SCIENCE. 



[N. S. Vol. XV. No. 388. 



PtCl^ and KCljAuClj 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 con- 

 centration 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 generalization is that con- 

 tained in the words 'electrolytes are salts,' 

 and his very instructive comparison of the 

 readiness with which a substance enters 

 into double decomposition and the readi- 

 ness with which it can be electrolyzed. 

 With the fairness to his predecessors which 

 is characteristic of him, he quotes an almost 

 forgotten statement of Gay-Lussac to some- 

 thing 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 authorized me 

 to ask our Chairman, Lord Kelvin, to send 

 your congratulations to Professor Hittorf 

 on his jubilee. 



We now come to another turning-point 

 in the development of the theory of electrol- 

 ysis, inseparably associated with the name 

 of Kohlrauseh.* 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 



* 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); XL, p. 653 (1880). 



time. An early result of Kohlrausch 's in- 

 vestigation was his discovery that 'all 

 acids which have been examined in strong 

 solutions show, for a definite proportion 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 con- 

 ductivity of the following acids are at the 

 following percentages: HNO3, 29.7 per 

 cent. ; HCl, 18.3 per cent. ; H^SO^, 30.4 per 

 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 electrolytic conductor.' He 

 refers the trace of conductivity in H^SO^ 

 to the dissociation into water and SO3 ob- 

 served by Marignac and by Pf aundler, and 

 observes that, as up to the present time we 

 know only mixtures which at ordinary tem- 

 perature conduct well, the supposition is 

 not unnatural that it is mixture that makes 

 electrolytes good conductors. And again, 

 if what has been said is correct, we must, 

 in order to have good conduction, protect 

 the wandering constituents from frequent 

 meeting with one another, and this service 

 is performed by the solvent, which makes it 

 possible for the ions to get over a part of 

 their journey— and so much larger a part 

 the more solvent there is— without reform- 

 ing molecules. It is this suggestion which 

 I ventured a little while ago to put into 

 an allegorical form. 



In order to compare the conductivity of 

 one electrolyte with that of another, it is 

 necessary that we choose comparable 

 quantities of the two, and there is no diffi- 

 culty in seeing that such comparable 

 quantities are those decomposed by the 

 same current of electricity — that is to say, 

 the electro-chemical equivalents of the 



