DOUBLE SALTS iiss. 
of as a waste product, whilst the carnallite that had separated 
from it is again worked up in the same manner as the original 
mineral. 
Van’t Hoff, in his Chicago lectures on ‘‘ Physical Chemistry 
in the Service of the Sciences,” has pointed out that application 
of the physico-chemical method to the carnallite problem reveals 
all the possible methods of splitting it up, methods of which 
the above process is only one amongst several. He proceeded 
to suggest another in which, instead of effecting the separation 
of the KCl by use of the solvent water and concentration of 
the solution obtained, temperature changes are resorted to in 
order to produce transformation, melting, and solidification. 
The process advocated (which has been patented by Van’t Hoff’s 
late colleague, Meyerhoffer) is as follows: 
The carnallite is heated in closed vessels to 168°, the 
transition point, when three-fourths of the KCl separates 
out and is removed by filtration, the water split off retaining 
in solution all the MgCi, and one-fourth of the KCl. The 
liquid is then cooled to 115°, when carnallite crystallises out, 
leaving a liquid which is an almost pure melted hydrate of MgCl. 
The solid carnallite is separated by filtration and again worked 
up, whilst the liquid, which at ordinary temperatures becomes 
solid, constitutes the waste product. 
The essential difference between the two methods—the 
empirical and that based on the application of physico-chemical 
methods—is that in the latter the MgCl, separated from the 
KCl is obtained, not as a mother liquor, but in the more 
convenient form of a solid. 
The number of illustrative cases of the application of physico- 
chemical methods to the special problems encountered in the 
formation and decomposition of double salts could, of course, 
be greatly increased; but even the slightest respect for pro- 
portion makes it necessary to pass on to another and altogether 
different aspect of this subject. 
The physico-chemical method, as developed on the basis of 
the phase rule, considers the existence of double salts in its 
dependence on temperature, pressure, and concentration, that 
is, on factors which it is possible to vary arbitrarily; and in 
this it differs essentially from the chemical method, whose 
object it must be to find an answer to the question: What 
relations must exist between two salts in order that—apart 
