480 M. R. Pictet on the Application of the Mechanical 



Indeed, let us suppose we have two volatile liquids possessing 

 the same density, the same tensions P and P', the same specific 

 heat c, but one of them having twice the cohesion of the other. 

 The internal work of disgregation of the one will be double 

 that of the other ; and consequently the latent heat X will be 

 twice as much as X', since latent heat is nothing else but that 

 which is absorbed in order to overcome the cohesion of the 

 liquid molecules. 



In this example the quantity T would be the same for both 

 liquids; but the second formula would give for A, — c (t f — t) 

 and \ r — c (t f — t) values which might vary from the single to 

 the double. 



Let us suppose, in the second place, that the two liquids 

 have X and A/ equal, c equal, but 8' = 2 8. The quantity, in 

 kilogrammetres, expressed by T and T' will be 



T = 



10333 (274 + *) *(£') 



1-2935x274 



1033 



T'= 



10333 (274 + ^(p) 



1-293x28x274 



replacing 8 f by 2 8 ; thus T = 2 T', while the heat withdrawn 

 from the refrigerant A will be constant in the two cases. 



It may likewise be that all the quantities X,, c, P, P', 8, &c. 

 vary from one liquid to the other, in such manner that the 

 equations have nothing general as a theoretic result, and that 

 they can only be applied for each liquid in particular by taking 

 the numerical coefficients from experimenters worthy of con- 

 fidence. 



Therefore, forming no hypothesis on the constitution of 

 liquids, but accepting the arbitrary and empiricism in this 

 matter, our two equations remain essentially independent. 



It is here that an hypothesis finds a place ; and we emphasize 

 the word, because it is purely and simply an hypothesis : we 

 suppose that the cohesion of liquids is constant for all ; we as- 

 sume that the mobility of the molecules which characterizes 

 the liquid state corresponds to an equal molecular or atomic 

 attraction — that X and X', referred to two molecules, are equal 

 and constant for all liquids without exception. This first 

 hypothesis furnishes a relation between equations I. and II. 



We will now make a second hypothesis, viz. that Carnot's 

 cycle applies to volatile liquids and to their changes of volume 

 when they are volatilized, and that it establishes the ratio 

 between the work expended and the heat absorbed. 



