•with Elastic Fluids. 169 



both cases. In favour of the inference that its mode of acting 

 is similar, we have at least a strong argument from analogy. 

 It is proved that the heat which dilates air or a gas, spends 

 its whole force in producing this single effect, and is concealed 

 from the thermometer : — why should it not follow the same 

 law when it expands a mass of iron, or a portion of water or 

 mercury ? There seems to be no kind of difference between 

 the two cases, except that, in one, the experimental proof is 

 at hand, and, in the other, it is placed beyond our reach. 

 But in the continued application of heat to solid and fluid 

 bodies, there are two memorable stages at which we are en- 

 abled to contemplate the mode in which it operates, while the 

 temperature remains constant, and while it rises without the 

 afflux of extraneous heat. These occur in particular relations 

 between the expansive force of heat and the cohesion ; when 

 the former overcomes the latter, and when it is overcome by 

 it. The melting of a solid body, and the conversion of a fluid 

 into vapour, are instances of the power of heat overcoming 

 the cohesive force ; and, during all the time the changes are 

 going on, the temperature remains constant ; the whole sup- 

 ply of extraneous heat being absorbed and employed in ex- 

 panding the new fluid or vapour. The reverse processes of 

 a fluid passing into a solid, and of a vapour condensing into 

 a fluid, are instances of the power of heat being overcome by 

 the cohesive force; and here the extrication of heat before 

 concealed, causes a rise of the thermometer till the transfor- 

 mation of the bodies is completed. The first instances are 

 similar to the absoi-ption of heat which always accompanies 

 the enlargement of the volume of an elastic fluid ; the second 

 resemble the evolution of heat when the fluid contracts its 

 bulk. By the remarkable phsenomena we have mentioned, 

 which were first accurately examined and explained by Dr. 

 Black, the argument for the generality of the law relating to 

 latent heat is much strengthened. 



We employ the terms latent heat and sensible or free heat, 

 not in reference to any hypothesis concerning the nature of 

 that power, but to denote effects actually observed when it 

 acts upon matter. Latent heat is that which expands bodies, 

 which produces this single effect and no other, remaining con- 

 cealed from the thermometer. Heat of temperature, or fiee 

 heat, on the contrary, affects our senses, and is ready to dii- 

 fuse itself around whenever the equilibrium is broken. In 

 the two cases, if it be allowed that the facts are etjually ge- 

 neral, the phraseology must be alike unexceptionable. The 

 modes of speaking relate entirely to modes of acting. Heat 



iV«i) Seiies. Vol. 1. No. 3. March 1827. Z in 



