On the Refraction, Dispersion, and Sensitiveness of Liquids. 487 



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 amyle, which contain the same number o t 

 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. 

 The replacement of hydrogen by a compound radical, aniline — amyl- 

 aniline ; and water, alcohol, ether (according to Williamson's theory). 

 Of hydrogen by oxygen — alcohol, acetic acid ; ether, acetic ether ; 

 and carvene, carvole, eugenic acid. Of hydrogen by peroxide of 

 nitrogen — benzole, nitrobenzole, dinitrobenzole (in solution) ; gly- 

 cerine, nitroglycerine ; and amylic alcohol, nitrate of amyle. Of 

 hydrogen by chlorine — benzole, chlorobenzole, terchlorobenzole ; 

 and the substitution of chlorine by bromine — terchloride of phos- 

 phorus, terbromide of phosphorus ; chloroform, bromoform ; and 

 bichloride of chlor-ethylene, bibromide of chlor-ethylene, bibromide 

 of brom-ethylene. When hydrogen is replaced by some other body, 

 there is generally an increase of the actual refraction and dispersion ; 

 but this is due to the increased weight, hydrogen having a very low 

 actual, but a very high specific influence on the rays of light. In 

 each of the five instances of two substitution-products, as, for 

 instance, chlorobenzole and trichlorobenzole, the lower one always' 

 retains in its optical properties an intermediate position between 

 the original substance and the higher product. 



These experiments on substitution sufficed to show, as the exa- 

 mination of isomeric bodies had done, that the special influence 

 exerted on the rays of light by the elements of a compound is 

 greatly dependent on the manner of their combination. 



The following is given as a generalization approximately, if not 

 absolutely true : — Every liquid has a specific refractive energy com- 

 posed of the specific refractive energies of its component elements, 

 modified by the manner of combination, and which is unaffected by 

 change of temperature, and this refractive energy accompanies it 

 when mixed with other liquids. 



