563 
1909-10.] On the Precipitation of Soluble Chlorides. 
The precipitation of certain soluble chlorides from aqueous solution on 
addition of concentrated hydrochloric acid is of special interest from this 
point of view. Not all soluble chlorides can be precipitated in this way; 
thus the extremely soluble chlorides of lithium and caesium are not pre- 
cipitated by hydrochloric acid. The chlorides of the other alkali metals 
may be precipitated by hydrochloric acid under certain conditions. 
Let it be assumed as an hypothesis that in the precipitations to be 
considered a tendency towards maximum specific conductivity is the deter- 
mining factor : — Then a solution of hydrochloric acid should tend to abstract 
water when its concentration is greater than that corresponding to maximum 
conductivity, because its conductivity is increased by dilution. Conse- 
quently, we are led to expect such solutions to precipitate sodium chloride 
from its saturated aqueous solution, for in a saturated solution of a salt, 
which crystallises without water of crystallisation, we may regard the whole 
of the water in the solution as taken up in dissolving the salt. On the 
other hand, according to the same hypothesis, whenever the concentration 
of the hydrochloric acid falls below that corresponding to maximum con- 
ductivity, the tendency to abstract water should cease, because further 
dilution diminishes the specific conductivity. We, therefore, should not 
expect such dilute solutions of hydrochloric acid to precipitate sodium 
chloride from its saturated solution ; on the contrary, we should regard such 
solutions as containing some water available for the solution of an additional 
quantity of solid salt. 
Looking at the matter from the present standpoint of the dissociation 
theory, the precipitation might be ascribed to a decrease in the dissociation 
of the salt, and to the consequent increase in the concentration of the un- 
dissociated molecules. We should, from this point of view, expect precipita- 
tion to occur only when the concentration of the chlorine ions in the solution 
of hydrochloric acid is greater than the concentration of the chlorine ions 
in the saturated solution to which it is added. 
Unfortunately, the applicability of this principle to concentrated 
solutions cannot be tested in a satisfactory manner, because the dissocia- 
tion theory in its present form does not enable us to calculate the ionic 
concentrations of concentrated solutions with sufficient accuracy, more 
especially in the case of strong electrolytes. 
For the purposes of a very rough comparison, the laws established for 
dilute solutions may be assumed to be applicable to highly concentrated 
solutions §uch as those we are considering. 
Table I., column VII., gives the ionic concentrations calculated on the 
assumption that even for saturated solutions of the chlorides of ammonium, 
