Page:Radio-activity.djvu/570

548 measured. The results are shown graphically in Fig. 107, where the ordinates represent the distance of the phosphorescent screen from the active wire, and the abscissae the number of layers of aluminium foil, each ·00031 cms. thick.

Fig. 107.

It is seen that the curve joining the points is a straight line. 12·5 thicknesses of foil absorbed the rays to the same extent as 6·8 cms. of air, so that each thickness of aluminium corresponded in absorbing power to ·54 cms. of air. For a screen of zinc sulphide, the phosphorescent action ceased at a distance of air of 6·8 cms., showing that the photographic and phosphorescent ranges of the α rays in air were practically identical.

The experiments with barium platino-cyanide and willemite were more difficult, as the β and γ rays from the active wire produced a luminosity comparable with that produced by the α rays. Fairly concordant results, however, were obtained by introducing a thin sheet of black paper between the active wire and the screen. If the luminosity was sensibly changed, it was concluded that the α rays still produced an effect, and in this way the point of cessation of phosphorescent action could be approximately determined. For example, with eight thicknesses of foil over the active wire the additional thickness of air required to cut off the phosphorescent effect of the a rays was 2·5 cms. for willemite, and 2·1 cms. for barium platino-cyanide.