APPLICATIONS OF CRYSTALLOGRAPHY, ETC. 581 



therefore probably have the composition Sg. Further, when 

 sulphur is heated from its melting to its boiling point it 

 undergoes a remarkable series of changes ; near the melting 

 point sulphur is a fairly limpid straw-coloured liquid which 

 does not change rapidly in colour or consistency until a 

 temperature not far below the boiling point is reached, when 

 it suddenly assumes a deep-brown colour and becomes so 

 excessively viscous that the containing vessel may be in- 

 verted without the liquid running out. It is scarcely con- 

 ceivable that a change of this kind can be due to anything 

 but a change in molecular complexity ; and, since changes 

 in molecular complexity certainly occur in gaseous sulphur 

 and almost as certainly in liquid sulphur, it is very possible 

 that the six or seven different polymorphous crystalline 

 modifications of solid sulphur have not all the same 

 molecular weight but that some are polymerides of others. 

 The physico-chemical investigation of crystalline sub- 

 stances may be confidently expected to yield brilliant results 

 in that department of chemistry which deals with tauto- 

 merism occurring amongst organic compounds ; this subject 

 is one which every organic chemist recognises as presenting 

 great difficulties and some of the ablest physical chemists 

 have for long past applied their most ingenious efforts to 

 its study. Many organic compounds are known which do not 

 behave consistently during the ordinary laboratory opera- 

 tions ; in some reactions they behave as if they had a certain 

 constitution whilst in others they seem to possess quite a 

 different constitution. Such substances are said to be 

 tautomeric and are supposed to possess each of the two 

 constitutions at different times, the conversion from the one 

 molecular constitution to the other being very readily 

 brought about. In certain cases two tautomeric compounds 

 can be isolated and examined ; their investigation in the 

 liquid state is, however, extremely difficult owing to the 

 complication introduced by the tendency to interconversion. 

 Two tautomeric forms, if in the solid state, should be much 

 more easily investigated than if they were in the liquid con- 

 dition, because in the former case the tendency to rearrange- 

 ment of the atoms composing the molecule would naturally 



