Of recent years much interesting work has been done to connect the atomic weight of an element with its power of emitting and transmitting various kinds of radiation. One may mention Mcclellands work on the secondary raditation given out by a substance exposed to the ß and γ rays of radium, and Prof. J. J. Thomson's results, which brought out the relation existing between atomic weight and the intensity of the emitted secondary Röntgen radiation. In each case an increase in atomic weight was accompanied by an increase in the amount of secondary radiation. Kleeman has obtained a similar result in the case of the secondary radiation produced by the γ rays from radium. Benoist in 1901, working with the absorption by various elements of a definite beam of Röntgen rays, obtained a smooth curve approximating to a rectangular hyperbola by plotting atomic weight against a factor related to λ/ρ (i. e., the absorption of unit mass per unit area), where ρ is the density of the screen, and λ is the coefficient of absorption. λ is defined by the exponential relation for a homogeneous beam I=I 0 e ~ xd , in which I 0 is the intensity of the incident beam, and I that of the emergent beam from a layer of thickness d . It follows from Benoist ’s curve that λ/p increases with the atomic weight, and more rapidly in the region of low atomic weights. Crowther measured the absorption by different elements of the ß rays from uranium, and obtained a periodic relation between atomic weight and λ/p .