 Observe that the air in the left arm occupies now only 
 one-half of its original volume. In other words, its pres- 
 ent density is double its original density. This demon- 
 strates that : 
 
 When air is reduced to one-half of its original volume, 
 or, when its density is doubled, it exerts double its original 
 pressure. 
 
 If mercury should be added to the right arm until the 
 excess of mercury in that arm is equal to twice the 
 height of the mercury in the barometer, then a total of 
 three atmospheres of pressure would be exerted upon the 
 balanced part of mercury in the lower part of the U- 
 tube. The air confined in the left arm in that case, how- 
 ever, would occupy only one-third of its original volume, 
 
 THE 
 
24 ELEMENTARY PHYSIOS 
 
 in other words would possess three times its original 
 density and exert three times its original pressure. 
 
 From a series of such experiments the following rules 
 can be derived : 
 
 The pressure of a gas varies inversely as its volume and 
 directly as its density. 
 
 The volume of a gas varies inversely as the pressure which 
 it exerts, or which is exerted upon it. 
 
 The density of a gas varies directly as the pressure it exerts, 
 or the pressure (^exerted upon it. 
 
 These rules are often expressed in a condensed form 
 known as Boyle's Law. 
 
 What is the meaning of the mathematical expressions 
 " varies inversely " and " varies directly " ? 
 
 22. Valves. One of the common forms of valve con- 
 sists of a piece of leather which is caused, by its weight, 
 to rest on or against an opening. One edge of this piece 
 of leather is attached, to prevent it from shifting its posi- 
 tion. When the pressures on both face and back of the 
 valve are equal, the weight of the valve keeps it closed. 
 If the pressure against the face of the valve (the side 
 in contact with the opening) is, in any manner, made 
 
 greater than the weight 
 of the valve plus what- 
 ever pressure may be 
 exerted upon its back, 
 the valve is pushed 
 open (Fig. 21, A). When 
 the weight of the valve 
 
 plus the pressure on its 
 FIG. 21. 
 
 back is greater than the 
 
 pressure on the face, the valve is pushed against the 
 opening (Fig. 21, B). In the latter case, when the differ- 
 
GASES AND LIQUIDS 25 
 
 ence in pressures is considerable, the valve is closed with 
 sufficient force to prevent any gas or liquid from passing 
 through the opening. 
 
 The pressures acting upon valves are usually caused by 
 gases or liquids, usually air or water. In the case of a 
 large valve, such as is used in the various kinds of pumps, 
 the valve is often stiffened by a piece of wood or iron 
 fastened to its back. 
 
 23. Piston. A piston is a round disk or plate which, 
 although it can be moved back and forth in a cylinder, 
 fits so tightly against the inner wall of the cylinder that 
 no air can pass between the piston and the wall. The 
 rod by means of which the piston is moved is called the 
 piston-rod. In some pumps an opening passes vertically 
 through the piston, at or near its centre, and this is cov- 
 ered by an outward-opening valve in order to permit the 
 passage of air in an outward direction only (Fig. 22, A, B). 
 When the piston is not provided with such an opening, 
 all passage of air from one side of the piston to the 
 other is prevented (Fig. 22, C, D). In this case the out- 
 ward-opening valve is placed within a tube leading out 
 laterally from the lower part of the cylinder. 
 
 In both kinds of pump there is an inward-opening 
 valve which closes the opening at the base of the cylin- 
 der. Where it is desired to force the piston down so far 
 that it will come in direct contact with the base as in 
 the air-pump, this inward-opening valve is placed in a 
 little depression formed at the upper end of the opening 
 passing through the base. 
 
 24. Pumps. Before either form of pump is set in opera- 
 tion, the same atmospheric pressure is found at all points 
 within and without the pump. When the piston is forced 
 downward, the air enclosed within the lower part of the 
 
ELEMENTARY PHYSICS 
 
 cylinder below the piston, is compressed. The air which 
 has been compressed exerts a greater pressure than the 
 atmosphere without. In consequence, the inward-open- 
 ing- valve, wherever placed, is forced shut, and the out- 
 ward-opening- valve is forced open (Fig. 22, A, C). Air 
 continues to escape through the opened valve, until the 
 pressure of the air remaining in the cylinder below the 
 piston is reduced so nearly to the pressure of the atmos- 
 phere without that it no longer holds up the valve. Then 
 the valve drops on account of its own weight. 
 
 -Ordinary afr 
 
 Fia. 22 A. 
 
 FIG. 22 B. 
 
 When the piston is raised, the air remaining in the 
 lower part of the cylinder is given room for expansion. 
 As it expands its density becomes less, and it then exerts 
 less pressure than the atmosphere without. In conse- 
 quence, the outward-opening valve is held shut and the 
 inward-opening valve is forced open (Fig. 22, B, D). The 
 air from the outside then enters the cylinder. When the 
 pressure of the air within the cylinder has increased to 
 
GASES AND LIQUIDS 
 
 such an extent that the pressure of the outside atmos- 
 phere is no longer sufficient to hold open the inward- 
 opening valve, the valve drops. 
 
 When the piston is lowered the second time, the air 
 just admitted to the lower part of the cylinder is com- 
 pressed in its turn, and is forced, in the manner already 
 described, through the outward-opening valve. The oper- 
 ation of the piston, therefore, causes an inflow of air 
 through the inward-opening valve at each rise of the 
 
 FIG. 22 C. 
 
 FIG. 22 D. 
 
 piston, followed by an outflow through the outward- 
 opening valve at each descent. In consequence, air 
 passes in an intermittent stream through the inward- 
 opening valve into the lower part of the cylinder, and 
 then out through the outward-opening valve. In order 
 to pump air out of any vessel it is necessary to connect 
 the vessel with the inward-opening valve. In order to 
 pump air into any vessel the vessel must be connected 
 with the outward-opening valve. 
 
28 
 
 ELEMENTARY PHYSICS 
 
 25. Air-Pump: Use of Pump-Plate and Bell-jar. In a 
 pump used for pumping- out air (Fig. 23) the opening 
 beneath the valve at the base of the cylinder is connected 
 by means of a tube with a second opening which passes 
 up through the centre of a flat circular plate of iron or 
 brass. This plate is called the pump-plate. The inner 
 wall of the opening through the plate is supplied with 
 
 FIG. 23. 
 
 a spiral screw thread, so that it is possible to screw ves- 
 sels, air-tight, into the top of this plate. Any hollow 
 vessel screwed into the pump-plate is then connected to 
 the pump by means of the tube already mentioned. For 
 this reason the base of the hollow sphere used to deter- 
 mine the weight of air ( 2), and the lower end of the brass 
 tube of the vacuum fountain vessel ( 7) are screwed into 
 
GASES AND LIQUIDS 29 
 
 the opening in the pump-plate, when it is desired to re- 
 move the air. 
 
 For the purpose of exhibiting experiments demanding 
 the removal of part or all of the air surrounding the ap- 
 paratus, tall, cylindrical glass vessels, called bell-jars or 
 receivers, are placed on the pump-plate. In order to pre- 
 vent air from entering between the pump-plate and the 
 base of the bell-jar, the lower edges of the jar are ground 
 perfectly flat, so that they may rest evenly upon the plate. 
 The entrance of the air may be prevented still more ef- 
 fectually by coating the base of the bell-jar with a thin 
 layer of tallow, and then forcing the jar against the 
 pump-plate with a slightly twisting motion. Softer fats 
 and oils are serviceable but are not as effective. 
 
 As soon as any considerable portion of the air within 
 the bell- jar has been removed, the pressure of the air 
 within is diminished so much that the exterior atmos- 
 phere presses the jar against the plate with sufficient 
 force to make it difficult, or, in the case of a large bell- jar, 
 practically impossible to remove the jar from the plate. 
 Under these conditions very little air can enter between 
 the base of the bell-jar and the pump-plate. 
 
 When it is desired to remove the jar, air must be ad- 
 mitted in some manner to the interior of the bell- jar. 
 This is provided for in some air-pumps by placing, on 
 one side of the tube connected with the base of the pump- 
 plate, a valve which can be opened whenever necessary. 
 In the case of the air-pump illustrated by the figure air 
 may be admitted between the pump and the manometer 
 at an opening usually closed by means of a screw plug. 
 
 26. Action of Air-Pump. At the beginning, the press- 
 ures of the air in the receiver, and of the air in the 
 cylinder of the pump, both below and above the piston, 
 
30 ELEMENTARY PHYSICS 
 
 are the same. In a good air-pump, the piston, when 
 forced to its lowest position, fits so snugly against the 
 base, that but a very slight quantity of air can remain in 
 the lower part of the cylinder. When the piston is raised, 
 this slight quantity of air expands so many hundred times, 
 in occupying the enormously increased space now pres- 
 ent below the piston, that its pressure becomes almost 
 nothing. At the moment the piston begins to ascend, 
 the pressure of the exterior atmosphere closes more 
 tightly the outward- opening valve in the piston, while 
 some of the air within the bell-jar and within the con- 
 necting tube forces its way through the inward-opening 
 valve into the lower part of the cylinder (Fig. 23). When 
 the piston is forced downward, the air which has entered 
 the lower part of the cylinder beneath the piston is com- 
 pressed ; this enables it to close the inward-opening valve 
 connecting with the passage to the bell-jar and to force its 
 way out through the outward-opening valve in the piston. 
 Each upward and downward movement of the piston is 
 a repetition of these processes. Each upward stroke per- 
 mits more air to flow from the bell-jar into the lower part 
 of the pump-cylinder, and each downward stroke forces 
 most of this air to pass to the space above the piston. 
 In consequence, the quantity of air remaining in the bell- 
 jar steadily diminishes. Since, in a good air-pump, the 
 slight quantity of air remaining in the lower part of the 
 cylinder exerts almost no pressure when the piston is 
 raised, the air in the bell-jar will continue to force its 
 way into the pump as long as it is able to overcome the 
 weight of the lower valve at the base of the cylinder. 
 The degree of exhaustion which can be secured by a 
 good air-pump is, therefore, determined to a large ex- 
 tent by the lightness of the lower valve. 
 
GASES AND LIQUIDS 33 
 
 the pipe beneath the lower valve, which is located at the 
 bottom of the pump-barrel. 
 
 In a pump which has not been used for any consid- 
 erable time, the piston is likely to become dry and to 
 shrink sufficiently to permit air to pass between it and 
 the walls of the pump-barrel. A small quantity of water 
 poured into the barrel will remedy this defect tempora- 
 rily. Why ? 
 
 29. Height to Which Water may be Raised by the Lifting- 
 Pump. In the vacuum fountain experiment ( 7), the 
 pressure of the air within the vacuum fountain vessel was 
 less than the pressure of the air on the surface of the 
 water in the jar into which the open end of the tube at 
 the base of the vessel was dipped. In consequence of the 
 greater pressure of the outer air, the water in the jar was 
 forced up into the vessel. In a similar manner, after 
 pumping has reduced the quantity of air in the pipe below 
 the pump-barrel, and has thus diminished the pressure 
 of the air which remains in the pipe, the pressure of the 
 outer atmosphere on the water in the well is able to push 
 a part of the water up the pipe. 
 
 The distance the water is forced up the pipe depends 
 upon the degree of exhaustion of the air in the pipe, and 
 upon the amount of pressure exerted by the outer atmos- 
 phere at the time of pumping. It has been shown that 
 under the most favorable circumstances, the atmosphere 
 at sea level can hold up only about 34 feet of water ( 8). 
 This is, therefore, the extreme height to which the atmos- 
 phere can push water up a pump-stocL A rise of 34 feet 
 demands a perfect vacuum in the pipe up which the water 
 is forced. This state of exhaustion is never reached by a 
 water-pump. In practice, the pump-maker does not ex- 
 pect the water to rise much higher than 28 feet. He, 
 
34 ELEMENTARY PHYSICS 
 
 therefore chooses a pump-barrel of such length that the 
 valve in the bucket is not more than 28 feet from the sur- 
 face of the water in the well. 
 
 In order to pump water from a deep well it is necessary 
 to construct a pump -barrel of such length that it will ex- 
 tend from the pump -spout down to within 28 feet of the 
 surface of the water in the well. The bucket is fastened 
 to a rod of such length that when the bucket is on its 
 downward stroke it descends to within one or two inches 
 of the bottom of the pump-barrel. In case the pump- 
 barrel is 100 feet long, a vertical height of nearly 100 feet 
 of water is lifted within the pump-barrel at every upward 
 stroke of the pump-rod, when once the water has begun 
 to flow out at the spout. Every down-stroke of the 
 pump-handle causes an up -stroke of the pump-rod and 
 bucket. 
 
 30. Detailed Description of Action of Lifting-Pump 
 
 When the lifting-pump is first set in operation it acts like 
 an air-pump in removing the air from the pipe below the 
 pump-barrel. When the piston, or bucket, is raised for the 
 first time, the air in the lower part of the pump-barrel ex- 
 pands so as to fill the extra space below the bucket. This 
 reduces the pressure of the air in the barrel below that of 
 the air in the pipe. The air in the pipe is able, therefore, 
 to push up the lower valve, and a part of the air in the 
 pipe enters the lower part of the barrel. Owing to the 
 diminution in the quantity of air in the pipe situated 
 below the lower valve, there is also a reduction of the 
 pressure of the air remaining in the pipe, so that the at- 
 mosphere presses more strongly upon the surface of the 
 water in the well than the air within the pipe does upon 
 the water immediately beneath it. This unequal pressure 
 of air causes water to rise in the pipe, until the downward 
 
GASES AND LIQUIDS 35 
 
 pressure of the air remaining- in the pipe together with 
 the downward pressure of the water which has risen 
 within the pipe balances the pressure of the atmosphere 
 on the water in the well. 
 
 Each successive stroke of the bucket permits more air 
 to escape from the pipe into the barrel. By this means 
 the pressure of the air remaining- within the pipe is re- 
 duced more and more, and this permits the pressure of 
 the outside air on the water in the well to force more 
 water into the pipe, so that it will rise to higher and 
 higher levels within the pipe. After each stroke the 
 water rises in the pipe until the downward pressure of 
 the air and of the water which has risen within the pipe 
 balances the pressure of the atmosphere on the water in 
 the well. 
 
 Finally, if the valve in the bucket is not too far above 
 the surface of the water in the well, some of the water in 
 the pipe is forced above the lower valve of the pump. 
 This leaves a small quantity of air between the bucket and 
 the surface of the water which has now risen above the 
 lower valve. At the next descent of the bucket, in ad- 
 dition to this last remnant of air, a part of the water in 
 the barrel is also forced above the valve in the bucket. At 
 the next ascent of the bucket no air remains beneath it in 
 the pump-barrel. Consequently, the water which is forced 
 up the pipe by the outside air follows immediately after 
 the bucket and fills the space below it. 
 
 At the next descent of the bucket the water at the bot- 
 tom of the pump-barrel cannot escape downward, but 
 is forced through the valve in the bucket into that part 
 of the barrel which is above. With each following up- 
 ward stroke of the bucket more and more water enters the 
 lower part of the pump-barrel, and with each downward 
 
36 ELEMENTARY PHYSICS 
 
 stroke more water is forced into that part of the pump- 
 barrel which is above the bucket (Fig. 24, A), until finally 
 a part of the water runs out of the spout near the top. 
 
 The pump is now started. Each downward stroke of 
 the bucket causes some of the water which has last en- 
 tered the lower part of the pump -barrel to pass up 
 through the valve in the bucket, and each upward stroke 
 lifts the water which is above the bucket sufficiently to 
 allow the upper part of the water thus lifted above the 
 level of the spout to run out (Fig. 24, B). 
 
 31. The Force-Pump. The force-pump, in its construc- 
 tion and action, is quite similar to the lifting-pump. 
 The outward-opening valve, however, is not attached to 
 the piston, but is placed somewhere within the tube 
 which leads laterally from the base of the barrel and 
 forms an outlet for the water which it contains. The 
 piston does not lift water as in the lifting-pump, but, 
 during its downward stroke, forces the water out of the 
 base of the barrel, through the tube (Fig. 25). 
 
 Neither form of pump produces a steady stream. The 
 flow of water is greater during the upstroke of the piston 
 in the lifting-pump, and during the down-stroke in the 
 force-pump. 
 
 The flow of water from a force-pump, however, may be 
 made much more steady by employing an air-chamber. 
 This air-chamber consists of a hollow vessel, open 
 only at the base. It is usually provided with two 
 openings, one for the entrance of water coming from 
 the pump, the other for the exit of water into a second 
 tube. In this case the name delivery tube may be applied 
 to the tube through which the water escapes from the 
 air-chamber. In the case of a fire-engine a nozzle is at- 
 tached to the farther end of the delivery tube where the 
 
GASES AND LIQUIDS 
 
 37 
 
 water escapes. The nozzle of the delivery tube is made 
 so narrow that the pump can force water into the air- 
 chamber more readily than the water can escape through 
 the nozzle. This causes the water to rise in the air-chamber, 
 and to compress the air, which 
 the water confines to the upper 
 part of the chamber. As the 
 water rises in the air-chamber, 
 the air imprisoned in its upper 
 portion is more and more com- 
 pressed. The compressed air ex- 
 erts a counter-pressure upon the 
 water in the air-chamber, and 
 forces it out through the nozzle 
 at the end of the delivery tube. 
 This action of the imprisoned air 
 continues to force out water 
 through the nozzle at the end of 
 the delivery tube even while the 
 piston is rising in the pump- 
 barrel. The valve in the delivery 
 tube is at the base of the air- 
 chamber and prevents the return 
 of the water to the pump during 
 the upstroke of the piston. 
 
 If the pump is worked with suf- 
 ficient rapidity the piston begins 
 to descend before the air in the 
 air-chamber can force out much of the water in the lower 
 part of the chamber. The more rapid the action of the 
 pump, the shorter is the time during which the pressure 
 of the air in the chamber may decrease, and the more 
 steady will be the outflow of the water at the end of the 
 
 FIG. 25. 
 
38 ELEMENTARY PHYSICS 
 
 nozzle. Moreover, the more rapid the action of the 
 pump, the greater is the amount of water forced up into 
 the air-chamber, the greater is the degree of compression 
 suffered by the air, and the swifter is the stream of water 
 forced out through the nozzle. 
 
 32. Stop-cocks. A stop-cock is a contrivance by means 
 of which the passage through a tube can be either partly 
 or entirely opened and closed at wilL The essential part 
 of a stop-cock consists in a slightly tapering stopper or 
 plug, commonly known as the spigot, inserted air-tight 
 into a hole which passes at right angles through the 
 walls of the tube. The plug is fastened in such a man- 
 ner, that, while it cannot be withdrawn, it can be readily 
 turned. A hole is drilled crosswise through the plug at 
 such a point that, when the plug is turned in one posi- 
 tion, this hole is in line with the tube and thus forms a 
 passage continuous with the tube. When, however, the 
 plug is turned either to the right or to the left, the hole 
 through the plug is turned away from the passage 
 through the tube, thereby preventing all movement of 
 liquid or gas farther than the walls of the plug. 
 
 Sometimes a stop-cock is used not merely to open or 
 to close the passage through a tube, but also, when occa- 
 sion requires, to connect the interior of the tube with 
 the outside. This can be accomplished by drilling a hole 
 transversely through the plug or spigot, as in the previ- 
 ous case, and then drilling another hole at a right angle 
 to the first, extending from the surface on one side of the 
 plug as far as, but not beyond, the other hole which 
 passes through the centre of the plug. In other words, 
 the two holes in the plug join so as to form the letter T. 
 
 When the hole which passes entirely through the plug 
 is in line with the tube, the second hole, which joins it 
 
GASES AND LIQUIDS 
 
 39 
 
 at right angles, points toward one of the lateral walls of 
 the tube. An opening is drilled through this wall of the 
 tube so that in one position of the plug the first hole 
 through the plug is in line with the tube and the second 
 hole is in line with the opening in the wall (Fig. 26, a). 
 In this position a fluid may pass through the plug out 
 into the open air, or in 
 the inverse direction. 
 This is the position of 
 some plugs used to ad- 
 mit air to the tube con- 
 necting the air-pump 
 with the bell- jar, when it 
 is desired to permit air 
 to return into a bell- jar 
 from which air has pre- 
 viously been removed. If 
 the plug is turned half 
 way around, the passage 
 through the tube is un- 
 obstructed, but there is 
 no connection with the 
 side opening (Fig. 26, b). 
 At certain intermediate 
 positions of the stop- 
 cock the passage through the tube is entirely obstructed 
 (Fig. 26, c). 
 
 33. Manometer. A manometer is an instrument for 
 determining the degree of exhaustion of the air from a 
 bell-jar or other vessel. It consists essentially of a closed 
 cylindrical vessel containing a narrow U-shaped tube 
 closed at one end. The closed arm is filled with mercury, 
 great care being 1 taken to exclude all air from this arm. 
 
 
 < - ~IJ 
 
 *~r~ 
 
 2- ,;: I /;?;;. ;,", " 
 
 
 FIG. 26. 
 
40 ELEMENTARY PHYSICS 
 
 The mercury is prevented from running over into the 
 open arm by the pressure of the air (Fig. 23). As soon 
 as the pressure of the air in the manometer is reduced by 
 pumping so much that it is no longer sufficient to sustain 
 the weight of all the mercury in the closed arm, the 
 mercury in this arm falls and a part runs up into the 
 open side. 
 
 The proportion of air still remaining may be deter- 
 mined by noticing the height to which the upper surface 
 of the mercury in the closed arm rises above that in the 
 open arm. If the mercury in the barometer, at the time 
 of the use of the manometer, rises to a height of 30 inches 
 and the difference in the level of the mercury in the two 
 arms of the manometer tube is 3 inches, only T V as much 
 pressure is exerted by the air in the manometer as by the 
 atmosphere outside. This indicates that -fa of the air 
 has been pumped out of the manometer. When all of 
 the air has been removed, the mercury assumes the same 
 level on both sides of the manometer tube. As soon as 
 the air is admitted to the vessel, the mercury is pushed 
 back into the closed arm of the tube. In order to have 
 the space under the bell-jar entirely free for experimental 
 purposes, the manometer is usually attached to the tube 
 connecting the air-pump with the bell- jar. The degree 
 of exhaustion of the air in the bell- jar is accompanied 
 by an equal degree of exhaustion in the manometer and 
 is determined as soon as the degree of exhaustion in 
 the manometer is known. 
 
 Since the manometer is used chiefly to determine the 
 amount of air remaining in a vessel after almost all of 
 the air has been removed, it is not necessary to use a 
 U-tube more than 8 or 10 inches long. In that case the 
 mercury will not fall until the degree of exhaustion of 
 
GASES AND LIQUIDS 41 
 
 air in the manometer is so great that the air can no 
 longer support the 8 or 10 inches of mercury present in 
 the closed arm at the beginning of the experiment. 
 
 34. The Tension of Liquid Films. Dip a pipe into soap- 
 suds and produce a soap-bubble at the bowl of the pipe. 
 Notice that it is necessary to blow with some force to en- 
 large it. When the mouth is removed 
 from the stem of the pipe, the film 
 contracts and the bubble becomes 
 smaller. 
 
 Wet a funnel thoroughly with 
 water. Form a film across the larger 
 end of the funnel by dipping it into 
 soapsuds. Blow into the funnel so 
 as to expand the film (Fig. 27, A) 
 
 and then remove the mouth. The film contracts and on 
 this account travels inward towards the narrow end of the 
 funnel (Fig. 27, B). 
 
 Make a wire ring about 4 inches in diameter, and twist 
 the ends of the wire so as to. form a handle. Fasten the 
 ends of a short piece of thread to points on the wire ring 
 about 3 inches apart, so that the thread will hang down 
 loosely for a short distance. Dip the ring into soap- 
 suds. A film forms across the entire ring and supports 
 the thread (Fig. 28, A). By means of a pencil the thread 
 can be moved from side to side without destroying the 
 film. Break the film in the narrower of the two spaces 
 between the ring and the thread. All of the film within 
 this space at once disappears. The film within the 
 larger space contracts and stretches the thread, so that 
 the latter forms a regular curve (Fig. 28, B). 
 
 Dip the ring a second time into the soapsuds and 
 pierce the film in the broader space between the ring and 
 
42 ELEMENTARY PHYSIOS 
 
 the thread. That part of the film within the narrower 
 space draws the thread against the wire ring. Catch 
 hold of the thread and pull it toward the central part of 
 
 the ring. The film is 
 re-formed. Let go. 
 The thread is pulled 
 back as though the 
 film consisted of 
 stretched rubber. 
 
 It is evident in all 
 of these experiments 
 that thin films of 
 liquids act more or 
 FIG. 28. less like stretched 
 
 membranes. 
 
 The experiments will be more likely to succeed if a 
 small quantity of glycerine is added to the soapsuds. 
 
 35. Surface Tension of Liquids. Fill a test-tube with 
 water. Keep the edges of the test-tube dry and add 
 water drop by drop. The- level of the water will rise 
 considerably above the edges of the tube without over- 
 flowing. It behaves as though it were covered by a thin 
 film attached to the edges of the glass. 
 
 Dip a narrow glass tube into water and then take it out. 
 A small part of the water which rose into the tube still 
 remains in the tube. The lower part of the water sags 
 below the level of the glass and behaves very much as if 
 it were held up by a sac or thin film attached to the 
 margin of the glass. 
 
 Hold a small needle horizontally within a short dis- 
 tance of the water in a vessel. Drop it. It floats on the 
 surface, but the part of the surface nearest the needle is 
 now depressed and forms a little trough at the bottom of 
 
GASES AND LIQUIDS 43 
 
 which rests the needle (Fig. 29). Press on the needle and 
 
 see how quickly, after it has once broken through the 
 
 surface, it falls to the bottom of the vessel. If the needle 
 
 is removed without breaking 
 
 through the surface, the surface 
 
 flattens again. The surface of 
 
 the water in the vessel behaves as though it consisted of 
 
 a stretched film which is depressed by the weight, of the 
 
 needle but which springs back as soon as the needle is 
 
 removed. 
 
 This imaginary surface film appears to have considera- 
 ble strength and can be subjected to considerable tension 
 before breaking. If, for instance, an aluminum medal 
 about as large as a silver dollar be carefully placed in a 
 horizontal position on the surface of the water, the medal 
 will float, although it forms a depression in the surface 
 of the water. 
 
 The individual hairs of a camel's-hair brush when dry 
 stand slightly apart, producing a somewhat bushy ap- 
 pearance. Dip the brush into water. The bushy appear- 
 ance remains while the brush is in the water. The mere 
 presence of water is not sufficient to alter its appearance. 
 Take the brush out of the water. The surface of the 
 water adhering to the brush at once contracts and draws 
 the individual hairs together again, giving the brush a 
 pointed appearance. The surface of a liquid acts as if 
 it were covered by a film always tending to contract. 
 
 The surface of a small quantity of mercury contracts 
 until it can contract no longer. It then forms a spher- 
 ical mass and may be called a drop of mercury. A drop 
 of water is merely a small quantity of water whose sur- 
 face has contracted so much that a spherical form is 
 assumed. Melted lead poured through a sieve at the 
 
44 ] EMENTARY PHYSICS 
 
 top of a high t*s-.tjer appears at the bottom as rounded 
 shot. 
 
 On account of phenomena similar to those here de- 
 scribed, it is convenient to refer to the surface of a liquid 
 as if it were covered by a surface film, and, since this 
 film appears to be stretched or in a state of tension, the 
 expression surface tension is used in the discussion of 
 many phenomena. An explanation of the cause of the 
 phenomena known as surface tension requires the use of 
 the molecular theory. After this theory has been taken 
 up, it is well to return to a consideration of these phe- 
 nomena. Briefly stated, the molecules within the liquid 
 are equally attracted in all directions by surrounding 1 
 molecules, and hence do not give any evidence of tension. 
 However, those at the surface are attracted only down- 
 ward and laterally, not upward. Hence the surface mole- 
 cules act as though they formed a laterally stretched 
 membrane held tight against the main body of water. 
 
 36. Glassworking. To cut a glass tube, lay it on the 
 table, and, by means of a single forward stroke with a 
 three-cornered file, make a short but deep scratch across 
 the tube. Placing the thumbs directly behind the scratch, 
 gently push with the thumbs against the tube, at the 
 same time pull with the hands, and the tube will break at 
 
 the tube is large, wrap it in 
 cloth before trying to break it. 
 Rotate the end of the tube 
 slowly in the flame of a Bunsen 
 burner until the flame turns 
 yellow. The glass is softened 
 
 by the heat and its surface is melted. In consequence 
 of the shrinking of the surface film of the glass the 
 
GASES AND LIQUID' 
 
 45 
 
 FIG. 31. 
 
 sharp edges at the end of the tube arc; Bounded. If the 
 end of the tube is held a longer time :' i the flame, the 
 j contraction of the 
 
 surface film begins 
 to diminish the size 
 of the opening, and 
 this may be contin- 
 ued until an opening 
 of any desired size 
 is obtained, or until 
 the opening entirely 
 closes. "When the tube has melted shut for a distance 
 back from the end exceeding the thickness of the walls 
 of the tube, blow quickly but evenly into the open end. 
 The surface film resists the expansion of the glass in all 
 directions so evenly that the glass takes the form of a 
 small bulb or sphere. If the melted glass at the end of 
 the tube is sufficiently soft, a sphere may be blown as 
 large as a soap-bubble. 
 
 To bend a glass tube heat evenly a length of two 
 inches, moving the tube to right and left and rotating it 
 in the luminous part of an ordinary gas flame (Fig. 31). 
 As soon as the glass becomes soft enough, bend it a 
 little at every point along the heated portion, but avoid 
 any sharp bend (Fig. 32). 
 If a length of two inches 
 is heated long enough to 
 be soft, and the ends of 
 the tube are gently drawn 
 apart, the glass will stretch 
 like taffy, and the diam- 
 eter of the tube will become narrower at this point. 
 If considerably heated and rapidly drawn apart, the 
 
 FIG. 32. 
 
46 ELEMENTARY PHYSICS 
 
 stretched part of the tube may become as fine as a coarse 
 hair. 
 
 In order to secure a glass handle for a platinum wire 
 ( 229), heat a small part of a glass tube and stretch it 
 moderately so that the diameter of the heated part is 
 reduced about one half (Fig. 31). If the heated part is 
 moderately stretched, the walls of the tube remain thicker. 
 
 A Cut this part with 
 
 |l a file as above de- 
 
 scribed. Insert the 
 platinum wire in this 
 end of the glass tube 
 
 and place it in the flame of the Bunsen burner (Fig. 33). 
 The glass will close in around the wire and hold it in 
 position. 
 
 In order to secure a platinum tip at the end of the de- 
 livery tube for experiments with burning hydrogen ( 149), 
 roll a piece of platinum foil around that end of a file 
 which enters the handle. Withdraw the hollow conical 
 tube thus formed and slip it into a glass tube (which has 
 been stretched and cut in the manner described above) 
 so that the pointed end of the platinum tube projects. 
 Heat the narrowed end of the glass tube until it closes 
 in around the larger end of the platinum tube. 
 
 The opening of a glass tube may be enlarged while it 
 is heated by spreading it with the narrow end of a file. 
 
 37. Attraction of Water for Water. The force with 
 which the different parts of a mass of water hold together 
 can be shown by the following experiment. 
 
 Secure a circular piece of ordinary window-glass, 
 4 inches in diameter. By means of sealing-wax, attach 
 three strings at equidistant points near the margin of 
 the disk. Tie the ends together and fasten them to the 
 
GASES AND LIQUIDS 
 
 47 
 
 hook beneath a delicate spring balance (8-ounce, gradu- 
 ated to J-oiince intervals) in such a manner that the disk 
 will hang perfectly horizontal. Determine the weight of 
 the disk. Place a vessel of water beneath the glass disk, 
 and lower the balance until the glass disk comes in con- 
 tact with the water. 
 
 Slowly lift the balance. The disk gradually rises and 
 pulls a small part of the water above the general level of 
 the water in the vessel (Fig. 34). As 
 the balance is lifted higher and high- 
 er, the index of the balance is pulled 
 lower and lower, showing that the 
 water in the vessel is pulling down 
 the disk. At the same time the 
 amount of water pulled by the disk 
 above the general water level in- 
 creases. Finally the disk flies up- 
 ward. It seems to have been pulled 
 away from the water in the vessel. 
 On examining the lower surface of 
 the disk, however, a small quantity 
 of water is seen clinging to its lower 
 surface. This shows that the sepa- 
 ration did not occur between the disk 
 and the water, but that the film of 
 water still clinging to the disk was 
 pulled away from the main body of 
 water remaining in the vessel. The 
 film of water does not remain spread evenly over the sur- 
 face of the disk, but soon collects together in little drops, 
 ( 35). 
 
 The balance indicates what force is required to pull the 
 water attached to the disk away from the water in the vessel. 
 
 FIG. 34. 
 
ELEMENTARY PHYSICS 
 
 38. Adhesion and Cohesion. It will be noticed that a 
 film of water separates from the water remaining in the 
 vessel and clings to the glass plate. This indicates that 
 the attraction of water for glass is greater than the 
 attraction of water for water. The attraction between 
 unlike substances, for instance between water and glass, 
 is called adhesion. The attraction between masses of 
 the same kind of substances, for example the attraction 
 of the water film for the water in the vessel, is called co- 
 hesion. The adhesion between water and glass is greater 
 than the cohesion between water and water. 
 
 39. Water Drawn Up on Vertical Walls of Glass. The 
 attraction of water for glass can be shown by other phe- 
 nomena. A phenomenon is not necessarily an unusual or 
 striking occurrence. Indeed, most phenomena are quite 
 ordinary or commonplace. Any change of position, form, 
 color, or chemical composition which takes place, in fact, 
 anything which happens, whether caused by man or by 
 natural forces, is often called a phenomenon in physics. 
 
 Examine the water which has been placed in a tumbler. 
 The surface of the water is flat except at the edges, 
 where it curves upward against the sides 
 of the glass. 
 
 Dip the lower part of a plate of glass 
 vertically into water. Notice how high 
 the water rises on the sides of the glass 
 (Fig. 35). Now dip two plates into water 
 so that they form a very small angle with 
 one another, the two vertical edges on 
 one side being in contact. The water 
 rises to a greater height between the 
 plates than it does on their outer surface, especially where 
 the distance between the plates is very small (Fig. 36). 
 
GASES AND LIQUIDS 
 
 49 
 
 FIG. 36. 
 
 In all of these experiments the 
 attraction between glass and 
 water causes the water to creep 
 up along the sides of the glass. 
 In the experiment last described, 
 the water rises to the greatest 
 height where the two plates of 
 glass come in contact with each 
 other, for at this point both plates 
 act at the same time, and from 
 within a very short distance, upon the same particles of 
 water. 
 
 40. Capillary Tubes, Take glass tubes of different sizes, 
 the internal diameter or bore of the largest tube not ex- 
 ceeding -J of an inch. Arrange them in a series, beginning 
 with the tube of the largest bore. Fasten the tubes ver- 
 tically in small openings bored through a short piece of 
 _ wood. The lower ends of the 
 tubes should be at the same 
 level. Dip the tubes vertically 
 into water. Notice in the vari- 
 ous tubes the relation between 
 the size of the bore and the 
 height to which the water 
 rises, and the degree and direc- 
 tion of curvature of the upper 
 surface of the water within 
 the tube ( 43). 
 
 Water rises to the greatest 
 height in the tube having the 
 smallest bore (Fig. 37). The 
 rule is that The vertical rise of 
 FIG. 37. the water varies inversely as the 
 
50 
 
 ELEMENTARY PHYSICS 
 
 - Gas caused 
 by vaporization 
 of the liquid 
 wing to the heat. 
 
 diameter of the tube. In a tube having- one-half the diam- 
 eter water will rise to twice the height. This principle is 
 shown in the most striking- manner by tubes whose inter- 
 nal diameter is exceedingly small. Tubes of glass have 
 been constructed whose internal diameter does not ex- 
 ceed that of a hair, and, since tubes of this size were often 
 used in experiments of this nature, the action of liquids in 
 
 narrow tubes is often re- 
 ferred to as the action of 
 liquids in hair-like or capil- 
 lary tubes, or, as capillarity. 
 The ascent of liquids in fine 
 capillary tubes is well illus- 
 trated in the ascent of oil or 
 alcohol along the wick of a 
 lamp. On approaching the 
 flame, the oil or alcohol va- 
 porizes or turns into gas. 
 Where this gas comes in 
 contact with the air it burns, 
 when once lighted. Close 
 to the wick the gas is given 
 off too rapidly to become 
 mingled with air and here 
 there is no flame (Fig. 38). 
 
 Dip tubes of different di- 
 ameters into mercury (Fig. 39), and notice the relation- 
 ship existing between the size of the bore, the amount of 
 depression of the mercury in the tubes below the general 
 level of the mercury in the vessel, and the degree and 
 direction of curvature of the surface ( 43). What is the 
 difference in the direction of the curvature of the surface 
 of the water and of the mercury in these tubes ? 
 
 PIG. 38. 
 
GASES AND LIQUIDS 
 
 51 
 
 41. Concave and Convex Surfaces. Kegularly curved 
 surfaces are distinguished either as concave or convex. If 
 an observer, on looking" at a 
 
 regularly curved surface, finds 
 that the edges curve toward 
 him, so that from his point of 
 view the surface seems to be 
 hollowed out, the surface is 
 said to be concave (Fig. 40). 
 When, however, the surface 
 from the centre to the margin 
 curves away from him, so as 
 to have the appearance of a 
 part of a solid globe or sphere, 
 the surface is said to be con- 
 vex. When viewed from the 
 side, regularly curved surfaces 
 are said to be concave when 
 the edges curve upward and convex when the edges are 
 curved downward. 
 
 Which of the surfaces in the experiments above are 
 concave and which are convex ? 
 
 42. Comparison of Cohesion of Water and of Cohesion of 
 
 Mercury with the Ad- 
 h e s i o n of These Sub- 
 stances to Glass. It has 
 been explained that 
 water creeps up on the 
 sides of a piece of glass 
 because the attraction 
 of water for water is 
 less than the attraction of water for glass. This accounts 
 for the concave surface of the water in the glass tubes. 
 
 FIG. 39. 
 
 Concave 
 
 FIG. 40. 
 
52 ELEMENTARY PHYSICS 
 
 When a small glass plate is dipped in mercury, the 
 mercury seems to shrink from contact with the glass 
 (Fig. 41). The attraction of mercury for 
 mercury is greater than the attraction of 
 mercury for glass. In consequence, the 
 main body of the mercury within the 
 vessel draws that part of the mercury 
 which is in contact with the glass tow- 
 ards itself so effectively, that, where the 
 upper surface of the mercury meets the 
 glass, the mercury is drawn away from 
 the glass and in consequence its margin 
 curves strongly downward. This has the effect of making 
 the upper surface of the mercury convex when placed 
 within a narrow glass tube. 
 
 If the attraction of mercury for glass were as great as 
 the attraction of mercury for mercury, the surface would 
 be perfectly horizontal, even at its actual contact with 
 the glass. 
 
 43. Capillary Action Due to Surface Tension. The sur- 
 face of a liquid acts as if it consisted of a stretched film 
 which seeks to contract or shrink ( 35). 
 
 If a plate of glass previously moistened is dipped in 
 water, the film of water adhering to the glass meets the 
 film forming the surface of the water at a right angle. 
 But these films tend to contract and so the corner at 
 which they meet is rounded off, and the water is said to 
 rise along the glass. The contraction of the film con- 
 tinues until the weight of the liquid raised by the film 
 above the general level of the water in the vessel bal- 
 ances the force with which the film is able to contract. 
 
 Moisten a glass tube of small bore so that the interior 
 is covered by a film of water. Dip the tube into a vessel 
 
GASES AND LIQUIDS 53 
 
 of water. The film of water lining- the interior of the 
 tube meets the film forming- the surface of the water 
 within the tube at first at a right angle, but the con- 
 traction of these films causes the angle of contact to be- 
 come rounded. Consequently the water rises along- the 
 walls of the tube. As the contraction of the films con- 
 tinues, the water rises also at the central portion of the 
 tube. Any further contraction of the films causes the 
 water to rise both along- the walls and in the middle 
 regions of the tube. 
 
 When the tube has not been previously moistened, a 
 thin film of water creeps up the walls of the tube owing 
 to the strong attraction of glass for water (' 38). The 
 contraction of this film where it comes in contact with 
 the water in sthe tube causes the same phenomena as 
 those already described. 
 
 When a narrow glass tube is dipped into mercury, the 
 mercury does not form a film on the glass. Conse- 
 quently any contraction of the surface of the mercury 
 within the tube is not accompanied by a rise of mercury 
 along the walls. On the contrary, the edges of the surface 
 of the mercury, where they come in contact with the glass, 
 are drawn away from the glass, since the attraction of mer- 
 cury for mercury is greater than the attraction of mer- 
 cury for glass. This gives the surface of the mercury 
 within the tube a convex curvature, and further contrac- 
 tion of this surface must be accompanied by a depression 
 of the mercury within the tube below the general level of 
 the mercury in the vessel. This depression of the mer- 
 cury in the tube continues until the force with which the 
 surface film of the mercury within the tube contracts is 
 balanced by the upward pressure caused by that part of 
 the mercury in the vessel which rises above the level 
 
54 
 
 ELEMENTARY PHYSICS 
 
 of the mercury which has been depressed within the 
 tube. 
 
 44. Siphon. Dip a narrow glass tube under water. 
 Place the finger over the upper end, and, holding it in a 
 vertical position,- remove the tube. The water is held in 
 by the pressure of the air against the surface film of the 
 water at the lower end of the tube. Remove the finger. 
 The water drops out. Explain. 
 
 Take a narrow glass tube about 15 inches long, with 
 an internal diameter not exceeding J-inch, and bend 
 it at 5 inches from each end, so as to form a rectangular 
 U-shaped tube. Fill the tube with water. Be careful to 
 permit no air-bubbles to remain. Cover one of the open- 
 ings with a finger and invert the tube, keeping the two 
 openings at the same level. The water is held in at the 
 open end of the tube by the pressure of the air against 
 the surface-film of water. Now remove the finger with- 
 out changing the level of the tube. The water is held in 
 at both ends by the pressure of the air (Fig. 42, A). The 
 
 FIG. 42 
 
 success of this experiment depends upon the ends of this 
 tube being held at precisely the same level. Any slight 
 change in the relative level of the openings at once 
 
GASES AND LIQUIDS 55 
 
 causes the water to escape from the lower opening 
 (Fig. 42, B). 
 
 Bend a glass tube, about 36 inches long, into a rec- 
 tangular U-shaped form, so 'that one arm will be 5 inches 
 and the other 25 inches long. Fill with water in such a 
 way that no air-bubbles remain, and, placing a finger 
 over one of the open ends, invert the tube. The pressure 
 of the air prevents the escape of the water. Now remove 
 the finger and the water flows out rapidly. Try the ex- 
 periment again, closing the other arm of the tube. From 
 which arm does the water escape in both cases ? A tube 
 of this form will serve very well as a siphon. 
 
 45. Principle of the Siphon. The fact that the water 
 flows from the shorter out through the longer arm, does 
 not mean that the pressure of the air is greater at the end 
 of the shorter than at the end of the longer arm. In fact, 
 the pressure at the end of the shorter arm is slightly less 
 than the pressure at the end of the other arm ; but so 
 slightly less that the two pressures are practically equal. 
 
 While the pressures of the air at the ends of the two 
 arms are practically equal to one another, that part 
 of the air pressure which is available for moving the 
 water from either opening through the tube to the other 
 opening, is not the same at the lower end of the short arm 
 as at the lower end of the long arm of the tube. A part 
 of the pressure of the air at the end of the shorter arm is 
 used in supporting the water in this arm, and only the 
 remainder of the pressure can be used to push the water 
 toward the other end of the tube. In the same manner, 
 a part of the air pressure at the end of the longer arm is 
 used to support the water in that arm. But this column 
 of water is longer than that in the other arm, and hence a 
 smaller part of the air pressure remains available foi 
 
56 
 
 ELEMENTARY PHYSICS 
 
 pushing the water from this side over toward the shorter 
 arm. Hence, in spite of the tendency of the air to hold 
 up the water in both arms, the water is pushed from the 
 side of the short arm toward the long one. 
 
 The greater the difference in the length of the two 
 arms, the greater will be the inequality of the weights 
 of water to be supported, the greater also will be the 
 difference in the amount of air pressure available for 
 pushing the water from one side of the tube toward 
 the other, and the greater will be the rapidity of the 
 flow of water out of the longer arm. 
 
 46. Use of Siphon. If, before removing the finger from 
 either end of the siphon, the end of the short arm be 
 dipped under the surface of the 
 water in a jar, and the finger be then 
 removed, the water will flow from 
 the longer arm as before (Fig. 43). 
 As the water flows out of the longer 
 arm it tends to leave behind it an 
 empty space or vacuum. The press- 
 ure of the air on the water in the jar 
 then forces more water up through 
 the shorter arm, keeps both arms 
 full of water, and thus makes possi- 
 ble a constant flow of water through 
 the siphon. 
 
 In the same manner it is possible to remove liquids 
 from barrels and from carboys which are too heavy to be 
 easily tilted, but which permit the insertion of a siphon 
 at the top. Siphons are often improvised from rubber 
 tubes for the purpose of emptying barrels of cider. 
 Glass siphons are especially constructed for the purpose 
 of removing acids from large glass bottles called carboys. 
 
 Fro. 43. 
 
GASES AND LIQUIDS 
 
 57 
 
 FIG, 
 
 The length of any arm of a siphon can be determined 
 by measuring- the vertical distance from the open end of 
 the tube to the highest point in 
 the siphon (Fig. 44). When the 
 lower part of the arm is dipped 
 into water, the length of the arm 
 is found by measuring the vertical 
 distance from the water surface to 
 the highest point in the siphon. 
 
 47. Downward Pressure of Liq- 
 uids on the Base of Vessels Having 
 Vertical Sides. Fasten a glass 
 cylinder, whose lower end has 
 been ground flat, in a vertical 
 position over a vessel. Attach a string to the hook at 
 the centre of a round brass plate whose upper surface is 
 perfectly flat, and whose diameter slightly exceeds the 
 diameter of the cylinder. Draw the loose end of the 
 string up through the cylinder and fasten it to a second 
 hook attached to the lower side of one of the pans of a 
 balance. At first arrange the apparatus so as to allow 
 neither the string nor the brass plate to touch the cylin- 
 der. Place weights on the other pan of the balance until 
 the beam is horizontal. Then move the balance until the 
 string is in the centre of the cylinder and raise the bal- 
 ance l>eam until the brass plate barely touches the base 
 of the glass cylinder. If the upper surface of the brass 
 plate is perfectly horizontal, it will now fit neatly against 
 the base of the cylinder. Pour water into the cylinder. 
 The water at once escapes at the base. 
 
 Place an additional weight of 2 ounces on the other 
 pan of the balance. The brass plate is now held up 
 against the base of the cylinder by a force of 2 ounces. 
 
58 
 
 ELEMENTARY PHYSICS 
 
 Therefore, it now requires a force greater than 2 ounces 
 to pull or push the plate away from the cylinder. Pour 
 a small quantity of water gently into the cylinder. It re- 
 mains in the cylinder. Add more water. When a certain 
 level is reached the water (Fig. 45) begins to run out in 
 drops between the base of the cylinder and the plate. If 
 
 FIG. 45. 
 
 the water is poured into the cylinder too rapidly, it runs 
 out at the bottom in a steady stream. 
 
 Kepeat the experiment. In each case water begins to 
 flow out when it has attained a certain level within the 
 cylinder. Measure the depth of the water in the cylinder 
 at the moment when the water begins to escape. Then 
 let this water flow into a small vessel and determine its 
 weight. It will weigh 2 ounces. This indicates that the 
 moment before the water began to escape, the weight in 
 
GASES AND LIQUIDS 59 
 
 the pan and the downward pressure of the water in the 
 cylinder exactly balanced. Any further addition of water 
 to the cylinder causes the escape of water at its base. 
 This is in accordance with the statement made in the 
 earlier part of the paragraph, that a force of more than 
 2 ounces is required to push the brass plate away from 
 the cylinder. 
 
 Now add 2 ounces to the weights already present on 
 the other pan. Again pour water into the cylinder, and 
 determine the depth of the water in the cylinder when 
 the water begins to escape at the base. This depth is 
 found to be twice that observed in the preceding part of 
 the experiment. Determine the weight of the water in 
 the cylinder as before. 
 
 Eepeat the experiment, using greater weights. 
 
 The observations so far made are numerous enough to 
 justify the following conclusions. The downward pressure 
 of water on the base of a cylinder varies with the depth of the 
 water. Since the cylinder was placed in a vertical posi- 
 tion, all the weight of the water rests upon the base. 
 Therefore, in the case of a vessel with a vertical wall, the 
 downward pressure of a liquid on the base is equal to the 
 weight of the liquid within the vessel. 
 
 The downward pressure exerted by a liquid on the base of 
 a vessel varies also with the density of the liquid. To show 
 this repeat the experiment, using first a saturated solu- 
 tion of salt water and then a lighter liquid like coal oil. 
 With cylinders of different diameter it may be shown 
 that the downward pressure exerted by a liquid varies with 
 the size or area of the surface which forms the base. 
 
 48. Downward Pressure of Liquids on the Base of Vessels 
 Whose Sides are Not Vertical or are Partly Vertical. In- 
 stead of a glass cylinder take a vessel having the form of 
 
60 ELEMENTARY PHYSIOS 
 
 a large glass funnel. The opening at the lower and nar- 
 rower end of the funnel should be exactly equal in size to 
 the opening at the base of the cylinder used in the pre- 
 ceding experiment. The base of the funnel must be 
 perfectly flat. Fasten the funnel in a vertical position 
 and arrange the brass plate, the balance, and the weights, 
 as at the beginning of the preceding experiment. In the 
 apparatus figured, a different device is used which serves 
 the same purpose as that described (Fig. 45). 
 
 Place, in succession, weights of 2, 4, 6, or more ounces 
 upon the free pan of the balance. After each addition to 
 the weights, pour water into the funnel and record the 
 height at which the surface of the water stands above the 
 brass plate when the water begins to escape at the base. 
 Compare these results with the results in the preceding 
 experiment. It is found that the same depth of water is 
 required to produce the same pressure on the brass plate 
 at the base of the funnel as at the base of the cylinder. 
 This is true, notwithstanding the fact that, in order to 
 secure the same depth of water in the funnel as in the 
 cylinder, a much greater volume of water is required. 
 From this we may draw the conclusion that in the case 
 of vessels with vertical or with spreading sides, the press- 
 ure of a liquid on the base varies ivith the depth of the 
 water, but is independent of the volume of water used. 
 
 Replace the funnel with a vessel having a form similar 
 to that of an argand lamp chimney (Fig. 45) narrow 
 throughout the greater part of its length, but widening 
 suddenly a short distance above its base. The base 
 should be ground flat and its diameter should equal the 
 diameter of the cylinder used in the first of this series 
 of experiments. Adjust the brass plate, balance, and 
 weights. Follow the same methods and record the facts 
 
GASES AND LIQUIDS 61 
 
 observed as before. Compare the records with those 
 previously obtained for the cylinder and funnel. 
 
 It is found that the same depth of water is required 
 in the chimney as in the funnel and in the cylinder 
 to produce the same pressure on the brass plate at the 
 base. This is true, notwithstanding- the fact that the 
 same depth of water in the case of the chimney requires 
 a considerably smaller volume than in the case of either 
 the funnel or the cylinder. Similar results are obtained 
 by using- vessels very irregular in form but having an 
 equal area at the base. From this we draw the conclu- 
 sion, that the pressure of water upon any horizontal surface 
 varies with the depth of the water above the surface, hut is in- 
 dependent of the volume of water used. On the same sur- 
 face a small volume of water produces as great a pressure 
 as an enormously greater quantity of water, provided 
 that the depth be the same. To produce this effect in a 
 striking manner, the smaller quantity of water must be 
 placed in a vessel having the same diameter at the base, 
 but a diameter at the top much smaller than in the vessel 
 used for the larger quantity of water. 
 
 49. The Upward Pressure of a Liquid on Any Area Is 
 Equal to its Downward Pressure on the Same Area at the 
 Same Distance beneath the Surface. Take three sticks, 
 each of them one inch square, but respectively 4, 6, and 
 8 inches in length. Bore a hole lengthwise in each stick 
 to within about a quarter of an inch of the opposite end. 
 Put in shot as ballast. Place two or three round pieces 
 of pasteboard over the shot and ram the shot down tight. 
 Close the openings with cork. Cut off the cork even with 
 the surface of the wood. Let the total weight of the 
 sticks, including the shot, pasteboard, and cork, be 2, 3, 
 and 4 ounces respectively. Place the three sticks under 
 
ELEMENTARY PHYSICS 
 
 a bell-jar and pump out the air in the jar. A great por- 
 tion of the air enclosed in the fibres of the wood is re- 
 moved at the same time. As soon as possible after 
 pumping- out the air, boil the sticks for a short time in 
 hot, melted paraffine, so that the surface of the wood 
 shall be soaked with parafftne. This will prevent the 
 sticks from becoming water-logged. 
 
 Place the three sticks in a tall glass jar containing 
 water (Fig. 46). Notice the depth to which each sinks 
 into the water. The base of the 2-ounce stick is about 3| 
 inches below, the base of the 3- 
 ounce stick is about 5j inches be- 
 low, and the base of the 4-ounce 
 stick is about 7 inches below the 
 surface of the water. The sticks 
 press downward on the water 
 upon which they rest with a force 
 of 2, 3, and 4 ounces respectively. 
 Nevertheless, they do not sink to 
 the bottom of the vessel. The 
 water upon which the sticks rest 
 is in each case pressing upward 
 with the same force with which the sticks are pressing 
 downward ; otherwise the sticks would either rise or sink. 
 The upward pressure of water on a surface one inch 
 square is, therefore, 2 ounces, 3 ounces, and 4 ounces at 
 levels approximately 3|, 5j, and 7 inches below the sur- 
 face. 
 
 The same result will be obtained no matter whether 
 the sticks be placed in a shallow or in a deep vessel. 
 The upward pressure of water on any area immersed in 
 it depends, therefore, not upon the depth of the water 
 beneath the area pressed upon, but upon the depth of 
 
 FIG. 46. 
 
GASES AND LIQUIDS 63 
 
 the water above this area ; in other words, upon the dis- 
 tance of this area beneath the surface of the water. 
 
 The downward pressure of water on any area is equal 
 to the weight of a column of the water having- vertical 
 walls and having- an equal area as a base ( 47). This 
 downward pressure may be determined mathematically 
 by finding- the area pressed upon, in square inches, then 
 multiplying- this by the distance of the area below the 
 surface of the water expressed in inches, and finally mul- 
 tiplying the product by the weight of a cubic inch of 
 water. 
 
 A cubic inch of water weighs .577+ ounce ( 57). Cal- 
 culate the downward pressures of water on areas one inch 
 square, at depths of 3J, 5j, and 7 inches. The downward 
 pressures are approximately 2, 3, and 4 ounces respec- 
 tively. 
 
 Compare the downward pressures, obtained by calcula- 
 tion, with the upward pressures as determined in the 
 present experiment. It will be found that on the same 
 areas placed at the same depths beneath the surface of 
 the liquid, the upward and downward pressures are 
 equal. This fact is of the greatest importance in ex- 
 plaining the apparent loss in weight of bodies immersed 
 in liquids ( 51). 
 
 50. Loss of Weight of a Body Immersed in a Liquid. 
 Attach a stone by means of a long string to a spring bal- 
 ance, and find its weight. Hold the balance over a jar of 
 water, lower the stone into the water, and determine its 
 weight again (Fig. 47). Notice that the stone apparently 
 loses in weight when immersed in water. Place your 
 hand beneath the body of a person swimming in water. 
 Notice how easily you can support his weight though 
 using only a finger or two ( 52). 
 
64 
 
 ELEMENTARY PHYSICS 
 
 51. Cause of This Apparent Loss in Weight. Place a 
 rectangular block of wood or of any other substance in a 
 
 vertical position beneath the 
 surface of the water in a glass 
 jar (Fig. 48). The only direction 
 in which water can press upon 
 the top of the block is down- 
 ward ; the only direction in 
 which it can press against the 
 sides is lateral; and the only 
 direction in which it can press 
 against the base is upward. 
 The various pressures against 
 the sides balance each other. 
 This is shown by the fact that 
 the block is not pushed toward 
 any side, but remains wherever 
 placed. If the pressures are 
 unequal, the block is pushed in 
 a direction from the stronger 
 force toward the weaker force. 
 The downward 
 and upward 
 
 pressures cannot balance one another, 
 since the upward pressure which is ex- 
 erted against the bottom of the.block at a 
 lower depth is greater than the downward 
 pressure which is exerted on the top of 
 the block at a less depth beneath the 
 surface of the water. 
 
 If the upper surface of the block have 
 an area of 2 square inches, and if this top be 5 inches 
 below the surface of the water, the downward pressure of 
 
 FIG. 47. 
 
 FIG. 48. 
 
GASES AND LIQUIDS 65 
 
 the water on the top of the block is about 5.75 ounces ( 47). 
 If the base be of equal area, and if it be 8 inches below the 
 surface of the water, the upward pressure of the water on 
 the base will be about 9.25 ounces ( 49). In this case the 
 upward pressure will be 3.5 ounces greater than the down- 
 ward pressure. If, therefore, the block weighs less than 
 3.5 ounces, it will rise and float; if it weighs exactly 3.5 
 ounces, it will remain in the position in which it was 
 placed ; and if it weighs more than 3.5 ounces, it will 
 sink. 
 
 Calculate the weight of the volume of water pushed 
 aside by the block. Its volume is 2 X 3 = 6 cubic inches. 
 Its weight is 6 X .577+ = 3.5 ounces approximately. 
 Therefore, in order that a body may sink, it must weigh 
 more than an equal volume of water. 
 
 Even though a body sinks in water, and thus shows 
 that it is heavier than the water, it nevertheless appar- 
 ently loses in weight. This is necessarily true since 
 the upward pressure against the base of the block im- 
 mersed is always greater than the downward pressure 
 on the top. An examination of the numbers in the pre- 
 ceding paragraph shows that a body immersed in a 
 liquid apparently loses in weight an amount equal to the 
 weight of a quantity of the liquid whose volume is equal 
 to the volume of the body immersed. 
 
 In ordinary language we say that a body immersed in 
 a liquid loses in weight because of the buoyancy of 
 the liquid. Buoyancy is not a separate force. It is sim- 
 ply the effect produced by the inequality between the 
 downward and upward pressures exerted by a liquid 
 upon any immersed body. 
 
 52. Buoyancy of Gases. Loss in weight occurs also 
 when a body is immersed in a gas. The loss is usually 
 
66 ELEMENTARY PHYSICS 
 
 small, because there is but a small difference between the 
 downward and the upward pressures of gases on areas at 
 nearly the same level. The loss in weight can easily 
 be detected by means of a delicate balance. A body 
 weighed in air weighs less than the same body weighed 
 in a vacuum. A boy weighing 125 pounds in a vacuum 
 will weigh about 2| ounces less when weighed in air. 
 In estimating the weight of the boy, the loss in weight 
 due to immersion in air is never considered. 
 
 The loss of weight in water is much more likely to be 
 appreciated, especially by boys who are in the habit of 
 swimming. A boy will lose by far the greater part of his 
 weight if he is weighed in water. If his lungs be kept 
 well inflated with air, his weight in water will be so small 
 that a slight exertion on his part will enable him to float. 
 Some persons can float, apparently without exertion, 
 while lying in the water, back downward, with their lungs 
 well inflated. 
 
 53. Pascal's Law. Select a bottle with a neck of such 
 an even bore, that a cork of the same diameter can be 
 moved up and down in the neck like a piston in a cylin- 
 der. Fill the bottle with water. Insert the cork. Since 
 the cork fits tightly, no water can escape. Suppose that 
 the neck of the bottle is of such a diameter that, when 
 the bottle is filled, the surface of the water within the 
 neck has an area of one square inch. Then the cork 
 inserted in the neck rests upon a water surface of one 
 square inch. Suppose the cork to be pressed down with 
 a force of one pound upon the square inch of water sur- 
 face with which it comes in contact in the neck. Then 
 imagine a hole with an area of one square inch to be cut 
 in the side of the bottle. It is evident that, disregarding 
 the pressure due to the weight of the water, a pressure 
 
GASES AND LIQUIDS 
 
 67 
 
 of one pound is required to keep the water from running 
 out of this hole. If the number of holes be increased, a 
 pressure of one pound will be re- 
 quired on each hole to prevent the 
 escape of the water (Fig 1 . 49). In 
 other words, every square inch of 
 surface within the bottle will sustain 
 a pressure of one pound, in addition 
 to the pressure which it already sus- 
 tains due to the weight of the water 
 alone. 
 
 The principle here involved may be 
 stated as a general rule known as 
 Pascal's Law. 
 
 Fluids enclosed in a vessel, when sub- 
 jected to pressure, transmit this pressure 
 undiminished in all directions so that 
 every surface in the interior of the ves- 
 sel, equal in area to the surface upon 
 which the pressure is exerted, receives a pressure equal to 
 that applied. 
 
 In case a wide and a narrow vessel are connected by a 
 tube at the bottom and almost filled with water, the level 
 of the water in the two vessels is the same. If a movable 
 piston be placed on the surface of the water in each ves- 
 sel, and a weight be placed on the smaller piston, this 
 weight will balance a weight placed on the larger piston 
 as many times greater as the area of the water surface of 
 the wider vessel exceeds that of the narrower one. 
 
 It is difficult to make clear the reasons for the facts 
 here presented. It may be best for the present to omit 
 any explanation. In the future, when the student knows 
 more about molecules and the laws governing their ac- 
 
 FiG. 49. 
 
ELEMENTARY PHYSICS 
 
 tion, the subject can be studied with more profit. The 
 practical application of this law is, however, of consider- 
 able importance. 
 
 54. Hydrostatic Pressure TTsed to Lift Weights. Take a 
 short brass cylinder, having a diameter of nearly six inches, 
 with an opening through its 
 base into which is inserted 
 the end of a short brass tube, 
 and with a brass plate which 
 works up and down in the 
 
 FIG. 50. 
 
 cylinder like a piston. This is the same apparatus as 
 that described in paragraph 12, but inverted. 
 
 Carry the apparatus into the hall and place it near the 
 staircase. Thrust the plate down against the bottom 
 of the cylinder. Place upon it a block of wood high 
 enough to project slightly above the margin of the cylin- 
 der. On top of the block put a board and ask one of 
 the pupils to stand upon it. Slip one end of a short piece 
 
GASES AND LIQUIDS 
 
 69 
 
 of rubber tubing over the brass tube at the base of the 
 cylinder and into the other end insert a stop-cock. Attach 
 to the other end of the stop-cock about thirty feet of 
 stout rubber tubing filled with water, and into the farther 
 end of the tubing insert a funnel. 
 
 Close the stop-cock. Pour sufficient water into the 
 funnel to nearly fill it. Carry this end of the tube to the 
 top of the stairway. Open the stop-cock 
 (Fig. 51). The water escapes into the 
 lower part of the cylinder and pushes 
 upward against the base of the brass 
 plate. The pupil is lifted up very slowly 
 but steadily. If the funnel be kept full 
 of water, the pupil will continue to be 
 lifted until the plate reaches the top of 
 the cylinder. 
 
 The pressure of the water on the area 
 at the bottom of the rubber tubing is 
 quite small. This is because the area 
 exposed here is quite small. The area 
 of the brass plate is several hundred 
 times larger than the area at the base 
 of the rubber tube. It sustains, there- 
 fore, several hundred times the pressure 
 sustained by the area at the base of this 
 tube. Although the downward pressure at the base of 
 the narrow tube is small, the upward pressure against the 
 broad brass plate is sufficient to lift the pupil. 
 
 55. Hydraulic Press. In the same manner, in the hy- 
 draulic press, the pressure exerted upon a small surface 
 of water by means of the piston of a sort of force-pump, 
 is transmitted to every equal area on the base of a very 
 large piston (Fig. 52). A small pressure by means of the 
 
 FIG. 51. 
 
70 
 
 ELEMENTARY PHYSICS 
 
 small piston causes a great total pressure against the 
 much larger base of the large piston. 
 
 The principle of the hydraulic press is used for many 
 purposes: to lift bridges, elevators, and even ordinary 
 
 JQL 
 
 FIG. 52. 
 
 barbers' chairs. The 
 newest forms of hoisting- 
 jacks, cider-presses, and 
 oil-presses make use of 
 the same principle. 
 
 56. Archimedes' Prin- 
 ciple. An overflow-can 
 consists of a brass vessel 
 about 3.5 inches in diameter and 6 inches high, supplied 
 near the rim with a short, nearly horizontal tube which 
 serves as a spout. Pour water into the can until it runs 
 out through the spout. After the water has ceased flow- 
 ing from the can, place beneath the spout a small brass 
 bucket whose weight has been previously determined by 
 means of a spring balance. 
 
 Attach a rock to the same balance by means of a thread. 
 Weigh the rock in air, then lower it slowly into the can 
 and weigh it again in the water. Determine the appar- 
 ent loss in the weight of the rock due to the buoyant 
 
GASES AND LIQUIDS 
 
 71 
 
 Overflow can 
 
 FIG. 53. 
 
 effect of the water. If the water caught in the bucket 
 was forced out of the overflow-can only in consequence of 
 displacement by the rock (Fig-. 53) 
 and not owing- to any jerking mo- 
 tion during the weighing of the 
 rock in the water, then determine^ 
 the combined weight of the bucket 
 and of the water which has run 
 over into it from the can. 
 
 Next find the weight of the 
 water displaced by the stone, by 
 subtracting the weight of the 
 bucket alone from the weight of the bucket and the 
 water. Compare the loss in weight of the rock due to 
 buoyancy with the weight of the water displaced by the 
 rock. They are the same. 
 
 From this we learn, that a body immersed in a liquid 
 apparently loses in weight an amount equal to the weight of 
 the liquid displaced. 
 
 This statement of fact is usually known as Archimedes^ 
 Principle. 
 
 57. Specific Gravity. The specific gravity of a substance 
 is a number which shows how many times heavier that 
 substance is than an equal volume of another substance 
 taken as a standard. The standard for solids and liquids 
 is distilled water at a temperature of 4 degrees Centi- 
 grade, or 39.2 degrees Fahrenheit. A reading of 4 degrees 
 on the Centigrade thermometer indicates the same temper- 
 ature as that indicated by the reading of 39.2 degrees on 
 the Fahrenheit thermometer. At this temperature water 
 has its maximum density ; in other words, at this temper- 
 ature a given weight of water occupies less space and 
 therefore is more dense than at any other temperature. 
 
72 ELEMENTAEY PHYSICS 
 
 In determining" the specific gravity of gases, hydrogen 
 subjected to a barometric pressure of 30 inches ( 14) is 
 usually taken as the standard. Sometimes air is taken 
 as the standard. 
 
 Iron has a specific gravity of 7.8. This means that any 
 piece of pure iron is 7.8 times as heavy as an equal bulk 
 of water. If, for instance, it is desired to find the weight 
 of one cubic foot of iron, it is sufficient to multiply the 
 weight of one cubic foot of water by 7.8. The weight of 
 one cubic foot of water has been found by careful weigh- 
 ing to be 62.42 pounds ( 49). A cubic foot of iron, 
 therefore, weighs 487.88 pounds. 
 
 58. Specific Gravity of Solids Heavier than Water How 
 is it possible to determine the fact that the specific gravity 
 of iron is 7.8 ? Take any piece of pure iron. Weigh it 
 in air. Determine the weight of an equal volume of 
 water. Divide the weight of the iron in air by the weight 
 of the equal volume of water. The quotient is the specific 
 gravity of iron. To determine the weight of an equal 
 volume of water, weigh the water which escapes into the 
 bucket when the piece of iron is lowered into an overflow- 
 can full of water. 
 
 It is possible to secure the weight of an equal volume 
 of water without using the overflow-can and the bucket. 
 From Archimedes' Principle we know that the apparent 
 loss in weight of any rock, immersed in water, is equal 
 to the weight of a mass of water equal in volume to the 
 volume of the rock. It is, therefore, sufficient to deter- 
 mine the apparent loss in weight of the rock in the water, 
 in order to know the weight of an equal volume of water. 
 For instance, if the rock weighs 5 ounces in air and ap- 
 parently 3 ounces in water, the so-called loss in weight 
 is 2 ounces. Hence the weight of a mass of water 
 
GASES AND LIQUIDS 73 
 
 equal in volume to the volume of the rock is also 2 
 ounces. 
 
 If the rock weighs 5 ounces in air and an equal bulk of 
 water weighs 2 ounces, the rock must weigh 2.5 times as 
 much as an equal bulk of water. 
 
 The greater weight of rock as compared with the 
 weight of water is believed to be due to the fact that 
 there is actually more material in a cubic inch of rock 
 than in a cubic inch of water. Eock is believed to be 
 more compact, or more dense. In accordance with this 
 idea, the particular kind of rock here used is said to be 
 2.5 times as dense as water. The number which specifies 
 how the density of any substance compares with the den- 
 sity of water is called the Specific Density of that sub- 
 stance. Therefore, the specific density of the stone in 
 the preceding example is 2.5. In other words, its den- 
 sity is 2.5 as great as the density of water. 
 
 Since the weight of a body is proportional to the 
 amount of material in it, the specific gravity of a sub- 
 stance is the same as its specific density. For this reason 
 these two terms are often used interchangeably. 
 
 The method used in determining the specific gravity of 
 any substance depends upon its weight, solubility, chem- 
 ical action when exposed to air or immersed in water, and 
 other properties. A few additional methods are described 
 in the following paragraphs. The underlying principle 
 in all of these methods is the comparison of the weights 
 of equal bulks of these substances and of water. 
 
 59. Specific Gravity of Solids Lighter than Water. 
 Attach a small block of wood to a balance and weigh it. 
 To the block fasten a stone heavy enough to drag the 
 block beneath the surface of water. Hold the spring 
 balance over a jar of water and lower it sufficiently to 
 
74 
 
 ELEMENTARY PHYSICS 
 
 permit the entire stone to dip beneath the surface of the 
 water, while all of the block remains in the air (Fig. 54, A). 
 Determine the combined weight of block and stone in 
 this position. Lower the balance still farther, until both 
 
 block and stone are under the 
 surface of the water (Fig-. 54, B). 
 Ascertain the weight of both 
 in this position also. The loss 
 
 I 
 
 FIG. 51. 
 
 FIG. 54. 
 
 in weight of the combination is due to the buoyant effect 
 of the water upon the block of wood. This loss in weight, 
 according to Archimedes' Principle, is equal to the weight 
 of a quantity of water whose volume is equal to the vol- 
 ume of the block. 
 
GASES AND LIQUIDS 75 
 
 If the combination, while the block is in air and the 
 stone is in water, weighs 28 ounces, and the combination 
 when the block is also under water, weighs 8 ounces, 
 then the weight of a volume of water equal to the volume 
 of the block is 20 ounces. If the weight of the block alone 
 is 14 ounces, the wood is .7 times as heavy, bulk for bulk, as 
 water, or its density is only .7 times as great as that of 
 water. In other words, the specific gravity or the specific 
 density of the kind of wood investigated is .7. 
 
 60. Specific Gravity of Liquids. FIRST METHOD. Find 
 the weight of an empty bottle. Weigh the bottle when 
 filled with water, and afterwards weigh it again when 
 filled with the liquid whose specific gravity is to be 
 determined. In this manner the weights of equal vol- 
 umes of water and of the liquid are obtained. To find 
 how many times heavier the liquid is than an equal vol- 
 ume of water, divide the weight of the liquid by the 
 weight of the water. The quotient obtained is the 
 specific gravity of the liquid. 
 
 SECOND METHOD. Weigh a solid in air, then in water, 
 and then in the liquid. The loss in weight of the solid 
 when weighed in water and the loss when weighed in the 
 liquid, are the respective weights of equal bulks of the 
 water and of the liquid which were displaced by the 
 solid. Hence, to find the specific gravity of the liquid, 
 divide the loss in weight of the solid when weighed in 
 the liquid by its loss in weight when weighed in water. 
 
CHAPTEE H 
 MOLECULAR PHENOMENA 
 
 61. The Contraction of Liquids Due to Loss of Heat Indi- 
 cates That Liquids Are Made Up of Particles Called Molecules. 
 Scientific study has often led to conclusions widely at 
 variance with opinions ordinarily looked upon as per- 
 fectly simple and self-evident. It is usually taken for 
 granted, for example, that water occupies all of the space 
 which it seems to occupy. Even a drop of water, if placed 
 under a microscope, appears perfectly compact, and re- 
 veals no empty spaces anywhere within the liquid. 
 Nevertheless, certain experiments suggest that water is 
 made up of particles and that empty spaces actually exist 
 between the particles. Among these experiments, the 
 following are simple and easily performed. 
 
 Fill a small Florence flask with water 
 having a temperature of about 72 degrees 
 Fahrenheit. Insert one end of a 12-inch 
 piece of fine glass tubing through the hole 
 in a rubber stopper. Force the stopper 
 into the mouth of the bottle, until the water 
 displaced by the stopper rises nearly to the 
 top of the glass tube (Fig. 55). The water 
 >e rendered more readilv visible from 
 
 FIG 55 
 
 a distance, if, before it is placed in the flask, 
 
 it is colored deeply by means of some aniline dye. Cool 
 the water in the flask by placing it in ice-water. The 
 
 76 
 
MOLECULAR PHENOMENA 77 
 
 water in the glass tube falls. That part of the water, 
 which before cooling filled the flask, must now occupy 
 less space, in order to permit a part of the water which 
 was forced up into the tube to return to the flask. 
 
