■ 372 Mr. Briggs on Distribution of Active Deposits of 



root of the molecular weight of the gas through which diffu- 

 sion takes place. The experiments with radium and thorium 

 emanations were made at a pressure of 20 cm., the time of 

 exposure being generally greater than 16 hours, and at 

 -a voltage of 720, except in the case of hydrogen sulphide, 

 :for which consistent results for either emanation or in the 

 experiments with RaB were only obtained when voltages as 

 high as 10,000 were used. Zinc electrodes were used for 



■ the gases ammonia, acetylene, and hydrogen sulphide. 

 Each result given in Table II. is the mean of several expe- 

 riments. It was found necessary to liquify and fractionate 



the gases methane and ethylene to free them from the 

 vapours of organic compounds produced during their pre- 

 par .tion. Hydrogen sulphide was obtained by heating 

 a solution of magnesium hydrosulphide, and was also 

 fractionated. 



VIII. Actinium Active Deposit. 



The experiments of Kennedy*, Lucian f, or of McKeehan J 

 might be used to test the theory given above for the case of 

 actinium emanation. Lucian's experiments are by far the 

 most suitable for this purpose, since direct recoil on to 

 the electrodes was negligible in his but not in the expe- 

 riments of the other two. Lucian's method was very similar 

 to that used by Wellish for radium active deposit. In 

 section V. it has been shown that the number of neutral 

 .atoms of Ac A diffusing to the walls of a vessel with parallel 

 plates 2 cm. apart, before transforming into AcB, is 1*8 per 

 cent, at atmospheric pressure. The number so diffusing, in 

 the vessel used by Lucian, which was a cylinder of 4'9 cm. 

 diameter with a central rod as cathode, will be of the same 

 order of magnitude and may be neglected. By means of a 

 formula given by Lucian, the amount of AcA positively 

 • charged which reaches the cathode before transforming 

 into AcB may be calculated. Taking 1*2 for the mobility 

 •of AcA, the fraction of the AcA positively charged reaching 

 the cathode in Lucian's vessel, before transforming into AcB, 

 is 14 per cent, at 600 volts, 1 7 at 980 volts, and 20 at 

 1700 volts. Let us consider 100 molecules of actinium 

 -emanation. The first transformation produces 100 — 82*4 

 or 17"6 initially neutral ;itoms of AcA. Practically all of 

 these break up before diffusing to the walls, and 100 — 93 

 or 7 per cent, of them, i. e. 1*23, become neutral atozns 



* Kennedy, Phil. Mag. xviii. p. 744 (1909). 

 + Lucian, Phil. Mag. xxviii. p. 761 (1914). 

 % McKeehan, Phys. Rev. x. p. 473 (1917). 



