June 6, 1902.] 



SCIENCE. 



885 



change ; this is wholly due to Clausius, who 

 arrived at his views as to the exchanges 

 going on in a solution in a way quite dif- 

 ferent from that followed by Williamson, 

 and quite independently. It was not then 

 kaown how closely double decomposition 

 and electrolysis are connected. We may per- 

 haps get a clearer idea of Clausius 's theory 

 by imagining the phenomenon to take place 

 on a scale such that we could see the indi- 

 vidual 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 de- 

 viate. 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 horses 

 are exactly alike, the horse such a dismount- 

 ed 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 with- 

 out men. And the quicker the average pace 

 the larger will be the proportion of dis- 

 mounted 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 dis- 

 mounted 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 to 

 the electrodes, so that a man looking for a 

 horse would on the whole rather go- in the 

 direction of lunch than away from it, and 

 if he got near the refreshment room before 

 he found a horse, he would look in there. 

 An objection was made to Clausius 's theory 

 that the same thing which he supposed to 

 happenin solution, say of hydrochloric acid, 

 ought also to happen in the gas. We are 

 not yet in a position to discuss this point 

 with much prospect of obtaining a perfectly 

 satisfactory explanation of the difficulty, 

 although some progress towards an intel- 

 ligible theory has been made, but at the 

 risk of being tedious I may indicate that 

 my allegory may show us that we need 

 not despair of finding in due time an 

 answer. Let us suppose that in the field 

 there are not only men and horses, but also 

 a large number of other moving objects, let 

 us say, by way of example, cows. It seems 

 plain that whether the presence of the cows 

 would increase the chance of a man being 

 dismounted or not, it would sensibly inter- 

 fere with his chance of catching a horse 

 if he were. And it will be admitted that 

 the nature and size of these other moving 

 objects must exercise an influence on the 

 proportion of horseless men and riderless 

 horses to the total number. But these other 

 moving objects represent the molecules of 

 the solvent, so that we need not be sur- 

 prised when we find that the electrolytic 

 conductivity is affected by the nature of the 

 solvent and 'that where there is no solvent 

 the conductivity is veiy small or even noth- 

 ing. 



A very important question was left only 

 partially answered by Faraday. It is, 

 "VVTiat substances are electrolytes? Fara- 

 day considered the water in dilute acid as 

 the electrolyte, and the acid as a substance 

 having the power of increasing the con- 

 ductivity of the water. When a solution 

 of sulphate of copper was electrolyzed, he 



