Dr Barkla, Phenomena of X-Ray Transmission. 267 



functions of the atomic weight of the substance subjected to a 

 fixed primary beam. 



Many experiments remain to be performed before any theory 

 regarding the exact processes taking place during the transmission 

 of X-rays through matter can be established. It is, however, 

 worth while pointing out that if an electromagnetic pulse in which 

 the electric force is uniform from front to back of the pulse passed 

 over a number of independent electrons with a definite period of 

 vibration, the absorption would be a periodic function of the thick- 

 ness of the primary pulse. The absorption coeflficient might be 

 written 



irm d 



1 o ^ 



1 — cos ZTT k 



where d is the thickness of primary pulse and h the wave-length of 

 the radiation emitted by the freely vibrating electrons, N the 

 number of electrons in unit volume, e and m the charge and mass 

 of an electron. This would give an absorption curve of somewhat 

 the same form as those shown, and would also show periodicity. 

 As we have no method of measuring the thickness of the pulse, 

 and do not know anything of the force distribution in the pulse, we 

 cannot strictly compare the experimental curve with this. 



It should also be pointed out that with a primary pulse 

 in which the force gradually increases from zero to a maximum 

 and gradually again to zero, the periodicity and indeed the 

 absorption of thicker pulses would practically disappear. In such 

 a case the periodicity would have to be explained by assuming 

 the existence of different absorbing, radiating and ionizable systems 

 in the same atom. 



The connection between absorption, ionization and intensity of 

 secondary radiation might be easily accounted for. It would only 

 be after such an absorption of energy that electrons would be 

 hurled out of an atom, and ionization be produced — (both as a 

 primary and secondary or subsequent effect of this ejection). The 

 disturbance in the atom would set up an electromagnetic pulse 

 characteristic of the atom. 



It seems very improbable for several reasons that the sub- 

 sequent bombardment of other atoms by the ejected electrons 

 produces an appreciable secondary X-radiation. The speed of 

 ejection has been found to depend on the penetrating power of 

 the primary radiation causing that ejection ; so also would the 

 character of the secondary X-radiation produced by these. This is 

 contrary to experimental fact. Again, the connection between 

 the penetrating power of the primary radiation necessary to 

 produce a certain secondary radiation and the penetrating 

 power of that radiation would be very difficult to explain. 