 The total amount of water present in the flask and tube 
 is the same both before and after cooling. This is shown 
 by the fact that in a vacuum ( 52) the water weighs pre- 
 cisely as much at the lower as at the higher temperature. 
 How, then, can water occupy less space at one tempera- 
 ture than at another ? It has been suggested that water 
 consists of separate particles, which are slightly distant 
 from one another, and that cooling is a process which 
 brings the particles of water closer together. These 
 separate particles have received the name of molecules of 
 water. 
 
 62. Increase of Heat Causes the Molecules of Liquids to 
 Move Farther Apart. If cooling is a process which brings 
 the molecules of water closer together, then the converse 
 also must be true, that heating is a process which causes 
 the distance between the molecules of water to increase. 
 
 Prepare the apparatus as in the case of the experiment 
 described in the preceding paragraph. Pour enough 
 water into the flask to fill the flask when closed and to 
 rise in the tube to only a short distance above the level 
 of the stopper. Place the flask on a piece of wire gauze 
 which rests on an iron ring stand. Heat it gently by 
 means of a Bunsen burner or alcohol lamp. The heat 
 affects the glass before it reaches the water. The mole- 
 cules of glass separate, the flask becomes slightly larger, 
 and the water in the tube, which is still cool, falls. 
 However, for the same increase in temperature, the in- 
 crease in volume of the water is much greater than the 
 increase in volume of the glass. Therefore, as soon as 
 
78 ELEMENTAKY PHYSICS 
 
 the water in the flask is also affected by the heat, the dis- 
 tance between the molecules of water also increases, and 
 the water soon rises rapidly in the tube until it runs out 
 at the top ( 120). 
 
 63. Solids are Composed of Molecules. The ball and ring 
 apparatus consists of a copper ball attached by a chain to 
 
 a wooden handle, and of a cop- 
 per ring- fastened at the end of 
 a rod which is also supplied 
 
 __ __ aaf -** r -^ w ^ a l ian( ^ e - When both ball 
 ~"* sssss ^ and ring are at the same tem- 
 perature, the ball will barely pass through the 
 ring (Fig. 56). 
 
 If the ring is cooled by means of ice or ice- 
 water, the ball can no longer pass through the 
 FIG. 56. ring. Cooling has caused the molecules of cop- 
 per to move closer together. The copper in the 
 ring now occupies less space, and, in consequence, the 
 diameter of the ring is shortened. The decrease in the 
 diameter of the opening through the ring may be meas- 
 ured by means of a vernier sliding caliper. 
 
 Permit the ring to return to its original temperature. 
 Then heat the ball. It increases in size. The ball is 
 now too large to pass through the ring. If, however, 
 the ring also is heated sufficiently, it becomes so en- 
 larged that the heated ball can once more pass through it. 
 It is necessary to come to the conclusion that copper as 
 well as water is composed of particles or molecules which 
 are not in direct contact with one another. Since the 
 separate particles have never been seen, even by means 
 of the strongest microscope, the molecules of both sub- 
 stances must be exceedingly small, and the distance be- 
 tween them must be very minute. 
 
 
MOLECULAR PHENOMENA 79 
 
 64. The Distance between the Molecules of Some Liquids 
 Is Sufficient to Permit the Interposition of Molecules of Other 
 Liquids. If water consists of molecules and these mole- 
 cules are separated by very minute, but none the less real 
 spaces, the question arises whether it is possible to insert 
 any substances into the spaces between the molecules of 
 water. The following experiment suggests that this may 
 be accomplished successfully. 
 
 Pour water into a long tube which is closed at one end 
 ( for example, a combustion-tube about half an inch in 
 diameter), until it rises to a level of 7 inches above the 
 base of the tube. Pour 7 inches of alcohol on top 
 of the water, so gently that the liquids do not 
 commingle (Fig. 57, A). Mark the upper limit of 
 the alcohol by a rubber band slipped over the 
 top of the tube. The water and alcohol seem to ^||| ||| 
 occupy all the space in the tube for a length of 
 exactly 14 inches. However, when the opening 
 at the top of the tube is closed by means of the 
 thumb, and the tube is repeatedly inverted and 
 
 turned back to its original position, the volume 
 
 FIG. 57. 
 of the mixture is decreased to such an extent 
 
 that the length of the tube occupied by the mixture is 
 shortened by about three-tenths of an inch (Fig. 57, B). 
 For some reason the two liquids, when mixed, occupy 
 less space than when kept separate. 
 
 This shortening of the liquid column can be explained 
 only by the supposition that the molecules of one liquid 
 may slip into the empty spaces which exist between the 
 molecules of the other liquid. Possibly the spaces be- 
 tween the molecules in both liquids are large enough to 
 permit in each case the entrance of some of the molecules 
 of the other liquid. 
 
80 ELEMENTARY PHYSICS 
 
 65. Small Size of Molecules Indicated by Gold Leaf and by 
 Soap-bubble Film. The smallest particles which can be 
 detected by means of the best microscopes have a diameter 
 of about jooloo ^ an i ncn - Gold has been hammered into 
 leaves so thin that the thickness of a single leaf does not 
 exceed gooijoo P ar * ^ an i ncn - ^ ^ were possible to remove 
 from this leaf a piece whose width and length did not ex- 
 ceed its thickness, the piece would be invisible even if 
 search were made with the best microscope. 
 
 A few moments after a soap-bubble has been blown, 
 rings of color make their appearance at the top of the 
 bubble and travel down its sides. Just before bursting, 
 a black spot becomes visible at the top of the bubble, 
 this spreads, and a moment later the bubble breaks. It 
 has been determined by investigators in physics that the 
 thickness of the colored part of the bubble immediately 
 below the margin of the black area, is about ^TQQ part of 
 an inch. The black part of the soap-bubble (just before 
 bursting) is much thinner according to some investi- 
 gators, about g^o^ooo of an inch. This thickness is about 
 100 times as small as the diameter of the smallest particle 
 visible by means of the best microscope. 
 
 Although the black part of the film breaks, the cohe- 
 sion between the molecules composing it is sufficiently 
 strong to enable the film to remain in existence for an 
 appreciable, although very brief, amount of time. It is 
 difficult to conceive of even this degree of cohesion, unless 
 the film consists of several layers of molecules. 
 
 If it be assumed that the number of layers of molecules 
 forming the film is at least equal to five, then the di- 
 ameter of the molecules forming the film does not exceed 
 soWooo f an i ncn - Molecules, however, are supposed not 
 to be in direct contact with one another, at least not 
 
MOLECULAR PHENOMENA 81 
 
 for any appreciable length of time. Therefore the actual 
 diameter of molecules must be much smaller than ^ ^ 000 
 of an inch. 
 
 From recent mathematical investigations it appears 
 that a single molecule of water taken from a soap film 
 may be as small as 250^00^000 ^ an i nc h i n diameter. 
 
 66. Relative Size of Molecules. Molecules of the same 
 substance and under the same conditions are believed to 
 be similar in every respect. The molecules of one kind 
 of substance, however, are believed to differ from those of 
 another kind. Among other things, they differ in size 
 and in weight. For instance, the molecules of mercury 
 are probably larger and heavier than those of water. 
 
 The actual size of molecules is unknown. According 
 to some calculations, difficult to explain here, the average 
 diameter of molecules is about 62 5UU 000 of an inch. It has 
 been calculated that if a globe of water, the size of a foot- 
 ball six inches in diameter, were magnified to the size of 
 the earth, the molecules of water would occupy spaces 
 greater than those filled by small shot and less than 
 those filled by footballs. These ideas may not be very 
 definite, but they are the most accurate which the present 
 state of science affords ( 78, 81). 
 
 67. Use of Litmus in Determining the Presence of Acids 
 or Alkalis. The value of the evidence furnished by the 
 experiment described in the next paragraph will be 
 better appreciated if the use of litmus in chemical ex- 
 periments be well understood. 
 
 The color of litmus is blue tinged with purple. It may 
 be purchased in the form of small cubes which may be 
 dissolved in water, giving to the water a characteristic 
 blue color. A single drop of any acid will change the 
 color of a considerable quantity of the litmus solution 
 
ELEMENTARY PHYSICS 
 
 from blue to red. The addition of several drops of any 
 alkali will change the color back to blue once more. 
 Any change to a red color in a litmus solution originally 
 blue indicates, therefore, the introduction of an acid. 
 Any change from red to blue indicates the introduction 
 of an alkali. 
 
 The more common acids are sulphuric acid, hydro- 
 chloric acid, and nitric acid. Among the common alkalis 
 are ammonia water and caustic potash. 
 
 68. Diffusion of Liquids. Fasten a long test-tube, one 
 inch in diameter, in a vertical position. Nearly fill it 
 with water distinctly colored with blue 
 litmus. Insert a thistle-tube so that its 
 lower end rests upon the bottom of the 
 test-tube. Pour hydrochloric acid into 
 the bowl of the thistle-tube so slowly 
 that, when the acid reaches the bottom 
 of the thistle-tube, its entrance will not 
 produce a perceptible current in the 
 colored water within the test-tube. As 
 soon as the acid reaches the bottom of 
 the test-tube, its presence is at once 
 detected by the change, from blue to 
 red, in the color of the litmus dissolved 
 in the water (Fig. 58). 
 
 Continue to add acid to the thistle- 
 tube until the red color extends a little 
 over half an inch above the bottom of the test-tube. 
 Allow the apparatus to stand for several days, without 
 disturbing either the liquids or the thistle-tube. The red 
 coloring gradually extends to higher and higher levels. 
 This indicates that the molecules of the acid are slowly 
 travelling upward into the litmus solution. If for sev- 
 
 FIG. 58. 
 
MOLECULAK PHENOMENA 83 
 
 eral days a record be kept of the height to which the red 
 coloring has crept, it will be found possible to determine 
 the rate at which the molecules of acid travel up the 
 tubes. 
 
 Different substances travel at various rates of speed. 
 Hydrochloric acid travels twice as fast as salt, salt travels 
 three times as fast as sugar, and sugar travels seven times 
 as fast as the albumen of an egg. The name albumen is 
 given by physiologists to the colorless liquid which sur- 
 rounds the yolk of an egg, and which after cooking be- 
 comes hard and white. 
 
 Two liquids in contact with one another may become 
 intermingled owing to the wandering of their molecules 
 (71). This slow process of intermingling is known as 
 diffusion. 
 
 69. Membranes for Illustrating Osmose of Liquids. In 
 many cases molecules pass readily through membranes 
 in which even the best microscope can detect no open- 
 ings. For experimental purposes, the best membrane is 
 the very thin membrane which lines the large masses of 
 suet (fat) found in the interior of the bodies of cattle. 
 These membranes may be easily obtained from any 
 butcher. By exercise of considerable care, this mem- 
 brane can be removed without rents or punctures. 
 
 70. Osmose of Liquids. If two different liquids or solu- 
 tions are placed on opposite sides of a membrane, the 
 molecules of both liquids or solutions may pass through 
 the membrane at the same time, but in opposite direc- 
 tions. This may be shown by the following experiment. 
 
 Cover the open end of the bowl of a thistle-tube with 
 a membrane and tie it securely by means of a string 
 wrapped around the neck of the bowl. Invert the thistle- 
 tube and insert it vertically in a jar nearly filled with 
 
84 
 
 ELEMENTARY PHYSICS 
 
 water. Fasten the thistle-tube securely in this posi- 
 tion. 
 
 Dissolve in water all the (blue) copper sulphate which 
 the water will hold. If the solution is not clear add a 
 few drops of sulphuric acid. Through a funnel pour the 
 solution into the upper end of the thistle-tube (the nar- 
 row end, opposite the bowl, in the present position of 
 the tube), until the level of the copper sulphate solution 
 in the tube is the same as the level of the water in the 
 jar. The level of the copper sulphate solution may easily 
 be adjusted to that of the water by first bringing-' the two 
 liquids approximately to the same level, and then raising 
 or lowering the thistle-tube until the surface of both 
 liquids is found at exactly the same level. Then allow 
 the apparatus to remain undis- 
 turbed for several hours. 
 
 In a short time the blue copper- 
 sulphate solution in the tube rises 
 above the level of the water in the 
 jar, and the height reached by the 
 solution increases for several hours 
 (Fig. 59). The rise of the copper- 
 sulphate solution in the thistle- tube 
 can be due only to the passage of 
 water from the jar, through the 
 membrane, up into the bowl of the 
 thistle-tube. 
 
 At the same time that part of the 
 water immediately below the thistle- 
 tube becomes tinged with light 
 blue. This indicates that part of 
 
 the copper-sulphate solution is passing from the bowl, 
 through the membrane, into the water in the jar. 
 
MOLECULAR PHENOMENA 85 
 
 The interchange of the molecules of different liquids 
 through the same membrane in opposite directions at 
 the same time is known as the osmose of liquids. 
 
 71. Varying Rates of Transference of Molecules through 
 Membranes during Osmose of Liquids. If the molecules 
 of copper sulphate passed downward through the mem- 
 brane as rapidly as the molecules of water move in the 
 opposite direction, the level of the solution in the thistle- 
 tube would not change. In the experiment described in 
 the preceding paragraph, however, the level of the copper 
 sulphate rises and that of the water falls. Owing to the 
 much greater width of the jar, the fall of the water level 
 is practically imperceptible. This change in level of the 
 two liquids is evidently due to the fact that the mole- 
 cules of water pass through the membrane more rapidly 
 than do the molecules of copper sulphate. 
 
 The comparative rates of speed shown during the os- 
 mose of various liquids and solutions has been studied. 
 Thus, for instance, water passes through a membrane 
 four to six times as rapidly as salt. Since a salt solu- 
 tion is colorless, its motion down into the water cannot 
 be made visible, and for this reason copper sulphate 
 was used in the preceding experiment (see also close of 
 68). 
 
 72. Osmose of Gases. Molecules of gases are much 
 more free to move than molecules of liquids. If there be 
 any cohesion between the molecules of a gas, it is so 
 slight that it is impossible to detect this cohesion experi- 
 mentally. 
 
 The molecules of different gases move with different 
 rates of speed. This may be shown by an experiment 
 very similar to the one preceding. 
 
 Cement to the upper edge of a funnel the open end of 
 
8G 
 
 ELEMENTAEY PHYSICS 
 
 a long-, cylindrical, unglazed earthen-ware jar, similar to 
 the jars used in two-fluid electric cells. Thrust the tube 
 of the funnel vertically through the opening in the rubber 
 stopper which closes one of the necks of a two-necked 
 bottle. Nearly fill the bottle with water. Through the 
 rubber stopper in the second neck thrust a glass tube 
 
 until the lower end of the 
 tube dips beneath the surface 
 of the water. 
 
 The experiment will prove 
 more interesting if before in- 
 serting the glass tube in the 
 stopper, the upper end of 
 the tube be drawn out until 
 the opening becomes very 
 small, as directed in 36. 
 
 Fill a bell -jar with hydro- 
 gen (a gas already mentioned 
 in 2, 3 and 4 ; see also 137, 
 138). Lift the bell-jar and, 
 without disturbing its ver- 
 tical position, place it above 
 the apparatus and lower it 
 
 ^^ over the earthen -ware jar 
 
 (Fig. 60). The hydrogen now 
 
 enters the earthen- ware jar so much more rapidly than the 
 air in the jar can move out in the opposite direction, that 
 the total quantity of gas within the earthen- ware jar is 
 increased. In consequence, the pressure of the mixture 
 of gases in the jar also increases. This mixture of gases 
 presses downward upon the water in the bottle with such 
 force that some of the water passes up the glass tube and 
 but through the small opening in the form of a tiny spray. 
 
 FIG. GO. 
 
MOLECULAR PHENOMENA 87 
 
 73. The Rate of Motion of Molecules of Gases. Mole- 
 cules of hydrogen move about four times as fast as the 
 average speed of the molecules forming the mixture of 
 gases called air. It has been calculated that hydrogen 
 travels at the rate of about 9 miles per second. This 
 does not mean that any one particle of hydrogen actually 
 travels that distance in a straight line in the period of 
 one second. In hydrogen which is subjected to the same 
 pressure as ordinary air, the molecules are probably so 
 close together that, before any molecule has gone 
 
 of an inch, it will strike some other molecule of hydrogen 
 which is moving in some other direction. While the 
 speed with which a molecule of hydrogen travels is very 
 great, the distance which it gets from its original posi- 
 tion in one second is, owing to these collisions, compara- 
 tively small. Still the total change in position, after a 
 number of seconds, is sufficiently great to give rise to 
 such phenomena as those just described in the preceding 
 paragraph. 
 
 74. Evaporation. The fact that the molecules of a liq- 
 uid are continually changing their position is shown by 
 the phenomena known as diffusion. Moreover, many 
 of the molecules at the surface of a liquid leave the 
 remainder of the liquid and move out into the air. If 
 these molecules do not return to the liquid from which 
 they came, the quantity of liquid contained in the ves- 
 sel is permanently diminished. That part of the liquid 
 which disappears, is said to evaporate. 
 
 75. The Evaporation of Solids, or Sublimation. Liq- 
 uids are not the only substances which evaporate. If 
 a piece of camphor be exposed to the air for any length 
 of time, it will evaporate completely. The molecules 
 leave the piece of camphor and wander over the room: 
 
88 ELEMENTARY PHYSICS 
 
 They take the form of a vapor, or gas, which is very op- 
 pressive to certain insects, and on this account camphor 
 is often placed where the vapor formed by its evaporation 
 will prove offensive to the moths which attack woollen 
 clothing. 
 
 Even ice evaporates. If a piece of ice be exposed to 
 dry air in a place where the temperature is much below 
 the freezing point, it will gradually disappear. It passes 
 directly into the state of a vapor without first taking the 
 liquid state. In this manner a thin coating of ice on a 
 stone often disappears during wintry weather, especially 
 when exposed to a strong wind. When clothing just 
 washed is hung out to dry on days when the temperature 
 is below the freezing point, the water in it freezes, and 
 the clothing becomes stiff. During the day, however, a 
 large part of the ice evaporates, especially in case a 
 strong wind is blowing. 
 
 The resins and oils found in various kinds of wood, 
 such as pine and cedar, also evaporate slowly. This 
 gives rise to the smell which is characteristic of the 
 various kinds of wood. 
 
 When a solid, that is heated, passes directly into the 
 state of a vapor without first becoming liquid, the proc- 
 ess is called sublimation. A cool body placed in the path 
 of a vapor may cause it to return again to the solid state, 
 in this case also without first assuming the liquid state. 
 
 76. The Cause of the Odor of the Rose. Investigators 
 in physiology have come to the conclusion that we can 
 smell only that with which we actually come in contact. 
 We say that we smell a rose ; but in reality we smell only 
 those particles which leave the rose and strike against 
 our nostrils. 
 The showy part of the flower of the rose consists of the 
 
MOLECULAR PHENOMENA 89 
 
 beautifully colored leaves, called petals. Upon these 
 petals are minute drops of oil. The oil evaporates. 
 This means that some of the molecules of the oil leave 
 the drops of oil found upon the petals, and do not again 
 return to the flower. These molecules of oil move away 
 in all directions, and strike against any bodies obstruct- 
 ing their path. The molecules may come in contact with 
 most parts of our body without our being aware of the 
 fact. Within the nose, however, are nerves especially 
 adapted to perceive the presence of even the very minute 
 quantities of oil which enter the nostrils. When these 
 nerves are bombarded by the molecules of oil coming 
 from the petals of the rose, we are aware of a sensation in 
 the nose, and say we smell the rose. 
 
 This fragrant oil is of considerable value in the manu- 
 facture of perfumery. About 2,700 pounds of roses are 
 required to obtain .9 pound of oil, and this weight of 
 roses may be raised on an area of about 1 acre. The 
 odor of pure rose-oil is peculiarly honey-like, and too 
 intense to be agreeable. Its entire deliciousness is de- 
 veloped only by a strong dilution with water, alcohol, or 
 other substances. 
 
 77. The Small Size of Molecules shown by the Sense of 
 Smell. Molecules of substances having disagreeable 
 odors often travel rapidly, and a small quantity is some- 
 times sufficient to cause discomfort to all the persons in a 
 large room. 
 
 A very small particle of musk, -^-5- of an ounce, will 
 keep a room scented for many years. The number of 
 molecules which leave the slowly evaporating musk must 
 be enormous. Nevertheless, the total loss in weight in 
 several years is so small that it cannot be determined 
 accurately even by means of a delicate balance. 
 
90 ELEMENTAKY PHYSICS 
 
 Dogs have been known to track men several days after 
 their feet have left imprints along- their line of wander- 
 ing. Some particles characteristic of the man or of his 
 clothing must have been left behind in his tracks. The 
 total quantity of the particles must be very small, since 
 they cannot be detected by means of a microscope. 
 Nevertheless, the total number of molecules left behind 
 in each imprint must be sufficient to enable the dog to 
 recognize their presence, even one or two days after the 
 tracks were made. Moreover, this is true notwithstand- 
 ing the fact that, in the meantime, many molecules must 
 have passed away from the imprints. Molecules must, 
 therefore, be very small ( 65, 66, 81). 
 
 78. Molecules of Solids May Enter between the Mole- 
 cules of a Liquid. When sugar is placed in water, it dis- 
 appears. The sugar is said to dissolve in the water. 
 It has not ceased to exist. If the water be removed by 
 evaporation, the sugar once more may be recovered. 
 When sugar dissolves, the molecules of sugar sepa- 
 rate and enter the spaces between the molecules of 
 water. The individual molecules of sugar are so small 
 that, when they have once become separated, they can- 
 not be recognized even under the microscope. When 
 copper sulphate is dissolved in water, the individual 
 molecules separate also, and can no longer be recognized 
 by means of the microscope. But they have not ceased 
 to exist. Even the color of the individual molecules is 
 retained, for every part of the water in which the copper 
 sulphate is dissolved shows the blue color. 
 
 79. The Cause of Solution. What causes the sugar to 
 dissolve in water I A difference in the relative strength of 
 certain molecular forces. One of these forces is the mu- 
 tual attraction of the molecules of sugar for one another, 
 
MOLECULAR PHENOMENA 91 
 
 which causes them to remain together and form a lump. 
 The second force is the mutual attraction between the 
 molecules of water. The third force is the mutual attrac- 
 tion between sugar and water. 
 
 The cohesion between the molecules of sugar is much 
 greater than that between the molecules of water. This 
 is shown by the fact that the point of a knife can be 
 thrust much more readily into water than into a lump of 
 sugar. In order to separate the molecules of sugar, 
 a third force must exist which is stronger than either 
 the cohesion of sugar or the cohesion of water. This 
 third force is the adhesion between the molecules of 
 sugar and those of water. 
 
 The mutual attraction between sugar and water is so 
 great that both the molecules of water and the molecules 
 of sugar are drawn apart. Sugar molecules draw apart 
 water molecules, and water molecules draw apart sugar 
 molecules, until finally the same proportion of sugar is 
 found in every part of the water. The solution has be- 
 come uniform. 
 
 80. The Small Size of Molecules Shown by Solutions. 
 When sugar and copper sulphate dissolve in water, their 
 molecules continue to separate until no particles of sugar 
 or copper sulphate can be recognized even under the 
 most powerful microscope. Since the smallest particles 
 which can be detected by means of the best microscopes 
 have a diameter of about ^QQQ of an inch, molecules of 
 sugar and of copper sulphate are smaller than 1U0 1 OUO of an 
 inch. 
 
 When T ^ 07} - of an ounce of indigo is dissolved in sul- 
 phuric acid, it gives a distinctly blue color to one cu- 
 bic foot of water. This is equivalent to 440 ^ () 00() of an 
 ounce of indigo to a drop of water. Nevertheless, each 
 
92 ELEMENTARY PHYSICS 
 
 drop of water is evenly tinged with blue. In order that 
 such a small quantity of indigo may give an even tinge 
 of blue to each drop of water, the number of molecules in 
 even 440,000,000 f an ounce of indigo must be large. In order 
 that many molecules may exist in 44U>OUO>OUO of an ounce, the 
 molecules must be exceedingly small ( 65, 66, 78). 
 
 81. The Sense of Taste. Absolutely insoluble materials 
 are tasteless. If the substances are not already dissolved 
 in some liquid, in order to be tasted they must be dis- 
 solved in the saliva, as soon as they are placed within the 
 mouth. Those substances which are most readily dis- 
 solved are most easily tasted. Sugar and salt are good 
 examples. In the juices of many fruits, sugar and other 
 substances are already dissolved. It is owing to the 
 presence of the dissolved sugar that the tastes of many 
 fruits are so agreeable. 
 
 The sense of taste is caused by the motion of the dis- 
 solved molecules. Located in different parts of the 
 mouth, especially upon the tongue, are certain nerves 
 which are very susceptible to the irritations caused 
 by the motions of some kinds of molecules, while indif- 
 ferent to the motions of other kinds. When the mole- 
 cules of many of the dissolved substances come in contact 
 with these nerves, a sensation is produced known as the 
 sensation of taste. The nerves are called the nerves ,of 
 taste. 
 
 82. The Amount of a Substance Which Can Be Dissolved by 
 a Liquid Depends upon the Temperature of the Liquid, The 
 quantity of a solid which will dissolve in a liquid de- 
 pends in some manner upon the distance between the 
 molecules of the liquid. As the molecules of the liquid 
 move farther apart, the space available for the molecules 
 of the solid increases, and a greater amount of the solid 
 
MOLECULAR PHENOMENA 93 
 
 can be dissolved. The distance between the molecules 
 of any substance depends upon its temperature, and in- 
 creases (with few exceptions), as the temperature in- 
 creases. The amount of a solid which can be dissolved 
 in a liquid depends, therefore, upon the temperature of 
 the liquid. 
 
 Water at 150 degrees Fahrenheit will dissolve more 
 sugar than the same quantity of water at 100 degrees 
 Fahrenheit. When sugar is placed in water already 
 saturated with sugar, the sugar last added falls to the 
 bottom of the solution and remains there undissolved. 
 
 Corresponding to each degree of temperature there is a 
 definite distance between the molecules of the liquid, and 
 a definite quantity of the solid which can be dissolved. 
 When at this temperature the liquid has dissolved all the 
 material which it can hold, it is said to be a saturated 
 solution. This solution will be a saturated solution as 
 Jong as its temperature remains constant. Raising the 
 temperature of a saturated solution, therefore, places it 
 in an unsaturated condition, for if its temperature be 
 raised, the liquid can dissolve more of the solid. 
 
 83. The Cooling of a Saturated Solution Causes the Ex- 
 clusion of a Part of the Dissolved Solid. If a saturated 
 solution is cooled, the spaces between the molecules 
 of the liquid decrease, all of the solid can no longer be 
 held in solution, and a part is crowded out of the liquid. 
 The quantity of the solid excluded depends upon the 
 amount of fallin temperature of the liquid. The forced- 
 out molecules of the solid settle upon the sides and bot- 
 tom of the vessel holding the solution. 
 
 84. Evaporation Causes an Exclusion of Fart of the Dis- 
 solved Material, In a saturated solution all the spaces 
 available between the molecules of the liquid are occu- 
 
94 ELEMENTARY PHYSICS 
 
 pied by the molecules of the dissolved solid. When a 
 part of the liquid evaporates, the molecules of the solid 
 which were between the molecules of that portion of the 
 liquid which has evaporated cannot pass into the spaces 
 between the molecules of the liquid which is left in the 
 vessel, because the spaces are already filled. These 
 molecules then settle upon the sides and the bottom of 
 the vessel. As the liquid continues to evaporate, the 
 crust of the solid material upon the walls of the vessel 
 increases in thickness. If all of the liquid evaporates, 
 all of the solid which was in solution is left behind in 
 the vessel. 
 
 The evaporation may be brought about either by sub- 
 jecting the solution to heat, or by exposing- it to air at 
 ordinary temperatures. In the first case, evaporation will 
 be rapid. In the second case, it will be much slower. 
 
 85. Crystallization. When molecules are slowly ex- 
 cluded from a saturated solution, either by very slow 
 cooling- or by evaporation, the molecules often do not 
 form a crust of even thickness on the walls of the vessel, 
 but collect in groups. If, before cooling or evaporation 
 is begun, a small fragment of the solid is lowered into 
 the liquid by means of a string, the excluded molecules 
 seem to collect by preference upon this fragment. In 
 this manner groups of molecules of unusual size may be 
 secured at the end of a string, and the quantity of mole- 
 cules settling upon the sides and the bottom of the vessel 
 may be considerably reduced. 
 
 If these groups of molecules be examined very care- 
 fully, it will be discovered that they consist of a combina- 
 tion of smaller groups called crystals. The surface of 
 each crystal is made up of a number of flat smooth faces 
 which join each other along straight edges. Crystals of 
 
MOLECULAR PHENOMENA 95 
 
 the same substance formed in the same solution usually 
 resemble each other very much in form, although their 
 faces are usually not all of the same form and size. Even 
 when the bases of these crystals are grown together so 
 that only the free surfaces of the crystals can be exam- 
 ined, their similarity in form may be recognized. In 
 order to make evident their similarity in form, crystals 
 must be placed so that the corresponding faces of the 
 different crystals which are inclined toward one another 
 at the same angles occupy the same relative positions. 
 
 86. Solids maybe Recognized by the Form of their Crystals. 
 A careful study of the crystals of different kinds of sol- 
 ids shows that crystals composed of the same kinds of 
 
 Salt Quartz Alum Sugar 
 
 molecules are constructed on the same general plan. This 
 is usually easily recognized when the crystal faces are 
 not large. On the other hand, when crystals composed of 
 one kind of molecules are compared with crystals com- 
 posed of another kind, it is noticed that the crystals dif- 
 fering in composition are usually dissimilar in form 
 (Fig. 61). It is possible, therefore, to recognize sub- 
 stances by the form of their crystals. It is not very 
 likely that this can be done readily and with confidence 
 by beginners, even in the case of crystals of very common 
 substances. But an examination of a few crystals, such 
 as those of salt, quartz, and alum, will give a fair idea of 
 the principle involved. 
 
 Crystals of salt have six principal rectangular faces, ar- 
 
96 ELEMENTARY PHYSICS 
 
 ranged like the faces of a cube. All the faces, therefore, 
 form angles of 90 degrees with the immediately adjoining 
 faces. 
 
 Crystals of quartz have the form of a six-sided prism, 
 terminated at each end (if both ends are free) with a six- 
 sided pyramid. The faces of the prism meet each other 
 at angles of 120 degrees. The faces of the prism meet the 
 faces of the pyramid at angles of about 140 degrees. The 
 alternate faces of the pyramid are sometimes slightly 
 rougher in appearance than the intermediate faces. 
 
 Crystals of alum are formed by a combination of two 
 geometric forms, the octohedron and the cube. The 
 typical octohedron consists of two similar pyramids, fac- 
 ing in opposite directions and united along their base. 
 Each pyramid consists of four triangular faces. All the 
 faces are identical in form ; they are equilateral triangles. 
 The faces of the octohedron meet along straight edges at 
 angles of about 110 degrees. The cube consists of six 
 rectangular faces meeting each other at angles of 90 de- 
 grees. In the combination, the faces of the cube usually 
 appear like little faces cutting off the corners of the octo- 
 hedron. The faces of the cube and those of the octohe- 
 dron meet at angles of about 125 degrees. 
 
 Crystals of sugar are more complex in form. While 
 not readily described in language which those not famil- 
 iar with geometry will easily understand, the fact that 
 they have a characteristic, form of their own (monoclinic) 
 is readily recognized. 
 
 87. Crystals Suggest that the Molecules of Different Sub- 
 stances Differ in Form No matter whether large or 
 small, whether single or in groups, whether presenting 
 their natural color or giving evidence of being stained 
 by foreign substances, crystals composed of the same 
 
MOLECULAR PHENOMENA 97 
 
 kinds of molecules possess the same general form. This 
 shows that they have been developed in accordance with 
 the same general plan. 
 
 It is difficult to understand how this can be possible, if 
 all molecules of the same kind do not follow the same 
 relative arrangement in all crystals. The remarkable 
 uniformity of the arrangement of the molecules is shown 
 in a striking 'way by the wonderfully even surface of the 
 flat faces of the crystals. Few objects in nature are 
 as absolutely smooth as are many of the faces on the 
 smaller-sized crystals of most minerals. The absolute 
 identity of the angle made by corresponding faces of 
 crystals composed of the same kind of molecules further 
 indicates the absolute identity in the arrangement of the 
 molecules. How is this arrangement possible, if mole- 
 cules of the same substance are not exactly alike in size 
 and form ? 
 
 The reason why crystals of different substances differ 
 in form must be because the molecules of different sub- 
 stances also differ in size and form. 
 
 88. Large Crystals Produced by Slow Cooling or Evapo- 
 ration. Molecules excluded from a saturated solution by 
 cooling or evaporation do not settle with equal readiness 
 upon all objects with which they come in contact. They 
 seem to display a kind of preference for certain loca- 
 tions. For instance, they are more apt to settle upon 
 rough bodies, such as strings and pieces of wood, than 
 upon smooth objects. But the greatest preference seems 
 to be shown for crystals composed of molecules identical 
 in kind with* those which are seeking a location. For 
 this reason the crystals first formed usually grow much 
 more rapidly than those which come into existence at a 
 later time. Even fragments of crystals lowered into the 
 
08 ELEMENTARY PHYSICS 
 
 solution by means of strings form good collecting cen 
 tres, if they consist of the same kind of molecules as 
 those which are being excluded from the liquid. By 
 their use the likelihood is much increased of securing 
 large crystals which may be removed easily from the 
 vessel. 
 
 In order to secure large crystals of salt, dissolve a con- 
 siderable quantity of salt in water, until the salt last 
 added remains undissolved even after considerable stir- 
 ring. Set aside the vessel containing the solution, in a 
 place where evaporation will take place very slowly. As 
 the water gradually evaporates, the crystals will develop. 
 In several weeks fine large crystals may be obtained. 
 
 In order to secure large crystals of alum or sugar, dis- 
 solve as much of the solid as possible in hot water. Tie 
 a piece of the solid at the end of a string and lower it 
 into the liquid. Then allow the saturated solution to 
 cool very slowly. In the case of sugar the temperature 
 of the heated water should not be raised above 140 de- 
 grees Fahrenheit and, while crystallization is taking 
 place, the temperature should never be allowed to fall 
 below 70 degrees Fahrenheit. 
 
 Good crystals of quartz can easily be purchased from 
 any dealer in minerals. (A. E. Foote, Philadelphia.) 
 
 89. Small Crystals Produced by Rapid Cooling or Evapo- 
 ration. In the formation of crystals of large size, the 
 rate of cooling seems to be a very important element. It 
 takes time for molecules to travel through any liquid. 
 If there be ample time, most of the excluded molecules 
 find their way to the crystals first formed, so that the 
 first formed crystals show the greatest growth. If, how- 
 ever, the solution be cooled rapidly, the molecules of the 
 solid in solution are excluded so rapidly and in such 
 
MOLECULAR PHENOMENA 99 
 
 great numbers that, before they can reach the crystals 
 already in existence, they form new groups at many 
 points within the liquid. These groups form new cen- 
 tres of collection, so that, in case of rapid cooling, crys- 
 tals are formed simultaneously throughout the liquid. 
 These crystals may at first be so small that they can be 
 recognized only by means of the microscope. If they are 
 very numerous, the solution simply assumes to the naked 
 eye a turbid appearance. 
 
 Granulated sugar and the finer grades of table salt are 
 produced by rapid evaporation accompanied by continual 
 stirring, in order that the small crystals, which are in 
 process of formation throughout the liquid, may not 
 grow together. When the crystals are of proper size 
 the liquid is withdrawn or the crystals are removed. 
 The rate of cooling may be so rapid that the molecules 
 cannot find time to arrange themselves in an orderly 
 manner. They then form upon the walls and the bottom 
 of the vessel a coating which gives no evidence of crys- 
 tallization. 
 
 The lavas which have poured out from the volcanoes in 
 different parts of the world have cooled with such vary- 
 ing degrees of rapidity that some of them have turned 
 into a mass of crystals, while others show no more than 
 the faintest trace of crystallization. Under the micro- 
 scope all gradations may be seen between lavas com- 
 posed entirely of crystals, and lavas which are practically 
 without trace of crystallization. 
 
CHAPTEE in 
 
 HEAT 
 
 90. The Three States of Matter. It is a familiar fact that 
 water may exist under three different forms. It may be 
 a solid, a liquid, or a gas. When solid, it is called ice, 
 when liquid, it is called water, and when gaseous, it is 
 called steam. The word water may also be used, in a 
 general sense, so as to include all forms of water. In 
 this case ice is looked upon as solid water, and steam as 
 water in the gaseous state. 
 
 It is not so well known that many other substances 
 also exist in three forms. The chief reason for this lack 
 of information is the fact that few substances are as use- 
 ful as water, in all three forms in which they can exist. 
 Few substances, therefore, have been studied as carefully 
 as water. Paraffine, sealing-wax, lead, and iron are all 
 solids at ordinary temperatures, but when paraffine is 
 used to form thin layers over jelly so as to exclude the 
 air, and when sealing-wax or lead is employed to seal 
 fruit jars, these substances are first melted. When it is de- 
 sired to cast lead into the form of bullets and iron into the 
 form of stoves, they must first be liquefied and then poured 
 into moulds. It is, therefore, well known that these sub- 
 stances exist both as solids and as liquids, but it is not 
 so well known that some of these can also exist as gases. 
 Nevertheless, paraffine can easily be heated sufficiently 
 
 100 
 
HEAT 
 
 to be turned into a gaseous form, and, in the enormously 
 heated atmosphere of the sun, lead and iron exist in the 
 form of gases. 
 
 Another reason why the existence of many substances 
 in all three forms is unknown, is because in many sub- 
 stances some of these forms require extremely high or 
 extremely low temperatures. Most metals, for instance, 
 assume the gaseous form only at very high temperatures^ 
 and many of the more common gases become liquid and 
 solid only at very low temperatures. For this reason, 
 air, which is composed almost wholly of two gases, one 
 part of oxygen and four parts of nitrogen, has been 
 known in the liquid, and even in the solid state, only 
 within the last twenty-five years. 
 
 As far as known all substances when sufficiently cooled, 
 assume the solid state. Some substances change from 
 gases into solids or from solids into gases, directly, with- 
 out assuming the intermediate liquid state ( 75). Many 
 solids, when heated beyond a certain temperature, do 
 not turn into liquids, but change their nature entirely. 
 Wood, for instance, when heated beyond a certain tem- 
 perature, does not become liquid, but burns ; it changes 
 into ashes and'certain kinds of gas. Changes of this kind 
 will be more fully studied later ( 195). 
 
 The general fact remains, that there are three states of 
 matter. The changes of matter from one state into an- 
 other are accompanied by interesting phenomena. The 
 phenomena will be studied in the following paragraphs, 
 water being taken as a familiar example. 
 
 91. The Temperature of Ice Is Variable. The tempera- 
 ture of a piece of iron exposed to the open air varies with 
 that of the surrounding atmosphere. In summer it is 
 warm, and in winter it is cold. 
 
PHYSICS 
 
 In a similar manner the temperature of a piece of ice 
 varies with the surrounding- atmosphere. At Werkojank 
 in Siberia the temperature of the air has been known to 
 fall to 90 below zero, Fahrenheit, or 122 below the 
 freezing- point. This is the coldest region so far dis- 
 covered on our globe. The temperature of the ice ex- 
 posed to the air in this locality must also fall to 90 below 
 zero. 
 
 In Ohio, temperatures of 15 below zero, or 47 below 
 the freezing point, are not frequent, but occur often 
 enough to be within the experience of anyone who has 
 lived within the State ten or more years. During the 
 existence of such cold weather, the temperature of ice 
 also falls to 15 below zero. Place the thermometer in 
 snow on some day when the temperature of the air is 
 much below the freezing point of water. When the 
 weather becomes warmer, the temperature of ice rises, 
 and when the temperature of the air exceeds 32 above 
 zero, the ice melts. 
 
 Ice-cream, at a temperature of 32 F., will melt readily 
 in summer time. But if its temperature be reduced con- 
 siderably below 32 F., it will remain in a frozen condition 
 until its temperature has risen to 32 F., when the process 
 of melting begins. For this reason caterers sometimes 
 lower the temperature of icercream which is intended 
 for evening parties, so much that it may become a little 
 warmer and still be in fit condition to eat several hours 
 after it has been delivered ( 114). 
 
 92. The Temperatures of Water and Steam are Variable. 
 That the temperature of water varies considerably is too 
 familiar a fact to require special mention. Between the 
 freezing point, 32 F., and the boiling point, 212 F., there 
 is a difference of 180 degrees. 
 
HEAT 103 
 
 It is not so well known, however, that the temperature 
 of steam may also vary considerably. At ordinary press- 
 ures of the atmosphere, at sea level, water is converted 
 into steam soon after its temperature reaches 212 F. 
 The steam which is formed also has a temperature of 
 212 F. If the steam is placed in a closed vessel, its tem- 
 perature may easily be raised considerably above 212 F. 
 The steam generated in the boiler of an engine always 
 rises above this temperature. When the temperature of 
 steam is so great that no water is present in a liquid 
 condition, even in the form of a mist, the steam is said to 
 be dry. 
 
 93. The Temperature at which Liquefaction and Solidifica- 
 tion Take Place. At ordinary pressures of the air, ice 
 melts at a temperature of 32 F. This temperature is 
 known as the melting point of ice. Other substances do 
 not melt at this temperature but have their own char- 
 acteristic melting points. Mercury, for instance, melts at 
 40 below zero, Fahrenheit, paraffine melts at 129 above 
 zero, sulphur at 239, tin at 449, lead at 619, zinc at 
 773, silver at 1749, copper at 1930, hard glass at 2012, 
 nickel at 2642, and iron at 2912 F. 
 
 Many substances melt at temperatures so low that they 
 are never seen in a solid state except in laboratories 
 where very low temperatures are produced artificially. 
 This is especially true of many gases, which are not only 
 never seen as solids outside of laboratories, but which 
 are never seen even as liquids, excepting under similar 
 conditions. 
 
 The fact that different substances melt at different de- 
 grees of temperature is utilized in a number of ways. 
 Lead, zinc, copper, and glass, for instance, may be read- 
 ily melted in a vessel constructed of iron. Why ? 
 
104 ELEMENTARY PHYSICS 
 
 If water is cooled sufficiently it freezes. The tempera- 
 ture at which it freezes is the same as that at which it 
 melts. In other words, the point of liquefaction is also 
 the point of solidification. This is also true of other 
 substances. 
 
 94. The Temperature at which Ebullition and Condensa- 
 tion Take Place. When subjected to a pressure of one 
 atmosphere, the ordinary pressure of the air at sea level, 
 water boils at a temperature of 212 F. This tempera- 
 ture is known as the boiling point of water. Other 
 liquids do not boil at this temperature but have their 
 own characteristic boiling point. Under a pressure of 
 one atmosphere, ether boils at 95, alcohol at 172, tur- 
 pentine at 319, and mercury at 662 F. 
 
 Some substances boil at such low temperatures that 
 they are never seen in the liquid state except in laborato- 
 ries. They are usually known only in the form of gases. 
 This is true of hydrogen, nitrogen, oxygen, and carbon 
 dioxide. 
 
 The fact that different liquids boil at different temper- 
 atures is utilized in the separation of various substances 
 by means of distillation. If a mixture of water and alco- 
 hol is heated above the boiling point of alcohol and be- 
 low that of water, the alcohol passes off in the form of a 
 vapor, while the water will be left behind. If the alco- 
 hol vapor is passed through a coil of tin or copper pipe 
 placed in cold water, the alcohol vapor is chilled and 
 returns to the liquid state. It may thus be secured 
 nearly free from water. 
 
 95. Change in Volume during Solidification and Liquefac- 
 tion. The passage of any substance from a liquid to a 
 solid state is invariably accompanied by a change of 
 volume. 
 
Volume ofutater before freezing. 
 
 HEAT 105 
 
 Water expands on turning into ice. It increases abou-t 
 ^ in volume (Fig. 62). The result is that when water 
 freezes in a pipe which has no 
 outlet, it often bursts the pipe. 
 The water in a pitcher freezes | roiu^^^/y^. ~~| 
 at the surface first. The layer 
 of ice thus formed serves as a 
 
 huge stopper, preventing the escape of the water beneath. 
 If any considerable part of the water in the lower part of 
 the pitcher freezes, the pitcher is likely to break. Since 
 water expands on turning into ice, ice is lighter than 
 water, and floats on the water. Beneath the layer of ice 
 covering the river during winter, the water still has a 
 temperature of 32 F., or slightly above. At this tem- 
 perature the kinds of fish found in northern climates can 
 live. 
 
 Most liquids contract on assuming the solid state. 
 Water is one of the most notable exceptions. Owing to 
 the abundance of water, and its great importance, both in 
 maintaining life and in making possible many lines of 
 manufacture, the exceptional behavior of water is more 
 familiar than the general behavior of other liquids. 
 
 When solids pass into the liquid state their change in 
 volume is exactly the reverse of the change which takes 
 place during solidification. 
 
 96. Changes in Volume during Ebullition and Condensa- 
 tion. The passage from a liquid to a gaseous state is 
 invariably accompanied by an increase of volume. 
 
 When water at a temperature of 212 F. is changed to 
 steam at 212 F., it does not change in temperature, but 
 it does change its state, and this change is accompanied 
 by an enormous change in volume. A cubic inch of 
 water at 212 is converted into 1696 cubic ipches, or 
 
106 
 
 ELEMENTARY PHYSICS 
 
 Volume of water 
 in form of Steam 
 
 FIG. 
 
 nearly one cubic foot, of steam having the same tempera- 
 ture (Fig 1 . 63). Water furnishes, bulk for bulk, a greater 
 
 volume of gaseous material than 
 
 O Volume of water ,-, , . . -. .-p-i . 
 
 in form of liquid any other liquid. This enormous 
 
 expansion is accompanied by 
 great force, and is utilized in all 
 the numerous kinds of steam-en- 
 gines. 
 
 The return from a gaseous to 
 a liquid state is always accom- 
 panied by a considerable de- 
 crease in volume. 
 
 97. For the Same Increase of 
 Temperature, Gases Expand More 
 than Liquids, and Liquids Expand 
 More than Solids. A considerable number of well-known 
 substances remain in the solid state when raised to a 
 temperature of 70 F. A large number are always in the 
 liquid state, and a much smaller number are always in 
 the gaseous state, at this temperature. 
 
 If now equal volumes of a variety of substances are 
 chosen, some of which are in the solid, and others in the 
 liquid, or gaseous, states at 70 F., and if their tempera- 
 ture is raised from 70 to 71 F., it is found that all in- 
 crease in volume, but that the amount of increase is not 
 the same in all cases. All gases expand at the same rate 
 for the same increase of temperature, but the rate of ex- 
 pansion of different liquids .and solids varies consider- 
 ably. In a general way it may be stated that for the 
 same increase in temperature, liquids expand on the 
 average about 20 times, and gases about 70 times as 
 much as solids. The figures secured will naturally vary 
 with the solids and liquids chosen for comparison. 
 
HEAT 107 
 
 This fact is true also of different states of the same 
 substance. For instance, the amount of increase in vol- 
 ume of water for a rise of one degree in temperature is 
 greater for water in the gaseous state than for water in the 
 liquid state, and for water in the liquid state than for water 
 in the solid state. These changes in volume without a 
 change of state are, however, very small as compared 
 with the changes in volume during any change from one 
 state to another ( 95, 96). 
 
 98. Effect of Pressure on the Melting Point. Water ex- 
 pands on turning into ice. Under a pressure of one at- 
 mosphere, water freezes at a temperature of 32 F. If 
 the pressure on the water is increased, there is a corre- 
 sponding resistance to the increase of volume, and water 
 must be cooled to a temperature below 32 F. before it 
 will expand violently enough to freeze. Pressure does 
 not assist in the freezing of water. It interferes. An 
 increase in pressure of a thousand pounds per square 
 inch, lowers the freezing point of water about 1 F. 
 
 When ice melts, it decreases in volume. An increase 
 of the pressure exerted on the ice tends to make it oc- 
 cupy less volume. If the temperature of the ice is as 
 high as 32 F., the ice can occupy considerably less vol- 
 ume by becoming water. Pressure, therefore, assists in 
 melting ice. 
 
 When particles of ice in the form of snow are pressed 
 together, they melt slightly at the surface. Immediately 
 on removing the pressure, the water formed turns again 
 into ice. The ice crystals become united at their surfaces 
 and a snowball is the result. When the rivers of ice 
 called glaciers pass over rocks, the ice is pressed against 
 the rock, and melts. The water thus formed turns into 
 ice again immediately on passing the obstruction. In 
 
108 ELEMENTARY PHYSICS 
 
 this way ice is able to flow down valleys and to follow 
 the irregular curves along the sides and the bottom of 
 the valley and yet may arrive at the foot of the glacier 
 in the form of compact ice. 
 
 Solids which expand on melting, when subjected to 
 pressure, will have their melting point raised instead of 
 lowered. 
 
 99. Effect of Pressure on the Boiling Point. Water sub- 
 jected to a pressure of one atmosphere boils at a tem- 
 perature of 212 F. When water at this temperature 
 turns into steam it increases in volume 1696 times. In 
 order to turn into steam, therefore, water must occupy 
 1696 times as much space, and in order to secure this 
 space it must push back the overlying air. 
 
 The force which water must exert in order to expand 
 into steam depends upon the pressure which the atmos- 
 phere exerts upon the water. If the atmosphere presses 
 upon the water with greater force, the water must expand 
 with greater force in order to turn into steam. If, on the 
 contrary, the pressure of the atmosphere upon the sur- 
 face of the water is less, the water need not exert so much 
 force in order to turn into steam. 
 
 The force which the molecules of water must exert in 
 order that water may turn into steam, must be suffi- 
 ciently energetic to cause the molecules to leave the 
 water remaining in the vessel, and to enable them to pass 
 off into space. The motions of molecules which enable 
 them to move about, become more energetic as their 
 temperature increases. Hence in order that molecules 
 of water may escape in the form of steam, their tempera- 
 ture must be greater when the atmospheric pressure is 
 greater, and their temperature need not be so high when 
 the atmospheric pressure is less. 
 
HEAT 109 
 
 Increase of pressure raises the boiling point of a liquid ; 
 decrease of pressure lowers the boiling- point. 
 
 100. Determination of Elevation by Boiling Point Since 
 the pressure exerted by air varies with the elevation of 
 the object pressed upon ( 13), the force which water 
 must exert in order to turn into steam also depends upon 
 the elevation above sea level. At a greater elevation less 
 force is required, at a lower elevation a greater force is 
 necessary. Hence at a greater elevation water boils at a 
 lower temperature, and at a lower elevation a higher tem- 
 perature is required. If a table be prepared giving ac- 
 curate information on the relationship between boiling 
 points, atmospheric pressures, and elevations, the boiling 
 point of water at any locality may be used as a means of 
 determining its elevation. 
 
 In a general way it may be stated that a lowering of 1 
 F. in the boiling point of water is equivalent to a rise in 
 elevation of approximately 538 feet. At Quito, the high- 
 est city in the Andes of South America, water boils at a 
 temperature of 194.2 F. This is 17.8 F. below the boil- 
 ing point of water at sea level. Therefore, the elevation 
 of Quito is approximately 17.8x538=9574.4 feet. At 
 what temperature does water boil at the locality in which 
 you live ? What, therefore, is its elevation above sea 
 level approximately ? 
 
 On the top of Mount Blanc water boils at a tempera- 
 ture of 185.8 F. Eggs cannot be cooked in water boil- 
 ing at this temperature. Many other cooking operations 
 requiring water boiling at higher temperatures cannot 
 be carried on in open vessels at these elevations. 
 
 In boilers connected with high-pressure engines the 
 boiling point is often raised to temperatures exceeding 
 300 F. 
 
110 ELEMENTAEY PHYSICS 
 
 101. Relation between Atmospheric Pressure and Evapo- 
 ration. When water evaporates, the molecules separate 
 and form a vapor which passes off into the air. The 
 greater the pressure of the atmosphere, the greater is 
 the difficulty with which the molecules of the water can 
 pass off into the air. Therefore, evaporation takes place 
 much more readily in localities where atmospheric press- 
 ure is low. 
 
 The relationship between atmospheric pressure and 
 evaporation are very similar to those between atmos- 
 pheric pressure and ebullition, or boiling. 
 
 In the evaporation of syrups during the manufacture 
 of sugar, the operation is carried on in large air-tight 
 chambers from which the air and the vapor formed by 
 the evaporation are removed by an air-pump. The space 
 above the syrup approaches so nearly to the conditions 
 of a vacuum, that the molecules of the water are far less 
 impeded in their efforts to leave the surface of the liquid. 
 Evaporation takes place, therefore, much more readily 
 and at much lower temperatures, when sugar is heated in 
 vacuum-pans. This is done in order to keep a tempera- 
 ture sufficiently low so as not to burn the sugar. 
 
 102. Difference between Intensity and Quantity of Heat. 
 A further study of the phenomena involved in the change 
 of a body from a solid to a liquid, or from a liquid to a 
 gaseous state, makes it desirable to have an accurate idea 
 as to the quantity of heat which is necessary to produce 
 these changes. 
 
 The conception of quantity of heat is not so familiar to 
 most persons as the conception of intensity of heat. The 
 intensity of heat shown by a body is known as its tem- 
 perature. The unit of temperature is called a degree. 
 The position of the top of a column of mercury at various 
 
HEAT 111 
 
 degrees of temperature is marked off on the scales of 
 thermometers, and by means of these instruments the 
 temperature of a body can be directly measured. 
 
 An ounce of red-hot iron (at a temperature of about 
 1000 F.) possesses a much greater intensity of heat than 
 boiling water at a temperature of 212. The intensity of 
 heat does not depend upon the quantity of material, but 
 merely upon the question, how hot it is. Therefore, an 
 ounce of red-hot iron has much greater intensity of 
 heat, notwithstanding the small quantity of iron, than a 
 barrel of boiling water. 
 
 Which, however, possesses the greater quantity of 
 heat ? Suppose that on a cold winter day the ounce of 
 red-hot iron and the barrel of boiling water be placed in 
 separate rooms, both of small size and having the same 
 temperature. Which will heat up the air in the room to 
 the higher temperature, the ounce of red-hot iron or 
 the barrel of boiling water ? Unquestionably, the latter. 
 Or, suppose that the ounce of red-hot iron and the bar- 
 rel of boiling water be placed in large tanks half full of 
 snow ? Which will melt the most snow ? Again, un- 
 questionably the barrel of boiling water. Which, there- 
 fore, possesses the greater quantity of heat ? The barrel 
 of boiling water. 
 
 From this it may be seen that the quantity of heat de- 
 pends not only upon the temperature of a body but also 
 upon the quantity of material contained in that body. 
 
 103. The Unit of Quantity of Heat. In order to com- 
 pare different quantities of heat, it is desirable to have 
 some method of measuring this quantity. When we 
 wish to measure length, we use a wooden or steel rule 
 upon which feet, inches, and parts of inches have been 
 carefully marked. When we wish to measure weight, it 
 
ELEMENTARY PHYSICS 
 
 is sufficient for ordinary purposes to use some form of a 
 balance accompanied by a. set of known weights. The 
 intensity of heat is measured by means of a thermome- 
 ter supplied with a scale upon which the degrees are 
 indicated. But there is no instrument to determine 
 the quantity of heat. Therefore the quantity of heat 
 contained in a body must be determined indirectly 
 by drawing conclusions from some of the effects which 
 the heat contained is able to produce. In practice, the 
 quantity of heat contained in a body is determined by 
 noticing through what range of temperature it will raise 
 a known quantity of water. 
 
 Just as there must be definite units in order to meas- 
 ure length, weight, and temperature, so there must also 
 be some definite unit in order to measure quantity of 
 heat. The unit here accepted is the quantity of heat 
 which will raise the temperature of one pound of water 
 from 32 F. to 33 F. This unit is called a calorie,. 
 
 Within ordinary ranges of temperature it takes very 
 nearly the same quantities of heat to raise a pound of 
 water one degree in temperature, no matter what be the 
 temperature of the water to which the additional heat is 
 applied. For this reason it is sufficiently accurate for 
 most purposes to assume that any quantity of heat which 
 raises one pound of water 1 F. above its original tem- 
 perature is equal to a calorie. Therefore, to raise one 
 pound of water 15 degrees would require 15 calories, and 
 to raise 10 pounds of water 8 degrees would require 80 
 calories. 
 
 This rule applies only to water in the liquid state. To 
 raise one pound of ice one degree in temperature requires 
 .504 calorie, and to raise a pound of steam through an 
 equal range of temperature requires .4805 calorie. 
 
HEAT 113 
 
 104. Heat Consumed daring Change of State, from Solid to 
 Liquid, or from Liquid to Gas. Heat is consumed in rais- 
 ing 1 a body from a lower to a higher temperature. Heat 
 is consumed, for instance, in raising a pound of ice from a 
 temperature of 10 F. to 32 F., in raising- the water from 
 32 to 212, and in raising- the steam from 212 to 250. 
 From the concluding- statements in the preceding- para- 
 graph it may be seen that the quantities of heat required 
 in the three cases just mentioned are about 11, 180, and 
 19 calories respectively. 
 
 Is any heat consumed in changing- a body from a solid 
 to a liquid or from a liquid to a g-aseous state ? If any 
 heat is consumed in any of these operations, the ther- 
 mometer does not indicate it. 
 
 If, for instance, ice be taken at 10 F. and be slowly 
 heated in a vessel over a Bunsen burner, a thermometer 
 imbedded in the ice will show a rise in temperature until 
 a temperature of 32 F. is reached. Then for some time, 
 notwithstanding the fact that heat is being continually 
 consumed by the ice, the thermometer will show no rise 
 in temperature. During this time the ice is melting. 
 
 As soon as all the ice has melted, the rise in tempera- 
 ture again begins, and the mercury in the thermometer 
 moves up steadily until it indicates a temperature of 212. 
 Then the mercury in the thermometer once more be- 
 comes stationary. Notwithstanding the fact that heat is 
 being consumed by the water, no rise in temperature 
 takes place. During this time the water is changing 
 into steam. When all of the water has been changed 
 into steam, the temperature once more rises provided the 
 steam is confined in a vessel so that it cannot escape. 
 If it is desired to raise steam to very high temperatures, 
 it must be enclosed in very strong vessels, otherwise the 
 
114 ELEMENTARY PHYSICS 
 
 rapidly increasing pressure of the steam may cause an 
 explosion. 
 
 Since the temperature of the ice when it begins melt- 
 ing is 32 ' F., and since the temperature of the water im- 
 mediately after melting has taken place is also 32, it is 
 evident that the heat consumed has not been utilized in 
 causing a rise in temperature, but must all have been 
 used in effecting the change of state from solid ice to 
 liquid water. 
 
 In the same manner all the heat which was applied to 
 the water while it was changing into steam, was con- 
 sumed in changing liquid water into gaseous steam. 
 None of it was utilized in effecting a rise in temperature. 
 
 105. Quantity of Heat Necessary to Change Ice to Water. 
 Heat is consumed in changing ice to water. In order 
 to determine the quantity of heat necessary for this pur- 
 pose, the experiment must be conducted so that all the 
 heat used shall be employed solely for converting ice 
 into water. None of it must be used to raise the temper- 
 ature of ice from some lower temperature up to the melt- 
 ing point, 32 F., and none of it must be used to raise 
 the water resulting from the melting of the ice from its 
 original temperature, 32 F., to any higher temperature. 
 This can be accomplished if the ice experimented upon 
 has a temperature of 32 F. at the beginning of the ex- 
 periment, and if the experiment is stopped the instant all 
 the ice has changed to water, before any of the vrLi/er 
 formed by the ice has been raised above its original 
 temperature. 
 
 From a number of experiments it has been learned 
 that, if one pound of ice having a temperature of 32 F. 
 be placed in one pound of water at 174 F., all of the ice 
 will be melted by means of the heat derived from the 
 
HEAT 115 
 
 pound of hot water. As fast as tne ice melts, the water 
 thus formed mingles with the water used in the melting 
 of the ice. The temperature of this mixture falls as the 
 ice melts, and after the ice is all melted the temperature 
 of the water (including both the original water used and 
 that derived from the melted ice), is found to be 32 F. 
 At this moment, the temperature of both the water re- 
 sulting from the melting ice and the water whose orig- 
 inal temperature was 174 is found to be the same, 
 namely, 32. This was also the temperature of the ice. 
 The ice turned to water without any change of temper- 
 ature. The temperature of the pound of water used in 
 melting the ice fell 142. 
 
 The change of state from ice to water was caused by 
 the quantity of heat supplied by the hot water during its 
 fall of temperature. Since, to fall one degree, a pound 
 of water must give out 1 calorie, to fall 142 degrees it 
 must give out 142 calories. Therefore 142 calories were 
 consumed in melting the pound of ice. One hundred 
 and forty-two calories of heat may also be supplied by 
 2 pounds of water falling 71, or by 3 pounds of water fall- 
 ing 47.33 degrees. If, therefore, a pound of ice at 32 is 
 placed in two pounds of water at a temperature of 103 
 F., or in three pounds of water at 79.33 F., the same 
 result is obtained as in the preceding experiment. Suffi- 
 cient heat is given out to melt all of the ice. If less heat 
 is supplied, all of the ice does not melt. If more heat is 
 supplied, the ice firsts melts and then the water thus 
 formed is raised in temperature. 
 
 106. Total Quantity of Heat Necessary to Melt Ice and to 
 Raise the Temperature of the Resulting Water. If it be de- 
 sired to change a pound of ice at 32 to water at a tem- 
 perature of say 60, this can be effected by placing the 
 
116 ELEMENTAEY PHYSICS 
 
 ice in water of such quantity and temperature that it will 
 give up sufficient heat both to change the ice to water 
 and also to raise the water from a temperature of 32 to 
 that of 60. The amount necessary is a total of 170 cal- 
 ories. This can be secured by taking 2 pounds of water 
 having a temperature of 145, or five pounds of water 
 having a temperature of 94. 
 
 One hundred and eighty calories of heat are necessary 
 to raise a pound of water from a temperature of 32, the 
 freezing point, to a temperature of 212, the boiling point. 
 Almost as many calories of heat, 142, are necessary merely 
 to change the ice to water, without raising its temperature 
 at all. All of this heat is necessary to weaken the cohe- 
 sion between the molecules to such an extent that the 
 molecules, which are only moderately movable in the 
 case of ice, may change their position with the greatest 
 of readiness in the form of water. This ready change of 
 position of the molecules is characteristic of all sub- 
 stances while in the liquid state. 
 
 Instead of using the heat supplied by water, the heat 
 given out by a Bunsen burner may be used in these ex- 
 periments. But in that case it would be difficult to esti- 
 mate the number of calories of heat supplied by the 
 burner during the change of ice to water. 
 
 107. Bough Estimation of Quantity of Heat Necessary to 
 Change Water to Steam. With the same source of heat, for 
 instance the flame from a small Bunsen burner, it takes 
 about 5| times as long to change any quantity of boiling 
 water into steam as it does to raise the same quantity 
 of water from the freezing point, 32, to the boiling point, 
 212. Therefore it may be said that it takes about 5^ 
 times as much heat to convert water into steam as it 
 does to raise water from the freezing to the boiling point. 
 
HEAT 117 
 
 Since it requires 180 calories to raise one pound of water 
 from the freezing- to the boiling point, it will require 
 5J x 180, or about 960 calories, to convert one pound of 
 water at 212 into steam. This is only a rough estimate, 
 and if the experiment be attempted, the results might 
 not be so satisfactory, but it will serve to give a concep- 
 tion of the quantity of heat necessary to cause the con- 
 version of water into steam. 
 
 108. The Quantity of Heat Necessary to Change Water 
 into Steam may be Determined Indirectly. The amount of 
 heat necessary to change water into steam is not usually 
 determined directly. Instead of finding how many cal- 
 ories of heat are necessary to change a pound of water 
 into a pound of steam, the experimenter determines how 
 many calories of heat the steam is able to give out when 
 it returns from its condition of steam to that of water. If, 
 for instance, a certain quantity of steam at a temperature 
 of 212, on changing to water at a temperature of 212, 
 can be shown to give out 100 calories of heat, it may 
 justly be assumed that 100 calories of heat are necessary 
 also to change that weight of water to steam. 
 
 The number of calories given off by the steam may be 
 determined by noticing the rise in temperature produced 
 in one pound of water by a given quantity of steam. If, 
 for example, any given quantity of steam is found to 
 raise the temperature of one pound of water from 60 to 
 80 it is a safe conclusion that the steam has given out 20 
 calories of heat. This is the method used in the follow- 
 ing experiment. 
 
 109. Exact Determination of Quantity of Heat Necessary 
 to Change Water into Steam. Place a Florence flask on a 
 piece of wire-gauze covering one of the rings of an iron 
 ring stand (Fig. 64). Half fill the flask with water.. 
 
118 
 
 ELEMENTARY PHYSICS 
 
 Close the flask with a one -hole rubber stopper and 
 through the single opening passing through the stop- 
 per thrust one end of a glass tube. 
 
 At some distance from the Florence flask place a sec- 
 ond vessel. Pour into the vessel a known quantity of 
 water, for instance, one pound. Close the vessel with a 
 rubber stopper perforated by means of three openings. 
 Through one opening thrust a glass tube so that its lower 
 end will extend nearly to the bottom of the water in the 
 
 FIG. 64. 
 
 vessel, and connect the upper end of this tube with the 
 Florence flask. Through the second opening insert a 
 chemical thermometer so that the bulb will dip into the 
 water. Through the third opening thjust a glass tube 
 bent in such a manner that if any of the escaping steam 
 condenses into water, the water will not be able to return 
 to the vessel. Determine the temperature of the water 
 in the vessel carefully by means of the thermometer. 
 Disconnect the Florence flask temporarily from the ves- 
 
HEAT 119 
 
 sel and heat the water in the flask until steam is given 
 off freely. Then quickly reconnect the flask with the 
 vessel and continue the formation of steam in the flask. 
 
 The steam formed in the flask will leave by means of 
 the glass tube. It will pass along the tube and will es- 
 cape at its other end into the water which occupies the 
 lower part of the vessel. As the steam rises through 
 this water, it is cooled by the water and returns to the 
 liquid form. In so doing it gives up to the water that 
 part of the heat which was formerly necessary to convert 
 the steam, from its original liquid, to its later gaseous 
 form. Any steam which escapes from the vessel must 
 still have a temperature of 212, otherwise it would no 
 longer be in the form of steam, and, since it cannot have 
 given up any part of its heat to the water, its escape will 
 not in any way affect the results of the experiment. 
 
 If the pound of water at the beginning of the experi- 
 ment had a temperature of 60 F., and, if after the en- 
 trance of steam for some time, its temperature has been 
 raised to 212, the steam has supplied 152 calories of heat 
 to the water in the vessel. It is now necessary to deter- 
 mine how much steam was necessary to supply 152 cal- 
 ories of heat. This may be determined by weighing the 
 water as soon as its temperature has arrived at 212 and 
 subtracting from this the weight of the water at the be- 
 ginning of the experiment. The additional weight is 
 evidently due to the addition of that part of the steam 
 which was condensed on entering the vessel. 
 
 If the weight of the water added to the vessel be .156 H- 
 of a pound, it is evident that .156+ of a pound of steam 
 was able to raise the temperature of 1 pound of water from 
 60 F. to 212 F. Therefore, .156 + of a pound of steam 
 gives up 152 calories of heat on changing from steam at 
 
120 ELEMENTARY PHYSICS 
 
 212 to water at 212. It must, consequently, have re- 
 quired 152 calories of heat to convert .156 + of a pound of 
 water, at a temperature of 212, to steam at 212. 
 
 From this it may be calculated that it would have re- 
 quired about 965 calories of heat to convert an entire 
 pound of water, at 212, to steam at 212. 
 
 In raising one pound of water from ice at a tempera- 
 ture of to steam at a temperature of 250, 1321.3 cal- 
 ories are needed. Of these only 214.3 calories are used 
 in raising the temperature. The remainder are utilized 
 in changing the ice to water and the water to steam. 
 In the diagram (Fig. 65) the number of calories needed 
 to raise the temperature is indicated by the length of the 
 
 Steam 250 
 Water 212^ ^ 18.2 -^ 
 
 Steam 212- 
 
 16.1 + 142 +180 +965 +18.2 -1321.3 Calories 
 o hich only 214.3 raise the temperature.. 
 
 ^ Water 32 
 
 FIG. 65. 
 
 vertical lines and the number needed to change ice to 
 water, and water to steam, by the length of the horizon- 
 tal lines. 
 
 110. Latent Heat of Water and Steam. When the changes 
 of ice to water, and of water to steam were first studied, 
 observers merely noticed that the mercury in the ther- 
 mometers failed to rise during the time in which these 
 changes were taking place, while at other times any addi- 
 tion of heat was at once followed by a rise in tempera- 
 ture. Observers, therefore, said that during the change 
 of a substance from a solid to a liquid state, and later 
 again during the change from a liquid to a gaseous state, 
 a definite quantity of heat in some way became hidden. 
 
HEAT 121 
 
 The heat which disappeared while these changes were 
 taking place was called latent heat, the word " latent " 
 meaning to lie hidden or concealed. Such expressions 
 as the latent heat of water and the latent heat of steam 
 are still in common use. In the preceding paragraphs 
 it has been shown that the latent heat of water is equal 
 to 142 calories, and the latent heat of steam to 965 cal- 
 ories. 
 
 When steam returns to water, or water to ice, the latent 
 heat is again given out. 
 
 111. Importance of the Large Latent Heat of Water. Ev- 
 ery pound of water, on changing from water at a tempera- 
 ture of 32 to ice at 32 F., must give out 142 calories of 
 heat. This quantity of heat must escape before the next 
 pound of water can turn into ice. 
 
 When the water at the surface of a lake or river freezes, 
 it not only gives out heat to the air above but also to the 
 water below. This causes the freezing of lakes and rivers 
 to be very slow. Therefore, the depth to which water is 
 frozen is very slight compared to the depth to which it 
 would be frozen, if the heat which it gave out on chang- 
 ing from a liquid to a solid state were very small. 
 
 During very cold weather, when the vegetables in the 
 cellar are in danger of freezing, some people, in order 
 to prevent serious injury, place a tub of water near the 
 vegetables. Every pound of water which freezes gives 
 out 142 calories of heat. This heat becomes dispersed 
 throughout the cellar, and the total quantity of heat 
 given out is often sufficient to prevent serious injury to 
 the vegetables. Fortunately, most winter vegetables do 
 not suffer from cold until a temperature of several de- 
 grees below the freezing point has been reached. 
 
 When ice is melted in the laboratory, the heat necessary 
 
122 ELEMENTARY PHYSICS 
 
 to change ice into water is supplied by a Bunsen burner 
 or some other artificial source of heat. In this case the 
 source of heat is evident. However when ice melts on ex- 
 posure to the open air, without any especial attempt being- 
 made to supply heat to the ice, the fact that it requires a 
 considerable quantity of heat to melt every pound of ice 
 is likely to be overlooked. From what source is this 
 heat obtained? From the surrounding objects. 
 
 The fact that ice, in order to melt, withdraws heat from 
 surrounding objects may be shown in a number of ways. 
 The ice in an ice-chest, on melting, withdraws heat from 
 the objects placed in the chest. When snow melts in the 
 early spring, the air frequently feels very chilly. So 
 much heat is withdrawn from surrounding objects, includ- 
 ing both the air and the snow in the immediate neighbor- 
 hood, as to prevent the rapid melting of the snow. If it 
 were not for this fact, disastrous floods, similar to those 
 which are often caused at present by thaws which follow 
 a considerable fall of snow, would be far more extensive 
 and frequent. Near Lake Erie, spring is often so much 
 retarded by the melting of the winter's ice and snow, that 
 the buds of trees usually do not begin growth until the 
 danger of late frost is past. 
 
 112, Importance of the Large Latent Heat of Steam. The 
 fact, that it requires 965 calories of heat to convert one 
 pound of water into steam, makes necessary the consump- 
 tion of enormous quantities of coal for the production 
 of steam in boilers. The heat which is given out, when 
 steam returns to the liquid condition, is utilized in the 
 heating of buildings. Every pound of steam which con- 
 denses to water inside of a radiator, used for heating, 
 supplies 965 calories of heat to the room. As the con- 
 densed water returns to the boiler, it continues to give 
 
HEAT 123 
 
 out heat, in consequence of the water falling to a tem- 
 perature below 212. 
 
