VIII. On the refractive indices of the elements

Abstract

The relation between the constitution of the molecules of compounds and their power to retard the velocity of light is a subject which has attracted many observers, and has led to results which are interesting, both from their general regularity and occasional discrepancy. It is singular, therefore, that so few attempts have been made to investigate that portion of the subject which should, theoretically, have occupied a prior place, the study of the refractive indices of the elements in the gaseous state and their relation to one another. No doubt this defect in our information is due to the difficulty of obtaining all but a very few elements in such a state that their index of refraction can be measured directly. Till recently, the only elements in which this had been done were H, N, O, Cl, Br, I, Hg, S, P, and As; and the numbers obtained in these instances did not seem to bear any relation to each other, or to any known constant. Thus, the refractivities of Cl, Br, and I increase with the increase in their atomic weights, though not proportionately. But O, with an atomic weight sixteen times that of H, only retards light twice as much, while the index of N is actually greater than that of O. Discouraged by these anomalous results, investigators have generally contented themselves with calculating values for the refractivities of the elements from the effects they produce in compounds, and have aimed at obtaining an empirical constant for each element which would satisfy an additive law in compounds. The numbers so obtained are found to vary with the constitutional formula of the compound in question, and the variations are sufficiently regular to afford assistance in analysing the constitution of complex molecules; but even the best results sometimes fail to fit in with the facts in a way which proves that our empirical formulæ are still far from the truth.