568 
Proceedings of the Koyal Society of Edinburgh. [Sess. 
This hydration equilibrium may be the first disturbed when concentrated 
hydrochloric acid is added to saturated solutions of those chlorides con- 
sidered in this paper, with the result that salt is precipitated. Considered 
in this way, there is qualitatively nothing in the modern theory of solution 
against the views which we have expressed earlier in the paper regarding 
this precipitation. The problem cannot be solved quantitatively with the 
aid of the law of mass action until we are better acquainted with the 
above hydration equilibria. 
In the more intricate cases where complex ions are formed we have in a 
saturated solution the equilibria — 
Solid salt undissociated salt, hydrated or not ions, hydrated or not 
complex ions, hydrated or not. 
The addition of concentrated hydrochloric acid to a saturated solution of 
a chloride which forms complex ions and crystallises with water of crystalli- 
sation would not necessarily precipitate the salt, owing to the variety of 
ways in which the equilibrium could readjust itself, or the tendency towards 
maximum specific conductivity be satisfied. It is thus not to be wondered 
at that maximum conductivity HC1 does not precipitate LiCl, MgCl 2 , CaCl 2 , 
SrCl 2 , ZnCl 2 , etc., but it is interesting to note that the chloride of the 
alkaline earths which comes nearest to being so precipitated is the one in 
which complex formation is certainly least marked, namely, barium chloride. 
We have endeavoured to explain according to current theory the 
facts of the precipitation of these chlorides by hydrochloric acid, and we 
find any explanation on the basis of the ionic theory very difficult, in so 
far as this theory involves the assumption of a concentration of chlorine 
ions in the saturated solutions of the chlorides in question equal to that in 
hydrochloric acid having maximum conductivity. So far as we can judge 
from the present data, this assumption does not seem tenable. On the other 
hand, the hypothesis of a tendency towards maximum specific conductivity 
has proved most useful, for by it the conditions necessary for these pre- 
cipitations are correlated and were predicted. 
Chemistry Department, 
Heriot-Watt College. 
{Issued separately December 23 , 1910 .) 
