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PHYSICS BY BALFOUR STEWART, Professor of Natural Philosophy, the Owens College, Mamhatcr, Author of ** Elementary Lessons in Physics.** WITH ILLUSTRATIONS. IK0tottt0: JAMES CAMPBELL & SON. - - 7 Entered accordiuj; to the Act of Parliament of Canada, in the year one thousantt eight hundred and eighty -one, by Jamrs Gampbbll AND Son, in the office of the Minister of Asjiculture. PREFACE, In publishing the Science Primers on Physics and Chemistry, the object of the Authors has been to state the fundamental principles of their respective sciences in a manner suited to pupils of an early age. They feel that the thing to be aimed at is not so much to give information, as to endeavour to discipline the mind in a way which has not hitherto been cus- tomary, by bringing it into immediate contact with Nature herself. For this purpose a series of simple experiments has been devised, leading up to the chief truths of each science. These experiments must be performed by the teacher in regular order before the class. The power of observation in the pupils will thus be awakened and strengthened; and the amount and accuracy of the knowledge gained must be tested and increased by a thorough system of questioning. The study of the Introductory Primer will, in most cases, naturally precede that of either of the above- named subjects; and then it will probably be found best to take Chemistry as the second and Physics as the third stage. The whole of the apparatus needed for all the ex- periments (except a few marked in the text with an asterisk) will be supplied by Messrs. J. J. Griffin and Sons, 22,-Garrick Street, Covent Garden, London, W.C., (or j£ic) 3 J. 8//. exclusive of packing. If prompt payment be made, Messrs. Griffin are willing to supply the set for jCi'j exclusive of packing cases, which may be either 9^. or 21s, according to quality. Those who have already purchased the ap- paratus for the Chemical Piimer need not purchase another Grove's battery, and in this case they must make their own arrangement with Messrs, Griffin, TABLE OF CONrENTS. Introduction. ART. FAG* Definition of Physics 1 i i „ of Motion . 2 2 „ of Fercc 3 4 The Chief Forces of Nature. Definition of Gravity 4 7 ,, of Cohesion ....... . 5 8 „ of Chemical Attraction 6 9 Uses of these three Forces . . . , 7 10 How Gravity acts. Centre of Gravity 8 1 1 'Xi\z Balance 9 13 The Three States of Matter. General Remarks 10 14 Dehnition of Solids 1 1 16 „ of Liquids 12 16 ,, of Gases . 13 16 Properties of Solids. General Remarks on Cohesion ....... 14 16 Bending 15 19 Strength of Materials . • '6 20 Friction. 17 20 Properties of Liquids. Liquids keep their size though not their sliape . . 18 21 They communicate pressure 19 21 Water-press explained 20 23 Liquids find their level .,.,., ^ .,., 21 24 TABLE OF CONTENTS. ^\ ART. V/ater- level and Spirit,-level ........ 22 Pressure ot deep Water . . . . • 23 Buoyancy of Water . 24 Flotation in Water 25 Comparative Density or Specific Gravity .... 26 Buoyancy of other Liquids 27 Capillarity 28 , . . Properties of Gases. Pressure of Air 29 Weight of Air .............. 30 Barometer explained^— Mercurial t^olunin . ... 31 Uses of the Barometer .... ^ ..... 32 Air-pump explained . . 33 Water-pump explained — limits of working; . . ,34 Syphon described ....'. 35 Moving Bodies. Definition of Energy . . . . . ... . .36 ,, of Work ...... •. •."... 37 Work done by a moving body 38 Energy in repose 59 Vibrating BooFis. Sound explained 40 What is Noise and what Music 41 Sound can do work 42 It requires a medium (Air) to carry k ..... 43 Its mode of moticn through the Air ..... .44 Its rate of motion , . . 45 Echoes or reflection of Sound 46 How to find the number of vibrations in one second coi responding to any rvote ....... ^ 47 Heated Bodies. Nature of I Icat (first notice) ........ 48 Expansion of bodies generally when heated ... 49 Thermometer described . , .50 How to make a Centigfad6 Thermometer . . . . 51 Expansion of Solids 52 „ of Liquidi .,*.,- ^ . - , 53 PACK 25 26 28 30 31 32 33 34 35 38 40 41 43 46 47 48 49 50 52 53 54 54 54 56 57 59 61 63 65 66 69 70 via TABLE OF CONTENTS. AKT. PAGB Expansion of Gases 54 Remarks on Expansion • 55 S.pecific Heat • . 5^ Change of state, with table of melting-points ... 57 Latent heat of Water 5^ ,, of Steam . . ....••• 59 Ebullition and Evaporation ..*,.••. 60 Boiling-point depends on pressure 61 Other effects of Heat , .> 62 Freezing mixtures 63 Distribution of Heat — General statement .... 64 Conduction of Heat . • • • 65 Convection of Heat 66 Radiant Light and Heat — General statement ... 67 Velocity of Light, how discovered 68 Reflection of Light, laws of 69 Refraction or bending of Light 70 Lenses and Images given by them 71 Magnifying-glasses and Telescopes 72 Different l^ending of different rays 73 Recapitulation 74 Nature of Heat (second notice) ...,...* 75 Electrified Bodies. Conductors and Non-conductors 76 ■f wo kinds of Electricity — their mutual action . . 77 They exist combined in unexcited bodies .... 78 Action of excited on unexcited bodies . . . . > . 79 The Electric Spark 80 Sundry Experiments — Electroscope 81 Action of roints 82 Electrical Machine described 83 Leyden Jar described 84 Energetic nature of Electrified Bodies ..... 85 Electric Currents , 86 Grove's Battery described 87 Properties of Current j heating, chemical, and mag- netic effects 88 Electric Telegraph .......'» 89 Concluding Remarks 90 Things to be remembered ........ Instructions regarding Apparatus . . . . . Ll3T OF Apparatus , , 70 71 72 72 75 76 78 79 81 82 83 85 86 89 89 91 95 97 99 100 102 103 106 107 109 no III 112 113 115 117 119 119 121 122 125 126 12S 130 13? SCIENCE PRIMERS. PHYSICS. INTRODUCTION. 1. Definition of Physics. — You have been told in the Chemistry Primer what sort of things we have around us. You have seen what the chemist does j how he weighs things and finds their quantity, and also how he finds that certain things are compound, and may be split up into two or more new things; while again other things are simple or elementary and cannot be so split up. In fact you have been told about the various kinds of things we have in the world, but you have not yet learned much about the affections or moods of these things. You are yourself subject to change of moods; sometimes you appear with a smile on yoiT face, and sometimes, perhaps, with a face full of frowns or tears ; sometimes, again, you feel vigorous and active ; sometimes dull and listless. Now if you think a little you will see that the things around you are subject to moods very much like yours. To-day the face of nature looks bright and happy, and SCIENCE PRIMERS, ^[introduction. full of smiles ; to-morrow the same face is dark and lowering ; the rain falls, the thunder roars, and the sea is tossed with waves and very stormy. Or again : let us take an iron ball which lies upon the floor ; it is cold and heavy to the touch, but let us put it into the fire, and when it comes out the same substance is there, but the state of it is very different ; if you now attempt to touch it, you will be sure to burn your fingers. Or again : if, instead of putting it into the fire, we put it into a cannon and discharge the cannon, it will come out with tremendous velocity, and will knock to pieces anything it touches. Thus you see that a cold cannon-ball is a very dif- ferent thing from a hot cannon-ball ; and also that a cannon-ball at rest is a very different thing from a cannon-ball in motion. Now if we see you crying and unhappy, we ask what is the cause of this mood, and we always find there is a cause ; or if we find you listless and sleepy, and wanting energy, we inquire what is the meaning of c.U this, and we find that it has a meaning and a cause. . So likewise when we find changes in the moods or qualities of dead matter we inquire what is the cause of these changes, and we always find they have a cause. This inquiry we shall make in the following pages, and you must attend well to the answer we get. You have already been told that this mode of questioning nature is called experiment. 2. Definition of Motion. — You must in the first place get a clear idea of motion. Motion means change of place. Some of you may have heard that this solid earth on which we dwell is in truth moving very fast round the sun, but we may, in the meantime, INTRODUCTION.] PHYSICS. put away this thought altogether from our minds, be- cause although the earth is moving very fast it carries us all along with it, and everything goes on as smoothly and quietly as if the earth were at rest. Well then, if I sit on a chair in a room I may say that I am at rest, but if I walk up and down the room I am in motion. Now in order to understand my movements, you must know something more than the mere fact that I am moving about ; you must know the direction or line in which I am moving, and you must also know the rate or velocity with which I am moving. You must try clearly to under- stand the meaning of this word " velocity ; " and to make you do so, let us suppose that I go out of doors and walk along a straight road for two or three hours, and always at the same pace. Well, I find that in one hour I have got four miles beyond my starting point, and that in two hours I have got eight miles beyond it, and I therefore say that I am walking on at the rate or with the velocity (for both words mean the same thing) of four miles an hour. But what if the rate be not always the same ? Sup- pose a railway train to be coming near a station, and just beginning to slacken its speed. The train is first of all moving, let us say, at the rate of forty miles an hour, but presently its velocity gets less and less, until when it arrives at the station it comes quite to a standstill. Now, how can we find its rate when ihis is always changing ? or what do we mean when we say that the train, before it began to slacken its speed, was moving at forty miles an hour ? We simply mean, that if the train had been allowed to move for ^ whole hoiur at the same rate it had before it SCIENCE PRIMERS. [iNTRonucTiON. began to slacken its speed it would have moved over forty miles. In fact, if instead of coming to rest at the station it had been an express train, and gone on, it would have been forty miles away an hour after we began to notice it There are different ways of expressing velocity: sometimes we speak of so many miles an hour, as we have done here, but sometimes it is better to use feet and seconds ; thus if I drop a stone down a well I Should say that it fell sixteen feet during the first second after it was dropped. Sixty seconds, you all know, make a minute, and sixty minutes make an hour. In this little book, when speaking of velocity or rate, we shall use feet and seconds more frequently than miles and hours, and speak of a body as moving at the rate of ten, or twenty, or thirty feet a second, as the case ma^ be. 3. Definition of Force.—Now what is it that sets in motion anything that was previously at rest ? Or what is it that brings to rest a thing that was pre- viously in motion ? It is force that does this. It is force that sets a body in motion, and it is force (only applied in an opposite direction) that brings it again to rest Nay, more, if it requires a strong force to set a body in motion, it requires also a strong force to bring it to rest You can set a cricket-ball in motion by the blow of your hand, and you can also stop it by a blow, but a massive body like a railway train needs a strong force to set it iu motion, and a strong force to stop it. That which is easy to start is easy to stop ; that which is difficult to start is difficult to stop. You see now that force acts not only when it sets a body in motiv D, but as tnily when it brings a body to rest VM^n^-^St^ti^^ _fil£.:£^.^& INTRODUCTION.] PHYSICS, V X 1. In fact that which changes th# state of a body is called force, whether that state be one of rest or of motion. Experiment i. — To prove this, take a tin pan with some peas in the bottom of it, and hold the pan in your right hand. Now quickly raise your right hand, with the pan in it, until your right arm is brought to a stop by a fixed bar of wood, which you tiave placed a little above it (your other arm held stiffly will do as w^U as the wood). Now what you have done is to make the pan with the peas rise quickly upi and then m Fig. 1. suddenly come to a dead stop. You have first, by the force of your arm, given an upward motion to the pan, and the pan has forced the peas to mount with it, since clearly they could not remain behind. Then, again, when your right arm holding the pan was mount- ing quickly, you allowed it to be stopped all at once by the bar of wood ; that is to say, the bar of wood forced your hand to stf;p, and your hand in its turn forced the pan, which you held tightly, to stop also. But this stopping force does not affect the peas which lie loosely at the bottom of the pan, so that they will SCIENCE PRIMERS. [forces t)F continue to mount up after the pan has been stopped, and many of them will fall over the edge and be scattered about upon the floor. Experiment 2, — Now put some more peas into the pan, having spilt the last ones; but iftstead of raising the pan quickly upwards, lower it as quickly as you possibly can. Here, the force of your arm makes the pan move down very quickly, but does not affect the peas which lie loosely on the bottom of the pan ; the result will be, that the peas will not follow the quick motion .of the pan, but will lag behind until at last they are all scattered about upon the floor. Let us now pause for a moment, and see what we really learn from these two experiments. We learn from the first, that after we have once set the peas in motion upwards, since the stopping force of the bar of wood does not affect them, they continue to move upwards after the pan has been stopped. It requires force to stop their upward motion, and this force we could not apply by means of the bar of wood, so that they continue to mount upwards until the force of the ea'-.h at last brings them downwards to the floor. You see, therefore, that it needs force to stop a moving body. Again, in the second experiment, we communicate a downward motion to the pan, but the force of our arm which does so, does not affect the peas which lie loosely on the bottom of the pan. They, therefore, keep their state of rest, and lag behind the pan until at last the force of the earth brings them downwards to the floor. You see, therefore, that it needs forcjft to start a body at rest. Force, therefore, may do two things j it may either t>F NATURE.] PHYSICS, 5f stop a body in motion, or it may set in motion a body at rest. But very often we find that a force, although present, does not appear to act. Now, why is this ? We reply, because it is prevented from doing so by another equal and opposite force. Thus, I hold a heavy weight in my hand ; if I open my fingeiK, the force of tke earth which acts upon the weight will bring it very soon to the floor ] but as long as I keep my fingers shut I prevent this force from acting. Or, imagine the same weight to lie on the table; if there, were no table, it would fall to the floor ; but the force of the earth which gives it a tendency to fall, is pre- vented from acting or is resisted by the table. The weight presses against the table, but the table with- stands this pressure. So that you have here two forces resisting or withstanding each other, the one being the weight, and the other the resisting force of the table. From all this we learn that force is that which changes the state of rest or of motion ot a body, but that very often force is resisted or prevented by an equal and opposite force, so that it is not able to do anything or to produce any eff"ect THE CHIEF FORCES OF NATURE. 4. Definition of Gravity. — I have thus told you what is the meaning of the word force, and now let us loc>k about us in order to see what are the chief forces with which we have to do, and to see also what part each plays, and what is its use* The most prqminent force is the attraction of the earth. If we let go a heavy thing out of oui handsj 8 SCIENCE PRIMERS, [forces we know where to look for it; we know that it will not mount towards the sky, nor will it move off sideways in some direction, but it will fall to the ground or earth. It falls dov/D, we say, and the very words up and down depend upon the earth's force ; so that if the earth had no force, we should not use such words at all. The word " up " denotes a difficult motion against the earth's force ; the word "down " an easy motion, by help of the earth's force. It is difficult ,to walk up a hill, but it is very easy to walk down. Now when we say that the earth attracts things, you must not think that all, or nearly all of the things which we see are moving towards the earth. You and I are not so falling, nor should we wish to be in such a very dangerous condition. Why are we not falling? Because we stand upon the floor; but if there were no floor, we should fall through to the ground, and the floor must be strong enough to support our weight, otherwise it would give way and we should fall. Sometimes a wooden floor or platform has been so filled with people that it has given way, and they have fallen to the ground, and many of the people have been killed or very much hurt. Thus you see that the earth attracts everything, but yet most of the things which we see around us are not moving towards the earth, because they are sup- ported by something else that is able to i-iist their weight. In fact, this property of things called weight is really caused by the attraction of the earth. This force which the earth exerts is called gravity* 5* Definition of Cohesion. — But there are oth^r forces besides that which the earth exerts. If we take a piece of string or of wire, and try to break it OF NATURE.] PHYSICS, ') into two parts, it exerts a force to prevent our doing so, and it is only when the force we exert is greater than the force with which it resists us that we succeed in breaking it. In fact the different parts or particles of the string or of the wire are held together by a force which resists any attempt to pull them asunder. And 30 are the various parts or particles of all solid bodies, such as w^ood, stone, metals, and so on. It is often very difficult to break a substance to pieces, or bend it, or powder it, or alter its shape or size in any way. Now that force which binds together the various particles of a body is called cohesion. You see now the difference between gravity and cohesion : gravity is that force which the earth exerts to pull bodies to itself, and which acts at a great distance ; so that, for instance, the moon, which is 240 thousand miles away, is attracted by the earth. Cohe- sion again is that force which the neighbouring particles of a body exert to keep each other together, but this force does not act except when the particles are very near each other ; for if once a thing is broken or ground to powder, its particle j cannot come easily together again. 