258 Dr BarUa, Plwiiomcna of X-Raij Transmission. 



Sadler* to be emitted only when the exciting primary radiation 

 was of more penetrating type. Also it W7i,s shown that beyond a 

 certain penetrating power of the primary, the intensity of this 

 secondary radiation from a given mass of snbstance was pro- 

 portional to the ionization produced by this primary beam in 

 a thin film of air. As we have no exact method of comparing the 

 intensities of beams differing in penetrating power, we are unable 

 to determine accurately the relative amounts of energy of different 

 beams reappearing as secondary radiation of this type. If, however, 

 w^e assume as approximately true that the ionizations produced in 

 a thin him of air by beams of equal intensity are proportional to 

 the absorptions of these beams in air, we may also say that the in- 

 tensity of this secondar}' radiation, beyond a certain penetrating 

 power of the primary radiation, is proportional to the coefficient 

 of absorption of the primary radiation in aluminium. The relation 

 between the intensity of secondary homogeneous radiation and the 

 absorbability of the primary radiation may thus be approximately 

 represented by curve II in tig. 4. Whatever the accuracy of the 

 assumption, the geueral features of the curve remain unquestion- 

 able, i.e. lirst, no appreciable intensity until the primary is more 

 penetrating; secondly, a rapid rise of intensity as the primary 

 is made more penetrating ; and thirdly, a decline of intensity, ap- 

 proximately, if not accurately proportional to the absorbability 

 of the primary radiation. Mr Sadler f has recently investigated 

 the curves in detail on the same assumption, using a series of 

 homogeneous beams as primary radiations, and has found the 

 same type of relation in all cases investigated. 



The secondary radiation from a large number of elements has 

 been studied, and the percentage absorption of the radiation by a 

 sheet of aluminium -01 cm. thick, as measured by the diminution 

 of ionization in an electroscope 7 or 8 cms. from the radiating 

 elements, has been published by the wTiter|. 



When subject to a beam of X-rays of only moderate penetrating- 

 power, the radiation reaching the electroscope from elements of 

 atomic weight between those of hydrogen and sulphur was almost 

 entirely a sc^ittered radiation, none of the homogeneous type 

 appearing. Later analysis of the radiations from other elements 

 by Barkla and Sadler showed that the radiations fi-om the elements 

 from Cr to Ag were practically homogeneous radiations, producing 

 in general, ionizations several hvindred times the ionization pro- 

 duced by the scattered radiation from the equal masses of the 

 light elements. A weak scattered radiation was mixed with these. 



* '' Homogeneous Secondary Eontgen Eadiations," Phil. Mag., Oct. 190S, 

 pp. 550 — 5S4. 



t •' Transformations of Eontgen Eays," Phil. Soc. Loud., April 23, 1909. 

 ij; " Secondary Eontgen Eadiations,"' Phil. 2Iag., June 1900. 



