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XL The Routgen Radiation from Substances of Low Atomic 

 Weight, By Charles A. Sadler, D.Sc, Lecturer and 

 Demonstrator in Physics, University College, Reading, late 

 (Hirer Lodge Fellow in the University of Liverpool, and 

 PAUL MeshAM, AI.Sc, Lecturer and Demonstrator in 

 Physics, University of Liverpool* . 



IT was fouinl by early investigators tbat when a beam oP 

 X rays Fell upon a substance an X radiation was excited 

 in it. This secondary radiation, as it was called, varied with 

 the nature of the substance in whieh it originated. With 

 elements of low atomic weigbts, the excited radiation was 

 approximately of the same type as the exciting beam. More 

 recently t, Barkla and Sadler have shown that from sub- 

 stances of atomic weight higher than 40, the secondary 

 radiation is characteristic of the substance and consists in 

 the main of one or more homogeneous beams. In the earlier 

 experiments investigators were considerably handicapped by 

 having to work with beams which were very heterogeneous 

 and whose character was continually liable to variation. 

 For example, when the secondary radiation from some par- 

 ticular element of low atomic weight was examined it was 

 found to depend upon the kind of primary beam used. The 

 discovery of the homooeneous beams ottered an enormous 

 simplification of the problem, since the character of these 

 beams is quite definite and can be reproduced with certainty 

 at any time. There is, however, one serious drawback in 

 their use. The intensity of the homogeneous beams can only 

 be a comparatively small fraction of the intensity of the 

 primary beam exciting them. Under the most favourable 

 circumstances the radiation scattered by a substance of low 

 atomic weight is o£ feeble intensity, and when the exciting- 

 radiation is considerably weakened, as it is when we use 

 homogeneous beams, special arrangements have to be made 

 to measure the intensity and other properties of the scattered 

 radiation. 



The authors made use of the special form of electrometer 

 described in previous papers, and found that by taking- 

 reasonable precautions they were able to obtain consistent 

 results. 



Carbon was chosen as a suitable substance to serve as a 

 radiator. 



In accordance with the results obtained by those who had 



* Communicated by the Authors. Part of the expenses of this 

 Research have been defrayed by a Government Grant. 



t Barkla and Sadler, Phil. Mag-. October 1908, pp. 550-584. 



