1902.] on the lont of Electrolytta. 31 



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 happen in solution, say of hydro- 

 chloric 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 intelligible 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 num- 

 ber 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 interfere 

 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 surprised 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 

 very small or even nothing. 



A very important question was left only partially answered by 

 Faraday. It is, What substances are electrolytes? Faraday con- 

 sidered the water in dilute acid as the electrolyte, and the acid as a 

 substance having the power of increasing the conductivity of the 

 water. When a solution of sulphate of copper was electrolysed, he 

 supposed that the water was primarily decomposed and that the 

 metallic copper was a secondary product reduced by the nascent 

 hydrogen. He says,* " I have experimented on many bodies, with a 

 view to determine whether the results were primary or secondary. I 

 have been surprised to find how many of them, in ordinary cases, are 

 of the latter class, and how frequently water is the only body electro- 

 lysed in instances where other substances have been supposed to give 

 way." From our present point of view many of us would rather say 

 that the direct electrolysis of water very rarely occurs, except to a 

 very small extent. 



In 1839 Daniell began a series of ingeniously devised and skilfully 

 executed experiments with the view of determining, in the case of 

 salt solutions, whether it is the salt or the water which is primarily 

 electrolysed. The results appeared in two letters from Daniell to 

 Faraday in 1839 f and 1840,| and in a paper by Daniell and W. A. 

 Miller in 1844,§ all published in the Transactions of the Eoyal 



* Faraday, Experimental Researches in Electricity, par. 751 (1834). 



t Daniell, Phil. Trans. 1839, p. 97. 



X Op. cit. 1840, p. 209. 



§ Daniell and Miller, Phil. Trans. 1844, p. 1. 



