MOLECULAR ARCHITECTURE 



THE RELATION BETWEEN CRYSTALLINE FORM 

 AND CHEMICAL CONSTITUTION 



By R. T. COLGATE and E. H. RODD 



It is evident that we may expect many fruitful results for molecular mechanics 

 (which forms a problem common to many provinces of natural science) from the 

 further elaboration of the data concerning those variations which take place in 

 crystalline form when the composition of a substance is subjected to a known 

 change ; and I therefore consider it useful to point out to the student of science 

 seeking for matter for independent scientific research this vast field for work which 

 is presented by the correlation of form and composition. — Mendeleef, Principles 

 of Chemistry, 1905, i. 10. 



Introduction 



Time out of mind, the scientific imagination has been strongly 

 impressed by the beautiful symmetry of the solids known as 

 crystals. Haiiy's great discovery of the rationality of crystal 

 indices^ led to this symmetry being traced to its source 

 in the definite uniform arrangement of the ultimate units or 

 molecules and was the first important step towards the funda- 

 mental conclusion since reached that crystals display symmetry 

 because they are homogeneous structures as defined by the 

 mathematician. 



Recent investigation in the department of modern chemistry 

 known as stereochemistry has shown that the homogeneity 

 displayed is not alone a regular repetition throughout space of 

 mere points or similar centres of force but that the chemical 

 atoms of which the space units of a crystal consist are them- 

 selves definitely arranged in the molecule. 



The conclusion has indeed been forced upon us that regularity 

 of atomic arrangement characterises all matter and that, strictly 

 speaking, the term amorphous (formless) is inappropriate as 

 applied to molecular structure ; the relative disposition of the 



' The indices of a crystal face are the reciprocals of the intercepts of the face, 

 supposed indefinitely extended, on the three crystal axes. The Law of Rationality 

 of Indices states that these are always whole numbers. The occurrence of a digit 

 higher than 3 in the indices of a crystal face is rare, 



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