GASEOUS REFRACTIVE INDICES 297 



In conclusion it will be convenient to summarise the points 

 which it is the object of this brief review to enforce. The study 

 of the refractive indices of elements and compounds is universally 

 admitted to be of importance in throwing light on the nature of 

 atoms and molecules ; but the comparative sterility of the work 

 which has been done on the indices of liquids and solids proves 

 that in these states the causes which tend to obscure the simpli- 

 city of result which we hope for are too powerful, and that it 

 is vain to hope for much addition to our knowledge in this 

 direction. It is, therefore, to the study of gaseous indices that 

 attention should be directed. The field of research is here 

 extremely wide, and it is comparatively untrodden ground. The 

 little work that has been done has led to the observation of a 

 series of simple relations which can hardly be believed to be 

 mere coincidences ; and which, by their very simplicity, seem to 

 be the outward and visible sign of some fundamental truth, 

 which ought to be pursued. Nor can the absence of any 

 plausible hypothesis be considered a good reason for neglecting 

 to push the inquiry further. The series 1, 2, 8, 12, 20, which 

 connects the refractivities of the inert gases, is anomalous from 

 a mathematical point of view ; but the periodic table of the 

 elements itself is not free from similar difficulties, and it is 

 possible that the anomalies of the one may help to explain 

 the anomalies of the other. In the region of the indices of the 

 compounds, such work as has been accomplished has sufficed to 

 prove that the additive law, which till recently held the field, is 

 so far from being true, even as a first approximation, that it must 

 certainly be abandoned. The discovery of the true law can 

 hardly be attempted without the accumulation of a large mass of 

 well-selected data. Hitherto, the work has been of a pioneer 

 character. What is now required is organised research on 

 typical series of the simplest compounds, selected with intelli- 

 gence. As examples of the existing dearth of material it may be 

 mentioned that no attempt has hitherto been made to find out 

 whether gaseous polymerisation has any effect on refraction, i.e. 

 whether two molecules are as effective in refraction when linked 

 together as when separate. A study of N0 2 and N 2 4 , of acetic 

 acid and of ozone would go far to set this question at rest ; but 

 this has not yet been attempted. Again, Dufet records that only 

 two of the hydrocarbons, methane and pentane, have been 

 investigated in the gaseous condition. A long list could be 



