34 Drs. Ramsay and Young on 



vapour-density calculated by the thermodynamic equation 



L _dp t 



^p5 



s 1 — s 2 dt J 



impossible values are obtained, unless it is supposed that 

 chemical dissociation takes place, which is absurd. 



4. We are told that the equation numbered (6) (p. 371 of 

 the ' Proceedings '), t = ai]r(p), is identical with our four 

 " laws; " and that " if (6) is true, then it follows that the ratio 

 of the absolute temperatures of two vapours to one another at 

 any pressure is the same as at any other pressure, or 6 = fct (7). 

 In fact, then, we can test laws I., II., III., and IV. by simply 

 testing (7) ; and as we find from Regnault and Rankine's 

 formula that (7) is untrue, there is nothing further to be said 

 about the four laws in question." We hope that no one has, 

 like Professors Ayrton and Perry, so misunderstood our 

 papers as to suppose that we imagine that " the ratio of the 

 temperatures of two vapours at any pressure is always the 

 same as at any other pressure." We do, indeed, notice that this 

 statement holds with very close approximation for like bodies, 

 such as chloro- and bromo-benzene.; but the whole of the second 

 and third parts of our memoir are devoted to an attempt, 

 which we deem not unsuccessful, to give a method whereby 

 the deviation from the above statement can be estimated. 



The didactic remarks so kindly directed to us at the begin- 

 ning of Messrs. Ayrton and Perry's critique, lead us to suppose 

 that the great importance of such problems in Chemistry is 

 not so generally known as we had imagined ; and we would 

 cordially agree that the cooperative principle may here with 

 advantage be applied, and physicist and chemist mutually 

 assist each other. 



The problem of dissociation has for long invited the atten- 

 tion of chemists. Many compounds decompose at a mode- 

 rately high temperature, and their constituents reunite when 

 the temperature is lowered. It is argued that, were it pos- 

 sible to attain a sufficiently high temperature, all the simpler 

 compounds at least would exhibit the phenomena of disso- 

 ciation. 



From the nature of gases, it is to be expected that a study 

 of the behaviour of dissociating gases would be the simplest 

 way of arriving at a knowledge of the subject. An obvious 

 criterion of the amount of dissociation is given by the vapour- 

 density of the partially dissociated body ; for if the dissociating 

 gas, and the simpler gases resulting from its dissociation, obey 

 Boyle's and Gay-Lussac's laws, then it is easy to calculate 