6. Definition of Chemical Attraction. — Be- sides these two forces there is the force of chemical attraction or affinity. You are told in the Chemistry Primer (Art. 4) that the two things coal and oxygen gas unite chemically together, and that carbonic acid gas is the result of their union. The coal and the oxygen gas are pulled together by a force which thev exert on each other as truly as ai stone is pulled towards the earth. In virtue of this force they rush together and unite, and the result is something quite different lO SCIENCE PRIMERS, [forces from either. Thii, then, is the force which we call chemical attraction, and which has this peculiarit|^ that it can only be exerted by different bodies ; for in chemistry it is only bodies of different kinds that rush together and unite after this fashion. 7. Use of these Forces. — Having now told you something about the chief forces of nature, let us try to find what part they play, and why they are there at all \ and I think we shall soon see that we should get on very badly without them. Let us begin by supposing that there was no such thing as gravity, and that the earth did not attract things to it. Now sometimes when we climb a steep hill we are tempted to think how pleasant it would be if we could go up as easily as we go down. How we wish there was no gravity! But it would be a terrible misfortune if on^^ of those spirits we read of were at once to grant us our request. There being no gravity there would of course be no weight, and we should then get up a hill easily enough, but if we jumped into the air we should remain there ; and possibly we might be able to leave this world altogether. The furniture of our houses would be found some on the floor, some on the roof, some floating about, and we ourselves could walk on the roof as easily as on the floor. The moon meanwhile, not being bound to the earth, would leave us for ever ; and in like manner the earth, being no longer bound to the sun, would leave it far behind and wander off among the stars. So much for gravity. Let us now see what would happen if there were no cohesion. If this force were absent, the particles of solid bodies would not adhere to one another, but they would all fall to pieces or OF ITATURE.] PHYSICS. II call for that rather to powder. The wood of our tables and chairs would fall to powder, and we should have no furniture ; and the bricks of our houses would do the same, so that we should have no houses. We should do the same ourselves, and in fine all things would resolve themselves into a huge mass of dust Finally, let us think what would happen if there were no such thing as chemical attraction. In the first place the fire would cease to burn because the carbon of the coals would no longer care to unite with the oxygen of the air. In the next place no two simple or elementary substances would unite together to forrfi a compound substance, but we should have nothing but about sixty simple substances consisting of a great number of metals and a small number of gases. There would be no variety in such a world, and indeed there would be no living in it, for our own bodies are compound ; and if chemical affinity were destroyed part of them would go up into the air and mix with it, while another part, consisting of a quantity of carbon, a little phosphorus, and some one or two metals, would fall to the ground, and thus we shoaid come to an end. HOW GRAVITY ACTSt 8. Centre oi Gravity. Experiment 3.— Let us now endeavour to find out what sort of a force gravity is, and for this purpose let us take this irregular sheet of iron and hang it up by a thread. You see i\ hangs in a particular way, and you also see that the line already drawn with paint on tl^^ sheet is in 12 SCIENCE PRIMERS. [now the same direction as the line of the thread. Next let us hang the sheet freely from some other point,* here again you have another white line in prolonga- tion of the thread, and you further see that these two white lines cut each other in a point marked g. Fi::. 0. Now let us hang up the sheet by some third point in its rim. As before, you have a white hne in pro- longation of the thread. Now you will easily see that these three white lines all cut one another at the same point Gj in fact, if you suspend the sheet from any point freely by. a thread, and draw a white line in prolongation of the thread, all sucli lines will cut one another in the same point g, so that this point will always be directly under the point from which the sheet is hung, and if you push the sheet to one side it will return agaih to its old position. Now what is this peculiar point g ? To find out let me « ■i^^iW'^ GRAVITY ACTS.] PHYSICS. n attach a string to G, and hang the sheet by the string; you see that the sheet will balance round g in all directions just as well as if its whole weigh were ccmdensed into the point G. Now G is what we call the centre of -gravity of the sheet; and if ,1 Fig, 3. I hang up the sheet freely by a siring, it will put itself in such a position that its centre of gravity G shall be as low as possible. If instead of hanging the sheet by a string I suspend it loosely upon a peg, it will still try to place the point G as low as it possibly can, and it will not hang as in fig. 3, but the point G will be immediately under the peg. 9. The Balance. — Every substance has a point G of this kind, which we call its centre of gravity. The balance which you see on page 28 has, like every- thing else, its point g — its centre of gravity. And it will endeavour, just like the sheet of iron, to place this point as low down as it possibly can. Now when there are equal weights in both scale- pans, this point g is somewhere directly under the point upon which the balance is swung ; and hence, if by pushing it I try to tilt it to a side, when freed it will ultimately return to its old position. In fact, when the weights in each pan are equal, it will always t4 SCIENCE PRIMERS. tnikEE keep this position, with the pointer pointing exactly in the middle ; so that if I am weighing a substance, and place this substance in the one scale-pan, and the weights in the other, and if the pointer points exactly in the middle, I am then quit» sure that the weights in the one scale-pan are exactly equal to *^ //eight of the substance in the other. But if the weights are not heavy enough, the beam of the balance will be tilted over by the substance in one direction ; while if the weights are too heavy, they in turn will tilt over the beam in the other direction. Experiment 4. — Suppose that I put this piece of metal into one of the scale-pans, and put weights equal to 150 grains into the other, the scale-pan with the metal in it sinks down, thereby showing that the metal is heavier than the weights. Next let me put weights equal to 250 grains into the other scale-pan. Now again it is these 250 grains that are too heavy, and you see that the scale-pan containing them sinks down, whereas before it was the other that sank. Thus the weight of the metal is somewhere between 150 and 250 grains. Let us therefore \xy a 200-grain weight, and you see that now the pointer points exactly in the middle, and the beam of the balance is exactly horizontal, showing that the weight of the metal is exactly 200 grains. THE THREE STATES OF MATTER. 10, You have seen that we cannot do without the various forces of nature, and that if one piece of matter were not drawn or attracted to another piece, STATES.] PHYSICS. IS there would be no such thing as a world at all. You have seen, too, that if there were no cohesion, there would be nothing but powder. I may now proceed to tell you that if everything possessed cohesion to a great extent, we should be nearly as badly off, for we should in such a case have neither liquids nor gases, neither water nor air. The particles of a bar of iron or steel possess very great cohesion, and it is very difficult to force them apart. But water and mercury have hardly any cohesion whatever, and the very slightest touch will scatter in all directions a quantity of water or of mer- cury. Yet these two liquids have still a little cohesion left, as you may see by the following experiments. Experiment 5. — Take a very small quantity of mercury from the bottle containing it, and put it on a flat glass surface. By pressing it you may split it up into small globules. Now these globules are a proof that the particles of mercury cling together. For, put another plate of glass above them, and you may by this means squeeze them flat ; but if you take away the glass, the mercury will resume its previous glo- bular shape. Experiment 6. — Sprinkle a few drops of water on an oily or greasy surface, and these will be found to have a rounded form, not unlike drops of mercury, showing that the particles cling to one another. On the other hand, the particles of gases, such as the air we breathe, have no tendency to keep together, but rather the reverse. Indeed they will separate from one another unless there is some force which keeps them from doing so. So that, you see, we have three very different states i6 SCIENCE PRIMERS. [rROrCRTIEfc It of matter, the solid, the liquid, and the gaseous ; and each of these states has certain properties which serve to distinguish it. 11. Definition of Solids. — A sclid body, such as a piece of iron or wood, resists any attempt to alter its shape or its size, always keeping the same size or volume and the same shape, unless it be violently destroyed. 12. Definition of Liquids. — A liquid like water, when kept in a bottle or other vessel, always spreads itself out, so as to make its surface level, but yet it will always keep its proper size or volume. You cannot by any means force g, pint of water into a half-pint measure; it will insist upon having its full volume, but it is not particular as to shape. 13. Definition of Gases.— A gas again has no surface; for if you put a quantity of any gas into a perfectly empty vessel, the SCIENCE PRIMERS. [PllOPERTlES hea^ mercury thirty inches high ; but it can do no more — the weight of this mercury pressing downwards ex- actly counterbalances the pressure of the air forcing it upwards, and hence on the one hand the column of mercury cannot push itself downwards, and on the other the pressure of air cannot push the column upwards, and we have therefore a blank space above the column. This experiment was devised by an Italian called Torricelli— the tube is called a Baro- meter, and the empty space at the top is called tfec Torricellian Vacuum. Most barometers are pro- vided with a scale of inches by which the height of the top of the column above the surface of mercury in the cistern may be accurately measured. 32. Uses of the Barometer. — The barometer is useful in many ways j for instance, we may by its means tell the height of a mountain. You were told (page 26) that the pressure is greater at the bottom of a deep vessel of water than near the top, and the sane thing takes place in this ocean of air in which wc live — the pressure is greater near the bottom of this aerial ocean than it is far up near the top. If there- fore we go to the top of a high mountain, we have a smaller weight of air above us than we had when down below, and in consequence the pressure of the air will be smaller at the top of the mountain than at the bottom. The air will not now be able to balance the same column of mercury as at the bottom, so that, in the barometer, instead of a column of mercury thirty inches high, we shall only have one of twenty-five inches or possibly of twenty inches, depending upoyi the height of the mountain. In fact the mercury will sink lower and lower down in the tube of the barometer EITIES more s ex- ingit nn of n the )lumn above by an 3aro- sd the repro- ight of lercury ometer by its re told )mof a le sane ^ich wc of this there- have a down air will at the ce the that, in thirty inches op the ill sink ometer OF GASES.] PHYSICS, 4t the higher up you rise in the air, and thus by means of the barometer you can tell to what height you have gone. The barometer is also useful in telling us when bad weal her is at hand. When the barometer falls, that is to say, when the top of the column of mercury gets lower in the tube and especially when it falls quickly, we may expect bad weather. On the other hand, if the mercury remains steady and high we may expect a continuance of fine weather. 33, Air-pump. — We have abready spoken about taking the air out of a jar, now this is done by the air-pump. You wi see how this instrument acts by means of the figure. But first of all I must tell Fig. 17. : . you what is meant by a valve. A valve is just a tightly fitting trap-door that closes a hole, and that can only open in one way — upwards, for instance. You have, most of you, seen trap-doors in floors that open upwards. Now in the figure you see to the left a bell- jar full of air, which fits tightly upon a plate. You see too coming out from the middle of the plate a tube which opens into the bell-jar on the left side, and into the cylinder or barrel oil the right, and thus connects the two together. You see also a piston or plug that can move up and down in the cylinder or banel. J— 4» SCIENCE PRIMERS, [properties Finally you see two valves or small and tightly fitting trap-doors, one of which is placed where the tube enters the bottom of the cylinder, while the other is in the piston itself. Both of these valves open upwards and not downwards. Now suppose we start with the piston at the bottom of the cylinder, and the valves shut, and begin to pull the piston up. In doing so we make an empty space which the air on all sides will try to fill up if it possibly can (Art 29). The air from above will try to press into this space, but it will not be able to get in, and all it can do will be to press against the outside of the upper valve and keep it tightly shut, since the valve does not open downwards. The air from the bell-jar will succeed better, for it will rush through the tube and press open the lower valve which opens upwards, and then get into the empty space. Let us now suppose that we have got the piston to the top of the cylinder, and that we are beginning to press it down. The push that we give to the piston, the piston gives to the air ; and the air in its turn communicates this push to the lower valve, which is kept shut. But the air within is more successful with the upper valve, for it pushes this open ; and so, as we continue to push down the piston, all the air that was in the cylinder below it is pushed out through the upper valve or trap- door. But this air which we have pushed out was part of the air that was originally in the bell-jar, so you see that in the first double or up-and-down stroke of the piston we have succeeded in squeezing out part of the air of the jar. Let us now repeat the same process, that is to say, raise the piston again, and the air from above will shut the upper valve, while the air from the SITIES ghtly i the other open jttom pull space >ssibly yS into 1 all it of the valve jell'jar e tube iwards, is now of the down. gives es this Jut the ve, for push ylinder or trap- raspart 50 you roke of part of process, ir from om the OP GASES.] PHYSICS. 43 bell-jar will rush along the tube, push open the lower valve and fill the empty space which we make when raising the piston. And when the piston descends once more, the lower valve is kept shut, while the air within pushes open the upper valve and gets out, and thus in every double stroke we get rid of part of the air in the bell-jar. Of course it is quite necessary in working the pump that the piston shall fit quite tightly into the cylinder ; for, if not, the air will get in from without, and therefore we shall not succeed in getting the air out from within. I have now told you the way in which the air-pump works, but you must not expect every air-pump to be precisely like the figure I have given you; the principle, however, of all air-pumps is the same, although the appearance may be very different in each. 34. Water-pump. — Having now told you about the air-pump, let us return for a moment to the baro- meter. You have seen how the pressure of air is just strong enough to hold up a column of mercury about thirty inches high. But water is much lighter, bulk for bulk, than mercury, and we might therefore expect the pressure of the air to hold up a much longer column of water than one of thirty inches. In truth, the pres- sure of the air will hold up a column of water very nearly thirty feet in height. This will enable you to understand the mode of ac- tion of the common pump. In the figure on the next page you have a sketch revealing the interior of such a pump. Below we have the reservoir from which we wish to pump the water up, and we have a tube leading from this reservoir up into the barrel of the pump. In thi9 barrel ^ou s^e a piston which fits tightly into the 44 SCIENCE PRIMERS, [PROPERTIES barrel, and in this piston there is a valve opening up* wards, while at the bottom of the barrel there is another valve also opening upwards. In fact, the barrel of the lifting pump is quite similar to that of the air-pump, and we may begin by supposing that the piston is at the bottom of the cylinder. Let us now raise up the piston, and just as in the air-pump, the air above will press down the upper valve and keep it shut. The air in the tube will on the other hand, rush up through the lower valve in order to fill up the empty space made by raising up the piston. When we lower the piston again, just as in the air- pump, the lower valve will be shut, and the valve in the piston will open and let out some air. In fact, we are now pumping out the air from the barrel and the tube. But meanwhile, what is the water in the reservoir doing ? The air from without continues pressing on the surface of the water in the reservoir ; but as we have been taking away the air in the tube, this pressure of outer air is no longer counter- balanced by that of the air in the tube; the outer air will therefore find itself unopposed, and will drive up the water into the tube, until at last, when all the air is taken away, the whole tube will be filled with water. This water will then enter the pump barrel through the lower, valve. But all this will not take place if the distance be- ^wc^p the surface of water in the reservoir and the Fig. xS. OP GASES.] PHYSICS. 4S the lower valve be more than thirty feet. For you have just been told that the pressure of the air will support a column of water thirty feet high, but if the column be higher than this it will not support it. So that if there be a greater distance than thirty feet between the sur- face of the reservoir and the pump barrel, the wrier will refuse to enter into the barrel, and do what, you can you will not be able to entice the water quite up into the barrel. If. however, the distance be not more than about twenty-six or twenty-seven feet, \aq pump will work well, and you will get the water to enter the barrel. Suppose now that you have got the barrel filled with water, and that you 'are pressing down the piston. As you do this the pressure you give the piston will be communicated > by the water to the lower valve, which ./ill be kept closed. On the other hand, the pressure of the water will force open the upper valve which opens upwards, and the water will get above the piston. Next time when you pull up the piston, you will pull up this water with i., and it will empty itself through the spout of the pump, and the water will now come out of the spout at every stroke. * Experiment 32. — To enable you to see witl. your own eyes what goes pn in a common pump, take a model in which the pump barrel is made ol glass, so that you can see into it. You will thus see that when we raise the piston, the upper valve shuts and the under one opens, while, as the piston descends, the under valve shuts and the upper valve opens. You quite understand that the piston of the pump must fit tightly on to the barrel, because otherwise the air will get ia from above and prevent the action. SometimeS| 4« !