Since the β rays from uranium are fairly homogeneous, and are at the same time penetrating in character, they are more suitable for such a determination than the complex rays of radium. I have in consequence made some experiments with uranium rays to determine the dependence of absorption on the density. The results obtained are given in the following table, where λ is the coefficient of absorption. Substance λ Density λ/Density Glass 14·0 2·45 5·7 Mica 14·2 2·78 5·1 Ebonite 6·5 1·14 5·7 Wood 2·16 ·40 5·4 Cardboard 3·7 ·70 5·3 Iron 44 7·8 5·6 Aluminium 14·0 2·60 5·4 Copper 60 8·6 7·0 Silver 75 10·5 7·1 Lead 122 11·5 10·8 Tin 96 7·3 13·2 It will be observed that the value of the absorption constant divided by the density is very nearly the same for such different substances as glass, mica, ebonite, wood, iron and aluminium. The divergences from the law are great, however, for the other metals examined, viz. copper, silver, lead and tin. In tin the value of λ divided by the density is 2·5 times its value for iron and aluminium. These differences show that a law for the absorption of the β rays depending only on the density does not hold for all substances. With an exception in the case of tin, the value of λ divided by the density for the metals increases in the same order as their atomic weights. The absorption of the β rays by matter decreases very rapidly with increase of speed. For example, the absorption of cathode rays in Lenard’s experiment (_loc. cit._) is about 500 times as great as for the uranium β rays. The velocity of the β rays of uranium was found by Becquerel to be about 1·6 × 10¹⁰ cms. per sec. The velocity of the cathode rays used in Lenard’s experiment was certainly not less than ⅒ of this, so that, for a decrease of speed of less than 10 times, the absorption has increased over 500 times. =85. Number of electrons stopped by matter.= An account will now be given of the experiments made by Seitz[136], to determine the relative number of electrons which are stopped in their passage through different thicknesses of matter. The experimental arrangement is shown in Fig. 31. [Illustration: Fig. 31.] The radium was placed outside a glass vessel containing an insulated brass plate _P_, the connection of which with a wire leading to the electrometer could be made or broken by a simple electromagnetic device. The β rays from the radium _R_, after passing through openings in a brass plate _A_, covered with thin aluminium foil, were absorbed in the plate _P_. The glass vessel was exhausted, and the charge communicated to _P_ by the β rays was measured by an electrometer.