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LVIII. On the Specific Ionization produced by the Corpuscles 

 given out by Radium. By J. J. E. DuRACK, 1851 Exhibi- 

 tion Scholar, Trinity College, Cambridge*. 



IN a former paper f I have shown that the corpuscles in the 

 Lenard-ray stream make on the average 0*4 % positive 

 and negative ions in travelling through one cm. of air at a 

 pressure of one mm. of Hg, or, if we suppose that a corpuscle 

 creates a pair of ions at each collision with the molecules, these 

 corpuscles make 0*4 collision under the conditions of distance 

 and pressure specified above ; this number measures what we 

 may call the specific ionization produced by the corpuscles 

 and will be denoted in what follows by the symbol a ; it must 

 be noted that we are dealing only with the ionization produced 

 in air, a of course depends on the gas ionized. 



Prof. Townsend § has shown that for the corpuscles || pro- 

 duced in air by Rontgen rays the number of ionizing 

 collisions reaches a constant maximum value (equal to 20 H) 

 when the velocity of the corpuscle exceeds a certain amount 

 (about 10 8 cms. per sec). 



I have tried to show ** that the very large difference in 

 the values of a obtained by Townsend and myself can be 

 accounted for by the difference in the velocities of the cor- 

 puscles in the two cases, the velocity of the Lenard rays in my 

 experiments being about 4 10° cms. per sec. 



If the difference be due to the difference in velocity, then, 

 according to the theory indicated, a must decrease as the 

 speed of the corpuscle increases, provided the corpuscle has 

 sufficient energy to produce ions at all the velocities considered. 



Now the velocity of the corpuscles given out by by radium 

 (usually called the deflectable Becquerel rays) has been 



* Communicated by Prof. J. J. Thomson, F.R.S. 



t Phil. Mag. ser. 6, vol. iv. 



\ Starke & Austin (Drude's Annalen. Band ix.) have recently shown 

 that 25 per cent, of the corpuscles in the cathode-rays are reflected on 

 striking the surface of an aluminium plate at perpendicular incidence, 

 this introduces an uncertainty in the value 04 given above, as it is not 

 certain what becomes of the corpuscles after reflexion, they being under 

 the influence of an electric field. If, in my former experiments, they 

 return to the plate kept at constant potential without producing further 

 ionization the number 04 will have to be reduced in the ratio of 4 to 3, 

 i. e. to 0'3 ; at present 04 must be regarded as the probable upper limit 

 of the quantity denoted by a. 



§ Phil. Mag. ser. 6, vol. i. 



j| See a paper by the same author in ' Nature,' vol. lxv. p. 413. 



% Further experiments have shown this number to be from 10 to 20 

 per cent, too large. Townsend, Phil. Mag. ser. 6, vol. iii. 



** Phil. Mag. he. cit. 



