RECENT PROGRESS IN CRYSTALLOGRAPHY 115 



In addition, the position of ammonium near rubidium in the 

 alkali series has been established, together with the interesting 

 corollary of the existence of intermolecular and interatomic 

 spaces. Lastly, the conception of topic parameters has been 

 developed into a valuable means of elucidating the nature of 

 the internal structure of crystals, and of expressing the 

 dimensions of the point system according to which the mole- 

 cules are arranged, besides affording some positive evidence 

 as to the arrangement of the atoms composing the molecule. 



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