740 



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



X-rays *, the penetrating power of which is characteristic 

 of the element emitting it. As these penetrating powers 

 vary considerably — the radiation from Cr being very soft 

 and that from Ag fairly penetrating — the above metals 

 furnish us with a series of nine homogeneous beams which 

 can conveniently be used for accurate investigations of many 

 X-ray phenomena. The variation in penetrating power is 

 shown by the following values for the coefficient of absorption 

 in aluminium of each radiation, the coefficient \ being- 

 defined by the equation I = I e~ Kx . 





Coefficient of Absorption 



in Al. 



adiator. (p — d ensity of Al.) 



Cr . . . . 136 p 



Fe . 







88-5p 



Co . 







71-6/0 



Xi . 







59-lp 



Cu . 







47-7/o 



Zn . 







39-4/0 



As . . 







22-5/0 



Se . . 







18-9/o 



Ag. . 







2-5/o 



Phenomena of Transmission. 



Before attempting to make accurate experiments on the 

 absorption of these radiations by various elements, it is 

 necessary to know something of the phenomena accom- 

 panying the transmission of X-rays through absorbing 

 substances. 



The secondary X-ray phenomena, which have an important 

 bearing on experiments on absorption, may be stated briefly 

 as follows : — 



When a beam of X-rays is transmitted through any 

 substance, secondary X-rays of the same penetrating power 

 are emitted by that substance in all directions. The 



* Mixed with the homogeneous radiation is an exceedingly weak 

 scattered radiation of the same penetrating power as the primary 

 radiation ; but the ionization produced by this is usually negligible 

 in comparison with that of the homogeneous radiation — in certain 

 cases not more than 1 per cent. The radiation from Ag was 

 transmitted through Al in order to get rid of most of the scattered 

 radiation. 



The evidence that these rays are X-rays is overwhelming. Some 

 of it has been given in the paper on "Homogeneous Secondary 

 Rontgen Radiations." 



