GASEOUS REFRACTIVE INDICES 275 



the effect of chemical substitution on optical properties. Their 

 general conclusion was that every body has a specific refractive 

 energy composed of the specific refractive energies of its com- 

 ponent elements, modified by the manner of combination, and 

 unaffected by change of temperature. 



Following up their inquiry, Gladstone gave, in 1870, a very 

 complete list of the "refraction equivalents" of the elements, 

 and in 1885 he published a revised list of these with several 

 important modifications. While Gladstone and Dale were 

 occupied with an extended survey of the refraction equivalents 

 of all the elements, Landolt was engaged on an elaborate 

 examination of those of carbon, hydrogen and oxygen in organic 

 compounds. By tabulating the difference between the numbers 

 representing the refraction equivalents of consecutive members 

 of homologous series, differing in composition by CH 2 , he 

 obtained a mean value for the equivalent of this group, and 

 hence the equivalent of each of the three elements separately. 

 His numbers were: for C, 5-00; for O, 3 # oo; and for H, i'30- 

 When these values are used for calculating the refraction 

 equivalents of a large number of carbon compounds of which 

 the refractive index has been measured, the agreement is satis- 

 factory, the difference between calculation and experiment being 

 generally less than 1 per cent. 



So good an agreement is only obtained, however, by com- 

 paring saturated compounds. To Bruhl we are indebted for a 

 series of researches in which it is proved that when two carbon 

 atoms in a molecule are connected by a double bond, the 

 refraction equivalent of the compound is increased by, approxi- 

 mately, 2, or two-fifths of the total effect produced by one atom 

 of carbon. If the number of double bonds is more than one, the 

 equivalent is proportionately larger. If the bond, instead of 

 being double, is triple, the increase is about i'8. In the results 

 of these researches we seem to perceive the outlines of a 

 fruitful field of work, which needed only patience and labour to 

 yield a plentiful harvest. Yet the outcome of further investiga- 

 tion has been very disappointing. They certainly established 

 the fundamental proposition that, to a first approximation, the 

 power of retarding light is inherent in the atoms of the elements, 

 and gave good grounds for believing that it is not materially 

 changed when they pass from one combination to another. And 

 they sufficed to prove, when Mendeleef's law came to be con- 



