68 
Proceedings of the Koyal Society of Edinburgh. [Sess. 
the properties of hydrochloric acid solution of maximum specific con- 
ductivity, found that acid of approximately this concentration would just 
fail to cause precipitation of certain chlorides from their saturated solutions ; 
that is to say, that what we have called the “ critical ” strength of acid, A, is 
approximately the same for all the salts studied by them. 
In the light of equation (4), we should expect to find that the salts 
examined by these authors should have roughly equal solubilities (B). And 
such proves to be the case with three of the four salts, whose values are : 
sodium chloride, 5*4N ; ammonium chloride, 5*7N ; rubidium chloride, 5 - 9N. 
Since the critical acidity, A, is stated to be between 5 0N — HC1 and 
5*7 N — HC1, it will be seen that these salts behave in exactly the same 
way as do many other chlorides in having volumes of k 0 nearly = 1. The 
fourth chloride, that of potassium, evidently has a slightly higher value 
for k 0 than is usual. 
The conclusions which would appear to be drawn by Gibson and Denison 
are that hydrochloric acid has a tendency towards assuming the condition 
of maximum conductivity ; that this fact can explain why the chlorides of 
sodium, potassium, ammonium, and rubidium are precipitated from their 
saturated solutions only by acid of strength greater than this “ maximal ” 
strength ( Proc ., loc. cit., p. 563; Denison, Trans . Faraday Soc., vol. viii., 
part i. (1912), p. 27) ; and that the chlorides of lithium, magnesium, calcium, 
strontium, and barium do not behave in the same way, possibly because of 
hydration or of complex-formation (Proc., loc. cit., p. 568). 
From what has been said, however, it is clear that the last-named group 
of chlorides in no way differs from the first-named when judged by the 
proper standard, namely, the course - of the solubility-acidity curves ; and 
that for the first-named group the approximation of the critical acidities to 
that of maximal acid is an accidental coincidence which depends upon the 
solubilities of the salts concerned. 
University College, 
London. 
(. Issued separately March 24 , 1913 .) 
