1912] Secondary Rays Excited by the Alpha Rays. 193 



From the numbers given in the table it will be seen that immediately 

 after the brass began to cool down to the temperature of the liquid air 

 the pressure in the apparatus rose slightly and then fell again as the 

 pumping was continued. The rate at which the polonium charged up, 

 however, steadily decreased as the brass plate cooled down. As the 

 effect of a rise in pressure would be to increase the ionisation current in 

 the chamber it follows from the occurrence of this decrease in the rate 

 of charging of the polonium that the secondary radiation from the brass 

 plate must have dropped off as its temperature lowered. 



The values of the a ray excited secondary radiation from the 

 brass plate at temperatures 20° C and -192° C as deduced from Tables 

 XIV and XV are given in Table XVI. 



Table XVI. — Brass in Air. 

 Secondary Radiation from Brass and Air Layer. 

 Temperature of Brass .... 20° C -192° C 



Secondary Radiation -215 -I73 



From these numbers it is evident that the secondary radiation 

 from the brass at a temperature of 20° C was about 25% higher 

 than it was under the same a ray bombardment at the temperature of 

 liquid air. 



If differences in " ray excited secondary radiation at low pres- 

 sures 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 brass 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 sup- 

 ports 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 temper- 

 ature of the room. 



V. — Summary of Results, 



I. The secondary radiation excited by the a rays of polonium in car- 

 bon was found to increase in intensity as the temperature of the carbon 

 was lowered from room temperature to the temperature of liquid air. 



I I . This increase in the secondary radiation from carbon as its tem- 

 perature was lowered has been shewn to be due to an increase in the 

 amount of gas occluded in the surface of the carbon. 



