100 
addition of hydrochloric acid, or sodium or potassium chloride to the satu- 
rated barium chloride solution. 
Similarly, potassium sulphate, though precipitated from its saturated 
solution by the addition of potassium chloride or sodium sulphate, gave 
no precipitate with sulphuric acid; and calcium sulphate was not thrown 
down by either sodium sulphate or ammonium sulphate but did separate 
slowly on the addition of potassium sulphate and more quickly with sul- 
phuric acid. 
Apparently, then, in these cases, the number of un-ionized molecules of 
‘the first is not increased by the addition of the second electrolyte, and the 
only plausible explanation seems to be that in these cases double salts are 
formed; e. g., lead-chloride-nitrate (PbCINO,), barium-chloride-nitrate 
(BaCINO,), hydrogen-potassium-sulphate (HIXSO,), calcium-sodium-sul- 
phate (CaNa.(SO,).?), and calcium-ammonium-sulphate (Ca(NH,).(SO,).?). 
Taking the case of lead chloride as an example, the addition of the 
lead nitrate must immediately increase the number of lead ions, but at 
the same time diminishes both this and the number of the chlorine ions by 
permitting the formation of lead-chloride-nitrate, so that the value of the 
product of the concentrations of lead and chlorine ions is not thereby 
increased, thus causing no rise in value of the concentration of the lead 
chloride, and, therefore, no separation of this substance as a precipitate. 
It does not follow that this peculiarity of behavior must accompany the 
formation of a double salt under similar circumstances, however, for it 
may be that the increase of the concentration of one kind of ion concerned 
may more than counterbalance the simultaneous decrease in this and that 
of the other kind of ion involved; this is simply a question of the value 
of the dissociation constants of the electrolytes present. 
It seems probable that double salts exist in solutions whenever there 
is a polyvalent acid in presence of two or more bases or a polyvalent base 
in presence of two or more acids, though other evidence of the existence 
of most of these compounds is at present lacking. Since, however, we 
know and recognize the existence of only those double salts which sepa- 
rate out from solutions of their constituents rather than these constituents 
in distinct crystals, the evidence so far accepted for the existence of such 
compounds is of a very limited kind, namely, only their solubility relative 
to that of their constituents. In other words, if these separate in prefer- 
ence to the double compound, the latter does not exist so far as this kind 
of evidence is concerned. 
