of the ol Particle from Radioactive Substances. 285 



been detected a few hours after the introduction of the 

 emanation. 



In order to examine this point more closely the experiments 

 were repeated, with the addition that a cylinder of thin sheet 

 lead of sufficient thickness to stop the a particles was placed 

 over the fine emanation tube. Preliminary experiments, in 

 the manner described later, showed that the lead-foil did not 

 initially contain a detectable amount of helium. Twenty-four 

 hours after the introduction into the tube A of about the 

 same amount of emanation as before, the yellow and green 

 lines of helium showed brightly in the vacuum-tube, and 

 after two days the whole helium spectrum was observed. The 

 spectrum of helium in this case after one day was of about 

 the same intensity as that after the fourth day in the experi- 

 ments without the lead screen. It was thus clear that the 

 lead-foil gave up the helium fired into it far more readily 

 than the glass. 



In order to form an idea of the rapidity of escape of the 

 helium from the lead some further experiments were made. 

 The outer cylinder T was removed and a small cylinder of 

 lead-foil placed round the thin emanation-tube surrounded 

 the air at atmospheric pressure. After exposure for a definite 

 time to the emanation, the lead screen was removed and 

 gested for helium as follows. The lead-foil was placed in a 

 glass tube between two stopcocks. In order to avoid a 

 possible release of the helium present in the lead by pumping 

 out the air, the air was displaced by a current of pure elec- 

 trolytic oxygen*. The stopcocks were closed and the tube 

 attached to a subsidiary apparatus similar to that employed 

 for testing for the presence of neon and helium in the gases 

 produced by the action of the radium emanation on water 

 (Phil. Mag. Nov. 1D08). The oxygen was absorbed by 

 charcoal and the tube then heated beyond the melting-point 

 of lead to allow the helium to escape. The presence of 

 helium was then spectroscopically looked for in the usual 

 way. Using this method, it was found possible to detect 

 the presence of helium in the lead which had been exposed 

 for only four hours to the a rays from the emanation. After 

 an exposure of 24 hours the helium yellow and green lines 

 came out brightly. These experiments were repeated several 

 times with similar results. 



A number of blank experiments were made, using samples 

 of the lead-foil which had not been exposed to the a. rays, 

 but in no case was any helium detected. In a similar way, 



* That the air was completely displaced was shown by the absence of 

 neon in the final spectrum. 



