Production of Helium l>y Padium. 601 



off, and the gases were pumped out and collected. During 

 the pumping operation the tube was heated so that the 

 sulohur melted. The tube was allowed to cool and a small 

 amount o£ pure oxygen was admitted. This was then pumped 

 out and added to the gases already obtained. The gases 

 were introduced into the burette B (rig. 2), were passed over 

 the heated copper oxide, introduced into the measuring por- 

 tion of the apparatus, and the volume of the helium was 

 determined in the usual manner. The gas measured was 

 then transferred to the spectrum tube S (fig. 2), where its- 

 spectrum was examined and found to be that of pure helium. 

 The volume of the helium found was 0*202 cubic millimetre. 

 The amount of radium emanation in the tube at the start 

 was the quantity in equilibrium with 0*126 gram of radium. 

 The production of helium at first would therefore be at the 

 rate of 0*126 x 3 x 0*107 cub. mm. per day ; the factor 3 being- 

 introduced because of the three a-ray changes (emanation, 

 radium A, and radium C). Owing to the disintegration of 

 the emanation the production of helium will decrease, how- 

 ever, and the total amount formed will be proportional 



to -(l — e~ Kt ), where X is the disintegration constant and 



t is the time. Taking the value of \ corresponding to a half- 

 value period of 3*85 days for the emanation, and taking the 

 day as the unit of time, the above expression gives 5*42 ; 

 i. e. the amount of helium produced in 21 days will be 



0*126 x 3 x 0*107 x 5*42 = 0220 cub. mm. 



The amount of helium found (0*202 cub. mm.) is in fair agree- 

 ment with the amount to be expected considering the 

 experimental difficulties. 



Production of Helium by Polonium, 



A determination was also made of the production of 

 helium by polonium. The sulphides of lead, bismuth, and 

 antimony obtained in the purification of the radium salt 

 (p. 589) were decomposed by digesting with strong hydro- 

 chloric acid containing a small quantity of potassium chlorate. 

 The solution was heated until the excess of hydrochloric acid 

 was largely removed, and was diluted with weak hydro- 

 chloric acid. This solution was poured into a strong solution 

 of ammonium sulphide. The precipitated sulphides of lead 

 and bismuth were removed and were again treated with 

 hydrochloric acid and potassium chlorate. An excess of 

 sulphuric acid was added to this solution. The mixture was 

 concentrated by evaporation and heated until the fumes of 



