to a variation in both these quantities.
Since the α rays differ either in mass or velocity, it follows that they cannot be ascribed to any single radio-active impurity common to all radio-active bodies.
100. Absorption of the α rays by gases. The α rays from the different radio-active substances are quickly absorbed in their passage through a few centimetres of air at atmospheric pressure and temperature. In consequence of this, the ionization of the air, due to the α rays, is greatest near the surface of the radiating body and falls off very rapidly with the distance (see section 98).
A simple method of determining the absorption in gases is shown in Fig. 36. The maximum current is measured between two parallel plates A and B kept at a fixed distance of 2 cms. apart, and