 113. Heat Withdrawn by the Evaporation of Liquids. 
 When water is converted into vapor by means of ordinary 
 evaporation, heat is consumed in the same manner as 
 when water is converted into steam, but the process is 
 much slower. The heat which is used to convert the 
 water to vapor is withdrawn from surrounding objects. 
 After a shower of rain the water evaporates and cools the 
 air by withdrawing from the air a portion of its heat. 
 The evaporation of water, sprinkled on the floor, cools 
 the air of a warm room. The evaporation of perspira- 
 tion during a hot day serves to cool the body. When 
 the air is already saturated with moisture, so that the 
 perspiration fails to evaporate promptly, much discom- 
 fort is felt. Fanning causes a more rapid evaporation of 
 the perspiration, and thereby produces a refreshing cool- 
 ness. In tropical countries water is cooled by being 
 placed in porous jars set in a draught of air. A small 
 quantity of water passes through the pores of the jar, 
 and, on evaporating, withdraws so much heat from the jar 
 that the temperature of the water remaining in the jar is 
 lowered considerably below that of water contained in 
 any vessel which does not permit ready evaporation. 
 
 Ether, when thrown in a fine spray, evaporates so 
 rapidly and withdraws such great quantities of heat 
 from surrounding objects that, when thrown on any part 
 of the body this part becomes numb on account of the 
 cold. In this manner, parts of the body are rendered 
 insensible to pain during surgical operations. 
 
 114, Cold Produced by Solution When a substance dis- 
 solves, it turns from a solid into what may be considered 
 a liquid form. It requires heat to enable it to make this 
 
124 ELEMENTARY PHYSICS 
 
 change of state. The necessary heat is withdrawn from 
 surrounding objects, the chief of which is the liquid in 
 which the substance dissolves. 
 
 If a certain quantity of the solid, called ammonium 
 nitrate, is thrown into an equal weight of water, the solid 
 dissolves and the temperature of the water falls about 
 45 F. If four parts, by weight, of crystallized chloride 
 of calcium are mingled with three parts of snow, both 
 the chloride of calcium and the snow become liquefied 
 and the temperature of the mixture falls about' 86. If 
 one part, by weight, of salt be mixed with two parts of 
 finely broken ice, both are liquefied, and if care is taken 
 to exclude the surrounding heat, the temperature is 
 lowered nearly 38. The salt causes the ice to melt more 
 rapidly than the ice would melt if the salt were not pres- 
 ent. The result is that a much greater degree of cold is 
 produced than would be the case if the ice were melted 
 alone. For this reason the cream in an ice-cream freezer 
 is surrounded by a mixture of ice and salt. Even if salt 
 alone were placed in water the temperature of the water 
 would be lowered to a perceptible extent ( 193). 
 
 115. Gases Cool as they Expand. A gas can be caused 
 to expand by increasing the quantity of heat which it 
 contains. This may be accomplished by means of a Bun- 
 sen burner or some other source of heat. It requires 
 heat to cause molecules to move apart. If a gas is al- 
 lowed to expand, by reducing the pressure which is 
 exerted upon it, the heat must be secured from some 
 other source. This source of heat is the gas itself to- 
 gether with all the objects enclosed within and surround- 
 ing the gas. Therefore, when a gas expands the tempera- 
 ture of the gas is found to decrease. 
 
 This principle is illustrated in the formation of clouds. 
 
HEAT 125 
 
 Air can absorb a certain quantity of moisture. This 
 quantity increases with the temperature of the air. 
 When air is heated it rises ( 127). As it ascends to 
 higher elevations, the heated air is subjected to less 
 pressure from the atmosphere. It therefore expands, 
 and, in order to expand, heat is necessary. This heat is 
 secured from the air and the moisture which it contains. 
 In consequence, the temperature of the air falls. When 
 air cools, it can no longer hold as much moisture. A part 
 of the moisture, therefore, leaves the air and collects in 
 minute drops. If the number of these drops is large, 
 they may be seen some distance above the earth in the 
 form of a cloud. The drops of water are so small and so 
 light that they seem to remain suspended in the air at 
 nearly the same levels. In reality, however, they settle 
 slowly through the air, but on reaching the warmer air 
 below are reabsorbed again, so that the falling of the 
 little drops of moisture which form clouds does not attract 
 attention. 
 
 116. Capacity of Different Substances for Heat. Some 
 substances ha,ve a greater capacity for heat than others. 
 For instance, the same amount of heat which will raise 
 one pound of water one degree would be sufficient to 
 raise the temperature of 33 pounds of lead one degree. 
 In other words, water has 33 times more capacity for 
 heat than lead. Copper has about three times as much 
 capacity for heat as lead. Iron has nearly four times as 
 much. On account of the great capacity of water for 
 heat, water is taken as the standard with which the 
 capacity of other substances is compared. 
 
 117. The Irregular Expansion of Water, As a rule, any 
 increase in the quantity of heat contained in any sub- 
 stance causes it to expand. This rule applies not only to 
 
126 ELEMENTARY PHYSICS 
 
 substances while in the solid, liquid, or gaseous condition, 
 but also to substances while changing from a solid to a 
 liquid, or from a liquid to a gaseous state. 
 
 Water is one of the most striking exceptions to this 
 rule. When water changes from the solid to the liquid 
 state, it contracts. In the liquid state it occupies only 
 .917 of the volume occupied by the ice just before melt- 
 ing. The result is that ice floats in water. 
 
 Most liquids expand on being heated from the melting 
 to the boiling point. They do not expand equally for 
 every equal increase of temperature, but they at least 
 expand. 
 
 Water, on the contrary, at first contracts and then ex- 
 pands. From 32 it contracts until a temperature of 39.2 
 is reached; from 39.2 to 212 it expands. The rate of 
 contraction and of expansion is unequal. From 32 to 
 39.2 the rate of contraction decreases until at 39.2 F. the 
 rate of contraction is very slight. Above 39.2 the rate of 
 expansion is at first very slight and then increases until 
 the boiling point is reached. At 200 F., for instance, an 
 increase in temperature of 1 F. causes an increase in vol- 
 ume six times as great as a corresponding increase of 
 temperature at 50. 
 
 The total decrease of volume between 32 and 39.2 F. 
 is onlyjpoQ-of the original volume. The total increase of 
 volume on raising water from a temperature of 39.2 to 
 that of 212 F., is about | s of its volume at 39.2 F. Ice, 
 therefore, floats in water at any temperature. 
 
 118. Heat Produced by Collision. Place a small piece of 
 iron on an anvil and pound it vigorously with a hammer. 
 It soon becomes warm, and, if it be pounded for a suf- 
 ficient length of time, it will become too hot to be held in 
 the hand. 
 
HEAT 127 
 
 Pounding 1 causes the molecules at the surface of the 
 iron to move violently. The molecules at the surface 
 strike the molecules just beneath and set these also in 
 motion. Molecules strike against molecules, rebound, and 
 strike against other molecules, until, finally, all the mol- 
 ecules within the body take part in the motion. In the 
 meantime, if the pounding be kept up, the heat of the 
 piece of iron continually increases. 
 
 119. Heat Produced by Compression and Friction. When 
 a bullet is fired against a stone, the lead of 
 which the bullet is composed is violently com- 
 pressed and becomes hot. As the lead is 
 stopped by the stone, the concussion is suffi- 
 cient to set the molecules of lead into violent 
 motion. Increase of heat is the result. 
 
 A sudden compression of air will also produce 
 heat. Attach a small piece of dry tinder to the 
 end of a piston fitting air-tight in the cylinder 
 of a fire syringe, and plunge the piston quickly 
 into the cylinder (Fig. 66). During the com- 
 pression of the air within the cylinder, the mole- 
 cules of air are set in motion and this generates 
 sufficient heat to ignite the tinder. FlG 66 
 
 Hold a piece of iron against the edge of a 
 dry, rapidly revolving grindstone. The grindstone knocks 
 off tiny fragments of the iron. These fragments are so 
 heated as a result of the concussion that, as they fly off, 
 they produce the appearance of a stream of sparks. 
 
 Rub the knuckles vigorously back and forth on the 
 sleeve of your coat. The friction of the knuckles against 
 the sleeve sets the molecules in violent motion and the 
 knuckles become hot. Indians start fires by rapidly 
 whirling the end of a soft piece of wood in the hollow 
 
128 ELEMENTARY PHYSICS 
 
 formed on the upper surface of a harder piece. The fric- 
 tion produces enough heat to set the softer wood on fire. 
 
 120. Heat a Form of Molecular Motion. Collision, com- 
 pression, and friction, all cause molecular motion, and 
 all result in an increase of heat. This suggests that heat 
 is the result of molecular motion, and that increase in 
 molecular motion results in an increase of heat. When 
 the temperature of a body is increased, the molecules are 
 set in more violent motion. They strike each other with 
 such force that they knock each other farther apart, and, 
 in consequence, the body expands. When a body cools, 
 the motion of its molecules becomes less violent, the mol- 
 ecules do not knock each other so far apart, and the vol- 
 ume of the body decreases ( 61, 62, and 63). 
 
 The nature of these molecular motions is not fully un- 
 derstood. The motions so far described partake chiefly 
 of the nature of a bombardment. The molecules move in 
 one direction until they come into violent contact with 
 other molecules, when they rebound and move in some 
 other direction. This form of motion from place to place 
 may be called a motion of translation. 
 
 When one molecule strikes against another molecule, 
 there is also another form of motion in addition to the 
 motion of translation. The molecule struck apparently is 
 caused to tremble. Since we do not exactly know what 
 a molecule is, or what form it possesses, we do not have 
 a very exact idea as to the nature of the trembling which 
 takes place in the material of a molecule after it has been 
 struck. But we may get some conception of such a mo- 
 tion by studying the motions of a very elastic rubber ball. 
 
 When a rubber ball (Fig. 67, a) is struck violently it 
 flattens for a moment transversely to the direction of 
 the blow (b). Then the displaced particles return so 
 
HEAT 
 
 129 
 
 violently to their original positions that they do not 
 cease their motion when the ball has resumed its original 
 shape, but for a moment the particles, which were moving- 
 outward, continue to move outward, and those which 
 were moving- inward continue to move inward. The re- 
 sult is that the flattened ball first returns to its origi- 
 nal shape (c) and then assumes the form of a slightly 
 elongated football (d). Then the particles which have 
 been moving outward, return, and those which have 
 been moving inward, move outward ; until the flattened 
 
 FIG. 67. 
 
 condition, assumed at first (b), is re- 
 sumed. This action continues for 
 some time, becoming less violent if 
 no further concussions occur, until 
 the motion finally ceases. This is 
 one of the many possible forms of vibration. The con- 
 cussion of molecules against each other may give to them 
 a vibratory motion somewhat of this character. 
 
 The motions which cause the molecular phenomena 
 called heat are, probably, a combination of the motions 
 of vibration and translation, and possibly also of other 
 forms of motion, such as rotation. 
 
 121. Conduction. Place one end of a very short bar of 
 iron on an anvil and pound this end vigorously with a 
 hammer. Not only will the end which was pounded 
 
130 ELEMENTARY PHYSICS 
 
 become hot, but, if the pounding- is continued long- 
 enough, the opposite end, which was not struck, will also 
 become distinctly warmer. The violent motions of the 
 molecules struck by the hammer are communicated from 
 molecule to molecule until even the molecules at the ex- 
 treme end of the bar of iron are set in violent motion. 
 
 Hold in your hand one end of an iron rod about eight 
 inches long, and place the other end in the flame of a 
 Bunsen burner. The end held in the hand also becomes 
 hot. The molecules brought into contact with the flame 
 are caused to move violently on account of the heat, and 
 this motion is communicated from molecule to molecule 
 until the molecules at the end held in the hand also move 
 violently. 
 
 In the case just described, both the bar and the rod 
 maintain their general form, showing that while these 
 molecules are in motion they do not change their relative 
 positions. If there were any considerable change in the 
 relative positions of the molecules, the general form of 
 the bar and rod could not be maintained. The communi- 
 cation of heat from one part of a body to another caused 
 by communication of motion from molecule to molecule, 
 not accompanied by any change in the relative positions 
 of the molecules, is called conduction. 
 
 122. Relative Conductivity of Different Solids. As a rule 
 the metals are the best conductors of heat. But different 
 metals vary greatly in the rapidity with which they con- 
 duct heat. Copper conducts heat about four times as 
 effectively as iron. Rocks are much poorer conductors 
 of heat. The conductivity of sandstone, for instance, is 
 only one -twentieth of that of iron. The various kinds of 
 wood are still poorer conductors. The wood of the fir-tree, 
 for example, conducts heat only ffa as readily as iron. 
 
HEAT 131 
 
 The temperature of the human body is 98.5 F. If on 
 a cold winter morning-, when the temperature of the air 
 in a room and of all the objects within the room is about 
 32 F., a person should step with bare feet in succession 
 on a piece of iron, a piece of wood, and the carpet, the 
 iron would feel colder than the wood, and the wood would 
 feel colder than the carpet. Iron conducts the heat away 
 from the foot more rapidly than wood, and wood conducts 
 the heat away more rapidly than the woollen carpet. In 
 consequence, the temperature of the foot is lowered much 
 more rapidly when in contact with the iron than when in 
 contact with the carpet. Therefore the iron feels colder, 
 although its temperature is practically the same as that 
 of the carpet. 
 
 Clothing for summer use should be made of material 
 which conducts heat away from the body fairly well. 
 Linen and cotton are the best materials in common use. 
 During the winter, clothing should be made of the poor- 
 est conductors of heat. Wool is one of the best non- 
 conductors in common use. Furs are used extensively in 
 northern countries. The feathers and down of northern 
 birds are such effective non-conductors that in some 
 countries they are sewn up in ticks and in this form are 
 used in the place of quilts as a covering for beds during 
 the winter months. The non-conductivity of loose wool, 
 furs, and feathers is largely due to the air imprisoned 
 between the fibres or hairs forming the material. 
 
 Paper is a good non-conductor. In early spring, and 
 late fall, plants may be protected by covering them with 
 paper. If, on a very cold night, several layers of ordi- 
 nary newspaper be placed between the quilts on a bed, 
 the warmth of the body will be better kept up. Several 
 newspapers wrapped around the body, underneath a 
 
132 ELEMENTARY PHYSICS 
 
 coat, will serve as a very effective protection against the 
 cold in stormy winter weather. 
 
 123. Heating of Glass Vessels. All varieties of glass are 
 poor conductors of heat. If hot water is poured into a 
 thick glass vessel having the ordinary temperature of the 
 room, the interior begins to expand strongly before the 
 heat has been conducted to the outer surface, and, in con- 
 sequence, the vessel often cracks. For similar reasons a 
 thick glass vessel is likely to crack if heat is suddenly 
 applied to the exterior by means of a Bunsen burner. 
 In order to avoid cracking on account of unequal expan- 
 sion, glass vessels used for chemical experiments requir- 
 ing heat are usually made of thin glass. 
 
 Even thin glass vessels require the use of certain pre- 
 cautions to avoid cracking. The greatest heat should 
 not be applied to the end of the test-tube, since this part 
 of the tube is often thinner than the sides. Apply the 
 heat gradually, moving the test-tube in and out of the 
 flame, and rolling it slightly between thumb and finger. 
 Beakers and flasks may easily become unequally heated 
 and crack. In order to avoid this, place the beaker on a 
 piece of brass wire-gauze (Figs. 64, 71) supported by a 
 ring stand. Brass conducts heat 150 times better than 
 glass. Hence the brass wire-gauze will conduct the heat 
 so rapidly to all parts of the base of the beaker or flask, 
 that the base of the beaker is more equally heated and 
 the risk of breaking it is very much lessened. But even 
 in this case the heat should be applied gradually at first. 
 
 Do not apply heat to that part of a test-tube which is 
 on a level with the top of the liquid inside. The dry 
 glass is likely to become so much warmer than that which 
 is kept cooler by contact with the liquid that the tube 
 may crack. For similar reasons do not place in the di- 
 
HEAT 
 
 133 
 
 ~F.IG. 68. 
 
 rect flame any test-tube to whose walls the drops of any 
 liquid adhere. 
 
 124. Liquids Poor Conductors of Heat. The fact that 
 liquids are not good conductors of heat can be shown in 
 quite a number of ways. In north- 
 ern countries large fires are often 
 built on the ice of rivers and ponds, 
 to furnish light and heat to skaters. 
 As soon as a thin layer of water is 
 formed beneath the burning wood, 
 the ice almost ceases to melt. If a 
 metal pan is floated upon water 
 (Fig. 68), and alcohol is placed 
 in the pan and set on fire, the 
 metal pan communicates the heat 
 quickly to the water, but the lower 
 part of the water remains cold for a very long time. 
 If the flame of a Bunsen burner is directed against the 
 
 upper part of a test tube 
 nearly full of water (Fig. 69), 
 the upper part of the water 
 may be made to boil while 
 the water in the lower part of 
 the tube remains quite cool. 
 125. Gases are Poor Conductors. Gases 
 are poor conductors of heat. It has 
 already been mentioned that the air en- 
 closed in loose wool, furs, and feathers, 
 contributes much toward the non-con- 
 ductivity of these substances. The air enclosed between 
 double doors and windows also serves as a non-conductor, 
 and prevents the escape of heat from houses. The air 
 enclosed between particles of sawdust placed within the 
 
 FIG. 
 
134 
 
 ELEMENTARY PHYSICS 
 
 FIG, 70. 
 
 walls of ice-houses is an important factor, during- the 
 summer, in preventing* the heat outside of the ice-house 
 from gaining- entrance to the ice within ( 217-223). 
 
 126. Convection of Liquids. Notwithstanding the fact 
 that liquids are poor conductors, heat is often readily 
 communicated from one part of the liquid 
 in a vessel to another part in the same 
 vessel. If a Bunsen burner be placed be- 
 neath a test-tube, inclined at a moderate 
 angle, the water at the base of the tube 
 becomes hot (Fig. 70). It expands, be- 
 comes lighter, and, in consequence, it rises 
 along the upper side of the inclined tube 
 and floats on top of 
 the water not yet 
 heated. At the same 
 time the water, which 
 is at a greater distance above the 
 flame, and which is still cold, comes 
 down along the lower side of the 
 inclined tube, and takes the place 
 of the water which is rising. This 
 colder water becomes heated in its 
 turn and also rises. The result 
 is the formation of a continu- 
 ous current which can be easily 
 recognized if fine cork-dust is 
 thrown into the water. In the 
 same manner, irregular currents are started in the water 
 in tea-kettles, or in coffee-pots, when placed over a fire 
 (Fig. 71). 
 
 In all of these cases the heated water expands and be- 
 comes lighter. The colder water is heavier. Both kinds 
 
 FIG. 71. 
 
HEAT 
 
 135 
 
 of water are pulled down by the attractive force of the 
 earth (gravitation). Since the heavier water is pulled 
 down with greater force, the heavier water will be drawn 
 beneath the lighter water, and the lighter water in con- 
 sequence will then rise to the top. 
 
 The water at the surface of the ocean within the tropics 
 becomes much more heated than that at the surface within 
 the Arctic regions. In consequence, the colder water of 
 the Arctic ocean gradually and very slowly settles be- 
 neath the warmer water of more southern regions, while 
 the heated water of the Torrid zone gradually passes 
 northward along the surface un- 
 til it reaches the Arctic Ocean. 
 This is one of the causes which 
 produce currents in the ocean. 
 
 127. Convection of Gases. 
 When air is unequally heated, 
 so that one part of the air be- 
 comes hot while the other re- 
 mains cold, the hot air becomes 
 lighter than the cold air. The 
 cold air therefore crowds down- 
 ward and takes the place of the 
 warmer air. Currents are pro- 
 duced in the air similar to those 
 produced in water, and, as a re- 
 sult of the currents, the entire 
 air finally becomes heated. 
 
 Place a piece of sheet-iron 
 on a ring stand over a Bunsen 
 burner (Fig. 72). Hold a paper 
 pin-wheel over the sheet-iron, 
 causes the wheel to rotate. 
 
 T 
 J 
 / 
 
 *', 
 
 ttV 
 
 M t 
 ^1 
 
 uL_u5 
 
 / i 
 
 FIG. 72. 
 
 The ascending hot air 
 If convenient, hold the same 
 
136 ELEMENTAET PHYSICS 
 
 wheel near the side of a stove-pipe and notice the same 
 result, caused in this case by the ascending currents of 
 heated air along- the sides of the heated stove-pipe. 
 
 If the door connecting- a warm room with a cold one is 
 opened slightly, and a lighted candle is held at the base, 
 the coming in of the cold air is readily detected by 
 the fact that the flame is blown inward. If the candle is 
 held near the top of the door, the flame is blown out- 
 ward, showing that the cold air is passing beneath the 
 warm air, and that warm air is rising above the cold air. 
 
 In order to ventilate a room during winter weather 
 without too great a loss of heat, the lower part of the 
 window should be raised. In order to cool the room 
 as rapidly as possible, the upper window should be low- 
 ered. 
 
 When fire is placed in a stove, the colder, heavier air in 
 the room pushes the much warmer and lighter air in the 
 stove up the chimney. The heated air is not drawn up 
 the chimney from above as is often supposed, but is 
 forced up by the weight of the colder air surrounding it. 
 
 128. Convection Due to Displacement of Molecules. Heat 
 is conveyed to the more remote parts of liquids and gases 
 by means of currents. The molecules in contact with the 
 source of heat are set in violent vibration. The heated 
 molecules are continually pushed aside, and give the 
 cooler molecules an opportunity to become heated in 
 their turn. The final result is that all the molecules 
 become heated. The transference of heat through liquids 
 and gases, by means of a circulation among the mole- 
 cules, is called convection. 
 
 Why the molecules of liquids and gases are not able to 
 communicate their vibrations readily from molecule to 
 molecule, as is the case with many solids, is unknown. 
 
HEAT 137 
 
 The result, however, is that the transference of heat in 
 liquids and gases is dependent upon currents. 
 
 129. Sensation of Heat and Cold cannot be Relied upon to 
 Determine Temperature. Although the body is usually able 
 to distinguish between heat and cold, it is not always a safe 
 guide. Take three basins, the first containing ice-water, 
 the second, lukewarm water, and the third, water as hot 
 as can be endured without injury. Place the left hand in 
 the ice-water and the right hand in the hot water. As 
 soon as the hands have grown somewhat accustomed to 
 the temperatures in the two basins, plunge both hands at 
 the same time into the lukewarm water. The left hand 
 now feels warm, while the right hand feels cool. In fact, 
 the left hand feels warmer than the right hand, not- 
 withstanding the fact that both hands are in the same 
 basin. 
 
 During winter weather, persons coming into a room 
 from the very cold air of the street, often call the air in 
 the room hot and close. A person working vigorously in 
 the same room may also consider the air too warm. Nev- 
 ertheless a thermometer hanging on the wall may regis- 
 ter a temperature of only 68, and most of the persons 
 seated in the room may consider the air a trifle cool. 
 A thermometer is the only safe guide for determining the 
 actual temperature of a room. We cannot rely alto- 
 gether upon our senses. 
 
 130. The Sensations of Temperature, Pain, and Touch. It 
 is commonly stated that man has five senses : seeing, 
 hearing, tasting, smelling, and feeling. The result of re- 
 cent investigations, however, indicates that a number of 
 distinct sensations have been usually classed together 
 under the heading, feeling. Among these are the sensa- 
 tions of touch, temperature, and pain. 
 
138 ELEMENTARY PHYSICS 
 
 Investigations indicate that there are as many kinds of 
 nerves as there are sensations ; that each kind of nervea 
 is so constituted as to be particularly sensitive toward 
 some special form of irritation. Thus, the nerves of sight, 
 if irritated by a pin, are not aroused to a true sensation of 
 touch or pain. 
 
 Each kind of nerve terminates at some special part of 
 the body. Thus, the nerves of sight terminate in the eye ; 
 those of hearing, in the ear; of taste, in various parts of 
 the mouth ; of smell, in the nose. No attempt was made 
 formerly to distinguish carefully between the areas in 
 which the nerves of touch, temperature, or pain terminate. 
 In consequence no distinction was discovered between 
 these three different kinds of nerves. They were classed 
 together as nerves of feeling, and it was believed that any 
 part of the skin capable of feeling one of these sensations 
 was also capable of feeling the other two. 
 
 However, it is now known that in certain diseases of the 
 spinal cord areas of skin may be mapped out in which 
 the sensation of pressure (touch) is lost, while that of tem- 
 perature remains ; areas may also be found in which the 
 sensation of temperature is lost and that of pressure re- 
 mains. Sometimes, in cases of persons under surgical 
 treatment, before the patient comes fully under the influ- 
 ence of ether or of chloroform, the sensation of touch re- 
 mains, while that of pain is lost. In each of these cases, 
 the facts indicate that one class of nerves has lost its 
 power to receive sensations, while the other class of nerves 
 mentioned still remains active. 
 
 Recent investigations indicate that there are two sets 
 of nerves of temperature: one to indicate warmth, the 
 other to indicate cold. In some diseases the patient can 
 appreciate warmth applied to the skin but not cold. The 
 
HEAT 139 
 
 areas of the skin in which the sensation of heat is percep- 
 tible, and the areas in which the sensation of cold can be 
 felt, have been carefully mapped out on various parts of 
 the surface of the body, and the discovery has been made, 
 that these areas are often distinct, although sometimes 
 they overlap. These areas appear to be the places where 
 the nerves capable of perceiving a rise of temperature, 
 and those capable of perceiving- a fall of temperature, ter- 
 minate. Since the sensations of heat and cold are often 
 localized in different areas, it seems reasonable to con- 
 clude that the sensations are felt by different nerves. 
 
 By a similar method of investigation it has been deter- 
 mined that there are special nerves to convey the sensa- 
 tion of pain. 
 
 131. Extremes of Temperature. By means of the electric 
 furnace, M. Moissan, of Paris, has secured temperatures 
 as high as 6,000 to 7,000 degrees Fahrenheit. The tem- 
 perature of the sun is considerably greater. Widely 
 different estimates have been made. Probably 16,000 to 
 18,000 degrees Fahrenheit is near the truth. Owing to 
 the scattering of the heat as it spreads away from the sun, 
 only one two-billionth of the total quantity of heat given 
 out by the sun ever reaches the earth. The remainder 
 passes to other points in space. 
 
 When liquid air is allowed to evaporate, temperatures 
 of about 312 degrees below zero, Fahrenheit, are pro- 
 duced. By allowing solid hydrogen to liquefy and then 
 to evaporate, Professor Dewar, of London, has succeeded 
 in getting temperatures as low as 432 degrees below zero. 
 Theoretically, the lowest temperature which it will ever 
 be possible to secure is 460 degrees below zero, Fahren- 
 heit. It is unknown whether there is any upper limit to 
 temperature. 
 
CHAPTEE IV 
 CHEMISTRY, ATOMS 
 
 132. Principles Underlying Recognition of Gases. Gases 
 play a very important part in many chemical phenomena. 
 Therefore, the ability to recognize gases is of the highest 
 importance. Some gases may be distinguished with some 
 exactness by their color, or by their peculiar odor. But 
 many gases are both colorless and odorless, and their 
 recognition depends upon a clear understanding of their 
 chemical properties. Among the most important of these 
 colorless gases are oxygen, hydrogen, carbon dioxide, and 
 nitrogen. 
 
 The action of these gases has been studied under many 
 kinds of chemical influences, and the action of each gas 
 under each of these influences is fully known. By a 
 comparison of the results obtained, it has been deter- 
 mined which actions or combinations of actions are char- 
 acteristic of each particular gas, and which therefore can 
 be used as a means of identification. Several of the im- 
 portant characteristics of oxygen, hydrogen, carbon diox- 
 ide, and nitrogen are given in the following paragraphs. 
 
 133. Pneumatic Trough. Procure a galvanized iron 
 trough (Fig. 73) constructed in accordance with the fol- 
 lowing directions : 
 
 Height, 8 inches. Width, 10 inches. Length, 18 inches. 
 Edges along the top strengthened by turning them over 
 a strong wire. A handle at each end of the trough for 
 
 140 
 
CHEMISTEY, ATOMS 141 
 
 convenience in lifting. Four holes, f of an inch in 
 diameter, near the upper edge of the trough ; each hole 
 3 inches from the nearest corner, the lower edge of the 
 hole not more than one inch below the top of the trough. 
 Across the centre of the trough, a permanent shelf, also 
 of galvanized iron, strengthened by turning the edges 
 over strong iron wire ; this shelf, 8 inches wide, so sup- 
 ported as to be 2 inches below the top of the trough ; in 
 this shelf, four holes f of an inch in diameter, each 
 
 FXG. 73. 
 
 hole 2 inches from the edge of the shelf and 2.5 inches 
 from the side of the trough. 
 
 Fill the trough with water. Lower a fruit-jar into the 
 water, and allow the water to rush in and fill the jar com- 
 pletely, so that no air remains in the jar. This can be 
 done easily if jars of proper size and form are selected. 
 Invert the jar in the water, raise it, and place it over one 
 of the holes in the shelf. If the mouth of the jar be kept 
 below the surface of the water, none of the water will es- 
 cape. It is held up by the pressure of the air. On this 
 account the trough is usually called a pneumatic trough. 
 
 Instead of the pneumatic trough here described, a pan 
 
142 ELEMENTARY PHYSICS 
 
 or basin may be used. Fill the jar with water, cover it 
 with a piece of wet pasteboard, invert it, place its mouth 
 beneath the level of the water in the basin, remove the 
 pasteboard and support the jar on a piece of the hoop of 
 a bucket bent into the form of a letter V placed horizon- 
 tally. Insert the end of the delivery tube ( 134) beneath 
 the mouth of the jar. 
 
 134. Method of Securing Oxygen, Crush four ounces of 
 potassium chlorate in a mortar, until it is about as coarse 
 as granulated sugar. Mix it thoroughly with four ounces 
 of manganese dioxide. Place in a te^t-tube enough of 
 this mixture to fill one-third of the tube (Fig. 73). Close 
 the top of the tube with a one-hole rubber stopper. 
 Through the hole insert a glass tube, and over the glass 
 tube, slip one end of a piece of rubber tubing about 30 
 inches long. Thrust the free end of this rubber tube 
 through the nearest opening in the side of the trough and, 
 leading it through the water, push it up through that hole 
 in the shelf which is directly beneath the mouth of an in- 
 verted jar filled with water. The friction between the 
 rubber tubing and the rough sides of the opening is 
 sufficient to hold the tubing in place. 
 
 Hold the test-tube in an inclined position, and heat it 
 by means of a Bunsen burner. At first apply a moderate 
 amount of heat to the upper part of the powder, shifting 
 the flame of the burner back and forth, so as not to injure 
 the test-tube by overheating a small part of the glass 
 while the rest of the tube is still comparatively cold. In- 
 crease the heat gradually until gas is given off freely, but 
 not with such rapidity as to visibly disturb the powder. 
 If the gas is given off too rapidly, hold the Bunsen burner 
 at a greater distance. As the supply of gas given off by 
 the upper part of the powder begins to lessen, apply the 
 
CHEMISTKY, ATOMS 143 
 
 heat to the lower part of the tube. Toward the end of 
 the experiment it will be necessary to increase the 
 amount of heat considerably in order to keep up the free 
 flow of the gas. This gas will flow through the rubber 
 tube and will escape into the water just within the mouth 
 of the jar on the shelf in the pneumatic trough. Do not 
 collect the gas until it bubbles freely through the water. 
 The rubber tube used to convey the gas is called the de- 
 livery tube. 
 
 As the gas escapes from the tube, bubbles of the gas 
 rise to the top of the inverted jar and collect there in such 
 quantities that they push the water toward the lower 
 part of the jar. As the gas accumulates, less and less 
 water is left in the jar. Finally gas fills the entire 
 jar and begins to escape at the bottom. Now move the 
 jar to some other part of the shelf, keeping the mouth of 
 the jar below the surface of the water. Prepare a fresh 
 jar for the collection of an additional supply of gas. 
 Jars filled with oxygen may be removed without losing 
 any of the gas by covering the mouth of the jar with wet 
 filter-paper or a wet blotter before lifting it out. 
 
 As soon as the flame of the Bunsen burner is removed, 
 the gas within the test-tube cools, shrinks in volume, and 
 tends to form a vacuum. The -pressure of the outside air 
 is then likely to force cold water through the delivery 
 tube into the hot test-tube. This almost invariably cracks 
 the test-tube. Therefore, the delivery tube should always 
 be removed from the pneumatic trough before removing 
 the test-tube from the flame of the Bunsen burner. 
 
 135. Properties of Oxygen. Oxygen is slightly heavier 
 than air. It may therefore be kept in bottles which are 
 not inverted, provided the mouths are covered with wet 
 filter-paper. When retained in an inverted bottle, the 
 
144 ELEMENTARY PHYSICS 
 
 mouth must dip into water, the latter serving as a 
 stopper. 
 
 Set fire to a long- splinter of wood. Blow out the 
 flame and, while the end of the splinter is still glowing-, 
 thrust it down into a jar filled with oxygen. The glow- 
 ing tip will burst into flame. Repeat the operation until 
 the supply of oxygen is exhausted. Oxygen supports 
 combustion. Heat the unwound end of a 
 stranded iron picture-wire red hot, dip it into 
 powdered sulphur, and, while the sulphur is 
 still burning, lower the wire into the jar filled 
 with oxygen (Fig. 74). The iron burns with 
 brilliant scintillations. Drops of a black melted 
 substance fall to the bottom and often crack the 
 jar, unless the bottom of the jar is covered by at least an 
 inch in depth of water. A piece of pasteboard held over 
 the top of the jar while the burning is taking place, will 
 prevent the too ready escape of the gas. 
 
 If a burning match is brought in contact with the 
 oxygen at the mouth of the jar, where the gas comes in 
 contact with the air, the oxygen does not burn, although 
 the match burns more brightly. Mixed with air, it does 
 not explode. When oxygen is mingled with lime-water, 
 the lime-water remains colorless and transparent. 
 
 136. Oxygen is Present in Ordinary Air. The glowing tip 
 of a splinter of wood will not burst into flame on being 
 held quietly in air, but it may be caused to burst into 
 flame by bringing it in contact with more air, either by 
 waving the splinter through the air, or by blowing air 
 against the glowing end of the splinter. In other words, 
 air acts as though it contained oxygen diluted with con- 
 siderable quantities of some other gas which does not 
 support combustion. 
 
CHEMISTKY, ATOMS 
 
 145 
 
 137. Method of Securing Hydrogen. Introduce into a 
 two-necked Woulfe bottle sufficient granulated zinc, or 
 sheet zinc cut into small pieces, to form 
 a layer at least half an inch deep (Fig-. 75). 
 Pour in water enough to raise the water 
 level about half an inch above the zinc. 
 In one of the necks of the bottle place a 
 rubber stopper, and through the hole in 
 the stopper thrust a thistle-tube until 
 the bottom of the tube is below the sur- 
 face of the water. In the second neck of 
 the bottle place another rubber stopper. 
 Through the hole in this stopper thrust 
 one end of a bent glass tube ; to the 
 other end of this tube attach the de- 
 livery tube. Fill with water two jars 
 on the shelf of the pneumatic trough, 
 as in the preceding experiment, and 
 introduce the open end of the delivery 
 tube into the mouth of one of these 
 
 jars. 
 
 75. 
 
 Pour into the thistle-tube a mixture 
 of one part of hydrochloric acid and three parts of water. 
 Hydrogen is given off at the place of contact between 
 the acid and the zinc. As this hydrogen passes over 
 into the jar, it carries with it the air which is in the 
 Woulfe bottle. The first jar of gas collected is a mixt- 
 ure of hydrogen and air. Since a mixture of hydrogen 
 and air explodes violently when set on fire, the first jar 
 of gas collected should be thrown away as soon as the 
 collecting of gas in the second jar has been started. 
 Of course, heat should not be applied at any time to 
 the Woulfe bottle, nor should any flame, large or small, 
 
146 ELEMENTARY PHYSICS 
 
 be allowed in the vicinity of the apparatus while the 
 collecting- of gases is going on. 
 
 In order to ascertain whether the hydrogen is pure, be- 
 fore the collecting- of gas in the second jar is begun, col- 
 lect a test-tube full of the gas in the same manner as in 
 the case of the jars. Raise the test-tube from the water, 
 being careful to retain its inverted position, and light the 
 gas quickly at the mouth of the tube. If the gas burns 
 quietly and without any explosion, it is pure. Since 
 there is only a small quantity of gas in the test-tube, the 
 test is not dangerous even if the hydrogen is still mixed 
 with air. 
 
 As soon as the production of gas becomes slow, more 
 hydrochloric acid should be added. In adding the acid, 
 never fill more than half of the bowl of the thistle-tube ; 
 otherwise any unexpectedly rapid production of gas may 
 produce such a great pressure within the bottle, that a 
 part of the acid may be pushed out of the top of the 
 thistle-tube. 
 
 138. Properties of Hydrogen, Place a bottle containing 
 hydrogen in an upright position and remove the paper 
 or glass covering the mouth, for three minutes by the 
 clock. Drop a lighted match into the jar. Why has the 
 gas escaped ? 
 
 Hydrogen weighs T V as much as air. Therefore in the 
 following experiments the jars or test-tubes containing 
 the hydrogen should be held in a vertical position, mouth 
 downward. 
 
 If a burning splinter of wood is thrust up into an in- 
 verted jar filled with hydrogen, the flame is extinguished 
 by the hydrogen (Fig. 76). Hydrogen does not support 
 combustion. However, the gas itself is set on fire where 
 it comes in contact with the air, at the open mouth of the 
 
CHEMISTRY, ATOMS 
 
 147 
 
 FIG. 76. 
 
 jar. The oxygen in the air supports the combustion of 
 the hydrogen. When hydrogen is mingled with lime- 
 water, the lime-water remains colorless and 
 transparent. 
 
 If the hydrogen in the jar is thoroughly 
 mixed with air, the hydrogen in all parts of 
 the jar can burn at the same time, and if 
 it is set on fire, it burns so rapidly and vig- 
 orously that violent explosion results. On 
 this account it is safer not to use a fruit- jar 
 full of hydrogen for these experiments. A 
 large test-tube filled with hydrogen is suffi- 
 cient for most experiments. 
 
 139. Method of Securing Carbon Dioxide. 
 In a two-necked Woulfe bottle (Fig. 75) place about one- 
 third of a tumblerful of marble or of limestone broken 
 into small lumps. Pour in water enough to cover the 
 bottom of the bottle. Insert a thistle-tube, connect the 
 delivery tube, and arrange for the escape of gas into the 
 jars on the shelf of the pneumatic trough, as in the pre- 
 ceding experiments. Then pour in concentrated hydro- 
 chloric acid enough to cover the marble. In order to se- 
 cure carbon dioxide unmixed with air, throw away the 
 first jarful of gas collected. 
 
 Since carbon dioxide is considerably heavier than air, it 
 may be collected without the use of the pneumatic trough. 
 Through the mouth of an erect, empty jar thrust the open 
 end of the delivery tube, so that the gas will escape into 
 the jar at the bottom. The air in the jar will be gradually 
 pushed out by the entering gas. The flow of the gas 
 should be allowed to continue for some time, in order to 
 be certain that the last remnant of air has been removed 
 from the jar. In order to retain the gas, the jar should 
 
148 
 
 ELEMENTARY PHYSICS 
 
 be covered with a piece of wet filter-paper or window 
 glass. 
 
 At the ordinary pressure of the air, water absorbs its 
 own volume of carbon dioxide. If it be desirable to re- 
 tain for any length of time the gas collected in the jar 
 over the pneumatic trough, slip a piece of ordinary 
 window glass under the mouth of the jar, and set the jar 
 aside in an erect position. 
 
 140. Properties of Carbon Dioxide. Since carbon dioxide 
 weighs 1J times as much as air, the jar containing this 
 gas should be placed in an erect position. If a burning 
 
 FIG. 77. 
 
 splinter of wood is thrust down into a jar filled with car- 
 bon dioxide, the flame is extinguished, and the carbon 
 dioxide does not ignite at the mouth of the jar, where it 
 comes in contact with the air. It does not explode when 
 mingled with air. 
 
 Pour carbon dioxide down a slanting wooden trough, 
 along which is arranged a row of short, lighted candles 
 (Fig. 77). The candles are extinguished in succession, as 
 soon as they are covered by the down ward- flowing but 
 invisible gas. 
 
 Pour water into a fruit- jar containing carbon dioxide 
 
CHEMISTRY, ATOMS 149 
 
 until it is one-third full of water. Close the jar air-tight, 
 shake it vigorously, and open it again under water, mouth 
 downward. A large part of the carbon dioxide has dis- 
 appeared. It has been taken up or dissolved by the 
 water. 
 
 When the gas is allowed to flow for a short time 
 through lime-water, the lime-water assumes a milky ap- 
 pearance. The same effect may be produced by pouring 
 a small quantity of lime-water into a jar containing car- 
 bon dioxide and shaking the jar violently. In conse- 
 quence of the shaking the gas and the lime-water be- 
 come intermingled. 
 
 In order to prepare lime-water, place a small lump of 
 quick-lime, such as is used by plasterers and masons, in 
 a jar full of water. Allow the jar to stand over night, 
 then pour off the clear liquid and, if it is not perfectly 
 clear, filter it. 
 
 141. Carbon Dioxide Present in Ordinary Air and in the 
 Breath. Place lime-water in a bottle closed with a rub- 
 ber stopper provided with two openings (Fig. 78). 
 Through one opening 
 thrust a glass tube be- 
 neath the surface of the 
 lime-water, until near 
 the bottom of the bottle. 
 Through the other open- 
 ing thrust a short tube 
 extending only a short 
 
 distance beneath the 
 
 _ FIG. 78. 
 
 stopper. Through the 
 
 second tube suck the air out of the upper part of the 
 bottle. Ordinary air runs down the first tube and bub- 
 bling up through the lime-water gradually produces a 
 
150 
 
 ELEMENTARY PHYSICS 
 
 milky appearance, thus showing the presence of small 
 quantities of carbon dioxide in ordinary air. 
 
 Eeplace the lime-water with a fresh solution and, re- 
 versing the process, blow the breath through the first 
 tube. The milky appearance is pro- 
 duced much more rapidly, thus show- 
 ing that the lungs give out a greater 
 quantity of carbon dioxide than they 
 take in. In other words, the lungs 
 increase the quantity of carbon dioxide 
 in the air. 
 
 142. Method of Securing Nitrogen. 
 Take a glass tube about 18 inches long, 
 and having an internal diameter of 
 about half an inch. Round the edges 
 at the ends by holding them a short 
 time in the flame of a Bunsen burner 
 and, after cooling, close both ends of 
 the tube by means of rubber stoppers. 
 With a file mark off transverse lines at 
 points 3, 6, 9, 12 and 15 inches from the 
 end of the stopper, closing the upper 
 end of the tube (Fig. 79). This will 
 leave a space of more than 2 inches 
 B between the last mark and the stopper 
 at the lower end of the tube. Fill this 
 space with the dark-brown liquid 
 known as potassium pyrogallate and 
 again close the tube. Air now occu- 
 pies a length of 15 inches (Fig. 79 A). Shake the liquid 
 back and forth in the tube for at least 15 minutes. Then 
 open the lower end of the tube beneath the water sur- 
 face. Water rushes up into the tube, showing that a 
 
 Fro. 79. 
 
CHEMISTEY, ATOMS 151 
 
 part of the air has been dissolved by the potassium 
 pyrogallate. 
 
 In the meantime a large part of the potassium pyro- 
 gallate, since it is a heavier liquid than water, escapes 
 into the water beneath. Close the tube and shake it 
 back and forth again. Most of the potassium pyrogallate 
 is now loosened from the walls of the tube and sinks 
 with the water into the lower part of the tube. Open 
 the lower end of the tube again below the water surface, 
 and raise or lower the tube until the water in the tube 
 and in the vessel are at the same level (Fig. 79 B). Then 
 close the tube again. That part of the air which remains 
 in the tube now occupies only -$ as much space as the 
 space originally occupied by the air. It moreover ex- 
 hibits entirely different properties. It is that part of air 
 known as nitrogen. 
 
 143. Presence of Nitrogen in the Air. Thrust a burning 
 splinter of wood into that part of the air remaining in 
 the tube after the preceding experiment. The flame is 
 extinguished at once. It is evident that the oxygen 
 which was formerly present in the air has been removed 
 by means of the potassium pyrogallate. Moreover the 
 experiment enables us to determine the relative propor- 
 tion of oxygen in ordinary air. Since ^ of the ordinary 
 air has disappeared at the close of the experiment, this 
 suggests that oxygen forms about ^ of the air. It was 
 seen in a former experiment that the proportion of car- 
 bon dioxide in ordinary air is small, in fact, only about 
 ^Vtf of any voltftne of ordinary air consists of carbon 
 dioxide. Very small quantities of other gases are also 
 present. Among these argon has recently attracted con- 
 siderable attention. The gas which remains in the tube 
 in the preceding experiment, however, is nearly all ni- 
 
152 
 
 ELEMENTARY PHYSICS 
 
 trogen. Nitrogen, therefore, forms about f of ordinary 
 air. 
 
 144. Properties of Nitrogen. Objects placed in nitrogen 
 do not burn. On the contrary, nitrogen extinguishes 
 burning objects. Under ordinary conditions it is im- 
 possible to set fire to nitrogen. Mingled with air, 
 it does not explode. Mingled with lime-water, it does 
 not produce a milky appearance. Under ordinary condi- 
 tions nitrogen is a very inactive or inert gas. It may be 
 distinguished from oxygen, hydrogen, and carbon dioxide 
 by the fact that it does not produce the effects produced 
 by these gases. This inactivity of nitrogen is its most 
 striking characteristic. It weighs slightly less than air. 
 
 145. Tests for Oxygen, Hydrogen, Carbon Dioxide, and 
 Nitrogen. The various properties of the gases so far dis- 
 cussed may be recorded in the form of the following 
 table. 
 
 Table indicatingthe 
 most prominent 
 tests of several 
 colorless gases. 
 
 Weight of the 
 gas compared 
 with weight 
 of air. 
 
 Causes a glow- 
 ing splinter of 
 wood to burst 
 into flame. 
 
 Enables metals 
 to burn. 
 
 Burns when ig- 
 nited where it 
 comes in con- 
 tact with air. 
 
 Explodes when 
 mixed with 
 a i r and ig- 
 nited. 
 
 Causes a white 
 precipitate in 
 lime-water. 
 
 Extinguishes 
 burning sab- 
 stances. 
 
 
 4-1 
 
 + 
 
 -\- 
 
 
 
 
 
 Hydrogen 
 
 1 
 
 T v 
 
 
 
 + 
 
 + 
 
 
 + 
 
 Carbon Dioxide 
 
 I 
 
 
 
 
 
 -f 
 
 -\- 
 
 Nitrogen 
 
 T 9 7T 7 n 
 
 
 
 
 
 
 -+- 
 
 
 
 
 
 
 
 
 
 From this table it may be seen which tests are most 
 suitable for the rapid identification of any of these gases, 
 provided no other gases are present. If the gas is color- 
 less and odorless, the bursting into flame of the glowing 
 tip of a splinter of wood introduced into the gas indi- 
 
CHEMISTRY, ATOMS 153 
 
 cates the presence of oxygen. The explosion of the gas 
 when it is mingled with air and set on fire suggests 
 hydrogen. The production of a milky appearance, when 
 the gas is passed through lime-water, identifies it as 
 carbon dioxide. The failure of all three of these tests 
 suggests nitrogen. 
 
 A much larger number of gases is known to chemists. 
 Sometimes two or three kinds of tests must be used 
 before one of these gases is fully identified. 
 
 146. Apparatus used to Determine the Composition of 
 Water. Cut off the bottom from a large wide-mouthed 
 bottle. If no glass cutter is at hand, this may be accom- 
 plished in the following manner. With a new three- 
 cornered file produce a sharp, deep scratch, about half 
 an inch long, near the bottom of the bottle. Heat the 
 end of a poker, or of an iron rod a little thicker than a 
 lead pencil, until it is red hot, then allow it to cool, until 
 the red color has disappeared. Hold the heated end of 
 the poker against one end of the scratch for about four 
 seconds. Reverse the position of the poker and hold it 
 against the other end of the scratch for about eight 
 seconds. Bring it back to its original position and hold 
 it there until the glass cracks. As soon as the glass 
 cracks, draw the poker along the surface of the bottle, 
 keeping it about one-eighth of an inch ahead of the 
 crack. The crack will slowly follow the poker around 
 the bottle. After the bottom of the bottle has been cut 
 off, the sharp edges can be removed with a file. 
 
 Secure a cork large enough to close the mouth of the 
 bottle (Fig. 80). Solder two thin strips of platinum 2J 
 inches long and inch wide, to two strong copper wires. 
 Thrust the wires down through the cork as far as the 
 platinum strips, at such distances apart that it is possible 
 
154 
 
 ELEMENTARY PHYSICS 
 
 to slip two test-tubes into the cut end of the bottle and 
 over the platinum strips. Cover the upper side of the 
 cork and the connection between the 
 copper wire and the platinum strips 
 thoroughly with sealing-wax. Insert 
 the cork in the bottle and add sealing- 
 wax around the edge of the cork, so 
 as to form a water-tight vessel, into 
 which the platinum strips project 
 from beneath. If, on trial, the vessel 
 permits a little water to escape, dry 
 thoroughly before adding any more 
 sealing-wax, otherwise the wax will 
 not adhere to the glass. 
 
 Apparatus for use in this experi- 
 ment, often more complicated, is sold 
 by dealers under the name of Appa- 
 ratus for the Electrolysis of Water. 
 147. Composition of Water. Fasten the bottle on a ring 
 stand and nearly fill it with water. Fill two long test- 
 tubes or combustion-tubes with water. Close the mouth 
 of each test-tube with the thumb or with a rubber stop- 
 per, invert the tube, place the end just closed under the 
 surface of the water in the bottle, and then remove the 
 thumb or stopper. The pressure of the air will hold in 
 the water. Slip the mouth of each test-tube over one of 
 the platinum strips (Fig. 80), and fasten the test-tubes in 
 a vertical position so that the tubes cannot move when 
 filled with gas. Remove water from the jar, using a 
 rubber tube as a siphon, until it is certain that if the 
 water in the test-tube were added to the water in the jar 
 there would be no overflow. If the water already con- 
 tains acid, fill the rubber tube with water ; close both 
 
 Fro. 
 
CHEMISTRY, ATOMS 155 
 
 ends with fingers, insert one end in the water in the 
 jar, lower the other end and remove the fingers. 
 
 To the copper wires projecting beneath the cork attach 
 the terminal wires coming from an electric battery or dy- 
 namo. For the electric battery, six potassium bichromate 
 cells (Fig. 128) will be sufficient. For the student un- 
 acquainted with electrical phenomena, it is sufficient to 
 know that the purpose of the battery or dynamo is 
 simply to send a current of electricity through the 
 water. 
 
 As soon as the electricity begins to flow, gas bubbles 
 up at the surface of both of the platinum strips and col- 
 lects at the top of the test-tubes, pushing out the water. 
 The rapidity of the accumulation of gas may be very much 
 increased by mixing sulphuric acid with the water in the 
 proportion of about one to ten. After the flow of elec- 
 tricity has continued for some time, it is seen that the 
 quantity of gas collected in one of the test-tubes is twice 
 as great as that collected in the other. 
 
 The gases are cojorless and their identity can be estab- 
 lished only by experiment. Allow the, gases to collect 
 until the tube containing the greater quantity of gas is 
 full. Remove the tube, keeping it in its inverted position, 
 and thrust up into its mouth a burning splinter. The 
 flame of the splinter is extinguished, but the gas burns 
 where it comes in contact with the air. Blow out the 
 flame. Mix the gas with air and bring a lighted match 
 near the mouth of the tube. The mixture explodes. The 
 quantity of gas used is usually too small to break the 
 tube. Hold the tube by means of a handkerchief wrapped 
 around its upper end if you have any reason to expect a 
 stronger explosion. The gas is hydrogen. 
 
 By the time the identity of the more rapidly accumu- 
 
156 ELEMENTARY PHYSICS 
 
 lating gas has been determined, the other test-tube will 
 probably be full. Hold the test-tube with the mouth up- 
 ward and thrust the glowing tip of a splinter of wood 
 down into the gas. The tip bursts into flame. The gas is 
 oxygen. 
 
 Only hydrogen is given off in one of the test-tubes, and 
 only oxygen collects in the other. If there were a mixt- 
 ure of these gases in either one of the tubes, there would 
 be an explosion when a burning splinter is thrust in, al- 
 though the violence of the explosion would depend upon 
 the proportion of each gas in the mixture. No other gas 
 except hydrogen and oxygen can be discovered. At the 
 close of the experiment, nothing is left in the bottle ex- 
 cept the sulphuric acid which was added at the beginning 
 of the experiment, and the water which has not yet been 
 separated into hydrogen and oxygen. In other words, 
 the experiment demonstrates that water consists of a com- 
 bination of hydrogen and oxygen. 
 
 Electricity has the power not only of separating water 
 into its component gases, but also of collecting the gases 
 at separate points. The oxygen collects at the platinum 
 strip ( + ) by means of which the current of electricity 
 enters the water, and the hydrogen collects at the strip 
 ( ) by means of which the current of electricity leaves 
 the water. 
 
 148. Relative Proportion of Hydrogen and Oxygen in Wa- 
 ter. It is evident that twice as great a volume of hydro- 
 gen is produced as of oxygen. If, however, these quanti- 
 ties of gas are weighed ( 2 ) it is found that the one 
 volume of oxygen weighs 8 times as much as the two 
 volumes of hydrogen. Any volume of oxygen, therefore, 
 weighs 16 times as much as an equal volume of hydro- 
 gen. 
 
CHEMISTRY, ATOMS 157 
 
 The composition of water may be represented as fol- 
 lows : 
 
 Water = Hydrogen + Oxygen. 
 (18) (2) (16) 
 
 The volumetric composition may be suggested in the 
 following manner : 
 
 Water = 
 
 (Hydrogen) (Oxygen) 
 
 149, Formation of Water from Oxygen and Hydrogen. If 
 
 two volumes of hydrogen and one volume of oxygen be 
 placed in a vessel and an electric spark be sent through 
 the mixture, the gases will unite and form water ( 173). 
 
 The production of water from hydrogen and oxygen 
 may, however, be shown in a much more simple form. 
 Attach a platinum tipped nozzle ( 36) to the end of a 
 delivery tube connected with an apparatus for the secur- 
 ing of hydrogen ( 137). Collect a test-tube full of 
 hydrogen, and thrust into it a burning splinter. If there 
 is no explosion, there is no admixture of air with the 
 hydrogen coming from the apparatus. Then, and not till 
 then, is it safe to light the gas escaping from the platinum 
 nozzle. A very dangerous and violent explosion may occur 
 if this precaution is not observed. The danger may be en- 
 tirely removed by placing the Woulfe bottle in a cage 
 constructed of coarse wire-gauze, permitting only the 
 thistle-tube and the delivery tube to protrude. Any tin- 
 ner can construct such a cage. In case of explosion the 
 broken glass is then caught by the wire-gauze. 
 
 After the hydrogen has been ignited hold the nozzle 
 within the upper part of a cold bell-jar, but not near 
 
158 ELEMENTARY PHYSICS 
 
 enough to crack the jar (Fig. 81). Water collects on the 
 sides of the interior of the jar. 
 
 A little thought will show that something unusual has 
 taken place. Heat does not ordinarily cause water to 
 
 settle on objects. On the con- 
 trary, heat is used to drive off 
 water which has settled on 
 any surface, by causing it to- 
 evaporate. 
 
 But when the hydrogen 
 burns, it unites with the oxy- 
 FJG 81 gen in the air, and forms 
 
 water. This water settles on 
 
 the glass. Directly over the flame the heat usually causes 
 the water to evaporate as rapidly as it settles there, and 
 the upper part of the bell -jar, therefore, usually remains 
 practically dry. On the sides of the bell- jar, however, 
 the water collects more rapidly than it evaporates, and 
 hence the sides of the jar are soon covered with a thin 
 mist, or even with large drops of water. 
 
 150. Different Action of Oxygen in Air and Oxygen in 
 Water. Oxygen in air shows all of the properties of pure 
 oxygen, with only this difference, its action is relatively 
 weaker. This is due solely to the fact that, in air, the 
 oxygen is diluted with four times its volume of nitrogen, 
 a very inactive gas. ( 136, 143.) 
 
 Oxygen in water, however, shows entirely different 
 qualities from pure oxygen. Thrust a burning stick into 
 pure oxygen or into air and it continues to burn. Thrust 
 it into water or steam and it is extinguished. Air is the 
 great supporter of combustion. Water is the great enemy 
 of combustion, the most commonly used fire extinguisher. 
 Moreover, the hydrogen with which oxygen is com- 
 
CHEMISTRY, ATOMS 159 
 
 bined in water, is not an inactive substance like nitrogen. 
 It lias already been shown that a mixture of hydrogen 
 and oxygen brought in contact with fire explodes vio- 
 lently, and that hydrogen in contact with air will burn. 
 In fact the heat produced by the union of hydrogen 
 and oxygen is the greatest heat produced outside of the 
 electric furnace. Neither the oxygen nor the hydrogen 
 when combined in the form of water act as these gases 
 usually do either when separate or mixed. 
 
 151. Mixtures, Compounds, and Components. The dif- 
 ferent action of oxygen in air and in water is explained 
 by the fact that air is merely a mixture, and each of the 
 gases in the mixture retains its own characteristic prop- 
 erties. Water, however, is not a mixture of two gases, 
 but a new substance formed from these gases, possessing 
 none of the properties of the gases from which it is formed. 
 A new substance formed by the union of two or more dif- 
 ferent substances, and having properties dissimilar from 
 those of any of the components, is called a compound. Hy- 
 drogen and oxygen are therefore components of the com- 
 pound^ water. They are said to be chemically united, not 
 mixed, and the force which causes the gases to unite is 
 called chemical affinity. These expressions are often found 
 convenient. It is well, however, to remember that to 
 have a name for a process is not equivalent to explaining 
 its nature. Although much has been learned about the 
 conditions under which substances unite, and about the 
 proportions in which they combine, practically nothing 
 is known as yet of the real nature of the force called 
 chemical affinity. 
 
 152. Analysis and Synthesis. The composition of a com- 
 pound substance may be determined either by separating 
 the substance into its components, or by taking the 
 
160 ELEMENTARY PHYSICS 
 
 necessary components and, by a chemical union of these 
 components, producing the compound substance whose 
 composition it is desired to know. The first process is 
 called Analysis (loosening- the components), the second 
 process is called Synthesis (putting together the com- 
 ponents). 
 
 In the preceding paragraphs the composition of water 
 was determined by both analysis ( 147) and synthesis 
 ( 149). There are several methods of analysis. The 
 separation of the components by means of an electric cur- 
 rent may be called the electrolytic method. The applica- 
 tion of a considerable quantity of heat at high tempera- 
 ture is often capable of driving the components of a 
 compound apart. This method of separation may be 
 called the thermal method of analysis. The red oxide of 
 mercury may be divided into its components by this 
 method. In order to recognize its components, it is 
 necessary to be able to identify both oxygen and mercury 
 readily. 
 
 153. The Properties of Mercury. When mercury falls 
 upon the floor it usually breaks into countless fragments 
 of all sizes, and each fragment, large or small, assumes the 
 form of a more or less flattened sphere. When mercury 
 is heated gently at the bottom of a test-tube, it evapo- 
 rates and collects again at the top of the tube, where the 
 glass is still comparatively cold. Often, along the top 
 of the tube, it forms a bright coating, called a mercury 
 mirror. A strip of copper wire, dipped into mercury and 
 rubbed, assumes a bright, silvery appearance. 
 
 154. The Composition of Red Oxide of Mercury. Place a 
 small quantity of red oxide of mercury (a powder) in a 
 test-tube. Close the mouth of the tube with a rubber 
 stopper through which passes a glass tube connecting 
 
CHEMISTRY, ATOMS 
 
 161 
 
 with the delivery -tube (Fig-. 82). Weigh the test-tube 
 with the stopper and powder. Connect the delivery-tube 
 with the test-tube and pneumatic trough, as in the appa- 
 ratus for the collection of oxygen ( 134), but, for the sake 
 of economy, use a wide 8-inch test-tube for the collection 
 of any gas that may be given off. Hold the test-tube in a 
 horizontal position, and tap it gently so as to spread out 
 the powder. Heat the powder, being careful to avoid 
 heating intensely any single small spot of the test-tube. 
 Gas begins to escape from the delivery-tube and bubbles 
 
 FIG. 82 
 
 up through the water. The first portion of the gas is 
 simply the air which filled the tube at the beginning of 
 the experiment and which is expanded by heat. This air 
 should be thrown away. The volume of the gas thrown 
 away should not be much greater than the total volume 
 contained in the delivery -tube and in the test-tube hold- 
 ing the powder. As the test-tube becomes hotter, gas is 
 given off from the oxide of mercury and collects in the 
 8-inch test-tube. As soon as the disengagement of gas 
 slackens, take the delivery-tube out of the water, taking 
 care that no water remains in the end of the tube. Then, 
 
162 ELEMENTARY PHYSICS 
 
 and not till then ( 134), remove the flame from the test- 
 tube. 
 
 Slip a piece of wet filter paper under the mouth of the 
 test-tube containing the gas and turn the test-tube into an 
 upright position. Remove the filter paper and thrust the 
 glowing end of a splinter of wood down into the tube. 
 The glowing tip bursts into flame. This indicates that 
 the gas is oxygen. 
 
 Weigh the test-tube containing the powder, together 
 with its contents, and the stopper, as soon as they have 
 cooled to the temperature of the room. There has been 
 a distinct loss in weight. This is due to the loss of the 
 oxygen which was driven off from the powder by the 
 heat. 
 
 The material remaining in the test-tube is now not all 
 red oxide of mercury. In the upper part of the tube are 
 found minute globules and also a coating of a bright me- 
 tallic-looking liquid. In order that this metallic coating 
 may not be driven off by the heat, the upper part of the 
 test-tube must not be heated. Remove the stopper. In- 
 vert the test-tube and tap its mouth vigorously on the 
 open palm of the hand. The metallic liquid forms a 
 large drop in the palm of the hand. The drop is proba- 
 bly coated with some of the red oxide of mercury from 
 which the oxygen has not been driven off. This coating 
 can be easily removed with the finger. A piece of clean 
 copper wire, if dipped into the drop and then rubbed, 
 assumes a bright, silvery appearance. If allowed to 
 fall, the drop breaks up into smaller drops of various 
 sizes, and even the smallest drop takes the form of a 
 sphere. The metallic liquid is unquestionably mercury. 
 
 If the red oxide of mercury is heated long enough, it 
 completely disappears. Nothing but mercury remains in 
 
CHEMISTRY, ATOMS 163 
 
 the test-tube, and the only substance which has left the 
 test-tube is the oxygen, of which there may be a suffi- 
 cient quantity to fill a small-sized jar. The red oxide of 
 mercury therefore consists of a combination of oxygen 
 and mercury. This fact was utilized in inventing a name 
 for the substance. 
 
 155. The Relative Proportion of Mercury and Oxygen in the 
 Red Oxide of Mercury. If the weight of the mercury pro- 
 duced from any given quantity of the red oxide of mer- 
 cury is compared with the weight of the oxygen, it is 
 found that the mercury weighs exactly 12J times as much 
 as the oxygen. 
 
 At ordinary temperatures there is a great difference in 
 the volumes occupied by the mercury and the oxygen ob- 
 tained from the red oxide of mercury, for, at ordinary 
 temperatures, mercury is in a liquid form, and oxygen is 
 in a gaseous form. The volume of the oxygen, therefore, 
 greatly exceeds that of the mercury, in fact, about 10,104 
 times at 68 F. 
 
 When both are in the gaseous state the difference in 
 volume is much less. If mercury is sufficiently heated it 
 turns into vapor. At high temperatures both mercury 
 and oxygen are in the gaseous condition. If, while both 
 are in the gaseous condition, at the same temperature (for 
 instance 700 F.) and under the same pressure, the mer- 
 cury and oxygen given off by any quantity of red oxide 
 of mercury be compared, the volume occupied by the 
 mercury will be found to be exactly twice the volume oc- 
 cupied by the oxygen. 
 
 This is another illustration of the fact that substances 
 combine in very simple proportions, if the proportions 
 are based upon the volumes of these substances while in a 
 gaseous condition ( 148). 
 
164 ELEMENTARY PHYSICS 
 
 The composition of the red oxide of mercury may be 
 represented as follows : 
 
 Red Oxide of Mercury _ Mercury Oxygen 
 
 (13.5) (12.5) (1) 
 
 The volumetric composition may be indicated in the 
 following manner : 
 
 Red Oxide of Mercury = 
 
 (Mercury) (Oxygen) 
 
 156. All Components of a Compound not Always Readily 
 Detected. In the preceding experiments, illustrating the 
 electrolytic and thermal methods of analysis, it was pos- 
 sible to recognize readily both components of the com- 
 pounds examined. In many compounds it is possible to 
 identify only one component with ease. 
 
 If, for instance, a solution of copper sulphate is poured 
 into the bottle prepared for the electrolysis of water 
 ( 146), and a current of electricity is sent through the solu- 
 tion, copper is deposited on one of the platinum strips. 
 The only source from which the copper could have come 
 is the copper sulphate, since the tops of the copper wire 
 holding the platinum strips are buried beneath the seal- 
 ing wax. Copper is, therefore, one of the components of 
 copper sulphate. The other components undergo chemi- 
 cal changes not here explained. They remain dissolved 
 in the water, and since they possess no distinctive color 
 they cannot be seen. 
 
 Instead of using the electrolysis apparatus, wires from 
 the battery or dynamo may be connected to two electric- 
 light carbons, and the ends of these carbons may be 
 dipped into the copper sulphate. When a current is sent 
 through the solution, the copper is deposited on one of 
 
CHEMISTRY, ATOMS 165 
 
 the carbons. The copper, adheres better to the platinum 
 or carbon, if it is deposited slowly by a weak current of 
 electricity. 
 
 If the current of electricity is allowed to flow until all 
 the copper in the copper sulphate solution has settled 
 upon one of the platinum strips or electric-light carbons, 
 the weight of the copper is found to be exactly ^\ or f J 
 of the entire weight of the copper sulphate crystals origi- 
 nally placed in the water. 
 
 Copper Sulphate = Copper + Other Components 
 (249) (63) (186) 
 
 157. Substitution, or the Replacement of one Component 
 of a Compound by another Substance. The presence of 
 copper in copper sulphate may be demonstrated by a 
 method which does not require the application of either 
 electricity or heat. 
 
 Pour a small quantity of a solution of copper sulphate 
 into a test-tube. Place a large bright wire nail in the 
 tube. Close the tube with the thumb and repeatedly tip 
 it, so that the solution covers the nail and recedes again. 
 Copper settles on the iron. 
 
 If the nail is taken out of the test-tube as soon as the 
 copper covering is distinctly seen (a few seconds are 
 usually enough) and if the coating is first allowed to 
 dry and is then vigorously rubbed with cloth, the character- 
 istic color of smooth copper may be recognized. The other 
 components of copper sulphate, however, remain in the 
 solution, as in the case of the preceding experiment, and, 
 since they exhibit no distinctive coloring, their presence 
 again escapes detection. 
 
 If the nail is allowed to remain in the copper sulphate 
 for a long time, the copper coating increases in thickness, 
 
166 ELEMENTARY PHYSIOS 
 
 but it no longer adheres well to the nail and may easily 
 be rubbed off. If now the nail is closely examined, it is 
 discovered that the surface is distinctly roughened, owing 
 to the disappearance of that part of the iron which once 
 formed the smooth surface of the nail. The pitted char- 
 acter of the surface may be distinctly recognized under a 
 lens, and a comparison with another nail of the same 
 kind shows that its diameter has been perceptibly dimin- 
 ished. The iron disappears while the quantity of copper 
 which settles on the nail increases. The iron takes the 
 place of the copper in the copper sulphate solution. 
 
 Copper has an attraction for the other components of 
 copper sulphate. From the experiment it appears that 
 iron also has an attraction for these components. In fact, 
 it seems to have a stronger attraction for them than does 
 copper. The iron therefore pushes the copper away from 
 the other components of copper sulphate and takes its 
 place. In other words, iron is substituted for copper in 
 copper sulphate and it forms a new substance, called iron 
 sulphate. The copper which is pushed aside settles upon 
 the iron. nail. 
 
 If enough iron is placed in the copper sulphate solu- 
 tion to replace all of the copper, the color of the solution 
 gradually changes from the blue of copper sulphate to 
 the green of iron sulphate. If the excess of iron with 
 its copper coating is taken out and the water is removed 
 by heating the solution in an evaporating dish, the iron 
 sulphate is left behind and may easily be recognized as 
 something quite distinct from copper sulphate. The 
 chemical change may be expressed as follows : 
 
 [Copper + Other components] + Iron = [Iron -+- Other components] + Copper 
 
 (63) (186) (55.6) (55.6) (186) (63) 
 
 (Copper sulphate) + Iron = (Iron sulphate) +- Copper 
 
CHEMISTRY, ATOMS 167 
 
 This is one of the simplest illustrations of the identi- 
 fication of one component of a compound by chemical 
 substitution. 
 
 158. Identification of Components of Compounds by Color 
 of Precipitates. In the cases of analysis of compounds 
 so far discussed, the components which were identified 
 were secured free from union with any other substance. 
 It is often possible to recognize the presence of one or 
 more of the components of a compound without securing 1 
 them in the free state, in other words without obtaining 
 them uncombined with any other substance. 
 
 Dissolve about 1.7 of an ounce of potassium bichro- 
 mate in one pound of water. Mingle a small quantity of 
 nitric acid with 4 times its volume of water. Pour a* small 
 part of the nitric acid solution into a test-tube and add a 
 little of the potassium bichromate solution. Nothing is 
 noticed beyond the mingling of the two liquids. 
 
 In another test-tube place a small piece of silver (coin), 
 add dilute nitric acid and heat for a short time in the 
 flame of a Bunsen burner. A part of the silver disap- 
 pears in the acid solution. Pour a little of the acid solu- 
 tion into a third test-tube, dilute it still farther with water 
 (pure distilled water), and again add a little of the potas- 
 sium bichromate solution. The mixture becomes clouded 
 with a dark red substance, which evidently can be due 
 only to the presence of silver in some form in the acid 
 solution. Most of this dark red substance promptly set- 
 tles to the bottom of the test-tube. This portion consists 
 of the heavier particles. The lighter particles remain 
 suspended for a long time in the liquid, but finally these 
 also are found at the bottom of the tube. Hence the dark 
 red substance is called a precipitate. 
 
 Now, chemists have tested the effect of pouring potas- 
 
168 ELEMENTARY PHYSICS 
 
 slum bichromate into the solutions of all sorts of sub- 
 stances whose compositions are known, and, while they 
 have succeeded in getting precipitates in some of these 
 cases, the color of none of these precipitates was dark 
 red, excepting in those cases where silver was present in 
 some form or other in the solution. Having once estab- 
 lished this fact, the presence of a dark red precipitate on 
 the addition of potassium bichromate to the solution of 
 any unknown substance is at once an evidence of the 
 presence of silver in this substance. 
 
 Chemicals which are used for the purpose of ascertain- 
 ing the presence of various substances in compounds are 
 often spoken of as reagents. Potassium bichromate is only 
 one of a great many reagents used to produce precipitates. 
 
 159. Several Tests Often Necessary to Determine Def- 
 initely the Presence of a Substance in a Compound. 
 Potassium bichromate produces precipitates only when 
 one of the following substances is present in the unknown 
 compound which is in solution : lead, silver, mercury, 
 bismuth, and barium. The presence of any precipitate on 
 the addition of potassium bichromate to the solution of 
 an unknown compound, therefore, is at once an evidence 
 of the presence of one or more of these five substances in 
 the compound. However, since many of these precipi- 
 tates are yellow, the formation of a precipitate on the 
 addition of potassium bichromate is not sufficient to de- 
 termine which metal is present in the compound. 
 
 But other reagents also form precipitates and the color 
 of these precipitates varies so much, that it is often 
 possible, if several -re agents are used in succession, to de- 
 termine with confidence the presence of a substance in 
 an unknown compound even in cases when the use of a 
 single reagent leads to no definite result. 
 
CHEMISTRY, ATOMS 
 
 169 
 
 This fact is shown by the following- table. In the first 
 column are placed the names of the different substances 
 which give rise to precipitates when the compounds con- 
 taining 1 them are dissolved in water, and when some one 
 of the three reagents named at the head of the following- 
 columns is added- Opposite the names in the first col- 
 umn are indicated the colors of the precipitates formed 
 on the addition of these reag-ents. When no precipitates 
 are formed, the space is left blank. 
 
 COMPOUNDS CONTAIN- 
 
 
 REAGENTS. 
 
 
 TO BE TESTED. 
 
 Potassium Bichro- 
 mate. 
 
 Potassium Iodide. 
 
 Hydrochloric 
 Acid. 
 
 Lead. 
 
 Yellow 
 
 Bright yellow 
 
 White. 
 
 Silver 
 
 Dark red 
 
 Yellow-white, darken- 
 
 White, 
 
 Mercury 
 
 Yellowish green 
 
 ing on exposure. 
 Greenish yellow to 
 
 blacken- 
 ing on ex- 
 posure. 
 White. 
 
 Bismuth 
 
 to red. 
 Orange yellow. 
 
 red. 
 Dark brown 
 
 
 Copper. . 
 
 
 White to yellow to 
 
 
 Antimony 
 
 
 brownish yellow. 
 Yellow. 
 
 
 Barium 
 
 Light yellow. 
 
 
 
 Palladium . 
 
 
 Black. 
 
