excited by tlie a. Rays of Polonium. 411 



Table XII I". 



Fresh Carbon in Hydrogen. 



Secondary Radiation from Carbon and Hvdrooen Laver. 



Temperature of Carbon 24° C. - 1 92° C. 



Secondary Radiation —1-17 —233 



Here, again, we find that both with oxygen and with 



hydrogen the results show that the secondary radiation was 

 much greater when the carbon was at a low temperature than 

 when it was maintained at a higher temperature. 



From the foregoing it is clear that this modification of the 

 intensity of the secondary radiation is attributable, as in the 

 case of air, to an increase in the amount of gaseous oxygen 

 and hydrogen occluded in the surface of the carbon by a 

 reduction of the temperature. 



From the experiments just described it follows that the 

 amount of secondary radiation obtained from an electrode 

 uuiler bombardment by a rays may be taken as a measure of 

 the density cf the gaseous layer at the surface of the elec- 

 trode when the latter is placed in a gas at a very low pressure. 

 If there were no difference in the secondary radiation from 

 a substance at different temperatures under the conditions 

 mentioned it would indicate either that there was no gaseous 

 layer at the surface of the substance bombarded, or else that 

 the density of the gaseous laver adhering to it was the same 

 at all temperatures. In the following section an experiment 

 is described which makes use of this conclusion. 



4. Experiments with Brass. 

 A peculiar effect was observed during the course of all 

 the experiments, namely, that it took a longer time to reduce 

 the pressure of the gas in the apparatus to a low value when 

 the walls of the vessel were cooled with liquid air, than when 

 the walls were at ordinary room temperature. At first this 

 effect was supposed to be due to some leak in the apparatus, 

 and the vessel was removed and carefully tested under pres- 

 sure for small holes or porous places in the brass. But 

 invariably it was found that no such holes or porous places 

 could be discovered. It was also found that when the 

 apparatus was put back and exhausted at ordinary tempera- 

 tures the pressure was reduced with the same speed as before 

 the apparatus was cooled with liquid air. There seemed, 

 therefore, to be no leak of air through the walls of the 