^CiENCE pRIMERt (MOVlNd however, if a pump is not much used, the leather or Other packing around the piston gets dry, and the pump will not act In that case, if a little water is thrown upon the piston, it wets the packing and serves to make it tight. 35* Syphon* — Before leaving this subject, let me describe to you an inr*rument called a syphon, of which the action depends, like the pump, upon the Fig. Z9. pressure of the air. I shall not, however, explain its principle. You see the syphon before you in the figure ; it is used for conveying liquids from a vessel at a higher to one at a lower level In the first place, you must invert the syphon tube, and completely fill it with water, keeping your finger at the end of the ihorter tube. Now place the shorter end beneath INti •or the r is and me , of the BODIES.] PHYSICS, 47 in its n the vessel )lace, ilyfiU rf the sneath the surface of the water in the higher vessel as in the figure, and remove your finger. Once you have done this, the water will, thereupon, flow in a continuous stream from the end of the longer tube into the lower vessel, and you may by this means remove the water completely from the upper into the lower vessel, pro- vided the short tube of the syphon be long enough to reach to the bottom of the upper vessel MOVING BODIES. 36. Energy. — You have been told (page i) about the moods or affections of things, and how a cannon- ball in motion is a very different thing from one at rest, or a hot cannon-ball from a cold one ; and you have also been told that one of our great ob- jects in this Primer is to find out something about these varying moods or affections of matter. We could not begin with this, for we had first of all to tell you about the things themselves, and you ought now to have a tolerably good acquaintance with solids, liquids, and gases ; it is time, therefore, that you learned something about the varying moods or affec- tions of things. You were told that bodies were sometimes full of energy, such as a cannon-ball in motion, and sometimes utterly listless and devoid of energy, such as a cannon-ball at rest, and in what follows we cannot do better than study the most conspicuous cases in which a body is full of energy. Now this happens when a body is in actual motion, or when it is in rapid vibration, or when it is heated, or when it is electrified, and we shall therefore class 4^ SCIENCE P/^/M£/^S. tMOVlNG energetic bodies under these four divisions. We shall first of all speak of bodies in actual motion, and uitder this head give you some idea of the way of acting of such bodies ; we shall then speak of bodies in vibration, such as a sounding drum or bell, and under this head we shall tell you something about sound. We shall next speak about heated bodies, and under this head tell you something about light and heat ; and lastly, when speaking about electrified bodies, you will hear about that mysterious thing called electricity. We cannot in this little Primer give you anything like a complete account of the various moods of bodies, or the various kinds of energy which they sometimes possess. This must be reserved for a more advanced stage; we can only give you a mere outline of the subject, telling you at the same time that it is one of very great importance. 37: l>efinition of Work.— When we say that a man is full of energy, we mean that he is full of the power of doing work ; and when we say that a thing is full of energy, we mean in like manner that it is full of the power of doing work. In fact, we measure the energy of anything by the amount of work which it can do before it is utterly spent. Now if we raise a pound weight one foot high, we do a certain amount of work, but if we raise it two feet high we do twice as much work, if three feet high three times as much work, and so on. If therefore we call the work of raising a pound weight one foot high one, we should call the work of raising it three feet high three. Again, the work of raising two pounds to any height is double the work of raising one pound to the same height, so that the work of raising two pounds threi^ ' fMT' " BODIES.] PHYSICS, 49 he it a mt as feet high would be six. In fact, multiply the number of pounds you raise by the number of feet you raise them, and the product will give you the work done. Let us now suppose that we point a cannon straight up into the air, and discharge a ball weighing i«^o lbs. with velocity just enough to make it mount up i,ooo feet before it turns ; we can tell at once from this how much energy the ball had when it was discharged. It had energy enough to carry loo lbs. (that is to say, itself) up I, coo feet, and consequently ^ergy enough to do work equal to i oo x i ,000 or 1 00,000. If we now put a larger charge of powder into the cannon, we shall make the ball come out with greater velocity. Sup- pose that now it can mount up 1,500 feet before it turns ; it has herefore energy capable of doing work equal to 100x1,500=150,000. In fact, you see at once that the greater the velocity or quickness with which the ball is shot out, the higher will it go, the more work will it do, and hence the greater energy will it have. 38. Work done by amoving body. — I cannot enter very fully here into the subject, but I will tell you that a body shot upwards with a double velocity will mount not twice but four times as high— a body with a triple velocity not thrice but thrice three times or nine times as high — and so on. You see therefore that a cannon-ball of double the velocity will do four times the jrk. But there are other ways of measuring the work of a cannon-ball than by seeing how high it can lift itself into the air, for we may fire it into wooden planks placed one behind the oAeft and we shall then find that a ball with a double UL 1 Sd SCIENCE PklMERS, [moving velocity will go through nearly four times as many planks, a ball with a triple velocity through nearly nine times as many, and so on. You thus see that a ball with a double velocity will have four times the destructive effect of one with a single velocity, and indeed in whatever way we measure its energy it will have four times as much energy as the other, 39, Energy in repose. — It is very easy to see that a body moving very fast has the power Ca doing a great deal of work, but besides this we have often energy in a quiet state, just as a man may be quiet, and yet able to do a great amount of work when he sets about it. Suppose there are two equally strong men fighting together, each with a heap of stones which fchey are throwing at each other, only the one with his pile of stones is standing on the top of a house, while the other man is standing at the bottom with his pile. I need not ask you which of the two is likely to win the day ; you will tell me at once the man at the top of the house. Now why has he the advantage ? He is not stronger or more energetic than the other — his advantage is therefore due to the stones ; it is clearly because his pile of stones is high up. He himself has not more energy than the man at the bottom, but his pile of stones has more energy than the pile of stones of the man at the bottom, and thus you see that the stones have an energy arising from the high position in which they are placed ; they are, in fact, capable of doing work, whether this be the very use- less work of knocking down a man or the very useful work of driving in a pile. Or let us ^mppose two water mills — one having a large tank or pond of water at a high level near it, while the other has a pond ot HODIBS.] Pb^s/cs. Si tank of water, but at a level lower down than that of the mill ; which mill is likely to work ? You will at . once tell me, the one with the pond of water at a high level, because the fall of water will drive round the wheel. You see, therefore, that there is a great deal of work to be got from a pond of water high up, or a head of vrater, as this is called — real substantial work, such as grinding com or threshing it, or turning wood or sawing it. On the other hand, there is no work at all to be got from a pond of water that is low down. Let us now compare a water-mill driven by a head of water with a windmill driven by the wind. The wind is like the cannon-ball, although not moving so fast, its energy being that of a body which is actually moving : it is in fact rushing against the sails of the windmill and driving them round ; and if we throw up a feather or a straw in a strong gale, we find that it is hurried away by the wind. But a water-mill has one decided advantage over a windmill, for in a wmdmill we must wait for the wind ; but if we have a water- mill with a good head of water we may turn the water on and off whenever we choose. We can keep our stock of energy and draw upon it whenever we have a mind. In fact, the energy of a body in motion is like ready money which we are in the act of spending, but the energy of a head of water, or of any body which is high up, is like money in a bank, which we may draw out whenever we want it. « ^ ■ ' • \ E9 $t SCIENCE PRIMERS. [VIBRATINO VIBRATING BODIES. 40. Sound. — A body that is changing its place is of course in motion, but it does not follow that every moving body changes its place as a whole; a top that spins round very quickly is in motion, but it does not change its place as a whole. Experiment 33. — Here is a wire which you see attached by one end to a support ; now if the other end be struck it goes backwaids and forwards rapidly, but the wire as a whok does not change its place. AVhen the pardcles of such a wire are moving back- wards and forwards, they are said to be in a state of vibration. In like manner, when a bell or a drum is struck the particles of the bell or drum are in a state of vibration, or when the string of a musical instrument is pulled and let go, the string is in a state of vibration. Now vibrating motion, just like motion from place to place, denotes enei^gy, and indeed the particles of a vibrating body are moving actively about from side to side ; if you try to stop them, they will give you a blow. If anything is in their way, they will give it a blow — the atmospheric air is, and they conse- quently give it a blow. Indeed each time the top ^ this vibrating wire comes back it gives the air a knock in the same direction. In fact, a vibrating body gives in a short time a great number of little knocks to the tiir. When the air is struck, it does not receive the stroke Fig. to. , '-r^- BODIES.] PHYSICS. 53 quietly, but strikes the air next it, and this in turn strikes the air next it, and so on, until the blow given to the air is carried over a great distance. At last this blow reaches your ear or mine, and we get a blow, which, however, does not aifect us in the same way as a blow that knocks us down, and therefore we do not call it a blow ; but we say that a sound has struck our ears — in fact we hear a sound. 41. What is noise and what music. — Now if the body that strikes the air just deals it one single blow, such as when a cannon is fired, the air, carries Uiis one blow to our ear, and we say that we hear a noise. If however the body that strikes the air be in vibration, and deal it a great number of little blowr> in one second, the air will carry these on and give i>ur ears just as many blows in one second, and then we say that we hear a musical sound. Thus you see a noise is a single blow given to our ear, but a musical sound is caused by a series of little blows following one another at regu- lar intervals. More than this, if the vibrating body which is the cause of this disturbance deals the air only a comparatively small number of blows in one second, then the air will of course only deal us the same num- ber in one second, and we shall hear a deep low note ; but if the vibrating body vibrates very quickly and deals the air a great number of blows in one second, the air will of course deal us just as many, and we shall hear a shrill high note. Thus you see a deep low note means a small number of blows dealt to our ears in one second, while a shrill high note means a great number of blows in the same time. A very shrill nr^e will y^ SCIENCE PRIMERS, [VIBRATIN6 be given by 20,000 blows in one second, and a very low note by 50 blows in the same time. 42. Sound can do work. — ^A musical note is pleasant, but a noise or single blow is disagreeable, and sometimes it hurts or even destroys the ear if it be a very violent one. Thus if a large cannon were discharged, the blow to the ear might in some cases destroy its hearing power; or if the sound struck against a pane of glass, the concussion mighi be s*) strong as to shatter the glass, and sometimes; in such cases as the explosion of a powder magazine all the windows in the neighbourhood are shattered to pieces. Thus you see that a loud noise is something with energy in it, and that it can do work — more especially work of a destructive nature. 43. It requires Air to carry it. * Experiment 34.— Let us try to ring a bell in a place where J there is no air, such as an exhausted receiver. There being no air, there will be nothing which the moving particles of the bell can give a blow to, and hence no sound will reach our ears. Jn fact, a bell that has been struck, or any other vibrating body, has in it a quantity of energy, of which it parts with some to the air, while the air in its turn parts with some to our ear. But if there be no air, there is nothing to carry to our ear the energy of the vibrating body. 44. Its mode of motion through Air. — Let us now think a little about the nature of this thing called sound, which is given out to the air by bodies in vibra- tion, and which is then carried to a great distance by the air itself. In the first place, when a cannon is discharged a mile or two off, do not imagine that the same partides BODIES.] PHYSICS. 55 of air travel all the way from the cannon to your ear. The particles near the cannon give a blow to those next ihena and then stop, the particles that have re- ceived the blow give in their turn a blow to those next thera and then stop, and so on, till the blow reaches your ear. What really happens will be made quite plain by the following experiment ^ * Experiment 35. — Let us take a series of elastic balls suspended in a row by separate threads, so as to Fig. 2x. hang loosely together, just touching one another. Let us now pull aside endways the first ball, and allovi it to give a blow to the second. What will happen ? The first ball having delivered its blow to the ^'^cond, will become quite still. The second will vv y quicWy transmit the blow to the third, and be- come still in its turn ; the third will do so likewise, until the impulse reaches the last ball of the series, which being the last will be put in motion l^ the blow. S6 SCIENCE PRIMERS. [vibrating Now the first ball may be likened to the particles of air which are next the cannon, and the last ball to the particles that are next your ear, and thus you see how the blow from the air next the cannon is transmitted to the air next your ear without the necessity of the same individual particles of air moving all the distance in order to carry it. Those of you who have played at croquet must have noticed what takes place when you croquet your ad- versary's ball. In this case you hold your own ball tightly under your foot while your adversary's is just touching it : you then by means of the mallet give a blow to your own ball, which docL not however move, but which transmits the blow to your adversary's ball with sufficient force to send it a great way off. We have here, therefore, a result the same as in the series of balls. 45. Its rate of motion. — Again, this impulse or blow which we call sound requires time in order to pass from the cannon to our ear. No doubt it travels very fast, as fast as a xifle-ball, but yet it does not pass instantaneously from the cannon to our ear. Most of you have no doubt seen a cannon fired a long distance off, and you then saw, first of all the flash and puff of smoke, and after a few seconds you heard the noise. Now these few seconds are the time which the sound or impulse took to travel from the can- non to your ear. You saw the flash the very moment the qannon was fired, and therefore, counting from its appearance, you know how long the sound took to travel from the cannon to you. Suppose, for instance^ that the cannon was 11,000 feet away, and that you reckoned ten seconds between Ihe flash and the report, BODIES.) PHYSICS, 57 you therefore conclude that sound takes ten seconds to pass through ii,ooo feet of air, or that it moves at the rate of i,iob feet a second, which is ]"~ett> near the truth. Sound will, however, pass through water much more quickly than through air, and by means of experiments made at the Lake of Geneva it has been ascertained that the rate of progress of sound through water is nearly four times as great as through air. Sound travels through wood or iron still faster — through wood, for instance, it travels from lo to i6 times as fast as through air, so that it would pass through more than two miles' length of Avooden logs in one second of time. 46. Echoes. — Suppose now that I stand in the centre of a large natural amphitheatre, having rocky cliffs all round me, and from this position let me dis- charge a gun — the noise or impulse will spread from the gun to the rocky cliffs and strike them, but something more will happen after that. The sound when it has struck the cliffs, finding it can get no further, will come back again, and in this particular case it will come back along the very same line that it went, travelling always at the rate of about r, 100 feet per second. The result will be that a few seconds after the gun has been fired I shall hear the sound that has travelled back from the cliffs just as if another gun had been fired. Now this sound is called an echo. You thus see that in the case of echoes we have the sound or impulse striking an obstacle and then reflected back from it, but it does not always come back in the same direction in which it goes ; this depends upon the shape of the surface against which it strikes. A very curious expeiiment is that which is 5» SCIENCE PRIMERS, [vibrating shown in the following figure. Place two large hoiiow reflectors at some distance from one another, and in a point called the focus of the one put a watch, while you place your ear in the focus of the other ; you will then hear the ticking of the watch very distinctly, just as if it were close to your car. The reason of this is that the blows given by the watch to the air strike against the left-hand reflector, and are reflected from Fig* 29. it indirections which bring them to the other reflector, from which they are ^hen all reflected into the ear. All this is shown in the figure. This property of sound makes a very nice experiment, but it has some- times proved inconvenient in practice : for instance, in the Cathedral of Girgenti in Sicily, it is related that the slightest whisper is conveyed from the great west- ern door to the cornice behind the high altar, and that unfortunately the former station was chosen as the place of the confessional. The result was that a lis- tener i^ced at the other station often heard what was never intended for the public ear, until at length this caineto be knowD, and another site was chosen. The BODIES.] PHYSICS. 