 
 
 
 
 
 Suppose potassium bichromate is added to the solution 
 of some unknown compound and a yellowish precipitate 
 is formed. If neither potassium iodide nor hydrochloric 
 acid form a precipitate when added to a fresh quantity of 
 the same unknown solution, barium is probably pres- 
 ent in the compound. If potassium iodide gives a dark 
 brown precipitate, bismuth is one of the components. In 
 this case, hydrochloric acid should produce no precipi- 
 tate. If potassium iodide produces a yellow precipitate, 
 the unknown compound* contains lead. In this case hy- 
 
170 ELEMENTARY PHYSICS 
 
 drochloric acid produces a white precipitate. If a small 
 quantity of potassium bichromate produces a yellowish 
 green precipitate, and if a greater quantity of the reagent 
 produces a distinctly greenish or reddish precipitate, 
 mercury is present. Suppose the addition of potassium 
 bichromate produces a reddish precipitate of such a tint 
 that it is impossible to determine whether silver or mer- 
 cury is present. The addition of potassium iodide should 
 at once decide this matter. 
 
 160. Method of Separation of Different Precipitates Formed 
 by the Same Reagent. "When a compound or a mixture 
 of compounds contains several substances which will 
 give precipitates with the same reagent, it is necessary 
 to separate these substances before their identity can 
 be fully established. This is especially true when the 
 different precipitates have about the same color. This 
 separation may often be easily accomplished if the differ- 
 ent precipitates vary considerably as to their solubility 
 in the different reagents. This may be illustrated by the 
 following example. 
 
 When dilute hydrochloric acid is added to all known 
 solutions of compounds, precipitates are formed only 
 when lead, silver, and mercury are present. Hence the 
 formation of a precipitate on the addition of hydrochloric 
 acid is at once an evidence of the presence of either lead, 
 silver, or mercury, or of any two of these substances, 
 or of all three of them. Since all of these precipitates 
 are white, it is impossible to determine from the color 
 alone which substance or substances are present. 
 
 The precipitates differ, however, considerably in the 
 readiness with which they dissolve in various substances, 
 and this fact may be utilized in the identification of the 
 metal present in the unknown compound. For instance, 
 
CHEMISTRY, ATOMS 171 
 
 the precipitate due to the presence of lead dissolves 
 readily in hot water, but those due to the presence of 
 silver and mercury do not dissolve in hot water. The 
 precipitate due to the presence of silver dissolves in warm 
 dilute ammonia water, while the precipitate due to the 
 presence of mercury turns black and does not dissolve. 
 The following method of procedure will make it possi- 
 ble to determine which substances caused the precipi- 
 tates due to the addition of hydrochloric acid. 
 
 Place a filter paper in a funnel supported on a ring 
 stand. Put a beaker directly beneath, and 
 pour the precipitates with the liquid in 
 which they were formed into the funnel. 
 The precipitates remain on the filter pa- 
 per; the liquids (filtrates) pass through 
 and escape at the bottom. Pour water 
 gently over the precipitates until they 
 are thoroughly clean. Then punch a hole 
 through the bottom of the filter paper, 
 and, with a little water, wash the precip- 
 itates into a second small glass beaker. 
 Add water to the precipitates, place the 
 beaker on a wire gauze on a ring stand (Fig. 83) and boil 
 for a short time. If all of the precipitate disappears, 
 only lead is present. 
 
 If all of the precipitate does not disappear after boil- 
 ing in plenty of water, filter mixture while hot. If any 
 lead is present, white needle-shaped crystals will appear 
 in the filtrate on cooling. The precipitate remaining on 
 the filter paper contains either silver or mercury, or both. 
 
 Wash the precipitate left on the second filter paper 
 into another beaker, add ammonia water, and warm the 
 mixture. If only silver is present, all of the precipitate 
 
172 ELEMENTAEY PHYSICS 
 
 disappears. If the precipitate turns black, mercury is 
 present. In order to determine whether silver is present 
 in addition to the mercury, filter the black mixture and 
 add dilute nitric acid to the filtrate, until the filtrate 
 changes the color of litmus paper from blue to red. If a 
 white precipitate appears, silver is present. 
 
 Lead is first separated from silver and mercury, and 
 then silver is separated from mercury. After the separa- 
 tion, the other tests with potassium bichromate and po- 
 tassium iodide may be applied. 
 
 161. Elements. The preceding- paragraphs by no means 
 describe all of the methods employed by chemists to de- 
 termine the components of a compound, but they at least 
 make evident that there are methods for attacking such 
 problems. 
 
 Chemists have been at work for many years in deter- 
 mining the components of all kinds of compounds, and, 
 wherever possible, in determining even the components 
 of these components. Whenever none of the methods at 
 the command of a chemist enable him to subdivide any 
 component into two or more other components, the last 
 component obtained is called an dement. It is impossi- 
 ble, of course, to determine definitely what substances 
 are elements, since new methods of analysis may in the 
 future enable the chemist to subdivide those which are 
 now universally considered as elements. But it is easily 
 possible to determine what substances have not yet 
 been subdivided and must therefore be considered ele- 
 ments for the present. 
 
 An element is a substance which cannot, by any method 
 at present known to chemists, be divided into two or more 
 dissimilar substances. 
 
 Nearly 80 substances are at present considered as ele- 
 
CHEMISTRY, ATOMS 173 
 
 ments ( 181). Most of these are known only to the chem- 
 ist, since they rarely exist uncombined in nature. Among 
 the most commonly known elements are the metals : 
 aluminum, copper, gold, iron, lead, mercury, nickel, 
 platinum, silver, tin and zinc. Brass is not an element 
 but a mixture chiefly of copper and zinc. Bronze is a 
 mixture of copper, tin and zinc. Arsenic, carbon, phos- 
 phorus and sulphur are elements. The gases, hydrogen, 
 nitrogen, and oxygen are elements. Some elements are 
 exceedingly common in nature, but are seen uncombined 
 with other elements in chemical laboratories only. 
 Among these are silicon and calcium. Aluminum has 
 only recently become one of those metals commonly seen 
 outside of the chemical laboratory. In fact, until re- 
 cently it was a very expensive metal, and difficult to ob- 
 tain. 
 
 162. Chief Elements Present in the Earth. Nearly 50 
 per cent, of the solid crust of the earth consists of oxygen, 
 and a little more than 25 per cent, consists of silicon, so 
 that the non-metallic elements, oxygen and silicon, form 
 nearly 75 per cent, of the crust of the earth ; while almost 
 all of the remainder is formed by the metals: aluminum, 
 8 per cent. ; iron, 5 per cent. ; calcium, 4 per cent. ; mag- 
 nesium, 2.5 per cent. ; sodium, 2.5 per cent., and potas- 
 sium, 2.5 per cent. The other rock-forming elements 
 titanium, carbon, hydrogen, phosphorus, manganese, sul- 
 phur, and chlorine form scarcely 1 per cent, of the crust. 
 
 The rock-forming compounds are usually known as 
 minerals. Compounds or mixtures of compounds which 
 are commercially valuable as sources of metals are usu- 
 ally called ores. 
 
 163. Chief Elements Found in Plants. Water forms by 
 far the largest part of most plants. While green they 
 
174 ELEMENTARY PHYSICS 
 
 contain from 75 to 91 per cent, of water. Even so-called 
 dry wood contains usually 15 per cent, of water. 
 
 After the water present in plants has been completely 
 evaporated, there remain, in a dried condition, those 
 compounds which once formed the more solid parts of 
 the roots, stems, leaves, flowers, and fruits. These or- 
 ganic compounds consist chiefly of carbon, oxygen, hy- 
 drogen, and nitrogen. If these compounds are 'burned, 
 the gaseous substances due to combustion escape, and 
 only the mineral constituents remain. The mineral con- 
 stituents, called ash when left as the result of burning, 
 form by far the smallest part of plants. Usually less 
 than 3 per cent, of a plant remains in the form of ash, 
 and many juicy plants contain less than 1 per cent, of 
 mineral material. In the ash, potassium, calcium, mag- 
 nesium, and phosphorus are always present in an appre- 
 ciable amount. Iron, chlorine, sulphur, and sodium are 
 also present, but usually in very small quantities. Sili- 
 con occurs in various grasses, sedges, rushes, and other 
 plants. Iodine is found in some sea plants. 
 
 164. The Chief Elements Present in the Animal Body, 
 It has been calculated that in an average man weighing 
 154 pounds, 116 pounds consist of water which may be 
 driven off as moisture, and 29 pounds consist of the flesh, 
 skin, blood, fat, and gelatine after all the water has been 
 driven off. If the body were burned, the water and the 
 organic compounds would disappear and only the min- 
 eral matter would be left behind. This would consist 
 chiefly of what was left of the bones, a total of only about 
 10 pounds. 
 
 Oxygen forms about 72 per cent, of the human body ; 
 carbon, 13.5 ; hydrogen, 9.1 ; nitrogen, 2.5 ; calcium, 1.3 ; 
 phosphorus, 1.15 ; sulphur, 1.47 ; sodium, .1 ; chlorine, 
 
CHEMISTliY, ATOMS 175 
 
 .085 ; fluorine, .08 ; potassium, .026 ; iron, .01 ; magnesium, 
 .0012 ; and silicon, .0002. The great quantity of oxygen 
 and hydrogen is largely due to the abundance of water in 
 all the tissues. Many organic compounds in the body 
 contain sulphur. Calcium and phosphorus are among 
 the most important constituents of bones. Sodium and 
 chlorine are present in the form of salt dissolved in the 
 various liquids of the body. 
 
 165. Organic Compounds. Plants and animals are able 
 to produce many compounds not otherwise found in nat- 
 ure. Among these may be mentioned, sugar starch, oil, 
 wood, fat, muscle, and bone. About 75 years ago it was 
 believed that chemists never would be able to duplicate 
 any of these compounds, and hence a very careful distinc- 
 tion was made between the compounds not produced by 
 living organisms, and those which could be produced only 
 by plants and animals. The first were called inorganic 
 compounds, and the second, organic compounds. At 
 present, however, many of the so-called organic com- 
 pounds, such as sugar and starch, can be produced in the 
 laboratory. 
 
 166. The Chief Difference between Animals and Plants. 
 If the student is acquainted with only the larger animals 
 and plants, he will not find it difficult to find many dis- 
 tinctions between these two divisions of the living world. 
 If he knows also the microscopic animals and plants, he 
 will find that many distinctions which can readily be 
 made between the larger forms cannot be retained as a 
 means of separation between all of the smaller animals 
 and plants. For instance, it was once supposed that ani- 
 mals are capable of motion and plants are not. But now 
 it is known that various parts of plants are in continual, 
 although very slow, motion ; some sensitive plants close 
 
176 ELEMENTARY PHYSICS 
 
 their leaves as soon as touched; some insect-catching 
 plants close their leaves too rapidly for the insects to 
 escape; and some microscopic plants in some stages of 
 their existence can swim around freely in the water even 
 in a direction opposite to that of the current. 
 
 There is, however, one difference which will serve bet- 
 ter as a distinction between animals and plants than any 
 of the rest. Animals can not manufacture organic com- 
 pounds from inorganic substances. They must obtain 
 substances which are already organic compounds, and 
 they use these to manufacture from them new organic 
 compounds. Plants, however, can manufacture organic 
 compounds from inorganic materials. Plants, therefore, 
 can live on the inorganic materials they secure from the 
 earth, the water, and the air; but animals must live on 
 plants, or on animals which have eaten plants. 
 
 167. Water and Air are the Sources of the Chief Con- 
 stituents of Plants and Animals. The chief constituents 
 of animal and vegetable bodies, aside from water, are the 
 carbon compounds. In addition to carbon, most of these 
 compounds contain hydrogen and oxygen, and often also 
 nitrogen, all of which are non-metallic elements. In 
 marked contrast with the comparatively small import- 
 ance of carbon, hydrogen, and nitrogen in rock-forming 
 minerals, is their great importance in animal and vegetable 
 compounds. 
 
 A large part of the hydrogen and oxygen used by 
 plants is secured from the water which is present in the 
 soil. Both animals and plants secure oxygen also from 
 the air, the process being called breathing. Nitrogen 
 forms four-fifths of the air, but plants take in nitrogen 
 by way of the roots. Carbon united with oxygen, in the 
 form of carbon dioxide ( 141), is present as a very 
 
CHEMISTRY, ATOMS 177 
 
 minor constituent of the air, but this small quantity is of 
 the greatest importance, since it provides most of the 
 carbon used by plants. The supply of carbon dioxide in 
 the air is continually replaced by the decay of animal and 
 vegetable bodies. 
 
 Animals secure carbon and nitrogen by eating plants 
 or animals that have eaten plants. 
 
 168. Elements are Usually Found Combined in Nature 
 
 Most elements unite so readily with other elements that 
 their natural condition may be said to be a state of com- 
 bination. Even some of those elements which are found 
 free in nature occur more frequently combined than un- 
 combined. This is true, for instance, of oxygen, carbon, 
 and sulphur. Nitrogen is also frequently found in com- 
 bination in the organic compounds present in animal 
 and vegetable bodies. 
 
 Some elements can unite with nearly all of the other 
 elements. This is true to a remarkable degree of oxygen, 
 chlorine, and sulphur. Other elements, like gold, appear 
 to unite with but few other substances. For this reason, 
 gold is often found as a pure metal in the form of nug- 
 gets. Elements which combine with difficulty with other 
 elements are said to be inert. Very few elements are 
 inert. If an element is desired in the free uncombined 
 state, it is usually necessary to secure some compound in 
 which the element is present, and then by some means 
 separate the element from the rest of the compound, in 
 such a manner that the element will remain separated 
 from all other substances. Some elements are so active 
 that special precautions must be used in order to retain 
 them in the free state. Phosphorus must be kept under 
 water. Potassium and sodium must be kept under naph- 
 tha or benzene. 
 
178 
 
 ELEMENTARY PHYSICS 
 
 169. The Formation of Compounds by Direct Combination, 
 Since a state of combination is the natural condition 
 for most elements, it is a comparatively easy matter to 
 illustrate the various methods of combination. 
 
 The direct combination of two elements is shown when 
 hydrogen is burnt in air. It unites with oxygen and 
 forms water ( 149). Chlorine is a poisonous gas, not suit- 
 able for experiment in an ordinary classroom not pro- 
 vided with special apparatus for carrying off obnoxious 
 gases. However, it may be interesting to know that if 
 
 FIG. 84. 
 
 equal volumes of chlorine and hydrogen are placed in 
 direct sunlight, they unite so rapidly as to cause a vio- 
 lent explosion. The compound formed is hydrochloric 
 acid. 
 
 When iron is burnt in oxygen ( 135), a black compound, 
 black iron oxide, is produced. Red hot carbon (charcoal) 
 placed in oxygen will burn and produce carbon dioxide. 
 
 If a current of chlorine is passed over iron filings heated 
 in a hard glass tube (Fig. 84 C), the chlorine and iron 
 unite with such rapidity that the iron becomes white hot. 
 
CHEMISTRY, ATOMS 179 
 
 Iron chloride is the result. To secure the chlorine, place 
 in a flask, A, a third of an ounce of manganese dioxide, 
 and an ounce of common hydrochloric acid, and gently 
 heat the mixture. To dry the gas, pass it through a tube 
 filled with lumps of calcium chloride, B. 
 
 Mix thoroughly j- of an ounce of fine iron filings with 
 J5 of an ounce of sulphur. Place about one-fourth of the 
 mixture in a test-tube, and heat until the mixture glows. 
 The sulphur unites with the iron, producing a bright 
 light and forming iron sulphide. 
 
 Mix thoroughly ^ of an ounce of coarsely powdered 
 sulphur with ^ of an ounce of copper filings. The mixt- 
 ure presents neither the lemon-yellow color of sulphur 
 nor the well-known color of copper. The sulphur and 
 copper, however, have not united. If the mixture be ex- 
 amined under the microscope, the particles of sulphur 
 may still be recognized among the particles of copper. 
 With the exercise of sufficient care and diligence, the 
 particles of sulphur may be picked out from the mixture, 
 and thus may be separated from the copper. If the mixt- 
 ure be thrown into water, the copper will sink to the bot- 
 tom at once, while the sulphur, which weighs less than 
 one-fourth as much, will sink more slowly through the 
 liquid. This method may also be used for the separation 
 of the sulphur from the copper 
 
 Put the mixture into a tube of hard glass (test-tube or 
 ignition-tube), and heat it with a Bunsen burner. The 
 sulphur melts, catches fire, and unites with the copper, 
 producing a brilliant light. When no further change 
 takes place, allow the tube to cool, break it, and examine 
 the contents. Sulphur and copper have disappeared as 
 such. A new substance has appeared, which, though con- 
 taining both the sulphur and the copper, has no external 
 
180 ELEMENTARY PHYSICS 
 
 resemblance to either, and possesses new properties. It 
 is the chemical compound, copper sulphide. 
 
 170. Formation of New Compounds by Substitution. 
 Many compounds are formed, not by direct union of the 
 elements, but by substituting- some other element for 
 one of the elements of a compound, thus forming a new 
 compound. Hydrochloric acid consists of hydrogen and 
 chlorine ( 169). When zinc is placed in hydrochloric 
 acid it takes the place of the hydrogen, and forms zinc 
 chloride. The hydrogen which is no longer combined 
 escapes ( 137). When iron replaces copper in copper 
 sulphate, a new compound is formed, iron sulphate ( 157). 
 
 (Hydrochloric 
 acid) 
 
 (Zinc 
 chloride) 
 
 
 ( Copper 
 -I Sulphur = 
 [Oxygen 
 (Copper 
 sulphate) 
 
 ( Iron 
 < Sulphur 
 ( Oxygen 
 (Iron 
 sulphate) 
 
 + Copper 
 
 Iron 
 
 171. The Formation of New Compounds by Double Sub- 
 stitution, or Double Decomposition. In order to illus- 
 trate the formation of compounds by direct combination 
 and by substitution, simple cases have been selected* 
 Usually the processes are more complicated but the prin- 
 ciples remain the same. One of the most interesting and 
 most common of these complicated cases is seen when 
 two compounds are brought into contact, and by a partial 
 or complete interchange of elements produce new com- 
 pounds. 
 
 When two compounds are brought in contact with each 
 other, one of the elements in one compound may be sub- 
 stituted for one of the elements in the second compound, 
 
CHEMISTRY, ATOMS 181 
 
 and the element displaced from the second compound 
 may take the place left vacant in the first compound. 
 This evidently results in an interchange of elements 
 between the compounds. The process may be called 
 double substitution, or, if emphasis be laid on the breaking 
 up of the molecules of the original substance preparatory 
 to the formation of new molecules, it may also be called 
 double decomposition. 
 
 Potassium iodide consists of potassium and iodine. If 
 solutions of lead nitrate and potassium iodide are brought 
 together, the lead and potassium exchange places, and the 
 new compounds, lead iodide and potassium nitrate, result. 
 The lead iodide is seen as a bright yellow precipitate 
 ( 159). The potassium nitrate remains in solution, but 
 if the water is filtered and then boiled the potassium 
 nitrate remains as a colorless or white substance resem- 
 bling salt in appearance. 
 
 IBS- - 
 
 (Lead nitrate) (Potassium iodide) (Lead iodide) (Potassium nitrate) 
 
 One of the most interesting cases of double decom- 
 position is the formation of common table salt and water 
 from two poisons, sodium hydrate or caustic soda, and 
 hydrochloric acid. Dissolve caustic soda in water (6 
 parts, by weight, of caustic soda to 50 parts of water). 
 Pour half a test-tubeful of this solution into a porcelain 
 evaporating dish, and add, drop by drop, diluted hydro- 
 chloric acid (4 parts of water to 1 part of the commercial 
 acid). Stir the mixture after the addition of each drop 
 and test with litmus paper ( 67), until the mixture no 
 longer turns litmus paper from red to blue. By this 
 time the solution is likely to have dissolved considerable 
 
182 ELEMENTARY PHYSICS 
 
 litmus from the litmus paper. If the number of drops 
 of acid used has been counted, the experiment can be 
 brought up quickly to about the same stage by taking 
 another half test-tubeful of the caustic soda solution, 
 adding at once the number of drops of acid used before, 
 and then testing once more by dipping as little of the 
 litmus paper in the solution as possible. If too much 
 acid has been used, a drop or more of caustic soda solu- 
 tion must be added. When red litmus paper is no longer 
 changed to blue, and blue litmus paper is no longer 
 changed to red, both hydrochloric acid and the alkali, 
 sodium hydrate, are gone. 
 
 Pour a few drops of the hydrochloric acid solution into 
 a test-tube, one-eighth full of water. Taste one drop of 
 the mixture on a glass stirring rod. Taste a similarly 
 diluted solution of the caustic soda. Taste a drop of 
 the mixture just formed in the evaporating dish, without 
 diluting the mixture. The tastes of the three are different : 
 sour, alkaline, salty. 
 
 Sodium has taken the place of hydrogen in hydro- 
 chloric acid, and has formed common table salt. At the 
 same time the hydrogen which w r as displaced has taken 
 the place vacated by the sodium, and has united with the 
 oxygen and hydrogen, and formed water. The solution 
 left in the evaporation dish is simply a solution of salt 
 in water. If the water is evaporated, the salt is left be- 
 hind. If the solution has not dissolved too much litmus, 
 the salt left in the evaporating dish is pure and white. 
 
 ( Sodium 
 < Oxygen 
 ( Hydrogen 
 
 ( Hydrogen 
 1 Chlorine 
 
 j Sodium 
 1 Chlorine 
 
 j Hydrogen 
 { Oxygen 
 
 (Sodium 
 hydrate) 
 
 (Hydrochloric 
 acid) 
 
 (Salt) 
 
 (Water) 
 
GHEMISTEY, ATOMS 183 
 
 172, The Law of Conservation of Mass. If any of the 
 preceding 1 chemical experiments be performed in a vessel 
 closed in such a manner that it is impossible for any 
 material to enter it or to escape from it, it will be found 
 that the total weight of the vessel and of its contents is 
 the same both before and after the cheinical change takes 
 place. In other words, there is neither gain nor loss of 
 material during any chemical change. The change con- 
 sists merely in a different arrangement of the elements. 
 A compound may be separated into its components or ele- 
 ments. Elements may unite to form a compound. Or, 
 the components of different compounds may first separate 
 and may then unite in such a manner as to form com- 
 pounds different from the original substances. In each 
 case the same fact is observed. 
 
 The total weight of all the products of a chemical change 
 is exactly equal to the sum of all the weights of all the sub- 
 stances affected by the change. 
 
 173. Law of Definite Proportions by Weight It has 
 been shown that the red oxide of mercury consists of 
 mercury and oxygen. It is possible to separate these 
 elements in such a manner that the weight of each can 
 be determined accurately. The weight of mercury and 
 oxygen present in red oxide of mercury depends, of 
 course, upon the quantity of the compound taken for 
 analysis. If, however, a number of analyses are made 
 and the weights of the component elements are deter- 
 mined, the interesting fact is discovered that in all cases 
 the quantities of mercury and oxygen are present in the 
 same relative proportion. If, for instance, 28.35, 49.95, 
 189, and 245.83 ounces of the red oxide of mercury were 
 analyzed, the following weights of mercury and oxygen 
 would be obtained : 
 
184 ELEMENTARY PHYSICS 
 
 Oxide of Proportion of Mercury 
 
 Mercury. Mercury. Oxygen. to Oxygen. 
 
 28.35 oz. = 26.25 + 2.10 12.5 : 1 
 
 49.95 = 46.25 + 3.70 12.5 : 1 
 
 189.00 = 175.00 + 14.00 12.5 : 1 
 
 245.83 = 227.62 + 18.21 12.5 : 1 
 
 If, in a similar, manner different quantities of water are 
 analyzed, the relative proportion of oxygen and hydrogen 
 present is found to be always the same : the weight of 
 the oxygen is in each case 8 times the weight of the 
 hydrogen. 
 
 The constancy of the relative proportion of the ele- 
 ments in different quantities of the same compound may 
 also be shown by carefully determining the weights of 
 the different elements which enter into combination dur- 
 ing the formation of some compound. 
 
 If 8 times as great a weight of oxygen as of hydro- 
 gen is imprisoned in any long test-tube inverted in water, 
 and if a spark of electricity is sent through the mixture 
 ( 149), all of the mixture enters into combination, and 
 forms water. Since the water formed from the gas occu- 
 pies much less volume than the gas, this results in a tem- 
 porary vacuum and the water in which the test-tube is 
 inverted rushes up, so that the water formed from the 
 mixture of oxygen and hydrogen and the water rushing 
 up into the test-tube from the outside completely fill the 
 tube. 
 
 If, however, other proportions are taken, all of the gase- 
 ous mixture does not enter into combination. If 1.72 
 ounces of hydrogen and 15.41 ounces of oxygen are placed 
 in a vessel and united by sending a spark of electricity 
 through the mixture (Fig. 85 A), and if the water is al- 
 lowed to enter the vessel co as to fill the partial vacuum, 
 it is found that the water resulting from combination, and 
 
CHEMISTRY, ATOMS 
 
 185 
 
 the water rushing in from the outside are prevented from 
 filling all the space in the vessel by the presence of some 
 gas, evidently a part of the original mixture. If the gas 
 left uncombined is examined it is found that the gas is 
 pure oxygen and that its weight is 1.65 ounces (Fig. 85 B). 
 Therefore all the hydrogen entered into combination, 
 but only 13.76 ounces of oxygen united with the hydro- 
 gen. But 13.76 ounces are exactly 8 times 1.72 ounces. 
 Therefore exactly 8 times 
 as much oxygen as hydro- 
 gen enters into combina- 
 tion, even if an excess of 
 oxygen exists in the mixt- 
 ure. If an excess of hy- 
 drogen had been used, only 
 J as much hydrogen, by 
 weight, as oxygen would 
 have entered into combina- 
 tion. The remainder of the 
 hydrogen would have re- 
 mained uncombined. 
 
 In actual experiments 
 the weights of the mate- 
 rials used are much smaller 
 than those indicated by 
 the figures here given, but 
 the principle remains the 
 same. 
 
 The same principle may be readily shown, provided a 
 delicate balance is at hand. Place different weights of 
 magnesium (in the form of ribbon) in porcelain crucibles, 
 and heat the crucibles until the magnesium unites with 
 the oxygen in the air, and produces a white compound, 
 
 FIG. 85. 
 
186 ELEMENTARY PHYSICS 
 
 magnesium oxide, easily reduced to powder by a mere 
 touch. The quantity of oxygen entering into combina- 
 tion, as shown by the increased weight of the material in 
 the crucible, varies in each case, but the proportion of 
 oxygen as compared with that of the magnesium is always 
 the same : in each case the weight of the oxygen entering 
 into combination is f of that of the magnesium. Of all the 
 oxygen in the air only that quantity enters into combi- 
 nation which is necessary to preserve this proportion. If 
 not enough oxygen were at hand, a part of the magnesium 
 ribbon would remain uncombined. 
 
 From these and similar facts, the following law, known 
 as the Law of Definite Proportions, has been derived: 
 
 The same chemical compound always contains the same 
 elements in the same proportion l>y weight. 
 
 The proportion of the weight of any element, forming part 
 of a compound, to the total weight of the compound is always 
 the same. 
 
 174. The Law of Multiple Proportions by Weight.- When- 
 ever hydrogen and oxygen unite directly, they unite in 
 one proportion only and always form water. However, by 
 a process of double substitution it is possible to secure 
 another compound in which these elements are united in 
 a different proportion. When, for instance, small quanti- 
 ties of barium dioxide are introduced in succession into 
 cold, dilute hydrochloric acid, the barium and hydrogen 
 exchange places, and two new compounds are formed, 
 barium chloride and hydrogen dioxide. 
 
 ( Barium _, \ Hydrogen j Barium , \ Hydrogen 
 
 \ Oxygen ~ " \ Chlorine " { Chlorine ~"~ \ Oxygen 
 
 Now when the hydrogen dioxide is analyzed it is dis- 
 covered that the proportion of oxygen is much greater 
 
CHEMISTRY, ATOMS 
 
 187 
 
 than in water ; in fact, there is 16 times as much oxygen, 
 by weight, as hydrogen, while in water the proportion of 
 oxygen is only 8 times that of hydrogen. This shows 
 that two elements can unite in more than one proportion, 
 in order to form compounds. 
 
 The hydrogen dioxide has very different properties 
 from water. It is a syrupy liquid, and weighs 1.45 times 
 as much as water. It corrodes the skin, destroys the col- 
 oring matter in plants, and is used in surgical operations 
 to kill any germs which may come in contact with the 
 wound. It readily loses half of its oxygen when heated 
 to 200 F., and the remainder becomes ordinary water. 
 Owing to the fact that hydrogen dioxide contains exactly 
 twice as much oxygen as water, the relation between hy- 
 drogen dioxide and water may be expressed in the follow- 
 ing manner ( 147) : 
 
 ( Hydrogen 
 
 -< Oxygen + 
 
 ( Oxygen 
 
 (Hydrogen dioxide) = 
 
 ( Hydrogen 
 I Oxygen 
 
 (Water) 
 
 = Oxygen 
 
 + (Oxygen) 
 
 (Hydrogen dioxide) 
 
 (Water) 
 
 (Oxygen) 
 
 Hydrogen dioxide, usually more or less diluted with 
 water, may be purchased under the name hydrogen per- 
 oxide. 
 
 Cases in which elements unite in more than one pro- 
 portion are rather common, but different conditions must 
 be utilized to induce the elements to unite in these differ- 
 
188 ELEMENTARY PHYSIOS 
 
 ent proportions, and each proportion produces a different 
 compound with different properties. 
 
 Nitrogen and oxygen unite in 5 different proportions. 
 When these elements unite in such proportions that the 
 weight of the oxygen is exactly .57 as great as that of the 
 nitrogen, nitrogen monoxide is produced. It is a colorless 
 and odorless gas, but possesses a sweetish taste. It is 
 used by dentists to deaden pain, and is often called laugh- 
 ing gas. 
 
 When nitrogen and oxygen unite in such proportion by 
 weight, that the oxygen weighs exactly 1.14 times as 
 much as the nitrogen, the compound nitrogen dioxide is 
 produced. It is a colorless gas, and is chiefly character- 
 ized by the fact that as soon as it is exposed to the air 
 it takes up more oxygen, and forms red vapors, nitrogen 
 tetroxide. 
 
 When the proportion by weight in which the elements 
 unite is 1.71 times as much oxygen as nitrogen, the 
 compound nitrogen trioxide is produced. It is a blue 
 liquid. 
 
 When 2.28 times as much oxygen as nitrogen unite, 
 the compound nitrogen tetroxide is produced. It is an 
 orange-brown liquid. Its vapor, however, is red, and is 
 very corrosive and dangerous to inhale. 
 
 When 2.85 times as much oxygen as nitrogen unite, 
 nitrogen pentoxide is produced. It is a white solid sub- 
 stance and is liable to explode. 
 
 An examination of the last four compounds shows that 
 the proportion of oxygen as compared with the nitrogen 
 is 2, 3, 4, and 5 times the proportion of oxygen present in 
 the first compound, nitrogen monoxide. 
 
 The chemical composition of these compounds may be 
 expressed in the following manner : 
 
CHEMISTRY, ATOMS 189 
 
 j Nitrogen 
 I Oxygen 
 
 (Nitrogen 
 monoxide) 
 
 fNitrogen 
 j Oxygen 
 lOxygen 
 
 (Nitrogen 
 dioxide) 
 
 fNitrogen 
 I Oxygen 
 I Oxygen 
 lOxygen 
 
 (Nitrogen 
 trioxide) 
 
 rNitrogen 
 Oxygen 
 -! Oxygen 
 Oxygen 
 VOxygen 
 
 (Nitrogen 
 tetroxide) 
 
 1 Nitrogen 
 Oxygen 
 Oxygen 
 Oxygen 
 Oxygen 
 Oxygen 
 (Nitrogen 
 pentoxide) 
 
 In many cases the series are not so complete. For in- 
 stance, iron and phosphorus unite in the following pro- 
 portions with chlorine: 
 
 {Iron flron ^Phosphorus ^Phosphorus 
 
 Chlorine J Chlorine J Chlorine I Chlorine 
 
 Chlorine j Chlorine j Chlorine J Chlorine 
 
 I Chlorine I Chlorine 1 Chlorine 
 
 Chlorine 
 l.Chlorine 
 
 If all the substances be studied in which the same 
 elements unite in more than one proportion, it will be 
 found that the numbers expressing the various combining 
 weights of one element with a given quantity of another 
 element are all multiples of some one number. These 
 facts can be expressed in the form of the following law, 
 known as the Law of Multiple Proportions. 
 
 When different weights of one element enter into combination 
 with a fixed weight of another element , the different weights 
 of the first element form simple ratios with each other. 
 
 175. Elements Often Exist in Compounds as Equal, Dis- 
 tinct, Separate Quantities. In the preceding paragraphs 
 it has been shown that the proportion, by weight, of oxy- 
 gen and hydrogen is exactly twice as great in hydrogen 
 dioxide as in water: moreover, that the oxygen acts as 
 though half of it were not as well united to the hydrogen 
 in hydrogen dioxide as the other half of the oxygen, since, 
 on heating the hydrogen dioxide, half of the oxygen 
 readily leaves the compound, and thus changes hydro- 
 
190 ELEMENTARY PHYSICS 
 
 gen dioxide into water. The other half of the oxygen 
 clings more tightly to the hydrogen, and it therefore re- 
 quires a much higher temperature to dissociate it from 
 the hydrogen in water. Even at ordinary temperatures 
 hydrogen dioxide slowly loses its oxygen, but above 
 200 degrees F. half of the oxygen passes off so readily 
 that the liquid seems to boil briskly. In order actually 
 to separate the oxygen from the hydrogen in water by 
 means of heat, a much higher temperature, 1230 degrees 
 to 1280 degrees Fahrenheit, must be applied to water 
 after it has been changed to the form of steam. 
 
 Since oxygen appears to exist in hydrogen dioxide in 
 two quantities, not equally well united to the hydrogen, 
 the question arises, may the oxygen in water be also sepa- 
 rated into two or more distinct quantities so that part 
 may be removed while the other remains in combination? 
 No method for accomplishing this is known at present 
 and chemists consider it impossible. Whenever any 
 quantity of oxygen is set free, the proportionate quantity 
 of hydrogen is also set free. The result is the complete 
 breaking up of the water affected into its component ele- 
 ments. Only the water which has not yet lost its oxygen 
 remains behind in the vessel. 
 
 However, a method is known of separating a part of 
 the hydrogen from water, leaving the rest in combination 
 with the oxygen. In this case the part of the water af- 
 fected loses exactly half of its hydrogen. In order to 
 cause this separation heat is not sufficient. A chemical 
 change is necessary. Some other element must take the 
 place of that half of the hydrogen which was separated. 
 One of the simplest means of accomplishing this is by 
 placing sodium in a vessel containing a small quantity of 
 water. Sodium takes the place of part of the hydrogen, 
 
O vjf I f 
 
 CHEMISTRY, ATOMS 
 
 191 
 
 forming- a new compound, sodium hydrate, while the hy- 
 drogen displaced escapes. If sodium is added until all 
 of the water has been changed to sodium hydrate, or if, 
 after some sodium has been added, the water not affected 
 is removed by boiling, the new compound becomes visi- 
 ble. It is a white crystalline solid, and is usually sold 
 under the name caustic soda. If this chemical is analyzed 
 it is found that the proportion of hydrogen compared 
 with oxygen is just half as great in sodium hydrate as in , 
 water. The other half of the hydrogen in water has been 
 replaced by sodium. 
 
 The removal of the second half of the hydrogen which 
 was originally a part of the water, but which is now a 
 part of the sodium hydrate, requires the application of 
 heat. In this case 40 parts by weight of solid sodium 
 hydrate should be heated with 23 parts of sodium. The 
 hydrogen remaining as part of the sodium hydrate after 
 the preceding experiment, then is also displaced by 
 sodium, and the compound sodium oxide is formed. 
 
 These chemical changes may be represented as follows, 
 designating the word sodium by the letters Na : 
 
 H 
 
 (Water) 
 
 (Sodium) (Sodium hydrate) (Hydrogen) 
 
 Na 
 
 
 
 (Sodium hydrate) (Sodium) (Sodium oxide) (Hydrogen) 
 
192 
 
 ELEMENTARY PHYSICS 
 
 If iron pyrites is heated for a long time at a high tem- 
 perature in a covered crucible, half of the sulphur is 
 driven off and the compound iron sulphide remains. This 
 may be represented diagrammatically as follows, the word 
 iron being represented by Fe : 
 
 (Iron pyrites) (Iron sulphide) (Sulphur) 
 
 As the result of many similar experiments it has been 
 learned that in many compounds one or more of the ele- 
 ments seem to exist as though in equal, distinct, and 
 separable quantities ; as though in water there were two 
 equal, separable quantities of hydrogen and one insepa- 
 rable quantity of oxygen, and as though in hydrogen di- 
 oxide there were two equal, separable quantities of hy- 
 drogen and two equal, separable quantities of oxygen. 
 
 176. The Molecules of Compounds Must Have the Same 
 Chemical Composition as the Compounds Themselves. Our 
 conception of molecules demands that molecules of the 
 same substance be considered exactly alike in every re- 
 spect. The fact that crystals of the same substance 
 have the same form, indicates that the molecules also of 
 these substances are alike in size and form ( 86, 87). 
 Since the same substances always consist of the same ele- 
 ments, combined in the same proportion ( 173), their 
 molecules must all be alike in chemical composition. 
 
 If the red oxide of mercury consists of mercury and oxy- 
 gen in the proportion, by weight, of 12.5 to 1 ( 173), 
 then each single molecule of this substance also consists 
 of mercury and oxygen united in this same proportion, 
 
CHEMISTRY, ATOMS 193 
 
 Moreover, whenever any element in any compound seems 
 to act as if it consisted of several equal, distinct, and sepa- 
 rable quantities ( 175), it may be assumed that in each 
 molecule of the compound this element consists of the 
 same number of equal, separable quantities. 
 
 For example, it may be assumed that in the case of a 
 molecule of water, there are two equal, distinct, and sepa- 
 rable quantities of hydrogen but only one distinct quan- 
 tity of oxygen. 
 
 Finally, since these equal quantities of the element en- 
 tering into combination seem to be indivisible into still 
 smaller quantities, it may be assumed that in the case of 
 each molecule also the equal, distinct, separable quanti- 
 ties of this element are not further divisible. 
 
 In the case of water, this theory assumes that each 
 molecule consists of two equal, distinct, separable quan- 
 tities of hydrogen, which are not further divisible into 
 smaller portions, but that there is only one such quantity 
 of oxygen. 
 
 177. Atoms. To the smallest, indivisible quantity of an 
 element present in any molecule the name atom has been 
 given. A molecule of water is therefore supposed to con- 
 sist of 2 atoms of hydrogen and 1 atom of oxygen, while a 
 molecule of hydrogen dioxide consists of 2 atoms of hy- 
 drogen and 2 atoms of oxygen : 
 
 oo Hydrogen 
 (J Oxygen 
 
 Every element is believed to have its own kind of atom. 
 The atoms of the same element are all alike, having the 
 same size, shape and weight, but the atoms of different 
 elements are unlike. Thus every atom of oxygen is like 
 every other atom of oxygen, but not like the atoms of any 
 
194 ELEMENTARY PHYSICS 
 
 other element. There are then as many different kinds 
 of atoms as there are elements. 
 
 Atoms have an attraction for each other, which causes 
 them to unite and to form molecules. This force of attrac- 
 tion which brings the atoms together is called chemical 
 affinity. Molecules may be considered as consisting of 
 groups of atoms held together by chemical affinity. 
 
 178. Chemical Changes Involving Large Quantities of a 
 Substance may be Discussed as if Affecting only One Mole- 
 cule. Since a molecule is nothing more nor less than the 
 smallest particle of any substance which can exist, it is 
 evident that no chemical change can take place in a body 
 unless the chemical character of the individual molecules 
 is altered. If the experiment is conducted in such a 
 manner that only a part of the molecules of the com- 
 pound are affected, then, at the close of the chemical opera- 
 tion, two compounds will be present : the unchanged por- 
 tion of the original compound, and as much of the new 
 compound as resulted from the chemical alteration of that 
 part of the original substance which was actually affected. 
 For the sake of simplicity, in order to make clear the 
 general principles underlying chemical changes, it is 
 always assumed that in each case under discussion the 
 process involved has been carried far enough to affect 
 every molecule. This being the case, the chemical 
 changes affecting the entire mass of the substance acted 
 upon are represented in miniature by the chemical 
 changes taking place in any one of the molecules. In 
 discussing chemical changes it is therefore usually con- 
 venient to discuss them as if affecting only one molecule 
 of each substance acted upon. 
 
 179, Multiple Proportions Explained by Existence of 
 Atoms. The theory that elements are not more or less 
 
CHEMISTRY, ATOMS 195 
 
 continuous substances but are made up of separate atoms 
 will explain a number of facts otherwise inexplainable. 
 It may be shown by chemical analysis that the same com- 
 pound always consists of the same elements united in the 
 same proportion. Water, for instance, consists of hydro- 
 gen and oxygen united in the proportion of 2 parts of 
 hydrogen to 16 parts of oxygen. According to the atomic 
 theory each molecule of water consists of two atoms of 
 hydrogen and one atom of oxygen, the oxygen atom 
 weighing 16 times as much as one of the hydrogen 
 atoms. Since each atom of oxygen is like every other 
 atom of oxygen, and since each atom of hydrogen is like 
 every other atom of hydrogen, the relative amount of 
 oxygen and hydrogen in each molecule of water must 
 always be the same. 
 
 Oxygen may unite with hydrogen in another propor- 
 tion, differing from that present in water. It is evident, 
 however, that if the second compound differs from the 
 first compound only in the relative amount of oxygen 
 present, each molecule of the second compound must 
 contain more than one atom of oxygen. It cannot con- 
 tain a fractional quantity of oxygen, since atoms are in- 
 divisible. This second compound is hydrogen dioxide. 
 Its molecules consist of 2 atoms of hydrogen and 2 atoms 
 of oxygen. Whenever one of the atoms of oxygen is 
 driven off by heat, a molecule of water is left behind. 
 
 Oxygen unites in more than one proportion also with 
 many other elements. It unites in 5 proportions with 
 the same weight of nitrogen producing 5 different com- 
 pounds. Since a molecule represents in miniature what 
 is true of the entire mass of a compound, the number of 
 atoms of nitrogen may be considered the same in every 
 molecule of all five compounds. In this case, however, 
 
196 ELEMENTARY PHYSICS 
 
 the number of atoms of oxygen present in the molecules 
 of the different compounds cannot be the same. The 
 conception of atoms of the same substance as equal and 
 indivisible excludes all possibility of the extrance of an 
 element into combination except in the form of entire 
 atoms. Therefore the number of atoms of oxygen pres- 
 ent in the molecules of each o^ these five compounds 
 must be 1, 2, 3, 4, or some other whole number of atoms. 
 
 According to chemists the number of atoms of nitrogen 
 present in the molecules of all of these compounds is 2. 
 With these two atoms of nitrogen are united 1, 2, 3, 4, 
 and 5 atoms of oxygen. Therefore, the relative weight 
 of oxygen united with nitrogen in these five compounds 
 must be 1, 2, 3, 4, and 5 times the relative weight shown 
 by that compound whose molecules contain only one 
 atom of oxygen. 
 
 180. The Weights of the Atoms of Elements Compared 
 with the Weight of an Atom of Hydrogen. It is impossible 
 to determine the actual weight of an atom, but if our ideas 
 as to the number of atoms in various compounds are 
 correct, it is possible at least to determine their relative 
 weights ; in other words, to determine how much greater 
 the weight of one kind of atom is than the weight of 
 another kind of atom. 
 
 In determining the relative weights, or what are be- 
 lieved to be the relative weights, of atoms, the weight of 
 one atom of hydrogen, which is considered the lightest 
 atom, has been taken as a standard. The weight of an 
 atom of hydrogen is, therefore, called 1, and the weights 
 of all other kinds of atoms are compared with this unit 
 ( 182). 
 
 It is known that in the formation of hydrochloric acid 
 1 part, by weight, of hydrogen unites with 35.2 parts, by 
 
CHEMISTEY, ATOMS 197 
 
 weight, of chlorine. If the theory accepted by chemists 
 be true that a molecule of this acid consists of 1 atom of 
 hydrogen and 1 atom of chlorine, then, if the weight of 
 one atom of hydrogen be called 1, the weight of 1 atom of 
 chlorine must be 35.2. 
 
 In the formation of water 1 part, by weight, of hydro- 
 gen unites with 8 parts, by weight, of oxygen. If it be 
 true that water consists of 2. atoms of hydrogen and 1 
 atom of oxygen, then 1 atom of oxygen weighs 8 times as 
 much as 2 atoms of hydrogen, or 16 times as much as 1 
 atom of hydrogen. 
 
 In ammonia gas 4f parts by weight of nitrogen unite 
 with 1 part by weight of hydrogen. If the molecule of 
 ammonia consists of 1 atom of nitrogen and 3 atoms of 
 hydrogen, then 1 atom of nitrogen weighs 4f times as 
 much as 3 atoms of hydrogen, or 14 times as much as 1 
 atom of hydrogen. 
 
 This discussion will be sufficient to show that if our 
 ideas about the number of atoms in the molecules of 
 various compounds are correct, it is possible to compare 
 the weights of the atoms united in the same compound, 
 and, by a series of comparisons, to compare the weight of 
 any atom with the weight of the atom of hydrogen. The 
 determination of the number of atoms in a compound is 
 a matter of theory. The reasoning involved is often too 
 difficult for an elementary text-book. Enough has been 
 said in preceding paragraphs to give some notions on this 
 subject ( 174179). 
 
 181. Names of Elements Represented by Symbols. In the 
 discussion of chemical phenomena it is often convenient 
 to be able to represent the names of elements by one or 
 two letters. Some names are represented by the initial 
 letter, capitalized always. Carbon is represented by C, 
 
198 
 
 ELEMENTARY PHYSICS 
 
 hydrogen by H, nitrogen by N, oxygen by O, phosphorus 
 by P, sulphur by S. When several names begin with the 
 same letter, two letters are used, the initial letter together 
 with a letter occurring later in the word. The first letter 
 is capitalized and the second is not. Aluminum is repre- 
 sented by Al, arsenic by As, chlorine by Cl, manganese 
 by Mn, nickel by Ni, platinum by Pt, zinc by Zn. In 
 selecting these letters the Latin names of the elements 
 are used. For that reason the letters used to represent 
 the names of some elements will not be recognized at 
 once by those not familiar with the Latin names. For 
 example, antimony (stibium in Latin) is represented by 
 Sb, copper (cuprum) by Cu, gold (aurum) by Au, iron 
 (ferrum) by Fe, lead (plumbum) by Pb, mercury (hydrar- 
 gyrum) by Hg, potassium [kaliumj by K, silver [argen- 
 tum] by Ag, sodium [natrium] by Na, and tin [stannum] 
 by Sn. The letters are called symbols of the elements. 
 
 182. Table of Elements, Including Their Symbols and 
 Atomic Weights. The following is a list of the more 
 common elements. The names of the metals [ 187] are 
 in italics. The non-metals [ 187] are printed in ordi- 
 nary type. The symbols are given in the second and the 
 atomic weight in the third column. A few substances 
 act both as metals and as non-metals. 
 
 Name 
 
 Symbol 
 
 Atomic Wt. 
 
 Name 
 
 Symbol 
 
 Atomic Wt. 
 
 Aluminum 
 
 Al 
 
 27.0 
 
 Chlorine 
 
 Cl 
 
 35.2 
 
 Antimony 
 
 Sb 
 
 119.0 
 
 Chromium 
 
 Cr 
 
 51.7 
 
 Arsenic 
 
 As 
 
 74.4 
 
 Cobalt 
 
 Co 
 
 58.6 
 
 Barium 
 
 Ba 
 
 136.4 
 
 Copper 
 
 Cu 
 
 63.0 
 
 Bismuth ? 
 
 Bi 
 
 206.5 
 
 Fluorine 
 
 F 
 
 19.0 
 
 Bromine 
 
 Br 
 
 79.4 
 
 Gold 
 
 Au 
 
 196.0 
 
 Cadmium 
 
 Cd 
 
 111.4 
 
 Hydrogen 
 
 H 
 
 1.0 
 
 Calcium 
 
 Ca 
 
 40.0 
 
 Iodine 
 
 I 
 
 126.0 
 
 Carbon 
 
 C 
 
 12.0 
 
 Iridium 
 
 Ir 
 
 192.7 
 
CHEMISTRY, ATOMS 
 
 199 
 
 Name 
 
 Symbol 
 
 Atomic Wt. Name 
 
 Symbol 
 
 Iron 
 
 Fe 
 
 55.6 
 
 Platinum 
 
 Pt 
 
 Lead 
 
 Pb 
 
 205.4 
 
 Potassium 
 
 K 
 
 Lithium 
 
 Li 
 
 7.0 
 
 Silicon 
 
 Si 
 
 Magnesium 
 
 Mg 
 
 24.0 
 
 Silver 
 
 Ag 
 
 Manganese. 
 
 Mn 
 
 55.0 
 
 Sodium 
 
 Na 
 
 Mercury 
 
 Hg 
 
 198.5 
 
 Strontium 
 
 Sr 
 
 Nickel 
 
 Ni 
 
 58.2 
 
 Sulphur 
 
 S 
 
 Nitrogen 
 
 N 
 
 14.0 
 
 Tin 
 
 Sn 
 
 Oxygen 
 
 O 
 
 15.8 
 
 Zinc 
 
 Zn 
 
 Phosphorus 
 
 P 
 
 30.8 
 
 
 
 Atomic Wt. 
 
 193.6 
 39.0 
 28.2. 
 
 107.1 
 23.0 
 87.0 
 32.0 
 
 118.0 
 65.0 
 
 183. The Formula of a Compound Records the Number 
 of Atoms of each Element Supposed to be Present in each 
 Molecule. The ability to represent a long- name by means 
 of a few letters is of great convenience, not only when it 
 is necessary to use the names of elements, but also when 
 it is desirable to use names of chemical compounds. In 
 the system in actual use no attempt is made to abbreviate 
 the names of compounds, but a combination of letters is 
 chosen which will indicate, at a glance, what is believed 
 to be the chemical composition of each molecule of the 
 compound. This combination of letters is called the 
 formula of the molecule. In these formulas each symbol 
 represents a single atom of one of the component ele- 
 ments. For instance, red oxide of mercury is represented 
 by the formula HgO, which indicates that every mole- 
 cule of the red oxide of mercury is believed to contain 
 one atom of mercury and one atom of oxygen. 
 
 When more than 1 atom of the element are present, a 
 number, which indicates how many atoms of the element 
 are present in the molecule, is written at the lower right- 
 hand corner of the symbol representing the element. The 
 formula of water, H 2 O, indicates that 2 atoms of hydrogen 
 and 1 of oxygen are present in each molecule of water. 
 
200 ELEMENTARY PHYSICS 
 
 In the same manner, a single molecule of potassium 
 chlorate may be represented by the formula KC1O 3 . 
 This indicates that each molecule of the substance con- 
 tains 1 atom of potassium, 1 atom of chlorine, and 3 atoms 
 of oxygen. 
 
 A group of letters enclosed in brackets, with a small 
 number after the second bracket, indicates that the en- 
 tire group of atoms represented by the letters enclosed 
 in the brackets must be multiplied by this number in 
 order to ascertain the number of atoms of each element 
 in the molecule of the compound. Thus Pb (NO 3 ) 2 , lead 
 nitrate, consists of 1 atom of lead, 2 of nitrogen, and 6 of 
 oxygen. In this case the group of atoms in that part of 
 the formula which is enclosed in brackets is believed to 
 be more strongly united than the other atoms, so that in 
 cases of substitution it often enters and leaves the mole- 
 cule as a group. At least the atoms included in the group 
 often enter the molecule from the same source. Thus the 
 NO 3 of lead nitrate may have been derived from nitric acid, 
 the hydrogen of the acid having been displaced by lead. 
 When it is desirable to express that several molecules 
 are present, a large figure representing the number of 
 molecules is placed before the formula. Thus, 3H 2 O rep- 
 resents 3 molecules of water. 
 
 184. How to Read a Formula. A glance at a formula 
 will at once suggest to the expert chemist the name of 
 the compound ; moreover, it will also suggest, even to one 
 not an expert chemist, the chemical composition of the 
 compound. In speaking of, or in writing about a sub- 
 stance, it is often more convenient to give the formula of 
 its molecule than to use its actual name. In this case 
 each figure and letter is read in the order in which it 
 occurs in the formula. A slight stop is made after every 
 
CHEMISTRY, ATOMS 201 
 
 figure indicating the number of atoms. Any figure given 
 before any of the letters of the formula have been men- 
 tioned is at once understood as giving the number of 
 molecules present. The reading of 3H 2 SO 4 is three H 
 two S O four, and 4K 2 Cr 2 O 7 is read four K two Cr 
 two O seven. 
 
 Head the formulas, P 2 O 5 , CO 2 , KC1O 3 , Ca CO 3 , ZnSO 4 . 
 In every case tell what elements and how many of their 
 atoms are present in each molecule. 
 
 185. The Formula of a Compound Indirectly Records 
 the Relative Combining Weights of the Component Ele- 
 ments. The existence of both molecules and atoms is 
 assumed in order to explain phenomena, often very 
 familiar phenomena, which otherwise are inexplainable. 
 Since it is not at all likely that molecules and atoms will 
 ever be seen, our ideas of them and of their properties 
 must necessarily vary with the increase of facts which 
 we try to explain when we assume the existence of mole- 
 cules and atoms. As our knowledge of chemistry increases 
 it is fair to assume that new methods of experimentation 
 may prove that elements which we now consider to be 
 present in molecules in the form of single atoms, may in 
 reality be represented by 2 or 3 atoms, or that elements 
 now believed to be present in the form of 2 atoms in each 
 molecule may later, on account of further discoveries, be 
 believed to be present in the form of 4 or 6 atoms in each 
 molecule, so that it will be necessary from time to time 
 to change some of the formulas in order to represent the 
 new facts. 
 
 However, while our conceptions of the number of atoms 
 of each element in the molecules of various compounds 
 may change, there is one fact which can be determined 
 with considerable exactness, regarding which there are 
 
202 ELEMENTARY PHYSICS 
 
 likely to be few changes of opinion in the future, and that 
 is the relative quantity of each element present in a com- 
 pound. If the element has been accurately identified, if 
 the balances are good, and if the chemical changes taking 
 place during all the processes of synthesis and analysis 
 have been correctly followed, there need be little change of 
 opinion as to the relative weights of the elements present. 
 In the effort to express the atomic constitution of each 
 molecule of the compound, the relative quantity of each 
 element present in the compound is also expressed, al- 
 though only indirectly. For instance, if the formula of 
 nitric acid is written as HNO 3 , this shows that, at pres- 
 ent, methods of chemical analysis suggest that 1 atom of 
 hydrogen, 1 of nitrogen and 3 of oxygen are present in 
 the compound. Since at present the weight of an atom 
 of nitrogen is assumed to be 14 times as great as the 
 weight of an atom of hydrogen, and that of an atom of 
 oxygen to be 16 times as great as that of an atom of hy- 
 drogen, it is evident that the relative proportions of hy- 
 drogen, nitrogen, and oxygen present in nitric acid must 
 be 1:14:48. This relative proportion of the three ele- 
 ments in nitric acid is known with certainty. It has been 
 determined by chemical analysis. 
 Fact But the existence of three atoms 
 
 n I I I I of oxygen, with only one atom of 
 
 ^ ; ^ : Q hydrogen and nitrogen in this 
 
 compound, is merely an assumption 
 (Fig. 86), based, however, upon facts 
 o C^\ (^Theory which are just as well established 
 N ^-. as those discussed in the previous 
 
 V_y paragraphs in connection with the 
 
 3 constitution of water and of 
 
 FIG. 86. T . -. 
 
 gen dioxide. 
 
CHEMISTRY, ATOMS 203 
 
 From the relative proportions of the elements present 
 in a compound, as established by actual analysis, and in- 
 directly recorded in the formula for a molecule of a com- 
 pound, it is possible to determine what weight of any 
 element is present in any compound. If, for instance, in 
 the case of nitric acid, the elements hydrogen, nitrogen, 
 and oxygen are present in the proportions 1 : 14 : 48, then 
 -fe of any quantity of nitric acid must be hydrogen, jf 
 must be nitrogen, and f f must be oxygen. In 16 ounces 
 of nitric acid, ^ of 16 .254 ounce of the acid must con- 
 sist of hydrogen, f of 16 = 3.556 ounces of the acid must 
 consist of nitrogen, and f f of 16 = 12.190 ounces of the 
 acid must consist of oxygen. 
 
 In fact, before a single formula is ever written, some 
 chemist analyzes the compound and determines the rela- 
 tive proportion by weight of all the elements which it 
 contains. These relative proportions of the elements he 
 records in his formula, but, instead of writing his formula 
 in the form of .254H, 3.556N, 190O, or 1H, 14N, 48O, he 
 attempts to express the results of his analysis in such a 
 manner that their application may be general. He there- 
 fore determines from his chemical analysis how many 
 atoms of each element are present in each molecule and 
 records the number of atoms in the formula. Then the 
 number of atoms of the various elements as recorded in 
 the formula indicates practically also the relative pro- 
 portion by weight of the elements present, to any one who 
 knows the relative weights of the different atoms. 
 
 186. Chemical Changes May Be Recorded in the Form 
 of an Equation. The amount of each element present in 
 any particular quantity of any substance may be deter- 
 mined by chemical analysis. Since not a particle of sub- 
 stance is either created or destroyed during any chemical 
 
204 ELEMENTARY PHYSICS 
 
 change, it is possible to express in the form of an equation 
 the relative amount of each element present in each sub- 
 stance at the beginning- of the experiment and in what 
 manner these elements are combined in the different prod- 
 ucts resulting from the chemical change. 
 
 In practice, no attempt is made to express in equations 
 the actual weight of the different materials taking part, 
 but only their relative weights. This may be accom- 
 plished by representing the chemical change as though it 
 were taking place between only 1, 2, or some other small 
 number of molecules of the substances present. The 
 manner in which the atoms are united before the chem- 
 ical change is indicated before the equality sign. The 
 manner in which the atoms are united after the chemical 
 change is indicated after the sign of equality. In other 
 words, the substances used are indicated on the left of 
 the sign of equality, and the substances resulting from 
 the chemical action are indicated on the right of this sign. 
 
 To illustrate, the chemical action which took place 
 when the potassium chlorate was heated together with 
 the manganese dioxide ( 134), may be represented by 
 
 KC1O 3 + MnO 2 = KC1 + 3O + MnO 2 , 
 
 which means that after the chemical change took place 
 the manganese dioxide was found unchanged, but the 
 potassium chlorate was found broken up into potassium 
 chloride, KC1, and oxygen. 
 
 The chemical action when phosphorus is burned in air 
 may be represented by the equation, 
 
 2P + 5O + 20N = P 2 O 5 + 20N, 
 
 which means that, after the chemical change has taken 
 place, the nitrogen in the air is found unchanged, but the 
 
CHEMISTRY, ATOMS 205 
 
 atoms of phosphorus and the atoms of oxygen have 
 united in the proportion indicated. 
 
 187. Classification of Elements. Elements are classi- 
 fied as metals and non-metals. The metals possess a pe- 
 culiar lustre which we call metallic. They are good con- 
 ductors of heat and electricity. When they are substituted 
 for the hydrogen in acids, they form salts. When chemi- 
 cal action takes place between the oxide of a metal and 
 water, a new compound called a base is produced. 
 
 The non-metals do not possess a metallic lustre. Most 
 of them are very poor conductors of heat and electricity. 
 When united with hydrogen, they generally form gaseous 
 compounds. When chemical action takes place between 
 the oxide of anon-metal and water, an acid is produced. 
 
 The most important metals and non-metals are given 
 in the table in 182. 
 
 The meaning of some of the terms used here are ex- 
 plained in the next section. 
 
 188. The Classification of Compounds. No system of 
 classification of compounds has been proposed, up to the 
 present time, which can in any sense be called perfect. 
 Nevertheless the classification now in use has been found 
 to be very convenient. 
 
 The four most important classes are oxides, acids, bases, 
 and salts. 
 
 An oxide is a combination of an element with oxygen. 
 H 2 O, C0 2 , Mn0 2 . 
 
 An acid is a peculiar combination of hydrogen with one 
 or more other elements. The combination is of such a 
 character that the hydrogen may be more or less readily 
 displaced by a metal, so that the hydrogen compound will 
 be changed to a metal compound. 
 
 2HC1 + Zn = ZnCl 2 + 2H. 
 H 2 SO 4 + Mg = MgSO 4 + 2H. 
 
206 ELEMENTARY PHYSICS 
 
 Some acids are formed by the combination of hydrogen 
 with only chlorine, bromine, iodine, or sulphur, but most 
 acids are formed by the union of hydrogen with two or 
 three elements, of which one is oxygen. 
 
 A base is formed by the union of a metal with one or 
 more of the group of atoms OH, called hydroxyl. Those 
 bases which contain the metals sodium, Na, potassium, 
 K, or the group of atoms NH 4 are also called alkalis, 
 Na(OH), K(OH). NH 4 (OH). 
 
 Acids and bases have several exactly opposite proper- 
 ties. Acids have an acid or sour taste ; they change lit- 
 mus-paper from blue to red. Bases have the taste of lye, 
 or an alkaline taste, and they change litmus-paper from 
 red to blue. Both acids and bases are chemically active 
 substances, but, when they are brought in contact, they 
 counteract each other, and, if the proportion be properly 
 adjusted, they entirely destroy the characteristic prop- 
 erties of each other. They neutralize each other. In the 
 neutralized solution the acid is no longer able to turn 
 blue litmus-paper red, and the base cannot turn red lit- 
 mus-paper blue. The reason for the inactivity of a neu- 
 tral solution is the fact that in a neutral solution no acid 
 or base is present. As soon as an acid and a base come 
 in contact with each other, chemical action takes place, 
 the metal of the base is interchanged with the hydrogen 
 in the acid, and two new compounds are formed. One of 
 these compounds is water. The other compound is a 
 substance called a salt. 
 
 HC1 + NaOH = NaCl + H 2 O. 
 
 Salts may also be produced by chemical reactions in 
 which no acids are present. 
 A salt has the properties of neither an acid nor a base. 
 
CHEMISTKY, ATOMS 207 
 
 It is a neutral substance. It has exactly the same com- 
 position as the acid from which it was derived, except 
 that the hydrogen has been displaced by some metal. 
 The name salt is given to this class of compounds, be- 
 cause most of these compounds, after having been dis- 
 solved in water, crystallize when the water is evaporated 
 and the crystals of the great majority of chemical salts 
 are transparent and colorless like table salt. A very con- 
 siderable number of chemical salts, however, are beauti- 
 fully and strongly colored. 
 
 189. Poisons and Antidotes. Both acids and bases are 
 usually poisonous, at least when taken in large quanti- 
 ties. If an acid has been swallowed, some base like 
 baking soda or lime-water is given to neutralize the acid. 
 If the substance swallowed be a base, some weak acid, 
 like weak vinegar (acetic acid), will serve as an antidote. 
 Different poisons require different antidotes, but the prin- 
 ciple involved in securing an antidote to a chemical 
 poison is usually to secure a chemical which, on entering 
 into chemical combination with the poisonous substance, 
 will form a harmless compound. Emetics and stomach- 
 pumps serve to remove poisons from the stomach, and if 
 used soon enough may rid the stomach of poison before 
 much has been taken into the system. 
 
 190. Names of Compounds When oxygen, chlorine, 
 bromine, iodine, sulphur, arsenic, carbon, all of which are 
 non-metallic elements, unite with only one other element, 
 and that element is a metal, the compound is called an 
 oxide, chloride, bromide, iodide, sulphide, arsenide, or 
 carbide of that metal. The metal is usually mentioned 
 first. For instance, PbI 2 is lead iodide. If it be desira- 
 ble to mention the number of atoms of the non-metallic 
 element present in the compound, the following prefixes 
 
208 ELEMENTARY PHYSICS 
 
 are placed before the name of the non-metallic element : 
 mono (one), di (two), tri (three), tetra (four), penta (five). 
 For example, MnO 2 is manganese dioxide ; PC1 3 is phosr 
 phorous trichloride. The prefix sesqui means one and a 
 half. It is used when the ratio of the metal to the non- 
 metal is 2 to 3 (=1 to 1J). For instance, Mn 2 O 3 is man- 
 ganese sesquioxide. 
 
 When a non-metallic element unites with hydrogen to 
 form an acid, the prefix hydro is used before the name of 
 this non-metallic element. For instance, HC1 is hydro- 
 chloric acid. 
 
 When an acid consists of hydrogen, oxygen, and some 
 other element, this other element gives the name to the 
 acid and the name ends in ic. For instance, HNO 3 is ni- 
 tric acid. 
 
 If two acids are formed by the same elements, and the 
 number of atoms of oxygen as compared with the number 
 of atoms of the other two elements is not the same, the 
 name of the acid which contains the smaller quantity of 
 oxygen ends in mis. For example, H 2 SO 4 is sulphuric 
 acid ; H 2 SO 3 , sulphurous acid. 
 
 When three elements unite to form three acids, each 
 with a different number of atoms of oxygen in the mole- 
 cule, the one containing the least oxygen has the prefix 
 hypo and the ending ous attached to the name of the acid. 
 For instance, H 3 PO 4 is phosphoric acid ; H 3 PO 3 , phos- 
 phorous acid ; and H 3 PO 2 , hypophosphorous acid. 
 
 Salts produced by the substitution of a metal for the 
 hydrogen in an acid are named by using both the name 
 of the metal and the name of the acid. But if the name 
 of the acid ends in ic, the name of the salt ends in ate, 
 and if the name of the acid ends in ous, the name of the 
 salt ends in ite. For instance, KNO 3 is potassium ni- 
 
CHEMISTKY, ATOMS 209 
 
 trate ; KNO 2 , potassium nitrite ; BaSO 4 , barium sulphate ; 
 BaSO 3 , barium sulphite. The connection between the 
 name of a salt and that of the acid from which it was de- 
 rived is indicated in the following table: 
 
 HN0 3 + 
 
 Nitric Acid 
 
 KOH 
 
 = KN0 3 + 
 
 Potassium Nitrate 
 
 H 2 O. 
 
 HN0 2 + 
 
 Nitrous Acid 
 
 KOH 
 
 = KNO 2 + 
 
 Potassium Nitrite 
 
 H 2 0. 
 
 H 2 SO 4 + 
 
 Sulphuric Acid 
 
 BaCl 2 
 
 = BaSO 4 + 
 
 Barium Sulphate 
 
 2HC1. 
 
 H 2 S0 3 + 
 
 Sulphurous Acid 
 
 BaCl 2 
 
 = BaS0 3 + 
 
 Barium Sulphite 
 
 2HC1. 
 
 The endings ic and ous appended to the names of the 
 metals in the names of compounds call attention also to 
 the quantity of oxygen, chlorine, or non-metal present. 
 For instance, Fe 2 O 3 is ferric oxide ; FeO, ferrous oxide ; 
 FeCl 3 , ferric chloride ; FeCl 2 , ferrous chloride ; Fe 2 (SO 4 ) 3 
 ferric sulphate ; Fe 2 (SO 4 ) 2 , ferrous sulphate. 
 
 191. Chemical Action Assisted by the Presence of Water. 
 Chemical action does not take place between lead 
 nitrate and potassium iodide ( 171) while they are in 
 the solid state. Even if they are ground together and in- 
 timately mixed, they do not combine. Each particle of 
 solid material is held together by the force of cohesion, 
 and, before chemical action can take place, this cohesion 
 must be overcome. Dissolving a solid in water loosens 
 the cohesion between the molecules ; more than that, it 
 weakens the force with which atoms hold together in the 
 molecule. The result is that chemical action will often 
 take place readily between solids dissolved in water, even 
 in cases in which no chemical action can be noticed when 
 these solids are mingled in the solid state. 
 
210 ELEMENTARY PHYSICS 
 
 192. Chemical Action Assisted by Heat. Increase of 
 heat causes the molecules of a solid to move farther apart. 
 Not only is the cohesion between the molecules lessened, 
 but also the force with which the atoms in each molecule 
 cling 1 together is weakened. In this condition atoms can 
 enter more readily into combination with other atoms. 
 When gunpowder is heated beyond a certain temperature, 
 chemical action between the various solids used takes 
 place with such violence that an explosion occurs. 
 
 Chemical action between solids in solution also is very 
 much increased by heating the solution. 
 
 193. Heat is Consumed during the Separation of Com- 
 pounds into Their Elements or Components. It has been 
 shown that increase of heat causes molecules to move 
 farther apart. This is true not only in cases where there 
 is no change of state (solids, 63 ; liquids, 62 ; arid 
 gases, 97, 115), but also in all cases of change of state 
 from liquids to gases ( 96, 104), and in most of the cases 
 of change from solids to liquids ( 95, 104). The same 
 fact may be stated in another form by saying that heat is 
 consumed during the separation of molecules. When the 
 molecules of solids separate on dissolving in a liquid, 
 heat is consumed, and, since the heat is usually taken by 
 the solid in large part from the liquid in which it dis- 
 solves, the liquid gets colder ( 114). When the mole- 
 cules of liquids separate during evaporation, heat is con- 
 sumed. This heat is secured chiefly from the air into 
 which the liquid evaporates. In consequence, the air be- 
 comes colder ( 113). 
 
 According to the same principle increase of heat causes 
 a separation of atoms ( 134, 154) already in combination 
 as molecules, and conversely the separation of atoms from 
 their combinations as molecules consumes heat. 
 
CHEMISTKY, ATOMS 211 
 
 194. When Substances Unite to Form Compounds Heat 
 is Produced. It has been shown that, when gases re- 
 turn to a liquid state, or when liquids return to a solid 
 state, heat is given out although there is no fall in tem- 
 perature ( 111, 112). It is also a matter of common ex- 
 perience that hot solids and liquids give out heat, when 
 they themselves are cooling or lowering in temperature. 
 Experiments have been devised to show that, while sub- 
 stances in solution are crystallizing, they give up heat to 
 the water in which they are dissolved or that the tempera- 
 ture of the water rises. 
 
 From these facts the general rule may be made, that, 
 while the molecules of substances come closer together, 
 they give up heat to surrounding bodies. The unusual 
 behavior of water has so far not been explained ( 117). 
 
 The coming together of atoms so as to form molecules 
 also produces heat. When elements unite very slowly 
 or in small quantities at any one time, the heat produced 
 may pass off into the air so readily that it may not even 
 be noticed. However, when the chemical action is rapid, 
 the production of heat can be detected readily. When 
 water is poured upon a piece of recently burned quick- 
 lime, the two substances unite to form slaked lime 
 [CaO -f H 2 O = Ca (OH) 2 ] and the heat produced is con- 
 siderable. This action may be seen whenever men slake 
 lime to make mortar. Fires are frequently started when 
 water gains access to ships or warehouses in which quick- 
 lime is stored. 
 
 195. Heat may be Produced with Sufficient Rapidity to 
 Cause Light. In some cases the chemical combination is 
 sufficiently rapid to cause light in addition to heat. 
 Straighten a piece of watch spring. Heat the tip red hot 
 and dip it into sulphur powder. As soon as the sulphur 
 
212 ELEMENTARY PHYSICS 
 
 adhering to the spring has ignited, dip the spring quickly 
 into a jar of oxygen. The iron will unite with the oxygen, 
 and sufficient heat will be produced to give rise to a brill- 
 iant sputtering light. In fact the light caused by the 
 ordinary burning of objects in air is due merely to the 
 great heat produced by the union of oxygen with the ele- 
 ments in the burning material. Ordinary combustion is, 
 therefore, due to rapid union of the elements in burning 
 bodies with the oxygen in the air. 
 
 Slow combustion is vastly more common than ordinary 
 combustion, but it often passes unnoticed. Most of the 
 chemical unions that take place in nature and even many 
 of those which take place in the laboratory, take place 
 without the production of light. In many of these cases 
 oxygen is one of the uniting elements. The uniting of 
 oxygen with other elements is called oxidation. 
 
 196, The Origin of Heat in the Animal Body One of 
 
 the most important cases of slow combustion is that which 
 is the cause of the heat in the animal body. The heat- 
 yielding food-stuffs that enter the body are compounds 
 which always contain carbon, hydrogen, and oxygen (fat, 
 sugar, starch, vegetables, etc.) and often also nitrogen 
 (meat). The materials of which our body is composed con- 
 sist of the same elements. When the compounds in our 
 body are decomposed they unite with the oxygen brought 
 in through our lungs and circulating through our blood. 
 The result is that the decomposition of the compounds in 
 our body is accompanied by a slow combustion due to 
 the union of oxygen with the elements which formed 
 part of these compounds. 
 
 Animal heat is not due to the decomposition of the 
 chemical compounds in our bodies, since the separation 
 of atoms, which takes place during decomposition, is 
 
CHEMISTRY, ATOMS 213 
 
 accompanied by absorption, not by emission of heat. 
 But the heat given out as the result of the union of addi- 
 tional oxygen to the decomposition products, is so much 
 greater than the heat absorbed during- the separation of 
 the atoms in decomposition, that on the whole more heat 
 is given out than taken up and the heat given out serves 
 to warm the animal body. 
 
