412 Mr. "W. M. Hicks on some Effects of Dissociation 



measured by comparison with the temperature of mercury or 

 some other substance ; and all we can deduce is that when the 

 temperature increases a certain degree, the pressures of all the 

 permanent gases increase in very nearly the same ratio with 

 one another ; but we have no proof that what we consider 

 equal increments of temperature corresponds to exactly equal 

 increments of mean kinetic energy. This is generally ac- 

 knowledged and the temperatures measured by the " air- 

 thermometer " are supposed to be correct, whilst the varia- 

 tions from the law Y — Kpt (when t is measured by the mercury- 

 thermometer) are set down to the account of the unequal ex- 

 pansibility of mercury at different temperatures. But even 

 on this supposition all that experiment tells us is that, in the 

 formula P = /cpt, k is the same for all permanent gases, and t is 

 measured from nearly the same zero-point. For instance, 

 suppose that f is the same for all gases at the same tempera- 

 ture ; then, (1 + £)0 being the same at the same temperature 

 for all gases, we should find in our experiments that k, is the 

 same and t is measured from the same zero. But if we 

 could by some means measure 9 correctly and then apply our 

 corrections to the temperatures as given by the mercury- 

 thermometer, we should find that the coefficient of expansion 

 of mercury increases still more with the real temperature than 

 is apparent from our comparisons of air and mercury, suppo- 

 sing, as is done, that equal increments of the pressure of air 

 correspond to equal increments of temperature, and not, as on 

 the theory of dissociation, to equal increments of the quantity 



Since it is highly probable that the relations between s, s lf s 2 

 are the same for all gases, the only condition that (1 + %)0 



may be the same is that 6 or — may be the same, which 



would mean that, if two molecules are to break up, the relative 

 velocity of the two impinging directly must be such — not that 

 the minimum blow is the same in all gases, but that the vis 

 viva must be the same, or that the force of cohesion is propor- 

 tional to the square root of the mass. As I see no likely hy- 

 pothesis on which this may be the case, I think it is not pro- 

 bable that (l + £)0 is rigorously independent of the kind of 

 gas. But that the theory may agree with experiment, it is 

 not necessary that (l + £)0 should be rigorously the same for 

 all gases ; in fact, experiment tells us it is not so, although the 

 variation is very small. 



16. If for all the permanent gases 6 be small compared 

 with our ordinary temperatures, then our ordinary tempera- 

 tures are some multiples of # , and the range within which 



