758 Dr. C. G. Barkla and Mr. C. A. Sadler on 



connected purely with X-rays with which we are experi- 

 mentally acquainted, nickel behaves as a normal element of 

 atomic weight 61*3. This is true of (1) the absorption by 

 nickel unconnected with the production of X-rays, (2) the 

 absorption connected with the emission of secondary X-rays, 

 (3) the character of the secondary rays emitted, (4) the 

 intensity of the secondary X-rays emitted ] 



Theory. 



If an attempt be made to account for these results on 

 Prof. Bragg' s neutral pair theory, we are at once led into 

 difficulties. The pairs constituting the homogeneous secon- 

 dary radiation characteristic of a particular element must 

 have been either in the primary radiation or in the atoms of 

 the element subject to that radiation. If we make the former 

 assumption we have to account for pairs varying enormously 

 in velocity being scattered in all directions with one velocity — 

 a velocity characteristic of the atom. If, on the other hand, 

 we assume that the pairs were originally in the atom, we 

 must account for their ejection with one velocity by assuming 

 some disruption to occur in the atom. The former of the 

 two assumptions is inconceivable, and it is against the latter — 

 the disruption theory — that much of Prof. Bragg's argument 

 has been directed. If, however, we change the point of view 

 and assume disruption, we must account for : — All the pairs 

 transmitted through a substance being undiminished in 

 velocity ; those moving with a velocity greater than a 

 critical velocity being stopped in greater proportion than those 

 moving more slowly ; those moving still more quickly being 

 stopped in smaller proportion ; and the equality of the velocity 

 required to produce instability with that acquired by another 

 pair due to the instability set up in the atom. Assumptions 

 might possibly be made to satisfy any individual result, but 

 in the combination we find the difficulties insuperable. 



The results of these experiments are, however, in their 

 general nature what would be expected on the sether-pulse 

 theory, as shown in previous papers*. 



A more detailed discussion of this theory wall be given 

 when several points have been further tested. 



It is possible that all the substances experimented upon 

 also emit a very easily absorbed secondary radiation, and 

 that the absorptions which have been observed in the case 

 of very "soft" radiations have been accompanied by the 

 emission of such radiations. 



* Phil. Mag. Oct. 1908, pp. 550-584, and Jahrhuch der JRadioaUivitcit 

 und EleMronik, iii. pp. 246-324. 