CHAPTEE V 
 SOUND 
 
 197. Sound Produced by Vibrations. Every body which 
 produces sound is in motion. When a tuning-fork, violin, 
 or bell produces sound, if the hand touches lightly the 
 prong- of the tuning-fork, the string of the violin, or the 
 margin of the bell, the motion may be distinctly felt. If a 
 light pith-ball suspended at the end of a thread is brought 
 
 into contact with these sounding 
 bodies (Fig. 87), it flies off, re- 
 turns, and flies off again, as if 
 repeatedly struck. The parts 
 in motion move short distances 
 only. On this account the mo- 
 tion is often not visible to the eye. 
 The parts in motion move to 
 and fro like the pendulum of a 
 clock, but with greater frequency. 
 Even the least frequent of these 
 to and fro motions amount to at 
 
 rIG. /. 
 
 least sixteen and a half per sec- 
 ond. This frequency makes the individual motions im- 
 perceptible, even if the distance covered is great enough 
 to be visible to the eye. But the hazy effect produced by 
 these rapid motions can be distinctly noticed. 
 
 198. The To and Fro Motions are Called Vibrations. 
 The to and fro motions are called vibrations. An entire 
 vibration includes both the to and the fro motion. The mo- 
 
 214 
 
SOUND 215 
 
 tion in one direction only, from the farthest point reached 
 in the right to the farthest point in the left, is but half of 
 a vibration (Fig. 88). Half of the motion from one side to 
 the other is called the amplitude of vibration. 
 The amount of time taken by the body to com- 
 plete an entire vibration is called the period of 
 the vibration. The number of vibrations com- 
 pleted by the body during one second is called 
 the frequency of the vibrations. When two or 
 three bodies complete the same number of vi- 
 brations per second, but move with varying 
 degrees of rapidity, so that some produce vi- 
 brations with great amplitudes, and other vi- 
 brations with smaller amplitudes, the vibra- 
 tions are said to vary in violence or intensity. FIG ^ 
 
 199. The Frequency of the Vibration Deter- 
 mines the Pitch of the Tone, If a sounding body vibrates 
 a few times per second, it will give a low tone. If it vi- 
 brates many times per second, it will give a high tone. 
 The lowness or highness of a tone is called the pitch of 
 the tone. The frequency of the vibrations of a sounding 
 
 body, therefore, de- 
 termines the pitch 
 of the tone which it 
 produces. 
 
 This may be 
 shown easily by a 
 FlG 89 simple experiment. 
 
 If a stiff piece of 
 
 card-board is held against the cogs of a rapidly turn- 
 ing wheel (Fig. 89) taken from the interior of some 
 clock, the card-board snaps from cog to cog, being first 
 lifted slightly by one. cog, and then allowed to slip past 
 
216 ELEMENTARY PHYSICS 
 
 its end so as to strike the next cog. This causes the 
 end of the card-board to move up and down or to vi- 
 brate. As long as the wheel moves slowly only a suc- 
 cession of snaps is heard, but as soon as the motion is 
 sufficiently rapid to cause the card-board to snap from 
 cog to cog at the rate of about 16 times per second, a 
 very low tone is heard in addition to the snaps. As the 
 motion of the wheel is increased, the vibrations produced 
 by the card-board are more frequent, and the tone heard 
 in addition to the snaps is not only louder but also higher 
 in pitch. If the wheel be rotated with sufficient frequency, 
 the pitch may rise high enough to be disagreeable. 
 
 200. The Frequency of Vibration of Various Tones. The 
 lowest C of the pipe organ is produced by 16J vibrations 
 per second, the lowest A of the grand piano by 27J vi- 
 brations per second, the lowest C of the ordinary piano 
 by 33 vibrations, and the lowest E of the double bass 
 violin by 41J vibrations. The highest C of the piano is 
 produced by 4224 vibrations per second, and the highest 
 D of the piccolo flute by 4752 vibrations per second. 
 Tones produced by vibrations more rapid than 4752 per 
 second are not considered useful for musical purposes, al- 
 though vibrations as frequent as 40,960 per second can be 
 heard. 
 
 The lowest tones which are audible to the ear are used 
 in music (pipe organ), although it is questionable whether 
 they have any distinct musical quality when the vibra- 
 tions number less than 32 per second. 
 
 The frequency of vibration of a mosquito's wings can 
 be determined by increasing the speed of the wheel of 
 the clock, until the card-board held against the cogs 
 gives out a sound of equal pitch. From the number of 
 cogs in the wheel and the number of turns per second, 
 
SOUND 217 
 
 the number of vibrations of the card can be determined. 
 Since the wings of the mosquito give the same tone as 
 that given by the card, the number of vibrations of the 
 wings of the mosquito must be the same as the number 
 of vibrations of the card. 
 
 In the same manner, the number of vibrations produced 
 by the vocal cords in the throat of a man or woman sing- 
 ing can be determined. The tone of G written on the 
 lowest line of the bass staff is produced by 96 vibrations 
 per second, middle C by 256 vibrations, and F, written on 
 the highest line of the soprano staff, by 682.6 vibrations. 
 It is possible to sing both lower and higher than these 
 tones. C above the soprano staff is produced by 1024 vi- 
 brations per second. 
 
 201. A Medium is Necessary to Transmit the Vibrations of 
 the Sounding Body to the Ear. Since the instrument pro- 
 ducing the sound and the ear perceiving the sound are 
 always at some distance from one another, something 
 must act as a means of transmission between the instru- 
 ment and the ear. This medium is usually the air. 
 
 An alarm clock placed under a bell- jar on the plate of 
 an air-pump can be heard easily, but if the air is pumped 
 out from the space within the bell- jar, the sound is much 
 less distinct. The sound could not be heard at all, if 
 its transmission through the plate of the air-pump could 
 in some way be prevented. A considerable quantity of 
 cotton batting placed under the clock will practically 
 accomplish this (Fig. 90). 
 
 "Water also may serve as a medium. If the head is 
 held under water while some one, a moderate distance 
 away, pounds together two stones, also held beneath the 
 surface of the water, the water transmits the sound so 
 effectively that the sensation is sometimes even painful. 
 
218 
 
 ELEMENTARY PHYSICS 
 
 Even solids transmit sound. In fact, some solids trans- 
 mit sound better than air. If the point of a pin is 
 scratched lightly against one end 
 of a long wooden pole, the sound 
 can be heard easily by a person 
 who places his ear against the 
 other end. However, if the ear is 
 held at some distance away from 
 the pole, but at the same distance 
 from the scratched end as before, 
 the sound, in this case transmitted 
 through the air, may not be audi- 
 ble. The cannonading at Antwerp 
 in 1832 was heard in the mines of 
 Saxony 320 miles away. What trans- 
 mitted the sound ? 
 
 202. The Sensation of Sound 
 The original cause of sound and 
 the physiological effect of sound 
 
 are two widely distinct things. The original cause of 
 sound is the vibration of the whole or of a part of some 
 musical instrument or other body. The physiological 
 effect of sound is the sensation produced by these vibra- 
 tions on the nerves of the ear (auditory nerves). The 
 physicist studies the original cause of sound, how sound 
 is transmitted from the original sounding body to the 
 ear, and the character of the action of the various mech- 
 anisms which transmit the vibrations from the air to the 
 nerves of the ear (fibres of the basilar membrane, 214- 
 215). The physiologist dissects all parts of the organ of 
 hearing and studies the effect produced by the vibrations 
 upon the auditory nerves. 
 
 If there is 110 air present to transmit these vibrations, 
 
 PIG. 90. 
 
SOUND 219 
 
 if there is no ear present to receive them, or if the vibra- 
 tions are too weak to affect the ear, there will be no 
 sensation of sound. Loudness of sound is the intensity 
 of the sensation produced in the nerves of the ear. Any 
 cause which will decrease the intensity of the vibrations 
 from the time when they leave the sounding- body to the 
 time when they reach the nerves in the ear, produces a 
 corresponding decrease in the loudness or strength of 
 the sensation in the auditory nerves. 
 
 203. Sound Transmitted through the Air by Waves of 
 Condensation and Rarefaction. When the prong of a tun- 
 ing-fork moves forward, it condenses the air immediately 
 in front of the prong, and, as the molecules strike against 
 molecules, this condition of condensation travels forward 
 and away from the fork. When the prong swings back 
 during the second half of its vibration, it tends to leave a 
 vacuum on the side where a moment before it had pro- 
 duced a condensation. As the nearest air particles rush 
 back to fill up this space, a condition of rarefaction 
 occurs at the points more remote from the fork, from 
 which the air particles have returned. This condition of 
 rarefaction is taken up successively at points more and 
 more removed, so that, while the molecules of the air are 
 in reality moving ~back towards the fork, the condition of 
 rarefaction may be said to be spreading outward from the 
 fork. Therefore, both the condition of condensation and 
 that of rarefaction travel outward from the fork. 
 
 If the fork continues to vibrate for some time, there will 
 be a regular succession of these condensations and rare- 
 factions moving away from the fork. Since the conden- 
 sations are produced by the forward motion of the fork 
 and the rarefactions by the backward motion, the rare- 
 factions will always be found between the condensations. 
 
220 
 
 ELEMENTARY PHYSICS 
 
 At any given time the condensations first formed will 
 have reached points in the air more distant from the fork, 
 while the condensations or rarefactions later formed will 
 not have travelled so great a distance (Fig. 91). 
 
 If the rate of vibrations given out by the tuning-fork be 
 rapid, the distances from condensation to condensation 
 will be short. If the rate of vibrations be slow, the dis- 
 tances between successive condensations will be greater. 
 As long as the rate of vibration remains the same, the 
 distances between successive condensations will be equal. 
 
 When the air is condensed at any point, it spreads 
 away from that point to all regions where the pressure is 
 
 FIG. 91. 
 
 less. The result is that the waves of condensation, as 
 they leave the fork, spread in all directions and form a 
 succession of spherical shells, the alternating shells being 
 represented by alternating conditions of condensation and 
 rarefaction (Fig. 92). 
 
 If an attempt be made to compare the series of conden- 
 sations and rarefactions to something which may be actu- 
 ally seen in nature, no better object of comparison can be 
 found than the succession of concentric crests and hol- 
 lows which move away from the point where a stone is 
 dropped into water. For this reason, the successive con- 
 densations and rarefactions produced in the air by sound- 
 ing bodies are usually referred to as waves of sound. 
 
SOUND 
 
 221 
 
 204. Although the Vibratory Motion is Transmitted 
 through the Air for Long Distances, the Individual Mole- 
 cules of Air Move Only Short Distances. A very crude 
 notion of the action of the molecules during- the trans- 
 mission of sound may be secured from the following ex- 
 periments : 
 
 If a dozen boys facing in the same direction form a line, 
 
 FIG. 92. 
 
 each boy placing his hands firmly against the shoulders 
 of the boy in front, a strong push against the boy in the 
 rear of the line causes the boy at the front to topple over 
 (Fig. 93). The motion is communicated from boy to boy. 
 The distance along which the motion is transferred is 
 much greater than the distance passed over by any one 
 boy. 
 
222 
 
 ELEMENTARY PHYSICS 
 
 If a large stone be dropped into a pond, a series of 
 waves will travel away from the stone to the margin of the 
 pond. Any corks floating' upon the water, however, will 
 merely bob up and down as the waves pass beneath them 
 and move slightly forward and backward without being 
 carried permanently nearer the shore. It is evident that 
 waves have been formed in the water without any con- 
 siderable forward motion on the part of the individual 
 molecules of water which transmit the waves. 
 
 In the same manner a molecule of air set in motion by 
 the prong of a tuning fork may strike against the mole- 
 
 FlG. 93. 
 
 cule in front, this molecule may strike against the next 
 molecule, and thus the motion may be transferred for 
 great distances from molecule to molecule without involv- 
 ing any great amount of motion on the part of any one 
 molecule. 
 
 205. The Vibrations of the Sounding Body Cause the In- 
 dividual Molecules of Air to Vibrate. A more exact no- 
 tion of the method of transmission of sound through 
 the air requires a study of the character of the motions 
 of the individual molecules. When the prong of a tuning- 
 fork moves forward, the molecules of air in front of the 
 prong- also move forward, and, as we have seen, the for- 
 
SOUND 223 
 
 ward motion is communicated from molecule to molecule, 
 until the molecules at a considerable distance from the 
 fork also move forward. When the prong of the tuning- 
 fork returns to its original position, it tends to form a 
 vacuum there where a moment before it produced a con- 
 densation. The pressure of the air in contact with this 
 side of the prong is now less than the pressure at a 
 greater distance from the fork. The molecules of air, 
 which a moment ago were moving away from the fork, 
 may now return. The molecules nearest the fork return 
 first. This diminishes the pressure of the air in the space 
 from which they return, and so the more distant mole- 
 cules may also move backward. This motion is taken up 
 by molecule after molecule, until even the molecules at a 
 considerable distance from the fork have taken up the 
 returning motion. 
 
 As the prongs of the tuning-fork continue to move for- 
 ward and backward, a corresponding forward and back- 
 ward motion is taken up by all the molecules of the air. 
 The molecules nearest the fork take up this motion first 
 and the molecules at a greater distance take it up later. 
 These to and fro motions are vibratory motions, and it is 
 by means of such motions that sound travels through the 
 air. 
 
 206. Loudness, or Intensity of Sound. The intensity of 
 the sensation produced in the ear depends not directly 
 upon the intensity of the vibrations of the molecules in 
 contact with the sounding body, but upon the intensity of 
 the vibrations of those molecules which come in contact 
 with the ear. If the molecules which come in contact 
 with the ear do not vibrate strongly, the sound will be 
 faint, even if the molecules in contact with the vibrating 
 body are in violent motion. This, however, is always the 
 
224 ELEMENTARY PHYSICS 
 
 case when the ear is at a considerable distance from the 
 sounding body. 
 
 When the air is condensed at any point, it spreads 
 away from that point to all regions where the pressure is 
 less. Therefore, the air condensed by the forward motion 
 of the tuning-fork tends to spread in all directions. As 
 the condition of condensation travels outward, it spreads 
 over more and more space, and, therefore, the degree of 
 condensation becomes less as the distance from the fork 
 increases (Fig. 92). Since these condensations are pro- 
 duced by the vibrations of the molecules of the air, a 
 lessening of the amount of condensation implies a lessen- 
 ing of the amount of space traversed by each molecule 
 during its vibrations. Therefore, the intensity of the vi- 
 brations of the molecules of air decreases as the distance 
 from the sounding body increases. The mathematical 
 expression for this statement is that the intensity of vibra- 
 tion varies inversely as the square of the distance. 
 
 Anything which prevents the spreading of the conden- 
 sations after they leave the sounding body will prevent, 
 to a considerable extent, the loss in the intensity of vibra- 
 tion as the vibratory motion is transmitted from molecule 
 to molecule. This principle is made use of in speaking- 
 tubes. 
 
 In an indirect manner, of course, the intensity of the 
 sensation of sound depends not only upon the distance 
 from the sounding body but also upon the intensity of 
 vibration of the molecules actually in contact with the 
 sounding body, because, if these molecules do not vibrate 
 strongly, the sound will be weak even at its source, and 
 may be entirely inaudible at a short distance from the 
 sounding body. This may be expressed by stating that 
 the intensity of the vibrations of the molecules in contact with 
 
SOUND 225 
 
 the ear depends upon the intensity of vibration (amplitude of 
 vibration) of the sounding body. 
 
 If only a few molecules of air are set in motion by the 
 sounding 1 body, their energy is soon lost in causing" the 
 neighboring 1 molecules to vibrate. But if the number of 
 molecules originally set in motion is very great, their vi- 
 brations will be transmitted much farther before they be- 
 come too weak to affect the ear. Therefore, the intensity 
 of vibration of the molecules in contact with the ear de- 
 pends not only upon the distance of the ear from the 
 sounding body and the intensity of vibration of the 
 sounding body itself, but also upon the number of mole- 
 cules of air set in motion by direct contact with the sound- 
 ing body. If a tuning-fork is mounted upon a box not 
 only is the small quantity of air in contact with the fork 
 set in motion but also the much greater quantity in con- 
 tact with the box, and the sound will be audible at a much 
 greater distance. 
 
 These facts are often stated in the form of the following 
 rules : 
 
 Loudness of sound varies inversely as the square of the 
 distance. 
 
 Loudness varies with the amplitude of vibration of the 
 sounding body. 
 
 Loudness varies with the area of the sounding body. 
 
 207. The Vibrating Air May Communicate Vibrations to 
 Other Bodies, Place two large tuning-forks, mounted on 
 boxes and producing exactly the same tone, about 12 feet 
 apart. Direct the open ends of the boxes toward each 
 other. Strike one of the tuning-forks once or twice sharp- 
 ly with a leather mallet, so that for half a minute it gives 
 out a loud sound. Then grasp the prongs of this fork with 
 the hand quickly, so that the fork can no longer vibrate. 
 
226 ELEMENTARY PHYSICS 
 
 If the ear be held near the open end of the other box 
 (Fig. 94), it will be noticed that the other tuning-fork has 
 in some manner been set in vibration, although it has not 
 been struck by any visible material. 
 
 The first tuning-fork causes the air in contact with it 
 to vibrate. This vibratory motion is transmitted from 
 molecule to molecule. When the molecules surrounding 
 the second fork move forward, they push the prong very 
 slightly in the same direction, and when a moment later 
 the molecules move in the opposite direction, this prong 
 of the tuning-fork is also allowed to return. 
 
 A single forward and backward motion of the air sur- 
 
 :EIG. 94. 
 
 rounding the second fork does not make this fork vibrate 
 sufficiently to produce an audible sound ; but the accu- 
 mulative effect caused by a regular succession of a large 
 number of such vibratory motions will finally cause the 
 second fork to vibrate sufficiently to produce a sound loud 
 enough to be heard if the ear be placed near the fork. 
 
 It is highly essential that the two tuning-forks should 
 produce the same tone ; in other words, that they should 
 vibrate at exactly the same rate. Otherwise, when the 
 vibrating air is ready to return at the end of its first half- 
 vibration, the prong of the tuning-fork may still be mov- 
 ing forward, and, when the molecules of air are ready to 
 
SOUND 227 
 
 go forward the second time, the fork may not have com- 
 pleted its first vibration. In this case there is not suffi- 
 cient agreement between the vibrations of the air and the 
 vibrations of the second fork to enable the air to set this 
 fork in vibration effectively. 
 
 If a loud, clear note be sung in front of a piano with the 
 dampers lifted from the wire (by the use of the loud 
 pedal), the wire having the same pitch will for a short 
 time give out the same tone. 
 
 When a body is made to vibrate by vibrations pro- 
 duced in the air by some other body, it is said to vibrate 
 in sympathy with the other body, and the vibrations are 
 called sympathetic. 
 
 208. Variations in the Pitch of Strings. Any one who is 
 familiar with the appearance of the interior of the grand 
 piano knows that the wires of the piano vary in length 
 and in thickness. Moreover, while some strings consist 
 of a single wire, others are formed by wrapping a layer of 
 wire around a thicker central wire in order to increase the 
 weight. In a general way it may be stated that the short 
 and thin wires produce the higher tones, and the long, 
 thick or heavy wires produce the low tones. Anything 
 which increases the weight of the wire lowers the tone. 
 For this reason, the layers of wire are wrapped around 
 the wires intended for bass tones. The more tightly a 
 string is stretched, the higher is the tone which it pro- 
 duces. 
 
 209. The Number of Vibrations Varies Inversely as the 
 Length of the String. The number of vibrations which a 
 string will produce in one second depends upon the length 
 of the string. A string half as long as another will pro- 
 duce twice as many vibrations ; one a third as long will 
 produce three times as many vibrations ; one a fourth as 
 
228 
 
 ELEMENTARY PHYSICS 
 
 long will produce four times as many ; one three-fourths 
 as long will produce four-thirds as many vibrations. The 
 mathematical rule is that the number of vibrations varies 
 inversely as the length of the string, provided no other 
 change be made in the string. 
 
 210. A String Vibrates in Parts in Addition to Vibrating 
 as a Whole. A string may be caused to vibrate as if it 
 were a series of separately vibrating strings fastened end 
 to end. If the string be touched lightly at the middle, 
 and struck at the same time at a point J of the length of 
 
 FIG. 95. 
 
 the string frpm the end, it will vibrate in two sections of 
 equal length. If the string be touched at a point one- 
 third of its length from the end, and struck at a point one- 
 sixth of its length from either end, it will vibrate in three 
 sections of equal length. 
 
 Cause it to vibrate in four sections, by touching it at 
 one-fourth its length from the end. Little V-shaped 
 pieces of paper hung across the wire at the ends of the 
 sections will not be thrown off, but pieces of paper placed 
 at the middle of these sections will fly off (Fig. 95). This 
 indicates that the vibrations are strong at the middle of 
 
SOUND 229 
 
 the four sections, but nearly at rest at the ends of the sec- 
 tions. In other words, the string is vibrating like four 
 sections of equal length. 
 
 A string vibrating in 2, 3, or 4 sections vibrates like 
 several strings, each J, J, or J as long as the entire string. 
 Each section, therefore, now produces 2, 3, or 4 times as 
 many vibrations as the string vibrating as a whole pro- 
 duced. This change can be recognized by the increase in 
 height of the pitch, as the string is made to vibrate in 
 more numerous sections. A string, which, when vibrat- 
 ing as a whole, produces C on the bass staff, will produce 
 middle C on the piano, when it vibrates in 2 parts ; G 
 on the soprano staff, when it vibrates in 3 parts ; and up- 
 per C on the soprano staff, when it vibrates in 4 parts. 
 
 A string can be made to vibrate in parts at the same 
 time that it is vibrating as a whole. Stretch a string so 
 that it produces middle C when vibrating as a whole. 
 Strike it strongly at a point one-sixth of its length from 
 the end. You will hear not only middle C, but also high 
 G above the soprano staff, showing that the string is at 
 the same time vibrating as a whole and also in three sec- 
 tions. 
 
 211. Instruments Producing the Same Fundamental Tone 
 are Distinguished by Their Overtones All musical instru- 
 ments vibrate in parts, while at the same time vibrating 
 as a whole. The higher tones due to the vibration of 
 the instrument in parts are called overtones. The tone 
 produced by the vibration of the instrument as a whole is 
 called the fundamental tone. Many instruments produce 
 at the same time nine or more overtones of different pitch. 
 Most of these overtones produce pleasing combinations 
 with the fundamental tone and increase the musical 
 quality of the sound produced by the instrument. 
 
230 ELEMENTARY PHYSICS 
 
 While all musical instruments produce overtones as well 
 as fundamentals, all do not produce the same overtones, 
 or at least do not produce the same overtones with the 
 same degree of loudness. Hence, the musical effects re- 
 sulting from the combination of fundamentals and over- 
 tones are different in different instruments. This makes 
 it possible to distinguish different kinds of instruments, 
 even when they are producing the same fundamental tone. 
 
 212. Many Vibrations may be Transmitted Through the 
 Air at the Same Time One might suppose that it would 
 be impossible for several sets of vibrations to pass 
 through the air at the same time without becoming hope- 
 lessly confused. However, that this is not true is shown 
 by every-day experience. The vibrations sent out by the 
 various instruments of a large orchestra all travel at the 
 same time through the air. Yet they can all be heard, 
 and a skilful conductor can at once pick out the instru- 
 ment which makes the slightest mistake. In the same 
 manner, a skilful director can distinguish the individual 
 voices of a chorus. 
 
 213. The Velocity of Sound. The velocity of sound in 
 air at the ordinary temperature (72 F.) is about 1130 feet 
 per second. This velocity increases and decreases at the 
 rate of 1 foot per second for every increase or decrease of 
 1 degree in the temperature of the air. The velocity in 
 water is 4} times as great ; in pine wood, 10 times ; and 
 in iron, more than 15 times as great as in air. Hence the 
 sound of a blow on the railroad track a quarter of a mile 
 away may be heard coming through the rail more than a 
 second before it can be heard through the air. 
 
 214. The Structure of the Ear. For the purpose of de- 
 scription, the passage between the opening into the ear 
 and the auditory nerves may be divided into three parts. 
 
SOUND 
 
 231 
 
 Ear drum 
 
 FIG. 
 
 The first part of this passage is a tube called the external 
 
 auditory canal. It is about one and a quarter inches in 
 
 length. For a distance of about 
 
 half an inch from the opening 
 
 into the ear, the canal wall is 
 
 cartilaginous and, therefore, 
 
 movable ; for the rest of its 
 
 extent, about three-fourths of 
 
 an inch, it consists of bone 
 
 covered by a thin skin. 
 
 The second part of this pas- 
 sage is called the tympanum or 
 middle ear. It is an air-holding cavity of irregular shape 
 hollowed out in the solid bone of the head. It is about 
 one-third of an inch long, one-fourth of an inch high, and 
 one-sixth of an inch wide. 
 
 Between the external and middle ear is stretched the 
 membrane known as the ear drum (Fig. 96). It is nearly 
 circular and has a diameter of nearly three-eighths of an 
 inch. Since the ear drum is stretched over the cavity of 
 the middle ear, this cavity is called 
 the tympanum (Latin for drum). 
 
 The third part is called the laby- 
 rinth or internal ear. It is a liquid- 
 holding cavity of irregular shape 
 hollowed out of the solid bone (Fig. 
 98). The lower half, occupying the 
 portion nearer the inner part of the 
 head, has the form of the interior 
 
 cavity of a snail-shell, and, on this account, is called the 
 cochlea, from the Latin word for shell (Figs. 98, 99) ; it 
 consists of two and one-half spiral turns. The upper and 
 more exterior half consists of three tubes, called semicir- 
 
 Anvil 
 
 Hammer 
 
 % ~ Stirrup 
 
 Bones of the Ear 
 FIG. 97. 
 
232 
 
 ELEMENTARY PHYSICS 
 
 Vestibule . 
 
 Round 
 Vfi 
 
 Bony labyrinth 
 FIG. 98. 
 
 cular canals, each tube bent in the form of a semicircle 
 and placed at right angles to the others. The open ends 
 
 of the semicircular tubes and 
 of the snail-like tube all open 
 into the central portion of 
 the labyrinth. 
 
 The two small entrances 
 from the middle ear to the 
 inner ear are called the oval 
 window and the round win- 
 dow, and are also each cov- 
 ered by a thin sheet of mem- 
 brane. Since both entrances 
 to the internal ear pierce the walls of the central portion, 
 this part is called the vestibule of the inner ear. The 
 inner ear itself is called the labyrinth, because of the 
 bewildering number of passages it contains. 
 
 In the middle ear, between the ear drum and the 
 membrane covering the oval window, is a series of 
 three irregular bones (Figs. 96, 97). To the first bone 
 attached to the ear drum has been given the fanciful 
 name of the hammer, and, since its thickened rounded 
 head rests against the 
 second bone, this bone is 
 called the anvil. The tip 
 of the second bone is at- 
 tached to the end of a third 
 bone which looks exactly 
 like a minute stirrup. The 
 flat end of the stirrup is 
 fastened to the membrane 
 stretched over the oval window opening into the ves- 
 tibule of the inner ear. Any motion of the ear drum 
 
 Hound window 
 with its membrane 
 
 Section of left cochlea 
 FIG. 99. 
 
SOUND 
 
 233 
 
 FIG. 100. 
 
 is at once transmitted by these bones to the oval mem- 
 brane. 
 
 Within the snail-like part of the inner ear, the cochlea, 
 is stretched the basilar membrane (Fig-. 100). This mem- 
 brane consists of 
 16,000 to 24,000 mi- 
 nute fibres, which are 
 stretched transverse- 
 ly across the length 
 of the passage in the 
 cochlea. These fibres 
 are looked upon as a 
 sheet of parallel 
 string's, similar to 
 the series of wires in 
 a piano. The longer fibres are found nearer the smaller 
 end of the spiral tube, where, at first thought, they would 
 scarcely be expected. These fibres are weighted with the 
 various cells of Corti, to which are connected the auditory 
 nerves. 
 
 A. large part of the structure of the ear has purposely 
 not been described. A fuller knowledge may be secured 
 from some large work on physiology (American Text 
 Book of Physiology, published by W. B. Sanders, Phila- 
 delphia.) 
 
 215. The Communication of Sound to the Nerves of the 
 Ear. When the molecules of air set in motion by the 
 vibration of some other body strike against the drum of 
 the ear, the ear drum is made to vibrate at the same rate 
 as the molecules of air. The vibrations of the ear drum 
 are communicated by the series of bones in the middle 
 ear to the membrane closing the oval window which leads 
 to the inner ear. The membrane transmits the vibra- 
 
 
234 ELEMENTARY PHYSICS 
 
 tions to the liquid filling the labyrinth. Among other 
 places, the vibrations reach the liquid filling the snail- 
 like cavity, or cochlea. Here the molecules of the vibrat- 
 ing liquid are in contact with the fibres of the basilar 
 membrane. Those fibres of the membrane which vibrate 
 naturally at the same rate at which the molecules of the 
 liquid in the cochlea are now vibrating, take up the 
 vibrations of the liquid. These vibrations are communi- 
 cated then to the cells of Corti, which weight the fibres, 
 and these finally produce a sensation in the auditory 
 nerves connected with the cells. The perception of this 
 sensation by the brain is called hearing. 
 
 Not only is the vibratory motion communicated from 
 body to body, but the same rate of vibration is taken up 
 in succession by the air, ear drum, bones in the middle 
 ear, membrane covering the oval window, fluid in the 
 cochlea, and, as far as we know, also by one or several 
 of the fibres of the basilar membrane. 
 
 The taking up of the vibrations by the basilar mem- 
 brane is another case of sympathetic vibration. It will 
 be remembered that, when a tone is sung loudly in front 
 of the piano, that wire of the piano which possesses the 
 same pitch begins to vibrate also. 
 
CHAPTER YI 
 BADTANT ENERGY HEAT AND LIGHT 
 
 216. Some Vibrations may Produce both the Sensation of 
 Heat and of Light. The sensations of heat and light 
 are entirely different in character and they are felt by 
 entirely different nerves. The nerves of temperature 
 terminate in the skin and extend to almost every part of 
 the surface of the body ( 130). The nerves of sight, how- 
 ever, terminate at only one spot, the eye. It therefore 
 might reasonably be supposed that the sensations of heat 
 and light are due to very different phenomena, just as 
 the sensations of sound and smell are unquestionably due 
 to entirely different causes. The following experiment, 
 however, suggests that the sensations of heat and light 
 may be due to exactly the same phenomena. 
 
 Place a piece of iron in a very hot flame. At first while 
 it rises in temperature, it gives out merely more and 
 more heat, and this heat may be felt by the nerves of 
 temperature. However, after a certain temperature has 
 been reached, the iron not only gives out heat, but also 
 emits light, not a bright light but at least a light that 
 can be seen a dull red. This suggests that if the in- 
 crease in temperature of the iron is due to an increase in 
 the rate and intensity of vibration of the molecules of the 
 iron ( 120), that light may be due to more rapid vibrations 
 and that slower vibrations are not able to produce the 
 sensation of light. If this idea is correct, the general 
 
 235 
 
236 ELEMENTAEY PHYSICS 
 
 statement can be made, that a great variety of vibrations 
 can produce a sensation in the nerves of temperature, 
 but only the more rapid vibrations can produce any sen- 
 sation in the nerves of sight. If red is the first color seen 
 when a body is raised in temperature, then all the other 
 colors should also be produced by the more rapid vibra- 
 tions. This has been shown to be true, but not by 
 means of the simple experiment here described. 
 
 If the temperature of the iron be increased above that 
 necessary to produce the dull red color, the iron will give 
 out a brighter red, then a reddish-orange, later a yellow, 
 and finally a white color. In following paragraphs it will 
 be shown that many other colors are given out, but that 
 these are so mingled together that until we learn how to 
 separate them, we are not able to recognize their presence 
 (. 231). For the present it will be sufficient to recognize 
 the fact that vibrations of many degrees of rapidity may 
 produce the sensation of heat, but that only the more 
 rapid vibrations can produce the sensation of light. 
 
 217. The Transmission of Heat by Contact of Molecule 
 with Molecule. Heat may pass from one part of a body 
 to another, by the communication of heat from molecule 
 to molecule. When the molecules do not change their 
 relative positions, as in the case of solids, this method of 
 transference of heat is called conduction ( 121). When 
 the molecules change their relative positions, as in the 
 case of liquids and gases, it is called convection ( 125, 
 127). In both cases heated molecules come in contact 
 with others not yet heated, and communicate to them a 
 part of their own heat. 
 
 When water is boiled in a kettle, the hot molecules in the 
 flame communicate a part of their heat to the molecules 
 of iron forming the exterior of the kettle. This heat is 
 
RADIANT ENERGY HEAT AND LIGHT 237 
 
 communicated from molecule to molecule until it reaches 
 the interior of the kettle. Here it is transmitted to the 
 molecules of water in contact with the kettle, and, as these 
 move away, other molecules of water may take their 
 places and also become heated. 
 
 218. The Air does not Transmit the Heat or Light from 
 the Sun to the Earth. Heat may also be transmitted from 
 molecule to molecule without direct contact, and without 
 any transmission by means of an intermediate set of mole- 
 cules. During a hot cloudless summer day the tempera- 
 ture at the surface of the earth rises frequently to 100 F. 
 However, as one ascends to regions above the earth, a 
 steady lowering of the temperature is observable. Near 
 the surface of the earth the temperature decreases at the 
 average rate of 1 F. for every increase in elevation of 
 about 300 feet. At an elevation of several miles above the 
 earth the rate of decrease of temperature is considerably 
 less. This is shown by the fact that Glaisher found a 
 total decrease in temperature of only 61 F. when he as- 
 cended in a balloon to a height of 5 miles. Nevertheless 
 we have every reason to believe that at a distance of 100 
 miles above the surface of the earth, the temperature 
 must be nearly 460 below zero, Fahrenheit. Since the 
 temperature of the air at the surface of the earth is higher 
 than the temperature of the air nearer the sun, it is evi- 
 dent that the heat and light of the sun reach the earth 
 without being conducted to the earth by the air. 
 
 219. No Air is Present in the Greater Part of the Space 
 Between the Sun and the Earth. The density of the air 
 diminishes as the elevation above the surface of the earth 
 increases. At an elevation of If miles the air is not f as 
 dense as at sea. level, at 3| miles it is less than half as 
 dense, at 6 miles it is less than J as dense ( 17). It is 
 
238 ELEMENTARY PHYSICS 
 
 evident that 100 miles above the surface of the earth the 
 air must be exceedingly rare. 
 
 How do we know that air of any degree of density is 
 present 100 miles above the earth ? Tiny particles of rock 
 flying through space frequently come in contact with the 
 atmosphere of the earth. Since the speed with which 
 the earth moves is about 18.5 miles per second, and since 
 the particles of rock are also moving rapidly, the total 
 speed with which the particles of rock and the earth ap- 
 proach each other may amount to as much as 45 miles per 
 second. When the particles of rock strike the molecules 
 of the air, the heat produced, as a result of the concus- 
 sions, is so great that both air and rock become white hot 
 and are then visible in the form of shooting stars. When 
 larger masses of rock strike the air, they not only give out 
 a much more brilliant light, but occasionally portions of 
 the rock reach the surface of the earth, and are then 
 called meteorites. When two observers in neighboring 
 towns notice the same meteor and record the angle at 
 which it appears above the horizon, it is possible to deter- 
 mine the elevation of the meteor, as soon as the distance 
 between the two towns has been measured. Observations 
 of a similar kind indicate that the tiny particles of rock 
 which become shooting stars occasionally become visible 
 at elevations between 70 and 100 miles above the surface 
 of the earth. Therefore at 100 miles above the surface 
 of the earth some air is still present. 
 
 Since no shooting stars ever appear at a greater eleva- 
 tion, it is evident that at still higher elevations the quan- 
 tity of air continues to decrease until finally air is prac- 
 tically absent. By far the greater part of the space be- 
 tween the earth and the sun cannot contain air. But 
 if air is present for only a short distance between the 
 
RADIANT ENERGY HEAT AND LIGHT 239 
 
 sun and the earth, then, along most, if not all, of the dis- 
 tance between the sun and the earth, the heat and light 
 of the sun must be transmitted by means of some other 
 medium than air. 
 
 220. Heat and Light may be Transmitted Through a 
 Vacuum. The possibility of the transmission of heat and 
 light through a vacuum may be demonstrated experimen- 
 tally. 
 
 Enclose the blackened bulb of a thermometer within a 
 glass globe. By means of a mercury-pump withdraw the 
 air which is in the globe, and melt shut the opening 
 through which the air was pumped out (Fig. 101). By 
 
 FIG. 101. 
 
 means of this type of pump it is possible to get an almost 
 perfect vacuum. Less than a millionth of the air is left 
 behind. 
 
 If a red-hot poker be held near the globe, the mercury 
 in the thermometer instantly rises. Since the interior of 
 the globe is practically a vacuum, air cannot have served 
 as a medium for the transmission of heat, at least in the 
 space between the walls of the globe and the bulb of the 
 thermometer. The part of the thermometer enclosed in 
 the globe can be seen as readily through the vacuum as 
 before the removal of the air. Therefore, light can also 
 pass through a vacuum. 
 
240 ELEMENTARY PHYSICS 
 
 221. In Order to Explain the Transmission of Heat and 
 Light Through Space in which no Material Substance Is 
 Known to Be Present, the Existence of Ether Is Assumed, 
 It is impossible to conceive of the transmission of heat 
 from the sun to the earth without the existence of some 
 intervening medium. For this reason scientists have 
 proposed the theory that all space which is apparently 
 empty is in reality filled with something which they call 
 the ether, and they have suggested that in all cases in 
 which heat is not transferred by some known medium, in 
 other words, by contact of molecule with molecule, the 
 heat is transferred in some manner by the ether. Know- 
 ing what phenomena of heat and light it was necessary to 
 explain by means of the ether, scientists then proceeded 
 to determine what must be the properties of the ether, in 
 order that it might show these phenomena. 
 
 They soon came to the conclusion that the ether must 
 be colorless, inconceivably light in weight, more elastic 
 than any substance actually known to exist, and so all- 
 pervading that it not only occupies all space ordinarily 
 considered as a vacuum, but even fills the space between 
 the atoms of ordinary matter. It is therefore believed to 
 be vastly more continuous than anything actually seen on 
 earth ; for the molecules of even the densest substances 
 are known to be not in actual contact with each other, 
 since every decrease in temperature at once brings them 
 closer together. This ether can be set in vibration by the 
 vibrations of the atoms imbedded within the ether. It 
 can transmit these vibrations to more distant parts of the 
 ether, and it can communicate this vibratory motion to 
 other atoms at a distance from those atoms which origi- 
 nally set the ether in motion. 
 
 This theory at first rested chiefly upon imagination, 
 
RADIANT ENERGY, HEAT AND LIGHT 241 
 
 and was merely an attempt to explain many phenomena 
 otherwise inexplainable. However, within recent years 
 this theory has been found capable of explaining so many 
 phenomena in heat, light, and electricity, that the exist- 
 ence of ether may be considered as proved. Some scien- 
 tists have even attempted to determine approximately its 
 density and elasticity (Principles of Physics, Daniell. 
 Macmillan & Co., New York). 
 
 A liquid known as ether is sold in drug stores. The 
 ether which transmits light has no connection with this 
 liquid. The ether known in theory is entirely invisible, 
 and, if it were not for the phenomena of heat, light, and 
 electricity, its existence would never have been suspected. 
 
 222. Radiation, When a pebble falls into a pool of 
 water, it starts a series of waves. The waves spread out 
 in every direction and set in motion every light object 
 floating upon the water. 
 
 According to the ether theory vibrating atoms start 
 waves in the ether surrounding each atom. These waves 
 are transmitted by the ether in every direction and strike 
 other atoms imbedded in other parts of the ether. In con- 
 sequence these more distant atoms also begin to vibrate. 
 
 The ether theory may be used to explain the transmis- 
 sion of heat and light from the sun to the earth without 
 the use of air. The atoms forming the sun vibrate at an 
 exceedingly rapid rate. This produces a vibratory motion 
 in that part of the ether surrounding each atom and gives 
 rise to waves traversing the ether. These waves pass 
 through the enormous spaces devoid of air but filled with 
 ether. On reaching the atmosphere surrounding the 
 earth, the wave motion of the ether gives rise to vibratory 
 motion in the atoms forming the air. At the surface of 
 tbe earth they cause the atoms present in the rock-forming 
 
242 ELEMENTARY PHYSICS 
 
 compounds to vibrate, since these also are imbedded in 
 the ether. In fact, the atoms of all substances at the sur- 
 face of the earth are more or less affected, since ether 
 pervades all space, even the space between the atoms of 
 the densest substances. 
 
 The vibratory motion of the atoms gives rise to the 
 phenomena known as heat. Some atoms take up the 
 vibratory motion more readily than others, and therefore 
 become hotter. On this account rocks become hotter 
 than air. When the rate and intensity of the vibrations 
 communicated to the atoms are sufficient, the vibratory 
 motion of the atoms gives rise to both heat and light. In 
 proportion as the ether sets the atoms imbedded within it 
 into more violent vibration, the intensity of its own mo- 
 tion decreases. Hence the heating effects of the sun do 
 not penetrate very far beyond the surface of objects struck 
 by its rays. 
 
 The transmission of vibrations capable of producing 
 the sensation of heat, or of both heat and light, is called 
 radiation. 
 
 223. Ether may Serve to Transmit Heat and Light even 
 when Air is Present. If, on a cold winter day before 
 the room has become warm, we stand before a large fire 
 in an open fireplace, the heat striking the body may be 
 so great that it even scorches us ; but if a large screen 
 be placed between us and the fire, we will at once feel 
 cold. Yet we know by experience that air does not be- 
 come cold instantly when the source of heat is removed. 
 Is the air between us and the fire actually warm ? 
 
 If we place a delicate thermometer behind a piece of 
 wood which will serve as a screen for the thermometer, 
 and then hold it thus protected between us and the fire, 
 we will learn that the temperature of the air is still low. 
 
RADIANT ENERGY KEAT AND LIGHT 243 
 
 However, the instant the thermometer is removed from 
 behind the piece of wood and allowed to face toward the 
 fire, the temperature rises considerably. Those parts of 
 our bodies which face the fire also feel much more heat 
 than can be accounted for by the actual temperature of the 
 intervening air, as indicated by the thermometer. The 
 vibrations of the heated atoms in the fire are transmitted 
 by means of the ether which is between the atoms form- 
 ing the air. As soon as the waves set up in the ether in- 
 crease sufficiently the rate of vibration of the atoms of 
 mercur\ r , the thermometer indicates a rise in temperature. 
 Waves of a still higher rate of vibration may produce both 
 the sensation of heat and of light. 
 
 224. Refraction of Light Passing Obliquely from Water 
 to Air, The waves set up in the ether spread in every 
 direction from the centre of disturbance. Each wave 
 may be looked upon as a spherical shell enlarging with 
 enormous rapidity. When waves strike obstructions 
 only the unobstructed parts of waves may continue their 
 progress. Hence light passes through rifts in clouds in 
 long sheets or beams. Through smaller apertures the wave 
 continues its course in the form of rays. Since every 
 part of a wave travels in a radiate manner directly away 
 from the centre of disturbance, that part of the wave which 
 continues its path through a small aperture is straight, 
 at least as long as the substance through which it passes 
 has the same density. Hence rays of light are straight. 
 
 However, when a ray travels through substances hav- 
 ing different degrees of density, the path followed by the 
 ray changes at each point where a new substance is en- 
 tered. The bending of a ray of light on passing from one 
 substance into another can be shown easily by the fol- 
 lowing experiment. 
 
244 
 
 ELEMENTAKY PHYSICS 
 
 Place a coin in the centre of a pan set on a low table. 
 The sides of the pan should be nearly perpendicular, and 
 almost 4 inches high. Take a position at such a distance 
 that, when you glance across the edge of the pan, only 
 the farther margin of the coin is visible (Fig. 102). The 
 light from the coin now passes in a straight line through 
 the air past the edge of the pan to the eye. Now let 
 
 FIG. 102. 
 
 some one fill the pan with water up to the brim, and then 
 lower the head until the light from the farther margin 
 of the coin is again seen just above the edge of the pan 
 (Fig. 103). The path of the light is now partly in water 
 and partly in air. That part of the path which extends 
 from the farther margin of the coin to the edge of the 
 pan passes in a straight line through the water. The re- 
 
RADIANT ENERGY HEAT AND LIGHT 245 
 
 maiuder of the path, from the edge of the pan to the eye, 
 passes in a straight direction through the air. But the 
 eye is no longer in its former position. The path of 
 light has been evidently bent downward at the surface 
 of the water near the margin of the pan. This bending 
 of the light while entering another substance at an ob- 
 lique angle is called refraction. 
 
 If the coin be watched while the basin is filling with 
 water, it will be noticed that the coin comes into view 
 and seems to move upward and toward the centre of the 
 
 FIG. 103. 
 
 pan as the water increases in depth. This effect is pro- 
 duced by the bending of the rays of light as they pass 
 from the surface of the water out into the air. By further 
 study it may be determined which rays are bent, what 
 was their former course, and why the coin seems to move. 
 The only purpose of the preceding experiment, however, 
 is to bring out the fact that light is bent downward on 
 passing obliquely from water into air. 
 
 225. Refraction of Light which Passes Obliquely from Air 
 into Glass, and from Glass into Air. Secure from a dealer 
 in physical apparatus a rectangular piece of plate-glass, 
 
246 
 
 ELEMENTARY PHYSICS 
 
 5 inches long and 3 inches wide, whose long-, narrow 
 sides have been ground so as to be flat and parallel, 
 and then polished so as to be almost perfectly trans- 
 parent. Lay the glass on the table, several inches from 
 the margin. Stick a pin into the table, several inches 
 behind the plate of glass, and then lowering the head to 
 the level of the table, look at the pin through the entire 
 width of the glass. Move the head toward one side 
 until the lower part of the pin as seen through the glass 
 
 FIG. 104. 
 
 is very far out of line with the upper part of the pin as 
 seen through the air, above the glass. Then, without 
 moving either the plate or the eye, stick a second pin 
 into the table, in contact with the more distant of the 
 polished faces of the glass, so as partly to hide the lower 
 part of the first pin looked at through the glass (Fig. 104). 
 This will mark the point where the light from the first 
 pin enters the glass. Stick a third pin into the table, in 
 contact with the nearer polished face, so as to hide the 
 
RADIANT ENERGY HEAT AND LIGHT 247 
 
 lower part of the second pin. This will indicate where 
 the light from the first pin would leave the glass and 
 re-enter the air if not obstructed by the second pin. 
 Finally, stick a fourth pin in the table, several inches in 
 front of the plate, so as to hide the third pin, which is of 
 course in plain p-iew. This will indicate in what direc- 
 tion the light from the first pin would pass on leaving 
 the glass if not obstructed by the other pins. 
 
 If the position of the head has not been changed, all 
 four pins appear to be in a straight line while the ob- 
 server is looking through the entire width of the glass. 
 However, if the head is lifted and the pins are connected 
 
 in their order by straight lines, it will be found that the 
 light from the first pin bends both on entering the glass 
 and on re-entering the air, but the bending in the two 
 cases is in opposite directions. 
 
 226, A Small Portion of a Spherical Wave is Practically 
 Flat. The cause of the bending of the rays of light in 
 passing from one substance into another may be explained 
 in the following manner : 
 
 The vibrations of atoms at the source of heat and light 
 produce spherical waves which spread through the ether 
 in all directions from the centre of disturbance outward. 
 In their spherical form they resemble the waves of sound 
 
248 ELEMENTARY PHYSICS 
 
 ( 203). At a considerable distance from the original 
 cause of disturbance the curvature of any small portion 
 of the front of a wave is very small (Fig-. 105). There- 
 fore, any small part of the wave front may be treated, in 
 a theoretical discussion, as if it were flat. 
 
 It may be interesting 1 in this connection to notice that 
 many people still believe that the surface of the earth is 
 flat, because the small portions of its surface which they 
 can see at any one time often appear flat. 
 
 227. Refraction of Waves Due to Variation in Amount of 
 Speed of Waves while they are Passing through Different 
 Substances. If a thick flat piece of glass is held in front 
 of a wave advancing through the ether in the air, a por- 
 tion of the wave will enter the glass. If the glass be held 
 obliquely, that end of the glass which is nearer to the 
 wave front will be entered first. 
 
 It has been found that light does not travel as fast 
 through glass as it does through air. Consequently the 
 
 velocity of that part of 
 the wave which enters 
 
 the glass first will be re- 
 tarded (Fig. 106), while 
 the remainder of the 
 wave continues to travel 
 with its original veloc- 
 ity, until it also enters 
 
 Air the glass. This permits 
 
 the lagging end of the 
 FJG IQ6 wave to catch up partly 
 
 with that end which en- 
 ters the glass first. In consequence the wave front swings 
 around so that it is more nearly parallel to the surface of 
 the glass, after it has entered the glass, than it was before. 
 
RADIANT ENERGY HEAT AND LIGHT 249 
 
 As soon as the entire wave front lias entered the glass 
 there is no further change in direction, since all parts of 
 the wave, while in the glass, travel with the same veloc- 
 ity. Since each part of a wave always moves in the di- 
 rection in which the wave front faces, it is evident that 
 the wave, on entering the glass, will take up a path 
 which is different in direction from its path in air. 
 
 The mathematical statement of this change of direction 
 is that light bends toward the perpendicular in passing 
 from air into glass. The perpendicular is supposed to be 
 erected at the point where the light enters the glass and 
 is supposed to be perpendicular to the surface of con- 
 tact between the air and the glass. 
 
 Any substance which retards the speed of the wave 
 more than glass will cause a greater change in the direc- 
 tion of the wave front, and, therefore, in the path taken 
 by this portion of the wave. 
 
 When a wave passes obliquely from glass into air, the 
 part of the wave which enters the air first moves faster 
 than the part of the wave still travelling through glass. 
 The result is that by the time the entire wave has entered 
 the air its front makes a larger angle with the surface of 
 the glass than it did while it was completely within the 
 glass. 
 
 The mathematical statement of this change of direction 
 is that light bends from the perpendicular on passing from 
 glass into air. 
 
 The direction in which light travels is similarly changed 
 on passing from any substance by which it is more re- 
 tarded, into another substance by which it is less retarded, 
 the amount of change in direction depending upon dif- 
 ferences in the amount of retardation offered by the two 
 substances concerned. 
 
250 ELEMENTARY PHYSICS 
 
 It is necessary to remember that when light is said to 
 pass through air, water, or glass, the waves are in reality 
 passing through the same substance, ether. In some 
 manner, the atoms imbedded within the ether retard the 
 speed of the waves passing through it, each kind of sub- 
 stance retarding the speed of these waves to a different 
 degree. 
 
 228. The Refraction of Light Passing Through a Prism. 
 When light passes from air into glass, it bends toward 
 the perpendicular. When light passes from glass into 
 air, it bends from the perpendicular. If a piece of glass 
 is so cut that the surface by means of which the light 
 leaves the glass has a different direction from that at 
 which the light enters ( 225), the light after leaving the 
 glass will have a different direction from that at which it 
 entered. By placing the second surface at the proper 
 angle, it is possible to make this surface bend tke light 
 still farther from its original course than it was bent by 
 the first surface. 
 
 Secure a large glass prism with flat ends, so that, when 
 the prism is set up on one of these ends, the prismatic 
 faces will be vertical. A triangular piece of plate glass 
 with polished edges or a bisulphide of carbon prism will 
 do just as well. Stick a pin into the table several inches 
 behind the prism and look at it through the sides of the 
 prism. Slowly turn the prism until the apparent position 
 of the pin, as seen through the sides of the prism, is as 
 far removed as possible from the actual position. Then, 
 without moving the head or the prism, place in contact 
 with the more distant face of the prism a second pin, 
 locating the point at which the light coming from the 
 pin to the eye enters the glass (Fig. 107). In contact with 
 the nearer face place a third pin marking the point at 
 
RADIANT ENERGY HEAT AND LIGHT 251 
 
 which the light leaves the glass. Between the eye and 
 the prism place a fourth pin, hiding the third pin, to 
 determine in what direction the light travels after leaving 
 the third pin. While looking through the glass the four 
 pins appear to be standing in a straight line. 
 
 Kemove the prism and connect the pins, in order, with 
 straight lines. It will be found that the path of the 
 light has twice changed its course, once on entering the 
 glass, and again on re-entering the air. Both changes 
 
 J 
 
 FIG. 107. 
 
 result in turning the light toward the same side of the 
 original path. Mark the positions of all pins, and of 
 the two faces of the prism through which the light 
 passed on its way from the pin to the eye. 
 
 229. The Refraction of Yellow Light Send a ray of yel- 
 low light through the same prism in such a manner that 
 it may follow as nearly as possible the path of light re- 
 corded in the preceding experiment, and locate the di- 
 rections followed by the yellow light after it has passed 
 
252 
 
 ELEMENTARY PHYSICS 
 
 through the prism. In order to do this with accuracy, 
 the following apparatus will be found convenient. 
 
 A source of yellow light. When the access of air to the 
 lower part of a Bunsen burner is not cut off, it burns with 
 a faint, bluish flame which is scarcely visible. However, 
 if sodium carbonate be placed in the flame, it gives off an 
 intense yellow light. The most convenient manner of 
 getting sodium carbonate into the flame is to dip one 
 end of a platinum wire in hydrochloric acid; then dip 
 
 PIG. 108. 
 
 this end into powdered sodium carbonate, and hold it 
 in the flame cf the burner (Fig. 108). The wire is sup- 
 plied with a glass handle ( 36). 
 
 A screen with a narrow slit to permit the passage of 
 only a narrow beam of light. This screen can be easily 
 made by boring a hole an inch or more in diameter through 
 a thin board about 8 inches wide and 12 inches long and 
 tacking two straight-edged strips of tin over this hole so 
 as to form a vertical slit about half as wide as the thick- 
 
RADIANT ENERGY HEAT AND LIGHT 253 
 
 ness of a pin. The hole should be bored in such a posi- 
 tion that, when the board is set on end on a table and a 
 Bunsen burner is placed by the side of it, the hole will 
 be on a level with the flame of the burner. On each of 
 the lower corners of the board nail a small strip of wood 
 to serve as a support to keep the screen in a vertical 
 position. 
 
 Set up a carbon bisulphide prism at a distance of about 
 6 inches from the slit of the screen, and place the Bunsen 
 burner on the other side of the screen at a distance of 
 about 10 inches from the slit. The prism, slit, and lu- 
 minous part of the flame should have the same elevation 
 above the table, so that when in position they lie in the 
 same horizontal plane. On looking at the slit through 
 the prism we find that the yellow light, resulting from 
 the use of sodium carbonate, comes through the slit and 
 is bent from its straight path by the prism in a manner 
 similar to that discussed in connection with the preced- 
 ing experiment. 
 
 230. The Refraction of Red Light. Use lithium car- 
 bonate instead of sodium carbonate. The flame is col- 
 ored strong red. The red light is also refracted by the 
 prism, but it is refracted less than the yellow light. If 
 both sodium carbonate and lithium carbonate are placed 
 in the flame at the same time, the flame is colored with a 
 mixture of yellow and red light. The prism, however, 
 will separate this mixture into its elements, and the yel- 
 low light and the red light are seen through the prism as 
 two distinct narrow vertical lines (Fig. 108). 
 
 If a prism can separate all compound colors in this 
 manner, it is possible to use a slit and prism in order to 
 determine what simple colors are present in any com- 
 pound color. 
 
254 
 
 ELEMENTARY PHYSICS 
 
 231. White is a Compound Color. The light of the sun 
 is white. This fact may be recognized readily when sun- 
 light falls upon a piece of paper on the floor. 
 
 Without shifting either the screen or the prism, remove 
 the Bunsen burner and, by the use of one or more mirrors, 
 reflect a little sunlight along the path formerly followed 
 by the yellow and red lights in their course through the 
 slit toward the prism ; as a result, a broad band of light 
 formed by a regular series of colors with red at one end 
 and violet at the other is seen through the prism. The 
 
 Ultra-red rays 
 (luyisible) 
 
 Red Or. Yell. Green Blue .Indigo 
 ; (Visible Rays) 
 
 Violet Ultra-violet rays 
 > (invisible; 
 
 FIG. 109. 
 
 order of occurrence of these colors is : red, orange, yel- 
 low, green, blue, indigo, and violet (Fig. 109). 
 
 If these colors be examined more closely, it will be 
 found that each so called color in reality consists of 
 many tints of color, so that there are many tints of red, 
 many tints of green and many tints of the other colors. 
 These tints grade into each other imperceptibly, so that 
 it is impossible to find any natural line of separation be- 
 tween the red and orange, orange and yellow, yellow and 
 green, or any other neighboring colors of this band. 
 Such lines ot separation are often drawn for the sake of 
 
RADIANT ENERGY HEAT AND LIGHT 255 
 
 convenience in discussing the different tints produced, 
 but their location is arbitrary. 
 
 It is, therefore, evident that the white light of the sun 
 consists of a great many colors, or at least of a great 
 many tints. The number of tints is so great that, even 
 when they have been separated by means of a prism, 
 they form a practically continuous band of color. While 
 it is convenient to speak of this band as consisting of 
 seven colors, it should be remembered that, in reality, it 
 consists of thousands of tints. The prism has separated 
 these colors in such a manner that they can be studied 
 separately. 
 
 White light is a compound color. It contains a greater 
 number of simple colors than any other compound color 
 known. In fact, it contains all of the simple colors. For 
 that reason, the colors present in white light are used as 
 the standard with which the simple colors found in all 
 other compound colors are compared. Among the colors 
 present in white light may be located the particular tint 
 of yellow produced by sodium carbonate, and the partic- 
 ular tint of red produced by lithium carbonate. Since 
 the simple colors present in white light can be recognized 
 only when separated by means of a prism, they are called 
 the prismatic colors, and the band of colors formed is 
 called the spectrum of white light. When the sun is the 
 source of the white light, the spectrum is commonly 
 spoken of as the solar spectrum. 
 
 232. The Growth of the Spectrum During Increase of 
 Temperature on Heating a Platinum Ball, Place a Bun- 
 sen burner in such a position that when a platinum ball, 
 held in the flame, becomes hot enough to emit light, the 
 light will pass, through the slit and the prism, toward 
 the eye, as in the preceding experiments. The first 
 
 \ 8 R A 
 THE 
 
 I / M I ./ me 
 
256 ELEMENTARY PHYSICS 
 
 light which the ball gives out on being- heated has a 
 dull red color. In tint it corresponds to the dull red at 
 the extreme limit of one end of the solar spectrum. 
 Looked at through the prism this dull red color is seen 
 as a vertical line. As the heating of the ball continues, 
 its color not only becomes more intensely red but it also 
 changes in tint. If during this change of color the light 
 coming from the ball is examined through the prism, it 
 is noticed that new tints of red are added to those 
 already formed so that the original vertical line of dull 
 red appears to broaden out into a band of red. Since 
 the growth of the band takes place in one direction only, 
 the dull red color first seen always forms one end of the 
 band, while the new tints of red are added in succession 
 to the other end. 
 
 The addition of new tints of red is only one of the 
 changes to be noticed on heating the ball. If the tints 
 of red successively added to those already present be ex- 
 amined closely, it will be seen that they become brighter, 
 or more intense. The examination of the ball through 
 the prism, therefore, shows that as the ball grows hotter, 
 new tints of red are added and that those already pres- 
 ent become more intense. 
 
 The brighter red color of the platinum ball as seen 
 without the prism is evidently due to the combined action 
 of all of the changes just described, but without the use of 
 the prism it is not likely that the exact nature of the 
 changes of color of the ball would ever have been detected. 
 The prism makes it possible to determine to what the 
 changes in the color of the ball are due. 
 
 As the heating of the ball is continued, the band seen 
 through the prism continues to grow by adding vari- 
 ous tints of orange-red, then of reddish-orange, orange- 
 
RADIANT ENERGY; HEAT AND LIGHT 257 
 
 yellow, yellow, yellowish-green, green, greenish-blue, 
 blue, bluish-violet and violet, always to the same side of 
 the band, until all of the colors seen in the solar spectrum 
 are present. In the meantime the intensity of the tints 
 first present increases as before. 
 
 The changes in the compound colors produced by the 
 addition of new tints as the heating progresses are far 
 less striking. First there is a mixture of the different 
 red tints ; then a mixture of red and orange, producing a 
 bright red effect ; then a mixture of red, orange, and 
 yellow, which gives a somewhat yellowish tinge to the 
 red. As the other colors of the spectrum are added to 
 those already present, the compound color varies from 
 reddish-yellow to white. Since the red tints of the spec- 
 trum are the first to appear when a platinum ball is heated, 
 these are the first colors to become very intense. There- 
 fore, red strongly predominates in the compound colors 
 produced during the earlier part of the heating of the ball. 
 
 233. Each Prismatic Color in the Spectrum is Due to a 
 Different Rate of Vibration. The original cause of sound 
 is the vibration of some body. This vibratory motion is 
 taken up by the air and is transmitted in the form of 
 waves. Increase in the rate of vibration of the sounding 
 body results in an increase in the number of waves sent 
 out per second. This is recognized by the ear as an in- 
 crease in pitch. Increase in the amplitude of vibration 
 of the sounding body causes stronger condensations in 
 the air. In consequence the waves move forward, not 
 more swiftly, but with greater energy. Hence, the sound 
 heard is louder. Anything which causes a greater quan- 
 tity of air to be set in motion, for instance, a sounding- 
 board, also increases the total energy with which the 
 waves move forward, and adds to the loudness of sound. 
 
258 ELEMENTARY PHYSICS 
 
 In the same manner, the original cause of light is the 
 vibration of atoms, or of the combinations of atoms called 
 molecules. This vibratory motion is taken up by the 
 ether and is transmitted in the form of waves. Increase 
 in the rate of vibration of the atoms results in an increase 
 in the number of waves sent forward by the ether in each 
 second. If these vibrations are sufficiently rapid, the 
 waves produce the sensation of light and color. Any 
 further increase in the rate of vibration of the atoms 
 results in a change of color, the tint produced being nearer 
 the violet end of the solar spectrum. Increase in the 
 amplitude of vibration of the atoms causes the waves set 
 up in the ether to move with greater energy. This is 
 recognized as an increase in the intensity of the color 
 produced. 
 
 Most sounding bodies produce simultaneously many 
 vibrations differing in intensity and rate of vibration. 
 This has already been discussed in connection with funda- 
 mental tones and overtones. The waves set up simulta- 
 neously in the air produce a combined effect, recognized 
 as the characteristic quality of the sound. 
 
 In a like manner, the various atoms of the same heated 
 body may differ so much in their rate and intensity of 
 vibrations that only the combined effect can be recognized 
 without the use of a prism. This combined effect may 
 also result in the perception of color, but it is a com- 
 pound color. 
 
 Something similar to this is unquestionably true also 
 in the case of heat, since heat and light are due to the 
 same phenomena. The phenomena of heat, however, are 
 usually investigated in an entirely different manner from 
 the phenomena of light, and we have no terms which are 
 strictly comparable with the terms pitch and loudness in 
 
RADIANT ENERGY, HEAT AND LIGHT 259 
 
 sound, or with tint and intensity as applied to the spec- 
 trum colors. The temperature of a body depends, of 
 course, on the rate of vibration of its atoms or of the 
 groups of atoms known as molecules, but the temperature 
 of a body as registered by a thermometer depends also 
 upon the amplitude of vibration of the molecules and upon 
 the number of the molecules which are vibrating ; in other 
 words, upon the size of the body. The quantity of heat 
 in a body depends also upon the rate and amplitude of 
 vibration of its molecules and upon the number of mole- 
 cules present in the body. However, a great quantity of 
 heat usually implies a large body, while a high tempera- 
 ture may be shown by a small body. 
 
 234. Vibrations whose Rates are too Slow to Produce 
 Light are Deflected Beyond the Red End of the Spectrum. 
 All waves pass through the ether at the same speed. 
 The waves due to the less rapidly vibrating atoms which 
 produce a red color, pass through the ether with the same 
 rapidity as the waves due to vibrations sufficiently rapid 
 to produce a violet color. Since each vibration of an 
 atom gives rise to one wave, a greater number of waves 
 must be produced in one second by a rapidly vibrating 
 atom than by one vibrating more slowly. If the rate of 
 vibration of an atom is regular, it will send off waves at 
 equal intervals of time. These waves must, therefore, be 
 an equal distance apart. If the distance from one wave 
 to the next is called a wave length, the length of the 
 waves produced by a rapidly vibrating atom must be 
 shorter than the length of the waves produced by a more 
 slowly vibrating atom. Hence the waves producing the 
 colors nearer the violet end of the spectrum must be 
 shorter than those producing the colors nearer the red 
 end. An examination of the solar spectrum shows that 
 
260 ELEMENTAKY PHYSICS 
 
 the larger waves producing colors near the red end of the 
 spectrum are deflected less than the shorter waves. 
 
 Vibrations of atoms too slow to give rise to the sensa- 
 tion of light also produce waves. These waves must be 
 longer even than those of the dull red at one end of 
 the spectrum. Although invisible, they also must pass 
 through the prism, but should be deflected less and hence 
 the search for them should be conducted beyond the red 
 end of the spectrum. How may these waves be detected 
 if they are not capable of producing the sensation of 
 light ? Some instrument other than the eye must be used. 
 A very delicate thermometer might be serviceable. A 
 thermopile, however, is better. 
 
 235. A Thermopile is Used to Detect Extremely Small 
 Variations in Temperature. A thermopile is an electrical 
 instrument which is capable of detecting much smaller 
 changes in temperature than even the most delicate ther- 
 mometer. Thermopiles have been constructed which are 
 capable of detecting a variation in temperature of less than 
 one-millionth of a degree. It is not necessary to describe 
 the instrument here, since a full description may be found 
 in any larger book on physics. A slit thermopile is most 
 convenient in detecting the presence of waves which have 
 been deflected by a prism. The method of its use is suffi- 
 ciently well explained in "A School Course in Heat," by 
 Larder, Sampson Low, Marston and Co. London. 
 
 236. Relative Intensity of Heat Produced by the Different 
 Waves Giving Rise to the Solar Spectrum. The use of the 
 thermopile may be illustrated by investigating the rela- 
 tive intensity of heat produced by the waves giving rise 
 to the solar spectrum. Cause a small quantity of light 
 from the sun to pass through a narrow slit and a prism, 
 and then allow it to strike the surface of a large piece of 
 
RADIANT ENERGY, HEAT AND LIGHT 261 
 
 white paper. The prism will deflect the different tints to 
 different positions on the paper, and the result will be the 
 appearance on its surface of a broad band of colors, the 
 solar spectrum. 
 
 By moving the thermopile in front of the paper so as to 
 permit the various tints to fall in succession upon the 
 thermopile, the relative intensity of heat given out 
 by the different waves producing 1 light can be compared. 
 Instead of a glass prism, a salt prism is used, since salt 
 absorbs far less of the wave motion than glass. 255. 
 
 By studies of this kind it has been discovered that the 
 lowest intensity of heat is produced by the short waves at 
 the violet end of the spectrum, and that the intensity of 
 heat increases on approaching the red end of the spec- 
 trum (Fig. 109). A similar fact is true of the spectrum of 
 the platinum ball. 
 
 The greater intensity of heat produced by the longer 
 waves due to the lower rates of vibration in the case of the 
 platinum ball is possibly due, not only to the greater in- 
 tensity of these vibrations, but also to the fact that, even 
 when the platinum ball has become white hot, the number 
 of molecules vibrating at a lower rate is probably still 
 much greater than the number of molecules vibrating at 
 a higher rate. The same supposition may also be made 
 to account for the similar phenomena just described in 
 the case of the spectrum of the sun. 
 
 X, 237. Relative Intensity of Heat Produced by the Invisible 
 Waves Deflected Beyond the Red and the Violet Ends of the 
 Spectrum. It has been shown that, when a thermopile is 
 moved from the violet to the red end of the spectrum, an 
 increase of temperature is indicated. If the motion of 
 the thermopile is continued in the same direction, be- 
 yond the red end of the spectrum, it is found that the 
 
262 ELEMENTARY PHYSICS 
 
 thermopile indicates an increase in intensity of heat 
 even after the dull red tint at the very extreme of the 
 visible part of the spectrum has been passed (Fig. 109). 
 This increase continues until a certain distance beyond 
 the end of the visible spectrum has been reached, after 
 which the thermopile indicates a decrease in the intensity 
 of heat. The fact that the thermopile is affected at all 
 indicates the presence of waves beyond the red end of 
 the spectrum and that these waves may be detected by the 
 thermopile even when they cannot be seen by the eye. 
 
 It is an interesting fact that the greatest intensity of 
 heat is produced, not by the waves causing the red colors 
 of the spectrum, but by invisible waves having a consider- 
 ably greater length, whose position is some distance be- 
 yond the red end of the spectrum. 
 
 The possibility of invisible waves producing great 
 intensity of heat is shown by the name of the Bunsen 
 burner, which is intensely hot, but which (when well 
 regulated) scarcely gives out any light. 
 
 If the thermopile is now moved in the opposite direc- 
 tion, from the red to the violet end of the spectrum, it is 
 found that invisible waves exist also in the region be- 
 yond the violet end of the spectrum (Fig. 109). These 
 waves are shorter than any visible in the spectrum, but 
 they are unable to produce the sensation of light. More- 
 over they cause even a less intensity of heat than the vio- 
 let rays. The farther the waves are beyond the violet end 
 of the spectrum, the less is the intensity of heat. 
 
 238. Ultra-Red and Ultra- Violet Parts of the Spectrum. 
 The word spectrum was originally used to designate 
 the band of colors produced by light passing through a 
 prism. Since the prism separates all waves passing 
 through it, both visible and invisible, and distributes them 
 
KADIANT ENERGY, HEAT AND LIGHT 263 
 
 in the order of their length, it is possible to speak of 
 the entire series of waves after they leave the prism, as 
 the spectrum. That part of this spectrum which includes 
 the waves capable of producing the sensation of light is 
 then called the visible spectrum. The invisible waves 
 which are deflected to points beyond the red end of the 
 spectrum form the ultra-red spectrum, and the invisible 
 waves which are deflected beyond the violet end of the 
 spectrum form the ultra-violet spectrum. 
 
 239. Nerves of Sight Not Sensitive to All Rates of Vibra- 
 tion. The vibrations which produce the dull red tint at 
 the red end of the visible spectrum have a rate of 392 trill- 
 ions per second ; the vibrations at the extreme limit of the 
 violet end of the visible spectrum have a rate of 752 trill- 
 ions per second. Some eyes are sensitive to lower and some 
 to higher rates of vibrations than others. This merely 
 corroborates the view, that the inability of the eye to see 
 all vibrations transmitted through the ether is due not to 
 any special difference between visible and invisible vibra- 
 tions, aside from their rate of vibration, but is caused by 
 a lack of sufficient sensitiveness in the nerves of the eye. 
 
 As already stated there are vibrations whose rate is 
 less than that giving rise to red at the extreme end of the 
 visible end of the spectrum. In addition to the rays 
 which are capable of giving rise to the sensation of heat, 
 but not of light, are found waves whose frequency is very 
 much less. Some of these are known as the Hertz waves, 
 and produce the results utilized in wireless telegraphy. 
 On the other hand, the frequency of the vibrations present 
 in X-rays very much exceeds the frequency shown by the 
 violet rays at the other end of the spectrum. 
 
 240. Natural and Artificial Sources of Light. In order 
 that light may be seen, there must be a source of light, a 
 
264 ELEMENTARY PHYSICS 
 
 source capable of producing those waves to which the 
 eye is sensitive. Very few bodies act as sources of light 
 or are self-luminous. Most bodies merely reflect or 
 transmit light. The sun is the most important source of 
 light. The moon merely reflects a part of the sunlight 
 which falls upon it. That part of the moon's surface 
 upon which the light of the sun does not fall remains 
 dark. The stars are also sources of light, as may be seen 
 easily on a night when the moon is not shining. The 
 most important artificial sources of light are those due to 
 rapid combustion : such as, the burning of wood, coal, 
 coal-oil, acetylene gas, and other materials. The light 
 given out by the flame is due to particles of carbon which 
 once were parts of the wood, or coal-oil, and which for a 
 moment are heated enough to give out light. Any parti- 
 cles not burned up before they leave the flame remain- un- 
 consumed and pass off as smoke. In the incandescent 
 electric light the carbon filament is heated white hot, but 
 is not burnt. 
 
 The flash of lightning is due to intense, rapid vibra- 
 tions of the atoms forming the air, set in motion by elec- 
 tricity. The light given out by the fire-fly is also due to 
 very rapid vibrations of atoms. The heat produced by the 
 fire-fly is small. This may be due to the small intensity 
 of the vibrations belonging to the red and ultra-red parts 
 of the spectrum, or to the small number of vibrating atoms. 
 
 241. Different Sources of Light Give Out Different Colors. 
 Different sources of light may produce different waves, 
 or may produce waves which are alike in length but dif- 
 fer in energy. It has already been shown that sodium 
 carbonate, placed in the flame of a Bunsen burner, gives 
 out yellow light, and that lithium carbonate gives out 
 red light. In reality the compounds are broken np, and 
 
RADIANT ENERGY, HEAT AND LIGHT 265 
 
 the elements, sodium and lithium, give out the yellow 
 and red light. A Other substances give out different colors. 
 The light from a coal-oil lamp consists of all the waves 
 present in sunlight, but the yellow and red colors are 
 relatively more intense and the blue, bluish-violet, and 
 violet colors less intense, than corresponding colors in 
 the light from the sun. Hence objects illuminated by 
 lamplight often appear to have colors differing from the 
 colors of the same objects seen in daylight. 
 
