452 [March 5, 



Other groups of homologous bodies were also examined. Benzole, 

 toluole, xylole, cumole, and cymole gave nearly the same numbers, 

 and no regular progression. Pyridine, picoline, lutidine, and collidine 

 showed an augmentation of the specific refractive energy, but a 

 diminution of the specific dispersion with the advancing series. 

 Chinoline and lepidine (which proved to be the most refractive 

 organic liquid known) showed an increase of each of the optical 

 properties by the addition of C 2 H 2 . Thus the influence of the 

 added increment on the rays of light differs in different groups, 

 just as it does in respect to the boiling-point. 



IV. The refraction, dispersion, and sensitiveness of isotneric 

 liquids. Several of the liquids, isomeric with the different members 

 of the benzole series, were examined ; some proved to be identical in 

 all optical properties ; others sensibly the same in actual refraction 

 and dispersion, though slightly different in density ; some again 

 identical in density, but differing in optical properties ; while other 

 isomeric bodies differed slightly in each of these respects. Several 

 hydrocarbons of the type C 20 H 16 , from essential oils, seemed to be 

 identical in actual refraction, notwithstanding slight differences of 

 their density. In dispersion, too, there were some variations ; but 

 not in sensitiveness. Other hydrocarbons, however, of the same 

 ultimate composition, but differing considerably in physical pro- 

 perties, differed also optically. Compound ethers, as valerianic 

 ether and acetate of amyl, which contain the same number of 

 carbon, hydrogen, and oxygen elements, though differently arranged, 

 are optically identical, as was partially shown by Delffs some years 

 ago. Aniline and picoline, each empirically C 12 H 7 N, are totally 

 different. The conclusion arrived at is that isomeric bodies are 

 sometimes widely different in these optical properties ; but that in 

 many cases, especially where there is close chemical relationship, 

 there is identity also in this respect. 



V. The effect of chemical substitution. By observing the amount 

 of change in the optical properties which results from a replacement 

 of one element by another, the chemical type remaining the same, it 

 seemed possible to arrive at a knowledge of the influence of the 

 individual elements on the rays of light transmitted by them. Of 

 the immense number of data required for the perfecting of such an 

 inquiry, the following are afforded by the experiments already made. 



