HEAT. 73 



have more heat than at first, or a creation of heat out of noth- 

 ing in other words, perpetual motion. 



Let us now assume that this 20 supplied in the first in- 

 stance was yielded by a body at 90, of such size and material 

 that its total capacity for heat is equal to that of the mass of 

 confined air : this body would be reduced in temperature to 

 70, in other words, our furnace would have lost 20 of heat. 

 Let the cold body of the same size and material, used as a 

 condenser, be at 30. In the first experiment, the body at 

 30 would bring back the piston to its original point ; but in 

 the second experiment, or that where the weight has been 

 removed, the body at 30 would not suffice to restore the pis- 

 ton : to effect this, the cold body or condenser must be at a 

 lower temperature. 



The question in Carnot's theory, which is not experi- 

 mentally resolved, and which presents extreme experimental 

 difficulty, is the following: Granted that a piston with a 

 superimposed weight be raised by the thermic expansion of 

 confined gas or vapour below it ; if the elastic medium be 

 restored to its original temperature by cooling, the weight in 

 depressing the piston will restore that portion of the heat 

 which has been lost by the expansion, and by the mechanical 

 effect consequent thereon ; but if the weight be removed when 

 at its maximum of elevation, and the piston be brought back 

 to its starting point by a necessarily cooler body than could 

 restore it if the weight were not removed, would the return of 

 the piston now restore the heat which had been lost by the 

 dilatation, or, in other words, would pulling the piston down 

 by cold restore the heat equally with the pressing it down by 

 mechanical force ? The argument from the impossibility of 

 perpetual motion would say no, for if all the heat were 

 restored, the mechanical effect produced by the fall of the 

 weight, or the heating effect which might be made to 

 result from this mechanical power, would be got from 

 nothing. 



4 



