negative charge communicated to P by a definite amount. A few of the numbers are given below, and expressed in terms of tin as unity.
| Substance | Thickness Tin = 1 |
|---|---|
| Lead | ·745 |
| Gold | ·83 |
| Platinum | ·84 |
| Silver | 1 |
| Steel | 1·29 |
| Aluminium | 1·56 |
| Water | 1·66 |
| Paraffin | 1·69 |
The thickness required to stop a given proportion of the β rays thus decreases with the density, but not nearly so fast as the density increases. These results are difficult to reconcile with the density-law of absorption found by Lenard from the cathode rays, or with the results of the ionization method already considered. A further experimental examination of the whole question is very much to be desired.
86. Variation of the amount of radiation with the thickness of the layer of radiating material. The radiations are sent out equally from all portions of the active mass, but the ionization of the gas which is measured is due only to the radiations which escape into the air. The depth from which the radiations can reach the surface depends on the absorption of the radiation by the active matter itself.
Let λ be the absorption constant of the homogeneous radiation by the active material. It can readily be shown that the intensity I of the rays issuing from a layer of active matter, of thickness