Royal Institution. 317 



Mr. J. Thomson saw that this supposed impossibility would not 

 result if the freezing-point of water were lowered by pressure, which 

 was experimentally proved to be the case by his brother. 



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, however its ex- 

 pansive force varied. Clement Desormes was also supposed to have 

 experimentally verified this law. If this were so, vapour raising a pis- 

 ton 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 per- 

 petual motion. Southern supposed that the latent heat was con- 

 stant, 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' experiments was, the 

 idea involved in the term latent heat ; by which, supposing the phee- 

 nomenon of the disappearance of sensible heat to be due to the ab- 

 sorption of a material substance, that substance, 'caloric,' was thought 

 to be restored when the vapour was condensed 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 

 assumjjtion which he did not make, and to which allusion will pre- 

 sently 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-con- 

 ducting for heat; part of this temperature will be due to the press- 

 ure exerted, since compression produces heat in air, while dilatation 

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

 the weiglit attached, will rise, and the temperature in consequence 

 of the exi)an.-ion 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 by exi)ansion ; 