59 ■****^**i*WiWi«i You see a large wheel a to the right, which is turned by a handle. Over the circumference or rim of this wheel we have a strong tight strap which passes over the axle of another wheel b. The result is that by mean: of the strap the axle of the wheel b will go round a great many times for a single turn of A, and the wheel b will itself of course move with its axle — ^in fact, b may be made to move round very quickly. You see, too, thai b is full of small teeth. Now there is a bit of card placed at e against the teeth of b, so that each tooth strikes the card as it passes. Each time the card is struck we hear a sound, because a blow is given by the card to the air. If there are loo teeth in the wheel b, there will be loo blows given to the air in the time that b goes once round. If b goes round once in a second, loo blows wiU be given to the air, and in consequence loo sounds Will strike our ear in one second, each single sound of which we shaH not be able to distinguish, but we shall hear an apparently continuous deep note. Now by driving the handle fast enough I can make b go round loo times in a second, and during each time it ^vill strike the card loo times ; the card will in this case be struck loo times loo, or 10,000 times in one second : 10,000 little blows will now strike the ear each second, and we shall hear ai continuous shrill note. Now when you wish to find the number of blows in one second corresponding to a given note, what you have to do is this. Turn the handle more and more quickly until the instrument by means of the card gives you a note precisely of the sams pitch as the note y ju have got to measure ; and when you have once BODIES.] PHYSICS. 6i IS got the proper speed, keep turning the handle for some time at the same speed, say for one minute or more. Now there is connected with the wheel b a dial (which is shown separately in a large scale lying below), and the dial registers how many times the card has been struck since you began to turn. You must therefore, when you yourself are turning the handle steadily at the speed which gives the right note, get another observer to note the position of the hand in the dial at the b'^Qinning and at the end of one minute. Suppose he finds out by the dial that during this minute the card has been struck 60,000 times, this will correspond to 1,000 times in one second, and hence you conclude that the note given out is that which corresponds to 1,000 blows given every second to the air. HEATED BODIES. 48. Nature of Heat. — You have seen that a body in actual motion may be said to possess eneigy, and also that the same may be said of a body in vibration. You have further seen that a body in vibration does not, in consequence, move about from one place to another, but remains at rest as a whole, while, however, its various particles are moving about alternately forwards and backwards. You have now to consider bodies in a heated state. First of all, what is heat ? Let us reply by supposing an iron ball to be put into the fire, and when white- hot suppose we take it out, put it on the scale-pan of a bidaiicei counterpoise it^ aii4 allow it to cqq). 6a SCIENCE PRIMERS. [heated Now if heat be something that has entered into the ball we should expect that as it cools it will grow con- tinually lighter. If, however, this experiment be properly made, it will be found that the iron ball does not lose weight as it cools, and therefore, whatever heat be, its presence has not made the ball one grain the heavier. Let me now suppose that I place myself upon a very delicate scale-pan, and while I am there, exactly counterpoised, let some water enter my ear. Of course I shall now be heavier than I was before. Suppose, however, that a sound enters my ear. Will the sound make me heavier ? Not one whit. It will strike what is called the drum of my ear, and set it vibrating, and I shall hear the sound, but I shall not be the least whit heavier in consequence of the entrance of the sound into my ear. In fact, while the entrance of water is the entrance of matter, and makes me heavier, the ^iittiince of sound is only the entrance of a kind of vibratory motion, and does not make me heavier. Now may not something of the kind take place in heated bodies ? May not the entrance of heat mean the entrance of some kind of vibratory or backward and forward motion, that does not add anything to the weight of the body ? We have strong reasons for thinking that heat is really a kind of vibratory motiooi so that when a body is heated each extremely small particle of it is moving about either backwards and forwards or round and round. But these particles are so very small, and cheir motion so very rapid, that the eye has no means of seeing what really takes place. Why dien, you will >ay> doe$ pot ^ h^t^ bcdy' BODIES.] PHYSICS. «3 give out a sound, if, as you tell us, its particles are in a state of rapid motion? Why does not such a body give a series of small blows to the air around it, just as a body in ordinary vibration does ? We reply, thai a heated body does give a series of blows to the medium around it ; and although these blows are such that they do not affect the ear, yet they affect the eye, and give us the sense of light You see now how great a likeness there is between a sounding body such as a bell and a hot body such as a white-hot ball. The particles of both bodies are in a state of rapid motion : those of the bell strike the air around the bell, and the air conveys the blows to our ear ; the particles of the hot ball also deal a succession of blows to the medium around the ball, and this medium conveys the blows to our eye. Thus when we experi- mented on vibrating bodies we used the ear, but when we experiment on highly heated bodies we use the eye. And in each case there are two divisions to the subject : for in vibrating bodies we have to study in the first place the bodies themselves, how fast they vibrate, in what way they vibrate, and so on, and in the second place we have to learn the rate at which the sound they give out is carried through the air ; so in the case of heated bodies, we have first of all to study the bodies themselves, and secondly to learn how fast the rays of light and heat which they give out travel through the air. 49. Expansion of bodies when heated. — When a body is heated, it almost always expands; that is to say, it gets larger in all directions. To prove to you that this is the case let us heat a solid, a Uquid| and a gas. 64 SCIENCE PRIMERS, [heated BOD * Experiment 36.— Let us take (fig. 24) a long metallic rod held tightly by a screw at one end, b. The other end is, however, free to expand, and in doing so it will press against the pointer, p, and in consequence this pointer will rise; if, therefore, the bar expands ever so little, this expansion will be seen very easily, for it will make the pointer alter its posi- tion, and rise np towards the top. Now let us place two or three lamps beneath the rod and heat it, and we shall find that the rod expands, and presses against the pointer so that it rises. If the lamps be withdrawn, the rod will cool, and in the course of a few minutes the pointer will have fallen into its old position. Expfii^iMENT 37.— Here is a hollow glass bulb which 1^ filled with water; let us now heat this glass bulb, and the water will rise in Ihe fine tube which is attached to the bulb. In this case both the glass bulb and the water expand^ but the water ex- pands more than the glass bulb, and hence it pushes its way upwards in the fine tube : indeed it expandis with such force that if there were no empty tube into which it might rise it would burst the bulb, now air it n expj 5< expe thini ing. BODIES.] PHYSICS, 65 Experiment 38. — To vary the experiment, let us now take a bladder which is about two-thirds filled with air and heat it over the fire, turning it round so that it may not burn. In a short time the air will have expanded so as to make the bladder appear quite full. 50. Thermometers. — You see from all these experiments that the tendency of heat is to make things expand, whether they be solids, liquids, or gases. And now let us particularly consider mercury in a bulb of glass, which like water will become expanded and run up the fine tube when heat is applied. Here we have in reality two things expand- ing. In the first place the bulb itself expands, so that if you were accurately to gauge the bulb when cold and when hot, you would find it to be slightly larger when hot. The bulb, however, does not ex- pand so much as the mercury, and in consequence the mercury is not content with occupying its old position in the tube attached to t^^e bulb, — it must have more room, and to get this it rises in the tube, xnd, the tube being very fine, a very small expansion of the mercury causes a very considerable rise in the tube, and is thus easily seen by the eye. In fact, the mere warmth of your hand will drive the mercury rapidly up the tube, and a mere breath of cold air will drive it down. An instrument of this kind is therefore very useful for telling whether one thing is hotter or colder than another, and answers very much better for this purpose than the feeling of touch. Suppose, for instance, that we place such an instrument with its bulb in a vessel of water, and leave it there for a few minutes, the top of the mercury will then keep a fixed position in the tube. Let us make a HI, f C6 SCIENCE PRIMERS. [heated mark, and note this position accurately. Let us now take the instrument out of this vessel of water and place it into another vessel also containing water. If this water be hotter than the first, the mercury will rise above the mark which we made — that is to say, the end of its column will now be higher up \ if, however, this water be colder than the first, the mercury will sink below the mark which we made ; and thus by observing the height of the meicury in the tube, we can at once tell whether the second' vessel of water be hotter or colder than the first. An instrument of this kind is called a thermo- meter, and I shall now tell you how a thermometer is made. 51. How to make them.— To make a thermo- meter, get a glassblower to blow a hollow bulb at the end >f a tube of glass, with a very fine bore, the other end of this fine tube being open to the air. Next heat the bulb in a flame; in doing this the air in the bulb expands (just as it did when we heated the bladder) ; but the other end of the fine tube being open, the expanded air gets out tlHFCWgh this end. Next, before the air has had IIm to cool, plunge the open end of the fine tube below the surface of a vessel containing mercury. The bulb, remember, now contains less air than it ^iU m first, part having been driven out by heat. Jhi^B air cools it shrinks into less bulk, and the fiiil^ of the air from without drives up the mer- c«y to occupy the vacant space, just as it drove up riie water in the water-pump (Art. 34). Part erf this mercury ^.viH therefore be driven into the bulb. We hava DO!? got a littk mercury ia the bulb, and w^ BODIES.] PHYSICS. 67 M- 80 TO- CO- 90. 30- n. -90 -ao yo -60 ■If- ■40 next take the bulb with the mercury in it and heat it well above the flame of a lamp — ^bulb, tube, and all The mercury will soon begin to boil, and its vapour will drive out the air before it, until bulb and tube will both be filled with the vapour of mercury. When this is done, we plunge the open end of^he tube once more into a vessel of mercury. As there is now no air in the tube or in the bulb, but only vapour of mercury, when this cools it will condense and there will be a vacuum, and the mercury in which the instrument is plunged will be driven up by the pressure of the outside air until it , fills both tube and bulb. We have thus filled both tube and bulb with mercury, and now before it has cooled we seal the open end by melting the glass, so as to keep the air out, and this part of the process is then complete. Having thus got our thermometer tube, we plunge it when sufficiently cool into a box containing pounded ice which is in the act of melting. The column of mer- cury of course falls in the tube because the ice is very cold : (you have been told that the column of mercury falls when ^v- "s- the bulb is plunged in a cold substance.) When the mercury ceases to fall, mark off with a file the posi- tion of the top of its column in the tule : this is the position which the top of the column will alway* occupy when the instrument is put into melting ice> or something equally cold. Having done this, next take the thermometer tube and plunge both bulb F 2 30 20 10 6S SCTEI^CE PRIMERS. [heated and tube into boiling water, and mark off the posi- tion of the top of the column as before. The column will now, of course, be very high, for the mercury will have expanded very much in consequence of the hot water. You have now got two marks on your fine tube — the'^one denoting the position of the top of the column of mercury when you plunge the bulb into melting ice, the other the position of the top of the column when you \ 'upf 'Jie bulb and tube into boiling water. You will . trwv^rds learn that the heat of boiling water is not <.^ -c . rnstant, but for the present we may regard boiling water liS having a fixed heat Having thus got two points marked or scratched with a file upon the tube of the thermometer cor- responding to the freezing and to the boiling points of water, the next operation is to divide the whole distance between these two points into loo equal parts. This is done by coating the whole tube with wax, and then making scratches in the wax-coating with the point of a needle at the proper places. If we then dip the whole tube into a solution of hydro- fluoric acid, this will not affect the wax, but it will affect the glass where the point of the needle has cut through the wax. After the tube has been taken out of the solution we shall therefore find that all the lines which we made with the point of the needle have eaten into the gla^s by help of the acid, and form, in fact, a scale of lines by the aid of which we may rise from the freezing to the boiling point of water through ICO steps or stairs, each step denoting some- thing hotter than the one below it, and colder than the on ! above it. Finally, let us call the lower step q degree, the BODIES.) piTYsrcs, ^ upper step loo degrees, and let us also number every ten steps between these two, and our thermometer is complete. Such an instrument is called a centigrade ther- mometer, which means a thermometer with a hundred steps ; and as this is the most convenient form of graduation, we shall always use it If a substance be of such a heat that when a ther- mometer is placed in it the end of the column rises to I o or 20 or 30, we say the temperature of the substance is 10 or 20 or 3* degrees, and so on. Melting ice therefore has the temperature of o deg ee (written 0°) on the Centigrade scale, and boilir j vater the temperature of 100 degrees (written lod") on the same scale; 20"* is a good summer heat, and 35** is about the heat of our blood, or blood- eat. In fact, such an instrument gives us a very accurate means of measuring temperature. 52. Expansion of Solids. — By a method similar to that of Experiment 36, only more accurate, we have found out how much rods made of glass or of metal expand between the freezing and the boiling points of water, that is to say between o"* and loo' of the thermometer, and the results are exhibited in the following table :— Expanuon between the freezing and the boiling points of water of a rod xoo,ooo inches long. , , , ♦ 85 inches. Glass Copper Brass Soft iron Cast iron Steel 171 188 120 109 114 » »> n 10 SCIENCE PRIMERS, [heated BODll Lead • Tin Silver Gold Platinum . Zinc Expansion between the freezing and the boiling points of water of a roc 100,000 inches long. 282 inches 196 192 144 87 298 u u » >f » 53. Expansion of Liquids. — Liquids expand more than solids when you increase their heat, but you cannot make an experiment upon a liquid rod, because a liquid cannot form a rod. In this case let us take a definite measure, such as a pint, and find what would be the overflow in pints of a liquid that occupied 100,000 pints at the freezing-point of water if it were raised to the boiling-point Now, if 100,000 pints of mercury were heated from o' to 100', or from the freezing to the boiling point, there would be an overflow of 1,815 pints; and if 100,000 pints of water were heated between the same range, there would be an overflow of 4,315 pints. It is found by such experiments that Liquids expand more than solids for the same increase of temperature, and that liquids expand more rapidly at a high than at a low temperature. 54. Expansion of Gases. — Gases expand through heat, and that to a great extent; but here we must bear in mind that other things besides heat will make gases expand. You remember the india-rubber ball that was put into a rieceiver and began to expsmd when the air was withdrawn from the recel to si heatj the we howl that' atm< V, BODIES.! PHYSICS. 71 receiver (Experiment 25). When, therefore, we wish to see how much a quantity of gas expands through heat, we must take care that the air which surrounds the gas does not change its pressure : in other words, we must take a bladder with some air in it, and find how much it expands when heated in the open air — that is to say, under the constant pressure of the atmosphere — between the freezing and the boiling points of water. When this is done, it is found that if a bladder not completely filled with air have a volume equal to 1,000 cubic inches at the freezing-point, its volume at the boiling-point will be 1,367 cubic inches. If therefore we have a large quantity of ice-cold water in a vessel and force this bladder containing 1,000 cubic inches beneath the water, we shall find the water rise in the vessel through * a space denoting 1,000 cubic inches — this being the increase of volume due to the bladder. But if we take the same vessel, only filled with boiling water, and plunge the bladder into it, we shall find the water rise through a space denoting 1,367 cubic inches — this being the volume of the bladder at this temperature. 