. Radio-activity . ownin Fig. 12 was employed. This was placed on a large lead plate•65 cm. thick the active substance being placed in a closed vesselbeneath. * Villard, G. E. 130, pp. 1010, 1178, Becquerel, C. R. 130, p. 1154, Rutherford, Phijs. Zeit. 3, p. 517, 1902. 12—2 180 NATURE OF THE RADIATIONS [CH, The discharge due to the natural ionization of the air in theelectroscope was first observed. The additional ionization due tothe active substance must be that produced by rays which havepassed through the lead plate and the walls of the following table shows
. Radio-activity . ownin Fig. 12 was employed. This was placed on a large lead plate•65 cm. thick the active substance being placed in a closed vesselbeneath. * Villard, G. E. 130, pp. 1010, 1178, Becquerel, C. R. 130, p. 1154, Rutherford, Phijs. Zeit. 3, p. 517, 1902. 12—2 180 NATURE OF THE RADIATIONS [CH, The discharge due to the natural ionization of the air in theelectroscope was first observed. The additional ionization due tothe active substance must be that produced by rays which havepassed through the lead plate and the walls of the following table shows that the discharge due to these raysdecreases approximately according to an exponential law with thethickness of lead traversed. Thickness of lead Eate of discharge •62 cms. 100 „ + -64 0108. 67 „ +2-86 „ 23 „ +5-08 „ 8 Using 100 grs. of uranium and thorium, the discharge due tothe rays through 1 cm. of lead was quite appreciable, and readilymeasured. The results showed that the amount of 7 rays was. 10 20 _ Thickness of Lead in 44. about the same for equal weights of thorium and uranium penetrating power was also about the same as for the radiumrays. IV] NATURE OF THE RADIATIONS 181 The writer showed that the absorption of the 7 rays fromradium was approximately proportional to the density of thesubstance traversed. A more detailed examination of the absorp-tion of these rays in various substances has been recently madeby McClelland*. The curve (Fig. 44) shows the decrease of theionization current in a testing vessel due to the j3 and 7 rayswith successive layers of lead. It is seen that the /3 rays arealmost completely stopped by 4 mms. of lead; the ionization isthen due entirely to the 7 rays. In order to leave no doubt that all the /3 rays were absorbed,the radium was covered with a thickness of 8 mms. of lead, andmeasurements of the coefficient of absorption X. were made foradditional thicknesses. The average value of \ was calculated
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