IN a letter to NATURE of October 31, 1907 (vol. lxxvi., p. 661), a copy of which arrived here recently, Mr. Barkla criticises a paper of mine which was published in the Philosophical Magazine for October. In that paper I tried to show how closely the properties of γ and X-rays were in agreement with the hypothesis that they consisted mainly at least of neutral pairs, and I pointed out that even the peculiar polarisation effects which Mr. Barkla had shown to exist might be explained, not unreasonably, as a consequence of the rotatory motion which such pairs would probably possess. I suggested that a pair might be more likely to become entangled with and deflected by an atom revolving in the same plane as itself.