 242. Most Objects are Visible Only While they Reflect 
 or Transmit Light. Most objects do not give out light of 
 their own. They are visible only by means of the light 
 which they receive from various sources of light, and 
 then either reflect or transmit. 
 
 If a room were closed so that not even a single ray of 
 light could enter it, and if no artificial source of light (a 
 lamp or electric light) were introduced, not a single object 
 in the room could be seen. During nights when the 
 moon is not shining, and when the clouds are thick enough 
 to absorb almost all of the light from the stars, we are 
 able to appreciate more fully the fact, that most objects 
 can be seen only because they reflect or transmit light. 
 
 Even a tiny ray of light illuminates many parts of a 
 darkened room. The light is reflected from object to ob- 
 ject, and scattered in so many directions by any uneven 
 surface, that a little light is finally reflected from most 
 objects in the room. In a similar manner, objects not in 
 direct sunlight receive the light of the sun after reflec- 
 tion from many other bodies. 
 
 243. Difference of Color of Opaque Objects Due to their 
 Inability to Reflect All or a Part of Certain Prismatic 
 Colors. All objects which are not transparent are visible 
 only when they reflect light. The light which they re- 
 
266 ELEMENTARY PHYSICS 
 
 fleet depends up6n the light which falls upon them. It 
 is a familiar fact that, when red light alone falls upon a 
 piece of white paper, the paper appears red. When yel- 
 low light alone falls upon it, the paper appears yellow. 
 But this is not true of all objects. All objects cannot 
 reflect the same color. Therefore the color of an opaque 
 object depends, not only upon the colors which fall upon 
 it but also upon the colors which it can reflect. 
 
 This may be shown by taking some source of light 
 which contains all of the prismatic colors, and investigat- 
 ing which of these colors different bodies can reflect. 
 
 By means of a mirror reflect a beam of sunlight through 
 a narrow slit into a darkened room. Let the light pass 
 through a prism, as before, and then let it fall upon a 
 sheet of white paper. The paper is white, because it re- 
 flects all the colors present in sunlight. It is able, there- 
 fore, to reflect each one of these colors singly, after all 
 the colors have been separated by the prism. This is the 
 reason for the use of white paper in this experiment. 
 Change the position of the paper, until the prismatic 
 colors spread over as large an area as possible. 
 
 Place a piece of red flannel in front of the different 
 colors of the spectrum. When red light falls upon it, it 
 has its usual strong red appearance. When other colors 
 fall upon it, it appears dark or black. Red is the only 
 color which it reflects well. Therefore, it appears red even 
 if all the prismatic colors present in sunlight fall upon it. 
 But when the light which falls upon the flannel does 
 not contain any red light, when for instance it contains 
 only green light, the flannel is not able to reflect the green 
 light, the only color which it receives. Since it fails 
 to reflect any color, it appears black. 
 
 Cover a smooth piece of wood with soot by means of a 
 
RADIANT ENERGY, HEAT AND LIGHT 267 
 
 lamp- or gas-flame. Hold it in various parts of the spec- 
 trum. It reflects none of the prismatic colors. Black, 
 therefore, is the absence of all color. Absolute blackness 
 is due either to a complete failure to reflect light, or to a 
 complete absence of light to be reflected. Most objects 
 called black are not absolutely black, but reflect a suffi- 
 cient quantity of some one prismatic color to make it 
 more appropriate to speak of the color as bluish-black, 
 greenish-black, or some other shade of black. 
 
 244. Compound Colors Due to Selective Reflection. If now 
 a great variety of substances be tested by holding them 
 in different parts of the spectrum, it will be found that 
 some substances reflect only one color well, and, therefore, 
 possess a color which approaches one of the prismatic 
 colors. Other substances reflect one prismatic color well 
 and two or three other prismatic colors in a very moder- 
 ate degree. The prismatic color which is well reflected 
 usually gives its name to the resulting compound color. 
 The other prismatic colors reflected determine the char- 
 acteristic tint. The general color effect produced by the 
 mixture is usually very different from the effect of any of 
 the prismatic colors : for example, greenish-blue, orange- 
 red. 
 
 Many substances reflect a number of prismatic colors 
 in such quantities that the compound color produced 
 has no resemblance to any of the prismatic colors. By 
 persons not students they are often looked upon as of 
 equal rank with prismatic colors. Brown, gray, drab, 
 lilac are compound colors. 
 
 Among the objects to be examined it is well to have 
 quite a number which are distinctly and strongly colored. 
 Light-colored objects appear light, because they reflect 
 a considerable amount of almost all of the other prismatic 
 
268 ELEMENTARY PHYSICS 
 
 colors, in addition to the prismatic colors which strongly 
 predominate ; for example, light-red ribbons. 
 
 245. Reflection of Heat. Since most substances are not 
 able to reflect all of the prismatic colors which form the 
 visible part of the spectrum, it is probable that most sub- 
 stances also cannot reflect all of the waves which form the 
 invisible part of the spectrum. Moreover, since both vis- 
 ible and invisible waves can produce the sensation of heat, 
 this is practically equivalent to saying that most sub- 
 stances vary considerably in their ability to reflect heat. 
 This subject is difficult to investigate, since most of the 
 waves producing heat are invisible, but it is well known 
 that some bodies reflect a much greater portion of the 
 total quantity of heat falling upon them than others. 
 Polished metals are among the best reflectors of heat. 
 Lampblack is the poorest reflector of heat known. 
 
 246. Heat May be Reflected by a Cold Surface. If we 
 stand at one side of an open fireplace, out of reach of the 
 direct heat, in a room that has not yet been heated for a 
 sufficient length of time to become warm, and, if some one 
 standing in front of the fire suddenly places a large bright 
 sheet of tin in such a position that the light of the fire is 
 reflected upon us, we not only instantly perceive the light, 
 but we are at the same moment aware of a considerable 
 quantity of heat. However, if we touch the tin quickly 
 we find that it is still cold. The heat that has come to us 
 is not the heat of the tin, but the heat of the fire reflected 
 by the tin. 
 
 247. Colorless Transparent Substances Transmit All of the 
 Prismatic Colors. Many transparent substances show no 
 indication of color. If one of these substances be held 
 anywhere along the path of light which is reflected by a 
 mirror or porte-lumiere into a darkened room and toward 
 
EADIANT ENERGY, HEAT AND LIGHT 269 
 
 a prism, it will be found that the transparent substance 
 has absorbed so little of the various prismatic colors pres- 
 ent in sunlight, that the spectrum on the screen remains 
 practically unchanged. In fact, these substances are 
 called colorless, because they transmit all colors un- 
 changed and, therefore, produce no perceptible effect 
 upon the colors of objects seen through them. 
 
 248. Differences of Color Transmitted by Transparent 
 Objects Due to Their Inability to Transmit All or a Part 
 of Certain Prismatic Colors. Any object seen through a 
 piece of red glass looks either red or black. The reason 
 for this may be readily seen if the red glass is held so 
 that the sunlight passes through the glass before it enters 
 the prism. The spectrum on the screen is changed in ap- 
 pearance. Most of the colors have evidently been stopped 
 by the red glass. Only the red colors have been allowed 
 to pass through freely. The other colors which are trans- 
 mitted, are transmitted only in small quantities. If a 
 kind of glass could be found which would transmit abso- 
 lutely nothing but red, only objects which give out, re- 
 flect, or transmit red light, could be seen through it. 
 Since only red light would be transmitted by this kind of 
 glass, all objects seen through the glass would appear red. 
 Objects not giving out red light, but giving out only other 
 colors, would be invisible and the spaces occupied by 
 them would appear black. 
 
 This does not mean that only those objects which ap- 
 pear red to the unassisted eye could be seen through a 
 piece of red glass. It should be remembered that most 
 of the colors occurring in nature are compound colors. 
 Most of these colors contain (in addition to other and 
 often more evident colors) more or less of the prismatic 
 colors called red. For instance, the color of an ob- 
 
270 ELEMENTARY PHYSICS 
 
 ject which appears blue to the naked eye may also contain 
 a small quantity of red, so that when the object is viewed- 
 through red glass, it will be visible, and will apparently 
 have a dull red color. How prevalent these red pris- 
 matic colors are in almost all of the combinations of 
 colors reflected by objects in nature, may be better ap- 
 preciated by looking at these objects through a piece of 
 red glass. It will at once become evident that almost 
 all objects reflect or transmit some reel color, although 
 the quantity of red color thus reflected or transmitted, 
 may be so small that it cannot be noticed until the red 
 glass is used, or until the prism separates the red from 
 the other colors. 
 
 Hold other kinds of colored glass in the path of the 
 sunlight, before it enters the prism. It will be found 
 that green glass transmits chiefly green ; blue glass, 
 chiefly blue ; other kinds of glass, other colors. In each 
 of these cases, the predominant prismatic color trans- 
 mitted gives name to the color of the glass. 
 
 When several prismatic colors are transmitted in con- 
 siderable quantities, their combined effect is often so dis- 
 similar to the effect of any single prismatic color, that 
 special names have been coined to designate the com- 
 bination of color thus produced. 
 
 Throw a spectrum on the wall and look at it through 
 differently colored pieces of glass. 
 
 249. Color of Glass Usually Recognized by Light Reflected 
 from Other Bodies. It is not necessary to hold the glass 
 between the eye and some source of light in order to 
 determine its color. No matter in what position the 
 eye is held, a sufficient variety of colors is usually re- 
 flected by various objects beyond the glass to enable the 
 glass to transmit to the eye about the same combination 
 
RADIANT ENERGY, HEAT AND LIGHT 271 
 
 of colors which it transmits when it is exposed to sun- 
 light. Therefore, if the glass be held between the eye 
 and the floor, the eye and a building, or the eye and a 
 plot of grass, a range of prismatic colors will be sent by 
 the various objects through the glass to the eye which 
 will be sufficient to give the glass its usual color. 
 
 250. Color of Light Transmitted Through a Succession of 
 Differently Colored Substances If a kind of glass could be 
 found which transmitted nothing but red, and another 
 kind which transmitted nothing but green, no light could 
 be transmitted through both glasses if they were held to- 
 gether. The red glass would transmit only the red light, 
 and this red light would be absorbed soon after it entered 
 the green glass, so that nothing could pass through both. 
 
 As a matter of fact, however, most green glass trans- 
 mits not only green light, but also a little red light, so 
 that objects seen through both red and green glass appear 
 dull red. In the same manner most blue glass transmits 
 not only blue light, but also a little red light, so that ob- 
 jects seen through both red and blue glass appear dull 
 red. Blue glass transmits a little green in addition to 
 blue, and green glass transmits a little blue in addition 
 to green, so that objects seen through both have a peculiar 
 bluish-green color affected a little by the red which is 
 transmitted by both kinds of glass. But a combination 
 of red, green, and blue glasses will absorb so much of all 
 the different colors, that very little light of any kind, ex- 
 cept a faint tinge of red, can pass through all of them. 
 The result is that no objects can be seen through a com- 
 bination of all of the three kinds of glass here mentioned, 
 unless they give out a very considerable quantity of red 
 light. Ordinarily objects do not do this, but the light 
 of the sun or of an electric light contains enough of 
 
272 ELEMENTAEY PHYSICS 
 
 the strong- red colors to be seen through all of these 
 glasses used tog-ether. 
 
 251. Effect of Thickness upon the Color of Transparent 
 Substances. The colors which enter glass, but which are 
 not transmitted, are absorbed. They are not absorbed at 
 once, but are absorbed more and more as they penetrate 
 deeper into the glass. The thickness of the glass will, 
 therefore, determine how much of these colors will be 
 absorbed. If a thin layer of colored glass examined by 
 means of a prism shows that it absorbs very much more of 
 some colors than of others, it is certain that thicker layers 
 may absorb all of the more readily absorbed colors, while 
 they will still transmit considerable quantities of those 
 less readily absorbed. The compound colors produced 
 by light passing through thicker layers of this kind of 
 glass will not contain some of the prismatic colors which 
 are transmitted through thinner layers. The colors of 
 certain kinds of glass, therefore, depend, to a certain ex- 
 tent, upon the thickness of the glass. 
 
 The influence of thickness upon the color of transparent 
 substances may be seen by placing an increasing number 
 of pieces of blue glass between the eye and the sun, or 
 by examining different thicknesses of litmus solution by 
 transmitted light. 
 
 252. Almost All Opaque Substances are Transparent in 
 Very Thin Sections. Since no substance is known which 
 reflects all of the light which falls upon it, some light 
 must enter every substance, even those substances called 
 opaque. A substance is called opaque when the light 
 which enters it is not able to penetrate far into the sub- 
 stance before it is stopped (absorbed). Since the light 
 which enters the opaque substance must penetrate some 
 distance before it is absorbed, light may pass through any 
 
BADIANT ENERGY, HEAT AND LIGHT 273 
 
 opaque substance if a thin enough layer of the substance 
 can be obtained. Hence every substance is transparent 
 to some degree. 
 
 In practice, it has been found that it is very easy to se- 
 cure sections of wood which are thin enough to be trans- 
 parent. There is such little difficulty in securing trans- 
 parent sections of even the blackest rocks that one of the 
 courses in geology (petrography) consists in a study of 
 the mineralogical composition of rocks by means of a 
 microscopical investigation of thin transparent sections. 
 
 Even gold is transparent, when it is secured in thin 
 films. Gold reflects a yellow color, but if thin films of 
 gold are placed between pieces of glass in front of the 
 light of a magic lantern, they will transmit green light. 
 Very thin films of silver transmit blue light. 
 
 Therefore, when we say that a substance is opaque we 
 mean that an ordinary thickness of the substance is opaque. 
 Sufficiently thin layers of this substance are almost cer- 
 tain to be transparent. 
 
 253. The Colors of Opaque Objects Partly Due to In- 
 ternal Reflection. When light is thrown upon glass, a 
 part is reflected by the surface of the glass, and a part, 
 after it has passed through the glass, is reflected by the 
 air at the rear of the glass. The result is that, if a lighted 
 candle be placed before a thick piece of plate-glass, two 
 images of the candle are formed. The front image is due 
 to the reflection of light by the front surface of the glass ; 
 the rear image is due to the reflection of light by the air 
 in contact with the rear of the glass. 
 
 If the glass is colorless, the light is not altered in color 
 as it passes through the glass and back again, after reflec- 
 tion, to the side of the glass from which it came. But if 
 the glass is colored, for instance if the glass is red, the 
 
274 ELEMENTARY PHYSICS 
 
 light which traverses the thickness of the glass and then 
 returns is red, for red glass absorbs all the other colors. 
 
 In the same manner, the light entering many so-called 
 opaque objects is reflected by different materials within a 
 very short distance of the surface, and so returns to the 
 original surface, containing only those rays which thin 
 sections of the opaque objects are not able to absorb. 
 
 It is probable that the colors of all opaque objects are 
 due to partial absorption of the colors which fall upon them. 
 While these bodies are called opaque, the light in reality 
 penetrates for a short distance and is reflected again to- 
 ward the surface, but only the light not absorbed during 
 the transition again reaches the surface, and gives the 
 characteristic color of the object. Any color reflected 
 from the actual surface of an object is probably reflected 
 practically unchanged. 
 
 254. Color of Pigments. Copper sulphate and potas- 
 sium bichromate both form transparent crystals. Copper 
 sulphate transmits blue chiefly ; it absorbs most of the 
 red, orange, and yellow, but transmits a little green 
 (Fig. 110, A). Potassium bichromate transmits orange- 
 red chiefly, the color noticed on examining the crystals ; 
 it absorbs most of the blue and violet, but transmits a 
 little green (Fig. 110, B). The solutions absorb and trans- 
 mit the same colors as the crystals. The result is that 
 when a copper sulphate solution is held in front of a po- 
 tassium bichromate solution the only color which is trans- 
 mitted to any considerable extent by both solutions is 
 green, and hence the light seen through both solutions 
 appears green, and a mixture of these solutions also ap- 
 pears green (Fig. 110, C, D). 
 
 If a very small quantity of finely powdered potassium 
 bichromate is added to a larger quantity of finely pow- 
 
RADIANT ENERGY, HEAT AND LIGHT 275 
 
 dered copper sulphate, and the mixture is then thoroughly 
 stirred, a slight change of color is noticeable. If the pro- 
 portion of potassium bichromate is gradually increased, 
 the mixture finally becomes a distinct light green, or ap- 
 ple-green. Any light which strikes the mixture pene- 
 trates it for some depth before it is reflected back to the 
 surface, and, since during its passage through the mixt- 
 
 r o y gr bl in 
 
 Colors transmuted 
 by copper sulphate 
 
 FIG. HO. 
 
 ure all the colors except green are absorbed, the mixture 
 appears green. Mix in the same manner chrome yellow 
 and ultramarine blue ( 244 and 253). 
 
 255. The Relative Transmission of Visible and of Ultra- 
 Red Waves Varies in Different Substances. Since most 
 substances are not able to transmit equally well all of 
 the prismatic colors which form the visible part of the 
 spectrum, it is probable that most substances cannot 
 
276 ELEMENTARY PHYSICS 
 
 transmit all of the waves formed by the invisible parts of 
 the spectrum. This fact is shown to a certain extent by 
 the following observations. 
 
 Rock salt appears to transmit the invisible ultra-red 
 waves about as well as the visible waves of the spectrum. 
 Hence prisms used to investigate the ultra-red part of 
 the spectrum are usually made of rock salt. A solution 
 of iodine in carbon bisulphide transmits none of the vis- 
 ible waves, but transmits a very considerable quantity of 
 the invisible, ultra-red waves. Alum and ice transmit 
 almost none of the invisible ultra-red waves, but transmit 
 most of the visible waves of the spectrum. Since most 
 of the heat given out by the light in a stereopticon 
 comes usually from the invisible ultra-red waves, a solu- 
 tion of alum in water is often used to shield lenses and 
 slides when images of microscopic objects are thrown 
 upon a screen by means of a magic lantern or stereop- 
 ticon. 
 
 Calcite, glass, and quartz transmit very little of the 
 heat due to those invisible, ultra-red waves whose rate is 
 low, but they transmit more of the more rapid ultra-red 
 waves which lie nearer the visible spectrum, and they 
 transmit most of the visible waves. 
 
 256. Good Reflectors of Visible and Invisible Waves are 
 Poor Absorbers of Heat. Those visible and invisible waves 
 which fall upon a substance and which are neither re- 
 flected nor transmitted, seem to disappear entirely. They 
 are said to be absorbed. What becomes of these waves ? 
 They increase the heat of the substance through which 
 they pass ( 257). 
 
 Those substances, therefore, which reflect the least light 
 (and which probably also reflect the smallest quantity 
 of the invisible waves) ought to show the greatest gain 
 
RADIANT ENERGY, HEAT AND LIGHT 277 
 
 in heat. This may be verified by an interesting experi- 
 ment. Place upon the snow, in direct sunlight, three 
 pieces of cloth, alike in every particular except in color. 
 Suppose the colors chosen are white, red, and black. The 
 snow will melt most rapidly around the black cloth, and 
 least readily around the white cloth. The black color is 
 evidently due to the absorption of most of the light of 
 the sun, and white is due to the least absorption, since it 
 evidently reflects all the colors to a considerable extent. 
 Therefore, the black cloth becomes warm more quickly 
 than the white cloth. What colors should be worn during 
 summer ? During winter ? 
 
 Opaque bodies, if they are sufficiently thick, absorb all 
 the light which enters them, with the exception, of course, 
 of that part of the light which suffers interior reflection 
 very near the surface of the body (251). Lamp-black 
 seems to absorb all waves (ultra-red and visible), and to 
 reflect none. Most metals absorb about 13 or 14 per 
 cent, of both the ultra-red and of the visible waves. They 
 must, therefore, be excellent reflectors of both kinds of 
 waves. For that reason certain metals were formerly 
 used extensively for mirrors. So-called glass mirrors are 
 panes of glass with a metallic coating (chiefly mercury) 
 upon the back. This coating does most of the reflecting. 
 The image reflected by the front of the glass is rarely 
 noticed. 
 
 257. Good Transmitters of Visible and Invisible Waves 
 are Poor Absorbers of Heat. It has already been shown 
 that substances vary considerably in their ability to trans- 
 mit all the waves which form the visible and the invisible 
 parts of the spectrum. Those substances which trans- 
 mit most of the waves, especially of those invisible ultra- 
 red waves which give evidence of the greatest intensity 
 
278 ELEMENTARY PHYSICS 
 
 of heat, will necessarily absorb but little heat and will 
 therefore remain comparatively cold even in direct sun- 
 light. Salt is one of the best transmitters and therefore 
 is a poor absorber of heat. Glass transmits much less, 
 and therefore absorbs much more of the heat. Alum, 
 however, transmits but little heat and absorbs so much 
 that it is used to protect other bodies from heat ( 255). 
 Most of the rock-forming minerals absorb more heat 
 than they transmit. Air, on the contrary, absorbs but 
 little heat. This fact may be recognized easily on a hot 
 summer's day by placing the hand, which is already in 
 contact with the air, on dry ground which has been ex- 
 posed to direct sunlight. The contrast between the tem- 
 perature of the ground and the temperature of the air is 
 great. 
 
 The atoms forming the surface of the body may be set 
 in vibration by the waves set up in the ether so readily 
 that the face exposed to direct sunlight may feel burning 
 hot, though, at the same time, the air in contact with the 
 face may have a considerably lower temperature. The 
 instant an umbrella is used as a screen, the air is felt to 
 be cooler. 
 
 Water vapor is a much better absorber of heat than air, 
 although both are very transparent to light. Therefore, 
 moist air becomes hot much more quickly than dry air. 
 
 258. Explanation, by Means of the Ether, of the Connection 
 between Good Absorption and Poor Transmission. In at- 
 tempting to explain the evident connection between good 
 absorption and poor transmission of waves, or between 
 poor absorption and good transmission, the following as- 
 sumptions are made : 
 
 When atoms do not obstruct, to any considerable ex- 
 tent, the transmission of vibrations through the ether 
 
RADIANT ENERGY, HEAT AND LIGHT 279 
 
 within which the atoms are imbedded, the vibrations are 
 transmitted through the ether with very little loss of en- 
 ergy ; but the atoms are not set into very active vibration. 
 Substances of this kind are good transmitters but poor 
 absorbers of heat. 
 
 However, when atoms obstruct strongly the transmis- 
 sion of these waves, the energy of the waves in the ether 
 is considerably diminished. If a sufficient number of 
 these obstructing atoms be present, the energy of the 
 waves passing through the ether within the interior of 
 the body formed by these atoms becomes so small that 
 the vibrations of the ether practically die out there. But, 
 during this time, the obstructing atoms begin to vibrate 
 violently, and are said to have absorbed the heat. Sub- 
 stances of this kind are good absorbers but poor trans- 
 mitters of heat. 
 
 259. Good Absorbers are Good Radiators of Heat. 
 Investigators have shown by experiment that any body 
 which absorbs heat readily gives it out again readily. 
 Such a body is said to radiate the heat which it has ab- 
 sorbed. The sand of the desert of Sahara, which becomes 
 heated quickly in daytime, cools down rapidly at night. 
 The dark soil in our climate absorbs heat readity. When 
 the sky is cloudless at night, the soil again gives up the 
 heat readily ; but if the sky be covered by clouds, this 
 heat will be absorbed by the clouds, and then given back 
 again to the air beneath, so that cloudy nights are not as 
 cold as cloudless nights. 
 
 260. The Chemical Effect of Waves Belonging to Dif- 
 ferent Parts of the Spectrum. If silver nitrate is placed 
 upon some animal or vegetable substance and exposed 
 to sunlight, the effect of the sunlight is to produce cer- 
 tain chemical reactions between the silver nitrate and 
 
280 ELEMENTAEY PHYSICS 
 
 the other substances, and these reactions give rise to a 
 change in color. In a similar manner, the light of the sun, 
 falling upon the various silver compounds used in photog- 
 raphy, causes a chemical reaction which, when it is fol- 
 lowed by other chemical changes during the development 
 pf the plate, produces a black color. 
 
 It might be supposed from this that all objects which 
 reflect much light will appear black after the plate ex- 
 posed in the camera is developed. However, when the 
 effects of various colors upon the photographic plate are 
 studied, it is found that some colors produce much less 
 effect upon silver compounds than other colors do. The 
 chemical action is usually greatest when the plate is ex- 
 posed to violet or bluish-violet colors, and it diminishes 
 as colors nearer the red end of the spectrum are used. 
 When the effects of the invisible waves beyond the violet 
 end of the spectrum are investigated, it is discovered that 
 these effects diminish in intensity, as the distance of 
 these invisible waves from the violet end of the spectrum 
 increases. 
 
 When the effect of light upon other substances which 
 are not silver compounds is examined, very different re- 
 sults are obtained. Certain compounds of iron are most 
 sensitive to waves which lie beyond the extreme red end 
 of the spectrum. Each chemical compound which is 
 likely to undergo chemical reactions when it is exposed 
 to light, evidently absorbs waves of a certain rate best, 
 so that the chemical action depends not only upon the 
 frequency of the waves, but also upon the particular sub- 
 stance exposed to these waves. 
 
 261. Effect of Distance of an Object upon the Distance 
 of the Image from the Lens. Hold a double convex lens, 
 or a lens both of whose faces are convex, a considerable 
 
RADIANT ENERGY, HEAT AND LIGHT 281 
 
 distance back from the window. Behind the lens place a 
 piece of paper in such a position that the light reflected 
 from some building may pass through the lens and strike 
 the paper. Move the paper back and forth, until some 
 position is found in which a fairly distinct image of the 
 building is seen upon the paper. 
 
 A much better image may be secured in the following 
 manner : Cut a hole in one end of a box, of such a shape 
 and size that a lens may be inserted without permitting 
 any light to pass between the margin of the lens and the 
 
 TIG, Hi. 
 
 wood surrounding the hole (Fig. 111). Instead of paper 
 use a piece of ground glass, fastened in a vertical frame, 
 which can be moved backward and forward within the box. 
 The end of the box opposite the lens should be left open, 
 so that the image upon the ground glass may be readily 
 seen. Secure an image of the same building and record 
 the distance of the ground glass from the lens. Now 
 carry the box to a point much nearer to the building, and 
 move the screen again until another image is formed. 
 The screen will be found to occupy a position farther 
 from the lens. Keplace the screen in the position it oc- 
 
282 ELEMENTARY PHYSICS 
 
 cupied when the first image was made. Now carry the 
 box to a position much farther removed from the building 
 than any position so far occupied. In order to secure an 
 image, the screen must be moved nearer to the lens. The 
 positions occupied by the screen at times when distinct 
 images are formed are called foci (singular, focus). From 
 the observations here recorded, it may be seen that there 
 are a great many positions or foci at which images may be 
 formed by the same lens, and that the distance of each 
 focus from the lens depends upon the distance of the ob- 
 ject from the lens. In photographic cameras the ground 
 glass often occupies a fixed position and the lens is moved 
 back and forth until the image falls upon the screen. 
 
 Instead of the building a gas flame or candle may be 
 used as an object, and a piece of white paper will serve as 
 a screen. The apparatus being placed upon a table, dif- 
 ferent positions may be given to the lens, and the screen 
 may be moved back and forth until a distinct image is 
 produced upon the paper. 
 
 262. Effect of Distance of an Object from the Lens upon 
 the Size of the Image on the Screen. If the size of the 
 image be noted in the three cases described in the preced- 
 ing paragraph, it will be found that the image is smaller 
 when the object is farther from the lens, and larger when 
 the object is nearer to the lens. In order to secure an im- 
 age of an object very near to the lens, it may be necessary 
 to move the screen back quite a distance from the lens. 
 For this reason it is impossible to secure a good photo- 
 graph if the camera be held very near to an object, unless 
 the screen or photographic plate can be moved quite a dis- 
 tance from the lens. These facts can be most readily illus- 
 trated by using the gas flame or candle as an object and 
 the paper as a screen as suggested in the last paragraph. 
 
RADIANT ENERGY, HEAT AND LIGHT 283 
 
 263. Effect of Convexity of Lens upon the Distance of 
 the Image from the Lens. If the lens in the opening of 
 the box is replaced by one having- a greater convexity, and 
 the box is carried to each of the three positions occupied 
 before, another series of images is secured. But, in each 
 case, the image is found nearer to the lens than it was 
 when the less convex lens was used. If the experiments 
 be repeated with a lens of less convexity, the distance of 
 the image from the lens will be found to be greater. 
 
 If a sufficient number of lenses of different convexity 
 are at hand, it is possible to secure an image of any ob- 
 ject, no matter at what distance, without moving the 
 screen. If the object is very far away, it is necessary to 
 use a lens of slight convexity. If the object is near at 
 hand, a lens of greater convexity must be used. 
 
 264. Reason why the Image on the Screen is Inverted. 
 It is evident that in the preceding experiments any light 
 coming from the top of the building and passing through 
 the lens strikes the screen near its lower margin. Any 
 light from the bottom of the building strikes near the top 
 of the screen. Light from the left side of the building 
 reaches the right side of the screen. In other words, the 
 image produced upon the screen is inverted. 
 
 265. The Image Most Distinct when the Margin of the 
 Lens is Not Used. When the entire surface of the lens is 
 used, it is found that the image is brighter, but not as 
 distinct. If the margin of the lens is covered in some 
 way, a considerable quantity of light is excluded. The 
 image is, therefore, not so bright, but it is much more 
 sharp or distinct. For this reason, cameras are provided 
 with stops or diaphragms, which make it possible to use 
 only the central part of the lens in bright weather. If 
 objects are in motion, so that it is necessary to take an in- 
 
284 ELEMENTARY PHYSICS 
 
 stantaneous picture, it may be necessary to use the entire 
 lens to secure a good photograph in a very short time. 
 Moreover, if it be very dark, the entire lens must be used 
 in order to secure sufficient light, if the exposure is to oc- 
 cupy only a short time. 
 
 266. Structure of the Eye. The essential part of the 
 eye consists of a lens, L, enclosed within the eyeball, near 
 the front, and a screen, called the retina, R R, which lines 
 the inner surface of the eyeball (Fig. 112). The image 
 formed by the lens must fall upon the retina. If the 
 
 image produced up- 
 on the retina is dis- 
 tinct, the object is 
 seen distinctly. The 
 position of the screen 
 is fixed. The lens can 
 be moved only a lit- 
 
 Optic Nerve ., ,, , , , 
 
 tie forward or back- 
 ward. There is only 
 one lens, and it must 
 serve to produce an 
 
 FlG ^ image of any object, 
 
 no matter at what 
 
 distance from the eye. This can be accomplished only 
 when the convexity of the lens can be changed at will, so 
 that at times it can serve as a moderately convex lens and 
 at other times as a strongly convex one. This changing 
 is made possible by a series of muscles. It is possible 
 to perceive the effort of the eye to change the convexity 
 of the lens, when objects at very different distances are 
 examined in succession. Near objects require a greater 
 convexity of the lens and distant objects a smaller con- 
 vexity. If the proper muscles are not able to give the 
 
 
RADIANT ENERGY, HEAT AND LIGHT 285 
 
 proper convexity to the lens in the eye, the image will 
 not appear where it is needed. The lens will form an 
 image in front or behind the retina. In this case the 
 image will be blurred or indistinct. If the lens cannot 
 be made sufficiently convex, convex spectacles must be 
 worn. If the lens be too convex, concave spectacles must 
 be worn to counteract this effect. Some persons need 
 spectacles only when they are reading. This means that 
 the eye can accommodate itself to all conditions, except- 
 ing when a very convex lens is necessary. 
 
 Immediately in front of the lens is suspended a circu- 
 lar curtain or diaphragm, called the iris, 1 1, which leaves 
 a circular opening, thepupil,P f in front of the lens. This 
 opening may be enlarged or diminished, so as to admit 
 the use of more or less of the lens, according to the 
 amount of light furnished by the object to be examined. 
 In a dark room the opening through the iris (the pupil) 
 is usually large. In very bright light, the opening is 
 much reduced in size. 
 
 267. The Sensation of Sight. The retina, upon which 
 the image falls, is a transparent layer lining the interior 
 
 of the eyeball back of 
 
 ^lj I 
 
 the crystalline lens. 
 The space between 
 the retina and the 
 lens and the space 
 between the lens and 
 the front of the eye- 
 ball is occupied by 
 transparent liquids. 
 only 
 
 FIG. 113. 
 
 Although the retina forms a layer 
 of an inch thick, it has a rather complicated 
 structure, so that, for convenience of description, it is 
 usually divided into ten layers (Fig. 113). These will be 
 
286 ELEMENTARY PHYSICS 
 
 found well described in the larger books on physiology. 
 For our purpose, it is sufficient to know that the nerves 
 of sight enter as a bundle (optic nerve) at the rear of the 
 eye (Fig. 112), directly opposite the lens, and, after pass- 
 ing through all the coats of the eye, spread out, so as to 
 reach every portion of the retina. 
 
 That part of the thickness of the retina which is far- 
 thest removed from the interior of the eye contains a layer 
 of bodies described as rods and cones (Fig. 113). The 
 vibrations set up in these rods and cones are believed to 
 be the cause of the sensation of sight. A number of facts 
 indicates this, the most interesting among which is the 
 following : The smallest star which can be seen in the sky 
 is one whose apparent diameter is so small that lines 
 drawn from the extremities ,of this diameter to the ob- 
 server on earth would form an angle of only 60 seconds. 
 The smallest distance which must exist between lines so 
 that they may still be recognized as separate lines is such 
 that lines connecting these lines with the eye give an 
 angle which varies from 64 to 73 seconds. The sizes of 
 the images formed upon the retina of the eye in these 
 cases have been calculated, and it has been determined 
 that the image of the star would have a diameter of .00017 
 of an inch, while the distance between the lines, in the 
 image, would vary between .00018 and .00021 of an inch. 
 Now the diameter of the rods and the cones varies be- 
 tween .00018 and .00022 of an inch. The exact character 
 of the connection between the rods and cones and the 
 nerves of sight has not been fully established. 
 
 268. Sensitiveness of the Eye to Different Colors of the 
 Spectrum. If the intensity of the different colors seen in 
 the spectrum produced by sunlight be examined, it will 
 be found that the greatest intensity of color is shown by 
 
RADIANT ENERGY, HEAT AND LIGHT 287 
 
 the tints nearest the yellow part of the spectrum. This 
 is due to the fact that the nerves of our eyes are more 
 sensitive to these tints than to any others. They are less 
 and less sensitive to the other tints as these recede either 
 toward the red or toward the violet end of the spec- 
 trum. Some experiments seem to show that the eyes are 
 more sensitive to the blue colors than to the red colors at 
 the extreme end of the red part of the spectrum. 
 
CHAPTER VII 
 MAGNETISM AND ELEOTEICITY 
 
 269. Natural Magnets. It was observed by the ancients 
 from the remotest antiquity that certain hard and very 
 heavy black stones possess the remarkable property of 
 attracting small pieces of iron. These stones were be- 
 lieved to possess a magical power, and are mentioned by 
 many ancient writers. Although they excited much won- 
 der, no practical use was made of them until about l^ie 
 twelfth century, when it was discovered that," if one of 
 these stones is suspended so as to be free to turn in any 
 horizontal direction, it always finally comes to rest in the 
 same position one end pointing- North, the other South. 
 
 It is now known that these stones are pieces of mag- 
 netic iron ore, and, where this ore occurs in abundance, it 
 is mined in order to secure the iron which is its chief con 
 stituent. Its chemical composition is Fe 3 O 4 . On ac- 
 count of its magnetic properties the ore is also called 
 magnetite. 
 
 270. Artificial Magnets. It is possible to give to a piece 
 of steel the same properties as those possessed by mag- 
 netite. If the same end of a piece of magnetite is rubbed 
 always in the same direction upon a knitting needle, the 
 needle will attract iron filings and small tacks, and, if sus- 
 pended by a thread, it will point approximately North 
 and South (Fig. 114). By means of powerful electric cur- 
 rents it is possible to give these properties even to large 
 
 288 
 
MAGNETISM AND ELECTRICITY 
 
 289 
 
 S. 
 
 FIG. 114. 
 
 bars of steel. With these steel bars much heavier weights 
 of iron or steel can be raised than can be lifted by any 
 piece of magnetic iron ore of the 
 same weight. The property of 
 attracting iron or steel and of as- 
 suming a North and South direc- 
 tion is known as magnetism. Bars 
 of steel possessing these proper- 
 ties are known as magnets. Pieces 
 of magnetite are sometimes re- 
 ferred to as natural magnets. 
 
 271. Soft Iron Readily Loses its Magnetism. It is pos- 
 sible to give magnetic properties even to an ordinary 
 piece of iron, but ordinary iron does not retain magnetism 
 wejl. Magnetize a piece of iron wire about a foot long and 
 one-eighth of an inch in diameter by rubbing it with a 
 strong steel magnet. Dip the end in iron filings and no- 
 tice the size of the cluster which adheres (Fig. 115, A). 
 Then bend or twist the wire roughly and dip the same 
 end into the filings. The magnetism is practically gone 
 (Fig. 115, B). Some kinds of iron can be bent or cut so 
 much more readily than others that they are called soft. 
 The softest kinds of iron lose their magnetism at the slight- 
 
 ^ = ^~~ == - ^ 6st jar as soon as 
 
 A j] the magnetizing 
 
 J| J| agency has been 
 
 removed. Steel is 
 the only material 
 which retains mag- 
 netism well , and is, 
 therefore, the only 
 substance used for permanent magnets, although even 
 steel loses its magnetism if roughly handled or heated. 
 
 FIG. 115. 
 
290 ELEMENTARY PHYSICS 
 
 272. Only Iron and Steel Can be Strongly Magnetized. 
 Iron and steel are the only substances which can be mag- 
 netized strongly, and they are also the only substances 
 which are strongly attracted by magnets. In the study 
 of the ordinary properties of magnetism, all other sub- 
 stances may be ignored. 
 
 273. Magnetic Transparency. All substances which are 
 not readily attracted by magnets permit magnetism to 
 act readily through them. They are therefore said to be 
 magnetically transparent. It is impossible to pick up 
 even a very small particle of iron by means of a piece of 
 glass, but, if a small pane of glass is held against the 
 lower end of a magnet, the force of the magnet acts 
 through the glass, and it is possible to pick up tacks 
 placed immediately beneath. The tacks are drawn up 
 against the glass, but the instant the magnet is removed 
 tke tacks drop. Almost all other substances are magnet- 
 ically transparent. Iron and steel are the only conspicu- 
 ous exceptions. Substances are less transparent mag- 
 netically in proportion as they give evidence of greater 
 magnetic properties. 
 
 274. Magnetic Poles. When magnets are dipped in fil- 
 
 ings or tacks it is found 
 that the filings or tacks 
 adhere in tufts, and that 
 the tufts are longest and 
 most abundant along 
 the edges near the ends 
 (Fig. 116). Magnetism is 
 FlG 116 therefore strongest near 
 
 the ends of magnets and 
 
 these ends are called poles. Every magnet has two poles. 
 
 Both poles can pick up the same quantity of tacks, never- 
 
MAGNETISM AND ELECTRICITY 291 
 
 theless they must be quite different from one another, 
 since one end always turns North and the other South. 
 
 That there is a difference between the poles of a mag-net 
 can be shown in another manner. Mark with chalk or 
 paint those ends of two magnets which 
 point North. Usually these ends are 
 already marked with the letter N. 
 Suspend one magnet horizontally by 
 means of a thread and hold near its 
 north-seeking- end the north-seeking 1 
 end of the other magnet. The two 
 poles actually repel one another (Fig. 
 117). This result would certainly not 
 be expected from anything previously 
 stated. Now hold the south-seeking 
 pole near the north-seeking pole of the 
 magnet. The two poles seem to attract each other. Re- 
 peat the experiment in as many forms as you may be able 
 to devise using other objects, such as the magnetic needle 
 in a compass, and the same result will always be found. 
 We may state these facts in the form of a rule as fol- 
 lows : Like poles repel each other ; unlike poles attract each 
 other. 
 
 275. The Earth a Magnet. This rule opens up another 
 question. Why do all magnets, when suspended, point 
 North and South ? This is a very practical question, 
 since the needle in every compass is merely a small mag- 
 net. A suspended magnet acts as though some south- 
 seeking pole at some northern part of the earth were 
 drawing the north-seeking pole of the magnet northward. 
 Or, as if some north-seeking pole situated at some south- 
 ern point on the earth's surface, were attracting the south- 
 seeking pole of the magnet southward. Now, when we 
 
292 
 
 ELEMENTAEY PHYSICS 
 
 examine the directions of the compass needles at all points 
 of the earth's surface, we find that in the northern hemi- 
 sphere their north-seeking- ends all point in a general way 
 to a point on the coast of North America, northwest of 
 Hudson Bay (Fig. 118). And in a general way in the 
 southern hemisphere the needles all point away from a 
 locality in the Antarctic Ocean. This locality does not 
 appear to be directly opposite the north magnetic pole 
 (Fig. 118, A). Therefore, there is apparently a south-seek- 
 ing pole near the North end of the earth which attracts 
 
 , v ^ca 
 
 A Pole 
 tic Pole 
 
 The Earth a Magnet. 
 FIG. 118. 
 
 the north-seeking pole of the compass needle and a 
 north-seeking pole at the opposite end of the earth 
 which attracts the south-seeking end of the needle. Ap- 
 parently, the earth itself is a magnet with its poles at 
 the North and South ends. Since only unlike poles 
 attract each other, the magnetism of the poles of the 
 earth and that of the attracted poles of the compass 
 needle must be exactly opposite. This may be shown 
 experimentally by placing a compass upon its side and 
 moving it back and forth over a horizontal magnet. 
 
MAGNETISM AND ELECTRICITY 293 
 
 276. The Compass The essential part of a compass is 
 the needle. This consists of a small magnetized bar of 
 steel supported on the sharp point of an upright pin by 
 means of a cap of brass, glass, or agate attached to its 
 centre. In this manner friction is reduced to a minimum 
 and the needle can be moved by the slightest attraction. 
 In the mariner's compass the apparatus is so arranged 
 that the needle will remain horizontal notwithstanding 
 the roll of the ship. 
 
 It was discovered by Columbus on his voyage to Amer- 
 ica, that the compass needle did not point to exactly the 
 same part of the sky while sailing across the Western 
 Atlantic as it did in Spain. At that time this could not 
 be explained, but now we know that the magnetic pole in 
 the Northern Hemisphere is not situated at the north 
 pole as marked in our geographies, but northwest of Hud- 
 son Bay on the peninsula called Boothia Felix. The first 
 pole is sometimes called the geographical pole, and the 
 second the magnetic pole. 
 
 By looking at a geography you see that the needle must 
 point west in Greenland, west of north in New England, 
 and east of north in San Francisco. It so happens that in 
 Ohio the needle points almost exactly north. The angle 
 between the direction of the compass needle and the true 
 North and South line is called the declination of the nee- 
 dle. The declination varies at different parts of the earth's 
 surface, and in order that a navigator may know in what 
 direction true North is located, he must have magnetic 
 charts which give the declination of the compass needle 
 at all points of the sea. Moreover, it is necessary to have 
 the most recent charts, since the magnetic poles of the 
 earth shift a short distance toward the East and West in 
 the course of a number of years. 
 
294 ELEMENTARY PHYSICS 
 
 277. Variation in the Direction of a Compass Needle 
 within the Field of a Magnet. If a compass be placed at 
 different points in the vicinity of a bar magnet, the direc- 
 tion of the needle will vary with the position of the com- 
 pass. The space within which the bar magnet can influ- 
 ence the direction of the needle of a compass may be called 
 the field of the magnet. In order to investigate the differ- 
 ent directions assumed by the needle within this field, 
 place the bar magnet on the centre of a large piece of pa- 
 per. Hold the compass at different points in the field, 
 and indicate in each case the direction of the needle by 
 means of an arrow drawn directly beneath it. The point 
 of this arrow should indicate the direction assumed by 
 that end of the needle which points North when not in the 
 vicinity of the bar magnet. 
 
 Continue the investigation until the entire field for a 
 distance of at least six inches from the magnet is covered 
 with arrows. It then becomes apparent that, although 
 the direction of the needle varies considerably within the 
 field of the magnet, this variation is in accordance with a 
 definite plan. The arrows around one end of the magnet 
 point away from the magnet, while those around the 
 other end point toward it. The arrows at intermediate 
 positions assume such directions that it is possible to 
 connect them by means of curved lines starting out from 
 one end of the magnet and finally entering the other. 
 This appearance is strengthened by continuing the pro- 
 duction of arrows until the paper is densely covered 
 with them. If long lines are used to indicate the direc- 
 tions of the needle, the arrows frequently cross one 
 another, but in proportion as the arrows are made 
 shorter, the appearance of curved lines leaving the one 
 pole of the magnet and entering the other is increased. 
 
MAGNETISM AND ELECTKICITY 295 
 
 If a pane of glass is placed on a bar magnet, and iron- 
 filings are sifted evenly over the surface, the soft iron-fil- 
 ings will temporarily become magnets. A gentle tapping 
 of the glass will 
 
 permit the filings F' ' , X 
 
 to arrange them- ;: V 
 selves in direc- IJH&.I,. 
 
 tions correspond- 
 ing to those 
 which would be 
 assumed under 
 the same condi- FIG 119 
 
 tions by minute 
 
 magnets (Fig. 119). In this case it becomes still more 
 apparent that the directions assumed by numerous mi- 
 nute magnetic objects under the influence of a large mag- 
 net coincide with curves which may be drawn leaving 
 one end of the magnet and entering at the other. These 
 curves are called lines of force. 
 
 278. Magnetic Lines of Force. It is not known precisely 
 why magnetic needles arrange themselves in accordance 
 with a definite plan within the field of a magnet. Within 
 recent years, however, a number of ideas have been steadily 
 gaming ground, and, at present, these ideas largely influ- 
 ence current views on this subject. 
 
 It may be noticed, in the first place, that the bar mag- 
 net influences the direction assumed by the compass nee- 
 dle placed within its field, although not in direct contact 
 with the needle. If the original magnetic force resides 
 in the bar magnet, this force must in some manner trav- 
 erse the air in order to reach the needle, and the force 
 must be conveyed by something, by some medium. Is this 
 medium the air ? This may be easily determined. If the 
 
296 ELEMENTAKY PHYSICS 
 
 bar magnet and compass needle be placed under a bell-jar 
 and the air exhausted, the action of the bar magnet upon 
 the needle will remain exactly the same, proving that air 
 is not the medium. 
 
 Since it is impossible to conceive of the action of one 
 object upon another without either direct contact or an 
 intermediate medium conveying the force indirectly, it is 
 necessary to search for some medium. This medium 
 which is present even in a space devoid of air is the 
 ether, whose presence has already been found necessary 
 to explain the various phenomena of heat and light. The 
 theory is somewhat as follows : 
 
 In some manner a magnet is able to influence the ether 
 in the air surrounding it. The space within which the 
 ether is affected is called HLQ field of the magnet. Within 
 this field the ether is strained, and the direction of the 
 strain is along curved lines. Small magnets are consid- 
 erably affected by the strain produced in the ether by 
 large magnets, and, if suspended or supported so as to 
 swing freely, are forced to assume positions parallel to 
 the direction of the strain. Another way of expressing 
 the same idea is to say that the needles take positions at 
 a tangent with the direction of the strain in the ether. 
 
 A compass needle may therefore be used to determine 
 the direction of the strain of the ether at various points 
 within the field of the magnet. As has already been 
 shown by the preceding experiment, the direction of 
 this strain is different in different parts of the field. The 
 continual change of direction in the strain of the ether 
 may be indicated by means of curved lines. These lines 
 are drawn so as to leave one pole of the magnet, curve 
 around through the field of the magnet, and enter at the 
 other pole. Lines drawn in this manner may be called 
 
MAGNETISM AND ELECTRICITY 297 
 
 lines of force. In reality they serve only to indicate the 
 direction in which the ether is believed to be strained. 
 There are no actual lines present in the ether. But draw- 
 ings representing- lines of force traversing the field around 
 magnets enable the student quickly to grasp the chief 
 properties of a magnetic field the direction of the strain 
 within the ether and the resultant direction assumed by 
 any small magnet placed within this field. 
 
 279. Direction Assumed by North-seeking 1 Pole of Mag- 
 netic Needle. The strain in the ether present in the air 
 surrounding the bar magnet determines not only the gen- 
 eral direction assumed by the compass needle, but deter- 
 mines also the direction in which the north-seeking pole of 
 the needle faces. In reality, the bar magnet does not affect 
 the north-seeking pole of the needle more than the south- 
 seeking pole, but it is convenient to determine the direc- 
 tion in which a needle faces by stating the direction in 
 which its north-seeking pole faces. If the lines of force 
 mapping the different directions of strain in the ether 
 be followed from the north-seeking pole of the bar mag- 
 net until they enter the south-seeking pole, the north- 
 seeking pole of the needle at every part of this entire 
 path will point forward, while the south-seeking end of 
 the needle points backward. 
 
 In order to facilitate the ready determination of the di- 
 rection assumed by the compass needle, it may be imag- 
 ined that the lines of force leave the north-seeking end of 
 the bar magnet and, after curving through the field, enter 
 at the south-seeking end. In this case the rule may be 
 given that the north-seeking ends of small movable mag- 
 nets placed within the field of a larger magnet are always- 
 urged to face in the direction in which the nearest lines 
 of force are passing. But it should be remembered 
 
298 ELEMENTARY PHYSICS 
 
 that actual lines of force do not exist. What we call lines 
 of force are merely useful drawing's by means of which we 
 record the changes in direction of the strain in the ether. 
 It will sometimes be found convenient to refer to lines of 
 force as though they really existed, but a failure to under- 
 stand their real nature as imaginary lines mapping the 
 direction of strain in the ether and not as lines having 
 actual existence will result only in a confusion of fact and 
 fancy during the study of the phenomena of magnetism. 
 
 280. Magnetic Induction. Place a pane of glass on a 
 soft -iron bar about 2J inches long. Sprinkle iron-filings 
 evenly over the surface and tap the glass. No change 
 occurs suggesting the presence of magnetism in the iron 
 bar. 
 
 Place the glass over the north-seeking end of a bar 
 magnet, sprinkle filings, and again tap the glass. The 
 filings now appear to be arranged in lines which radiate 
 from the north-seeking pole of the magnet, curve around 
 its sides and then enter from all directions at the south- 
 seeking pole. It is this appearance that first suggested 
 the idea of lines of force. The lines indicate in what di- 
 rection the ether in the field around the magnet is strained. 
 
 The ether within the body of the magnet is also strained. 
 Lines of force therefore should be drawn so as to indicate 
 both the direction of the strain of the ether in the field 
 around the magnet and also of the ether within the body 
 of the magnet. Lines of force drawn in this manner ap- 
 pear to traverse complete circuits (not circles). They 
 may be imagined to pass from the north- seeking pole 
 through the field toward the south-seeking pole, and 
 then to complete their circuits by passing through the 
 body of the magnet from the south-seeking pole toward 
 the north-seeking pole. An examination of the lines in- 
 
MAGNETISM AND ELECTRICITY 299 
 
 dicated by the iron-filings will show that poles are not 
 definite points at the ends of magnets, but rather indefinite 
 regions, near the ends, from which the lines appear to 
 radiate, or toward which they appear to converge. 
 
 Lift the pane of glass carefully, so as not to disarrange 
 the filings. Fold a piece of paper neatly over the north- 
 seeking end of the magnet, and press this end of the 
 magnet against a soft-iron bar. The paper prevents 
 actual contact between the magnet and the soft iron. 
 Then replace the glass, lowering it carefully so that the 
 appearance of lines of force radiating from the north- 
 seeking pole of the magnet is restored. Again tap the glass. 
 The filings rearrange themselves, and many lines of force 
 now appear to radiate from the sides and farther end of the 
 soft-iron bar as well as from the end of the magnet. This 
 indicates that the ether within and around the soft-iron 
 bar now is also strained. It should therefore show the 
 same magnetic properties as the magnet, although possi- 
 bly in a lesser degree. This may be readily demonstrated 
 by placing the farther end of the soft-iron bar in a pile of 
 tacks while the north-seeking pole of the bar magnet with 
 its paper cover is still held against the other end of 
 the bar. In this condition the soft-iron bar picks up 
 tacks. 
 
 As soon as the magnet is withdrawn from the soft- 
 iron bar, the tacks drop off. After the removal of the 
 magnet, cover the soft-iron bar with the pane of glass, 
 sprinkle iron-filings, and tap the glass. As might be ex- 
 pected from the dropping off of the tacks when the mag- 
 net is removed, no appearance of lines of force is pro- 
 duced around the soft iron when not in the immediate 
 vicinity of the magnet. 
 
 The magnet is said to induce magnetism into the soft- 
 
300 
 
 ELEMENTARY PHYSICS 
 
 iron bar. Since soft iron does not retain magnetism well, 
 the bar is only temporarily a magnet. It should scarcely 
 be necessary to state that the strain in the ether as shown 
 by the iron-filings is not due to the tapping of the glass. 
 The tapping merely assists the iron-filings in rearranging 
 themselves so as to make evident any magnetic strain 
 which may be present in the ether. 
 
 281. Magnetic Permeability of Soft Iron and Air. It 
 was seen in the preceding experiment that the filings re- 
 arranged themselves after the soft-iron bar had been 
 added to the magnet and the glass had been again tapped. 
 
 Hfl 
 
 FIG. 120. 
 
 This indicates that the presence of soft iron within 
 the field of a magnet changes the direction of the strain 
 of the ether within the field. This may also be shown by 
 other experiments. 
 
 Place a bar of soft iron a short distance in front of the 
 poles of a large horseshoe magnet. Cover bar and mag- 
 net with a pane of glass and place a second bar of soft iron 
 directly over the first. Sprinkle iron-filings evenly over the 
 surface and tap the glass. Notice to what extent the 
 lines formed by the iron-filings may be distinctly traced 
 beyond the soft-iron bars (Fig. 120). Now gently raise 
 
MAGNETISM AND ELECTRICITY 
 
 301 
 
 the glass, remove both soft-iron bars, and carefully re- 
 place the glass in its original position over the magnet. 
 Tap the glass again. The lines, formed by the filings, 
 passing from the north-seeking to the south-seeking pole, 
 may now be traced distinctly to a much greater distance 
 in front of the magnet (Fig. 121). This suggests that in 
 the first case the soft-iron bars change the direction of 
 the strain in the ether in the field in front of the magnet, 
 and also distinctly weaken the strength of that part of 
 the field which lies beyond the soft iron. Instead of a 
 horseshoe magnet, two bar magnets may be used. Place 
 
 . 121. 
 
 the magnets side by side, about 3 inches apart, with the 
 like poles pointing in opposite directions. Use one pair 
 of poles. 
 
 This principle has been found useful to a certain extent 
 in protecting the works of a watch from magnetism. If 
 the case of a watch be constructed of soft iron, the part of 
 the field enclosed within the case is much weaker and the 
 works are in a measure protected. The appearance of the 
 watch is of course improved by plating the case with 
 gold. 
 
 A study of the preceding experiments suggests not only 
 
302 ELEMENTARY PHYSICS 
 
 that soft iron alters the direction of the strain in the ether 
 in the field surrounding a magnet, but that it appears to 
 take up a large part of the strain in this part of the field, 
 leaving the ether beyond in a less strained condition. On 
 this account soft iron is said to be more permeable to mag- 
 netism than air. 
 
 282. Physical Theory of Magnetization. Magnetize a 
 straightened piece of a broken watch-spring, such as may 
 be obtained from any jeweler. Dip the entire piece un- 
 der iron-filings; on its removal iron -filings adhere to the 
 ends but not to the middle. This shows that for the 
 greater part of the length of the magnetized watch-spring 
 the strain of the ether lies chiefly within the body of the 
 spring and is practically parallel to its length. At the 
 ends of the spring where the filings adhere the direction 
 of this strain changes. When studied in connection with 
 iron-filings the appearance is as if lines of force pass out 
 and enter chiefly at the ends of the magnet. 
 
 With a wire nipper, cut the straightened spring at a 
 point where no filings adhere. On dipping in iron-filings, 
 filings are found to adhere at the cut end. The lines of 
 force now appear to come out at a part of the mag- 
 netized spring where a moment before no magnetism was 
 shown. This indicates a change in the direction of the 
 strain at the point where the spring was cut. Continue 
 the cutting, and even the smallest fragment of watch- 
 spring will show magnetism at each end. In each case 
 the part remaining appears to be a complete magnet, 
 but smaller and weaker. It is possible to conceive that 
 this cutting might be continued indefinitely and that if a 
 single molecule of iron could be secured, each end of this 
 molecule would show magnetic properties, one end being 
 north-seeking and the other end south-seeking. 
 
MAGNETISM AND ELECTRICITY 303 
 
 In accordance with this view a magnet consists of a 
 combination of many magnets, each molecule being* a 
 tiny magnet. If the north-seeking ends of several mag- 
 nets are held together, their combined effect is greater 
 than that of any one of the magnets. In the same man- 
 ner, many millions of molecules of iron with their north- 
 seeking poles all directed in the same direction ought to 
 show considerable magnetism (Fig. 122, A). If these 
 molecules became considerably disarranged, the north- 
 
 S 
 
 FIG. 122. 
 
 seeking poles and the south-seeking poles might coun- 
 teract each other (Fig. 122, B). This principle may be 
 illustrated by laying the north-seeking pole of one mag- 
 net upon the south-seeking pole of another and then 
 trying to lift up tacks while these ends are in contact 
 with one another. It will be found that in this condition 
 the magnets will lift up less tacks than if either magnet 
 had been used alone. In fact, it frequently happens that 
 when the north-seeking and south-seeking poles are used 
 together, scarcely any tacks can be lifted. 
 
304 
 
 ELEMENTARY PHYSICS 
 
 According- to this theory, the magnetizing- of an iron or 
 steel bar consists simply in drawing the corresponding 
 ends of all the molecules in the same direction by strok- 
 ing the bars of which they are. a part by means of some 
 strong magnet, always in the same direction. The like 
 poles of the molecules are thus caused to face in the 
 same direction. In this position they act together as a 
 unit. Twisting the magnetized bar, pounding it, or heat- 
 ing it, considerably disarranges the molecules. The 
 poles of the molecules now face in so many directions 
 that they counteract each other, and the bar no longer 
 shows pronounced magnetic effects. The bar is said to 
 be demagnetized. 
 
 STATIC ELECTRICITY 
 
 283. Electrification by Rubbing. Suspend a pith ball, a 
 quarter of an inch in diameter, at the end of fine silk 
 
 thread about ten inches long. Hold the 
 end of an ebonite rod near the pith ball. 
 The pith ball is not affected in any man- 
 ner by the presence of the ebonite rod. 
 Now rub the end of the ebonite rod with 
 flannel and bring it near the pith ball. 
 The pith ball is at first attracted, but, 
 after contact has been broken, it is re- 
 pelled (Fig. 123). Eubbing has 
 given the ebonite rod some 
 properties which it did not pos- 
 sess before. The rod is said to 
 be electrified. 
 
 284. Electrification by Contact with an Electrified Body. 
 Suspend two pith balls at the same level by means of 
 
 FIG. 123. 
 
MAGNETISM AND ELECTRICITY 305 
 
 two silk threads. Bring- the upper ends of the threads 
 in contact with each other. The pith balls strike against 
 one another and show no unusual properties. Now bring 
 a strongly electrified ebonite rod into contact with the 
 pith balls. Withdraw the rod and again hold together 
 the upper ends of the silk threads. The pith balls re- 
 main apart, showing that they repel each other. The 
 pith balls, in this case, have become electrified, not by 
 rubbing, but by contact with a body which was elec- 
 trified. In this condition, the pith ball is often said to 
 be charged with electricity, and the experiment indicates 
 that two bodies electrified in t/ie same manner repel each 
 other. 
 
 285. Conductors. Suspend two pith balls from a glass 
 support by silk threads which are tied together at the 
 top, thus bringing the pith balls at the lower ends of the 
 threads in contact with one another. If the silk threads 
 are touched at their upper ends with a strongly electri- 
 fied ebonite rod, the pith balls remain in contact. If the 
 experiment be repeated with balls suspended by cotton 
 thread, the pith balls repel each other strongly. This 
 indicates that cotton conducts electricity fairly well from 
 the electrified ebonite rod to the pith balls, but silk is 
 evidently a very poor conductor. 
 
 286. The Electroscope. A pair of pith balls suspended 
 at the lower ends of cotton threads, tied together at the 
 top, may be used to detect the presence of electricity 
 in a body. The mutual repulsion of the pith balls on 
 bringing any object in contact with the cotton threads at 
 once indicates that the object is electrified. There are, 
 however, much better conductors than cotton, and much 
 lighter bodies than cotton thread and pith balls. By 
 means of these bodies it is possible to detect much 
 
306 
 
 ELEMENTARY PHYSICS 
 
 lighter charges of electricity than by means of the thread 
 and pith balls. All metals are better conductors of elec- 
 tricity than cotton. Several of these metals can be se- 
 cured in the form of exceedingly thin sheets. Tin, alum- 
 inum, and gold foil are examples of metals in this form. 
 When sufficiently thin, sheets of these metals are so light 
 that they move at the slightest breath of air. If two thin 
 strips of foil are fastened at the lower end of a metal 
 rod, and an electrified body is brought into contact 
 with the supporting rod, the electricity spreads readily 
 throughout the rod and a part 
 
 passes into the two .^ 'A strips of 
 
 foil which at once // .^^ " V repel each 
 
 other. The 
 strips of 
 
 foil are so light, that bodies, which are so 
 weakly electrified that contact with pith 
 balls produces no perceptible repulsion, 
 will, when brought in contact with the 
 supporting rod, cause the strips of foil to 
 repel each other violently. 
 
 This principle is used in the construc- 
 tion of an instrument for the detection and 
 study of charges of electricity, called the 
 electroscope (Fig. 124). SLis consists of a 
 brass rod with a knob "the top, passing 
 through a rubber stopper into a flask. 
 The lower end is broadened and serves as 
 the place of attachment of two thin strips 
 FIG. 124. of gold leaf If an electrified ebonite rod 
 is brought in contact with the knob, the electricity 
 passes down the brass rod into the gold leaves, and 
 consequently the gold leaves repel each other. 
 
MAGNETISM AND ELECTRICITY 
 
 307 
 
 287. Electricity Passes from Points of Strong Electrifica- 
 tion to Points of Less or No Electrification. If a slightly 
 electrified ebonite rod is brought in contact with the 
 electroscope, the gold leaves diverge slightly. If a 
 more strongly electrified rod is used, the gold leaves 
 diverge more. If an electroscope with strongly diverg- 
 ing gold leaves is brought in contact with an unelectri- 
 fied electroscope, by means of a long copper wire, the 
 leaves of the second electroscope diverge. If by the 
 
 FIG. 125. 
 
 same means a strongly electrified electroscope is brought 
 in contact with a moderately electrified electroscope, the 
 strongly diverging leaves of the first electroscope col- 
 lapse partly, while those in the second instrument di- 
 verge more (Fig. 125). 
 
 In performing theso experiments, the copper wire must 
 not be allowed to come in contact with the table, the 
 hand, or any other material which will conduct electric- 
 ity. It may be handled most readily by twisting the 
 wire around glass rods which may be held in the hand. 
 
308 ELEMENTAEY PHYSICS 
 
 These experiments show that electricity passes from 
 points of strong electrification to points of less or even 
 of no electrification. Whatever may be the true nature of 
 electricity, this transfer of electricity must be due to 
 some force or to a relative inequality of forces residing 
 in different objects. In the last experiment both the 
 electricity in the strongly electrified electroscope and 
 that in the less electrified instrument tend to move away 
 from these objects to points of still weaker electrification. 
 However, when the two are brought in contact with one 
 another by means of the copper wire, the electricity ap- 
 pears to move from the more electrified to the less elec- 
 trified body, and the force with which it moves must 
 unquestionably depend upon the difference of the two 
 forces urging the electricity in the two electroscopes 
 toward some other point. 
 
 288. Potential, Electromotive Force. The force with 
 which the electricity in each electroscope tends to move 
 to some other point, is called the potential of the electric- 
 ity in that instrument. The force with which a part of 
 the electricity in the more highly charged instrument is 
 urged to move from this electroscope to the one less 
 strongly charged, is due to the difference of potential be- 
 tween these instruments. The force with which a part of 
 the electricity moves in consequence of this difference of 
 potential is often spoken of as though it were a single 
 separate force, instead of an effect due to the difference in 
 strength of two forces, and is then called the electricity 
 moving force, or the electromotive force. 
 
 289. Electrification Caused by Contact of Two Different 
 Substances. In the preceding experiments, ebonite was 
 electrified by rubbing it with flannel. Many other bod- 
 ies may be electrified in a similar manner. Glass is fre- 
 
MAGNETISM AND ELECTRICITY 309 
 
 quently electrified by rubbing it with silk. In fact, the 
 rubbing together of any unlike substances at once results 
 in electrification. Bub the coat sleeve vigorously over a 
 piece of paper and place the paper against the wall. It 
 adheres to the wall, showing that it has become electri- 
 fied. Rub another piece over the varnished surface of a 
 school-room desk, and the paper will show less tendency 
 to slide down the inclined surface than it exhibited be- 
 fore it was rubbed. From this it might appear that rub- 
 bing is essential to the electrification of objects. 
 
 It is becoming evident, however, from more recent 
 observations, that the essential element in electrification 
 is mere contact between unlike bodies. Mere contact 
 between ebonite and flannel produces a state of electrifi- 
 cation, but, since both ebonite and flannel are very poor 
 conductors of electricity, it is difficult for electricity to 
 spread from the points touched to other parts, and the 
 state of electrification is therefore practically confined 
 to points of actual contact. Rubbing brings many more 
 points on the ebonite rod in actual contact with many 
 parts of the flannel, and hence the degree of electrifica- 
 tion of the rod is very much increased by rubbing. 
 
 Moreover, since ebonite is a non-conductor, only the 
 end rubbed shows evidence of electrification ; but if 
 one end of a good conductor, such as a brass rod, is 
 rubbed, the entire rod gives evidence of electrification. 
 In order to prevent the escape of electricity through the 
 hand, the brass rod must, of course, be held by means of 
 some non-conducting material for instance, at the end of 
 a glass rod, or by means of a piece of silk. 
 
310 ELEMENTARY PHYSICS 
 
 CURRENT ELECTRICITY 
 
 290. A Continuous Current of Electricity Maintained by 
 Chemical Action. Whenever any solid is brought in con- 
 tact with any liquid, for instance, by dipping- the solid 
 in the liquid, the solid is electrified. The evidence of 
 electrification may be made especially clear if both solid 
 and liquid are good conductors. When any two solid 
 conductors are dipped rnto the same liquid conductor, 
 they will, as a rule, be found to differ in their degree of 
 electrification. Hence, the charges of electricity in the 
 two solids will be at a different potential, or will differ in 
 the force with which they tend to move to other points. 
 In consequence of this difference of potential, the elec- 
 tricity tends to move from the point of higher potential 
 to that at lower potential, as soon as the solid conductors 
 are joined by means of wire which is also a conductor. 
 The result is a current of electricity. 
 
 It does not differ essentially from the current of elec- 
 tricity produced by two electroscopes electrified to dif- 
 ferent potentials, connected by means of a conducting 
 wire. 
 
 When a strongly electrified electroscope is connected 
 with a weakly electrified electroscope, the current of elec- 
 tricity lasts only an instant while the potentials in the 
 two electroscopes are being equalized. This takes place 
 so quickly that the current is only momentary. When, 
 on the contrary, two different solid conductors are 
 dipped in a liquid conductor which attacks at least one 
 of them chemically, the chemical action keeps up a 
 difference of potential in the two solids, even after they 
 have been connected a long time by the wire. It may 
 be that chemical action results in a continual renewal 
 
MAGNETISM AND ELECTRICITY 
 
 311 
 
 of contact between the solids and the molecules of the 
 liquid, so that the inequality of the charges of elec- 
 tricity in the solids is renewed as rapidly as these 
 charges are equalized by the connecting- wire. In other 
 words, owing to chemical action, there is a continuous 
 current of electricity through the wire. 
 
 291. The Electric Cell. When copper and zinc are 
 dipped into dilute sulphuric acid, both copper and zinc 
 are electrified ; the potential of copper, 
 where in contact with the air, is higher 
 than the potential of that part of the zinc ' 
 which extends up into the air. Hence 
 a current of electricity flows from the 
 copper through the wire to the zinc, as 
 soon as these metals are connected by 
 means of wire (Fig. 126). 
 
 The zinc is attacked by the sulphuric 
 acid. It continues to be attacked even 
 after the flow of electricity through tho 
 connecting wire has been started. In 
 consequence of this continued chem- 
 ical action, the difference of potential 
 between the copper and zinc is kept up, and the flow of 
 electricity continues. The zinc gradually wears away 
 and must be replaced from time to time. The sulphuric 
 acid solution must also be replenished. 
 
 Instead of copper, carbon is frequently used, and in- 
 stead of sulphuric acid, sal ammoniac, copper sulphate, 
 or a mixture of sulphuric acid and potassium bichro- 
 mate is often employed. The combination of copper and 
 zinc, or of copper and carbon plates with the chemical 
 solution, including the jar containing the solution, is 
 usually called an electric cell or battery. 
 
 FIG. 126. 
 
312 
 
 ELEMENTARY PHYSICS 
 
 The sal-ammoniac cell is the cell most frequently used 
 for electric bells and buzzers. It consists of zinc and 
 carbon plates, rods, or cylinders dipped into a solution of 
 sal ammoniac. 
 
 In the cell commonly used for telegraphic instruments, 
 the gravity cell (Fig. 127), the metals employed are zinc and 
 copper. The copper, in the form of thin sheets, is placed 
 in the bottom of the cell, and the zinc is suspended in the 
 upper part of the liquid. When the zinc is cast into the 
 form of diverging bars, it slightly resembles the spread- 
 ing toes of some birds, and in that case the gravity cell 
 is often called a crowfoot battery. Crystals of copper 
 sulphate are thrown into the water and dissolve. After 
 the cell has been used a short time, zinc sulphate is 
 formed. The solution of zinc sulphate is lighter than 
 that of copper sulphate, and therefore rises to the top of 
 
 the cell. The contrast be- 
 tween the colorless zinc sul- 
 phate solution and the blue 
 copper sulphate solution be- 
 neath is very distinct in a cell 
 which has been in use for 
 several days. The zinc sul- 
 phate is in contact with the 
 zinc plate. The copper sul- 
 phate is in contact with the 
 sheets of copper. The chem- 
 ical action is complicated. 
 The zinc wears away. The 
 copper in the copper sulphate 
 becomes separated from the 
 
 copper sulphate and settles on the copper plate. The 
 copper plate, therefore, increases in thickness, but the 
 
MAGNETISM AND ELECTRICITY 313 
 
 copper sulphate in the solution must be replenished 
 from time to time by the addition of more copper sul- 
 phate crystals. 
 
 The potassium bichromate cell consists of carbon and 
 zinc plates placed in a solution consisting- of sulphuric 
 acid, potassium bichromate, and water. 
 
 The plate having the higher potential, usually copper 
 or carbon, is called the positive plate. The plate having 
 the lower potential, almost invariably zinc, is called the 
 negative plate. For the purpose of connecting wires to 
 these plates, binding posts are added. The current 
 starts 011 that part of the copper or carbon plate which 
 is out of the liquid, passes through the wire to the 
 zinc plate, and then returns through the liquid to the 
 copper or carbon plate. 
 
 ELECTROMAGNETISM 
 
 292. Compass Needle Affected by a Wire Bearing an Elec- 
 tric Current. Connect the carbon plates of four potas- 
 sium bichromate cells to one another by means of wires. 
 Connect in the same manner the four zinc plates. Any 
 wire connecting one of these carbon plates with one of 
 the zinc plates will now conduct the electricity from all 
 of the carbon plates to the zinc plates. The four cells 
 now act like one huge cell. Cells arranged in this man- 
 ner are said to be connected in parallel. Use a connect- 
 ing wire about five feet long, and fasten a length of 
 about one foot in such a manner that the current will 
 pass vertically upward. Bend the remainder of the wire 
 so as to keep it as far distant from this length as pos- 
 sible. 
 
 The plates of potassium bichromate cells are usually 
 
314 
 
 ELEMENTARY PHYSICS 
 
 so arranged that all the plates can be raised at the same 
 time from the jar (Fig. 128). The plates are raised to pre- 
 vent the liquid from attacking the zinc plates when the 
 cells are not in use. While in this condition no current 
 can flow through the wire. Hold a small compass, not 
 more than an inch in diameter, on various sides of the 
 
 FIG. 128. 
 
 vertical part of the wire. In all positions of the com- 
 pass the needle points northward. Lower the plates 
 into the liquid and the needle now points southward on 
 the west side of the wire, eastward on the south side, 
 northward on the west side and westward on the north 
 side. If intermediate points be tried, the needle will be 
 found to point in intermediate directions. In conse- 
 
MAGNETISM AND ELECTRICITY 315 
 
 quence of the electric current some force evidently urges 
 the needle of the compass to occupy positions at variance 
 with its usual position when held at a distance from 
 objects conducting- electric currents. 
 
