Journal of the Chemical Society (Resumed)
THERE were three distinct lines of research which led up t o ths discovery of refraction equivalents. The first was the influence of temperature ou the refraction of light by liquids ; the second, the refraction of mixtures or combinations as compared with that of their constituents; and the third, the refractive indices of different members of homologous series of organic compounds. AB to the first of these, it must have been frequently observed that a liquid bent the rays of light differently
... when it was heated, but the late Rev. Baden P o w e l l seems to have been the first t o investigate the matter, and the Rev. T. Pelharn Dale and myself, carrying out his idea, and with his instrument, found that both the refraction and the dispersion always decrease as the temperature rises.* On further examination we observed a close relation between the change of density and the change of the refractive index minus unity, which we termed the " refractive energy," and which is expressed in the symbolic language of opticians as p, -1. The product of this refractive energy with the volume, that is ( p -1) vol., or, which is the same thing, the energy divided by the density, that is we called the " specific refractive energy," and we came to the conclusion that in the case of liquids this specific refractive energy is a constant not affected by temperature. Nevertheless me recognized " some influence, arising wholly or partially from dispersion, which we have not been able to take into account, but which gives rise to the slight progression of most of the calculated products, and perhaps t o the non-inversion of the sensitiveness of water at 4" C., remarked on already by J a m i n and ourselves."* This general conclusion was subsequently confirmed by the experiments of Professor Landol t of Bonn, and a rigorous investigation of the matter by W u l l n e r t has shown that we a! were justified in our belief that pzl, though the best simple formula, does not express the whole truth. The same is exhibited by the very careful determinations of the refraction of water at temperatures ranging from 0 ' t o 100' C., lately published by Kuh1mann.S The value of'this investigation in its bearing on refraction equivalents was, that it drew attention t o the specific refractive energy p Z 1 . Heretofore N e w t o n' s " Absolute refkactive d d -1 d power," p L , had generally engrossed the thoughts of physicists . As to the second line of research, that of the refraction of mixtures, solutions, and eimple combinations, D ulon g attempted to show, in regard to gases, and H 6 c k in regard to some liquids, that the absolute refractive power of a mixture is the mean of the absolute refractive powers of its constituents. Mr. Dale and myself, however, on examining the mixtures of substances 80 widely apart in their refractive power as bisulphide of carbon * On the other hand, however, we remarked that isomeric liquids are not always identical in refractive energy, and that * Brit. Ausoc. Report, 1863. -f Ibid. The details of many of these experiments are given in the Quarterly Trans., See. p. 13.