THE interesting results recorded by Mr. Eve in the preceding letter on the relative conductivity of gases for very penetrating Rontgen rays removes the strongest objection that has been urged against the common belief that the γ rays are an extremely penetrating type of Rontgen rays. All the experimental evidence so far obtained is now in agreement with the view that the γ rays are very penetrating Rontgen rays which have their source in the atom of the radio-active substance at the moment of the expulsion of the β or kathodic particle. For example, I have found that the γ rays from radium always accompany the β rays, and are always proportional in amount to them. In radium the β and γ rays appear only in the third change occurring in the radio-active matter which causes “excited activity,” i.e. in the fourth of the chain of radio-active products which result from the disintegration of the radium atom.