﻿412 Relation between X-ray Wave-lengths and Absorption. 



occurs when a characteristic radiation traverses a substance 

 o£ less atomic weight is accompanied by, and to some extent 

 dependent upon, the production of secondary radiation. 

 Zinc rays are highly absorbed by nickel, and at the same 

 time zinc rays excite nickel rays. But nickel rays cannot 

 excite zinc rays, and the absorption o£ nickel rays by zinc is 

 relatively small. 



When, therefore, we find that silver absorbs its own rays 

 on no higher a scale than it absorbs those of Eh and Pd, 

 which are of less atomic weight (the atomic weights of 

 Ag, Pd, and Eh are 108, 107, and 103, their atomic num- 

 bers 47, 46, and 45), we conclude that none of the rays 

 emitted by these three substances can excite any of the 

 silver rays. This is the case although many of the waves 

 are shorter than one or more of the characteristic silver 

 waves. 



If we examine the palladium curve, we find all things the 

 same, except that the short silver wave 0*495 is highly 

 absorbed by Pd, and no doubt excites Pd rays. Why should 

 it be able to do so ? It cannot be merely because it is 

 shorter than some of the Pd waves, because the Pd wave 

 0*516 cannot excite Ag waves of greater length. 



The most probable condition would seem to be that the 

 exciting wave must be shorter than all the characteristic 

 waves of the substance in which it excites those waves. 

 It will be observed that the /3 ray of Ag is just shorter than 

 the 7 ray of Pd. Further examination of parallel cases 

 will be necessary, of course, before this statement can be 

 generalized. It seems likely, however, that certain pecu- 

 liarities in Barkla's table of absorption coefficients can be 

 explained by its aid. For example, Barkla states that the 

 mass-absorption coefficients of Ni for the rays emitted by 

 Ni, Cu, Zn are respectively 56*3, 62*7, 265. We should 

 explain this on the ground that neither nickel ray can excite 

 the characteristic rays of nickel, because of course neither 

 is shorter than the shortest ; of the copper rays, one, the 

 weaker, can excite the nickel spectrum and is highly 

 absorbed, but the other, the longer and stronger, cannot 

 do so and is not specially absorbed. The net result is that 

 the copper rays, though shorter on the whole than the nickel, 

 are nevertheless the more highly absorbed as a whole. Both 

 the zinc rays can excite the nickel rays because both are 

 shorter than the shortest nickel ray ; the zinc rays are very 

 highly absorbed in consequence. It may be that the charac- 

 teristic rays of a substance form a system which can only be 

 excited as a whole. 



