565 
1909-10.] On the Precipitation of Soluble Chlorides. 
1. In no case was precipitation observed when the concentration of the 
hydrochloric acid added did not exceed 17 per cent. HC1. 
2. When the concentration was 19 per cent. HC1 or higher, precipitation 
was observed invariably. 
3. The mean of a number of observations gave 18'5 per cent. HC1. 
or 5‘5 mols. per litre as the lowest concentration of HC1 which caused 
precipitation. 
In order, however, to make sure that we were not dealing with something 
of the nature of a coincidence dependent on the temperature being in the 
neighbourhood of 18° C., the experiments were repeated with solutions of 
potassium chloride and sodium chloride saturated at 0° C. This lowering 
of the temperature involves a very considerable decrease in the solubility 
of the potassium chloride, and a comparatively slight decrease in the 
solubility of the sodium chloride. There seems, however, reason to suppose 
that the fall in temperature may not involve so great a relative change in 
the ionic concentrations of the saturated solutions of these two salts as one 
might infer from the great difference between their temperature coefficients 
of solubility, for the temperature coefficient of conductivity is greater for 
sodium chloride than for potassium chloride. Be this as it may, the 
hypothesis was found to be as applicable at 0° C. as at 18° C. Precipitation 
was only observed on adding acid solutions containing at least 18*0 per cent. 
HC1, and it was invariably well marked when the concentration was as 
high as 19 per cent. HC1. According to as yet unpublished determinations 
by one of us, the maximum specific conductivity at 0° C. occurs at a slightly 
higher concentration than at 18° C. This change is very slight, and does 
not affect the present argument. 
If a greater ionic concentration in the acid than in the saturated salt 
solution be really a necessary condition for precipitation, as the dissociation 
theory seems to demand, then the experiments described above clearly 
prove that the calculated ionic concentrations given in col. VII. of Table I. 
cannot be even approximately correct. 
The maximum acid, however, has probably a concentration of 
chlorine ions at least equal to that of the saturated salt solutions. 
Granting that this is so, it does not explain why the saturated salt 
solutions, which are all precipitated by hydrochloric acid of the same 
concentration, are in no case precipitated by a solution containing less 
hydrogen chloride. Moreover, at equal molecular concentration up to 4 
normal and at 18° C., the specific conductivities of potassium chloride and 
ammonium chloride are almost identical. At 18° C. the solution of 
potassium chloride is saturated at a concentration of 4 normal, while that 
