RADIO-ACTIVITY 191 



active change are excessively minute, and no other 

 method at present known enables us to detect the 

 final inactive products as they are formed. It 

 is, however, not improbable that, by the slow 

 accumulation of material which must of necessity 

 go on when a radio-active body is kept for a 

 long time, the inactive products will be obtained 

 eventually in amounts sufficient to be distin- 

 guished by the spectroscope or even by ordinary 

 chemical analysis. In this connection attention 

 was soon called to the fact that in all radio-active 

 minerals lead is found and considerable quantities 

 of helium gas are occluded. 



Sir William Ramsay and Mr Soddy, by 

 spectroscopic methods, detected helium in the 

 gases evolved from a sample of radium, originally 

 prepared from pitch-blende and kept as a solid 

 for some months. The spectrum of helium was 

 invisible when the emanation was first collected 

 and examined, but soon appeared and gradually 

 increased in intensity with the lapse of time. 



Similar results were then obtained by Dewar 

 and Curie, who, moreover, traced the disappear- 

 ance of a minute volume of the emanation. This 

 has been explained by the idea that the resulting 

 helium, being projected in the atomic state with 

 great velocity, penetrated the glass walls of the 

 vessel and thus occupied no volume. The decrease 

 in the volume of a minute quantity of emanation 

 was also observed by Ramsay and Soddy. 



This question, however, has been settled finally 

 by the researches of Rutherford and his fellow- 

 workers. Measurements of the maofnetic and 

 electric deflection of the a rays had indicated that 

 the a particles ejected by any single active sub- 



