154 Mr. W, E. Grove, on Inferences from [Jan. 25, 



In the effects of dilatation and contraction by heat and cold, when 

 applied to produce mechanical effects, and consequently in the 

 theory of the steam-engine, this subject possesses a greater practical 

 interest. Watt supposed, that a given weight of water required 

 the same quantity of what is termed total heat (that is, the sensible 

 added to the latent heat) to keep it in the state of vapour, whatever 

 was the pressure to which it was subjected, and consequently, how- 

 ever its expansive force varied. Clement Desormes was also sup- 

 posed to have experimentally verified this law. If this were so, vapour 

 raising a piston with a weight attached would produce mechanical 

 power ; and yet the same heat existing as at first, there would be no 

 expenditure of the initial force ; and if we suppose that the heat in 

 the condenser was the real representative of the original heat, we 

 should get perpetual motion. Southern supposed that the latent 

 heat was constant, and that the heat of vapour under pressure 

 increased as the sensible heat. M. Despretz, in 1832, made some 

 experiments which led him to the conclusion that the increase was 

 not in the same ratio as the sensible heat, but that yet there was an 

 increase ; a result confirmed and verified with great accuracy by 

 M. Regnault, in some recent and elaborate researches. What seems 

 to have occasioned the error in Watt and Clement Desormes' ex- 

 periments was, the idea involved in the term latent heat ; by which 

 supposing the phenomenon of the disappearance of sensible heat to 

 be due to the absorption of a material substance, that substance, 

 * caloric,' was thought to be restored when the vapour was con- 

 densed by water, even though the water was not subjected to 

 pressure ; but to estimate the total heat of vapour under pressure the 

 vapour should be condensed while subjected to the same pressure 

 as that under which it is generated, as was done in M. Despretz 

 and M. Regnault's experiments. 



Carnot's theory, that the mechanical force is produced by the 

 transfer of heat, and that there is no ultimate cost or expenditure of 

 heat in producing it, was founded in part on similar considerations ; 

 it is true that mechanical motion may be produced by the transfer 

 of heat from a higher to a lower temperature, without ultimate 

 loss, or, strictly speaking, with an infinitely small loss, but not, as 

 he seemed to think, an available mechanical force, except upon an 

 assumption which he did not make, and to which allusion will 

 presently be made. Thus, let a weight be supposed to rest on a 

 piston confining air of a certain temperature, say 50°, in a vessel 

 non-conducting for heat ; part of this temperature will be due to the 

 pressure exerted, since compression produces heat in air, while 

 dilatation produces cold. If the air be now heated, say to 70°, the 

 piston, with the weight attached, will rise, and the temperature in 

 consequence of the expansion of the air will cool somewhat, say to 

 69% (the heat of friction of the piston may be taken to compensate 

 the power lost by friction).: if now a cold body be made to abstract 

 20^^, the piston descending will, by its pressure, restore the 1° lost 



