130 Mr. L. Simons on the Beta-Ray Emission from 



placed in order of atomic number along the horizontal axis 

 of fig. 7 (PL III.). This process is repeated for other 

 distances from the screens. Thus, since the curves figs. 5 

 and 6 (PI. III.) refer to equal total ionizations, a high 

 ordinate on fig. 7 (PI. III.) at any point indicates a larger 

 proportion of higher velocity electrons passing that plane, 

 and vice versa. 



The question arises that the marked variations in the 

 shapes of the curves of figs. 5 and 6 (PL III.) might be due 

 entirely to the ionization in the chamber by the variable 

 tertiary X-radiation from the successive screens, superposed 

 on the ionization by the ^-radiations which has a uniform 

 function for each of the screens. Some of the reasons for 

 this view being un I enable are set down below. Though this 

 tertiary X-radiation must be present, the ionization produced 

 by it, in comparison with that produced by the ^-radiations 

 from the screens, must be negligibly small : Barkla and 

 Shearer neglected this effect in their work already cited. 



(1) The total ionization in the air in the chamber with 



carbon ends produced by the incident beam of 

 X-rays is quite small, even at atmospheric pressure, 

 in comparison with that produced by the ^-radiations 

 from the screens, and it is presumed that the effect 

 of the tertiary X-radiation from the screen would 

 only be a fraction of that due to the secondary 

 X-radiation. 



(2) The linear portions of the curves in fig. 3 (PL III.) 

 could not be parallel to each other if the effect of 

 tertiary radiation were appreciable. The greater the 

 effect of tertiary radiation, the more sloping would 

 these lines become. 



(3) For a given substance, the thinner the radiating film, 

 the smaller will be the ratio of ionization by tertiary 

 X-rays to that by /3-rays in a comparatively thin 

 layer of air. The absorption coefficient of the /3-rays 

 in the radiating film itself is extremely great, whilst 

 that of the tertiary X-rays is comparatively small. 

 If a film were prepared of such a thickness as just 

 to absorb the /3-rays generated at its face in contact 

 with the graphite, increasing the thickness would not 

 increase the tertiary X-ray emission. 



(4) The apparent independence of the shape of the curves 



in fig. 7 (PL III.) on the relative dimensions of the 

 curves in figs. 3 and 4. If the ionization by the 

 tertiary X-rays from the screens were finile, it would, 

 of course, be included in the dimensions of the curves 

 figs. 3 and 4 (PL III.). 



