DOUBLE SALTS 141 
chlorides is yellow at ordinary temperature, showing the charac- 
teristic colour of FeCl,; on raising the temperature to about 25°, 
the colour changes to red, indicating the presence in the solution 
of the double salt.’ 
These various investigations, taken in conjunction, justify 
the inference that the existence of double salts in solutions of 
definite concentration has been established. 
All said so far has been solely classificatory and has only 
brought us to the recognition of the distinctive properties of 
double salts. Quite separate from this is that side of the study 
of double salts which since 1887 has stimulated so much interest 
and has produced so great an output of work. Any historical 
review of the recent growth of physical chemistry, of the exten- 
sion of its scope and the deepening of its foundations would 
doubtless have to say something about the 1887 volume of the 
Zeitschrift fiir physikalische Chemie; and that not only, or not even 
chiefly, because this was the first volume of the journal which, 
initially a sign of the rapidly increasing activity in a special 
branch of chemistry, has become so instrumental in stimulating 
that activity, but because in it are contained papers which 
have already become classical as the starting-point and the 
foundation for new and most important theories. 
In that volume Van’t Hoff wrote on “ The Part Played by 
Osmotic Pressure in the Analogy between Solutions and Gases,” 
Arrhenius on “The Dissociation of Substances Dissolved in 
Water”; and in it appeared another contribution by Van’t Hoff 
entitled ‘The Transformation Temperature in Chemical Decom- 
position.” This last-named paper was the first in a long series 
of experimental and theoretical investigations, the results of 
which are summarised in the book published by Van’t Hoff in 
1897 under the title Zhe Formation and the Splitting of Double 
Salts. Starting from the observation that the formation of a 
double salt from its components and the reverse change occur 
at a definite temperature, above which temperature only the 
double salt and below which only the two constituent simple salts 
can exist, or vice versa, Van’t Hoff, in the pioneer paper referred 
to, gives those examples which now are found in every text-book 
of physical chemistry (Table II.). When the simple crystalline 
1 Hinrichsen and Sachsel, “Uber die Bildungs- und Léslichkeitsverhaltnisse 
der Doppelchloride des Eisens und der Alkalimetalle,” Zs. Dhyszk. Chem. 50, 
1905, p. 90. 
