Production of Helium by Radium. 595 



were pumped out through the large pump. The stop-cock a 

 was closed, liquid air was again placed around the tube C 

 and the mercury in A was lowered to the point a. The 

 gases in A were in this manner again exposed to the cooled 

 charcoal, and after an interval of about thirty minutes the 

 mercury was raised in A and the volume and pressure of 

 gas in E was again determined. Following this, the uncon- 

 densed gases in the tube C were transferred into A, the 

 liquid air was removed and the tube ( 1 was warmed up. Any 

 gases condensed in C were pumped out through the large 

 pump. This cycle of operation was repeated, a total of five 

 times giving a series of five separate measurements of 

 pressures and volumes. The results of the second and third 

 showed a slight diminution in the values, but the hist three 

 were in satisfactory agreement within limits of 1 per cent. 

 and indicated that the purification of the helium had been 

 carried as far as it could be by treatment with the cooled 

 charcoal. The helium in the apparatus was then introduced 

 into the spectrum tube S and its spectrum examined. It 

 was found to be essentially pure helium, in this experiment 

 no other precautions had been taken to remove the hydrogen 

 from the mixture of gases evolved by the radium salt except 

 the sparking of the mixture with oxygen and the exposure of 

 the residual gases to cooled coconut charcoal in lie manner 

 just. described. The copper-oxide tube T was not attached to 

 the apparatus until after the first determination had been 

 made. As it is well known, from the valuable researches 

 of Sir James Dewar and others, that tie treatment with 

 charcoal alone is not efficient in removing the last traces of 

 hydrogen under these conditions, an experiment was carried 

 out to determine to what extent the presence of free hydrogen 

 might have affected the results. A small volume of hydrogen 

 was therefore introduced into the apparatus and put through 

 the same series of operations as had been carried out with the 

 gases obtained from the radium salt. It was found that the 

 gas remaining after the first treatment with cooled charcoal 

 became rapidly less, and after three cycles of exposure 

 decreased to an amount which was under 1 per cent, of the 

 volume of the helium obtained from the radium salt. It 

 seemed improbable, therefore, that any serious error could 

 have arisen on this account. 



Another matter which required attention was the question 

 as to whether the helium produced in the radium salt had 

 been completely set free on heating the salt. Before the 

 salt was heated, the tube containing it was carefully ex- 

 amined to determine what proportion, if any, of the radium 



