284 Prof. Rutherford and Mr. Royds on the Nature 



experimentally. At intervals after the introduction of the 

 emanation the mercury was raised, and the gases in the outer 

 tube spectroscopically examined. After 24 hours no trace 

 of the helium yellow line was seen ; after 2 days the helium 

 yellow was faintly visible ; after 4 days the helium yellow 

 and green lines were bright ; and after 6 days all the stronger 

 lines of the helium spectrum were observed. The absence 

 of the neon spectrum shows that the helium present was not 

 due to a leakage of air into the apparatus. 



There is, however, one possible source of error in this 

 experiment. The helium may not be due to the a particles- 

 themselves, but may have diffused from the emanation 

 through the thin walls of the glass tube. In order to test 

 this point the emanation was completely pumped out of A r 

 and after some hours a quantity of helium, about 10 times 

 the previous volume of the emanation, was compressed into- 

 the same tube A. 



The outer tube T and the vacuum-tube were removed and 

 a fresh apparatus substituted. Observations to detect helium 

 in the tube T were made at intervals, in the same way as- 

 before, but no trace of the helium spectrum was observed 

 over a period of eight days. 



The helium in the tube A was then pumped out and a 

 fresh supply of emanation substituted. Results similar to* 

 the first experiment were observed. The helium yellow 

 and green lines showed brightly after four nays. 



These experiments thus show conclusively that the helium 

 could not have diffused through the glass walls, but must- 

 have been derived from the a. particles which were fired 

 through them. In other words, the experiments give a 

 decisive proof that the a particle after losing its charge is art 

 atom of helium. 



Other Experiments. 



We have seen that in the experiments above described 

 helium was not observed in the outer tube in sufficient 

 quantity to show the characteristic yellow line until two days 

 had elapsed. Now the equilibrium amount of emanation 

 from 100 milligrams of radium should produce helium at the 

 rate of about *03 c.mm. per day. The amount produced in 

 one day, if present in the outer tube, should produce a bright 

 spectrum of helium under the experimental conditions. It 

 thus appeared probable that the helium fired into the glass 

 must escape very slowly into the exhausted space, for if the 

 helium escaped at once, the presence of helium should have 



