S8 Ml" Crowiher, On the Relative Ionization 'produced hy 



On the Relative Ionization produced by Rontgen Rays in 

 different Oases. By J. A. Crowther, B.A., St John's College. 

 [Communicated by Sir J. J. Thomson.] 



[Bead 23 November 1908.] 



It may be of interest to place on record the following results 

 which have been obtained during the course of a series of experi- 

 ments, still in progress, on the passage of Rontgen rays through 

 gases. The apparatus was that described in a previous paper*. 

 A beam of Rontgen rays was fired through the gas, between two 

 parallel plate electrodes, care being taken to avoid any portion of 

 the beam falling upon either of the electrodes. If this precaution 

 is neglected very different values are obtained owing to the large 

 amount of soft secondary j3 radiation given off by the electrodes 

 under the action of the primary rays. 



The measurements given under the column headed "soft 

 rays " were made with the softest rays which would produce any 

 appreciable amount of ionization, and the aluminium window was 

 made very thin (about -^ mm.) in order to cut off as little as 

 possible of the soft radiation. The measurements labelled " hard 

 rays " were made with the bulb as hard as it was possible to work 

 it with a Rudge induction coil worked by a turbine mercury 

 interrupter. 



It has been suggested that the relative ionization would tend 

 to follow a density law as the rays became harder. The results 

 obtained in the present experiments do not give any indication of 

 such a result. Compared with air as a standard, methyl iodide, 

 methyl acetate and carbon dioxide shew a decrease as the rays 

 get harder in the relative amount of ionization produced ; hj'drogen 

 and ethyl bromide, on the other hand, give a distinct increase ; 

 while ethyl chloride and carbon tetrachloride remain nearly 

 constant. The exceedingly small value obtained for hydrogen 

 with very soft rays (only 1 °/^ of that of air at the same pressure) 

 is very remarkable. The increase in the case of ethyl bromide, 

 the value for which for soft rays is already considerably above 

 that required on a density law, may also be noticed. 



* Proc. Camb. Phil. Soc. Vol. xv. p. 34, 1908. 



