5.2 On the Inter changeability of Heat and Mechanical Action. 



But this conclusion is now generally ignored, or rather set 

 aside, in favour of a very different one, founded, as I am forced 

 to think, upon a misconception of the very important elementary 

 question in kinetics, How is a weight lifted up ? The quantity 

 of heat gained or lost, we are told, in the supposed case depends 

 upon the work expended upon its generation, or upon that done 

 by its destruction. In the first case, if the piston descends 

 through the space 6>, the work expended is JPdt? or P6>. In 

 the second case, if the gas drives the piston up before it, the 



work done is 1 pdv. In which it may be observed, en pas- 



sant, that the constancy of P affects the form of the result very 

 materially when it descends and causes condensation, but has no 

 influence upon it when it is raised. But the far more serious 

 objection to the doctrine is, that weight raised or resistance over- 

 come is precisely that kind of work done by a force in which no 

 conversion into heat or motion takes place at all ; and it excludes 

 the only case in which such conversion does take place, which, 

 as we have shown, is that in which force acting upon matter free 

 to move, itself passes into motion. From the second of the 

 above equations, f(p — p)dv = c, we see that so long as the forces 

 above and below the piston remain equal to each other no vis 

 viva is generated. The piston may rise or it may descend, but 

 the motion will not be due to either of the antagonistic forces, 

 whose function it is to reduce each other to nullity. The office 

 of the elasticity of gas in raising a superincumbent weight is 

 simply and exclusively that of giving it statical support at every 

 point of the rise. The force P— p generates acceleration, the 

 forces p—p make unaccelerated rise or fall possible: and it is 

 the singular infelicity of the modern doctrine that heat is created 

 by the expenditure of work, that the definition of the work ex- 

 pended does not include the case in which alone motion or heat 

 is created, and does include only the cases where no motion or 

 heat is created. I do not think I can more distinctly contradict 

 every part of the received doctrine on this subject than by 

 stating simply what appears to me to be unquestionable, as 

 truth — that no force employed in equilibrating resistances ever 

 becomes converted into heat, and that no heat is ever generated 

 except by forces acting on bodies verging on the state of motion, 

 and offering no resistance to the action of the forces. 



Milland, June 21, 1870. 



