in this ring on the assumption that it was the only one which influenced the refraction of ordinary light; the results they arrived at indicate that there is a close connexion between the number of these electrons and the chemical valency of the atom. In fact, they suggest that this number may be equal to the electro-positive valency of the element. It cannot, I think, be maintained that the experiments of Drude and others on the indices of refraction do more than suggest this identity. Many of the results differ considerably from those which would follow from it. We need not, however, I think, attach any very great importance to these discrepancies, as many assumptions were made in the course of the work for the sake of simplicity which may turn out not to have been well founded; it was assumed, for example, that there is only one period in the visible and ultra-violet light portion of the spectrum which enters into the expression for the refractive index, and this period was chosen not because it had been observed in the spectrum, but so as to fit in with the measurements of the refractive index. We must remember, too, that one or more of these mobile electrons in the outer ring may leave the atom when it enters into chemical combination, and that their arrangement is altered by the proximity of other atoms; as many of the substances used by Drude were compounds, the number of electrons in the ring may not have been the same as when the atom was in the free state.
The strongest evidence in favour of the close connexion between the number of electrons in the outer ring and the valency of the elements comes from the chemical properties of the elements, and especially the various types of chemical compounds they can form. Very many of these are simply explained by supposing that near the outside of the atom there are mobile