566 
Proceedings of the Royal Society of Edinburgh. [Sess. 
of ammonium chloride is not saturated until a concentration of 5*67 normal 
has been reached. Curves showing the variation of specific conductivity 
with concentration for these two isomorphous salts are nearly coincident 
up to a concentration of 4 normal, and beyond this the curve for ammonium 
chloride appears almost as a direct continuation of that for potassium 
chloride, the specific conductivity rising steadily (K 18 = 0*333) at 4 normal 
to (K 18 = = 0*420) at 5*67 normal. It is difficult to see how this can be 
brought into agreement with the suggestion of equal concentration of 
chlorine ions in these two saturated solutions. 
Unquestionably, the total ionic concentration is greater in the case of 
the saturated solution of ammonium chloride, and the only way that equal 
chlorine ion concentration can be imagined is by assuming that above 4 
normal a considerable complex formation takes place. On this assumption, 
however, the complex ions would have to have nearly the same mobility as 
simple chlorine ions, judging by the close similarity between the two curves. 
A similar precipitation of certain nitrates by nitric acid of greater 
concentration than that corresponding to maximum specific conductivity 
has been observed, but there are certain facts in connection therewith 
which cause us to leave this subject for a further communication. Sulphates 
behave quite differently, requiring a much higher concentration of acid be- 
fore salt separates out. This is doubtless* due to the formation of acid salts. 
Two questions arise out of our discussion : “ Can we predict from current 
theoretical considerations which chlorides should be precipitated by hydro- 
chloric acid of concentration higher than that of maximum conductivity ? ” 
and “ Why is it that the minimum concentration of acid causing precipita- 
tion is that of the acid with maximum specific conductivity ? ” 
To the latter question current theories give as yet no really satisfactory 
explanation, and we can only expect one when we have more experimental 
facts regarding the properties of this maximum acid and of saturated 
solutions. As regards the former question we may consider the matter as 
follows : — 
When a salt dissolves, it may (a) dissolve with a molecular weight 
higher than that given by the simplest formula for the salt; as we 
generally say, the salt molecules may associate in solution. There may be, 
however, no process of association at all. It is doubtful whether the phrase 
molecular weight of a solid has any definite meaning. The molecules in 
solution may be many times the size represented by their simplest mole- 
cular weight (but still dissociation of complex molecules into simpler ones 
may have taken place on solution, assuming a high molecular weight in 
the solid form). This dissociation would be different in different solvents. 
