698 SCIENCE PROGRESS 



constitution and various physical properties of organic sub- 

 stances, such as their refractive index, optical dispersion and 

 magnetic rotation in solution. The great majority of such 

 investigations deal with substances in the liquid form or in 

 solution. Very few attempts have been made to correlate the 

 physical properties of solid bodies with their chemical constitu- 

 tion. Consequently if it be found possible to discover a more 

 intimate connection between chemical constitution and crystal- 

 line form than heretofore, a great advance will have been made. 



The study of organic compounds with the idea of investiga- 

 ting their ultimate crystal structure will be simplified by the 

 accurate knowledge we possess of the constitution of many. 

 The constitution or mode of linking of the atoms in inorganic 

 compounds other than the very simplest is at present very 

 obscure ; should it be found possible in the case of organic com- 

 pounds to discover the connection existing between chemical 

 constitution and crystal structure, it might well become possible 

 to apply the knowledge thus gained to the solution of the problem 

 of the constitution of inorganic bodies. 



The fact is well known that benzenoid organic compounds 

 crystallise with greater facility than paraffinoid derivatives, many 

 of these latter being oils which crystallise only at low tempera- 

 tures. The benzene derivatives on this account afford the more 

 suitable material in commencing the crystallographic study of 

 organic compounds. In their first paper {J.C.S. Trans. 1906, 

 p. 1675) Barlow and Pope have considered at length the crystal 

 structure both of benzene and of a number of its derivatives. 



The Crystalline Structure of Benzene 



We have to consider first the data from which the internal 

 structure of the crystalline hydrocarbon is determined, regarding 

 it, of course, as a homogeneous close-packed assemblage of atomic 

 spheres of influence of carbon and hydrogen. The composition 

 of benzene is CoHg, so there are to be in the corresponding 

 assemblage of spheres equal numbers of spheres of diff'erent 

 kinds representing respectively carbon and hydrogen. Further, 

 as the fundamental valencies of carbon and hydrogen are in the 

 ratio 4:1, by hypothesis the volume of each carbon sphere of 

 influence is to be taken as four times that of each hydrogen 

 sphere. Again, it must be possible to fashion the structure 



