MOLECULAR ARCHITECTURE 



355 



or by Tutton " Molecular Distance Ratios " and are represented 

 by % : -v/r : o). They are calculated from the formulae : 



V.^ 



c sm A sin /3 sin y, a ' 



where V is the molecular volume. The above formulae are for 

 the general case of an anorthic crystal. 



It was hoped with the aid of these topic parameters to throw 

 light on the internal crystalline structure of substances which 

 were chemically related and to a limited extent these expecta- 

 tions were realised. To take an example, it is found that 

 when in ammonium iodide, which exhibits cubic symmetry, the 

 four hydrogen atoms are displaced by four methyl groups, the 

 resulting compound, tetramethylammonium iodide, is tetragonal 

 and one of its topic parameters is approximately equal to each 

 of the three parameters of ammonium iodide. That is to say, 

 the introduction of the four methyl groups has expanded the 

 structure of ammonium iodide equally in two axial directions 

 leaving it unchanged in the third. Substantially the same 

 change is produced if ethyl instead of methyl groups be intro- 

 duced into the molecules. The following table exhibits the 

 relationships between ammonium iodide and its derivatives 

 which are revealed by the topic parameters : 



Similar investigations have been carried out with the sul- 

 phates of the alkali metals and with the double salts formed 

 by these with the sulphates of the bivalent metals, with the alkyl 

 substituted ammonium chloroplatinates and with several other 

 series of related substances. 



The study of molecular volumes and molecular distance ratios, 

 however, was not found to lead to any considerable advance in 

 solving the problems involved in the correlation of crystalline 

 form and chemical constitution. The difficulty which had arisen 

 was to find the means by which to reconcile the mathematical 



