DOUBLE SALTS . 137 



II. The ratio of the quantities of the two component salts 

 is a simple molecular one ; in 7/2MA . /zM'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 n 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 4 . 8H 2 ; colourless LiCl and green CuCL.2H 2 give a 

 compound of the formula CuLiCL,. 2H2O, 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 + FeCyl 2 = K 4 FeCy 6 

 2KCI + PtCl 4 = K.,PtCl 6 

 3K 2 C,0 4 + Cr 2 (C,0 4 ) 3 = 2K 3 Cr(C,0 4 ) 3 , 



