274 



THE POPULAR EDUCATOR. 



the procpss. In order to determine the source of this heat, he 

 constructed a metal cylinder weighing about a hundredweight, 

 which was caused to rotate against a blunt steel borer. After the 

 lapse of half an hour the temperature of the cylinder was found 

 to have increased from 60 to 130, while the particles of 

 metal worn off only weighed 837 grains. It was evident, then, 

 that the heat which had elevated the temperature of the heavy 

 cylinder 70 could not have been evolved by a change of 

 capacity for heat in this small quantity of metal. 



In another experiment the cylinder was immersed in a vessel 

 containing about two and a-half gallons of water, and made to 

 rotate against the blunt borer as before, and in the course of two 

 hours and a half the water was caused to boil by the heat thus 

 evolved. The supply of heat thus obtainable appeared, indeed, 

 to be inexhaustible. The power which drove the cylinder was 

 in this case converted into heat, just as when the brake is 

 applied to a train the wheels are seen to smoke and give off 

 sparks, owing to the motion of the train being converted into 

 heat, and thereby destroyed. We see now the reason why 

 grease is applied to the axles of wheels and to pieces of 

 machinery ; if it be absent the friction is increased, and then 

 a portion of the power is wasted by being converted into heat, 

 instead of being employed to do the work of the machine. 



In these experiments the quantity of heat produced has been 

 so great as at once to be observed : very often, however, the 

 amount is so small that it cannot be well shown, even by an 

 ordinary. thermometer. The mode, therefore, usually adopted in 

 rendering its presence manifest is to use a thermo-electric pile. 



If a bar of bismuth and one of antimony be joined end to 

 end, and the point of junction heated, a faint current will pass 

 between the ends of the bar, and will deflect the needle of a 

 galvanometer. When several such compound bars are employed, 

 much greater sensitiveness is obtained. The bars are bent in 

 the middle, so that the alternate junctions may be at one side 

 of the pile, as seen in Fig. 2. The ends are then connected, by 

 the binding screws seen on the top in Fig. 1, with a delicate 

 galvanometer, and we have thus a means of rendering visible 

 the faintest amount of heat. So delicate, indeed, may this 

 instrument be made that the warmth of the hand, when held 

 at a distance of several feet, will visibly deflect the needle. An 

 instrument of this kind is of great service in all researches on 

 heat ; it is, in fact, almost indispensable, and hence frequent 

 reference will be made to it in these lessons. 



The next sources of heat which we must refer to are per- 

 cussion and compression. An illustration of the production of 

 heat by the former has already been given in the experiment of 

 letting fall a leaden ball. A piece of soft iron, too, may be 

 rendered red-hot by a few skilful blows on an anvil ; and a blow 

 or two with a hammer on an ordinary nail will at once raise its 

 temperature sufficiently to affect the thermo-electric pile, and 

 often to ignite a lucifer. 



The best means of exhibiting the effects of compression is by 

 the compression syringe represented in Fig. 3. A piece of stout 

 glass or metal tube closed at one end, and having an internal 

 diameter of about half an inch and a uniform bore, has a piston 

 fitted tightly to it : in the tinder side of this piston is a small 

 cavity in which a small fragment of tinder is placed. The tube 

 is now placed against a wall or some support, and the piston 

 quickly forced into it. So much heat is produced by the sudden 

 compression of the air that the tinder is ignited, and when 

 hastily drawn out will be found red-hot and smouldering. 



When a jet of hydrogen gas is allowed to strike upon very 

 finely divided platinum it sometimes renders it red-hot, and 

 thus the gas becomes ignited. This may partly be attributed 

 to condensation of the gas in the pores of the platinum, and 

 partly also to chemical action. 



This property of spongy platinum is sometimes turned to 

 account in Dobereiner's lamp, which is shown in Fig: 4. It 

 consists of two glass vessels, A and B, the neck of A reaching 

 to the bottom of B. A piece of zinc (z) is placed in the lower 

 one, and diluted sulphuric acid is poured over it ; the upper one 

 is then fixed tightly in its place, the neck being ground so that 

 it may fit air-tight. The action of the acid on the zinc gives off 

 hydrogen, which drives the liquid into the upper vessel, and 

 thus leaves the zinc dry. As soon as the tap in the tube H is 

 turned the gas escapes, and coming into contact with the 

 spongy platinum contained in D, is ignited. The acid then 

 passes again into B, and a fresh supply of gas is generated. 



