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, z.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 ¢ransition 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 ¢ransition point 
