POLYMORPHISM AT HIGH PRESSURES. 169 



It is evidently useless to try to generalize from only the two groups 

 found here. This conclusion does seem justified, however; similarity 

 of phase diagrams between corresponding phases is evidence of iden- 

 tity of structure of a higher order than is concerned in the ordinary 

 run of chemical or crystallographical phenomena. In general, the 

 identity of structure must be complete enough to allow continuous 

 series of mixed crystals, but this identity is not necessarily far reaching 

 enough. Instances of such far reaching identity must be rare; it is 

 all the more important to investigate other cases. 



Similarity of phase diagrams means not only an identity of struc- 

 ture so complete that similarly arranged edifices are possible, but also 

 means that the fields of force surrounding the elements are so similar 

 that corresponding edifices are stable. This has a bearing on the 

 custom of many crystallographers of classifying a substance as di- 

 morphic if it can crystallize in limited proportions with another 

 substance of different symmetry. Such a "dimorphism" cannot be 

 of broad significance ; it merely means that the similarity of the build- 

 ing stones of the two substances is great enough so that if sufficient 

 compulsion is applied to the one set they may combine in limited pro- 

 portions in the edifice appropriate to the other. A very wide range 

 of similarity or dissimilarity is evidently included in a classification 

 so elastic as this. 



It is also the custom, or rather a matter of definition, to class a sub- 

 stance as polymorphic if it has more than one modification, stable or 

 not. This again, has no well defined significance. Given a number 

 of identical building blocks, it would evidently be possible to build 

 these with our hands into a large variety of assemblages corresponding 

 to different crystalline systems. Most of these arrangements would 

 be very unstable, but all would persist for a small interval of time. 

 This means that in this sense every substance is polymorphic in a very 

 complicated way. In practise, however, not very many substances 

 under ordinary conditions happen to form assemblages that are com- 

 paratively stable. But it is conceivable, and likely, that under differ- 

 ent conditions of inoculation or subcooling the number of substances 

 with unstable polymorphic forms (monotropic polymorphy) should 

 be very largely increased. The point is that this kind of polymorphism 

 is not of absolute significance, and the more we extend the list of poly- 

 morphic substances by increased skill in manipulation, the less signi- 

 ficant does it become. There is no denying, of course, that the easy 

 and persistent polymorphism shown by phosphorus, for example, 

 is significant — it is merely impossible to draw a sharp line. Further- 



