Recoil Atoms in Ionized Air. 72.3 



increased as the distance was decreased, the potential 

 difference being constant. In these experiments no external 

 ionizing agent was employed. Recoil experiments were also 

 made using plates of different polish for the active deposit 

 plates from which the recoil atoms were ejected. The 

 number of recoil atoms collected on a negatively charged 

 electrode was greater from the surfaces of higher polish. 



These experiments were carried out two years ago in 

 Professor Rutherford's laboratory. It was the hope of the 

 writer to continue them, but after leaving England it was 

 found impossible to do so at once. Since work in this par- 

 ticular field is at present attracting other investigators, it whs 

 decided to publish the results obtained in these preliminary 

 experiments. 



Experimental Method. 



The experiments consisted in collecting actinium D by recoil 

 from the actinium active deposit. The air through which 

 the recoil atoms had to pass was ionized by the a-particles 

 from radium emanation and its products enclosed in a glass 

 tube whose walls were thin enough to permit the a-particles 

 to pass through. 



Two metal plates, large enough to ensure a uniform field 

 at the centre, were held apart by an ebonite plate having a 

 circular hole through the centre. The size of the hole was 

 the same for all the distances used. The lower of the two 

 plates was exposed to actinium emanation, so that the area of 

 the active deposit was considerably smaller lhan the opening 

 in the ebonite. In the recoil experiments the lower plate 

 was charged positively, while the upper plate, which served 

 to collect the recoil atoms, was charged negatively. The 

 exposure to the actinium emanation was at least five hours 

 with a potential difference of about 200 volts. In order 

 to be certain that the amount of the active deposit was the 

 same in all the experiments, the a-radiations and the /S- 

 radiations were measured at convenient times and their 

 values reduced to the time of removal from the emanation 

 These values, respectively, did not vary more than 8 per cent, 

 for the different experiments. It was found necessary to 

 use the same plate on which to collect the active deposit in 

 all the experiments. 



The time for the exposure in the recoil experiments was 

 twelve minutes, starting immediately after removing the 

 plate with the active deposit from the emanation. The plate 

 with the actinium D obtained by recoil was then placed in a 

 definite position under a ft-n\y electroscope, and a decay 



