34 Professor A. Crum Brown [Jan. 31, 



being completely hydrolysed. He also showed that such double salts 

 as KCN,AgCN; 2KCl,PtCl4; and KCl,AuCl3, have potassium for 

 their only cation, the silver, the platinum and the gold forming part 

 of the anion. He also showed that 2KI,Cdl2 behaves as a single 

 salt with K as cation when the concentration is great, but as two 

 salts with cadmium as well as potassium as cation in dilute solution. 

 In these and in similar cases, Hittorf made a valuable contribution 

 to the theory of double salts. But perhaps the most striking generali- 

 sation is that contained in the words " electrolytes are salts," and 

 his very instructive comparison of the readiness with which a sub- 

 stance enters into double decomposition and the readiness with which 

 it can be electrolysed. With the fairness to his predecessors which 

 is characteristic of him, he quotes an almost forgotten statement of 

 Gay-Lussac to something like the same effect. 



Ladies and Gentlemen, — I wish here to tell you that within the 

 last three weeks, Professor Hittorf entered on the fifty-first year 

 of his professorship. The officials of the Royal Institution have 

 authorised me to ask our Chairman, Lord Kelvin, to send your con- 

 gratulations to Professor Hittorf on his jubilee. 



We now come to another turning point in the development of 

 the theory of electrolysis, inseparably associated with the name of 

 Kohlrausch.* It is to Kohlrausch and to those who worked with 

 him that we owe the methods for the accurate determination of the 

 conductivity of electrolytes. I need not give a description of the 

 apparatus. It is now used in every laboratory, and by means of it a 

 series of observations of the conductivity of an electrolyte can be 

 made at different concentrations in a very short time. An early result 

 of Kohlrausch's investigation was his discovery that " all acids which 

 have been examined in strong solutions show, for a definite propor- 

 tion of water, a maximum of conductivity," and he shows that for 

 many other electrolytes there is a solution which conducts better 

 than one either a little more or a little less concentrated. Thus the 

 maxima of conductivity of the following acids are at the following 

 percentages : HNO3 29 • 7 per cent. ; HCl 18 • 3 per cent. ; H2SO4 

 80-4 ])QV cent.; HC2H3O2 16 '6 per cent. "The maximal acetic 

 acid conducts at least 38,000 times better than concentrated acetic 

 acid." In connection with this he says, " we do not know one single 

 liquid, which at ordinary temperature is, by itself, a good electro- 

 lytic conductor." He refers the trace of conductivity in H2SO4 to 

 the dissociation into water and SO3 observed by Marignac and by 

 Pfaundler, and observes that as up to the present time we know only 



* Kohlrausch and Nippoldt, Pogg. cxxxviii. p. 280 (1869); Kohlrausch, 

 Pogg. Jubelband, p. 290 (1874); Kohlrausch and Grotrian, Pogg. cliv. pp. 

 1 and 215 (1875); Grotrian, clvii. p. 130 (1876); Kohlrausch, clix. p. 233 

 (1876). Gottinger Nachrichten, 1876, p. 213. Wied. vi. p. 1 (1879); xi. p. 653 

 (1880). 



