144 SCIENCE PROGRESS 



corresponds to the melting point. At the outset of his dealings 

 with double salts, Van't Hoff had drawn attention to the fact 

 that co-existence of the three phases double salt, one simple 

 component, and solution, can occur only within a certain range 

 of temperature, and that there is only one temperature (the 

 transition point) at which there can be equilibrium between 

 the solution and the three solid phases double salt and two 

 simple salts. 



Soon after, Roozeboom, 1 applying the principle and the 

 terminology of the phase rule, recognised the transition point 



TABLE III 



of a double salt as the quintuple point of a non-variant system 

 composed of three components present in five phases, this point 

 being the intersection of curves for five univariant systems, each 

 composed of four phases (Table III.). 



Roozeboom's paper was followed in quick succession by the 

 publication of much experimental and theoretical work on the 

 lines by him initiated. 2 The greatest amount of attention was 



1 "Die Umwandlungstemperatur bei wasserhaltigen Doppelsalzen und ihre 

 Loslichkeit," Zs. physik. Chem. 2, 1888, p. 513. 



2 Meyerhoffer, " Die gesattigten Losungen der Verbindungen von Cuprichlorid 

 mit Kaliumchlorid," Zs. physik. Chem. 5, 1890, p. 96; " Zur Kenntnis der 

 Umwandlungsintervalle," ibid., 9, 1892, p. 641. Vriens, " Dampfspannung des 

 Kupferkaliumchlorids und seiner Losungen," ibid., 7, 1891, p. 194. Schreine- 



