excited by the a Rays of Polonium. 407 



gradually increased from zero upwards. The curves drawn 

 on the first half of fig. 4 and of fig. 5 show a remarkable 

 difference from the corresponding curves in fio-s. 2 and 3. 

 The curves of figs. 2 and 3 drop gradually with the time 

 and come to a constant value, while the curves of figs. 4 and 

 5 first decrease and then increase to an approximately 

 constant value. The only difference in the conditions under 

 which the experiments were made was a difference in the 

 temperature of the carbon. When the carbon was at room 

 temperature or at a temperature of 110° C. the rate of 

 charging of the polonium decreased with the time before 

 becoming constant, while when the carbon was at —78° C. 

 or at —192° 0. the rate of charging decreased slightly and 

 then increased. The cause of this difference is readily seen 

 when it is remembered that carbon occludes more air at a 

 low temperature than at a high, and that the a, rays excite 

 secondary rays in the air layer on the surface of the carbon as 

 well as in the carbon itself*. When the temperature of the 

 carbon is high the air readily comes away from the surfac3 

 of the carbon, and hence the secondary radiation decreases. 

 When the temperature of the carbon is low the air does not 

 come away, but becomes denser and denser at the surface 

 of the carbon as it oozes out of the interior, and also as it 

 comes out of the cooled brass walls of the apparatus. (It 

 will be shown later that air is expelled from brass as it is 

 cooled.) Therefore, instead of a decrease there is set up a 

 gradual increase in the secondary radiation as the air layer 

 becomes denser. 



The final value of the secondary radiation from the carbon 

 and the air layer at its surface was obtained in each case by 

 taking the difference between the constant rates of charging 

 of the polonium without and with the applied magnetic field. 

 In the following table the values of the secondary radiation 

 with the carbon at the several different temperatures are set 

 down. 



Table V. 



Fresh Carbon in Air. 

 Secondary Radiation from Carbon and Air Layer. 



Temperature of Carbon... 110° C. 23° C. -78° C. -191° C. 

 Secondary Radiation -124-5 -120-6 -151 -228 



It will be seen from the table that when the carbon was 



* Pound ; Trans. Canadian Institute, 1912; Phil. Mag. May 1912. 

 2 E 2 



