406 Prof. C. G. Barkla on the Spectra of 



Dealing with the various elements in this way, a large 

 number of fluorescent radiations have been identified. As 

 we have no method of determining the wave-length of the 

 radiation with which we deal, it is necessary to classify the 

 various X-radiations by their absorption in some standard 

 substance. This is not always a complete definition of an 

 X-radiation, for, as has been shown, radiations of three 

 different penetrating powers are absorbed to the same extent 

 in copper, or in iron, or in zinc, &c. This is analogous to 

 saying that refrangibility of light in one substance does not 

 always completely define the light. But just as the refrangi- 

 bility of light in a substance whose absorption bands are far 

 removed from the radiation used is a sufficient measure of 

 the character of the radiation, so the absorption in aluminium 

 is sufficient to define the Rontgen radiation used, for alumi- 

 nium does not emit a characteristic fluorescent radiation 

 within the range of penetrating power here dealt with, nor 

 near to it on its more absorbable side. In the following 

 table, column 3 gives the absorbability of the fluorescent 

 radiations as measured in aluminium. The quantity tabu- 

 lated is \/p, where A, is defined by the equation I = I e~ Xx 

 representing the intensity of radiation transmitted through 

 aluminium sheets of density p. 



In a number of cases the characteristic radiations from 

 elements were obtained by using compounds containing the 

 elements in combination with light atoms. The only effect 

 of these light atoms was then to add a little scattered radia- 

 tion to the fluorescent radiation which it was desired to 

 study. The effect of this was, however, negligible. The 

 radiations from Br, Sr, Rb, I, Ba, and Ce were obtained 

 from the compounds bromal hydrate, strontium hydroxide, 

 rubidium carbonate, iodoform, barium hydroxide, and cerium 

 oxide. 



It is seen that the radiations fall into two distinct series, 

 here denoted by the letters K and L*. Though only these 

 two have so far been found, it seems highly improbable that 

 these, consisting of radiations of penetrating power most 

 easily experimented upon, include all fluorescent radiations 

 actually emitted by elements. In support of this we will 

 look first at the phenomena of absorption. As shown in 

 fig. 2, the absorption in a substance R of rays softer than 



* Previously denoted by letters B and A (Proc. Carnb. Phi]. Soc. 

 May 1909). The letters K and L are, however, preferable, as it is 

 highly probable that series of radiations both more absorbable and more 

 penetrating exist. 



