892 



SCIENCE. 



[N. S. Vol. XV. No. 



ical decomposition of the substance in the 

 solution. Even if the consequences of this 

 proposition ghould 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 properties of the nascent 

 state, the easy decomposability by the 

 weakest galvanic current, the phenomena 

 of internal friction), are directly in favor 

 of the view that in all dilute solutions a 

 more or less complete decomposition of the 

 molecules of the dissolved substances takes 

 place. Besides, this conception adapts 

 itself well to the opinions developed by L. 

 Meyer, W. Ostwald and S. Arrhenius on 

 the state of the molecules of dissolved sub- 

 stances, as it only goes a step further and 

 fixes numerically the degree of the decom- 

 position. ' ' 



An objection was taken to Planck's argu- 

 ment. It was said that as his formula con- 

 tains 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 ob- 

 vious. 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 multi- 

 plied by the latent heat of one molecule 

 of the solvent, and similarly in other cases. 

 So that it makes no difference what molec- 

 ular 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 obviotis resemblances 

 to the dissociation 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 dis- 

 tinction consists in the products of the 

 electrolytical dissociation being charged, 

 the one set with positive, 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 dissocia- 

 tion 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 Avater or into a 

 more dilute solution, as is shown by the 

 fact that when two solutions of the same 

 electrolyte of different concentration are 

 in contact there is a difference of electric 

 potential between them, but they cannot be 

 separated to any weighable extent in this 

 way. In order to separate from one an- 

 other two gases uniformly mixed, a certain 

 calculable amount of work has to be done, 

 so that after a gas has been dissociated and 

 wholly or partially converted into a mix- 

 ture of the two gaseous products, some work 

 has still to be done to get them separately. 

 So it is also in the case of electrolytic dis- 

 sociation ; but while in the former case the 

 decomposition work is the main thing, and 

 the separation work very small, in the lat- 

 ter it is quite the other > way. Here the 

 heat of dissociation, that is, the work spent 

 in decomposing the electrolyte into its 

 ions, is small (indeed sometimes negative), 

 while the work to be done to separate the 



