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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[1] and by Owens[2]. 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[3] 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

  1. ↑ Rutherford, Phil. Mag. Jan. 1899.
  2. ↑ Owens, Phil. Mag. Oct. 1899.
  3. ↑ Rutherford and Miss Brooks, Phil. Mag. July, 1900.
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