POLYMORPHISM 



By F. D. CHATTAWAY, F.R.S. 



When matter passes from the gaseous or liquid to the solid 

 state it generally assumes a definite geometric form, this being 

 the external mark of the crystalline condition, that is, a condition 

 in which the molecules are assumed not to be distributed in 

 space at random, but to have an ordered arrangement. 



Towards the end of the eighteenth century Haiiy, as a result 

 of his work upon crystals, came to the conclusion that each 

 individual substance has an invariable crystalline form, peculiar 

 to itself. For many years this conclusion was accepted without 

 question until Mitscherlich proved that certain substances were 

 capable of assuming two entirely distinct crystalline forms. This 

 phenomenon was designated by him " dimorphism," but since 

 it is now known that the number of forms is not limited the more 

 comprehensive term " polymorphism " is generally used. 



This discovery was made by Mitscherlich in 1821 when he 

 was investigating the arsenates and phosphates of the alkali 

 metals and of ammonium in Berzelius' laboratory at Stockholm. 

 In the course of his work Mitscherlich noticed that sodium 

 dihydrogen phosphate, NaH 2 P0 4 . H 2 0, did not crystallise in 

 the same form as the similarly constituted sodium dihydrogen 

 arsenate, NaH 2 As0 4 . H 2 0, as he expected from his recently dis- 

 covered Law of Isomorphism. 



Thinking that this might be the result of some impurity 

 he repeated the crystallisation several times, getting on some 

 occasions crystals of the same form as before, but on others 

 crystals with angles almost identical with those of the arsenate. 

 He found that these latter crystals on solution or recrystallisa- 

 tion would often give him the form which differed from that 

 of the arsenate. 



It is curious that no one had previously reached the same 

 conclusion as Mitscherlich, for various similar cases had been 

 known for some considerable time, as, for example, the much- 

 discussed one of calcite and aragonite. These, however, were 

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