DOUBLE SALTS 143 



by the change of colour from blue to green. Van't Hoff recog- 

 nises these and similar transformations to be of essentially the 

 same kind as those occurring in fusion, of which the reversible 

 change ice-water is the simplest example ; or as those occurring 

 in allotropic transformation, of which the reversible change of 

 rhombic to monoclinic sulphur at 96 is an instance. What 

 all these cases have in common is, that the systems concerned 

 are in their composition heterogeneous, i.e. made up of non- 

 miscible portions which, themselves homogeneous, are marked 

 off from one another by bounding surfaces, and which can be 

 mechanically separated from one another, such portions having 

 by Willard Gibbs been named phases. Moreover, in the 

 changes occurring there is the complete transformation at a 

 definite temperature, named the transition point, of one phase 

 into another, and the equilibrium condition is independent of 

 the total amount present of each phase. Thus the formation and 

 the splitting up of double salts — that is, the reversible change 

 of one solid phase into two other solid phases — belongs to the 

 class of complete heterogeneous equilibria ; and the investigation 

 of the effect on the equilibrium condition of changes of tem- 

 perature, pressure, and concentration are problems which it has 

 become possible to solve by the application of the phase rule. 

 In the formation of a double crystalline salt which separates 

 from an aqueous solution of the two component simple salts, 

 we deal with a case of three components (viz. the two simple 

 salts and water) which can form five phases : three solid (the 

 two simple salts and the double salt), one liquid (the solution), 

 and one gaseous (the aqueous vapour). But in the formation 

 and decomposition of double salts in contact with solution, 

 which ordinarily occurs in open vessels, there is absence of 

 the possible gaseous phase ; moreover, the change of solid 

 phases into one another being accompanied with generally 

 very little volume change, the influence of pressure on the 

 equilibrium condition is practically negligible. These are the 

 two properties characteristic of the special class of heterogeneous 

 systems named by Van't Hoff condensed systems. Van't Hoff 

 says : " In the investigation of the splitting up and of the 

 formation of double salts, we follow out a special case of 

 the study of so-called condensed systems, which exhibit peculiar 

 transformation phenomena analogous to those of solidification 

 and of fusion. The temperature named the transition point 



