excited by the a, Rays of Polonium. 403 



became fairly constant. This constant value, as shown in 

 the previous paper*, was reached when the density of the 

 layer of gas occluded in the surface of the carbon was in 

 equilibrium with the pressure of gas in the vessel, that is, 

 when there was no readjustment going on in the gaseous 

 layer. After the rate of charging of the polonium had 

 become fairly constant a weak magnetic field was established 

 by passing a small current through the electromagnet, and 

 the rate of charging of the polonium was again found. Then 

 larger and larger currents were sent through the electro- 

 magnet, and readings were taken until the rate of charging 

 of the polonium became constant. This last constant charge 

 which came to the polonium was composed of the ol ray 

 current leaving the polonium and the ionization current 

 through the gas. The magnetic field was used to deflect the 

 slow-moving secondary rays coining from the carbon, and 

 the constant rate of charging of the polonium denoted that 

 the magnetic field had deflected them all. The influence of 

 the magnetic field in deflecting the a rays or the ionization 

 current through the gas was probably very small. Accord- 

 ingly the difference between the first constant value of the 

 rate of charging of the polonium and the last constant value 

 was a measure of the magnitude of the secondary ray current 

 from the carbon excited by the a ray bombardment. 



In the following tables the results obtained with air-filled 

 carbon, fii^t at a temperature of 110° C, second at room 

 temperature, third at a temperature given by a mixture of 

 solid carbonic acid gas and ether, namely —78° C, and, 

 lastly, at liquid-air temperature — 192° C, are given. After 

 each experiment air was allowed to remain in the apparatus 

 at atmospheric pressure until the next experiment. Before 

 beginning the set of experiments a piece of carbon, the 

 surface of which was freshly cut and which had never before 

 been exposed to a rays, was placed in the appai^atus. For 

 comparison all the readings were reduced to the same date 

 by the use of the standard equation 



the half decay period of the polonium being taken as 140 

 da) sf. 



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

 f See ' Physical and Chemical Constants/ by Kaye and Labv, 

 p. 107. 



