582 SCIENCE PROGRESS. 



be much less and because crystallographic methods could 

 be used to ascertain with ease and rapidity which of the two 

 substances was under examination ; up to the present, how- 

 ever, crystallographic methods have not been applied to the 

 study of tautomerism. 



xA-lthough our knowledge of tautomerism apparently has 

 much to gain from crystallography, yet a protest is necessary 

 against a highly reprehensible practice which has recently 

 been adopted by a certain school of organic chemists. 

 Large numbers of organic compounds exist in several 

 polymorphous forms, and naturally enough there are 

 amongst these substances some, the chemical behaviour 

 of which can be equally well represented by two tautomeric 

 constitutions ; in these cases it seems to have become cus- 

 tomary to assign one constitution to the one crystalline 

 modification, and a different one to the second modifica- 

 tion. This has been done in such cases as that of benzoin 

 with apparently no regard to anything but the fact that two 

 molecular configurations are possible, and that two poly- 

 morphous forms exist ; it need hardly be pointed out that 

 such a method of creating new compounds is hardly likely 

 to redound to the credit of organic chemistry. 



Should the view advocated above, namely, that the 

 physical molecules or the units of the structure of crystal- 

 line substances are in the majority of cases the chemical 

 molecules, we hold the key to many problems which have 

 hitherto successfully resisted all attempts at their solution. 

 The close crystallographic relationships often existing be- 

 tween substances which are not isomorphous, but, never- 

 theless, are more or less closely related in a chemical sense, 

 have always been amongst the most curious and interesting 

 of crystallographic phenomena ; these analogies are termed 

 morphotropic relationships, and have doubtless escaped 

 notice in hundreds of cases owing mainly to the fact that 

 the crystallographic examination of organic compounds has 

 been usually entrusted to mineralogists, who, naturally 

 enough, are not alive to the chemical interests at stake. 

 Here, again, the recital of an actual case will best serve 

 to exemplify what is meant by a morphotropic relationship. 