55. Remarks on Expansion. — Liquids and solids expand with immense force. If ycu were to fill an iron ball quite full of water, shut it tightly down by means of a screw, and then heat the ball ; the force of the expansion would be great enough to burst the ball In large iron and tubular bridges allowance must be noade so that the iron has room to expand ; for in the middle of summer the bridge will be somewhat longer than in the middle of winter, and if it has not room 71 sc/ej^CjE primers. [heated to lengthen out, it will be injured by the force tend- ing to expand it. There is an arrangement for this purpoi&e in the Menai Tubular Bridge. We take advantage of the force of expansion and contraction in many ways — for instance, in making carriage wheels. The iron tire is first made red-hot, and in this state is fitted on loosely upon the wheel ; it is then rapidly cooled, and in so doing it contracts, grasps the wheel firmly, and becomes quite tight 56. Specific Heat. — Some bodies require a greater amount of heat than others in order to raise their tem- perature one degree. The quantity of heat required to raise a pound weight of any substance one degree is called its specific heat. Water has a very great specific heat ; that is to say, it requires more heat to raise a pound of water one degree than it does to raise almost any other substance. The heat required to raise a pound of water one degree will raise through one degree 9 lbs. of iron, 1 1 lbs. of zinc, and no less than 30 lbs. of mercury or gold. Experiment 39. — To convince you of the great specific heat of water, let us take 2 lbs. weight of mercury and heat it to 100°, or the boiling-point of water, and let us then mix it with i lb. of water at an ordinary temperature. Now note the height of a thermometer placed in the water both before and after the mixture, and you will find that it has hardly risen more than 5° in consequence of the hot mercury being poured in. 57. Change of state.— Yqu have already heard about the three states of matter — the solid, the liquid, and the gaseous. I have now to tell you that substances x>'hen heated piss first from the solid to the lioj^id, and BODIES.] Pli YSICS. 73 then from the liquid to the gaseous state. You are told in the Introductory Primer that ic », water, and steam have precisely the same composition, and that ice becomes water if it be heaicil, while w?ter becomes steam if we continue the heat The very same ':hange will happen to other substances if we treat them m the same way. Let us, for instance, take a piece of the metal called zinc and heat it ; after some time it will melt, and if we still continue to heat it, it will at last pass away in the shape of zinc vapour. Even hard, solid iron or steel may be made to melt, and even driven away in the shape of vapour ; and by means of an agent called electricity (of which more hereafter) we can probably heat any substance suffi- ciently to drive it away in the state of vapour or gas. We cannot, however, cool all bodies sufficiently to bring them into the solid or even into the liquid state. Thus, for instance, pure alcohol has never been cooled into a solid; but we know very well that all we have to do is to obtain greater cold in order to succeed in freezing alcohol In like manner, we have never been able to cool the atmo- spheric air sufficiently to bring it into the liquid form ;. but we know very well that all we require in order to succeed is to obtain greater cold. You must not, however, imagine from what I have said, that cold means anything else than the absence of heat A cold body is a body which has little heat, a^d a still colder body has still less heat ; but even the coldest body which we can produce has a little heat left. Do not be guided in this respect by your feeling of touch. Two bodies may be of the &ame temperature^ as shown by the thermometer; 74 SCIENCE PRIMERS, [heated and yet the one may feel much colder to you than the othei \ and if you keep one hand for some time ill very cold and the other in very hot water, and then plunge them both into water of ordinary heat, this water will seem hot to the one hand and cold to the other. Do not therefore be guided by any- thing else than the thermometer, or imagine that cold is anything else than the absence of heat. 1 o return to our subject. Probably all bodies, if we could cool them enough — that is to say, take away enough of their heat — would assume the solid state ; and then, when each was again heated sufficiently, it would become liquid, until at last, if still heated, it would be driven off in the shape of gas or vapour. There would, however, be a great difference between the different bodies in the ease with which they would yield. Ice soon melts if we apply heat ; tin or lead require to be heated to 200 or 300 degrees before they will melt ; iron is more difhcult to melt than leid \ and platinum is more difficult than iron. A body very difficult to melt is called refractory. In the following table we have the temperature at which some of the most useful subs Ice melts at Phosphorus Spermaceti Potassium Sodium Tin . Lead . Silver . Gold . Iron , ances begin to melt, o" 49" ^r i,ooo° . 1,250'' • 1,500^ BODIES.] PHYSICS, 75 Platinum is so difRcult to melt that we cannot tell At what temperature it does so. And carbon is still more difficult to melt — indeed in the very hottest fire the coal or carbon is always sc'id ; and no one ever heard of the coals melting down and trickling out Uirough the furnace bars. We thus see that the same sort of change takes place in all bodies through heat ; that is to say, if we could reach a temperature sufficiently low, all bodies would become solid like ice, and if we could reach one sufficiently high, all would become gaseous like steam : in fact, the change that takes place is always of the same kind, and we cannot do better than use water as a type of all other things in this respect, and study the . behaviour of this substance under heat, beginning with its solid state when it appears in the shape of ice. . 58. Latent heat of Water. — Let us take some very cold ice, pound it into small pieces, and put the bulb of our thermometer into this pounded ice. Let us suppose that the reading of our instrument shews a temperature 20 degrees below the point we call 0°. Now let us heat the ice, and its temperature will rise like that of any other solid under like circum- stances until it comes to 0°, but at this point it will svop, and rise no fiir^^hcr as loiii^ as any ice remains. What then does the heat do if it does not raise the temperature above this point? We reply, it melts the ice. At first the heat is wholly spent in raising the temperature of the very cold ice, but when this temperature has reached o® the heat has quite a different office to perform ; its power is now wholly spent ill melting the ice, and when the ice ij ^il aeited 76 SCIENCE PRIMERS. ^tmm the water has only the temperature o^, being no hotter than melting ice. In fact, water at o** is equal to ice at o% together with a laige ametil of heat, which we call latent heat because it does dot affect the thermometer. Experiment 40. — ^You may prove this by putting some pounded ice into a tin pan and heating it over a lamp until there is only a little ice left If you then plunge a thermometer into the melted ice, you will find that the temperature will hardly be above o", or in fact the melted ice will be as cold as the ice before it was melted. 59* Latent heat of Steam.— We have now changed our ice iuto water, and if we continue to heat this water its temperature will rise in the ordinary manner, like that of other bodies, until it reaches the boiling-point or loo^ Its temperature will then stop rising, and if we continue to heat the water we shall only convert it into steam of which the temperature is 100^ and no more. In fact, just as it took a large amount of heat to convert ice at the fre<;zing-point into water at the freezing-point, so does it take a large amount of heat to convert water at the boiling-point into steam at the boiling-point So that we are entitled to say — steam at 100° is equal to water at 100^, together with a large amount of heat which we call latent, because it does not affect the thermometer. Experiment 41. — You may prove this by boiling some water in a flask and putting the thermometer first into the boiling water and then into the ateam. They will both be found to have the same te«npe- rature, or, in other words, steam is no hotter than boiling; water. ra aomss.] PHYSICS. 77 Thus you see that ice requires latent heat to bring it into water, while water again requires latent heat to bring it into steanu Now we can measure how much heat it will take to bring a pound of ice at o° to a pound of water at the same temperature, and we find that it will take as much heat to do this as it would to raise 79 pounds of water one degree in temperature, and this is what we mean when we say that the latent heat of water is equal to 79. In a similar manner it has been found that the latent heat of steam is 537 ; that is to say, it will take as much heat to change a pound of water at 100° into steam of the same temperature as it would to raise 537 pounds of water one degree in temperature. It thus takes a good deal of heat to melt ice, and it therefore takes a good deal of time to do sa In^ deed it is much better that this is the case, for what would happen if ice at the melting-point were to change into water at once when heated ever so little ? It would render uninhabitable a lai^ge part of the globe, for the ice of the mountains would on some fine spring day be at once liquefied, and the water would rush down in such overwhelming torrents as to sweep every- thing before it, and large tracts of low-lying land would be buried under water. In like manner, it is much better for us that it takes a large amount of heat to convert water at the boiling-point into steam ; for sup- pose that water at this point were at once converted into steam by heating it ever so little, there would then be an explosion in evei r tea kettle and in every boiler, while a steam-engine would be an v\tter impossibility. You have already been told that steam is a gas like air, and you have learned in the Introductory Primer J^md^aiL.i. 78 SCIENCE PRIMERS. [heated that you cannot see true steam. Wl i a kt ^k \^ boiling l^«^pidly, }.>u may have noticed that you do net Fee ,i:ythmg quite close to the spoilt of the kf ;s!e, I it about half an inch beyond it you see a cloud. Or, again, when a locomotive gives out its steam you do not see anything quite close to the funnel, bult a little distance above it you see a cloud. Now this invisible thing that comes out is true steam, but the visible cloud consists only of very small drops of water, formed from the steam as it cools; it is not therefore steam, but water. True steam is invisible, like the air or any other gas. 60. Ebullition and evaporation. — I have now told you something about the steam which is given out when water boils. I do not, however, mean to say that no steam is given out before it boils, for this would be contrary to fact : all of you must have noticed that a pan of water put on the fire gives out steam long before it begins to boil. Doubtless, too, you Buist have noticed that any wet thing or thing full of water gets dry near the fire —that is ^[f- <;ay, its water goes away in the shape of sieam. ':m / when steam or vapour (for both words mean the same thing) is given out by water which is not boiling we call it evapora- tion, but if the water boils we call it ebullition. The difference is simply this. When you heat water over the fire, the heat has at first two things to do. It has in the first place to heat the water, and in the HciXt place it evaporates part of the water ; l>ut when the temperature of the water has risen to 100° or the boiling-point, the water cannot be heated above this : all the t' ngth of the ^xt is then spent in converting the writ' mto steam, and this stenn) escapes not only »>r >*ss] P//VSICS. 79 *u imr<. f'-'ym uia top of the water but from the very bottom cuso, so that we hear a noise which we ral' boiling as the bubbles of steam rise through the vvatei and escape into the air. 6i. The boiling-point depends on pressure. — ^.I have now to tell you that the temperature or heat at which water bolis is not a perfectly fixed point like that of melting :ce, but depends upon the pressure of the air. If the pressure of the air be lessened, water will boil below ioo°. You remember you were told that the pressure of the air is less at the top of a lofty moun- tain than at the bottom, because at the top you have a less depth, and therefore *^ less weight or pressure, of air above you. Well, at the top of Mont Blanc in Switzerland, which is three miles high, water will boil at 85°; and if a traveller were to try to boil an egg in a pan ai the top of Mont Blanc, he might boil it for hours, but it would not harden, because Ss** is not high enough to harden the white of an egg. On the other hand, if we were to boil water at the foot of a very deep mine the boiling-point would b^^ considerably above 100°. Experiment 42. — You will see by the fallowing very simple experiment that the temperature of th«5 boiling-pomt depends upon the pressuu of gas or air upon the surface of the water. Let us take a glass flask and fill it half full of water, then cause the water to boil for some time, until the stea a has driven out all the air from the upper part of the flask, so that we have only water, and the vapour of water in the flask. Now cork it tightly, and, withdrawing it from the lamp, invert it as in fig. 26. When it has ceased boiUi^g, lake a sponge and ^.our some cold 8o SCIENCE PKlAfERS, [heated water on the flnsk, when boiling will again begin. I'he reason of thi« is, that before the cold water was pou'-ed on there was a considerable pressure of vapour upon the water of the flask, and this presrare kept it from boiling, but the effect of the told water was to condense this vapour, and therefore to lessen its Fig '3^ pressrire ; and since water boils more easily at a low pressure than at :i high, the water in the flask began, as you saw, immediately to boil. Before leavmg this part of our subject, I ought to tell you that some bodies expand while others con^ tract in the act of m^^'dng ; that is to say, in passing from the solid to th 1' -aid state. Experiment 4^^.-- Here is some ice, for instance, vrhich is lighter th=^n water, as you will see by the fact that th ice is at present floating upon the water. In passing from ice to water there is, therefore, a great contraction of substance, and in passing from water to DOMES.] PI/YSfCS, 8i ice — in the process of freezing— there is a great expansion. This expansion takes place with great force, and if a thick iron vessel be filled with water and then shut by a stop cock by causing the water to freeze, you may burst the iron vessel. Casl iron con- tracts like ice when it melts, or, which is the same thing, expands like water when it freezes or gets solid, and in consequence, if liquid iron be run into a mc uld, when it gets solid it will ex|)4nd so as to fill all the crevices of the mould ; it can thus be cast in a mould. On the other hand, gold, silver, and copper expand when they melt, and contract when they become solid ; they will not therefore, like cast iron, run into the crevices of a mould, and therefore coins made of these metals cannot be cast in a mould, but must be stamped. All substances, however, expand very greatly when tlu y are converted into gas, and a cubic inch of boil- ing water will be converted into steam occupying nearly 1,700 cubic inches. C2. Other effects of Heat. — You have now seen that heat expands bodies or makes them larger, and tliat it also causes them to change their state, pass- ing from solids to liquids and from liquids to gases as the heat continues to be applied. You have seen how powerful an agent heat is ; how the strongest and hardest bar of iron will by it be changed into a white- hot mass as soft as treacle, and if heated still more will be driven off in the shape of gas. Heat affects bodies in many other wa5rs, and more especially it promotes the operation of ciiemical attrac tion. Thus at a low temperature coal will not combine with the oxygen of the air, and we may keep o\ir coals for any length of time in our coal-cellar. But when heat is applied combination t^Vps nlace ; and lu this iiL o «a SCIENCE PRIMERS. pHEATE!) BO combination in its turn produces heat, the process of combination goes on, and the coal is said to burn. In like manner in the experiment (Chemistry Primer, Art. 6) where sulphur and copper combine together, heat is first of all applied in order to promote combi- nation, but v/hen this has begun heat is generated, and the process goes on of itself, without requiring any more heat from a lamp. 63. Freezing mixtures. — Again, chemical union, you have been told (Chemistry Primer, Art. 7), pro- duces h^at, and this is always true ; nevertheless some- times two substances which have a tendency to form a solution mix together with the production of cold and not of heat. Thus common salt and snow have a tendency to form a solution, and they will do so with the production of very considerable cold, or, to speak more correctly, with the absorption of a very con- siderable quantity of heat. ExPEKiMENT 44. — To prove this, let us rapidly mix some melting ice or snow and some salt together, and place in ta? mixture the bulb of our thermometer. The mercury in the tube wiJl soon fall below o*, thereby showing that this mixture is colder than melting ice. Now what is the reason of this ? it is to be found in the fact that after these two substances have become mixed together we have a liquid and not a solid — in iBsict, we have strong brine. Now you have been told that heat is swallowed up, or becomes latent, when bodies pass from the solid into the liquid state — for instance, when ice becomes water. The brine, there- fore, being a liquid, swallows up part of the heat of the snow and salt, and. the consequence is that we have a vel b( oti b^ BODIES. J Physics. 83 very cold liquid as the result of the union of two solid bodies. Thus when two solid bodies dissolve each other, we have very frequently a lowering of tempe- rature on account of the heat which is swallowed up by the liquid. Such bodies are said to form freezing mixtures. In like manner, if we have a liquid that evaporates very fast we find it to be intensely cold, because in order to become a vapour or gas it requires a great deal of heat, and gets it where it can : thus if you drop some ether upon your hand it feels very cold, and soon flies away in the shape of gas ; in fact, it has robbed your hand of a large quantity of heat in order to pro- duce this vapour or gas. Ve:y low temperatures, very intense cold, may sometime^ be produced by causing certain liquids to evaporate very rapidly. Experiment 45. — To prove this let me pour some water into a shallow vessel, and place it along with a pan containing strong sulphuric acid under the re- ceiver of the air-pump, and exhaust the air. As the pressure of the air is withdrawn the water will evaporate very rapidly, and in order to do so will take away so much heat from its own substance that ii will be turned into ice. 64. Distribution of Heat. — Let us now pro- reed to another part of our subject, and consider the tendency which heat has to distribute itself. A hot body will not always remain hot, but it will part with its heat to the colder bodies that are around it; and il will always insist upon doing this, but it will do it in different ways according to circumstances. Experiment 46. — For instance, let us put a poker into the fire ; some of the heat of the fire gets into G 2 84 SCIENCE PRIMERS, [heated Vk that part of the poker which is in the fire, and this passes along the poker until it heats that end which is farthest away from the fire, and you will at last find it too hot to touch. This passage of heat along the poker is called conduction of heat. Experiment 47. — Again, let us take a flask two-thirds full of water, and heat it from below. As the lower particles of water are heated they expand, and there- fore get lighter ; they consequently rise tc the top for the same reason that a cork rises in water, and are replaced by colder and, therefore, heavier particles from above. A new set of particles are thus con- tinually subjected to the heat of the lamp, and in process of time the whole water will get heated and begin to boil This process is called convection of heat. Neither of these processes will, however, account for the heat that reac^ies us from the sun. Whether in conduction or convection the heat is carried by means of particles of solid or liquid matter, but we have reason to think that there are no such particles between us and the sun, while we know that the sun's light and heat takes less than eight minutes to come from the sun to us over a distance of 90 millions of miles. Evidently, then, the heat which comes to us from the sun moves with an immense velocity, and does not reach us in virtue of boating up the particles between the sun and ounp^lves. In fact, in a very cold day, when the air is very cold and anything but heated, the sun's rays may be very powerful Now the process by which heat comes to us from the sun or any other very hot body is called radiation of heat. us BODIBS.] PHYSICS. We have thus three very different ways in which # heated body communicates its heat to a cold one namely, conduction, convection, and radiation. Lei us now consider these in order. 