Dr Barkla, Phenomena of X-Ray Transmission. 259 



The writer has recently investigated more closely the radia- 

 tions from Sn, Sb, I (which have been recorded as elements 

 emitting a radiation of variable penetrating power). It has been 

 found that these consist of a very easy absorbed radiation and a 

 very penetrating homogeneous radiation superposed. The absorp- 

 tions of the penetrating portions of the beams from each element 

 are shown in fig. 1 on curve B. The percentage absorptions of the 

 soft radiations from these elements have not yet been determined, 

 but they are roughly indicated on curve A in fig. 1. Though a 

 full analysis of the radiations from W, Pt, Au, Pb, Bi, etc., has 

 not yet been made, there is strong evidence that the observed 

 radiations from these elements are also principally homogeneous 

 radiations characteristic of the elements emittinsr them. 



Atomic Weight of Radiator 



Fig. 1. 



Note : Continuous lines connect points which have been accurately 



determined. 



— Discontinuous lines pass through approximate positions of points, 



Dotted lines are hypothetical. 



A knowledge of these characteristic radiations is sufficient to 

 account for the variations in the intensity and in the character of 

 the secondary rays emitted by different substances, and for what 

 have been regarded as anomalies in secondary radiation phenomena 

 by a number of investigators. Thus the intense radiation from 

 ethyl bromide when subject to a beam of ordinary penetrating 

 power is that given by the element bromine — an element in the 

 group of substances which emit homogeneous rays in considerable 

 intensity when subject to an ordinary primary radiation. The 

 variable behaviour of tin bichloride and of methyl iodide may also 

 be explained in terms of the results for tin and iodine already- 

 published. The variations are due to the fact that a beam of 

 X-rays of ordinary penetrating power contains only a small 



