MOLECULAR ARCHITECTURE 713 



not. Further, Barlow and Pope have been able to explain the 

 two crystalline forms of silica, namely quartz and tridymite, on 

 the assumption that in the assemblages representing the crystal 

 structure, the volume of the silicon sphere of atomic influence is 

 but slightly larger than that of oxygen, the mean valency volume 

 of all the spheres being 2. Other crystallographic evidence too 

 supports the assumption that the volume of the sphere of atomic 

 influence of silicon is 2, not 4. If then carbon be unique among 

 the elements in having a fundamental valency of 4, it is not 

 surprising that it should be unique in so many other respects. 



Sufficient has now been said to give the reader an idea of the 

 progress of chemical crystallography during the last few years. 

 The Barlow-Pope theory is unquestionably a great step forward 

 and its advent should stimulate endeavours to elucidate the 

 relationship undoubtedly existing between chemical constitution 

 and cr^^stalline form. 



