Prof. H. E. Armstrong. Electrolytic 



[Mar. 25, 



IV. " Electrolytic Conduction in relation to Molecular Composi- 

 tion, Valency and the nature of Chemical Change: being 

 an Attempt to apply a Theory of ' Residual Affinity.' " 

 By Henry E. Armstrong, Ph.D., F.R.S., Professor of 

 Chemistry, City and Guilds of London Central Institution. 

 Received March 11, 1886. 



In ray recent address to the Chemical Section of the British 

 Association at Aberdeen, I have specially called attention to the 

 " affinity " of negative elements — chlorine, oxygen, snlphur, &c. — for 

 negative elements ; and I have sought to show that the formation of 

 so-called molecular compounds is largely, if not entirely, an outcome 

 of this peculiarity of negative elements. I have also ventured to 

 suggest " that in electrolysing solutions, the friction arising from the 

 attraction of the ions for each other is perhaps diminished, not by 

 the mere mechanical interposition of the neutral molecules of the 

 solvent — in the manner suggested by F. Kohlrausch — but by the 

 actual attraction exercised by these molecules upon the negative ion 

 in virtue of the affinities of the negative radicles." In this passage 

 I but vaguely hinted at a modification of the curreut theory of 

 electrolysis which had occurred to me ; as further consideration of 

 the question, especially of Ostwald's electrochemical studies, has 

 strengthened my views, I am led to think that it may be justifiable 

 to submit them for discussion. 



It is usual to divide bodies into three classes according to the mode 

 in which they are acted on by an electromotive force : metals forming 

 one class, electrolytes a second, and dielectrics a third. In making 

 this division, perhaps the fact is not sufficiently borne in mind that' 

 some compounds — silver chloride, for example — are per se electrolytes, 

 while others — such as hydrogen chloride and water — are individually 

 dielectrics, but behave as electrolytes when conjoined. On this 

 account, it appears to me desirable to distinguish between — 



(a) Metals. 



(b) Simple electrolytes — compounds, like silver chloride, which in 

 the pure state are electrolytes. 



(c) Pse^cfo-dielectrics — compounds like water, hydrogen chloride 

 and sulphuric acid, which behave as dielectrics when pure, but as 

 electrolytes when mixed with other members of their own class. 

 Conducting mixtures of members of this class may conveniently be 

 termed composite electrolytes. 



(d) Dielectrics. 



