116 Dr. J. Stark on the Theory of Ionization bij 



the conclusions hithen*to reached may be sunmuirized as 

 follows : — 



(1) A radioactive gas or emanation can be obtained by 

 drawing air over hot cop])er, or by bubbling it through hot or 

 cold mercury. 



(2) By repeated circulation through mercury very consi- 

 derable activity can be obtained, activity of quite a difterent 

 order from that of metals as ordinarily observed. 



(3) The mercury emanation deposits radioactive matter on 

 the walls of the vessel containing it. This deposit remains 

 after blowing ont the gas, and possesses at first perhaps \ 

 the activity of the latter. This induced activity falls to half 

 value in 20 minutes. 



(4) The emanation itself decays in activity according to an 

 exponential law^ falling to half value in 3*18 days (24 hours 

 each) . 



XV. liemarks on the Theory of Ionization by Collision of Ions 

 loith JSeidral Molecules, 



To the Editors of the Pldlosopldcal Magazine. 



Gentlemen, 



A. SCHUSTER-^ (1884) suggested the view that electric 

 particles, ions, produced new ions out of neutral gas mole- 

 cules by collision. Ph. Lenardf (1894") found that cathode- 

 rays gave conductivity to a gas through which they passed ; 

 some years afterwards these rays were recognized as ne- 

 gatively charged particles. J. J. Thomson | and J. S. 

 Townsend § have (1900) used that view of ionization by 

 collision of ions ; the first explained by its aid some phe- 

 nomena of the glowing discharge, the second the important 

 observation made independently by himself that with low 

 gas-pressure and secondary ionization the current, after 

 having attained saturation, increases again if the potential- 

 difference of the electrodes is increased. This observation 

 is already contained in Stoletow's || researches on the photo- 



* A. Schuster, Proc. Roy. Soc. xxxvii. p. 336 (1884). 



t Ph. Lenard, Wied. Ann. li. p. 240 (1894). 



X J. J. Thomson, Nature, Ixi. March 1900, p. 458; Phil. Mag. (5) 1. 

 p. 279 (1900), (6) i. p. 308 (1901). 



§ J. S. Townsend, Nature, Ixii. August 1900, p. 340 ; Phil. Mag. (6) 

 i. p. 198 (1901), iii. p. 557 (1902) ; Townsend andKirkby, Phil. Mag. ((3) i. 

 p. 630 (1901). 



II A. Stoletow, Jouni. de Phys. (2) ix. p. 468 (1890). 



