414 Secondary Rays excited by a, Rays of Polonium. 



Table XVI. 



Brass in Air. 



Secondary Radiation from Brass and Air Layer. 



Temperature of Brass 20° C. -192° C. 



Secondary Radiation —215 —173 



From the numbers given in the table it is evident that the 

 secondary radiation from the brass at a temperature of 20° 0. 

 was about 25 per cent, higher than it was under the same 

 a ray bombardment at the temperature of liquid air. 



If differences in a ray excited secondary radiation at low 

 pressures be taken to connote differences in the quantities of 

 gas occluded at the surface of the substance bombarded, the 

 meaning of this smaller secondary radiation from the bra*s 

 at liquid air temperature is that the brass held less gas in its 

 surface at liquid air temperature than at the temperature of 

 the room. This experiment therefore strongly supports the 

 explanation given above of the greater difficulty experienced 

 in pumping the air from the brass chamber at —192° C. 

 than in making the same exhaustion when the apparatus was 

 maintained at the temperature of the room. 



5. Summary of Results. 



1. The secondary radiation excited by the a. rays of polonium 

 in carbon was found to increase in intensity as the tempera- 

 ture of the carbon was lowered from room temperature to 

 the temperature of liquid air. 



2. This increase in the secondary radiation from carbon as 

 its temperature was lowered has been shown to be due to an 

 increase in the amount of gas occluded in the surface of the 

 carbon. 



3. Since it has been shown that gases occluded in such 

 substances as carbon contribute to the secondary radiation 

 excited at the surface of these substances by a rays, it follows 

 that the procedure adopted in this investigation constitutes a 

 new method of studying the phenomena of occlusion. 



4. The results of the experiments described in this paper 

 also go to show that with a metal like brass the amount of 

 a gas retained in its surface when it is placed in a vacuum is 

 less at the temperature of liquid air than at ordinary room 

 temperature. 



In conclusion, I desire to thank Professor McLennan for 

 the kindly interest he has shown throughout the course of 



this research. 