The next and perhaps the most important source of heat is 

 chemical action. Nearly all chemical combinations are attended 

 with the production of a greater or less degree of heat. If wo 

 take some sulphuric acid, and pour it into a vessel containing 

 water, the heat thus evolved will at once be seen. When the 

 act of combination goes on very rapidly, light is often produced 

 as well as heat, and the term combustion is then commonly 

 applied to the change. In reality, however, it is as much com- 

 bustion -R-hen a piece of iron slowly rusts in the air as when 

 iron wire is burnt in oxygen gas ; and further, the same amount 

 of heat is evolved during the whole process, whether the com- 

 bination take a shorter or a longer time. 



If a quantity of tan or other dry vegetable matter be con- 

 sumed in a fire, a large amount of heat is given off, but it is 

 very soon over ; if, on the other hand, this matter be allowed to 

 decay so as to form a hot-bed, a much less degree of heat will 

 be produced, but it will continue for a much longer period, and 

 on the whole the same total quantity will be produced as in the 

 former case. 



In calculations of this kind it is necessary to have some unit 

 to express the quantity of heat produced, for the thermometer 

 merely reveals to us its intensity. The unit, accordingly, which 

 has been selected as being simple and convenient is the quantity 

 of heat required to raise 1 pound of water 1 in the Centigrade 

 scale, that is, 1| Fahrenheit. This amount, however, is 

 not absolutely uniform in all parts of the scale, and the interval 

 between Cent, and 1 Cent, is therefore usually fixed upon. As 

 a result of many experiments, tables have been compiled which 

 show the number of these thermic units which would bo pro- 

 duced by the combustion of 1 pound by weight of different 

 substances. The following are a few of these numbers : 



Hydrogen . . 34,000 

 Oil of turpentine 10,080 

 Olive oil . . 9,860 



Coke . . 7,000 Alcohol . 7,180 

 Coal . . 8,000 Phosphorus . 5,900 

 Tallow . 8,000 Wood (dry) . 4,000 



Though these numbers represent the amount of heat actually 

 produced by combustion, it is but rarely that we can obtain and 

 use anything like this amount, a large portion being always 

 wasted. In the steam-engine, for instance, the work accom- 

 plished by any amount of fuel is seldom more than \ of the 

 theoretical amount, and often falls far short even of this. A 

 large amount is given off with the smoke in the chimney, and 

 much is lost by being communicated to the machinery and 

 given off by radiation. 



In an ordinary fireplace, too, only a small fraction of the heat 

 generated is serviceable in warming the room, the greater 

 portion ascending the chimney, and being occupied in producing 

 the upward draught. On this account many other modes of 

 warming are more economical. The open fire, however, remains, 

 and probably will remain, the most popular on account of its 

 pleasant and comfortable appearance. 



In most cases the substance consumed combines with the 

 oxygen of the air. Heat, however, is produced by other com- 

 binations, as, for example, by that of hydrogen with chlorine. 



If a little sulphuric acid be dropped upon a mixture of pow- 

 dered sugar and chlorate of potash, the chemical action will be 

 so intense that sufficient heat will be generated to inflame the 

 mixture : this mode of producing heat is sometimes employed. 

 The ordinary lucifer match is tipped at the end with a compound 

 which is decomposed at a very low temperature. The friction of 

 the match against the box is sufficient to raise it to this degree, 

 and then the compound inflames and ignites the wood. 



Vital action is another source of heat, the temperature ot 

 the human body being above that of the surrounding air. This 

 may, however, be regarded as a result of combustion, for a 

 portion of the food taken into the system is really consumed, 

 that is, its carbon unites with the oxygen of the air, and by 

 this slow combination heat is produced which maintains tha 

 temperature of the body. 



The only other source of heat which we shall refer to now is 

 electricity. We have seen already how in the thermo-electric 

 pile heat is converted into electricity, and we shall find more 

 fully in our Lessons on Electricity how it may in turn be con- 

 verted into heat. A simple illustration of this is seen if a piece 

 of thin platinum wire be taken and made to form part of the 

 circuit round which a powerful electric current is passing; the 

 wire will very shortly become white-hot, and even be fused. 

 It is in this way that cannon and torpedoes are fired by the 

 agency of an electric current. 



