2 Ri'TilKRFORD & ROVDS, TJic Nature of the a Particle. 



amount from 150 mgs. of radium, was compressed by 

 raising a column of mercury into a fine glass tube about 

 r5 cms. long. The walls of this glass tube were 

 sufificientl}' strong to withstand atmospheric piessure but 

 thin enough to allow the greater part of the expelled a 

 particles to be fired through them. After a number 

 of trials, Mr. Baumbach succeeded in blowing a number 

 of such fine tubes for us. The emanation tube was 

 surrounded by a larger cylindrical glass tube about 8 cms. 

 long and 1-5 cms. diameter. This was first exhau.sted by a 

 pump and the exhaustion completed by means of a 

 charcoal tube immersed in licjuid air. By means of 

 another side tube connected with a mercury reservoir, the 

 gases formed within the outside tube could be compressed 

 into a small vacuum tube attached to the top aiid their 

 spectra examined. 



The tube containing emanation was about l/iOO 

 mm. thick. The stopping power of the glass for the a 

 particle corresponded to less than 2 cms. of air, so that 

 the a particles expelled from the emanation itself, radium 

 A and radium C escaped through the emanation tube, 

 and were fired into the walls of the outer glass tube. 

 Twenty-four hours after the introduction of the emana- 

 tion, no trace of helium was detected on compression of 

 the gases into the vacuum tube ; at the end of two days 

 the helium yellow line was seen faintly ; after four days, 

 the yellow and green lines came out brightl)^ and after 

 six days practically the whole helium spectrum was 

 observed. 



An experiment was then made to test whether the 

 helium observed could have diffused from the emanation 

 through the thin glass walls. For this purpo.se, the 

 emanation was rei)laced by about ten times its volume of 

 helium and a new outer tube and vacuum tube placed in 



