57^ SCIENCE PROGRESS. 



we do know that the point-system or structure changes, but 

 we do not know that the unit of the structure akers at the 

 same time ; and since the change of point-system or of 

 arrangement of the molecules is sufficient to explain all the 

 facts, it is an offence against the law of parsimony to assume 

 that the physical molecule changes during the intercon- 

 version of two polymorphous modifications of a substance. 

 It should, perhaps, be pointed out that capillarity determina- 

 tions have shown that many substances have not the same 

 molecular weight in both the liquid and the gaseous state, 

 the free particles in the liquid being apparently aggregates 

 of gaseous particles ; whether these polymolecular aggregates 

 persist in the solid state is not yet known, but it would seem 

 that no complication of this kind should affect salts, these 

 being, according to Crompton, monomolecular in the liquid 

 state. 



Although Tutton's purely crystallographic work indicates 

 that in a number of cases the chemical molecule is identical 

 with the physical molecule in the crystalline state yet it is 

 not to be expected that our knowledge of the relative 

 weights of these molecules will be in the main derived 

 from arguments deduced from the measurement of series 

 of crystalline salts ; the investigation is of so laborious a 

 character and the interpretation of the results so liable to 

 error as to suggest that the methods of physical chemistry- 

 are likely to lead more rapidly and surely to the desired 

 end, Nernst has indeed indicated a method which will 

 probably be found of general applicability, and which de- 

 pends on a remarkable property of chemically related 

 crystalline substances. Two compounds when very closely 

 related chemically are nearly always obtainable in crystal 

 of almost, though usually not quite, identical form ; the 

 structures in which the physical molecules of two such 

 substances are arranged are thus of the same pattern, and 

 the compounds are said to be isomorphous. A striking 

 peculiarity of isomorphous substances lies in their power of 

 crystallising together from solutions containing them both,, 

 crystals being deposited which contain the two salts in 

 varying proportion, depending on the composition of the 



