We shall first consider the results obtained on the absorption of β rays by measuring the variation of the ionization current, when screens of different thickness are placed over the active substance. When the active matter is covered with aluminium foil of thickness ·1 mm., the current in a testing vessel such as is shown in Fig. 17, is due almost entirely to the β rays. If a uranium compound is used, it is found that the saturation current decreases with the thickness of matter traversed nearly according to an exponential law. Taking the saturation current as a measure of the intensity of the rays, the intensity _I_ after passing through a thickness _d_ of matter is given by $$ \frac {I} {I₀} = e^{–λ d} $$, where λ is the constant of absorption of the rays and _I₀_ is the initial intensity. For uranium rays, the current is reduced to half its value after passing through about ·5 mm. of aluminium. If a compound of thorium or radium is examined in the same way, it is found that the current does not decrease regularly according to the above equation. Results of this kind for radium rays have been given by Meyer and Schweidler[133]. The amount of absorption of the rays by a certain thickness of matter decreases with the thickness traversed. This is exactly opposite to what is observed for the α rays. This variation in the absorption is due to the fact that the β rays are made up of rays which vary greatly in penetrating power. The rays from uranium are fairly homogeneous in character, _i.e._ they consist of rays projected with about the same velocity. The rays from radium and thorium are complex, _i.e._ they consist of rays projected with a wide range of velocity and consequently with a wide range of penetrating power. The electrical examination of the deviable rays thus leads to the same results as their examination by the photographic method. Results on the absorption of cathode rays have been given by Lenard[134], who has shown that the absorption of cathode rays is nearly proportional to the density of the absorbing matter, and is independent of its chemical state. If the deviable rays from active bodies are similar to cathode rays, a similar law of absorption is to be expected. Strutt[135], working with radium rays, has determined the law of absorption, and has found it roughly proportional to the density of matter over a range of densities varying from 0·041 for sulphur dioxide to 21·5 for platinum. In the case of mica and cardboard, the values of λ divided by the density were 3·94 and 3·84 respectively, while the value for platinum was 7·34. In order to deduce the absorption coefficient, he assumed that the radiation fell off according to an exponential law with the distance traversed. As the rays from radium are complex, we have seen that this is only approximately the case.