 293. A Magnetic Field Encircles a Wire Bearing an Elec- 
 tric Current. It has already been shown that bar magnets 
 are able to influence the direction of the compass needle 
 ( 277). It is believed that this influence is due to a 
 strain in the ether in the field surrounding the magnet. 
 The direction of strain of the ether in a magnetic field 
 may be indicated by lines called lines of force. In the 
 field of a magnet the strain is represented by lines leav- 
 ing in all directions at one pole, curving around through 
 the air, entering from all sides at the other pole, and trav- 
 ersing the length of the magnet, thus forming complete 
 circuits. 
 
 The experiment with the compass needle held near a 
 wire carrying a current of electricity ( 292) suggests that 
 the ether is strained also in the field surrounding a wire 
 while an electric current is passing. The different posi- 
 tions occupied by the compass needle indicate that the 
 lines of force representing this strain must be drawn in 
 the form of circles surrounding the 
 wire. This may be shown also by 
 means of iron-filings, sprinkled on a 
 sheet of card-board. If a wire is passed 
 vertically through the card-board and a 
 heavy current is turned on, a slight 
 jarring of the card-board causes the 
 filings to arrange themselves in circles FlG . 129. 
 
 surrounding the wire (Fig. 129). No 
 part of the path of these lines of force passes through 
 the wire ; nothing corresponding to poles exists, and 
 
816 ELEMENTAEY PHYSICS 
 
 hence there is no radiation of lines of force as at the 
 poles of magnets. If a very small compass needle is held 
 at various distances from the wire, it becomes evident 
 that the magnetic influence of the current is greatest in 
 the immediate vicinity of the wire, and becomes rapidly 
 less as the distance from the wire increases. This may 
 be represented by diminishing the number of lines of 
 force as the distance from the wire increases. 
 
 The direction assumed by a compass needle in the field 
 of a magnet may be determined mechanically by the rule 
 that the needle points forward at every part of any line of 
 force if followed from the north-seeking to the south- 
 seeking pole. 
 
 There is also a convenient mechanical rule for deter- 
 mining the direction assumed by a needle placed in the 
 magnetic field of a wire carrying a current. If the wire 
 is grasped by the right hand so that the fingers encircle 
 the wire and so that the thumb points in the direction 
 in which the current is flowing, the tips of the fingers will 
 point in the direction in which the north-seeking pole of 
 the needle will face if placed on the same side of the 
 wire. From this thumb and finger rule it is at once evi- 
 dent that if a current is sent downward through a vertical 
 wire, the lines of force must travel around the wire in a 
 direction exactly opposite to that followed when the cur- 
 rent is sent upward through the same wire. Hence all 
 the positions of the compass will be reversed. Show 
 this by actual experiment. 
 
 294. Direction of Current Determined by Means of the 
 Compass. If a wire is held in a horizontal position directly 
 above a needle so that the current passes northward over 
 the needle, the thumb and finger rule given in the pre- 
 ceding paragraph indicates that the north-seeking end of 
 
MAGNETISM AND ELECTRICITY 
 
 31 
 
 the needle must turn westward. If the current is sent 
 southward over the needle, the north-seeking end of the 
 needle turns eastward. 
 
 Determine in what direction the needle will point if you 
 send the current northward under the needle, or if you 
 send it southward beneath the same. In what direction 
 do the lines of force pass 011 the side of the wire when the 
 current passes northward? When the current passes 
 southward ? 
 
 295, A Loop through which an Electric Current is Passing 
 Behaves like a Magnet. Bend the wire connected with the 
 potassium bichromate cells 
 in such a manner that a part 
 of it forms a single horizon- 
 tal loop, and so that the cur- 
 rent goes westward along the 
 northern curve of the loop 
 (Fig. 130). Making use of 
 the thumb and finger rule, 
 and representing the direc- 
 tion of the strain in the ether 
 around the wire by means of lines of force, it will be 
 seen that the lines of force come upward at all points 
 along the interior of the loop, curve around the top of 
 the wire, pass downward along the outer part of the loop, 
 and re-enter at the lower face. In this form of represen- 
 tation the lines of force leave the upper face of the loop 
 and radiate in all directions just as they leave the north- 
 seeking pole of a magnet. At the lower face of the loop 
 the lines of force re-enter, as is true also of the lines of 
 force at the south-seeking end of the magnet. In both 
 cases the lines of force indicate merely the direction of 
 the strain in the ether, and the direction toward which 
 
 FIG. 130. 
 
318 
 
 ELEMENTARY PHYSICS 
 
 the north-seeking- end of the compass needle within the 
 field points. Therefore the character of the strain of the 
 ether on the upper face of the loop is similar to the strain 
 at the north-seeking- pole of a magnet. From this it 
 might be expected that the upper face of the loop ought 
 
 to possess magnetic prop- 
 erties similar to the north - 
 seeking pole of a magnet, 
 while the lower face should 
 possess properties similar 
 to those of a south-seeking 
 pole. 
 
 If the current of electrici- 
 ty is sent in an opposite di- 
 rection around the loop, or 
 if the wire is coiled so that 
 the current passes in an op- 
 posite direction, the upper 
 face of the loop becomes 
 south-seeking, and the low- 
 er north-seeking. 
 
 296. The Electromagnet. 
 Bend a wire so as to pro- 
 duce a succession of hori- 
 zontal loops forming a long, 
 close coil. If the ends of 
 the wire be connected with 
 the potassium bichromate 
 
 cells so that the current goes westward along the north- 
 ern curve of the loops, the upper face of each loop will 
 tend to be north-seeking while the lower face will be 
 south -seeking (Fig. 131). The combined effect of the 
 different loops is such as to cause a considerable in- 
 
 FlG. 131. 
 
MAGNETISM AND ELECTRICITY 319 
 
 crease in the strain of the ether, especially along* the in- 
 terior of the coil. This is represented diagrainmatically 
 by drawing- the lines of force in such a manner that many 
 of the lines of force pass from loop to loop lengthwise 
 through the centre of the coil, thus increasing the total 
 number of the lines of force passing- upward throug-h the 
 centre of the individual loops. 
 
 We have already learned that soft iron is much more 
 permeable to magnetism than air. The ether within the 
 soft iron appears to be strained more readily than the 
 ether in the surrounding air. The result is that, if a bar 
 of soft iron is placed lengthwise along the centre of the 
 coil, the strain of the ether along the interior of the coil 
 is much increased, and the magnetic effects exhibited at 
 the ends where the soft iron projects are also greatly 
 strengthened. This may be represented diagrammatical- 
 ly by increasing the number of lines of force traversing 
 the length of the coil when soft iron is placed along its 
 interior. The combination of the coil with the soft- 
 iron core shows, therefore, far greater magnetic proper- 
 ties at the end of the coil than when the coil is used 
 alone. 
 
 The end of the coil or of the soft iron within the coil 
 from which the lines of force are represented as coming 
 out, acts like a north-seeking pole of a bar magnet. The 
 opposite end at which the lines of force are represented 
 as coming in, acts like a south-seeking pole. 
 
 A modification of the thumb and finger rule will enable 
 any one quickly to determine the magnetic properties of 
 the poles of any coil through which a current is passing. 
 Place the thumb along one of the loops of the coil in such 
 a manner as to point in the direction in which the cur- 
 rent is passing. Imagine that the fingers grasp the wire 
 
320 ELEMENTARY PHYSICS 
 
 of this loop, but that the tips of the fingers lie within the 
 centre of the coil. The end of the coil toward which the 
 finger tips would point is the north-seeking end of the 
 coil. This may be easily tested by means of a compass 
 needle, whose north-seeking pole should be repelled by 
 this end of the coil as long as a current of electricity is 
 passing. 
 
 It should be noted that the strain in the ether in the 
 field around a wire is dependent entirely upon the pres- 
 ence of a current in the wire. When there is no current, 
 the ether is not strained. The coil containing the soft 
 iron is therefore a magnet only while a current of elec- 
 tricity is passing through the coil. For this reason the 
 combination of the coil with its soft-iron core is called an 
 electromagnet. 
 
 If the coil consists of many loops and if a sufficient 
 current of electricity is passed through the coil, an elec- 
 tromagnet can be made much more powerfully magnetic 
 than any permanent steel magnet of the same weight. 
 Electromagnets are employed for many instruments 
 (electric bells, telegraph instruments) in which it is nec- 
 essary that the magnets employed should be magnetic 
 only temporarily. In these cases, permanent magnets 
 would be useless. 
 
 Electromagnets are usually made in the form of two 
 short coils, traversed lengthwise by pieces of so'ft iron. 
 These pieces of soft iron are connected at one end by a 
 transverse piece, thus having a slight resemblance to a 
 permanent horseshoe magnet. The object to be at- 
 tracted usually consists of a fourth piece of soft iron held 
 a short distance away from the poles of the electromag- 
 net by a spring. This movable piece of soft iron is 
 called the armature. Most of the lines of force from the 
 
MAGNETISM AND ELECTRICITY 
 
 321 
 
 north-seeking pole of the electromagnet pass through 
 the air to the nearest part of the armature, through the 
 armature, thence from the other end of the armature 
 through the air to the south-seeking pole of the electro- 
 magnet, and then return through the soft iron to the 
 north-seeking pole. 
 
 As soon as a current of electricity is passed through 
 the coils of the electromagnet, the armature is attracted. 
 The instant the current of electricity is broken, the elec- 
 
 . 132. 
 
 tromagnet loses its magnetism and the spring attached 
 to the armature draws the armature away. 
 
 297. The Sounder Used in Telegraphy, In the sounder, 
 an instrument used in telegraphy (Fig. 132), the poles, 
 N S, of the electromagnet face upward and the arma- 
 ture, A, attached to a short horizontal brass tube, is 
 placed directly over the poles. Ordinarily the armature 
 is kept at a short distance from the electromagnet by 
 a spring attached to the farther end of the brass tube. 
 As soon as a current of electricity is passed through the 
 electromagnet, the armature is drawn down and the end 
 
ELEMENTAEY PHYSICS 
 
 of the brass tube to which the armature is attached strikes 
 against a vertical post, P, and the instant the current of 
 electricity ceases, this end of the brass tube is drawn up 
 by the spring at the opposite end already mentioned, 
 and strikes against the lower side of a brass arch, C. 
 The clicks against the vertical pillar and those against 
 the arch differ so much in the character of the sound that 
 they may be easily distinguished. By means of these 
 sounds, it may be readily determined how often and for 
 what length of time the current of electricity has been 
 passed through the electromagnet. 
 
 In telegraphy, a current of electricity sent through the 
 electromagnet for a short time is represented by a dot. 
 One sent through for a longer time is represented by a 
 dash. And one sent through for a still longer length of 
 time is represented by a dash twice or three times as 
 long. The various letters of the alphabet, the Arabic 
 numerals, and the signs used in punctuation, are repre- 
 sented by a combination of such dots and dashes. Any 
 one expert enough to recognize from the various clicks 
 how often and for what length of time the current of 
 electricity was sent through the electromagnet of the 
 sounder, can easily determine what letters, figures, 
 and signs it is desired to convey by means of the tele- 
 graph. 
 
 The following table represents the system in ordinary 
 use : 
 
 B 
 
 * 
 
 j 
 
 A ,.f. 
 
 W 
 
MAGNETISM AND ELECTRICITY 
 
 323 
 
 > 
 6 
 
 NUMERALS. 
 3 
 
 8 
 
 4 
 
 i 
 
 9 
 
 Period, 
 
 '- m 
 
 PUNCTUATION. 
 Comma, Semi-colon. 
 
 Quotation. 
 
 - J* - mmm 
 
 ^aj^ejntjiesis^ Interrogation. Colon. Paragraph, 
 
 298. The Relay Used in Telegraphy. When the distance 
 to be telegraphed is very great, the resistance of the tele- 
 graph wire to the passage of the current of electricity 
 may be so great that the electromagnet of the sounder 
 
 does not become magnetic enough to pull down the arma- 
 ture in opposition to the action of the spring which 
 tends to hold the armature up. In that case another in- 
 strument is used called the relay (Fig. 133). This instru- 
 ment is essentially like the sounder. The electromagnet 
 
324 ELEMENTARY PHYSICS 
 
 of the relay is, however, fastened in a horizontal position. 
 The armature, A, is a thin vertical piece of soft iron sup- 
 ported by a horizontal brass bar in such a manner as to 
 be readily movable. Ordinarily a very light spring draws 
 the armature a slight distance away from the electromag- 
 net. However, as soon as a current of electricity passes 
 through the electromagnet, the armature is drawn over 
 towards the magnet, N S. 
 
 Through the upper end of the armature of the relay 
 passes a small platinum peg, B. This peg during its for- 
 ward and backward motions strikes against the tips of 
 two horizontal screws, C D, passing through the upper 
 end of a brass post bent at the top into the form of an 
 arch, P. The clicks against these screws, although faint, 
 may be used for purposes of telegraphy just as were the 
 much louder sounds produced by the horizontal brass 
 tube in the case of the sounder. In order to make the 
 electromagnet of the relay as strong as possible, not- 
 withstanding the weak currents passing through the 
 wires, the coils of the electromagnet are made of many 
 turns of fine wire. By using fine wire it is possible to 
 get many more turns of wire on the same electromagnet 
 and the magnetic strength of the electromagnet is much 
 increased. 
 
 In actual practice the clicks of the armature in case of 
 the relay are never used for purposes of telegraphy. But 
 when the armature of the relay touches the screw, D, on 
 the side nearest the electromagnet it completes the path 
 for a second current of electricity which passes through 
 a second circuit whose total length is usually only a few 
 feet. In this second circuit, by means of the connections 
 at E and F, are placed an ordinary sounder and also an 
 electric cell, so that whenever a current of electricity 
 
MAGNETISM AND ELECTRICITY 325 
 
 passes through the electromagnet of the relay, the touch- 
 ing of the platinum peg in the armature against the metal 
 tipped screw permits a current to pass from the cell 
 through the sounder. When no current passes through 
 the electromagnet of the relay, the armature, A, is not 
 attracted : It may then be drawn away from the electro- 
 magnet by the weak spring. This stops also the cur- 
 rent in second or local circuit. Even if the platinum 
 tip, B, strikes against the screw, C, no current can flow 
 since the tip of this screw is made of ebonite, a non-con- 
 
 ductor ( 317) of electricity. On account of the short 
 length of the second circuit, the current of this circuit is 
 much stronger. The sound produced by the sounder is 
 much louder than that produced by the relay, and the 
 sounder is the only instrument to which the telegrapher 
 pays any attention. 
 
 299. The Telegraph Key. In order to start and to stop 
 the current of electricity quickly whenever desired, a 
 telegraph key (Fig. 134) is used. This is so constructed 
 that when the key is pressed down there is no break in 
 the path of the electricity. But when the finger is raised, 
 
326 ELEMENTAEY PHYSICS 
 
 the key is thrown up by a spring, S, there is a break in 
 the circuit at B, and the current ceases to flow. 
 
 300. The Earth Forms a Part of the Circuit for the Elec- 
 tric Current Used in Telegraphy. A single wire may con- 
 nect all the stations for a distance of several hundred 
 miles. In this case, the ends of the wires at the extreme 
 stations are connected with metal plates buried so deep 
 in the ground that they are always moist (Fig. 135). The 
 electricity then passes from the electric cells to the ex- 
 treme end of the wire into the ground and then returns 
 through the moist earth by way of the plate at the op- 
 posite end of the line. That part of the circuit which is 
 formed by the earth is usually known as the ground cir- 
 cuit. 
 
 301. A Message Can Be Heard at all Stations on the 
 Same Line. Since all the relays at all of the stations are 
 connected by the same wire (Fig. 135), when one sounder 
 is heard all the sounders at the other stations on the 
 same line are also in operation. Any message heard at 
 one station may therefore also be heard at all of the 
 others. Unless called for by his particular number, 
 the telegrapher need pay no attention to the message 
 sent. 
 
 302. Arrangement of Electric Cells for Telegraphic Pur- 
 poses. When it is desired to send a current of electricity 
 through a long wire or through anything which offers a 
 great resistance to the passage of the current, the cells 
 are arranged in series. In this case the zinc in each cell 
 is connected with the copper or carbon of the next cell in 
 such a manner that the current of electricity must pass 
 from cell to cell before it can get out into the main wire 
 (Fig. 135, explanatory drawings). When it is desired to 
 produce a strong current through a short wire, or through 
 
MAGNETISM AND ELECTRICITY 327 
 
 Ground Plate 
 
 Gro.und Plate 
 
 Zinc plate 
 
 Carbon 01 copper plate 
 
 | I ^sjji-g^e dentate cell 
 
 Four cells arrangedjn56rie5 
 
 | I | | l|| Also /our cells arrangetLiii series 
 Heetromagnet of Sounder and Relay 
 
 ure of Sound'er and .Relay with Spring 
 
 j ^ Armat 
 
 *^f Key whichjnay he closed "by side lever 
 
 FIG. 135., 
 
328 
 
 ELEMENTARY PHYSICS 
 
 anything offering- little resistance, the parallel arrange- 
 ment of cells is used (Fig. 128). For purposes of teleg- 
 raphy a number of gravity cells arranged in series is 
 commonly employed. 
 
 303. The Electric Bell, For ringing an electric bell any 
 kind of sal ammoniac cell will do. Instead of a key, a push 
 button is used, which is more simple in construction than 
 
 FIG. 130. 
 
 the telegraph key, but serves for exactly the same pur- 
 pose, to open and to close the circuit. 
 
 In the electric bell (Fig. 136), the armature, A, is held 
 by a spring, S, a short distance away from the poles of 
 the electromagnet. The spring consists of a flat strip of 
 brass, one end of which is fastened to a metal post, P, and 
 the other end is narrowed and so bent that when there is 
 no current in the electromagnet, the narrowed end of the 
 spring, B, presses against the tip of the screw, C. The 
 current after passing through the electromagnet enters 
 
MAGNETISM AND ELECTRICITY 329 
 
 the post supporting- the spring 1 , P, to which the armature 
 is fastened. It passes along- the spring-, S, to the nar- 
 rowed end, B, and there it enters the screw, C. 
 
 As soon as the current passes through the electromag- 
 net, the electromagnet becomes magnetic, attracts the 
 soft-iron armature, A, notwithstanding the action of the 
 spring, and in this manner draws the narrowed end of the 
 spring away from the screw. As soon as the tip of the 
 spring leaves the screw, the path of the current is broken, 
 the flow of electricity ceases, the electromagnet is no 
 longer magnetic, the armature is no longer attracted, and 
 the spring to which it is attached, on assuming its orig- 
 inal position, throws the armature back. This, however, 
 brings the tip of the spring against the screw, the current 
 is re-established, and the armature is once more attracted. 
 The armature continues to fly back and forth as long as 
 the hand is pressed against the push button. A tapper is 
 attached to the end of the armature and a bell is placed 
 on the same side as the electro-magnet. Whenever the 
 armature is drawn over, the tapper strikes against the 
 bell. 
 
 304. Galvanometers, If a wire is bent around a compass 
 so that the current goes northward over the needle and 
 southward under the needle, both the current above the 
 needle and the current below the needle will tend to 
 throw the north-seeking end of the needle westward. 
 This may be easily shown by applying the thumb and 
 finger rule for both parts of the wire. If the direction of 
 the strain in the ether around the wire above and below 
 the needle be represented by lines of force, it is evi- 
 dent that the lines of force due to both the current above 
 and below the needle will point in the same direction 
 within the space between the wires where the needle is 
 
330 
 
 ELEMENTAEY PHYSICS 
 
 situated. The wire bent in the manner directed forms a 
 single loop. If instead of a single loop a great many turns 
 of wire are taken the current will pass frequently north- 
 ward over the needle and southward under the same, and 
 the north-seeking- end of the needle will be thrown farther 
 toward the west. 
 
 In the case of any given coil, the amount of deflection 
 of the needle toward the west will increase with the in- 
 crease in strength of the current. Upon this principle 
 
 137. 
 
 instruments are constructed for the especial purpose of 
 determining the strength of currents. They are called 
 galvanometers. 
 
 The tangent galvanometer (Fig. 137) usually consists of 
 either few turns of thick wire or of many turns of thin 
 wire, forming a coil eight inches or more in diameter. 
 At the centre of this coil is placed a short, thick compass 
 needle. To this needle, transversely to its length, is 
 attached a very light aluminum pointer which indicates 
 the distance through which the needle has turned tow- 
 
MAGNETISM AND ELECTRICITY 
 
 331 
 
 ard the east or toward the west. This distance is read 
 in degrees by use of a scale attached to the interior of the 
 box containing the needle. The coil is placed in a north 
 and south position and from the amount of deflection of 
 the needle toward the east or toward the west, from the 
 strength of the magnet, from the size of the wire used in 
 
 -Silk fiber 
 
 Mirro 
 
 the coil, and the number of turns of the wire, the strength 
 of the current may be determined. 
 
 Some galvanometers are made of a great many turns of 
 wire. The magnetic needle is made very light, and is 
 suspended from a single thread of silk so that even a very 
 weak current will serve to deflect the needle. Such in- 
 struments may be called sensitive galvanometers. When 
 a very light tiny mirror is fastened to the needle, any 
 
332 ELEMENTARY PHYSICS 
 
 motion of the needle may be detected by throwing light 
 upon the mirror so that it will be reflected upon the wall 
 or upon a screen. Any motion of the mirror causes the 
 spot of light thrown upon the screen to move. An instru- 
 ment of this type is called a mirror galvanometer (Fig. 
 138). A sensitive mirror galvanometer will be found ex- 
 tremely useful in demonstrating the presence of the weak 
 currents produced during many of the following experi- 
 ments. 
 
 305. An Electric Current Generated in a Wiro while it is 
 Cutting Lines of Magnetic Force. It has already been 
 shown that the direction of the strain in the ether may 
 be represented by lines of force which leave the north- 
 seeking end of the magnet, curve around through the air, 
 and enter the south-seeking end. In the case of a per- 
 manent horseshoe magnet many of the lines of force pass 
 in a nearly straight direction from the north-seeking pole 
 of the magnet to the south-seeking pole (Fig. 121). If 
 the magnet is so held that the north-seeking pole of the 
 horseshoe magnet is directly above the south-seeking 
 pole, the direction of strain may be represented by lines 
 of force passing directly downward within the space be- 
 tween the poles. If then the magnet is turned so that 
 the poles face northward (Fig. 139), and a horizontal wire 
 is moved southward within the space between the poles, 
 the direction of motion is transverse to the direction of 
 strain in the ether. Since this direction of strain is rep- 
 resented by lines of force, it may also be said that the 
 wire cuts the lines of force. 
 
 The result of the motion of the wire across the mag- 
 netic field is a change in the direction of the strain in the 
 ether in the immediate vicinity of the wire. It has al- 
 ready been shown that a piece of soft iron placed in the 
 
MAGNETISM AND ELECTRICITY 
 
 333 
 
 field of a mag-net alters the direction of the strain. The 
 mere placing 1 of a piece of copper or other metal which is 
 not magnetic does not sensibly alter the direction of the 
 strain, but the motion of a copper wire across the magnetic 
 field in some manner causes a change in the direction of 
 this strain. The strain of the ether directly in front of the 
 moving wire appears to be increased, and the direction 
 of the strain at the side and the rear is altered in such 
 a manner as to curve partly or entirely around the wire. 
 This may be represented diagrammatically by drawing 
 
 .Direction of motion of wire 
 
 the lines of force so as to be crowded together on the 
 side in front of the moving wire, with the lines of force 
 nearest the wire entirely encircling it, leaving a some- 
 what smaller number of lines of force in the space im- 
 mediately in the rear. 
 
 The circular strain around the wire is the combined 
 result produced by the original strain of the ether from 
 pole to pole, the increased strain in front of the moving 
 wire, and the decreased strain behind. 
 
 It has been demonstrated that when a current passes 
 through a wire the strain may be represented by lines of 
 
334 ELEMENTARY PHYSICS 
 
 force which pass around it in circles. From this it may 
 be expected that the direct opposite is also true, so that 
 when by any means the direction of strain in any mag- 
 netic field is so altered that it may be represented by lines 
 of force passing around a wire, at least in the immediate 
 vicinity of the wire as in the case of the present discus- 
 sion, a current will tend to pass along the wire. If the 
 wire is connected with a sufficiently sensitive mirror gal- 
 vanometer, the existence of a current, when the wire is 
 moved between the poles of the magnet, may be easily 
 proved. 
 
 By reversing the thumb and finger rule, the direction 
 of the current in the wire may be predicted. The direc- 
 tion of the strain in the ether in front of the moving 
 wire always remains similar to the original direction 
 of the strain in the magnetic field between the poles, 
 even in the immediate vicinity of the wire where the 
 lines of force are represented as encircling the wire. 
 Therefore, grasp the wire with the right hand so that 
 the finger tips touch that side of the wire toward which 
 the wire is moving and at the same time point toward 
 the south-seeking pole of the magnet. The extended 
 thumb indicates the direction of the current in the 
 wire. 
 
 In the preceding case a horizontal wire is supposed to 
 be moving southward between the northward-facing poles 
 of a horseshoe magnet, the north-seeking pole being above 
 the south-seeking one. The application of the thumb and 
 finger rule indicates that the current passes eastward 
 through the wire. If the wire is moved northward while 
 the poles remain in the same position, the current passes 
 westward through the Avire. 
 
 It is noted in a preceding paragraph that a magnetic 
 
MAGNETISM AND ELECTRICITY 
 
 335 
 
 field encircles a wire only while a current is passing 
 through the wire. The reverse is also true. In order that 
 the magnetic field may produce a current in a wire its 
 strain must be of such a character as to encircle the wire* 
 so that it may be represented by lines of force passing 
 around the wire. This is true only while the wire is 
 kept in motion transversely to the original strain in the 
 magnetic field of the magnet. Therefore the current 
 in the wire ceases as soon as the motion of the wire 
 ceases. 
 
 306. The Dynamo. If a single loop of wire placed within 
 the space between the poles is so rotated that the upper 
 part of the wire loop moves southward at the same time 
 that the lower part of the loop moves northward (Fig. 
 146, A and B), a current will pass eastward in the upper 
 part of the loop at the same time that another current 
 passes westward in the lower part. 
 A brief examination will at once show 
 that these currents in no manner in- 
 terfere with each other, but that one 
 current follows immediately behind 
 the other around the loop. 
 
 If instead of a permanent horseshoe 
 magnet a very power- 
 ful electromagnet (Figs. 
 140, 141) is used, if in- 
 stead of a single loop of 
 wire a coil consisting of 
 several hundred loops 
 is employed (Figs. 141, FlQ 140 
 
 145), and if the coil is 
 
 rotated more rapidly so as to complete a greater number 
 of revolutions per second, the quantity of current pro- 
 
336 ELEMENTARY PHYSICS 
 
 duced by a single revolution of the coil is very much in- 
 creased. 
 
 We have already learned that the strain of ether in a 
 magnetic field is always very much increased by the pres- 
 ence of soft iron. If, therefore, the space between the 
 poles within which the coil rotates is filled as much as 
 possible with soft iron, the strength also of this field is 
 very much increased, and the rotation of the coil within 
 this stronger field produces a stronger current of elec- 
 tricity. In diagrammatic drawings this is represented by 
 drawing a greater number of lines of force through the 
 field when the space between the poles is largely occu- 
 pied by soft iron. The rotation of the coil then causes 
 the wire to cut a greater number of lines of force. In 
 practice, the field within which the coil rotates is most 
 effectively filled with soft iron by constructing a soft-iron 
 core, with grooves left for the wire coils. The wire form- 
 ing the coils is wrapped lengthwise in the grooves around 
 this core. Those parts of the inner sides of the poles of 
 the electromagnet which lie nearest to the coils are given 
 a concave curvature so as to leave the air space between 
 the coils and the poles of the magnet as small as pos- 
 sible (Fig. 140, N, S ; Fig. 141, P). 
 
 The iron core with its coils is called the armature (Fig. 
 145, D). The electromagnet in whose field the armature 
 revolves is called the field magnet (Fig. 140). When pro- 
 vision is made to permit the current to pass from the 
 coils of the armature into other wires so that the cur- 
 rent may be utilized, the combination is called a dynamo 
 (Fig. 141). 
 
 307. The Current Produced in a Loop of Wire Rotated in 
 the Field of a Horseshoe Magnet Fluctuates. If attention 
 be again confined to a single loop of wire revolving iu 
 
MAGNETISM AND ELECTRICITY 337 
 
 the field of a permanent horseshoe magnet (Fig. 139), it is 
 seen that twice during- every revolution, while the loop is 
 horizontal, the wires move practically parallel to the direc- 
 tion of strain in the field. Under these circumstances, the 
 ether is not strained more on one side of the wire than on 
 
 FIG. 141. 
 
 the other. In the diagrammatic representation of the 
 field of the magnet it will be seen that during- this part of 
 the path of the wire of the loop the lines of force are not 
 crowded more on one side of the wire than on the other. 
 Therefore, during this part of its revolution no circular 
 field is produced around the wire and the wire carries no 
 
338 
 
 ELEMENTARY PHYSICS 
 
 FIG. 142. 
 
 current. The maximum current, on the contrary, is pro- 
 duced when the wires of the loop are moving directly 
 
 across the direction of strain 
 of the magnetic field. This 
 takes place while the rota- 
 ting loop occupies a practi- 
 cally vertical position. Twice 
 during every revolution, the 
 current increases from zero 
 to the maximum current and 
 then falls again to zero. This 
 
 is represented diagrammatically in the figure (Fig. 142) 
 by varying the thickness of the circular path followed by 
 each wire of the rotat- 
 ing loop, from nothing, 
 where the wire moves 
 parallel to the lines of 
 force, to the maximum 
 thickness where it cuts 
 most directly across the 
 lines of force and thus 
 produces the strongest current. The irregularity of this 
 current may be largely decreased by employing two loops 
 
 at right angles with one an- 
 other (Fig. 143), or three loops 
 at angles of sixty degrees. In 
 figure 144 there are four coils 
 connected with each other in 
 such a manner that the current 
 can pass from coil to coil, D, 
 and yet escape from the dy- 
 namo at the proper point (B). By this means there is al- 
 ways one part of one coil very near that part of its course 
 
 FIG. 143. 
 
 FIG. 144. 
 
MAGNETISM AND ELECTRICITY 
 
 339 
 
 in which it produces its maximum current. This prevents 
 the current from decreasing- very much in strength at any 
 time, and, therefore, makes it more steady. Some dynamos 
 
 FIG. 145. 
 
 have armatures constructed upon this principle ; they 
 consist of a considerable number of coils of wire placed 
 at different angles (Fig. 145, D). 
 
 308. The Current in the Coil Changes in Direction Twice 
 during every Revolution. If one-half of the wire loop is 
 painted red and the other half green, it becomes evident 
 
 Conducting 
 ring- 
 
 Brush 
 
 FIG. 146. A 
 
 FIG. 146. B 
 
 that the red half of the wire carries an eastward going 
 current while it passes from the extreme north position 
 southward (Fig. 146, A). However, during the second 
 
340 
 
 ELEMENTARY PHYSICS 
 
 half of its revolution, while returning- from the south to 
 the north side, this half of the wire loop is traversed by 
 a westward-flowing 1 current. In the meantime, the cur- 
 rent through the green wire has changed from a west- 
 ward current to an eastward current (Fig. 146, B). The 
 result is that the current in the coil changes in direction 
 twice in the course of every revolution. 
 
 309. Alternating Currents. A current continually vary- 
 ing in direction is said to be alternating. All currents 
 produced by coils rotating in the fields of magnets are 
 
 alternating, at least while 
 the currents are still trav- 
 ersing the coil. If the ends 
 of the wires of a rotating 
 coil were fastened directly 
 to wires leading to any in- 
 strument, it is evident that 
 the wires would soon be- 
 come twisted and broken. 
 This is prevented in the 
 following manner. 
 
 One end of the axle carry- 
 ing the armature with its coil carries also two brass rings. 
 One of the ends of the coil is fastened to one ring and the 
 other end of the coil to the other ring (Fig-. 146). These 
 rings must be separated by some insulating material so 
 that the current cannot pass directly from one ring to 
 the other. As the armature rotates and the current 
 changes in direction, the current tends to leave the 
 dynamo first by one ring and then by the other. If two 
 thin strips of metal press against these rings, the cur- 
 rent will pass from the rings, first into one and then into 
 the other of these strips. To these strips of metal, per- 
 
 FlG. 147. 
 
MAGNETISM AND ELECTRICITY 341 
 
 manent wires leading to various electrical instruments 
 may be fastened. In that case the current will enter al- 
 ternately first one and then the other wire brush, pro- 
 ducing- an alternating current in the wire connected to the 
 brush. The two rings may be called the conducting rings, 
 and the thin strips of metal, the brushes. The thin strips 
 of metal are often replaced by thick pieces of carbon 
 (Fig. 147^) B^ and formerly were constructed of actual 
 wire brushes. In figure 141, the carbon brushes are en- 
 closed in brass holders, B, and only the lower ends of the 
 carbons can be seen, where they are in contact with the 
 commutator, C. A dynamo with conducting rings can 
 produce only an alternating current. 
 
 310. An Alternating Current Changed into a Direct Cur- 
 rent by Means of a Commutator If instead of two rings, a 
 single ring is used, and if this ring is cut transversely into 
 two equal halves, the ends of the coil connected to the 
 half rings will send a current first into one and then into 
 the other half ring. The two half rings are then known 
 as a split-ring commutator (Fig. 148). In this case the 
 brushes are placed on opposite sides of the commutator 
 at such an angle that they come in contact with both 
 halves of the commutator only at the moment at which 
 the coil is producing practically no current. In other 
 words, while the wires of the coil are moving parallel to 
 the lines of force. If the coil be so rotated that the 
 upper part of the coil always moves toward the south, 
 then that half of the coil which is moving southward will 
 always produce an eastward- moving current. By placing 
 the brushes at the proper angle, it may be so arranged 
 that the eastward-moving- current, no matter by which 
 half of the loop it is produced or by which half of the 
 commutator it leaves, will always leave by the same brush, 
 
342 
 
 ELEMENTARY PHYSICS 
 
 while the returning- current will always re-enter by the 
 other brush. In other words, a split-ring- commutator 
 
 may be used to secure 
 currents passing 1 in one 
 direction only. 
 
 By the use of a suf- 
 ficient number of coils 
 placed at various an- 
 gles with each other 
 (Fig. 145, D), in con- 
 nection with an in- 
 creased number of 
 segments in the com- 
 mutator (Figs. 141, 145, 
 147, C), a current may 
 
 be secured which is not only constant in direction, but 
 also fairly constant in strength. Dynamos constructed on 
 this principle are called direct current dynamos (Fig. 141). 
 
 311. The Motor. If 
 a strong current of 
 electricity be sent by 
 way of the brushes in- 
 to the single coil of 
 wire found in a simple 
 dynamo while the coil 
 is in a horizontal posi- 
 tion, in such a manner 
 that the upper face of 
 the coil becomes 
 north-seeking, the pja 148 B 
 
 north-seeking pole of 
 
 the magnet being directly above, the north-seeking mag- 
 netism of the coil will oppose the north-seeking magnetism 
 
MAGNETISM AND ELECTRICITY 
 
 343 
 
 of the magnet. The upper face of the coil and the north- 
 seeking- pole of the magnet, directly above, will mutually 
 repel each other (Fig. 149), and, if the opposing magnetic 
 effects are sufficiently strong, the coil will tend to move so 
 that the north-seeking face of the coil will turn downward. 
 
 If a half -ring commutator is used, the current may be 
 switched into the coil in such a manner that at the mo- 
 ment the north-seeking face of the coil is directly oppo- 
 site the south- seeking face of the magnet, the direction of 
 the current throughout the entire coil is changed. What 
 was the north - seeking ^ 
 face of the coil now be- 
 comes the south-seeking 
 face. The same condi- 
 tions are therefore re- 
 produced as at the be- 
 ginning. The coil will 
 therefore rotate once 
 more and in the same 
 direction. In other 
 words, when a divided- 
 ring commutator is used, 
 the coil continues to re- 
 volve as long as it is supplied with a sufficiently strong 
 current of electricity. The direct-current dynamo has 
 become the direct-current motor. 
 
 A motor is, therefore, the exact reverse of a dynamo. 
 Power is necessary to revolve the coil of a dynamo and 
 electricity results. Electricity is necessary to set in 
 operation the coils of a motor and motion results. 
 
 312. Induction. Any wire carrying a current is sur- 
 rounded by a field within which magnetic effects are per- 
 ceptible, These magnetic effects are strongest near the 
 
 FIG. 149. 
 
344 ELEMENTARY PHYSICS 
 
 wire and become gradually imperceptible as the distance 
 from the wire becomes greater. Any increase in strength 
 of the current passing through the wire is accompanied 
 both by an increase of strain in the ether in every part of 
 the magnetic field which already surrounds the wire and 
 by an extension of the magnetic field to a greater dis- 
 tance from the wire. Conversely, any decrease in the 
 strength of the current results in a decrease of both the 
 strength and the extension of the magnetic field. 
 
 If around one coil of wire a second coil is wrapped in 
 such a manner that the wires of the two coils at no point 
 come in contact with each other (Fig. 151), it is evident that 
 no current of electricity can pass directly from the first coil 
 into the second. However, the magnetic field around the 
 inner coil may extend to, and beyond the outer coil. Any 
 increase in the strength of the current passing through 
 the inner coil must result in an increase in the strain of 
 the ether around the wire in each part of this coil. Tkis 
 increased strain is in part transmitted outward, affecting 
 the magnetic field even beyond the outer coil. This 
 spreading of the increased strain seems to be slightly 
 checked on reaching the wire forming the outer coil. In 
 consequence, the direction of strain of the ether in im- 
 mediate contact with the wire forming the outer coil is 
 altered to a circular strain and this results in a current 
 through the wire. 
 
 The direction of the current in the wire of the outer 
 coil due to the increase of current in the inner coil may 
 be determined mechanically by the use of the line of force 
 idea. Any increase in the strength of the current passing 
 through a wire may be represented by an increase in the 
 number of lines of force in every part of the field sur- 
 rounding the wire and also by the spreading of these 
 
MAGNETISM AND ELECTRICITY 
 
 345 
 
 lines of force so as to occupy a larger field. Conversely, 
 any decrease in the strength of the current may be 
 imagined to result in a shrinking of the lines of force ac- 
 companied by a diminution of their number and also by 
 a decrease in the size of the field. 
 
 Primary current 
 
 increasing in 
 
 strength 
 
 FIG. 150. 
 
 If in the vicinity of one wire carrying a current there is 
 a second wire (Fig. 150), any increase in the strength of 
 the current passing through the first wire must result in 
 an increase in the number of lines of force passing around 
 this wire. In consequence, the lines of force spread to a 
 greater distance from the first wire and strike against the 
 
346 
 
 ELEMENTARY PHYSICS 
 
 second wire, which so far has not contained any current. 
 But as soon as the lines of force of the first wire strike 
 against the second wire, they begin to wrap around it 
 and this results in the production of a current in the sec- 
 ond wire, opposite in direction to the current present in 
 the first wire. 
 
 By studying on which side of the wires of the outer 
 coil the lines of force spreading from the inner coil are 
 momentarily held in check, it is possible to determine in 
 what direction the lines of force will pass around the 
 wires of the outer coil, and what must be the direction 
 
 n 
 
 Secondary Current 
 
 J\ (\ J\ AJ\fU\AJ\OfU\fU 
 
 Induction due to increase 
 in strength of current 
 
 A in strength ol ci 
 in primary coil 
 
 FIG. 151. 
 
 taken by the current in the outer coil. The current in- 
 duced in the secondary coil is found always to be opposite 
 in direction to that present in the primary coil (Fig. 151). 
 In one particular, the process is the exact reverse of 
 that which is followed when a current is produced in a 
 wire by moving it so as to cut directly across the lines of 
 force between the poles of a magnet. In one case, the 
 spreading of lines of force causes the lines of force to 
 move toward and strike against the wire. In the other 
 case the wire is moved toward and strikes against the 
 lines of force. In both cases the lines of force wrap 
 around the wire and a Current results in the wire. 
 
MAGNETISM AND ELECTRICITY 347 
 
 When the current in the inner coil decreases, the lines 
 of force again contract. The result is a current in the 
 opposite direction from that first produced in the outer 
 coil. Increase and decrease of current in the inner coil 
 therefore results in an alternation in the direction of the 
 current in the outer coil. 
 
 The two coils of wire used in the manner here described 
 may be called induction coils. Although two coils are 
 always present, the combination is usually referred to in 
 the singular number, as an induction coil. The name 
 Ruhmkorff coil is frequently employed when, at one 
 point in the outer coil, there is a break or spark gap, 
 across which the secondary current must leap in order to 
 complete the circuit. Induction coils are used with tele- 
 phones. They are usually placed in the box below the 
 transmitter. Ruhmkorff coils are used for wireless teleg- 
 raphy ( 322) and Eoentgen rays ( 323), although Holtz 
 and Wimshurst induction machines may also be employed. 
 
 313. The Telephone. In the case of telephones, the in- 
 strument into which you speak is called the transmitter, 
 and that by means of which you hear is called the re- 
 ceiver. Two wires lead from the telephone instrument to 
 the telephone station and from there pairs of wires lead 
 off to other telephones in the same town or district. The 
 connections between the wires from your telephone and 
 those leading to any other house are made at the central 
 station. 
 
 In the most recent systems in use the electric cells 
 (storage cells) are all located at the central station. As 
 soon as the receiver is lifted from its hook, the trans- 
 mitter is in electrical connection with the central office, 
 and there the connection is made with any other tele- 
 phone. 
 
348 
 
 ELEMENTAEY PHYSICS 
 
 314. The Transmitter. The transmitter (Fig. 152) con- 
 sists of a wooden box with an opening in front, behind 
 which is a thin piece of iron, called a diaphragm, against 
 which you talk. The sound waves produced by the voice 
 strike against this diaphragm and push the central part 
 forward. Between each sound wave the 
 diaphragm flies back. When it is re- 
 membered that sounds of high pitch are 
 caused by the production of many thou- 
 sands of successive sound waves in a sin- 
 gle second, it is almost in- 
 conceivable that even the 
 thinnest metal plate should 
 respond to all of these waves. 
 Nevertheless this is actually 
 the case, and every wave 
 produced by the voice, or by 
 any other sound-producing 
 instrument, affects to some 
 degree the motion of the di- 
 aphragm. A screw connects 
 the centre of the diaphragm 
 with the cover of a round 
 brass box which, however, 
 looks more like a round flat 
 piece of brass as seen from 
 the rear, on opening the door 
 of the transmitter. The cover of this box consists of a 
 thin sheet of mica to the centre of which is attached a thin 
 brass disk. The screw already mentioned connects the 
 diaphragm with the brass disk. As the diaphragm moves 
 backward and forward the brass disk also moves in the 
 same manner. To the inner side of the brass disk and to 
 
 FIG. 152. 
 
MAGNETISM AND ELECTKICITY 349 
 
 the base of the box directly opposite are fastened two 
 round and highly polished carbon disks. Between these 
 disks the box is filled with minute carbon grains. 
 
 The current comes from the battery at the central tele- 
 phone station to the receiver and passes by a thin con- 
 necting wire to the brass disk on the cover of the box. 
 Thence it passes through the carbon disk attached to the 
 brass disk, through the carbon grains to the carbon disk 
 at the base of the box, and finally through the brass box 
 itself by another thin wire to the wire returning to the 
 station. 
 
 Whenever a sound wave throws the diaphragm for- 
 ward, the cover of the box presses the carbon grains to- 
 gether so effectively as to cause them to transmit elec- 
 tricity better than when the diaphragm moves back and 
 the carbon grains are slightly less crowded. Every mo- 
 tion of the diaphragm therefore causes a corresponding 
 change in the strength of the current passing through the 
 transmitter, and therefore through the wires leading to 
 the central office and thence to the other telephone. It is 
 difficult to conceive that the slight variations in compres- 
 sion of the carbon grains should cause such distinct varia- 
 tions in the strength of the current that the hundreds or 
 thousands of the sound waves produced by the voice in 
 the coiirse of a single second, result in a corresponding 
 number of variations in the current of electricity pass- 
 ing through the transmitter. This, however, is the case. 
 
 315. The Receiver. The receiver (Fig. 153) consists of 
 a long, round bar magnet with the north-seeking end 
 almost touching the centre of the metal diaphragm. The 
 cover of the receiver may be easily unscrewed and the soft 
 iron diaphragm removed. There is then seen a small 
 spool surrounding the north-seeking end of a permanent 
 
350 
 
 ELEMENTAEY PHYSICS 
 
 bar magnet (Fig. 154). The spool is wrapped with many 
 turns of very thin wire covered with silk. The ends of 
 this wire pass lengthwise through the receiver, from the 
 spool, along the magnet, to the inner ends of the binding 
 posts at the rear of the instrument. 
 
 When a current is sent through the coil of wire on the 
 spool in one direction, the end of the coil facing in the 
 same direction as the north-seeking pole of the permanent 
 magnet becomes also north-seeking (Fig. 154, A). The 
 magnetic effect produced by both the permanent magnet 
 and by the coil is now greater than that produced a nio- 
 
 FlG. 153. 
 
 ment before by the magnet alone. The thin soft iron 
 diaphragm is therefore attracted more strongly, and is 
 drawn slightly nearer to the end of the magnet. When, 
 however, the current is sent in an opposite direction 
 through the wire, the end of the coil facing in the same 
 direction as the north-seeking pole of the magnet be- 
 comes south-seeking (Fig. 154, B). The south-seeking 
 magnetism of the coil slightly counteracts the north - 
 seeking magnetism of the bar magnet. The magnetic 
 effect produced by both magnet and coil is then less than 
 the effect usually produced by the magnet alone. The 
 
MAGNETISM AND ELECTKICITY 
 
 351 
 
 diaphragm is attracted even less than when no current 
 in any direction is passing- through the coil. In conse- 
 quence the diaphragm springs back to a position slightly 
 more distant from the end of the permanent magnet than 
 when no current is passing. Any change in the direc- 
 tion of the current through the coil results in a variation 
 in the degree of attraction of the diaphragm, and this 
 causes the diaphragm to move backward and forward. 
 
 316. The Use of Induction Coils in Telephone Circuits. 
 It has been shown ( 312) that any variation in the 
 
 FIG. 154. 
 
 strength of a current of electricity passing through a coil 
 of wire surrounded by another coil must result in a cur- 
 rent passing through the outer coil. Any increase in the 
 strength of the current in the inner coil produces a cur- 
 rent in the opposite direction in the outer coil, while any 
 decrease in the current in the inner coil results in a cur- 
 rent in the same direction in the outer coil. It has also 
 been shown ( 314) that the variable pressure produced 
 by a succession of sound waves striking the diaphragm of 
 a telephone transmitter is able to produce a variable cur- 
 rent through the transmitter. 
 
352 ELEMENTARY PHYSICS 
 
 If, therefore, the variable current passing through the 
 transmitter, during* the use of a telephone, be sent 
 through the inner one of two induction coils (Fig. 151), 
 secondary currents of electricity will be produced in the 
 coil immediately surrounding the first. These secondary 
 currents will vary in strength and direction with the vari- 
 ations in the strength and direction of the current in the 
 first coil. If the inner coil consists of comparatively few 
 turns and the outer coil is formed by a much greater num- 
 ber of turns of wire, the current produced in the outer 
 coil will move with greater violence along a greater 
 length of wire than could be passed over by the current 
 in the inner coil. 
 
 By connecting the two ends of the wire in the outer 
 coil with the two binding posts at the rear of the receiver, 
 the variations in the strength and direction of the current 
 in the outer coil may be used to produce currents vary- 
 ing in strength and direction in the coil wrapped around 
 the end of the permanent magnet in the receiver. This 
 will result in forward and backward motions in the dia- 
 phragm of the receiver. By properly connecting the 
 ends of the wire of the outer coil with those of the re- 
 ceiver, every motion of the diaphragm in the transmitter 
 will be imitated by the diaphragm in the receiver. 
 
 This would be true if the wires leading from the trans- 
 mitter were connected directly with the wires of the coil 
 around the spool in the receiver. However, when induc- 
 tion coils are employed, which possess a much greater 
 number of turns of wire in the outer coil than in the inner 
 one, the force with which the current produced in the 
 outer coil tends to move is much greater than the force 
 shown by the current in the inner coil. Hence the sec- 
 ondary current produced in the outer coil speeds on from 
 the outer coil to the receiver with much greater force 
 
MAGNETISM AND ELECTRICITY 353 
 
 than the current which comes from the transmitter to the 
 inner coil of the induction coils. The secondary current 
 produced in the outer coil, therefore, can overcome a 
 much greater resistance ( 318) and pass through a much 
 greater length of wire than that traversing the inner coil. 
 The use of the induction coils is therefore to secure a 
 current which will tend to move through the street wires 
 to the receiver with much greater force than the current 
 coming directly from the transmitter, and at the same 
 time to secure a current whose variations in strength 
 and direction are a precise duplicate of those coming 
 from the transmitter. 
 
 317. Action of the Telephone. Each sound wave caused 
 by the voice pushes the diaphragm of the transmitter for- 
 ward, and this produces a better contact between the car- 
 bon particles. The result is an increased flow of electricity 
 through the first coil. During this increase of current, a 
 momentary current passes in the opposite direction in 
 the second coil, and by proper connections with the spool 
 in the receiver, the diaphragm in the receiver may like- 
 wise be caused to move forward. During the interval 
 between two sound waves, the diaphragm of the trans- 
 mitter flies back, and in consequence the carbon particles 
 of the transmitter are in looser contact with one another 
 and a smaller current flows on to the first coil. The de- 
 crease of current in the first coil is accompanied by a 
 momentary current in the second coil having the same 
 direction as that in the first coil. The direction of the 
 current in the spool of the receiver, is in consequence 
 reversed. The diaphragm of the receiver, therefore, 
 moves in the opposite direction. In this simple manner 
 the diaphragm of the receiver is caused to imitate all the 
 motions of the diaphragm of the transmitter. 
 
 The diaphragm of the transmitter is set in motion by 
 
354 ELEMENTARY PHYSICS 
 
 the waves of the voice. The motions of the diaphragm 
 of the receiver, however, set the air in motion, and these 
 motions reproduce the original sound. 
 
 318. Conductors. The ability of different substances to 
 conduct electricity varies considerably. Metals are the 
 best conductors, but different metals vary considerably in 
 conductivity. Silver and copper are the best conductors. 
 Soft iron and platinum conduct only one-sixth as well. 
 German silver conducts about one-twelfth as well. As 
 compared with this carbon is a poor conductor. The 
 power to conduct electricity varies not only with the sub- 
 stance, but also with the area of cross section of the wire, 
 fibre, or rod which conducts the electricity. On this ac- 
 count a thick rod of carbon may serve to conduct elec- 
 tricity much better than a thin copper wire. 
 
 The electricity may pass also more readily through a 
 short piece of wire than through a longer one. Electric- 
 ity may pass most readily through a short thick piece of 
 wire constructed of the best conducting material, silver or 
 copper. It may pass far less readily through a long 
 thin piece of an only moderately good conductor, such as 
 German silver. Any body which does not readily permit 
 the flow of electricity through it, is said to offer resist- 
 ance to the flow of electricity. It is often desirable not to 
 have too great a flow of electricity through certain ma- 
 chines when they are first started. This may be managed 
 by causing the current of electricity at first to flow 
 through a considerable length of some poor conductor of 
 electricity. After the machine has once started in opera- 
 tion, the current is caused to flow through a much shorter 
 length of the poor conductor, thus causing the larger 
 part of the current to flow through the machine. Boxes 
 with coils arranged for the purpose of controlling the 
 
MAGNETISM AND ELECTRICITY 
 
 355 
 
 amount of flow of electricity are often called resistance 
 boxes. The resistance boxes at the front end of electric 
 street cars control the quantity of current admitted to the 
 motor beneath the car, and are hence called controllers, 
 Resistance coils are also used in connection with stereop- 
 ticons for the purpose of regulating the flow of electricity 
 through the lamp. 
 
 319. The Incandescent Lamp. Anything which offers re- 
 sistance to the flow of electricity becomes heated in con- 
 sequence. The same flow of electricity through the same 
 
 Brass contact with 
 wall oflsocket 
 
 FIG. 155. 
 
 length and thickness of copper, platinum, and carbon 
 will heat the platinum more than the copper, and the car- 
 bon far more than either of the other two. This may be 
 seen in the case of an ordinary incandescent electric light 
 (Fig. 155). In this case the carbon filament within the 
 lamp becomes very much more strongly heated than the 
 very short pieces of platinum which connect the ends of 
 the carbon filament within the lamp to the copper wires 
 at its base. The result is that the carbon filament be- 
 comes white hot, while the copper and platinum wires 
 become heated, but not sufficiently to produce light. 
 
356 ELEMENTARY PHYSICS 
 
 320. The Arc Light Fasten two pointed pieces of car- 
 bon to the wires leading from a powerful voltaic battery 
 or from a dynamo in such a manner that, when the car- 
 bons are allowed to touch, the circuit is closed. Bring 
 the points of the carbons in contact for a few seconds and 
 then slowly separate them. The points of the carbons be- 
 come heated to incandescence, and a luminous arc is seen 
 to extend between them. 
 
 When the carbons are permitted to touch, a current passes 
 through them. As they are separated, the current leaps 
 across the small gap and volatilizes 
 some of the carbon. This carbon 
 vapor, being a partial conductor, 
 permits the current to flow across 
 the gap, provided the gap is not 
 too wide. The ends of the carbons 
 are more brilliant than the vapor 
 between them. The tip of the car- 
 bon connected with the wire from 
 the positive pole of a battery or 
 FIG. 156. dynamo is slowly consumed so that 
 
 it possesses a cup-shaped cavity 
 
 called the crater (Fig. 156). It is this crater which is the 
 hottest and most luminous part of the light. 
 
 Since the carbons, slowly waste away, it is necessary to 
 keep moving them toward each other in order that the 
 distance between them may remain practically uniform. 
 In most arc lights in use this is accomplished by an auto- 
 matic arrangement controlled by an electromagnet. 
 
 321. The Production of Electric Waves by Induction Coils. 
 In one of the instruments utilizing induction coils, called 
 the Ruhmkorff coil (Fig. 157), the current sent through 
 the inner coil is automatically broken and re-established 
 
MAGNETISM AND ELECTKICITY 
 
 357 
 
 many times in the course of each second by a specially 
 devised mechanism. The violent fluctuations in the 
 strength of the current passing through the inner coil, 
 produced in this manner, result in alternating currents 
 in a second coil of wire, wrapped around the first. The 
 number of turns of wire in the outer coil is much greater 
 than that used for the inner coil. In large instruments 
 the outer coil is made of very thin wire and many thou- 
 sands of turns of wire are present. Hence the force with 
 
 Induction or Ruhmkorff Coil 
 
 FIG. 157. 
 
 which electricity is urged to move onward is so much 
 greater than that present in the inner coil that the elec- 
 tricity can jump across a considerable gap through the 
 air if the circuit belonging to the outer coil is anywhere 
 broken. 
 
 Since the current of electricity in the outer coil alter- 
 nates rapidly in direction, in consequence of the rapid 
 automatic breaking of the circuit belonging to the inner 
 coil, the passage of electricity through the air or spark 
 gap also alternates in direction. The result of these al- 
 ternating discharges of electricity through the air is the 
 
358 ELEMENTARY PHYSICS 
 
 production of a succession of sparks accompanied by os- 
 cillations or waves in the ether. The production of these 
 waves in the ether appears to be much facilitated by at- 
 taching- a long vertical wire to some part of the circuit 
 belonging to the outer coil. 
 
 A closer examination of the discharges of electricity 
 through the air-gap has shown that the passage of elec- 
 tricity across the air-gap is not accomplished by a single 
 discharge on each change in direction of the current, but 
 that each apparent single discharge consists in reality of 
 a very rapid succession of discharges alternating in di- 
 rection. The setting up of waves in the ether is there- 
 fore a somewhat complicated process. 
 
 322, Wireless Telegraphy The oscillations or waves in 
 the ether produced by a large Kuhmkorff coil, to which a 
 vertical wire has been attached, may be used for purposes 
 of wireless telegraphy. At the sending station an ordi- 
 nary telegraph key is inserted in the circuit including 
 the inner coil of the Kuhmkorff coil and the electric cells 
 (Fig. 157). By this means, the length of time during 
 which electricity flows through the inner coil may be 
 controlled as in ordinary telegraph instruments. The 
 production of currents of electricity in the outer coil 
 takes place only when the telegraph key is depressed, 
 and ceases the moment the key is permitted to spring 
 back. The production of waves in the ether is due to an 
 alternating discharge of the electric currents in the outer 
 coil across the spark-gap. 
 
 At the receiving station are two small but complete cir- 
 cuits (Fig. 158). In the first circuit are several gravity 
 cells, which send a current through the electromagnet of 
 a relay and through a glass tube containing nickel filings 
 held in place loosely by metal (silver) disks. This tube 
 
MAGNETISM AND ELECTEICITY 
 
 359 
 
 is called the coherer. Loose nickel filings under ordinary 
 circumstances offer a considerable resistance to the flow 
 of electricity, but when struck by electric waves or oscil- 
 lations they, in some mysterious manner, become good 
 conductors of electricity. Therefore, a good current of 
 
 Vertical wire attached to 
 tin lasin near the ceiling. 
 
 FIG. 158. 
 
 electricity passes through the coherer at the receiving 
 station as long as the key at the sending station remains 
 depressed, and during this time the electromagnet of the 
 relay attracts its armature. As soon as the key at the 
 sending station is permitted to spring back, the produc- 
 tion of electric waves or oscillations ceases, the nickel fil- 
 
360 ELEMENTARY PHYSICS 
 
 ings lose their conductivity, and the armature of the relay 
 is permitted to be drawn back. 
 
 The armature of the relay serves as a key to a second 
 circuit at the receiving- station. This circuit includes a 
 sounder, as in ordinary telegraphy. It also includes an 
 electromagnet whose armature is supplied with a tapper. 
 An electric bell may be fitted up for this purpose. This 
 tapper continues to strike against the coherer as long as 
 the current flows through the circuit. By this means 
 the nickel filings in the coherer are shaken up and lose 
 their conductivity much more completely than by the 
 mere cessation of electric waves in the ether. On this 
 account the electromagnet with its tapper is called a de- 
 coherer. As long as electric waves arrive from the Ruhm- 
 korff coil, the nickel filings are caused to cohere the in- 
 stant they are decohered by the tapper. Practically this 
 results in uninterrupted cohering. But as soon as the 
 waves cease to arrive, the hand having been taken off the 
 key at the sending station, the last stroke of the tapper 
 leaves the nickel filings too loose to convey a good cur- 
 rent. Hence all electric action ceases also at the receiv- 
 ing station. 
 
 323. Roentgen Rays. The ends of the wires forming the 
 outer coil of a Ruhmkorff coil (Fig. 159) may be inserted 
 at the extremities of a large tube and the points of entry 
 closed air tight. The discharge of electricity will then 
 take place through the air enclosed within the tube. If 
 an opening exists in one side of the tube through which 
 the air may be pumped out, the effect of decrease of 
 density of air upon the character of the discharge of elec- 
 tricity through the air-gap may be studied. 
 
 Dry air of ordinary density offers a considerable resist- 
 ance to the passage of electricity, and the discharge of elec- 
 
MAGNETISM AND ELECTKICITY 
 
 361 
 
 tricity through the air is by a bright, clearly defined path. 
 As the density of the air becomes less the discharges of 
 electricity through the air become more frequent and the 
 light less intense. Moreover, the path followed by the elec- 
 tricity while passing through the air is less distinct, and 
 the air surrounding it begins to glow with a bluish or pur- 
 ple light. As the density of the air becomes less, the path 
 taken by the electricity gradually becomes invisible, while 
 
 Fluoroscope 
 
 ^Primary -wires attached to 
 "dynamo or powerful 'battery. 
 
 FIG. 159. 
 
 the purple glow of light extends until it fills the entire tube. 
 After about fj-jj- of the air has been withdrawn from the 
 tube, the glow of light due to the passage of electricity 
 takes the form of a succession of regions of stronger light 
 alternating with regions of comparative darkness. Finally 
 when nearly all the air has been removed the discharge of 
 electricity through the tube becomes nearly invisible. 
 
 The passage of the current through the tube is now ac- 
 companied by the production of waves at the end of one 
 
362 ELEMENTAKY PHYSICS 
 
 of the wires entering- the tube, the end by which the elec- 
 tricity is supposed to leave the tube, the cathode end. 
 These waves have the power of passing through many 
 substances ordinarily called opaque, such as flesh, leather, 
 most kinds of clothing, paper, wood, ebonite, and many 
 other substances. They pass with much less readiness 
 through most metals, and through thick glass. Since 
 these rays are capable of affecting the chemicals on a 
 photographic plate, it is possible by their means to take 
 photographs of coin enclosed in leather pocket-books, or 
 of bullets lodged in the flesh, or of broken bones in the 
 case of a broken arm or leg. These rays have also the 
 power of making certain chemicals, such as calcium tung- 
 state, glow in the dark. By holding the hand in a dark 
 room between the source of the rays and a sheet of card- 
 board covered with calcium tungstate, the bones may be 
 seen as dark shadows within a slightly darkened area 
 due to the passage of the rays through the flesh (Fig. 
 160). Tubes especially constructed for the study of dis- 
 charges of electricity through nearly perfect vacua are 
 called Crooke's tubes. The sheet of cardboard covered 
 with calcium tungstate is known as a fluorescent screen. 
 It is usually placed at one end of a box whose opposite 
 end fits closely against the face. This leaves the interior 
 of the box dark, and makes it unnecessary to perform the 
 experiment in a dark room. The box with its fluorescent 
 screen at the end is called a fluoroscope. The rays here 
 described are called either X-rays or Roentgen rays. 
 
 324. Electricity and Magnetism Are Phenomena of the 
 Ether. It will be noticed from the preceding paragraph 
 that electrical phenomena do not cease when the air has 
 all been practically removed from a Crooke's tube. From 
 this it may be concluded that electrical phenomena, al- 
 
MAGNETISM AND ELECTEICITY 
 
 363 
 
 though profoundly affected by the presence of other sub- 
 stances, are after all mainly phenomena of the ether. The 
 
 FIG. 160. 
 
364 ELEMENTARY PHYSICS 
 
 passage of electrical oscillations through the air suggest 
 the same conclusion, since the passage of these oscillations 
 in wireless telegraphy not only vastly exceeds in speed 
 the most rapid transmission of vibrations of sound through 
 the air, but equals in speed the transmission of light 
 through space. Since magnetic phenomena always ac- 
 company the passage of electricity, magnetism must also 
 be a phenomenon of the ether. In what way various sub- 
 stances produce the various known modifications of these 
 phenomena of the ether is at present unknown. 
 
 Electricity gives rise to so many curious phenomena 
 and permits so many interesting applications to useful 
 ends that it is impossible to describe more than a small 
 part in an elementary book. The choice of the material 
 to be selected under these circumstances must be de- 
 termined therefore chiefly by its usefulness as an intro- 
 duction to the subject. Since this is a matter of judg- 
 ment, each writer or teacher is liable to make a different 
 selection. The pupil will find an interesting continuation 
 of this subject in Sylvanus Thompson's Elementary Les- 
 sons in Electricity and Magnetism. 
 
CHAPTER Yin 
 
 MECHANICS 
 
 325 V Gravitation It requires considerable exertion 
 merely to hold a cannon-ball, even if no effort is made to 
 carry it for any distance. The ball acts as if something- 
 were forcing it downward and yet nothing- touches the 
 ball except the hand and the air. It cannot be the air 
 which is urging- the ball downward. The air, rather, re- 
 sists the effort of the ball ; for have we not learned that 
 the upward pressure of the air on' the lower surface of the 
 ball must be greater than the downward pressure of the 
 air on the top of the ball ( 51, 52) ? And, does this not 
 leave an excess of upward acting force to counteract, to a 
 slight extent, the effort of the ball to move downward ? 
 And, if the ball is allowed to drop, does not the friction 
 of the air through which the ball must drop retard the 
 motion of the ball slightly? 
 
 No cause for the motion can be detected. There is no 
 visible connection between the ball and the earth. As far 
 as can be seen there is nothing either pushing or pulling 
 the ball. The only fact of which we are absolutely cer- 
 tain is that the ball seems urged toward the earth by a 
 certain force which we call gravitation. The strength of 
 this force can be measured by fastening the ball to a 
 spring balance by means of a string. 
 
 326. Directions in Which Gravitation Acts on the Surface 
 of the Earth. The direction in which the force acts is 
 
 365 
 
366 ELEMENTARY PHYSICS 
 
 shown by the direction of the string-. This direction is 
 straight downward. No matter to what portion of the 
 surface of the earth the ball be taken, the string from 
 which it is suspended will always point downward. Does 
 this mean that the ball always tries to move in the same 
 direction ? We who have studied geography know that 
 it means anything but that ; for the earth is round, and 
 down at one point of the surface of the earth is an en- 
 tirely different direction from down at any other point. 
 
 In fact down practically means toward the centre of 
 the earth. No matter upon what portion of the earth the 
 ball may be, it will try to move toward the centre of the 
 earth. 
 
 Even at the same part of the surface of the earth, down 
 does not mean in the same direction at all times ; for the 
 earth turns round on its axis, and the direction of a line 
 pointing toward the centre of the earth changes at every 
 instant. A man at the equator may look up at noon and 
 again at midnight. He has looked up in both cases and 
 yet he has looked in exactly opposite directions. Why 
 do not those people fall off the earth who live on the side 
 opposite us ? Because something forces them toward the 
 centre of the earth just as we are forced toward the centre 
 of the earth. 
 
 327. The Force of Gravitation Due to the Earth is Less at 
 Points More Distant from the Centre of the Earth. The force 
 with which a body is urged toward the earth is not the 
 same in all localities. It is less at the tops of high moun- 
 tains than at the level of the sea. The greater the dis- 
 tance from the centre of the earth, the weaker is this force. 
 This fact can be determined experimentally with wonder- 
 ful exactness, but the experiments involve a very exact 
 knowledge of the laws of the pendulum ( 337-342). 
 
MECHANICS 367 
 
 When we move away to points more distant from the 
 centre of the earth, the force with which a body is urged 
 toward the earth decreases much more rapidly than the 
 distance of the body from the earth increases. In fact, it 
 has been found that the strength of the force diminishes in 
 proportion as the square of the distance of the body from the 
 centre of the earth increases. 
 
 328. The Earth Attracts the Atmosphere and the Moon. 
 Notwithstanding- this rapid diminution of force, the air 
 from a distance of fully 100 miles is urged toward the 
 earth, so that, as the earth moves through space, it carries 
 the air with it. The atmosphere is not the only material 
 which follows the earth on its journey through space. 
 No matter in what direction the earth may be travelling 
 in its path around the sun, it is always accompanied by 
 the moon. Although the moon is 240,000 miles away, it is 
 continually being urged toward the earth. If it were not 
 for the fact that the moon has a motion of its own which 
 tends to carry it away from the earth, the moon would 
 not only keep near the earth, but would long ago have 
 come crashing into the earth. 
 
 We do not know whether the force urging the moon 
 and other objects toward the earth is in the nature of a 
 push or a pull. It is usually assumed, however, that it 
 is a pull and the earth is said to attract the moon and 
 other objects. v 
 
 329. The Earth is Attracted by the Sun. The earth is 
 not the only body which attracts other objects. It moves 
 around the sun at an enormous rate of speed. If some- 
 thing were not urging the earth toward the sun, it would 
 fly off and leave the sun just as a stone fastened to a 
 string and whirled rapidly around through the air will 
 start off in a straight line the instant the string breaks. 
 
368 ELEMENTARY PHYSICS 
 
 The sun seems to attract the earth, just as the earth at- 
 tracts the moon. In the same manner the sun attracts 
 the planet Jupiter, and Jupiter attracts five moons. 
 
 330. Every Object Attracts Every Other Object Nor is 
 the power of attraction confined to the heavenly bodies. 
 Every object attracts every other object. Yery interest- 
 ing- experiments have been devised in order to prove this. 
 In one of these experiments, it may be shown that two 
 thin glass globes filled with mercury attract each other 
 in a distinctly perceptible manner. 
 
 If every object attracts every other object, not only does 
 the earth attract the iron ball, but the ball must also 
 attract the earth. From this follows another and more 
 striking conclusion, that not only does the ball fall toward 
 the earth, but the earth also falls toward the ball. This 
 is actually the case. But, in order to understand why 
 the motion of the earth toward the ball is not percepti- 
 ble, it is necessary to understand what is meant by quan- 
 tity of motion. 
 
 331. Velocity, Mass, and Momentum. A body moving 
 from one position to another occupies time. The amount 
 of time consumed by a body in passing over a given dis- 
 tance depends upon its rate of motion, or how fast the 
 body is moving. This rate of motion of a body is called 
 its velocity. In determining the velocity of a moving 
 body not only must the distance passed over be con- 
 sidered, but also the time which is consumed. A body 
 which moves a distance of 5 feet in one second is said to 
 have a velocity of 5 feet per second. 
 
 The quantity of motion of a body is called momen- 
 tum. Common experience has taught us that, if two 
 bodies, one heavy and the other light, are moving with 
 equal velocities, it is much more difficult to stop the heavy 
 
MECHANICS 369 
 
 body than it is to stop the light one. In this case the 
 body having the greater amount of material, or mass, 
 has the greater quantity of motion. If one body is mov- 
 ing faster than another body of equal mass, the more 
 rapidly moving one is the harder to stop. The momen- 
 tum of a moving body depends, therefore, upon both its 
 mass and its velocity. 
 
 If two bodies have equal velocities, but the mass of one 
 is four times the mass of the other, the quantity of motion 
 of the greater is four times that of the lesser body. If 
 one of two bodies of equal mass is moving four times as 
 fast as the other, its quantity of motion is four times that 
 of the more slowly moving body. A body which has both 
 four times the mass and four times the velocity, as com- 
 pared with another body, has 4 X 4 or 16 times the quan- 
 tity of motion possessed by the other body. Therefore, 
 the quantity of motion, or momentum, of two bodies may 
 be compared by comparing the product of the mass and 
 the velocity of one body with the product of the mass 
 and the velocity of the other body. If a ball weighing 
 4 pounds moves 4 feet in one second, it has the same 
 momentum as a body weighing 1 pound moving 16 feet 
 in 1 second. This does not mean that both balls would 
 do the same amount of injury, if they should strike 
 another object, but that their momentum is the same. 
 
 332. Both Bodies Which Attract Each Other Will Fall 
 toward Each Other if Free to Move. If the earth falls tow- 
 ard the ball, while the ball is falling toward the earth, 
 the momentum of both the earth and the ball is the 
 same. But, in this case, since the mass of the earth 
 is enormously greater than the mass of the ball, the 
 distance moved by the earth in one minute must be 
 entirely imperceptible, notwithstanding the fact that 
 
370 ELEMENTAKY PHYSICS 
 
 the ball may have moved thousands of feet toward the 
 earth. 
 
 It is, scarcely likely that anyone will ever be able to 
 prove by means of direct observations, that the earth 
 falls toward the ball. But it has been proved by as- 
 tronomers that the earth and moon fall toward each 
 other, and that the quantity of motion, when the earth 
 falls toward the moon, is the same as the quantity of mo- 
 tion when the moon falls toward the earth. Why this 
 does not cause the earth and moon to rush toward each 
 other and to break each other up into fragments, cannot 
 be understood, until the character and causes of curvi- 
 linear motion are known ( 345). 
 
 333, Uniform Motion. A body which moves over equal 
 distances in successive equal intervals of time is said to 
 have uniform motion. For instance, if a body continues 
 to move over a distance of 5 feet each second, it neither 
 increases nor decreases in velocity and its motion is uni- 
 form. A body which moves a distance of 5 feet each 
 second will in 6 seconds move 5 x 6 = 30 feet. Hence, 
 to find the distance passed over when the uniform veloc- 
 ity and the time are given, multiply the velocity by the 
 time. 
 
 Calling the velocity v, and the time t, and the entire dis- 
 tance passed over s, the above rule can be stated in the 
 language of mathematics by s vt. Such a statement is 
 called a formula- After the formula for a given class of 
 problems has once been secured, there is no longer any 
 need of going through the long reasoning process re- 
 quired either in obtaining the formula, or in working 
 out each problem independently. All that is required is 
 to substitute for the letters in the formula their respec- 
 tive values as given in the problem. This results in an 
 
MECHANICS 371 
 
 equation which can be readily solved. If it is required 
 to find the distance passed over in 10 seconds by a body 
 having- a uniform velocity of 8 feet per second, -substi- 
 tute, in the formula s = vt, 8 for v, and 10 for t, and 
 the result is s 8 x 10. Whence s = 80 feet. 
 
 It is often convenient to have the formula so written 
 that the quantity whose value is to be found is placed by 
 itself on one side of the sign of equality. If instead of 
 the above problem, it were given that a body moved 80 
 feet in 10 seconds, to find the time, the 80 would be sub- 
 stituted for s in s = vt and 10 for t, which would give the 
 result 80 = v x 10, or 80 lOv ; whence v = 8. By alge- 
 
 o 
 
 braic processes s = vt can be readily changed into v = - 
 
 o t 
 
 and t = In the problem just given it would be more 
 
 convenient to use the formula in the form of t = than 
 
 v 
 
 in the original form of s = vt. 
 
