364 Mr. Briggs on Distribution of Active Deposits of 



Also, by making use of the fact that RaA, ThA, and AcA are- 

 isotopes, as also are RaB, ThB, and AcB, the difference in the 

 distribution of radium, thorium, and actinium emanations 

 in an electric field may be quantitatively explained. The 

 experiments by which the percentage of RaB initially posi- 

 tively charged in various gases, and the efficiency of recoil 

 of RaB from the electrodes were measured, will now be 

 described. 



The experiments consisted in collecting RaB by recoil 

 from a plate made active with RaA, first in the gas under 

 observation and then in air. This gave the ratio of the 

 fraction of RaB initially positively charged at the end 

 of the recoil path in the gas, to the corresponding fraction 

 for air. In a second experiment the fraction for air was 

 found. 



Two metal plates 6*7 cm. in diameter were separated by a 

 vertical ebonite plate 1*5 cm. thick, having a hole 4*8 cm. in 

 diameter in the centre. Stop-cock grease was used to make- 

 the contact surfaces air-tight. Through a three-way tap, air 

 or the gas under observation having passed through drying 

 and filtering tubes could be admitted. Another three-way 

 tap led to a pump and manometer. When the active plate 

 was negatively charged, or it there was no field between 

 the plates, and the vessel filled with a dry dust-free gas, the 

 opposite plate received a negligible amount of RaB, which 

 was probably chiefly due to the initial disturbance caused by 

 exhausting and filling the vessel, and not to the presence of 

 negative RaB atoms. All the gases used in these experiments 

 were tested for negative atoms by means of exposures in 

 cylindrical vessels, in which the gas was mixed with radium 

 emanation, and in all cases the activity on the central rod 

 when positive was no greater than the amount that should 

 have reached it by the diffusion of neutral atoms. There is 

 no evidence, therefore, of the presence of either negative RaA 

 or RaB atoms. In the experiment with the plates uncharged 

 a layer of neutral RaB atoms is formed close to the active 

 plate, and the result obtained shows that praclic ally all the 

 neutral atoms return to this plate. A large fraction, depending 

 on the gas in the vessel, of the total activity, was deposited on 

 the cathode when the active plate was positive. It was found 

 that this amount with the pressure of the gas at 15 cm. did 

 not increase perceptibly when the voltage was increased from 

 160 to 720. 



In an experiment three plates were used. Plate (1) was 

 activated with RaA by exposure to radium emanation for 

 about half a minute, the portion activated being an area 



