138 Proceedings of the Koyal Society of Edinburgh. [Sess. 
XII. — On the Electrical Conductivity of Aqueous Hydrochloric 
Acid, saturated with Sodium Chloride; and on a new form 
of Conductivity Cell. By F. D. Miles, B.Sc., A.R.C.S. Com- 
municated by Principal A. P. Laurie, D.Sc. 
(MS. received January 16, 1915. Read March 1, 1915.) 
The objects of this communication are to give the results of determinations 
of the electrical conductivity and composition of a range of aqueous 
solutions of hydrochloric acid saturated with sodium chloride at 18 ° C., 
and to describe a form of conductivity cell which has been found of great 
use for solutions which are saturated or contain a volatile solvent. 
The specific conductivity of the aqueous solution of any one of the 
strongly ionised mineral acids attains to and then decreases from a 
maximum, as the concentration is increased. From a study of the changes 
in electrical conductivity which accompany chemical changes in homo- 
geneous systems, the late Professor John Gibson was led to the general- 
isation that “ Homogeneous chemical systems which undergo change, either 
of themselves or under the influence of the electro-magnetic vibrations 
which we call Might,’ change so that their specific electrical conductivity 
is increased, unless when coerced in the opposite direction by stronger 
chemical affinities.” * 
In the communication referred to, several examples were noted of a 
ver}^ interesting type of system to which this general statement was found 
to apply. In these examples a relatively large mass of an aqueous solution 
of a mineral acid was the main constituent, and there was also present in 
each case a relatively small mass of some reagent which could undergo a 
chemical change either with the acid or with the water. It was found 
that the initiation and continuance of such a chemical change was de- 
termined by the concentration of the acid being greater or less than 
the concentration at which the conductivity of the solution was a 
maximum. An acid of lower concentration than this, tended, therefore, 
to become stronger, and vice versa, the system gaining in conductivity 
in each case. 
It is evident from these, and from other facts, that some change in the 
constitution of mineral acid solutions may reasonably be looked for, which, 
though not sharply coincident with the attainment of maximum conductivity, 
* Trans. Roy. Soc. Edin., xlviii, p. 130. 
