30 ProfeBsor A. Orum Brown [Jan. 31, 



the molecules an exchange of ions should take place here and there, 

 and perhaps comparatively rarely." 



In this connection we may look at the views expressed by 

 Williamson in his paper on the theory of etherification.* He 

 says, " we are thus forced to admit that in an aggregate of 

 molecules of any compound there is an exchange constantly going on 

 between the elements which are contained in it. For instance, a drop 

 of hydrochloric acid being supposed to be made up of a great number 

 of molecules of the composition CIH, the proposition at which we 

 have just arrived would lead us to believe that each atom of hydrogen 

 does not remain quietly in juxtaposition with the atom of chlorine 

 with which it first united, but, on the contrary, is constantly changing 

 places with other atoms of hydrogen, or what is the same thing, 

 changing chlorine." Williamson founded this opinion on the ob- 

 served facts of double decomposition. He made no application of 

 this view to the case of electrolysis, and indeed does not explicitly 

 mention the temporary detachment of the atoms during the process 

 of exchange ; this is wholly due to Clausius, who arrived at his views 

 as to the exchanges going on in a solution in a way quite different 

 from that followed by Williamson, and quite independently. It was 

 not then known how closely double decomposition and electrolysis 

 are connected. We may perhaps get a clearer idea of Clausius's 

 theory by imagining the phenomenon to take place on a scale such 

 that we could see the individual ions. Let us then imagine a large 

 field with a large number of men in it, each mounted on a horse. 

 We shall further suppose that all the men are exactly alike and that 

 all the horses are exactly alike. They are moving at random, most 

 of them at about the same rate but a few of them faster, a very few of 

 them considerably faster, a few of them slower, a very few of them 

 considerably slower, than the average. They move in straight lines 

 until they meet an obstacle which makes them deviate. This obstacle 

 will often be another man and horse. The collision will give both a 

 shake, and will sometimes dismount one or both of the riders. 

 When this happens each will look for a horse, and as all the horses 

 are exactly alike the horse such a dismounted man finds and mounts 

 will not always be the one he came down from. But in any case 

 there will be always in the field some men without horses and some 

 horses without men. And the quicker the average pace the larger 

 will be the proportion of dismounted men and riderless horses to the 

 total number of men and horses. And this not only because there 

 will be more and, as a rule, more violent collisions, but also because 

 the dismounted men will have more difficulty in catching horses, 

 although to keep up the analogy of the ions we must suppose the 

 horses to be as anxious to be caught as the men are to catch them. 

 If it does not make my allegory too grotesque we might suppose places 

 with attractions for men and for horses respectively, to correspond 



* Williamson, Ohem. Soc. Jonrn. iv. p. Ill (1852). 



