Page:Radio-activity.djvu/184

 a distance from the source will increase with the thickness of the layer of active matter, but will reach a maximum for a layer of a certain thickness. The greater proportion of the ionization, due to unscreened active matter, is thus entirely confined to a shell of air surrounding it not more than 10 cms. in depth.

Fig. 35.

99. The α rays from different compounds of the same active element, although differing in amount, have about the same average penetrating power. Experiments on this point have been made by the writer and by Owens. Thus in comparing the relative power of penetration of the α rays from the different radio-elements, it is only necessary to determine the penetrating power for one compound of each of the radio-elements. Rutherford and Miss Brooks have determined the amount of absorption of the α rays from the different active substances in their passage through successive layers of aluminium foil ·00034 cm. thick. The