194 SCIENCE PROGRESS 



identical volumes in these two classes of compounds is erroneous. 

 There is invariably a considerable contraction in the ring 

 structure as compared with the open-chain hydrocarbon of 

 identical molecular weight. 



Kopp found that a difference of CH 2 in what he regarded 

 as a comparable homologous series corresponded to a difference 

 of 22 in molecular volume, and he deduced the separate atomic 

 volumes for C and H in all compounds as respectively n 

 and 5* 5 or 2 : i . It has now been shown that the normal values 

 of C and H in all compounds of open chain structure at their 

 boiling-points are respectively 14-7 and yy or 4:1, and this 

 relation is preserved at their critical points. 



When the difference in constitution between aliphatic and 

 aromatic compounds was recognised, the attempt was made 

 to explain the contraction which occurred in passing from 

 the open-chain compound to that of ring structure by assum- 

 ing a different value for single-linked and double-linked 

 carbon, whilst that of the hydrogen atom remained constant. 

 Thus in benzene three carbon atoms were assumed to have 

 the value iro, the other three having the value 14-0, whilst 

 the volume of the hydrogen atom was uniformly 3*5. 



There is, however, no necessity to assume that variable 

 linkage has any influence on the ratio of the atomic volumes 

 of carbon and hydrogen. The 4 : 1 ratio still holds good 

 both at the boiling and critical points and at all equally 

 reduced pressures. At the same time, in passing from the 

 aliphatic to the aromatic class of compounds the atomic 

 volumes of carbon and hydrogen do actually undergo con- 

 traction without the characteristic ratio being affected. 



It is further established that the values thus obtained are 

 equally true for unsaturated and saturated compounds : the 

 values for corresponding members of the n paraffin, olefin 

 and acetylene series are simply dependent on their composi- 

 tion. This is contrary to the conclusion of Buff, who sought 

 to show that the atomic volume of carbon was greater in 

 unsaturated than in saturated compounds. The evidence that 

 unsaturation exerts no special effect would, however, seem to 

 admit of no further doubt. 



The scope of this article will not permit of a detailed ex- 

 amination of the various constitutive effects which have been 

 found to influence molecular volume, such as complexity, 