65. Conduction of Heat^We have spoken about thrusting a poker into the fire, and told you that at last the other end of the poker will be too hot to hold. But if, instead of a metal poker or rod, a glass or stoneware rod were thrust into the fire, the other end of this rod would never get very hot, because stoneware does not conduct heat nearly so well as metal. Wool and feathers are still worse conductors, and this is why these substances have been provided by nature as the clothing of animals ; for the heat of an animal is generally greater than that of the surrounding substances, and this heat is not readily conducted off through the garment of wool, feathers, or fur, with which the animal is cladc So in the case of boilers of engines ; when we wish to keep in the heat, we supply them with steam jackets or coverings made of a non- conducting substance. A bad conductor may be used not only to keep in heat, but also to keep it out ; flannel, for instance, may be used to wrap round our bodies in order to keep the heat in, or it may be used to wrap round a block of ice which we wish to preserve in order to keep the heat out. Ih fact, heat cannot readily pass through flannel whether it be going from within out- wards or from without inwards. Experiment 48.— It is very easy to show you that different substances have different conducting powers for heat You see, as in the figure, two rods or w)reS| one vr, IMAGE EVALUATION TEST TARGET (MT-3) // 4go 1.0 I.I 11.25 ■Si 121 125 ■^ Uii 12.2 Hi ... ■■■ ui us m |4£ 12.0 ■ 1.8 lliiii U |L6 S% V] "V*' '/ /A Photographic Sciences Corporation 23 WEST MAIN STMET WEBSTER, N.Y. MSEO (716) •73-4S03 o ^ ^ /.. ^ 86 SCIENCE PRIMERS. fHEATEf) of copper and or** of iron, with their ends together, at which they are K. ted by means of a lamp. After the lamp has burned for some time, let us take two little bits of phosphorus, and place one of them at the end of the copper rod furthest away from the flame. It will soon take fire. Now place the other piece on the iron rod at the same distance from the lamp as the burning phosphorus, and it will not take fire. This shows us that the beat of the lamp is conducted more powerfully along the copper than along the iron* Fig. 27. The conduction of heat explains the action of the safety lamp which was devised by Sir Humphry Davy for the use of miners, but this very useful lamp has already been fully described in the Chemical Primer (Art. 41). 66. Convection of Heat. — If we take a vessel full of water, and float on its surface a vessel full of boiling oil, we shall find that the heat of the oil will be conducted very slowly indeed downwards through BODIES.] PHYSICS. 87 the liquid ; in fact, a few inches down, the rise of temperature will be hardly perceptible. Bui if instead of heating the vessel with water in it from above we heat it from below, as in the figure, we shall find that in a very short time the whole water will be heated and begin to boil. In fact, as we have already stated, the heated particles getting lighter rise, and are re- placed by colc'er and heavier particles from above, so Fig. 28. that we have a current as is shown by the arrows in the figure, the heated water ascending in the middle and the cold water coming down the sides. We have several good examples of convection in nature \ for instance, in a lake which is cooled at its surface by the action of intense cold. The surface particles are first cooled, and gi tting heavier they sink 88 SCIENCE PRIMERS. [light from down and are replaced by lighter and warmer particles from beneath, so that in a short time the whole body of T/ater becomes cooled down to a temperature about 4° above the freezing-point; after that temperature the water, contrary to the usual practice of things, expands when further cooled instead of contracting ; and when ice is formed, this ice, being decidedly lighter than water, floats on the top. Now, had ice been heavier than water, it would have fallen down to the bottom as it was formed, a fresh surface would -thus have been exposed, and the whole lake would soon have !)ecome one mass of ice. But as it is, the cold can only freeze the second layer of water through the ice of the first, and this is a very slow process, so that there is no danger of a lake being permanently frozen. In the air again we have strong convection currents due to heating ; for it is on this account that the hot air of a fire goes up the chhnney, being replaced by cold air from the room ; and we have the very same thing on a large scale in the great system of wimls, for at that part of the earth called the equator, where the sun is most powerful, the air when heated mounts up just as the air of a fire mounts up the chimney. This air is then replaced by currents blowing along the surface of the earth from the poles or colder portions of the earth. We have thus at the equator, a system of upward currents which carry off the hot air to the poles in the upper regions of the air, and we have also currents bio win Roil s, we shall see a bright slit and nothing more; in fact, the slit will serve as an opening through which we may see the bright sun beyond. Let us now introduce the prism as in the figure \ when we have done so, our eye at E will no longer see the slit If, however, we move our eye towards the thick part of the prism, we shall at last catch hold of the light from the slit, but it will be now very much changed in appearance. It will not now reach our eye in the shape of a bright thin slit, as formerly, but it will appear as a broad band or ribbon of light of many colours, beginning with red at the one end, and passing gradually and in order through orange,, yellow, green, blue, and indigo, into violet at the other extremity. All this may be easily explained by what we have already said, bearing in mind that white sunlight is in reality composed of all the different colours mixed to- gether. Not only, therefore, are the rays bent in their passage through the prism, but they are unequally bent. And we shall have for each variety of light its own appropriate slit in its own appropriate position. We shall therefore have a multitude of little bright images of the slit lying side by side, forming, in fact, a band or ribbon of light rather than a i^it ; the red being at one end, because the red rays are least bent, and the violet at the other end, because the violet rays are most bent. This variously coloured ribbon of light is called a spectrum ; and if it be the light of the sun which we employ to light up our slit, then we get the solar spectrum^ * 74. Recapitulation. — I have now told you a good deal about radiant light and heat You have in the ftni place learned that» as you begin to heat bodiesi diey glv# HEATEJD BODIE.'>.] PHYSICS. i03 out first of all dark rays, but that as you raise their lemperature, the rays become luminous and capable of affecting the eye. You have then been told something about the reflection of these rays from polished sur- faces. You have also been told how their direction is bent when they pass through water and glass; and how a glass prism bends the rays towards its thickest part. You have next been told that a lens bends the rays all round towards its centre or thickest part ; and how, if you allow sun-light to fall upon a lens, you get a small bright image of the sun ; which image will set fire to a sheet of paper or bum the hand. You have also learned that the moon or a planet will give by means of a lens an image of the same kind ; and how, if you approach such an image with a magnifying glass and look into it, you really see a very large moon or a very large planet, and that this com- bination of two glasses is called a telescope. Finally, you have been told that differently coloured rays of light are bent by a prism into different places, so that a prism separates all the elements of a compound ray of light And now, before concluding, let us study a little the nature of heat. 75. Nature of Heat. — We have already compared heat to sound, and told you that a heated body is an energetic body. Let us now take up this comparison once more. In sound we have two things to study: first, the body which vibrates ; and secondly, the im- pulses which this body sends through the air to our ear, imd which make us hear a sound. How you wore told that a heated body is one in iirliidi ^ snifti particles are in very rapid vibration^ tD4 SCIENCE PRIMERS, [naturi and that just as a vibrating body gives out sound, which strikes the ear, so a heated body gives out light, which strikes the eye. But how is a body made to vibrate ; a bell or a drum, for instance ? — only by giving it a blow. You bring the heavy hainmer or tongue quickly against the side df thie bell, and the bell begins te vibrate ; now this hammer or tongue before it strike? the bell is a body in rapid motion, and therefore pos- sesses energy, or can do work. Well, what becomes of its energy after it strikes the bell ? It has, in truth, given up its own energy to the bell, for the bell is now vibrating, and you have already been told that a vibrat- ing body is one with energy in it. Thus the energy of the blow given to the bell has not been lost, but has only been transferred from the hammer to the bell. Now let us suppose a blacksfaith places a piece of lead upon his anvil and brings down his hammer upon it with a heavy blow. You hear a dull thud, but there is no vibration like that of the bell. What becomes therefore of the enerjgjr of the blow? It is not transformed into vibrations like those of the bell, which can strike the ear — into what therefore is it changed ? or is it changed into anything ? We reply that it is changed into heat. The blow has heated the lead and set all its particles vibrating, although n©t in the same way as those of the bell ; and if the blacksmith strikes the piece of lead long enough, I dare say he will even melt the ipad. No doubt some of you have spent much energy in rubbing a bright button on a piece of wood. Nowvlwit has become of all the energy you have:^nt upcm^e button ? We reply, it ha»been tnmsformed itito hei^ a» you will easily find out by put^g ^e bt&tton qukldy ^^. OF HEAT.] PIJYSICS. 105 on the back of your own hand or on the back of your n^hbour's. Experiment 52.— To show you how the energy of a blow is changed into that other kind of energy which we call heat, let us take one of those wax matches tipped with phosphorus called vestas, and, placing it upon the hearthstone, strike it a blow with a hammer or stone ; you will now find that the heat developed has been sufficient to set the phosphorus on fire. You thus see that friction produces heat, and you may have noticed that on a dark night sparks Hy out from the break-wheel, wluch stops the motion of a railway train. In all such cases> actual visible energy is being changed into that form of energy which we call heat, the differ- ence being that in visible energy the body moves as a whole, and all its particles move in the same direction at the saipe moment, while in heat the various particles move backwards and forwards rapidly, while the body, as a whole, is at rest. You thus see that visible energy can be changed into heat, and I have further to tell you that he^t can to some extent be transformed back into visible energy. In the case of a steam-engine what is it that does all the work ? Is it not the fire that heats the water of the boiler? and in this case part of the heat-energy of the burning coals actually and truly changes itself into the visible energy with which the piston moves up and down, and the fly-wheel moves round and round. In fact, all the work done by steam-engines is work got put of heat Thus you see we can not only change actual energy into beat, but, in the steam-engine, we can change heat back again into actual eneigy. io6 SCIENCE PRIMERS, [ELEcrRiFXKD ELECTRIFIED BODIES. 76. Conductors and Non-conductors. — It was known more than two thousand years ago that when a piece of amber is rubbed with silk, it attracts light bodies ; and Dr. Gilbert, about three hundred years ago, showed that many other things, such as sul- phur, sealing-wax and glass, have the same property as amber. Here you see the faint and small beginning of our knowledge of electricity, a knowledge which has of late years grown so wonderfully i« to enable us to send messages between Europe and America in less than one second of time; *ExpERiMENT 53.— Let us take a metal rod, having a glass stem, and rub the glass with a piece of silk, both silk and glass being well heated and quite dry. The glass will now have the power of attracting little bits of paper or elder pith, but only at that place where it has been rubbed. The glass has, in fact, by rubbing, acquired a new property, but this property cannot spread itself over its surface. So much for glass. Suppose now that we take the metal rod and touch with it the prime conductor of an electric machine in action, we shall find that the metal rod has acquired the same proper- ties as the glass; that is to say, it will attract light bodies like paper or elder pith, but all parts of the rod of metal will have the same property, and not merely that part which touched the electric itmchine. In fact, the electiic influence can spread itself oveir a surface of metal, though it cannot r.ver one of gi|U^» Glass, therefore, is said to be a non*cosiductor tf frRlFlED BCfDIES.] PHYSICS. V0I 'S It \o that ittracts mdred as sul- [erty as of our has of osend an one .1 ivinga )th silk ^ glass paper 5 been juired t itself 5 now prime shall oper- light f the I not hine. vttsc electricity, iivhile metal is called a conductor. In fact, neither heat nor electricity can easily spread itself over glass, but both c" > easily spread themselves over metal ; charcoal, acids, soluble salts, water, and the bodies of animals are likewise good conductors of electricity, although not so good as the metals; while, on the other hand, india-rubber, dry air, silk, glass, wax, sulphur, amb^r, shellac, are all very bad conductors. If we wish to succeed in experiments with elec- tricity, it is absolutely necessary to keep the electricity once we have got it ; we must, in fact, surround it on all sides by non-conducting bodies. It is, therefore, of great importance to make our experiments in dry air, and to make the body which has the electricity stand upon a glass support. 7|. Two kinds of Electricity. Experiment 54. — I have now to convince you that there are two opposite kinds of electricity. To prove this let us make^use of the apparatus you see in fig. 39, con- sisting of a small pith ball suspended by means of a silk thread to a glass support. First of all let us rub a glass rod with silk, and with the rod so rubbed touch the pith ball. The glass end will com- municate electricity^ to the pith ball, and it will not be able to get away^ because the silk thread, the glass support, and the air (if dry) around the pith ball are all non-conductors. Now, if you notice, you will $ee that after the glass rod has been made to touch the pith ball, this ball will no longer be attracted to the glass rod, but will,. on the other hand, be repelled by it Let us next rub a stick of sealing-wax with a piece of warm, dry flannel, and bring the stick so rubbed. io8 SCIENCE PRIMERS, [ELECTRIFIE^ near to the pith ball. It will now be found that the pith ball, which was repelled by the excited glass, will be attracted to the excited sealing-wax. It thus appears that a pith ball first touched with excited glass will be afterwards repelled by excited glass, but will be attracte<^ ' excited sealing-wax. Now if we had reversed oijr plan of operations, and had first of all touched the pith ball with excited seai- F'g 39- ing.wax, instead of excited glass, it would then have been repelled by excited sealing-wax, but attracted by excited glass. We learn from this that there are two kinds of elec- tricity; namely, that which we get from excited glass, and that which we get from excited sealing-wax. Now when we touched the pith ball with excited glass, we communicated to it part of the electricity of the glass j and as it was afterwards repelled by excited the rill [ith JS, Id BODIES.] PHYSICS. to9 glass, we conclude that bodies charged with the same kind of electricity repel one another. On the other hand, the pith ball, if charged with excited glass, will be attracted to excited sealing-wax ; or if charged with excited sealing-wax, it will be at- tracted to excited glass, and hence we conclude that bodies charged with opposite kinds of elec- tricity attract one another. 78. They exist combined in unexcited bodies. — We may suppose that every substance has in it a quantity of these two kinds of electricity mixed together, and that what we do in rubbing is merely to separate the two electricities from one another. Accordingly, when we rub a piece of seal- ing-wax with a piece of flannel, all that we do is to separate the two kinds of electricity — the one kind keeping to the sealing-wax, while the other remains behind upon the flannel. In like manner all that we do when we excite glass with silk is to separate the two electricities, one remaining on the glass while the other adheres to the silk. The same thing holds in all cases where electricity is developed by friction, and it is impossible to produce the one electricity ivithout, at the same time, producing just as much of the other also. In fine, we do not create electricity ; but, according ,;o this view of it, we merely separate the two opposite kinds from one another. Tlie electricity which appears in a stick of glass when it has been rubbed with silk is called positive, and that which appears in a stick of sealing-wax, when it has been rubbed with flannel, is called negative. These are merely terms used in order to distinguish i^lireen the two kindi of electricity. 