56 



THE POPULAR EDUCATOR. 



tc ja r.tcfjt auS, benn tie Suft tft fctjr fctynetbcnb, unb \<3) furcate, bap @te 

 ftd> tie -&dnbe erfrieren roerben. 12. @o lange tcr SB tub im Dften ifl, 

 luirb e fait unb ttocfen fcleiben. 13. >c8 lana.cn abcr entlicty mute, 

 mactyte tc$ Srieben mit mcincn grcunben. 



EXERCISE 172 (Vol. III., page 326). 



1. A patriot would rather die than become a traitor. 2. The first 

 Christians preferred suffering the severest persecutions to forsaking 

 their belief. 3. One does not suffer such a thing to be told him twice. 

 4. I have not seen one of my brothers for three years. 5. A friend of 

 mine was drowned some years ago in the Danube near Vienna. 6. To 

 travel is good, if one has money ; and to live agreeable, if one has no 

 cares. 7. It is better to live in a free country than in a despotic one. 

 8. It is pleasant to travel in the society of lively friends. 9. In pros- 

 perity man but too easily forgets what he is. 10. Many distinguished 

 and noble men have been forgotten. 11. It should not satisfy a man to 

 know what is right, but he ought also to endeavour to do right. 12. 

 It affords me satisfaction to know that you are all still well. 13. How 

 little is often sufficient to make a man happy ! 14. He handed him. 

 the paper after he had read it himself. 15. This was sufficient to 

 satisfy him. 16. The cook prepares the food. 17. He has produced 

 this little confusion on purpose. 18. The cook tasted the soup before 

 she served it up. 19. We must try if we cannot help him yet. 20. 

 Just taste this wine (to see) if it is sweet enough. 21. He told me to 

 remember him to you. 



HEAT. IV. 



CONVERSION OP HEAT INTO FORCE SPECIFIC HEAT MODES 

 OF ASCERTAINING TRANSMISSION OF HEAT CONDUCTION. 



ILLUSTRATIONS of the conversion of heat into motive power, as 

 described in our last lesson, are frequently met with. One of 

 the best of these is afforded by the steam-engine. If we enter 

 any large factory where steam-power is employed, we find 

 different machines at work. In one place, it may be, heavy 

 weights are being raised or moved ; in another, large pieces of 

 metal are being turned or cut into shape, or other operations 

 being carried on with apparent ease by the aid of machinery. 

 For all this a considerable amount of force is evidently required, 

 and the question arises, Whence does all this force come ? The 

 machines, we know, cannot create it ; it is evident, therefore, 

 that the source of it must be sought for in the heat produced 

 by the combustion of the fuel in the furnace. 



If the supply of fuel be diminished, and consequently a 

 Bmaller quantity of heat be produced, less work will be accom- 

 plished ; and if we could in any way ascertain exactly the 

 amount of heat carried away by the hot air up the chimney, and 

 that lost by radiation and conduction, and dissipated in other 

 ways, we should find that there was still a portion of that pro- 

 duced by the combustion of the fuel left unaccounted for ; this 

 balance would be exactly equivalent to the amount of work that 

 had been performed. Allowance must, of course, be made in 

 this calculation for the force required to impart motion to 

 the machinery itself. 



A portion of the force thus produced is often re-converted 

 into heat. If we stand by a drilling-machine, or lathe, in which 

 a piece of iron is being shaped, we shall find that the turnings 

 or borings are frequently too hot to be touched with any degree 

 of comfort, although the mass of metal and the tool were both 

 quite cold. The motion of the machinery is here converted into 

 one of the particles of the iron, which manifests itself in the form 

 of heat. In this way we learn that heat, like matter, cannot be 

 destroyed, but only converted into other modes of motion. 



In our first lesson we selected as our thermal unit the quan- 

 tity of heat requisite to raise a pound of water 1 in the Centi- 

 grade scale. Now we should at first suppose that the same 

 amount of heat would raise the temperature of a pound of any 

 other substance to the same extent. Experiment, however, the 

 philosopher's grand resort, soon shows us that this is not the 

 case. 



Let us provide three sources of heat of equal intensity or, 

 better still, an oil or water bath, capable of holding three large 

 beaker glasses. Equal weights of water, oil of turpentine, and 

 sulphuric acid should now be put in these, and a thermometer 

 should likewise be placed in each beaker. Now apply a power- 

 ful source of heat, such as a Bunsen's gas-burner, and watch 

 the thermometers. The heat applied to each vessel is, of course, 

 the same, but the thermometer in the sulphuric acid will soon 

 be seen to be rising more rapidly than the others, that in the 

 turpentine comes next, while that in the water is lowest of all. 



