244 Mr. J. J. Thomson on the 



bine to form PC1 5 , or the reverse action when PC1 5 is decom- 

 posed into these two gases. 



Let us suppose that the molecules of a gas A can combine 

 with the molecules of another gas, B, without either of them 

 being decomposed, and that one molecule of A combines with 

 one of B, to form one of a compound 0. Let m, n, p be the 

 number of molecules at any time of A, B, C respectively, let 

 us suppose the gases placed in a closed vessel, and let M and 

 N be the total number of molecules of A and B respectively 

 in the vessel ; then 



m+p=M, and n+p=l$i. 



Let us suppose that the molecules of A and B remain 

 together in the compound C for a time t, and let a molecule 

 of A remain apart from a molecule of B for the time r/n ; then, 

 just as in paragraph (1), we have 



dp _mn _p 

 dt~~ 7 T 



dm p mn 



Hi t 



dn_p _ mn 

 ~di~T 7~ 



Now when things have got into a state of equilibrium 



dp dm dn 

 dP ~dP dt 



all vanish, and we have 



mn p 



7 -? 



or 



(M.-p)(N-p)=^p (6) 



This equation gives the quantity of the compound C formed 

 when the elements A and B are mixed in any proportion. To 

 consider the simplest case, first let M = N, so that equal quan- 

 tities of the gases A and B are mixed together, or, what 

 amounts to the same thing, we may suppose that we have 

 initially a quantity of the compound C in a vessel, and then 

 by some means or another it gets dissociated. In this case, 

 putting M = N, we have 