116 SCIENCE PktMERS, (electrifieB 79. Action of excited on uncxcited bodieb. —We have seen that electricities of the same kind rapel, while electricities of opposite kinds attract each other, but we have still to learn what will happen in the following case. Let a (fig. 40), be a large ball of hollow brass, and let the tube to the left hand of ft be also of brass ; also let these stand upon a glass support, so that any electricity which a has may not be able to get away. Now let B and c be two vessels having' their upper parts made of brass, only capable of being separated from one another at the middle part, where Fig. 40. you see the line in the figure ; and let both b and C stand upon glass supports, so that any electricity which either of them has, may not be able to get away. Let us begin by supposing that A has received f charge of positive electricity, and that in thf flie^ time B and c are unelectrified. Now push B d^d € ^ towards a. Since b and c are not electnfied* tlM»ir M|^ BODIES.] PHYSICS. Ill Mr electricities arc not sejyarated from each other, but mixed together; however, when you push them up to A, the positive electricity of a attracts the negative electricity of b to its side, and repels the positive away to the extreme right of c, as you see in the figure. If we now pull c away from b, and finally pull B away from a, we shall thus have got a quantity of negative electricity in b, and a quantity of positive in c, both separate from each other, while the elec- tricity in A will be the same as before. We have, in fact, made use of the electricity in a to separate part of the two electricities of b and c from each other, and a is still as ready as ever to help us again. Now this distant action or help, rendered by the electricity of a in separating that of b and c, is called electric induction. 80. The electric spark. — We may, however, per- form our experiment iu a somewhat different manner. Let us now bring b and c slowly towards a, and con- tinue to do so. When a and b are very near together, we shall have the positive electricity of a and the nega- tive electricity, which has been induced to appear oxi b, separated firom t ach other by only a small thickness of air until at last they will be so strong and the film of air so ^n, diat they will rush together and unite in the ^oifm of a ispark. The consequence will be that a will ha^ lost a portion of its positive electricity, and b will Hii^lost all its negative. If we now pull b and c away li^e will still be the positive charge at c, which has nCMt gone away ; in fact, while a has lost part of its posi- ItVie electricity, c will have gained just as much, so that die result is virtually the same as if part of the elec- Idd^ of a hsMl fone over to c. 112 SCIENCE PRIMERS, [%\XJ^mmm %mmM\ :aitimtiiiitm Fig. 41. 8i. Sundry experiments. — Whatire said about electrfc may be easily illustrated by a few simple and striking experiments; but it wmk be remembered that in all these experiments the glass of the apparatus must be quite dry and warm. Experiment 55.~-Here you see in the figure an instru- ment by which we can detect electricity, called the gold leaf electroscope. In or- der to show you its action, let me first of all communicate to the knob at the top (see Appendix) a slight charge of positive electricity. Now this charge runs to the gold leaves which are electrically connected with the knob, and then these leaves, being both charged with the same kind of electricify, begin to repel each other as you see in the figure. The electroscope is now in action. ^ ^ Experiment 56.— Having thus charged the elec^o- scope with positive electricky, let us bring near its knob an excited glass rod, when the gold leaves will diverge still more. The reason of this is that the positive elec- tricity of the excited glass decomposes the neutiid electricity of the knob attracting the negative to it«i|| and repelling the positive to the gold leaves. 3^ therefore, the leaves had been previously chargpl with jwsitive electridty, they will now ^ms^ widely. iieeleis tricity, s that the The res sealing* knob a negativ were p of this tricity collapj Exi or cot Let ui electri spark, with I farthe bally Tb cause the II of tli as p posit two ^ cons 4* eleci lotl and S BOt>{£S.] PHYSICS, 113 Experiment 57. — If we now bring near the knob of die electroscope, charged as before with positive elec- tricity, a stick of excited sealing-wax, we shall first find that the gold leaves will collapse instead of diverging. The reason is that the negative electricity of the excited sealing-wax decompcses the neutral electricity of the knob attracting the positive to itself, and driving the negative to the gold leaves. But since the gold leaves were previously charged with positive electricity, part of this charge will be cancelled by the negative elec- tricity driven towards them, and they will consequently collapse. Experiment 58. — Here we havfe a hollow brass ball or conductor, supported on an insulating glass stand. Let us now bring this insulated conductor near the electric machine when in action, and we shall get a spark, but it will be very feeble. Let us now touch with our finger that part of the hollow ball which is farthest from the m>xhine, and the spark given to the ball will now be much more intense. This illustrates what we said in Art So about the cause of the spark. In fact, the positive electricity of the machine pulls towards itself the negative electricity of the hollow ball, and drives away the positive as far as possible. If, however, this ball is insulated, the positive cannot be driven away sufficiently far, nor the two electricities be separated sufficiently well, and the consequence is you have but a feeble spark. But when you touch the hollow brass ball, the positive electnctty of the ball is driven through your body to the earth, the electricities are thus well separated, and there is a |^1 spark. Sa^ Actiim of points.— In the last experiment, HI. I 114 SC/ENCJE PRIMERS, Ielectrifieo BODIES.) if you cor'uiue to touch the brass ball, and the electric machine is worked it the same time, a succession of sparks will pass through your body to the earth, and these will cause yoii to feel a somewhat unpleasant sensation. The spark from the electric machine may in truth be compared to a flash of lightning — a flash of lightning being, in fact, a very long spark. Now, just as when a man is struck by lightning the elec- tricity passes through his body to the earth, so when we grasp or touch the ball of the last experiment, the electricity goes through our body to the earth. Experiment 59. — Now let us attach a point to the hollow ball, and place this point next the conductor of the machine, touching the ball as before with our flnger. It will now be impossible to get a spark from the machine, but there will be instead a continuous rush of electricity. In facit, anything pointed carries off the electricity just as rapidly as it i? produced, and does not give it time to gather so as to form a spark. We now see the use of the pointed metallic conductors that are placed above lofty buildings, to protect them from lightning strokes. These pointed metallic con- ductors, running down into the earth, carry off the electricity in a silent manner, just as the point did in Experiment 59 ; and ju$t a^ the point protected my finger from a spark in the one case, so does the light- ning conductor protect the building from a flash m stroke of lightning in the other. Franklin, an American philosopher, was the first to find out that lightning and electricity are the same thhif — the only difference being that a flash of lightning is often several miles in length, whereas an electric spiik is only a few inches. 83. : positioi niachin we ha^ electric lecting One the ek revolvi rcvo onf UfHU id \ cm leal of ^WMKO BODIES.] pnvsiai. »t5 to the :tor of 5nger. n the 5 rush es off , and park. ctors them con- ■ the id in I my gbt- i Of tto 83. Electrical Machine.— You are now in a position to understand the construction of an electric machine. Such a machine is composed of two parts ; we have first of all an arrangement for producing electricity, and we have next an arrangement for col- lecting it. One of the best known machines is that in which the electricity is produced by a large plate of glass revolving, as ivL fig. 42. As the plate of glass Fig. 4», revolves, it is rubbed against by two sets of rubbers, m% above and the other below. These rubbers are UjHl^Uy made of leather stuffed with horse-hair, so as ^ press rather tightly against the glass. They are coated with a soft metal, which is spread over the leatli^', and this metal is generally made of one part of m^ one of tiD» and two of mercury melted a tt6 SCIENCE PRIMERS, [electrifi^ BODIES.] together. There is a metallic chain which connects these rubbers with one another, and with the earth. Now when the glass plate is turned round, positive electricity is produced in the glass, while negative electricity is produced in the rubbers. The negative electricity of the rubbers then passes along the me- tallic chain which is connected with the rubbers, and is conducted by means of this chain to the earth, through which it spreads until it is scattered and diffused — in fact completely lost. We have thus got rid of the negative electricity, and there is now left the positive electricity on the glass. Now sur- rounding the glass we have two brass rods, which are united to a latge metallic surface called the con- ductor, which you see in the figure. This conductor stands upon ^ass supports, so that it is able to keep what electricity it gets. The two large rods near the glass plate are moreover armed with metallic points. Now you have already been told that points have a great tendency to draw off electricity. The consequence is that these points draw off, or collect, the positive electricity of the glass and carry it to the conduct<»r, where it remains, since the conductor stands upon glass supports. By turning the glass plate sufficiently long, we may thus accumulate a large amoual of posi- tive electricity in%is condiK:tor. Experiment 6o. — If, when Hie conductor of the elec^ madiine is charged with ^ectricity, I {ilace my i«nger near it, a spark passes between the c^- ductor and my finger. The reason is that the posilivt electricity of the conductor separates tc^ two nnects earth. )ositiVe Jgative igative le roe- 's, and earth, I and us got now V sur- :h are con- luctor keep arthe ointa avea lence sitive ICtOf, upon ently posi- ^e »Iace C0n- itive ilec- ear^ dirough my feet, but, on the other hand, attract- ing the negative to itself. The two electricities — ^namely the positive in the conductor, and the negative in my finger — then rush together through the air and unite with each other, and in so doing they form a spark. 84. Leyden jar. Experiment 61. — When you thus approach your finger or your knuckle to* an electric machine, you feel a pricking sensation when the spark passes, but that is all; you do not get a severe shock. In order to get a shock you must use a Leyden jar, such as you see in fig. 43. This is a glass jar, the inside of which is coated with tinfoil, as well as the outside up to the neck. A brass rod with a knob at the end is con- nected with ihe inside coat- ing, and is kept tight by being passed through a cork which covers the mouth of the jar. Thus the jar ha s two coatings, an inside andun ou'side one, and these are quite separated from each other, as far as electricity is cofii^med, inasmuch as glass does nol^onduct electri- ^Mb^ Now suppose I take the jar by its outside coating in )||yhand, and h«ld the inside knob to the conductor oC A fleets machine at work. The positive elec- ir^H irom the ccmductor will then get into the inside coa^g of the j«r. It will then decompose the two electrt|iities of the outside coating, driying away the ponlti^ through my hand and body generally to the 1 Fix- 43» irS SCIENCE PRIMERS, [BtiCTRiFiED earth, and attracting the negfttive. In fact thipg will be a battalion of positive electricity in the inside coatit^ facing an opposite battalion of negative electricity in the outside coating, the two longing very much to meet, but unable to do so for the glass. So intent are these two electricities on watching each other that they will remain close at their post while I put some more positive electricity into the interior. This second charge will then act precisely like the first ; it will decompose anew the two electricities of the outside coating, driving positive electricity from the outside through my hand to the earth, while negative elec- tricity will remain in the outside coating, facing the new battalion of positive electricity which has been introduced inside. We have now two inside and two outside battalions watching one another, and by con- tinuing this process we accumulate a large quantity of opposite electricities in the two coatings of such a jar. If we wish to discharge the jar, we make use of a discharging rod, such as you see in the figure. It should be . held by its glass handles, and one of the knobs should be made to touch the outer coating of the jar while the other is gradually brought near the knob connected with the interior of the jar \ wh«n the two knobs are near to- gether a bright i|)ark is seen, accompanied with a report, and the \2x 10 i^is- charged. If we wiitr to feel the shock oui let us grasp t'.e coating by one of our hands, and 4|pj^roacli tbe Fig. 44. nODIES. ..■""."■—"■— towafc the di Or if 1 hands, tiide c the in! the be —Fro vincec in it the 3: is ac< is ver last I a seel Now a jar electi whicl Ag quire the e hard see 1 widi work t wht] CTRIFIED MODIES.] PHYSICS, tI9 ft will be coadi^ ricity in luch to tent are ler that ut some second it will outside outside VQ elec- ing the IS been md two by con- ntity of h a jar. ise of a ire. It of the ting of ear the ith the ; when ear to- ark is with a ii#is- to towards the knob connected with the inside coating, the discharge will then take place through our body. Or if several wish to feel the shock, let them all joih hands, and let the one at the one end grasp the out- dde coating, while the ow^ at the other end touches the inside knob, and the shock will then pass through the bodies of all. {5. Energetic nature of electrified bodies. — from what has been said jom must now be con- vinced that electricity is something which has energy in it You see that the two opposite electricities of the jar rush together and unite, and that the union is accompanied by a flash and a report. This, flash is very bright while it lasts ; and although it does not last longer than the twenty-four thousandth part of a second, it nevertheless implies considerable heat. Now heat means energy, and we thus see that when a jar is discharged, that kind of energy which we call electricity is changed into that other form of energy which we call heat and light. Again, since electricity is an energetic thing, it re- quires labour or work to produce it; ^ou do so by turning the electric machine^ but such a machine is particularly hard to turn on account of the electricity. You thus see that there is nothing for nothing ; if you widi to obtain an energetic agent, you must spend wcark in doing so. On the other hand, there is no ^ifppearance of energy when the two electricities pipRbine, but only, a change from the form of' elec- iPppity- inta that of heat 86. Electric curi-ents.— You have seen that when you hold a pointed conductor tiear an electric madiitie at work (Art 82) there is a continuous stream ISO SCIENCE PRIMERS, [electrified BODIES. or current of electricity, which passes through die point and through your hand to the ground. We have, however, a much better means than the electric machine gives us of obtaining powerful Fig- 45. electric currents. We shall now briefly describe this method, which was first discovered by -n Italian called Volta, and which has therefore been named the Voltaic battery. This arrangement is shown in the above figure. Here you see to the extreme left a plate marked Cy which means a plate of copper. Next you see a plate of zinc marked z, which is soldered to a wire connecting it with the plate of copper in the second vessel In the second vessel you have another plate of zinc, which is similarly connected with the copper in the third vessel. Finally, to the extreme right you have a Single plate of zinc. Suppose now that the vessels are filled with a mixture of su^^uric acid and water, and that we attach wires to the copper at the left-hand end, and also to the zinc at the right- hand end, and that we bring these wires together. (These wires are called the pole-wires of the batteiy;) It will now be found that there is a current of positfye electricity passing round and round through the cii^i^ in the direction of the arrow-heads. Let us tmt^ how it wire a and g( it ent< then ] coppei next the m by the and fi the li origin 87. descri many of ob It currei but a curre batte batte Gro\ singl beinj eartl parti wel asy hav< this tal;( l-'5 ' 'i^^Hfi^fitm^.y*'''-^ " "". ',v- .■' BODIESj p//ys/cs. Ul how it goes. In the first place, it comes firom the wire attached to the extreme left-hand copper plate, and goes, as in the figure, through the long wires until it enters the extreme right-hand plate of zinc ; it then passes through the liquid till it reaches the copper plate, from which it passes along the wire to the next zinc plate ; it then passes through the liquid of the middle vessel to the copper plate, and from that by the wire to the zinc plate of the left-hand vessel ; and finally from the zinc plate of this vessel through the liquid to the same plate from which it started originally. ' 87. Grove's Battery. — ^The arrangement now described was that used by Volta, but since his time many improvements have been made in the method of obtaining a current of electricity. It was found that with Volta's arrangement the current, though strong at first, very soon became weak ; but a method has been devised by which the electric current can always be kept at the same strength. A battery by which this is done is called a constant battery, and one of the best is that invented by Grove (see fig. 48). In this battery, instead of a single vessel we use a double one, the outer vessel being made of glass, while the inner is made of porous earthenware. The outer glass or stoneware vessel is partly filled with diluted sulphuric acid. Within it we have a plate of zinc (amalgamated on the outside), as you see in the figure,, while within the glass vessel we have a porous vessel, made of unglazed porcelain. Into this porous vessel is poured strong nitric acid, and into thi$ nifric acid is put a thin plate of platinum, which takes flic place of the copper in Volta's arrangement. 12» SCIENCE PRIMERS, [electrified ^DIES. Now when this battery is in action, the zinc dissolves in the dilute sulphuric acid, and during this process hydrogen gas is given off, But this hydrogen does not rise up in the sh?ipe of bubbles, but finds its way into the porous vessel which contains the strong nitric acid. It there decomposes the nitric acid, taking some oxygen to itself, so as to become water (hydrogen and oxygen forming water), and thereby turning the nitric into nitrous acid, which shows its presence by strong orange- coloured fumes. Thus the hydrogen does not reach the platinum plate ; indeed it is to prevent its doing so that this arrangement was made, for it was found that in Volta*s original battery the hydrogen given out as the zinc dissolved adhered to the copper plate, in con- sequence of which the force of the battery became much weakened. 