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SCIENCE PROGRESS 



salts must be isomorphous. The series of combinations thus 

 formed, named mixed crystals, are of the nature of solid 

 solutions', their physical properties are continuous functions of 

 the composition ; their chemical properties are those of the 

 two constituent simple salts, i.e. the analytical reactions are 

 those of the ions of the two simple salts. Thus KC10 4 and 

 KMn0 4 are isomorphous and form mixed crystals of all shades 

 of depth of colour, from almost colourless through pink to deep 

 purple, according to the relative amounts of colourless KCIO 



TABLE I 



Classification of Combinations wMA . nWA formed from 

 two Simple Salts MA and M'A 



I. Mixed Crystals 



II. Double Salts 



III. Salts of Complex 

 Acids 



Series 

 from 



m = 

 n = 



m = o 

 ioo 



(i) Composition 



.1 

 in and n = simple whole numbers. 



iooj fm 



o ) in 



(2) The physical properties of the solid wMA . nM'A are 



additively those of the con- 

 stituent salts, continuous 

 functions of the chemical 

 composition in terms of the 

 simple salts. 



in no definite way related to those of 

 the constituent simple salts. 



(3) The chemical properties of solutions ofmMA . nM'A are. 



those of the ions of the simple salts, i.e. those 

 of M, M', and A. 



not those of the ions of the 

 simple salts MA and MA, 

 but of ions M and M'A w+ „. 



and deep purple KMn0 4 present in the crystals. The same 

 additive relation holds for other physical properties, such as 

 specific volume, refractive index, etc. Moreover, a solution of 

 the mixed crystals will give the reaction for the potassium 

 cation and the perchlorate and permanganate anion. Retgers, 1 

 through his investigations of the relation between the physical 

 properties and the composition of whole series of such salts, 

 has supplied us with definite principles whereby to recognise 

 mixed crystals, the chief interest attaching to this class of complex 

 salts being in connection with the study of isomorphism. 



1 "Zur Kenntnis des Isomorphisms," I. -XII., Zs. physik. Chem. 4-20, 

 1 889- 1 896. 



