1902.] on the Ions of Electrolysis. 39 



weight. He says, in conclusion, " This formula claims exact 

 numerical validity. It gives for most substances a greater mole- 

 cular number than that usually assumed, i.e. a partial or complete 

 chemical decomposition of the substance in the solution. Even if 

 the consequences of this proposition should require an essential 

 modification of the generally prevailing views as to the constitution 

 of solutions, I do not know any fact which shows it to be untenable. 

 Indeed, many observations in other departments (the proportionally 

 strong affinities of dilute solutions, which remind one of the proper- 

 ties of the nascent state, the easy decomposability by the weakest 

 galvanic current, the phenomena of internal friction), are directly in 

 favour of the view that in all dilute solutions a more or less com- 

 plete decomposition of the molecules of the dissolved substance takes 

 place. Besides, this conception adapts itself well to the oi^inions 

 developed by L. Meyer, W. Ostwald and S. Arrhenius on the state of 

 the molecules of dissolved substances, as it only goes a step further 

 and fixes numerically the degree of the decomposition." 



An objection was taken to Planck's argument. It was said that 

 as his formula contains the ratio of the molecular numbers of the 

 solute and of the solvent, it could not be inferred that that of the 

 solute is greater than its formula leads to, for it might be that 

 the molecular number of the solvent is less than that indicated by 

 its formula. Planck's answer was immediate and obvious. In any 

 expression in which the molecular number of the solvent appears, 

 there also appears as a factor the molecular weight. For instance, 

 in the formula for the depression of the freezing-point the molecular 

 number of the solvent is multiplied by the latent heat of one mole- 

 cule of the solvent, and similarly in other cases. So that it makes 

 no difi'erence what molecular weight we assume for the solvent, and 

 the use of its molecular number is merely a convenient way of 

 expressing its quantity. 



This increase in the number of the molecules, or splitting into 

 ions, was called " electrolytic dissociation." It will be seen that it 

 is what Lodge in 1885, in speaking of Clausius's theory, called 

 dissociation. But while it has some obvious resemblances to the dis- 

 sociation of a gas, there are very striking differences between the cases, 

 and perhaps some of the difficulties in the way of the acceptance 

 of the theory may have arisen from the use of the same word for two 

 things differing so much. We need not, however, discuss the name, 

 but it is well to look for a little at the essentially different nature of 

 the things. This essential distinction consists in the products of 

 the electrolytical dissociation being charged, the one set with posi- 

 tive, the other set with negative, electricity, so that, while in the body 

 of the solution they can move about independently, they cannot be 

 separated by diffusion as the products of the dissociation of a gas can. 

 It is true that the quicker moving ions can, to a small extent, forerun 

 the slower moving ions, and diffuse a little further into pure water or 

 into a more dilute solution, as is shown by the fact that when two solu- 