9 What we have now described is only a single vessel or cell, as it is called, of Grove's battery. In a large battery of this kind there may be 50 or loo cells — the wire that is attached to the platinum of one cell being connected with the zinc of another, in a man- ner precisely similar to that of fig. 45, the only differ- ence being that instead of copper we have platinum, and instead of a single vessel a double one of the nature now described. Also, the positive current passes through the liquid from the zinc to the plati- num plate, just as it passed through the liquid from the zinc to the copper plate in Volta's arrangement. 88. Properties of the current. — Let us now see what an electric current can do ; that is to say, let us perform a few simple experimqits. Experiment 62.—Make a Grove's battery tteodjr for action, and introduce a bit of very fine p!*«,tinti»j wire bl the c\ will come Ex] for a^ 'RIFJBD [solves jrocess |es not into acid ygen ygen into |ange- each pgso tliat it as con- -ame esse! arge Is^ cell lan- fer. im, the ?nt iti. w Y fODIES.] PHVS/CS. 123 wire between the two pole wires of the battery ; when the connection is made, and the current passes, it will be found that the fine platinum wire will be- come red-hot. Experiment 63. — Make a Grove*s battery ready for action, and insert its two pole wires into two Fig. 46 inverted vessels containing water, as in fig. 46. It will be found that the current decomposes the water, a*id that oxygen gas will appear in the one vesisei and hydrogen gas in the other. The oxygen gas will appear at the pole which is connected with the platinum plate, while the hydrogen will appear at tfteat which is connected with the zinc plate. Thus j^u see that a voltaic battery has the power of Iteiomposing water. It has also the power of de- ^^f^isiposing very many compound liquids. Experiment 64. — Here we have some copper wire covered with thread so as to insulate it, and this cc^per wire is wound round a thick piece of irou 124 SCIENCE PRIMERS, [ELECTRiFiEn BOWl shaped like a horse-shoe ; now let us connect the two proles of our battery with the two extremities of the copper wire which goes round the iron. If the battery be now in action, it will be found that the iron has acquired the power of attracting other iron towards it, so that a plate of iron will be held up, as in the figure, with a heavy weight attached to it As soon, however, as the connection between the horse- shoe and the battery is broken this power is lost, and the weight which the iron has been supporting will drop down* at once. Experiment 65. — -Take a bit of ^'^ '*^* hard steel, such as a knitting-needle, and attach it to the iron of the horse-shoe in the last experiment while the current is passing. This needle will have gained certain properties which (unlike the soft iron) it will not lose when the current is broken, but will retain ever afterwards. For instance, if we suspend the needle round the middle by means of a very fine silk threat, and let it swing horizontally, it will always point in one direction, and this direction will be nearly north and south. The needle will, in fact, have become a compass needle, always pointing in one direction, and thus enabling the mariner when out at sea to steer his vessel in the proper course. A piece of hard steel possessing these properties is called a magnet. Experiment 66.— rLet us now suspend a magnetic needle horizontally upon a pivot It will poiixl nearly north and south. But let us now bring near it n wjtf tltfot foul soutl righj II its 11 ' .u^aMWMIWe^'^'M*!^'-'^^''*™*'^''''**''^"''* n RiFiEn BODIES.] PHYSICS, !15 through which a current is passing, and it will be found that the needle will no longer point north and south, but it will place itself so as to lie across or at right angles to the wire conveying the current. If we now break the current, the needle will resume its usual direction. Experiment 67. — We may render the last experi- ment more marked by means of an arrangement such -T Fig 48. • as is sketched in the above figure. Let us aup^- / pose that we have our battery at one end of tW^ room, while two wires covered with thread are carri^ from the two poles of the battery quite to^ jjther end of the room, and are there joined together, so th^ the battery is now in action. Furthermore ypiirsee at the end most remote from the battery arslppended magnetic needle, which is placed ne^r the^ire,;«nd , which will be violently deflected wh^n th^ current passes. Now if anyone at the very opposite comer of the room should disconnect the wire fro^ one of the poles of the battery, that very moment the cur-' rent will cease to flow, and the magnetic needle will resume its ordinary position. 89. Ele^ctric Telegraph. — It thus appears that 4 -}'-<:■■ ■ jft" 126 JSCIEJ^CE PRIMERS, t^LECTkiFiEb BODll by disconnecting the wire from the battery at one end of the room the needle is made to move at the other at the very same moment This action would take place even if the wires connected with th6 poles were carried loo or even i,ooo miles away before they were joined together. If a magnetic needle were placed beside the wire conveying the current, even though the wire should be i,ooo miles from the battery, it would be deflected, but as soon as the other extremity of this wire i,ooo miles away was disjoined from the pole of the battery, the current would cease to pass, and the magnetic needle would return to its usual position. You thus see how it is possible, by making and breaking contact of a wire with the pole of a battery, to move a magnetic needle i,ooo miles away. In fact we have here the principle of the electric telegraph, which performs such wonders in the way of information, telling us what takes place in America a few seconds after it happens. I cannot, however, enter more fully into the subject, but at least you see that it is possible to agitate a magnetic needle i,ooo miles away, and, just as in the alphabet for the deaf and dumb, these signals may be made the means of conveying inf^nnation. 90. Cone usion. — You* have now learned what the electric current can do. How, in the first place, it can heat a fine wire through which it passes; how, secondly, it can decompose wat^ and other compounds; how, thirdly, it can make a piece of soft iron into a powerful though tempo- rary magnet; how, fourthly, it can make a pi|pet of hard steel into a permanent mai^et; and fiMiiy and| ren( dist .«*iKiirt.i >ne end le other W take 'S were ywere placed though tery, it remity )m the pas^ usual e, by with netic Jctric e way aerica ^ever, u see t^ooo deaf is of vhat i k ake BODI£S.] PHYSICS, 127 Illy and lastly, how it can deflect the compass needle, rendering 'it thereby possible to telegraph to great distances. We cannot enter more fully into this very interesting subject, but in conckision let me remind you that you have now learned something about the active moods of matter. We spoke first of all about moving bodies, then about vibrating bodies, then about heated bodies, and lastly about electrified bodies; and we have tried throughout to show you that the activity which a body may possess is never really lost. It may, no doubt, pass to some other body, or it may change its form, going from visible energy into sound, or into heat, or into electricity, or changing about in many different ways, b tit it is really lost no more than a particle of matter is lost. Indeed just as the science of Chemistry is built upon the principle that matter only changes form, going from one combination to another, and does not absolutely disappear, so the science of Physics is founded upon the principle that activity or energy only changes form, and never absolutely disappears. This, however, is a principle the full development of which must be reserved for a future stage. THINGS TO BE REMEMBERED. A POUND avoirdupois is equal to 7,000 grains. If a stone be dropped from the hand, it will fall through 16 feet during the first second of time. Steel is the strongest metal, but gold is the most malleable; for a cubic inch of gold can be beaten out so as to cover the floor of a room 50 feet long and 40 feet wide. The diamond is the hardest solid ; that is to say, it can scratch everything else, but nothing else can scratch it A cubic inch of water weighs nearly 252 grains; and, therefore, four cubic inches weigh nearly 1,000 grains. 100 cubic inches of air weigh 31 grains. 100 cubic inches of carbonic acid weigh 47 grains. 100 cubic inches of hydrogen only weigh 2 grains. The pressure of the atmosphere will support a column of mercury 30 inches high, and a column of water more than 30 feet high. Sound travels through air at a velocity of about 1,100 feet in one second of time. If a musical string vibrates 50 times in one secon^, it emits a deep, low note; if it vibrates 10,000 tkatB in one second, it emits a shrill, high note. Tlj heat quire heat Lil Tl twei # THINGS 10 BE REMEMBERED, 129 The heat required to melt a pound of ice would heat 79 pounds of water one degree. The heat re- quired to boil away a pound of boUing water would heat 537 pounds of water one degree. Light travels through space nearly at the rate of 190,000 miles in one second of time. The spark from a Leyden jar lasts only the twenty-four-thousandth part of one second. INSTRUCTIONS REGARDING APPARATUS. The apparatus to be used should be set up on the table before the lesson, and the teacher should make sure that he can perform without difficulty the various experiments. After the lesson the apparatus ought to be put away carefully into its appropriate place. Care must be taken that the piston of the air-pump is rendered tight in its cylinder by means of lard. Care must also be taken that the receiver fits well upon its bed-plate, and for this purpose it must be well greased with lard. , When this is done, the receiver ought to move smoothly and without noise on its bed-plate ; but if there is a grinding noise it shows that some hajd substance is present, and the bottom of the receiver must then be carefully cleaned and greased anew. This remark applies to the hemi- spheres (fig. 15), as well as to the glass receivers. In order to fill the box of Experiment 28 with Qarbonic acid gas, the tube conveying the gas should descend very nearly, but not quite, to the bottom of the box. To fill the same box (Experiment 29) with hydrogen, the tube conveying the gas should ascend very nearly to the bottom of the box, which is now uppermost The whole apparatus for Experiment 45 must be placed in a cool room some hours before the ex-^ periment is made. « Gi phoi phosl little! pap< imTkUCTIONS REGARD WG APPARA TVS. 131 n the make arious ght to pump lard. upon well ceiver )n its !h0W5 >ttom and leniK with 3uld 1 of :en, irly be ex- • « Great care ought to be taken in handling phos- phorus, which very easily takes fire. The stock of phosphorus should be kept under water, and the little bits cut off should be well dried in blotting- paper before being used. When the mercury is tarnished, take a piece of paper and make it into a funnel, having a pin-hole at its bottom. Pour the mercury gently into this funnel, and let it run through the pin-hole into a vessel prepared for it It will now be quite bright Care must be taken that the mercury is not con- taminated with other metals. A small portion should be kept separate for amalgamation in the battery. Before the electrical machine is used the glass plate ought to be well warmed. For this purpose it ought to be placed endwise towards the fire, and the handle ought to be turned round occasionally, so as to expose to the fire the various parts of the plate. If these instructions be not attended to, the glass may probably crack. The electroscope ought not to be charged highly, otherwise the gold leaves will be driven to the sides of the jar and be torn. To charge the electroscope give the Leyden jar a single small spark from the machine— then touch the electroscope with its knob. The insulating glass supports of the conductor ought also to be warm and dry. Finally, the Leyden jar and everything made of glass with which any electrical experiment is to be made, ought to be warm and dry. In the Grove's battery the zinc ought to be well amalgamated (see Chemical Primer), and the varic is K 2 n^ SCIENCE PRIMERS. metals ought to be quite bright at the points where they are connected with the battery. The outer cells ought to be charged with one part by measure of strong sulphuric acid and eight parts of water. The porous vessels of the Grove's battery ought to be well steeped in water after the battery has been in use ; and the zinc and platinum plates ought likewise to be well cleaned. In Experiment No. 66, it is necessary to fill with mercury the two little brass cups into which the ends of the battery wires are plunged. DESCRIPTION OF APPARATUS. Ko. ofEx- r^"r\/ periment. A '• »• I, 2.— Tin pan, with peas ........ o i o 3. — Iron plate with four strings o I 6 4. — ^Balance to carry 2 lbs. in each scale ; beam two feet long i 12 o Piece of metal weighing 200 grains ...010 Set of weights, 600 grains to | grain ... o 10 6 5.— 2 lbs. mercury in a bottle . . . . . • o 10 o Two pieces of glass two inches square . . 004 6. — Apparatus unnecessary. 9, 10. — Beam of w^ood 019 Two 4-lb. weights ...030 15.— Plumoline ...........010 Stoneware dish for mercury .......006 16.— Tube for showing level of water ....026 17.— Metal cylinder with two tubes and stoppers .060 Tube with moveable bottom and cord ..030 Water-jar for tube . ........010 Indigo solution .... * 005 18, 19.— Substance weighing 1,000 gmins, same spe- cific gravity as water ... ....026 20. — Hollow brass cylinder .......026 ^^m^ Bucket to contain it .......,•026 ^W Apparatus for attaching the bucket to balance 01 6 21. — See Experiment 18. 22.- Block to illustrate flotation O o 3 24. — ^Apparatus unnecessary. 134 SCIENCE PRIMERS. No. of Ex- p . periment. / V 25. — Tate's air-pump . , 3 13 6 Bell-jar receiver ..026 Two india-rubber balls . . , . . . . 003 26.— Jar with neck and flange ......026 Two pieces of india-rubber for it . . . . o 10 27, 28, 29. — Box with strings 006 30.— Magdeburg hemispheres 056 Brass cock for ditto ........030 31' — Tube for barometer ........010 Glass mortar for cistern .......010 Funnel for filling barometer . . . . . . 002 33- — Vibrating wire on support ...... o i o 37. — Model thermometer ...050 Centigrade thermometer 040 38.— Bladder two-thirds filled with air ...,006 ",9. — Further apparatus unnecessary. 40. —Use tin pan of Experiment I. 41. — Use flask of Experiment 42. 42.— Flask for boiling water, and cork in dupli- cate ...,,,, 030 Triangleand wire gauze to support flak . .015 •3» 44* — No special apparatus necessary. 45* — ^^^ to Jiold sulphuric acid in vacuo^ and shallow vessel to hold water 038 46. — No apparatus necessary. 47.— Use flask of Experiment 42. 48. — Wires to show unequal power of iron and copper to conduct heat ......010 50.— Use tin pan of Experiment i. 51.— Apparatus to show image of candle , . . o lo 6 52. — Apparatus unnecessary. 54. — Electric pendulum .....,.,.020 Several pieces of elder-pith ...... o o 6 • 55.— Electroscope o 12 o DESCRIPTION OF APPARA TUS, 135 No. of Ex- periment. Electrical machine, 16-inch plate . Box of amalgam 56.— Rod, half brass, half glass , . . Rod of glass covered with jed-wax Piece of silk Piece of flannel ...... 57.— No additional apparatus. 58^ 59.— Brass ball, with point, on insulated 60. — No apparatus required. 61. — Leyden jar, pint size .... Discharger 62.— Grove's battery, 4 cells in frame Yard of fine platinum wire . . 63. — Voltameter 64. — Electro-magnet 65.— Knitting-needle and thread , . 66 —Apparatus for Oersted's experiment 67.— Thirty feet of covered wire . . . stand rrice. £ s. d, o 4 o o o o o 4 I 2 2 o o o 6 6 6 6 .036 .040 .030 . I 18 o .006 . o 10 6 .060 .002 .056 .013 £19 3 8 \ X As a wish has been expnsst i to havs a cheaper if iesi compute set of apparatus^ the following is offered as an alternative list. DESCRIPTION OF APPARATUS. No. of Ex- Pitct. piiriiMnt /^ ». d I, 2. — ^Tin pan, with peas (supplied by experi- menter). ooo 3. — Iron plate with four strings ......016 4.»-13alance to carry 2 lbs. in each scale ; beam twa feet long. I 12 O Piece of nietal weighing 200 grains . . . o I O Set of weights, 600 grains to ) grain . . . o 10 6 5. — 2 lbs. mercuiy in a bottle 0x00 Two pieces of glass two inches square • .004 6. — Apparatus unnecessary. 9, 10. — Beam of wood (supplied by experimenter) .000 Two 4-lb. weights Ditto ....000 15. — Plumbline '..oio Stoneware dish for mercury 006 16. — Tube for showing level of water ....026 17. — Metal cylinder with two tubes and stoppers (not supplied) , 000 Tube with moveable bottom and cord ...030 Water-jar for tul/e 010 Indigo solution ..005 18, 19. — Substance weighing 1,000 graiiis, same spe- cific gravity as water ....... o 2 6 Carried forward > • 3 6 3 No. oi £x- pcriment. 20. — H B A 21. — S 22. — B 24. — A 25.—^ ] r 26.— J I 17, 28, 29 30.— 1 31-' 33.— 37.- 38.- 39- 40.— 41.- 42.- 43»44*- 45-- SCIENCE PRIMERS. 137 No. oi Ex- Price, pcriment {^ s. d. Brought over ..363 20. — Hollow brass cylinder * o 2 j Bucket to contain it 026 Apparatus for attaching the backet to balance o i 6 21. — See Experiment 18. 22. — Block to illustrate flotation 003 24. — Apparatus unnecessary. 25. — Air-pump .......••••.220 Bell-jar receiver •••026 Two india-rubber balls ..•••••003 26. — Jar with neck and flange 026 Two pieces of india-rubber for it • • . • o I o If, 28, 29. — Box with strings ........006 30. — Magdeburg hemispheres .......05^ Brass cock for ditto ....•••.030 31. — Tube for barometer 010 Glass mortar for cistern 010 Funnel for filling barometer ......002 33. — Vibrating wire on support o I c 37. — Model thermometer 050 Centigrade thermometer .......040 38. — Bladder two-thirds filled with air ....00 6 39. — Further apparatus unuecesrory. 4a — Use tin pan of Experiment i. 41.- Use flask of Experiment 42. 42. — Flask for boiling water, and cork in duph- cate 030 Triangle and wire gauz . to support flask . . c i 5 43,44. — No special apparatus necessary. 45. — This experiment cannot be shown with the air-pump of this list . 000 Carried forward ..774 1 38 DESCRIPTION OF APPARA TUS, No. ol Ex- Priee. (teriment £ g. d. Brought over ..774 46. — No apparatus necessary. 47. — Use flask of Experiment 42. 48. — Wires to show unequal power of iron and copper to conduct heat ...... o I o 50. — Use tin pan of Experiment i. 51. — Apparatus to show image of candle ... o 10 6 52. — Apparatus unnecessary. 54. — Electric pendulum 020 Several pieces of elder-pith ......006 55. — Electroscope *, .076 Electrical machine ..iibo Box of amalgam ...010 56. — Rod, half brass, half glass .......026 Rod of glass covered with red-wax ...026 Piece of silk ..* 006 Piece of flannel 006 57. — No additional apparatus. 58, 59. — Brass ball, with point, on insulated stand . o 3 I ) lo b > 2 > o > 7 10 » I ' 2 > 2 > O ' O o 6 6 o o 6 6 6 6 3 ^ 4 o 3 o 12 6 o 6 o o 6 o O 2 5 6 » o i^a^gp*"