208 rEerortT—1890. 
point of view here indicated. The table is, in fact, merely a summary ot 
results as quoted by Wiedemann (‘ Elec.’ vol. 2), and represents the ions 
as indicated by the older experiments in electrolysis. The subject has 
not been specifically dealt with recently, but the modern work bearing 
on it will be brought under review in the section on the migration of 
ions. 
It must further be remembered ! that in the case of acids (where one 
of the ions is hydrogen) it is not possible by quantitative analysis to draw 
a distinction between the resultant effect of the motion of the positive ion 
and the deposition on the electrode. The whole result of the electrolysis, 
as far as the cathode vessel is concerned, is to develop a certain amount 
of hydrogen, and possibly increase or diminish the amount of free acid. 
Hence the distinction between primary and secondary development of the 
hydrogen fails. 
Some light might be thrown on the problem of the identification of 
the ions by the consideration of the heat-equivalents of the chemical 
action at the electrodes which should, if thoroughly understood, furnish 
evidence of distinction between the primary results of electrolysis and the 
secondary effects at the electrodes. I have already alluded to one case, 
namely, that of the representation of the heat-equivalent of the dilu- 
tion of a solution as an electromotive force, being possibly evidence of 
the complexity of the ions; but taking the evidence that I have been able 
to consult and arrange, it does not appear that the thermodynamic theory 
of electromotive force is sufficiently far advanced for it to be used with 
confidence as a means of determining the ions in electrolysis. 
We pass on now to the consideration of the ions in mixed solutions. In 
this case the substances set free at the electrodes are more liable to be 
due to secondary actions than in the case of a solution of a single salt, so 
that for some time it was supposed that the ions depended on the current 
density. An accouut of the earlier observations on this subject is given 
in Wiedemann, ‘Elec.’ 2, p. 593, from which it appears that at all current 
densities the current is divided between the two dissolved salts, but the 
ions due to one of them react upon the solution, and thus is explained the 
actual appearance of only one set of ions. 
Of recent work we may refer to 8. P. Thompson’s paper on the Hlec- 
tro-Deposition of Alloys (‘ Proc. Roy. Soc.’ 1887, p. 387), and to a paper 
by Arrhenius on Isohydric Solutions (Wied. ‘ Ann.’ vol. 30, p. 51, 1887, 
and ‘ British Association Rep.’ 1886, p. 315). 
By this latter paper we may infer (from the fact that the conductivities 
of certain mixtures are the sum of what would be the conductivity of 
each if the other were removed) that the presence of the one salt in solu- 
tion does not affect the partial conductivity of another salt in the same 
solvent, provided that the concentrations are of certain values, and hence 
that the two salts are resolved into ions independently. Salt solutions 
which are of such concentration that, when mixed, the conductivities may 
be regarded as the algebraic sum of the conductivities of each salt 
separately, are called by Arrhenius isohydric solutions. And the general 
law is established that solutions which are isohydric with the same solu- 
tion are isohydric with each other, and thus a table of isohydric solutions 
formed. Bender, in two papers, Wied. ‘ Ann.’ 22, p. 179, 1884, and 
Wied. ‘ Ann.’ 31, p. 872, 1887, publishes the results of a number of 
1 Hittorf, Wied. Ann. 4, p. 410, 1878. 
