ARCHITECTURE OF MOLECULES 225 



the general character of the compound, but if hydrogen is to be 

 replaced by oxygen two atoms of the former are replaced by one 

 atom of the latter element. 



The elements which stand in the same vertical column in the 

 periodic scheme are generally very similar in chemical character, 

 and the magnitude of the sphere of influence of the atoms of 

 elements in such a case increases but slowly. Consequently the 

 difference between two such elements as potassium and rubidium 

 would be but small, and the one may replace the other without 

 affecting the general arrangement or marshalling of the other 

 atoms present in a compound. In this way the relation of 

 valency and isomorphism becomes apparent. Measurements of 

 the crystals of some isomorphous groups, comparing potassium 

 sulphate, K 2 S0 4 , with caesium sulphate, Cs 2 S0 4 , for instance, 

 show that while the volume of the entire molecule is changed, 

 the spheres of influence of the atoms present preserve the same 

 ratios, and the same crystal form with but slightly changed axial 

 ratios is retained. 



Professor Richards of Harvard has adduced a considerable 

 amount of experimental evidence in favour of the idea that 

 atoms are compressible. How far the compressibility of an atom 

 may affect the volume of its sphere of influence is uncertain, but 

 the independence of the atom, the retention of its mass and, 

 approximately, its volume are facts which up to the present 

 have been very generally accepted. 



The theory of Barlow and Pope, which has just been very super- 

 ficially reviewed, grew out of the study of crystals, their form, 

 structure, optical properties, and relation to ch'emical composi- 

 tion. It has now been under consideration by the chemical world 

 during ten years, and though much yet remains to be learned, 

 partly owing to the scarcity of crystallographers, the theory in 

 its broad aspect may be regarded as having established its posi- 

 tion among accepted chemical doctrines. The theory in no way 

 conflicts with the tetrahedral hypothesis as to the atom of carbon 

 and elements related to carbon, and the assumption as to the 

 volume of the carbon atom being four times that of a hydrogen 

 atom may be regarded as consistent with all that is known of 

 carbon compounds. 



Help has come from an entirely new direction, and has arisen 

 out of discoveries which have been made as to the action of 

 crystalline substances on X-rays. The origin of the work is an 

 Q 



