the Fluorescent Runt yen Radiations. 409 



It is important to notice that there is no evidence of any 

 abrupt change in the character of the radiation with a change 

 in the atomic weight of the radiating substance. It would, 

 for instance, not be surprising to find groupings of the 

 elements similar to those based on chemical properties. No 

 indication of anything of the sort has been obtained. 



In early papers the elements were for convenience divided 

 into groups according to the characteristics of the radiation 

 they emitted when exposed to primary beams of ordinary 

 penetrating power. The elements of different groups ap- 

 peared at first sight to be fundamentally different in their 

 behaviour, but it was later shown by the writer that the 

 behaviour of one element is typical of all. The observed 

 differences were due simply to wide differences in the relation 

 between the penetrating power of the primary radiation and 

 the fluorescent radiations. 



An increase in the atomic weight of an element is invariably 

 accompanied by an increase in the penetrating power of the 

 fluorescent radiation of the same series. This is not always 

 apparent without analysis of the radiations owing to dif- 

 ferences in the relative proportions of the fluorescent radia- 

 tions from various elements under exposure to the same 

 primary radiation. The relation between the atomic weight 

 of an element and the absorbability (as measured by the 

 absorption in aluminium) of its fluorescent X-radiations is 

 shown in fig. 3 (p. 410^. The range of absorbability is, how- 

 ever, too great to admit of accurate representation of the 

 relative absorptions of the most penetrating radiations. A more 

 convenient method of exhibiting the relation throughout the 

 entire range is that of plotting the logarithm of the absorb- 

 ability against atomic weight of the radiator, absorbability 

 being A/p, where \ is given by the equation I = I e~ Xj , 

 representing transmission through aluminium of density p 

 (fig. 4, p. 411). 



It will be observed that for each series the value of log X/p is 

 approximately a linear function of the atomic weight of the 

 radiating element. It is remarkable that this should hold 

 closely over such a range in penetrating power. 



As all the principal phenomena connected with the trans- 

 mission of X-rays through matter — absorption, ionization, 

 secondary radiation — are determined by the character of the 

 fluorescent radiations characteristic of the elements consti- 

 tuting the matter traversed*, it is essential to a complete 



* Proc. Camb. Phil. Soc. Mav 1000. 



