DOUBLE SALTS 137 
II. The ratio of the quantities of the two component salts 
is a simple molecular one; in mMA.uM’A m and n are very 
simple whole numbers. Obviously any series of mixed crystals 
must comprise members the composition of which also exhibits 
such a simple molecular ratio, and cases of confusion have in 
consequence not been infrequent ; but there is the distinguishing 
feature that whilst for mixed crystals a variation in the pro- 
portion of the simple salts present in the solution from which 
they are deposited will produce a series of gradually varying 
composition, in the class now under consideration the number 
of combinations obtained is very small, generally only one; 
and when it is more than one, the change in composition is 
sudden, m and ” remaining simple whole numbers. Moreover, 
in this second class the physical properties, such as colour, 
specific volume, crystalline form, solubility, etc., etc., are not in 
any definite way related to those of the constituent simple salts, 
but generally differ widely from these Thus, to take as an 
example the easily apprehended property of colour, the following 
constitute striking cases: white acetate of calcium and green 
acetate of copper, if brought together in equi-molecular 
quantities, yield a magnificently deep blue salt of the formula 
CaCuAc,.8H,O; colourless LiCl and green CuCl,.2H,O give a 
compound of the formula CuLiCl,.2H.O, which, in a spirit of 
unusual rebellion against the predominant blue and green of 
cupric salts, crystallises in garnet-red needles; and similar 
examples might be multiplied. Compound salts of this type, 
when in solution, give the analytical tests of the ions of the 
constituent salts, and they are termed double salts, a name which 
until comparatively lately was indiscriminately applied also to 
that kind of complex substances formed from two simple salts 
which constitutes the third and last class. 
III. In this class, as in the preceding one, the ratio of the 
quantities of the two component salts is a simple molecular 
one; but the compound salt is distinct from a mixture of the 
components not merely in respect of the physical properties of 
the solid, but also in respect of the chemical properties of the 
solution. Typical examples of such cases are 
4KCy + FeCy!,= K,FeCye 
2KCl1 + PtCl, = K,PtCl, 
3K2C.0, + Cr.(C,0,)s = 2K 5Cr(C,0,)s, 