 334. Uniformly Accelerated Motion. Anyone who 
 watches a train starting from a station can observe that 
 the velocity of the train is constantly increasing until full 
 speed is attained, when its motion is practically uniform. 
 On the contrary, when a train approaches a station, its 
 speed becomes slower and slower until the train stops. 
 If it be desired to determine the velocity of the train at 
 any point while its speed is either increasing or decreas- 
 ing, it will be necessary to determine what distance the 
 train would pass over in a second, minuto, or hour, with a 
 uniform velocity, equal to the velocity of the train at the 
 point in question. If a ball rolling down hill has at the 
 end of the third second a velocity of 20 feet per second, 
 it will move during the fourth second a distance of 20 
 feet, provided its velocity does not change after the third 
 second. 
 
372 ELEMENTARY PHYSICS 
 
 A force, acting upon a body which is free to move, pro- 
 duces motion in that body. As long as the force con- 
 tinues to act in the same direction, the body will continue 
 to gain in velocity. The rate of gain in velocity is called 
 acceleration. If the force is uniform, the acceleration will 
 be uniform. If a body started from rest and had a 
 velocity of 5 feet per second at the end of the first second, 
 a velocity of 10 feet per second at the end of the second 
 second, and a velocity of 15 feet per second at the end of 
 the third second, it can be seen that the rate of gain in 
 velocity per second is 5 feet per second. If a body start- 
 ing from rest has a uniform acceleration for each second 
 of 5 feet per second, it will have at the end of 10 seconds 
 a final velocity of 10 x 5 = 50 feet per second. Hence to 
 find the final velocity for a body starting from rest, when 
 the time and acceleration are given, multiply the accel- 
 eration by the time. 
 
 Calling the final velocity v, the acceleration a, and the 
 number of seconds t, we can express the above rule by 
 the formula v at. From this formula can be derived 
 
 111- V 3 J. V 
 
 by algebraic processes a == -' and. t = -' 
 
 t a 
 
 If a body start' from rest with a uniform acceleration of 
 8 feet per second, it will have a velocity at the end of 10 
 seconds of 10 x 8 = 80 feet per second. Since the body 
 started from rest, its initial velocity is 0, and, since its 
 final velocity is 80 feet per second, its average velocity 
 for the 10 seconds is J of 80 = 40 feet per second. Then 
 the distance passed over is 10 X 40 = 400 feet. 
 
 Calling the acceleration a, and the number of seconds 
 t, the final velocity is at. The average velocity then is \ 
 
 of at ' Multiplying the average velocity by the 
 
 at at* 
 
 time we have x t = - or J at\ Hence the distance (s) 
 
 A 2l 
 
MECHANICS 
 
 373 
 
 passed over may be represented by the formula s = J at 2 . 
 That is, the distance passed over by a body starting from 
 rest with uniform acceleration is equal to one -half the 
 acceleration multiplied by the square of the time. 
 
 By algebraic processes the formula 6- = \ at 2 can be 
 changed into the following forms : 
 
 2s . f2s v 2 
 
 a = ' t \/ ' and s 
 
 t* V a 2a 
 
 335. Falling Bodies. It has been ascertained by experi- 
 ment that, if a body is free to fall, it will 
 acquire during each second of its fall an 
 increase in velocity of 32 feet per second 
 (32.16 ft. at New York City). This is true of 
 all bodies, whether large or small, light or 
 heavy, provided there is nothing to resist 
 their fall. The reason a piece of paper does 
 not fall as fast as a lead bullet, when the two 
 are dropped simultaneously from the top of 
 a tower, is because in proportion to its weight 
 the paper meets with more resistance from the 
 air than does the bullet. If, however, the two 
 had been dropped in a vacuum instead of in 
 the air, they would both have struck the 
 ground at the same time (Fig. 161). 
 
 It is true that the farther we get above the 
 earth's surface, the less is the force of gravity, 
 but, unless the body under consideration is 
 falling from a very great height, the force of 
 attraction at any locality may without pro- 
 ducing any appreciable error be considered 
 constant. Since this force is constant, the ac- 
 celeration due to gravity is uniform and the laws of fall- 
 ing bodies are the same as those of uniformly accelerated 
 
374 ELEMENTAKY PHYSICS 
 
 motion. Instead, however, of representing the accelera- 
 tion due to gravity by a, it is customary to represent it by 
 g, the initial letter of the word gravity. We may, therefore, 
 re-write the formulas already secured for bodies having 
 uniformly accelerated motion, so as to make them apply 
 to falling bodies. Substituting g for a in the formulas 
 for bodies having uniformly accelerated motion ( 334), 
 we have the formulas for falling bodies, v = gt, s = J gt 2 , 
 
 2s ' A /~2s v 2 
 
 g = , t V , and s = 
 t* g 2g 
 
 336. Method of Determining the Distance a Body Falls 
 during Any Particular Second. If it be desired to find how 
 far a body will fall during the tenth second, find how far 
 it will fall during 10 seconds, then how far it will fall dur- 
 ing 9 seconds ; from the first subtract the second, and the 
 remainder is the distance it will fall during the 10th sec- 
 ond. To make this general, suppose it is desired to find 
 the distance fallen during the tf-th second. The distance 
 fallen during t seconds is Jgt 2 , and during t 1 seconds 
 is ig(t - I) 2 , igt 2 - |g(t - 1) 2 = ig (2t - 1), the distance 
 fallen during the t-th second. That is, to find the dis- 
 tance passed through by a body falling from rest during 
 any given second, multiply twice the number of seconds 
 minus 1 by one-half of the acceleration due to gravity. 
 
 337. The Pendulum. Fasten a small iron or lead ball 
 to the end of a thread and attach the other end of the 
 thread to a fixed point to serve as a support. The thread 
 assumes a vertical position, and is in line with the direc- 
 tion in which the force of gravity acts. Pull the ball 
 to one side and let go. The ball vibrates to and fro. 
 This thread and ball constitute a pendulum. It can be 
 seen that as the ball is drawn to one side, it must also 
 be raised. When the hand lets go of the ball, the ball 
 
MECHANICS 375 
 
 begins to descend, but, on account of the string- it can- 
 not descend directly downward, but it descends as much 
 as it can and its path is the arc of a circle of which the 
 string is the radius. "When the ball has reached its 
 original position, it is as low as it can get, but, in falling 
 to this position, the ball has acquired a velocity, and its 
 momentum carries it past this lowest point and the ball 
 begins to ascend on the opposite side. The ball con- 
 tinues to ascend on the opposite side until its motion 
 has all been spent, when it again begins to descend and 
 the operation is repeated. 
 
 In investigating the theory of the pendulum, it is con- 
 venient to make use of an ideal or simple pendulum 
 which may be denned as a heavy material point sus- 
 pended from a fixed point about which it is free to move, 
 by a string without weight and incapable of stretching. 
 Of course, no such pendulum actually exists. 
 
 All pendulums which do not comply with the conditions 
 of an ideal or simple pendulum are called compound 
 pendulums. Since there can be no string without weight 
 and no material body without size, all pendulums in 
 actual existence are compound. The nearest approach 
 which can be made to a simple pendulum is to suspend a 
 small metal ball by means of a fine thread. 
 
 338. Terms Defined. A simple Vibration or Oscillation 
 is the motion of the pendulum from one extreme of its 
 swing to the other extreme. The term vibration is fre- 
 quently used to denote a simple vibration. 
 
 The Centre of /Suspension is the point of support about 
 which the pendulum vibrates (C, Fig. 162). 
 
 A Complete Vibration is the motion of the pendulum 
 from one extreme of its swing to the other extreme and 
 back again (A D A or B D A B). 
 
376 
 
 ELEMENTAKY PHYSICS 
 
 
 
 B 
 
 FIG. 162. 
 
 The Amplitude of Vibration is half the angle described 
 by the swinging pendulum (angle A C B or B C D). 
 
 The Period of Vibration of a pendu- 
 lum is the time required to make a 
 simple vibration. 
 
 The Rate of Vibration is the number 
 of simple vibrations made by the pen- 
 dulum in one second. 
 D 339. The Rate of Vibration Depends 
 upon the Length of the Pendulum. 
 Suspend two lead balls by means of 
 threads of equal length so that the balls will hang side 
 by side without touching. Cause them to vibrate so that 
 the amplitude of one vibration is larger than that of the 
 other. They both require equal times to make a vibra- 
 tion. Each successive vibration of a pendulum requires 
 the same amount of time equal to 
 that required by the preceding 
 one. If, however, the pendulum 
 be removed to a place where the 
 force of gravity is greater, it will 
 vibrate faster, but if taken to a 
 place where the force of gravity 
 is less, it will vibrate more 
 slowly. 
 
 Suspend a third ball by means 
 of a shorter thread and cause it 
 to vibrate. The shorter pendu- 
 lum vibrates faster than the 
 longer one. It has been found 
 
 that a pendulum 4 times as long as another pendulum 
 vibrates half as fast, and one 9 times as long vibrates 
 only one-third as fast. 
 
 FIG. 163. 
 
MECHANICS 377 
 
 340. The Law of the Pendulum. The law of the pen- 
 
 dulum is commonly expressed by the formula t = w * 
 
 O 
 
 in which -* 3.1416, 1 = length of the pendulum, and g 
 the acceleration due to gravity. The development of 
 this formula involves the use of higher mathematics, and 
 hence would be out of place here. This formula con- 
 tains three quantities, either two of which being known, 
 the third can be found. Thus in any locality where g is 
 known, the length of a seconds pendulum can be com- 
 puted, or, if the length and g are known, the time of 
 vibration can be found, and, if the time of vibration and 
 the length are known, g can be determined. The length 
 of a pendulum which beats seconds, when at sea-level in 
 the latitude of New York City, is 39.1 inches. 
 
 341. Length of Compound Pendulum. Most compound 
 pendulums in use consist of a ball or bob suspended by 
 means of a rigid rod. Since a short pendulum vibrates 
 faster than a long one, the particles in its upper part are 
 retarded by those below, and the particles below are ac- 
 celerated by those above. But there must be one point 
 which is neither accelerated nor retarded, that is, the ac- 
 celeration given it by the particles above is counteracted 
 by the retardation of the particles below it. This point 
 is called the centre of oscillation. The distance from the 
 centre of suspension to the centre of oscillation consti- 
 tutes the theoretical length of the pendulum. 
 
 It has been ascertained that if a pendulum be sus- 
 pended at its centre of oscillation, the rate of vibration 
 is not changed ; hence, the centre of suspension and the 
 centre of oscillation are interchangeable. This fact 
 furnishes the means of ascertaining the length of a 
 compound pendulum. 
 
378 
 
 ELEMENTARY PHYSICS 
 
 ---is 
 
 Fig-. 164 is that of a reversible pendulum with two 
 movable weights, M and N, which can be so adjusted that 
 the pendulum requires the same length of time to 
 complete a vibration whether suspended at A or at 
 B. This pendulum vibrates in the same time as the 
 pendulum, S O, which consists of an iron ball 
 suspended by a thread. 
 
 342. Uses of the Pendulum The 
 
 most familiar use of the pendulum 
 is that of serving as a regulator of 
 the motion of clocks. It also fur- 
 nishes the most precise and most 
 convenient way of measuring the 
 force of gravity. The first experi- 
 mental proof which showed that the 
 earth is flattened at the poles was 
 obtained by counting the vibrations 
 of a pendulum in different latitudes. 
 The United States Coast and Geo- 
 detic Survey uses the pendulum 
 exclusively in all of its gravity 
 work. The pendulum used by the 
 United States is enclosed in an air- 
 tight chamber from which the air 
 can be exhausted to any press- 
 ure desired. 
 
 343. Inertia If a stone lie on the 
 ground, it will remain in the same po- 
 sition until some force causes it to 
 move If a book has been placed upon 
 a desk and afterwards is gone, it is 
 certain that its removal was. due to some force'. I The book 
 has no power of setting itself in motion. 
 
 I /H\ 
 
 ,A I* 
 
 T 
 
 1 
 
 ;m \ 
 
 
 
 d 
 
 3 
 
 
 
 JIN 
 
 
 
 
 
 UN 
 
 a 
 
 o 
 
 
 
 1 
 
 
 HM 
 
 >y<- 
 
 <$Z o 
 
 If 
 
 A 
 
 
 
 A 
 
 g 
 
 S] 
 
 4-O- 
 
 FIG. 164 
 
 FIG. 1G5. 
 
MECHANICS 379 
 
 If a ball is already in motion, force is required to 
 change the character of that motion. Force is required 
 to make the ball move faster or to make it move more 
 slowly, or to make it move to one side, or to stop it 
 entirely. 
 
 When a ball is moving over any ordinary surface and 
 force is no longer applied to it, the ball will soon come 
 to rest. The force which resists or opposes the motion 
 in this case is friction. If the friction is made very small 
 by securing a smooth surface for the ball to roll upon, 
 the ball will roll for a much longer time. The resist- 
 ance of the air also helps to stop the ball. If it were 
 possible to secure ice whose surface is perfectly smooth 
 and flat, in a region where there is no air, a boy, moving 
 rapidly along this surface, would not be able to stop 
 himself. There would be no friction, either on the part 
 of the ice or on the part of the air, to decrease the speed 
 of his motion. 
 
 Every body continues in its state of rest or in its state 
 of uniform motion in a straight line unless it is compelled 
 by some force to change that state. Any change which a 
 body may exhibit is at once the evidence of the existence 
 of some force. Force may be described as anything 
 which produces, or tends to produce, opposes, or changes 
 motion in any manner. The inability of a body to start 
 to move from a state of rest, or to change the character 
 of its motion when it is already in motion, is called 
 inertia. 
 
 344. Units of Force, There are two systems of units 
 used in measuring force : the absolute units and the grav- 
 ity units. 
 
 In the gravity system the unit is the pound, or, when 
 the metric system of units is used, it is the kilogram or 
 
380 ELEMENTARY PHYSICS 
 
 the gram. The pound and the kilogram or gram are 
 the names of units used in measuring both mass and 
 force. By the mass of a body is meant the amount of 
 matter which it contains, not its weight. By its weight 
 is meant the force with which it tends to move to- 
 ward the earth. Mass and weight are not identical. 
 The pound mass is the mass of a piece of platinum 
 carefully preserved at the national capital. The force 
 pound is the force of the attraction which the earth 
 exerts upon a body on its surface whose mass is one 
 pound. The kilogram is the force of the attraction 
 which the earth exerts upon a body having a mass of 
 one kilogram. 
 
 The mass of a body remains constant wherever the 
 body may be placed, while the attraction of the earth for 
 it varies with the latitude and altitude of the place where 
 the body is located. A body if weighed at the equator 
 would weigh less than if weighed at the poles, and its 
 weight would be less on the summit of a mountain than 
 at the base. If a body having a mass of one pound 
 should be transported to the moon, and then to the sun, 
 it would be found that the body is attracted on the sur- 
 face of the moon by a less force, and on the surface of 
 the sun by a greater force than on the surface of the 
 earth. Since the force of gravity varies at different 
 points on the earth's surface, the gravity unit also varies, 
 and, for this reason, it is not suitable for precise scien- 
 tific investigations. 
 
 Momentum has been defined as the quantity of motion 
 of a body and is found by multiplying the mass of a body 
 by its velocity. Change of momentum in a given time is 
 proportional to the force which acts ; therefore, the force 
 may be measured by this change of momentum. A force 
 
MECHANICS 381 
 
 which in one second would give a body of ten pounds 
 mass a velocity of 2 feet per second, would give a body 
 of one pound mass a velocity of 20 feet per second. 
 
 A force which, if acting- upon a mass of one pound for 
 one second, will produce a change of velocity of one foot 
 per second, is called a poundal. A force which, if acting 
 upon a mass of one gram for one second, will produce a 
 change of velocity of 1 centimetre per second, is called a 
 dyne. These are absolute units whose values are invari- 
 able. 
 
 On the earth a body having a mass of one pound is 
 said to weigh one pound. If this body be allowed to fall 
 it will acquire in one second a velocity of 32.16 feet per 
 second (g at New York City is 32.16 feet). Since the force 
 of gravity acting upon a body of one pound mass for 
 one second gives it a velocity of 32.16 feet per second, a 
 force of one pound must be equal to 32.16 poundals. 
 Therefore, to change pounds to poundals, multiply the 
 number of pounds by 32.16, or g. Likewise, a body of one 
 gram mass, when free to fall, acquires in one second a 
 velocity of 980 centimetres per second. Hence a force of 
 1 gram is equal to 980 dynes. In each case it will be ob- 
 served that one gravity unit is equal to g absolute units. 
 
 Let ^represent the force, m the mass of the body, and 
 a the acceleration ; then, since force may be measured 
 by change of momentum, which is the mass multiplied by 
 the acceleration, we have the formula F ma (poundals 
 or dynes). 
 
 345. Curvilinear Motion and Centrifugal Force. One 
 force may cause a body to move, but a second force, act- 
 ing not in the same direction as the first, is necessary to 
 cause the body to move in a curve. 
 
 When a stone is fastened to a string and whirled, a part 
 
382 
 
 ELEMENTARY PHYSICS 
 
 PIG. 166. 
 
 of the force exerted by the hand is used in urging- the 
 stone forward, while the other part is used in pulling 
 
 the stone toward the hand. 
 After the stone is once set 
 in motion, very little force 
 is required to keep it mov- 
 ing, since all of the resist- 
 ance to be overcome comes 
 from the air. 
 
 If the string, in any po- 
 sition, be cut by means of 
 a very sharp knife, the 
 stone will start away in a 
 straight line which forms 
 a right angle with the po- 
 sition which the string 
 
 occupied the instant it is cut. Thus it can be seen that 
 
 the stone is continually being pulled in toward the hand, 
 
 in order to keep it from 
 
 going away in a straight 
 
 line. If the direction in 
 
 which the stone actually 
 
 moves is compared with 
 
 the straight path along 
 
 which the stone would 
 
 move, if it were not con- 
 tinually pulled aside, it is 
 
 seen that the pull of the 
 
 hand practically moves 
 
 the stone toward the hand. 
 
 The outward pull which 
 
 is experienced on whirling the stone about the hand 
 
 by means of a string is called centrifugal force. The 
 
MECHANICS 383 
 
 pull exerted by the hand is known as centripetal force 
 and is equal to and opposite in direction to the centri- 
 fugal force. 
 
 Suppose a body to be moving- in a circle of radius r 
 with the uniform velocity v, so that in the space of time t 
 it will have gone from A to E. Then arc A E = vt. In 
 passing from A to E the body has been drawn aside from 
 the straight path A D toward the centre C, a distance equal 
 to D E (or A B). A uniform force tends to give a body 
 uniformly accelerated motion ( 334). Therefore, since 
 the force pulling the body toward the centre is uniform, 
 the acceleration given this body toward the centre must 
 be uniform. 
 
 A B = the distance the body has been drawn aside from 
 the straight path A D in time t. Therefore A B = \ at 
 ( 334). If t is taken very small, the difference in length 
 between the chord A E and the arc A E becomes too 
 small to be readily recognized ; the chord A E and the arc 
 A E may, for purposes of this discussion, be considered 
 identical. By geometry it can be proven that the tri- 
 angles AEG and ABE are similar. 
 
 /. A G : A E :: A E : A B. 
 
 A G, being the diameter, is equal to 2r. A E, the dis- 
 tance passed over by the body with uniform velocity v, is 
 equal to vt. A B, the distance moved due to the ac- 
 celeration a toward the centre, is equal to \ a&. Sub- 
 stituting these values in the above proportion, 
 
 2r : vt : : vt : \ at\ 
 Whence v 2 t 2 = \ at? x 2r t 
 
 v* 
 and a = 
 
 v* T 
 
 Substitute for a in formula F = Ma t 
 
 T 
 
 I -mr 9 
 
 Centrifugal force, or F. 
 
384 ELEMENTARY PHYSICS 
 
 346. Work. Work is the overcoming of resistance in 
 producing or maintaining motion. The amount of work 
 which a force (F) acting upon a body has accomplished, 
 depends entirely upon the distance (s) the force has act- 
 ually moved the body in opposition to some resisting 
 force ( W Fs). A force may act, yet do no work, pro- 
 vided no motion results. 
 
 The unit of work used by English-speaking engineers 
 is the foot-pound, which is the work done in moving a 
 body one foot against a resistance of one pound. It is 
 best illustrated as being the work done in lifting a pound 
 weight through a vertical distance of one foot. All re- 
 sistance to be overcome, however, does not consist in 
 lifting weights against gravity. Work is done when a 
 board is torn loose from the side of a house, or in draw- 
 ing a body horizontally over the surface of another 
 body. 
 
 The kilogram-metre is the work done in moving a body 
 one metre against a 1 resistance of one kilogram. 
 
 The absolute units of work are the foot-poundal and the 
 erg. 
 
 The foot-poundal is the work done in moving a body 
 one foot against a resistance of one poundal. 
 
 The erg is the work done in moving a body one centi- 
 metre against a resistance of one dyne. 
 
 Power. Power in mechanics is the rate at which an 
 agent can work. The horse-power is the unit of power 
 and is that power which can do 33,000 foot-pounds of 
 work in one minute, or 550 foot-pounds in one second. 
 The horse-power was introduced by Watt so that the 
 values of the engines could be compared with the work 
 of horses. A horse cannot work quite at this rate, but a 
 margin was allowed to prevent the mistake of making a 
 
MECHANICS 385 
 
 horse-power too small. A man can work at the rate of 
 a little less than \ of a horse-power. 
 
 347, Energy. Energy is the ability to do work. Boys 
 have learned that an apple hanging beyond their reach 
 on a tree can be brought to the ground by a stone or club. 
 The moving stone possesses the ability to tear the apple 
 loose from the tree and gravity then causes the apple 
 to fall. The stone possesses the ability to do work be- 
 cause of its motion. 
 
 If a string be tied to a stone lying on a shelf and the 
 string be passed over a pulley and fastened to a lighter 
 object on the floor below, the stone, when free to fall, will 
 raise the body from the floor. The stone by virtue of its 
 position up on the shelf is able to perform work. 
 
 There are two types of energy, kinetic and potential. 
 Kinetic energy is the energy of a body by virtue of its mo- 
 tion. Potential energy is the energy of a body by virtue 
 of its position in reference to other bodies, or of the 
 relative position of its parts. Any body elevated above 
 the surface of the earth, a coiled or bent spring, or any 
 elastic body changed from its natural shape possesses 
 potential energy. In fact, a body possesses potential 
 energy if any condition is present which is capable of 
 producing motion in that body, such as strain, gravita- 
 tive separation, chemical separation, and electrification. 
 
 348. How Energy is Measured, Energy can be measured 
 only by the work it can do, therefore the units of energy 
 are the same as the units of work. 
 
 Suppose a body of a mass of m pounds start vertically 
 upward with a velocity of v feet per second, then the 
 
 9 
 
 distance (s) it will rise is The work done in raising 
 
 . v 2 
 
 a mass of m pounds s feet is ms foot-pounds. But s 
 
386 ELEMENTARY PHYSICS 
 
 (335). Multiplying- both members of this equation 
 
 by m, we have ms (the kinetic energy of the mass) J 
 
 2g 
 
 rto/i ri\% 
 
 Therefore representing the kinetic energy by E,E = 
 foot-pounds. 
 
 It has been shown that when a constant force acts upon 
 a body with mass m producing acceleration a, that F = 
 Ma poundals or dynes. Kepresenting the distance by s, 
 the work done is Fs foot-poundals or ergs. But s 
 
 *?? ^ QJ 
 
 * Multiplying s \>yF Ma, member by member, 
 
 we have fs (the kinetic energy of the mass) = 
 
 A 
 
 Hence E\ mrf foot-poundals or eggs. 
 
 349. Conservation of Energy. If a body be projected 
 vertically upward against gravity, it has kinetic energy 
 sufficient to lift its weight a certain height depending on 
 the strength of the projecting force. As the body rises, 
 its kinetic energy is expended in overcoming the resist- 
 ance of gravity, until when it comes to rest, the whole of 
 its kinetic energy has been expended, or rather changed 
 into potential energy. By virtue of the work done on 
 the body during its ascent, it has, at the instant, of com- 
 ing to rest, an amount of potential energy equal to the 
 amount of kinetic energy expended in lifting it to the 
 height attained. It then falls with uniformly accelerated 
 motion, changing the potential energy into kinetic, and 
 acquires more and more kinetic energy, until, at the in- 
 stant it reaches the point from which it was projected, it 
 has the same amount of kinetic energy as when it started 
 up that is, an amount sufficient to lift its own weight to 
 the height from which it fell. 
 
 At the beginning of the ascent, and at the end >f '> 
 
MECHANICS 387 
 
 descent, the body had the same amount of kinetic en- 
 ergy ; at the end of the ascent, and the beginning of the 
 descent, it had only potential energy. 
 
 At any point of the ascent it had enough potential 
 energy to lift it to the height already attained, and 
 enough kinetic energy to lift it the remainder of the 
 height yet to be attained. At any point of the descent it 
 had enough kinetic energy to lift it back to the point from 
 which it started to fall, and enough potential energy to 
 lift it from the lowest point of fall to its present position. 
 The sum of kinetic and potential energies remains con- 
 stant. 
 
 A body in motion on the earth's surface will sooner or 
 later come to rest after the propelling force ceases to act, 
 because the motion of every body on the earth is opposed 
 by friction. What has become of the energy of the mov- 
 ing body ? It has been transformed into heat. A bullet, 
 fired against a stone, is stopped by the stone. The bullet 
 is warmed by its impact against the stone. 
 
 When a mechanical engineer examines a dynamo for 
 the first time, he cannot understand why so much driv- 
 ing power is needed to make the armature rotate. He 
 pees th.-.t the friction of the bearings and of the brushes 
 agai 1 st the commutator is so small that it can absorb 
 but a small part of the energy delivered by the engine. 
 Some cause other than friction alone must be looked for. 
 It is found in the fact that, when the wires of the arma- 
 ture cut the lines of magnetic force, a resistance or drag 
 is experienced by the wires, and the energy required to 
 overcome this drag is converted into the energy of an 
 electric current. 
 
 heat is a form of energy can be seen in a locomo- 
 drawing a train of cars. Here it is the heat from the 
 
388 ELEMENTARY PHYSICS 
 
 burning- coal that supplies the engine with the required 
 energy. 
 
 That an electric current is a form of energy can be 
 seen in any city when an electric car is passing down the 
 street. In the locomotive it is the heat, in the electric 
 car it is the electric current which is transformed into 
 mechanical motion. 
 
 Energy which is apparently lost is merely transformed 
 into some other form. When it is transformed into heat, 
 the heat may be radiated away into space and become 
 lost or dissipated, but it cannot be annihilated. Neither 
 can energy be created. This can be summed up as one 
 of the greatest conceptions of modern science, namely 
 the conservation of energy. Energy can be transformed 
 from one form into another, but the sum total of all the 
 energy in the universe remains constant. 
 
 350. Machines. Of all animals man alone makes use 
 of devices for the advantageous application of force. 
 Such devices are known as machines. Machines are so 
 designed as to employ force for the attainment of a de- 
 sired result. By their use man can do work which other- 
 wise would be impossible. He is also thus enabled to 
 employ forces of nature, such as the force of the wind, 
 the energy of running or falling water, the energy in fuel, 
 and the strength of other animals. Machines have been 
 devised for utilizing even the force of the tides and the 
 heat of the sun. 
 
 The force which is applied to a machine is called the 
 power; and the resistance to be overcome is called the 
 weight. You cannot get more work out of a machine 
 than is put into it, because energy cannot be created. In 
 reality there is no machine in which there is no friction, 
 
MECHANICS 389 
 
 and, as a result, no machine turns out as much useful work 
 as is put into it. Some of the energy is transformed into 
 heat which is radiated away, and, as far as utility is con- 
 cerned, may be considered wasted. The ratio of the useful 
 work done by the machine to the total work done upon 
 the machine is known as the efficiency of the machine. 
 
 To find the work done by the power, it is necessary to 
 multiply the power by the distance through which it 
 moves. To find the useful work done, multiply the 
 weight by the distance through which it is moved. If 
 a machine could be made in which there were no friction, 
 we should have a, perfect machine. In such a machine the 
 work done by the power is equal to the work done upon 
 the weight. In the discussion of machines they will be 
 considered as perfect. Hence, we can deduce the gen- 
 eral law of machines : The power multiplied by the dis- 
 tance through which it moves is equal to the weight multiplied 
 by the distance through which it is moved. 
 
 If the machine under consideration has any parts which 
 bear the same ratio to each other as the ratio of the dis- 
 tance through which the power moves to the distance 
 through which the weight is moved, the power and the 
 weight can be respectively multiplied by the dimensions 
 of these parts. For illustration, if both the power and 
 the weight move in circles, the power multiplied by the 
 radius (or diameter) of its circle is equal to the weight 
 multiplied by the radius (or diameter) of its circle. 
 
 From the general law of machines it can be seen that 
 the faster the power moves as compared with the weight, 
 the greater is the weight which can be moved> Hence, 
 greater force on the weight can be had at the expense of 
 speed, while greater speed can be obtained at the e^- 
 pense of force. In practice, when the weight is a body 
 
390 ELEMENTARY PHYSICS 
 
 moved against resistance, a little additional power is re- 
 quired to impart motion to the body and also to the parts 
 of the machine which partake of motion. After* the re- 
 quired speed has been established, then the power and 
 weight bear the relation to each other as suggested in 
 the general law of machines. 
 
 351. Simple Machines. Every machine, however com- 
 plicated it may be, is but a combination of one or more 
 
 of the simple ma- 
 ^^^^^--^^^i^ ^'^^^^^^^ chines, or mechanical 
 
 + ^^^~ m Weight arm * 
 
 p power Arm F w powers. There are 
 
 Lever of the first class. , . 
 
 FlQ 168 . six simple machines 
 
 the lever, the wheel 
 
 -P and axle, the pulley, 
 
 Power Arm . 
 
 |^^^^ the inclined plane, 
 
 the screw, and the 
 
 Lever of the second class. 
 
 352. The Lever.- 
 The lever is a rigid 
 
 t Power Arm bar free to move 
 
 ^^^^^^ 
 
 r<_^ about a fixed point 
 
 w Weight arm . 
 
 Lever of the third class. OT aX1S Called l 
 
 FIG. 170 crum. The part of 
 
 the lever between the 
 
 power and the fulcrum is called the power arm. The part 
 of the lever between the weight and the fulcrum is called 
 the weight arm. 
 
 There are three classes of levers. A lever of the first 
 class is one in which the fulcrum is between the weight 
 and the power In a lever of the second class the weight 
 is between the power and the fulcrunie In a lever of the 
 third class the power is between the weight and the ful- 
 crum. 
 
MECHANICS 
 
 391 
 
 FIG. 171. 
 
 353. Law of the Lever. The power multiplied ly its dis- 
 tance from the fulcrum is equal to the iveight multiplied l)y 
 its distance from the 
 fulcrum. 
 
 In the case of the 
 lever, the power and 
 the weight move in 
 circles with the ful- 
 crum as the centre, the 
 power arm being- the radius of the circle whose circum- 
 ference is traversed by the power, and the weight arm 
 being the radius of the circle 
 whose circumference is traversed 
 by the weight. Since the ratio of 
 the radii of these circles is the 
 same as the ratio of their circum- 
 ferences, it is clear that the law of 
 the lever is derived from the gen- 
 eral law of machines. 
 
 354. Wheel and Axle. The wheel 
 and axle consist of a wheel fast- 
 ened to an axle so that they turn 
 together. The 
 power is applied 
 to the circumfer- 
 ence of the wheel, and the weight is fast- 
 ened by means of a cord to the circum- 
 ference of the axle. 
 
 The power passes over a distance 
 equal to the circumference of the wheel 
 in the same time that the weight passes 
 over a distance equal to the circumference of the axle. 
 Hence, the power multiplied Ijy the circumference (or radius 
 
 FIG. 172. 
 
 FIG. 173. 
 
392 
 
 ELEMENTARY PHYSIOS 
 
 FIG. 174. 
 
 or diameter) of the wheel is equal to the weight multiplied 
 by the circumference (or radius or diameter] of the axle. 
 
 Instead of a wheel a crank 
 may be used, as in a windlass. 
 Sometimes the w T heel and the 
 axle do not have the same axis, 
 as when motion is communi- 
 cated from wheel to wheel by 
 means of belts or cogs. 
 
 355. The Pulley. A pulley 
 consists of a wheel free to turn 
 about an axis. The wheel is 
 
 usually placed within a frame called the block. If, while 
 the power is acting, the block is stationary, the pulley is 
 said to be fixed; if the block moves, the pulley is said 
 to be movable. The rope or string which passes over the 
 wheel of the pulley has the same tension at every point. 
 
 In a single fixed pulley (Fig. 175), the weight and 
 power move equal distances ; hence the power is equal to 
 the weight. There is no gain of either force or speed, 
 but merely a change of direction of the force applied. 
 
 If a continuous 
 cord pass about a 
 movable and a fixed 
 pulley (Fig. 177), 
 the movable pul- 
 ley sustaining the 
 weight is supported 
 by two parts of the 
 cord, and the power 
 must pass through 
 
 twice the distance passed over by the weight. The 
 weight, therefore, is equal to twice the power. If the 
 
 > 
 
 FIG. 175. 
 
 FIG 17G. 
 
MECHANICS 
 
 393 
 
 cord pass about one movable and two fixed pulleys, the 
 pulley sustaining the weight is supported by three parts 
 of the cord and the power must 
 pass through three times the 
 distance passed over by the 
 weight. Therefore, the weight 
 is equal to three times the 
 power. In a similar manner it 
 can be shown that, when one 
 continuous cord is used, the 
 power must pass over a dis- 
 tance as many times greater 
 
 FIG. 177. 
 
 7 than the distance passed over by 
 
 the weight, as there are parts of the 
 
 cord sustaining the weight. Hence, 
 the law of the pulley: The iveight 
 is equal to the power multiplied l)y 
 the number of parts of the cord sus- 
 taining the weight. 
 
 356. Inclined Plane Any surface 
 inclined to a horizontal surface is 
 an inclined plane. In rolling a 
 barrel into a wagon, it is much 
 easier to roll the barrel on a plank 
 having one end on the ground and 
 the other end resting on the wagon 
 so as to form an inclined plane, 
 than it is to lift the barrel vertical- 
 ly into the wagon. A part of the 
 weight of the barrel produces press- 
 ure on the plank, while the rest of 
 its weight tends to make it roll down the plank to the 
 ground. All the force which is required is that needed 
 
 FIG. 178. 
 
394 
 
 ELEMENTARY PHYSICS 
 
 FIG. 179. 
 
 W 
 
 to overcome the tendency of the barrel to roll down the 
 inclined plane. jThe steeper the plank the greater must 
 be the force required to roll the barrel into the wagon.) 
 If the plank is in a vertical position this force must be 
 equal to the weight of the barrel. 
 
 In Fig. 179, A C is the 
 length of the plane, B C the 
 vertical height, A B the 
 base, a the angle of the 
 plane. 
 
 In the inclined plane the 
 power must move over the 
 entire length of the plane, 
 while the weight is raised 
 the vertical distance from 
 fche foot of the plane to the 
 top. Hence, the power multi- 
 plied by the length of the plane 
 is equal to the weight multi- 
 plied by the vertical lieight of 
 the plane. 
 
 FIG. isi. If the power acts, in a line 
 
 parallel with the base, the 
 
 power must act over a distance equal to the length of the 
 base of the plane while the weight is raised the vertical 
 distance from the foot of the plane to the top. Hence, 
 when the power is applied in a line parallel to the base, the 
 power multiplied by the base is equal to the weight multiplied 
 by the vertical height of the plane. 
 
 357. The Wedge If it is desired to raise one end of a 
 
 stone block, this can easily be accomplished by forcing 
 an inclined plane under that end of the block, as shown 
 in Fig. 182. This is the principle of the wedge, which 
 
 FIG. 180. 
 
 W 
 
MECHANICS 
 
 395 
 
 has usually the form of a double inclined plane with the 
 bases placed next to each other. The law of the inclined 
 plane, where the power is applied par- 
 allel to the base, applies also to the 
 wedge ; but, since the wedge is usually 
 driven by percussion, it is difficult to 
 apply any law other than that the 
 smaller the angle of the planes the 
 greater is the weight which can be 
 overcome. 
 The wedge is of great service in 
 
 FIG. 182. 
 
 FIG. 183. 
 
 splitting wood and in raising great weights short dis- 
 tances, such as raising ships, separating layers of rock, 
 etc. 
 
 358. The Screw, The screw consists of a cylinder with 
 
 Pitch 
 
 Pitch 
 
 FIG. 181. 
 
 a rib or thread passing spirally around it. The principle 
 involved is that of the inclined plane, as can be shown by 
 
396 
 
 ELEMENTARY PHYSICS 
 
 cutting- a piece of paper in the shape of a right triangle 
 and wrapping it around a lead pencil (Fig. 185). The 
 hypotenuse of the triangle traces out the path of 
 the thread. The distance between two contiguous 
 turns of the thread measured along the axis of the 
 cylinder is known as the pitch of the screw. The 
 screw works in a block on the inside of which are 
 threads corresponding to those of the screw. This 
 block is called the nut. 
 
 The power is usually applied to a wheel or lever 
 fastened to the upper part of the screw. In order 
 to turn the screw once, the power must pass over a 
 distance equal to the circumference of the wheel or 
 the circle of which the lever is the radius. In one 
 turn of the screw the weight is raised a distance 
 equal to the pitch of the screw. Hence, the power 
 multiplied by the circumference of the wheel is equal 
 to the weight multiplied by the pitch of the screw. 
 
 359. The Great Importance of Mathematics in 
 Modern Physics. Modern Physics rests 
 upon the mathematical development of 
 the ideas of momen- 
 
 FiG. 185. 
 
 turn, force, energy, 
 work, power, and other similar 
 quantities. While it is possible 
 to get an insight into many 
 physical and chemical prob- 
 lems without their use, they 
 are the foundations upon which 
 must be built all further prog- 
 ress. The mathematical de- 
 velopment of these ideas is, therefore, the first care of 
 any larger work 011 physics- It is hoped the more ele- 
 
 PlG. 186. 
 
MECHANICS 397 
 
 mentary work here presented will encourage students to 
 undertake the more advanced work based upon this 
 mathematical foundation. 
 
 EXERCISES 
 
 1. Is it possible ever to make a perpetual motion machine ? 
 
 Why? 
 
 2. If a stone be dropped from the top of a high monument, the 
 
 stone will fall a little to the east of the vertical line. 
 Explain. 
 
 3. Why cannot a man lift himself up by stepping into a 
 
 bushel basket and lifting on the handles ? 
 
 4. Why is a bicyclist apt to fall if he attempts to turn a 
 
 corner rapidly when the street is slippery ? 
 
 5. When a train starts, a passenger can feel his body pressing 
 
 harder against the back of the seat. Explain. 
 
 6. When a marksman shoots at a target, does the bullet 
 
 travel in a straight or curved line ? Explain. 
 
 7. If a ball be fired from a cannon pointing vertically upward, 
 
 will it fall back into the mouth of the cannon ? 
 
 8. How can a driver make it easier for his horses to draw a 
 
 heavy load up hill ? 
 
 9. If a large hole were made extending from one side of the 
 
 earth through the centre to the opposite side and a ball 
 were dropped into this hole, what would become of the 
 ball? 
 
 10. Why does the mud fly off of your shoe when you kick? 
 
 11. Why do fast revolving grindstones often break ? 
 
 12. Why is more coal consumed in running a train a given dis- 
 
 tance in which numerous stops are made than is con- 
 sumed by the train in running the same distance with- 
 out stops ? 
 
 13. Why is it so difficult to hold out a heavy weight at arm's 
 
 length ? 
 
 14. What effect has the placing of a live fish in a bucket even 
 
 full of water upon the weight of the bucket and its con- 
 tents? 
 
398 ELEMENTARY PHYSICS 
 
 15. A stone dropped into a well strikes the bottom in 3 
 
 seconds. How deep is the well ? 
 
 16. A man can pump 200 Ibs. of water per minute to a height 
 
 of 16 feet ; how many foot-pounds of work does he do 
 in an hour ? 
 
 17. A twenty H. P. engine is employed to pump water from 
 
 the bottom of a mine 400 feet deep. How many cu- 
 bic feet of water will it raise in 24 hrs. ? (1 cu. ft. of 
 water = 62 Ibs. ) 
 
 18. What force is required to roll a ball weighing 300 Ibs. up 
 
 an incline having a vertical rise of 10 ft. to every 100 
 ft. of the incline, when the force is applied parallel to 
 the plane ? When it is applied parallel to the base? 
 
 19. How far will a body, starting from rest, fall during the 
 
 first second ? During the eighth second ? 
 
 20. What is the kinetic energy of a stone of 10 Ibs. mass mov- 
 
 ing with a velocity of 15 ft. per sec. ? What is the mo- 
 mentum of the stone ? 
 
 21. What weight can be raised by a force of 100 Ibs. by means 
 
 of one fixed and one movable pulley ? 
 
 22. What is the length of a pendulum beating half seconds at 
 
 a place where the seconds pendulum is 39.1 inches? 
 
 23. The pitch of a screw is ^ inch. The length of the lever 
 
 used in turning the screw is 2 ft. What power must be 
 applied to produce a pressure of one ton ? 
 
 24. The radius of the wheel is 2 ft. and of the axle 4 in. What 
 
 weight can be raised by a power of 50 Ibs. ? 
 
 25. A body of mass 2 Ibs. is attached to the end of a string a 
 
 yard long, and is whirled round at an uniform rate, 
 making twenty revolutions in a minute. What is the 
 tension in the string? 
 
 26. A force of 30 dynes acts for 10 seconds upon a body resting 
 
 on a smooth horizontal plane, and imparts to it a veloc- 
 ity of 100 centimetres per second. What is the mass of 
 the body ? ' 
 
INDEX 
 
 ABSOLUTE UNITS, 381, 384 
 
 Absolute zero, 139 
 Absorption, of ether waves by atoms, 
 
 242 
 of heat during change of state, 115, 
 
 116 
 of light, its relation to reflection, 274 
 
 to transmission, 269-275 
 of radiant heat, its relation to radi- 
 ation, 279 
 to reflection, 276 
 to transmission, 275-278 
 Acceleration. 372 
 
 due to gravity, 373 
 Acid, definition of, 205 
 effect of on litmus, 82 
 Adhesion, 48 
 Affinity, chemical, 159 
 Air, density of, 17-19 
 transmits sound, 217 
 but not radiant heat or light, 237, 
 
 243 
 
 chamber, 36 
 
 gap in magnetic circuit, 321 
 gap in Ruhmkorff coil, 357 
 pressure, 5-15 
 pump, 28, 29 
 mercury air pump, 239 
 liquid, 139 
 Albumen, 83 
 Alcohol lamp, 50 
 Alkalis, 206 
 
 effect of on litmus, 82 
 Alphabet, code in telegraphy, 322 
 Alternating currents, 339-341, 347, 351, 
 
 357 
 discharge across spark gap, 358 
 
 Altitude, determined by barometer, 
 15 
 
 determined by boiling point of 
 
 water, 109 
 Alum, 96, 276, 278 
 Aluminum, 173 
 Ammonia water, 82 
 Amplitude of vibration, 215, 376 
 Analysis, chemical, 160 
 
 of color by prism, 254, 267, 269 
 
 of sound, 229 
 Aneroid barometer, 16 
 Animals distinguished from plants, 
 
 176 
 
 Animal body, origin of heat in, 212 
 Antidotes, 207 
 Archimedes' Principle, 71 
 Arc light, 356 
 
 Armature, 320, 324, 328, 336, 339 
 Atmosphere, attracted by earth, 367 
 
 chemical composition of, 151 
 
 height of, 238 
 
 pressure of, at sea-level, 14 
 
 total pressure on human body, 19 
 Atomic theory, 193, 194 
 Atoms, explain multiple proportion in 
 compounds, 194 
 
 weight of, 196 
 Attraction, capillary, 50 
 
 electric, 301 
 
 magnetic, 288-291 
 
 BAROMETER, aneroid, 16 
 mercurial, 15 
 
 used to measure elevations, 15 
 to predict changes of weather, 1 6 
 
 Base in chemistry, 205 
 
 403 
 
404 
 
 INDEX 
 
 Battery, see Cell 
 
 Beam of light, 243 
 
 Bell, electric, 338 
 
 Bichromate cell, 313, 314 
 
 Black, color, 267 
 
 Boiling point, 104 
 effected by pressure, 108 
 used to determine elevation, 109 
 
 Boyle's Law, 21-24 
 
 Brass, 173 
 
 Bronze, 173 
 
 Brushes of dynamo, 341 
 
 CALORIE, 112 
 Camphor, 87 
 Capacity for heat, 125 
 Capillarity, 50 
 
 cause of, 53 
 Capillary tubes, 49, 50 
 Carbon dioxide, 2, 147-149, 152, 177 
 Caustic potash, 82 
 
 soda, 191 
 Cell, electric, 311 
 
 connected in parallel, 313 
 in series, 326 
 
 copper sulphate, crowfoot, or grav- 
 ity, 312 
 
 potassium bichromate, 313 
 
 sal ammoniac, 312 
 
 terminals -f- and 313 
 Centimetre, 399 
 Centre, of oscillation, 377 
 
 of suspension, 375 
 Centrifugal force, 382, 383 
 Centripetal force, 383 
 Charge of electricity, 305 
 Chemical action as source of elec- 
 tricity, 310 
 
 action due to light, 279 
 
 affinity, 159, 194 
 
 analysis, 160 
 
 combination, 178 
 
 double decomposition, 180 
 Circuit, electric, 313 
 
 earth a part of, 326 
 
 magnetic, 297, 298, 315 
 
 primary and secondary, 346 
 
 Clouds, formation of, 125 
 
 Code used in telegraphy, 322 
 
 Coherer, 359 
 
 Cohesion, 48 
 
 Coil, in dynamo, 336 
 
 induction coil, 347, 351, 356 
 Collecting rings, 341 
 Colorless transparent pbjects, 268 
 Colors, black, 267 
 
 compound, 254, 267 
 
 depend on rate and intensity of 
 vibration of atoms, 257-259 
 
 intensity of, 256-259, 286 
 
 of flames and other sources of light, 
 252-257, 264 
 
 of non-luminous objects depend on 
 colors received from luminous 
 objects, 265-267, 270 
 on colors absorbed, 272-275 
 on colors reflected, 265-367 
 on colors transmitted, 269-273 
 
 effect of thickness, 272 
 
 of opaque objects, 365-267, 273 
 
 of pigments, 274 
 
 of transparent objects, 269-272 
 
 prismatic, 255 
 
 simple, 255 
 
 tint, 256-259 
 
 white, 254, 255, 266 
 Commutator, 341 
 Compass, 293 
 
 used to determine direction of elec- 
 tric current, 316 
 
 Component tones of notes, 239, 230 
 Compound colors, 254 
 Compounds, 159 
 
 classification of, 205 
 
 formulas of, 199 
 
 how named, 207 
 
 inorganic, 175 
 
 organic, 175 
 
 new , formed by substitution, 180 
 Concave surface, 51 
 Concussion produces heat, 126 
 Condensation, change of volume, 105 
 
 phase of sound waves, 219 
 Conduction, of electricity, 305-309, 354 
 
INDEX 
 
 405 
 
 Conduction, of heat, 130, 236 
 
 of magnetic force, 299 
 Conservation, of energy, 386 
 
 of mass, 183 
 Controller, 355 
 Convex surface, 51 
 Convection, cause of, 136 
 
 of gases, 135 
 
 of liquids, 134 
 Cooling, during evaporation, 123 
 
 during expansion, 124 
 Copper plate of cell, negative plate 
 
 with positive terminal, 313 
 Copper sulphate cell, 312 
 Core, soft iron, 319, 320 
 Crooke's tube, 361, 362 
 Crystallization, 94 
 Crystals, alum, 96 
 
 how produced, 94 
 
 quartz, 96 
 
 salt, 95 
 
 sugar, 96 
 Current of electricity, 310 
 
 direction of, 313 
 
 due to" chemical action, 310 
 
 due to fluctuation of current in 
 neighboring wire, 345 
 
 due to motion of wire across mag- 
 netic field, 335, 336 
 
 fluctuation in strength, 336 
 
 induced, 343 
 
 momentary, 335 
 
 primary and secondary, 346 
 Curvilinear motion, 381 
 
 DECLINATION, 293 
 
 Decoherer, 360 
 
 Decomposition of water by electrol- 
 ysis, 154 
 
 Deep-sea fish, phosphorescent, 19 
 
 Definite proportions by weight, 183, 
 186 
 
 Delivery tube, 143 
 
 Demagnetization, 304 
 
 Density, of air, 17-19 
 of ether, 240 
 
 Diaphragm of lens in camera, 283 
 Difference of potential, 308-310 
 Diffusion of liquids, 82 
 Direct current dynamos, 342 
 Direction of needle in magnetic field, 
 
 294-297, 314, 315 
 
 Direction of strain in the ether, affect- 
 ed by soft iron, 299-301 
 around a wire carrying a current, 
 
 315 
 
 around a wire moving across a mag- 
 netic field, 333 
 
 around a wire while current in neigh- 
 boring wire fluctuates, 345 
 in an electromagnet, 318 
 in field around a magnet, 295-298 
 within a loop carrying a current, 31 7 
 Distance, action at a, 240, 296 
 Distance of object, determines position 
 
 of image, 280 
 
 determines size of image, 282 
 Dogs, scent of, 90 
 Double substitution, 180 
 Dynamo, 335-337 
 Dyne, 381 
 
 EARTH, attracted by sun, 367 
 
 as a magnet, 291, 292 
 
 south-seeking pole at north end of, 
 292 
 
 part of circuit in telegraphy, 326 
 Ebullition, 105 
 Electric, arc, 356 
 
 bell 328 
 
 cell, 311-314 
 
 charge, 305 
 
 circuit, 313 
 
 conduction, 305-309, 354 
 
 current, 307, 310, 332, 345 
 
 light, 355, 356 
 
 potential, 308-311 
 
 waves, 356, 361 
 
 Electricity, a phenomenon of the ether, 
 364 
 
 current ,310 
 
 induced 343 
 
 static , 304 
 
406 
 
 INDEX 
 
 Electrification, by contact and separa- 
 tion, 309, 310 
 
 by contact with electrified body, 304 
 
 by chemical action, 310, 311 
 
 by induction, 343 
 
 by motion of wire across magnetic 
 field, 332 
 
 by fluctuation of current in neigh- 
 boring wire, 345 
 
 by rubbing, 304, 309 
 Electrolysis of water, 154 
 Electromagnet, 318 
 
 armature of, 320 
 
 comparative strength of, 320, 324 
 Electromotive force, 308 
 Electroscope, 305 
 Elements, 172 
 
 inert, 177 
 
 in animal body, 174 
 
 in plants, 173 
 
 chief present in earth, 173 
 
 usually found combined in nature, 
 177 
 
 symbols of, 197 
 Energy, 385 
 
 conservation of, 386 
 
 dissipation of, 388 
 
 formula for, 386 
 
 how measured, 385 
 
 kinetic, 385 
 
 potential, 385 
 Equations, 203 
 Erg, 384 
 
 Ether, chemical, evaporation of, 123 
 Ether, a theoretical substance, 240 
 
 transmission of heat and light, 240- 
 243, 278 
 
 magnetic strain of, 296, 315, 333, 364 
 
 waves in, 241, 248, 356, 361 
 Evaporation, 87 
 
 cooling due to, 123 
 
 effect of pressure on, 1 10 
 
 of camphor, 87 
 
 of ice, 88 
 
 of solids, 87 
 
 of syrups, 110 
 
 effect of on crystallization, 97-99 
 
 Expansion due to heat, 106 
 
 to cold water on cooling below 39.2 
 F., 126 
 
 water on freezing, 105 
 Extent of atmosphere, 238 
 Eye, structure of, 284 
 
 FALLING BODIES, 373, 374 
 Field of magnet, 294, 296 ; see Mag- 
 netic field 
 Field magnet, 336 
 Film, soap bubble, thickness of, 80 
 Fire-engine, 36 
 
 Fish, deep-sea, luminosity of, 19 
 Flow of electricity, 307, 311, 333 
 Fluctuation of strength of current in 
 
 loop of dynamo, 338 
 Fluoroscope, 362 
 Focus of lens, 282 
 Foot-pound, 384 
 Foot-poundal, 384 
 Force, 379 
 
 centrifugal, centripetal, 382, 383 
 
 lines of force, 295-298, 315, 333, 344, 
 345 
 
 units of force, 379 
 Force pump, 36 
 Formula, 370 
 
 chemical, 199-203 
 
 for falling bodies, 374 
 Freezing mixtures, 123, 124 
 Frequency of vibration, 215 
 
 of sound waves, 216 
 
 of waves of light, 263 
 Friction, heat produced by, 127 
 Front of wave practically plane, 247 
 Fulcrum, 390 
 Fundamental tones, 229 
 
 GALVANOMETER, mirror, 332 
 
 sensitive, 331 
 
 tangent, 330 
 Gases, buoyancy of, 65 
 
 cooling due to expansion of, 124 
 
 osmosis, 85 
 
 poor conduction of heat, 133 
 
 rate of motion of molecules in, 87 
 
INDEX 
 
 407 
 
 Gases, relation between pressure and 
 
 volume, 24 
 Glass- working, 44, 45 
 Gold-leaf, thickness of, 80 
 Gravitation, 365 
 
 Gravity, acceleration due to, 373 
 Gravity cell, 312 
 Ground, with ground plate, part of 
 
 electric circuit, 327 
 
 HEAT, molecular motion, 128 
 capacity of substances for, 125 
 cause of in animal body, 212 
 chemical action assisted by, 210 
 conduction, 130, 236 
 convection, 134, 136 
 produced by collision, 126 
 by compression, 127 
 by fricton, 127 
 by electric currents, 355 
 by uniting of elements to form 
 
 compounds, 211 
 quantity of, 110 
 unit of quantity of, 112 
 withdrawn during evaporation, 123 
 during separation of compounds 
 
 into their elements, 210 
 Heat, radiant, 235-241 
 absorption of, 276-279 
 reflection of, 268, 276 
 transmission of, 277-279 
 variation in different parts of spec- 
 trum, 260-262 
 waves, invisible, 259-263 
 
 visible, 263 
 Horse-power, 384 
 Hydraulic press, 69, 70 
 Hydrochloric acid, 82 
 Hydrogen, 2 
 flame of, 147 
 production of, 145 
 tests for, 152 
 Hydrogen dioxide, 187 
 Hydrostatic pressure, 68 
 
 ICE, evaporation of, 88 
 melting point of, 103, 114 
 
 Ice, temperature of, 101 
 Image, distance from lens, 280 
 
 influenced by convexity of lens, 283 
 
 inversion of, 281-283 
 
 size, 282 
 
 Incandescent lamp, 355 
 Inclined plane, 393, 394 
 Induction, electric, 322, 343 
 
 magnetic, 298 
 
 Induction coils, 347, 351, 356 
 Inertia, 379 
 
 Inorganic compounds, 175 
 Intensity of color, 256-259, 286 
 
 of heat, 260, 261 
 
 of light, 256-259, 286 
 
 of sound, 224, 225 
 
 of vibration, 215, 223 
 Internal 'reflection of light, 273 
 Invisible spectrum, 261, 263, 208 
 
 waves of heat, 261 
 
 KEY, telegraph, 325 
 Kilogram, 380, 399 
 Kilogram-metre, 384 
 Kinetic energy, 385 
 
 LATENT HEAT, 120 
 
 of water, 121 
 
 of steam, 121 
 Lava, 99 
 
 Lenses, effect of convexity on distance 
 of image, 283 
 
 effect of margin on image produced 
 
 by centre of lens, 283 
 Lever, 390, 391 
 Lifting pump, 33, 34 
 Light, 235 ; see Color 
 
 absorption of, 272, 275 
 
 electric, 355, 356 
 
 reflection of, 265-267, 273 
 
 source of, 264 
 Lightning, 264 
 Limit of hearing, 216 
 Lines of force, 295 ; see Direction of 
 strain in the ether 
 
 complete circuits, 297. 315, 333, 345 
 
 number of, 319, 336, 345 
 
408 
 
 INDEX 
 
 Liquefaction change of volume during, 
 
 104 
 
 Liquid air, 139 
 Liquid films, tension of, 41 
 Liquids, contraction due to loss of 
 heat, 76 
 
 diffusion of, 82 
 
 osmosis of, 83 
 
 poor conductors of heat, 133 
 
 pressure downward, 57, 59 
 
 upward, 61 
 
 specific gravity, 75 
 Litmus, effect of acids on, 81 
 
 effect of alkalis on, 82 
 Loop, bearing current, acts like a mag- 
 net, 317 
 
 rotated in a magnetic field carries a 
 
 current, 335, 338 
 Loops and nodes of strings, 228 
 Loudness of sound, 219, 223 
 
 MACHINES, efficiency of, 389 
 
 general law of, 389 
 
 simple, 390 
 
 useful work of, 389 
 Magnet, artificial, 288 
 
 earth a magnet, 291 
 
 electromagnet, 318 
 
 field magnet, 294-297 
 
 loop bearing a current acts like a 
 
 magnet, 317 
 Magnetic circuit, 297, 315, 333 
 
 conductivity of, 299, 319 
 
 declination of, 293 
 
 field, 294, 296, 315, 333, 344, 364 
 
 induction, 298 
 
 influence on direction of needle, 294 
 297, 315, 330 
 
 lines of force, 295-298, 815 
 
 needle of compass, 293 
 
 permeability, 300, 302 
 
 poles, 290, 291 
 of earth, 292 
 
 substances, 290 
 
 transparency, 290 
 Magnetism, 289 
 
 ether theory of, 296 315 
 
 Magnetite, 288 
 Magnetization, 302-304 
 Manometer, 39 
 Mass, 369 
 
 distinguished from weight, 380 
 Matter, states of, 100 
 Mechanics, 365 
 Medium, of electric waves, 356, 361 
 
 of magnetic force, 296, 315, 333, 364 
 
 of radiant heat and light, 240-243, 
 278 
 
 of sound, 217 
 Melting point, of ice, 103 
 
 effect of pressure on, 107 
 Membrane, diffusion through, 83-85 
 
 drum of ear, 231 
 Mercury, barometer, 15 
 
 cohesion compared with adhesion to 
 glass, 51 
 
 convex surface in glass tubes, 51 , 52 
 
 depressed in capillary tubes, 50 
 
 drop of, why spherical, 43 
 
 mirror, 160 
 
 properties of, 160 
 
 red oxide of, 160 
 Metals, 198, 205 
 
 conductors of electricity, 354 
 
 of heat, 130 
 Meteorites, 238 
 Metre, 399 
 Metric tables, 399 
 Microscope, size of smallest particles 
 
 detected by, 80, 91 
 Minerals, 173 
 Mixtures, 159 
 Molecular theory, 77 
 
 of heat, 128 
 
 of magnetism, 302 
 
 Molecules, driven farther apart by 
 heat, 77, 78 
 
 motions of, in heat, 128 
 in transmitting sound, 221, 222 
 
 of compounds, 192 
 
 relative size, 81 
 
 small size indicated by gold-leaf and 
 by soap-bubble film, 80 ; by sense 
 of smell, 89 
 
INDEX 
 
 409 
 
 Momentum, 368 
 Monochord, 228 
 Monochromatic light, 251, 253 
 Moon ; attracted by earth, 367 
 Morse code, 322 
 Motion, curvilinear, 381 
 
 quantity of, 368 
 
 uniform, 370 
 
 uniformly accelerated, 371 
 formula for, 373 
 Motors, 342 
 Multiple proportion by weight, 186- 
 
 189 
 
 Musical pitch, 216 
 Musk, 89 
 
 NEGATIVE, pole of magnet is south- 
 seeking, 290 
 
 pole of earth is at north end, 292 
 
 terminal of electric cell is at top of 
 
 zinc plate, 313 
 Nerves of hearing, 234 
 
 of pain, 137 
 
 of temperature, 137 
 
 of touch, 137 
 
 of sight, 263, 286 
 Nitric acid, 82 
 Nitrogen, amount in air, 176 
 
 dioxide, 188 
 
 how secured, 150 
 
 monoxide, 188 
 
 pentoxide, 188 
 
 present in air, 151 
 
 properties of, 152 
 
 tests for, 152 
 
 tetroxide, 188 
 
 trioxide, 188 
 
 Nodes of vibrating strings, 228 
 Non-conductors, of electricity, 309, 
 325 
 
 of heat, 131 
 Non-metals, 198, 205 
 
 OPAQUE OBJECTS, color of, 265, 273 
 Ores, 193 
 
 Organic compounds, 175 
 Oscillating discharges, 357 
 
 Osmose, of gases, 85 
 
 of liquids, 83 
 Overflow can, 70 
 Overtones, 229 
 Oxidation, 212 
 Oxide, definition of, 205 
 Oxygen, amount in air, 144 
 
 amount in human body, 174 
 
 production of, 142 
 
 properties of, 143 
 
 tests for, 152 
 
 PAIN, sensation of, 137 
 Parallel connection of cells, 313 
 Pascal's Law, 67 
 Pendulum, 374 
 
 amplitude of vibration, 376 
 
 centre of oscillation, 377 
 of suspension, 375 
 
 compound, 377 
 
 law of, 377 
 
 length of compound pendulum, 377 
 of seconds-pendulum, 377 
 
 motion described, 375 
 
 period of vibration, 37(5 
 
 rate of vibration, 370 
 
 reversible, 378 
 
 simple, 375 
 
 uses of, 378 
 
 Perception of color, 263, 285 
 Period of vibration, 215, 376 
 Permanent magnet, 289, 349 
 Permeability, magnetic, 300, 302, 319. 
 
 336 
 
 Phenomenon denned, 48 
 Phosphorus, how stored, 177 
 Pigments, color of, 274 
 Piston 25 
 Pitch, 215 
 
 in musical instruments, 216 
 
 range in human voice, 217 
 
 varies with length in strings, 227 
 Pitch of screw, 396 
 Plane, inclined, 393*, 394 
 Plane wave front, 247 
 Platinum wire, how fastened in glass 
 tube, 46 
 
410 
 
 INDEX 
 
 Pneumatic trough, 140 
 
 Poles of earth, geographic, 292 
 
 difference in action of, 291 
 
 magnetic, 292 
 
 of electromagnets, 319 
 
 of magnets, 290 
 Position of image, 280-283 
 Positive pole of earth is in Antarctic 
 regions, 292 
 
 of magnets, north-seeking, 290 
 
 terminals of cells at top of carbon 
 
 or copper plates, 313 
 Potassium bichromate cell, 313, 314 
 Potential, 310, 311 
 Potential energy, 385 
 Pound, 380 
 Poundal, 381 
 Power, 384 
 Precipitates, 167 
 
 separation of, 170 
 Pressure, effect on boiling point, 108 
 
 effect on melting point, 107 
 
 hydrostatic, 67, 68 
 
 relation to volume in gases, 24 
 Primary circuit or current, 346 
 Primary colors, 255 
 Prism, refraction of light by, 250 
 
 of rock salt, 276 
 Prismatic colors, 255, 257 
 Propagation of waves of sound; see Air 
 
 of radiant heat, light, and electric- 
 ity ; see Ether 
 Pulley, 392 
 Pump-barrel, 32 
 Pump-plate, 28 
 Pumps, 25-38 
 
 valves of, 25 
 
 QUALITY OF SOUND, 229 
 Quantity of heat, 110, 111 
 
 unit of, 111 
 
 Quantity of matter ; see Mass 
 Quantity of motion ; see Momentum 
 Quartz, crystals, 96 
 
 RADIANT ENERGY, 235 
 Radiant heat, 235 
 
 Radiation of heat, 241-243 
 
 in relation to absorption, 279 
 Rain foretold by barometer, 16 
 Rarefaction and condensation in waves 
 
 of air, 219 
 Rays of light, 243 
 
 Roentgen, 360 
 Reagents, 168 
 Receiver of air-pump, 29 
 
 of telephone, 349 
 Red light, refraction of, 253 
 Reflection, of heat, 268 
 
 in relation to absorption, 276 
 
 internal, 273 
 
 of light, 265-267, 270, 276 
 Refraction, of light, 243 
 
 cause of, 248 
 
 from denser to rarer medium, 245 
 
 from rarer to denser, 246 
 
 of red light, 253 
 
 of yellow light, 251 
 
 through a prism, 250 
 Relay, 323 
 Repulsion, of like electric charges, 305 
 
 of like poles, 291 
 Resistance, 355 
 Rings, conducting, 341 
 Rock salt prism, 276 
 Roentgen rays, 360 
 Rose, cause of odor, 88 
 Rose-oil, 89 
 Ruhmkorff coils, 347, 356 
 
 SAL AMMONIAC CELL, 312 
 Salt, crystals of, 95 
 Salts, how formed, 205, 206 
 Saturated solution, 93 
 Screen, magnetic, 301 
 Screw, 395 
 
 law, 396 
 
 pitch, 396 
 
 Sea-level, air pressure at, 14 
 Secondary circuit or current, 346 
 Segments of vibrating strings. 228 
 Self-luminous bodies, 264 
 Sensation, of hearing, 218 : 233 
 
 of pain, 137 
 
INDEX 
 
 411 
 
 Sensation of sight, 285 
 
 of taste, 92 
 
 of temperature, 137 
 
 of touch, 137 
 
 Sensitive galvanometer, 331 
 Sensitiveness of eye to different colors, 
 
 286 
 
 Series connection of cells, 326, 327 
 Shooting stars, 238 
 Simple machines, 390 
 Simple pendulum, 375 
 Siphon, 54-56 
 
 principle of, 55 
 
 use of, 56 
 Smell, cause of, 88 
 Soap-bubble, thickness of film, 80 
 Sodium, color of flame, 251 
 
 how stored, 177 
 Soft iron, armature, 320, 336 
 
 influence on direction of strain in 
 
 a magnetic field, 299, 300, 319 
 Solar spectrum, 255 
 
 relative heat in different parts of, 
 
 260 
 
 Solidification, change of volume dur- 
 ing, 104 
 Solids, relative conductivity of heat, 
 
 130 
 Solution, cause, 90 
 
 cold produced by, 123 
 
 saturated, 93 
 
 unsaturated, 93 
 Solutions, effect of heat on, 92 
 
 effect of evaporation, 93 
 
 effect of cooling on, 93 
 
 small size of molecules indicated by, 
 
 91 
 Sound, 214 
 
 due to vibration of sounding bodies, 
 214 
 
 intensity and loudness, 223 
 
 pitch, 216, 227 
 
 quality due to overtones, 230 
 
 sensation, 218, 233 
 
 transmitted by vibration of mole- 
 cules in air, 222 
 
 transmitted by waves in air, 219 
 
 Sound velocity, 230 
 Sounder, 321 
 Spark, electric, 357 
 Specific density, 73 
 Specific gravity, defined, 71 
 
 of iron, 72 
 
 of liquids, 75 
 
 of solids heavier than water, 72 
 
 of solids lighter than water, 73 
 Spectrum, growth during rise of tem- 
 perature, 255 
 
 invisible, 268 
 
 solar, 255 
 
 ultra -red and ultra-violet, 263 
 
 visible part of, 263, 268 
 States of matter, 1 00 
 Static electricity, 304 
 Steam, dry, 103 
 
 latent heat of, 120, 122 
 
 temperature, 103 
 Stop-cocks, 38 
 Straight path of light, 243 
 Strain of ether, direction of, 296, 297, 
 315 
 
 influenced by soft iron, 299, 319, 336, 
 
 344 
 Strings, vibration of, 227 
 
 vibration in parts, 228 
 Sublimation, 87 
 Substitution, chemical, 165 
 
 new compounds formed by, 180 
 Sugar, crystals of, 96 
 
 granulated, 99 
 Sulphuric acid, 82 
 Sun, attraction for earth, 367 
 Sun's light, color is white, 254 
 
 transmitted by ether, 240 
 Surface tension of liquids, 42 
 Symbols of elements, 197 
 Sympathetic vibrations, 226, 234 
 Synthesis, chemical, 160 
 
 TANGENT GALVANOMETER, 330 
 
 Taste, 92 
 
 Telegraph, code or alphabet, 322 
 
 key, 325 
 
 relay, 323 
 
412 
 
 INDEX 
 
 Telegraph sounder, 821 
 
 wireless, 358-360 
 Telephone, 347-354 
 
 receiver, 349 
 
 transmitter, 348 
 
 use of induction coil, 351 
 Temperature, defined, 110 
 
 decrease with elevation, 237 
 
 extremes of, 139 
 
 measurement of, 110 
 
 of ice, 101 
 
 of steam, 103 
 
 of water, 102 
 
 sensation of, 13 < 
 
 unit of, 110 
 Tension, of strings, 227 
 
 of surface film of liquids, 42 
 Terminals of cells, 313 
 Terrestrial magnetism, 292 
 Theory, atomic, 193, 194 
 
 ether, theory of radiant heat and 
 light, 240 
 
 of electricity, 358, 362 
 
 of magnetism, 295, 315, 362 
 
 lines of magnetic force, 295 
 
 molecular, 77 
 
 molecular theory of heat, 128 
 
 molecular theory of magnetism, 302 
 Thermopile, 260 
 Thistle tube, 82 
 Thumb and finger rule, 316, 317, 319, 
 
 329, 334 
 
 Tint of color, 259 
 
 Tones, frequency of vibration or pitch, 
 216 
 
 fundamental, 229 
 
 overtones, 229 
 
 Translation, motion of molecules, 128 
 Translucency, 272 
 
 Transmission of heat, by contact, 
 236 
 
 by radiation, 241-243 
 
 in relation to absorption, 277 
 Transmission of light, 268-275 
 
 selective transmission of visible and 
 
 invisible waves, 275, 276 
 Transmission of sound, 217 
 
 Transmission of many sound waves 
 simultaneously, 230 
 
 by liquid in labyrinth, 234 
 Transmitter, 348 
 
 Transparency of thin sections, 272 
 Transparent objects, colorless, 268 
 
 color due to selective absorption, 269 
 Tubes, capillary, 49, 50 
 
 ULTRA-RED AND ULTRA-VIOLET SPEC- 
 TRUM, 263 
 U-tubes, balancing of liquids in, 20 
 
 VACUUM, falling bodies in a, 373 
 transmission through, of electricity, 
 
 361 
 
 of electrical discharges, 360 
 of heat, 239 
 of light, 239 
 of magnetic force, 296 
 Vacuum fountain, 7 
 Vacuum pans, 110 
 Valves, 24 
 
 Variation, magnetic, 293 
 Velocity, 368 
 accelerated, 371, 372 
 average, 372 
 of electricity, 364 
 of molecules in gases, 87 
 of molecules in liquids, 83 
 of sound, 230 
 uniform, 370, 371 
 uniformly accelerated, 372 
 Vibrations, 375 
 amplitude of, 215, 375, 376 
 frequency of, 215, 223 
 intensity of, 215, 223 
 period of, 215, 376 
 of molecules in heat, 128, 129 
 of molecules in sound, 222 
 of sounding bodies, 214 
 of strings, 228 
 
 of atoms in radiant heat and light, 242 
 rate of, 376 
 
 Visibility, depends on light received, 
 and then reflected or trans- 
 mitted, 265 
 
INDEX 
 
 413 
 
 Voice, 217 
 
 Volume, change during liquefaction, 
 
 or consolidation, 104 
 during vaporization, 105, 106 
 
 WATER, attraction for glass, 47 
 
 attraction for water, cohesion, 46 
 
 boiling point of, 104 
 
 capillary action of, 48 
 
 chemical action assisted by, 209 
 
 composition of, 154 
 
 cohesion compared with adhesion to 
 glass, 51 
 
 drop of, why spherical, 43 
 
 electrolysis, 154 
 
 expansion due to freezing, 105 
 
 freezing point of, 104 
 
 irregular expansion of, 125 
 
 maximum density of, 71 
 
 poor conductor of heat, 133 
 
 pressure downward, 59, 60 
 
 pressure upward, 61 
 
 production of. 157 
 
 relative proportion of oxygen and hy- 
 drogen, 156 
 
 standard for specific gravity, 71 
 
 weight of one cubic foot of, 72 
 
 weight of one cubic inch of, 63 
 Water pump, action like air pump, 32 
 Wave front practically flat, 247 
 
 Waves, of condensation and rarefac- 
 tion, 219 
 electric, 356, 361 
 in ether, 241, 243 
 motion of individual molecules in 
 
 air, 219 
 
 of light, chemical effects, 279 
 simultaneous transmission of many 
 
 waves in air, 230 
 Wedge, 394, 395 
 Weight, atomic, 196 
 distinguished from mass, 380 
 loss due to immersion, of body in a 
 
 liquid, 63 
 variation with latitude and altitude, 
 
 380 
 
 Werkojank, 102 
 Wheel and axle, 391 
 Wheel, Savart's, 215 
 White light, 254, 255, 266 
 Wireless telegraphy, 358, 359 
 Work, 384 
 
 X-RATS, 362 
 
 YELLOW LIGHT, refraction of, 251, 
 253 
 
 ZERO, absolute, 139 
 Zinc plate of cell, positive plate with 
 negative terminal, 313 
 
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