HARRIET BROOKS (MRS. FRANK PITCHER), who died in Montreal on April 17, was well-known in the years 1901–5 for her original contributions to the then youthful science of radioactivity. A distinguished graduate of McGill University, she was one of the first research workers with Prof, (now Lord) Rutherford in Montreal. She observed that the decay of the active deposit of radium and actinium depended in a marked way on the time of exposure to the respective emanations and determined the curve of decay for very short exposures. This work, which was done before the transformation theory of radioactive substances was put forward, assisted in unravelling the complex transformations which occur in these deposits. With Rutherford she determined the rate of diffusion of the radium emanation into air and other gases. These experiments were at the time of much significance, for they showed that the radium emanation diffused like a gas of heavy molecular weight—estimated to be at least 100. Miss Brooks entered the Cavendish Laboratory, Cambridge, in 1903 and continued her radioactive investigations. In a letter to NATURE of July 21, 1904 (vol. 70, p. 270) she directed attention to a peculiar type of volatility shown by the active deposit of radium immediately after its removal from t he emanation. In the light of later results of Hahn and Russ and Makower in 1909, it is clear that the effect was due to the recoil of radium B from the active surface accompanying the expulsion of an α-particle from radium A. This method of separation of elements by recoil ultimately proved of much importance in disentangling the complicated series of changes occurring in the radioactive bodies.