If we now further observe the time taken by each to attain any 

 given temperature, as, for instance, 200, we shall learn that 

 the water takes nearly three times as long as the acid, and 

 more than twice as long as the turpentine. 



Now in each minute each must receive the same quantity of 

 heat ; it is clear, then, that different amounts of heat are 

 required to raise the same weights of different substances to the 

 same temperature. This fact, which is a very important one, is 

 usually accounted for by saying that different bodies have 

 different capacities for heat, or, as it is more commonly ex- 

 pressed, different specific heats. 



Another experiment, which the student may easily repeat, will 

 render this much more clear. Take a number of balls composed 

 of various substances, such as lead, copper, iron, tin, bismuth, 

 and glass (Fig. 18). Immerse them all for a short time in hot 

 oil of a known temperature, or in some other way bring them all 

 to one temperature, and then place them a little distance apart on 

 a sheet of wax about half an inch thick. The balls will melt the 

 wax at very different rates. If their temperature was high at 

 first, the glass will soon melt through the wax, and fall ; the iron 

 and copper likewise sink rapidly, and in a short time they too will 

 pass through it, the iron being a little in advance of the copper. 

 The tin ball comes next, and may just be able to be seen under- 

 neath, while the lead and bismuth sink but a little way, and 

 there remain : though they had the same temperature as the 

 rest, the amount of heat they possessed was only sufficient to 

 melt a very small portion of the wax. 



This experiment suggests to us a mode of ascertaining the 

 specific heat of different bodies, which is frequently adopted. 

 It consists in ascertaining the amount of ice which a given 

 weight of the substance is able to melt after being raised to a 

 high temperature. We know that when water becomes melted, 

 142 of heat become latent. The thermal unit is, however, 

 always reckoned in the Centigrade scale, instead of in Fahren- 

 heit's, and 142 Fahr. (of heat) is about equal to 79 Cent. 

 We may say, then, that 79 thermal units are required to melt a 

 pound of ice. The substance to be tested is therefore carefully 

 weighed, and raised to a high temperature, which is ascertained 

 and noted. It is then placed in a dry cavity in a lump of ice, 

 covered over by a slab of the same material, and left until it is 

 reduced to the freezing point. The moisture is then carefully 

 absorbed from it and from the cavity by a previously weighed 

 cloth, and thus the exact amount melted is at once shown. 

 From this the specific heat may be calculated, and in this way a 

 table can be drawn up, showing the specific heats of different 

 substances. 



Water is always taken as the standard, and the specific heat 

 of other bodies compared with that of an equal weight of this 

 substance. This is partly done as a matter of convenience ; it 

 is found, however, that the specific heat of water is greater than 

 that of any other substance. This fact is an important one in 

 the welfare of the globe. The sea, as is well known, always 

 tends to preserve a uniform temperature, so that islands do not 

 suffer from the same extremes of heat or cold as continents do. 

 The reason is that, on account of its great specific heat, a large 

 amount of heat is requisite to produce even a small variation in 

 the temperature of any mass of water, and hence it is very slow 

 in manifesting these changes. In this way the sea serves as a 

 great equaliser of temperature, absorbing a great deal of heat 

 when the temperature is high, and giving it out again as it 

 falls. 



As it is often difficult to procure a lump of ice large enough to 

 use in the mode described above, the apparatus represented in, 

 Fig. 19 was devised and used by Lavoisier and Laplace in their 

 investigations on specific heat. It consists of three concentric 

 metal vessels fitted with covers, as may be seen more clearly 

 by the sectional view (Fig. 20). The substance, M, to be tested is 

 weighed, and its temperature ascertained ; it is then placed in 

 the inner vessel, the spaces between that and the next, and also- 

 between the middle and outer vessels, being filled with pounded 

 ice. The outer layer prevents any heat from without reaching: 

 the middle vessel, and the water produced from this issues by 

 the tap E. A separate tap, D, carries off the water melted by 

 the heat of M ; this is received in a glass, and measured, and 

 shows the amount of heat given off by the substance in cooling. 

 The main drawback to this apparatus arises from the fact that 

 some of the water remains among the interstices of the ice, and 

 therefore the amount received in the glass is somewhat less than 



