752 Prof. C. Gr. Barkla and Mr. Gr. Shearer on the 



for L and the very much smaller number of M electrons 

 together; while the Sb and I curves show the ionization- 

 pressure relation for K electrons in addition. The similarity 

 indicates that the K electrons have approximately the same 

 velocity as the L electrons *. Interpreting the above results 

 in the way most unfavourable to this conclusion, — that is 

 assuming a real difference of shape between the Ag and Sn 

 curves and the Sb and I curves, — there is indication that 

 K electrons are emitted with a velocity about 8 per cent, less 

 than that of the L electrons. This is possible, though we 

 hesitate, especially after examination of the results obtained 

 with Cu and As radiations, to say that such a difference has 

 been established in any case. If, for instance, the K 

 electrons had a velocity 10 per cent, less than that of the 

 L electrons, they would be completely absorbed at less than 

 2/3 of the final pressure necessary to absorb the L electrons, 

 and a curve of the shape shown by the broken line in fig. 3 

 would be obtained for a compound radiation of the K and 

 L electrons. The method is therefore a sensitive one, and 

 the close agreement between the actual curves shows that the 

 ionization-pressure curve for K electrons is almost identical 

 with that of the L electrons. The only conclusion we can 

 draw from this is that the velocities of ejection of the K and 

 L electrons do not differ by more than about 8 per cent, if 

 at all. 



Similar experiments were made with the electrons ejected 

 from copper by copper and arsenic radiations of series K. 

 Copper radiation (series K) when incident on copper, pro- 

 duces an emission of L, a smaller number of M electrons, 

 and possibly N" electrons associated with the emission of 

 fluorescent radiations of still lower frequency. The L elec- 

 trons, however, predominate. Arsenic radiation when 

 incident on copper causes the emission of K electrons as 

 well, and in greater number than the L electrons. As the 

 electrons ejected by copper K-radiation only penetrate 

 about 1 mm. of air at atmospheric pressure, the ionization- 

 chamber was reduced in depth to 1 cm. and the contained 

 air was replaced by hydrogen. The direct ionization of the 



* It is evident that the curves might have differed in shape owing to 

 a different distribution of the electrons in space when these were ejected 

 from the plate by X-radiations of different wave-lengths. This would 

 have admitted the possibility of an approximate balance of two changes 

 in the shape of the curve, in which case the different velocity of K 

 electrons would have been hidden. Apart from the improbability of 

 this we have the evidence that there was no appreciable change in the 

 shape of the curve due simply to change of wave-length of the exciting 

 X-radiation, in the perfect similarity of the Ag and Sn curves. 



