DOUBLE SALTS 141 



chlorides is yellow at ordinary temperature, showing the charac- 

 teristic colour of FeCl 3 ; on raising the temperature to about 25 , 

 the colour changes to red, indicating the presence in the solution 

 of the double salt. 1 



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 

 1 897 under the title The 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 Ldslichkeitsverhaltnisse 

 der Doppelchloride des Eisens und der Alkalimetalle," Zs. physik. Chem. 50, 

 1905, p. 90. 



