370 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



compounds, which are similar both in chemical. type and in 

 crystalline form, possess the property of forming crystals 

 in which the two constituents are mixed as intimately as 

 the salt and water in an aqueous salt solution. Crystals of 

 this kind are described as ISOMORPHOUS MIXTURES or SOLID 

 SOLUTIONS. They are formed whenever two or more closely- 

 related substances are allowed to crystallise together from 

 solution or from the fused state. Thus if 



potash alum ..... KAl(SO 4 ) 2 ,i2H 2 O 

 ' soda alum ..... NaAl(SO 4 ) 2 ,i2H 2 O 

 and chrome alum .... KCr(SO 4 ) 2 ,i2H 2 O 



are dissolved together in water, the crystals which separate 

 will contain all three substances, the colour ranging from a 

 pure white to a deep purple according to the proportion 

 of chrome alum which they contain. So also in the case of 

 minerals, garnets of the type 

 Ca 3 Al 2 Si 3 O 12 or 



which have crystallised from a magma of complex com- 

 position, are all mixtures of isomorphous substances, in 

 which a part of the calcium has been replaced by magnesium, 

 ferrous iron, or manganese, and part of the aluminium by 

 ferric iron, or by chromium. But in all this complex 

 substitution (known to mineralogists as VICARIOUS REPLACE- 

 MENT) the type remains constant and the general formula 



3 [Ca,Mg,Fe,Mn]0,[Al,Fe,Cr] 2 3 , 3 Si0 2 



may be used to represent every member of the series. 



The formation of isomorphous mixtures is a valuable test 

 of chemical similarity, because they cannot be produced from 

 substances of different type, even when absolutely identical 

 in crystalline form ; on the other hand, a difference of 5 in 

 the angles of analogous compounds need not prevent the 

 formation of an isomorphous mixture, since composite 



