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XXXV. Typical Cases of Ionization by X-Rays. By Charles 

 G. Barkla, M.A., D.Sc, Professor of Physics, King's 

 College, London *. 



IN a letter to ' Nature' (April 15, 1909) and in a short 

 preliminary paper on " Phenomena of X-Ray Trans- 

 mission " (Proc. Camb. Phil. Soc. May 17, 1909) the writer 

 showed that many apparent anomalies of ionization might be 

 explained in terms of a few simple laws, and that so far as 

 experiments had then gone, the behaviour of one substance 

 might be regarded as typical of all. Further experiments 

 have confirmed these conclusions, but as accurate measure- 

 ments of ionization involve the study of absorption, secondary 

 X-radiation and secondary corpuscular radiation, the publi- 

 cation of the results in detail cannot yet be undertaken. It 

 is sufficient for the present to study the results of experi- 

 ments on two substances, carbonic acid gas and ethyl bromide 

 vapour, as these show all the marked phenomena hitherto 

 observed. 



Secondary X-radiations from Fe, Ni, Cu, Zn, As, Se, Br, 

 Sr, Mo, Ag, Sn, and Sb — with special treatment in some 

 cases — furnished homogeneous beams of X-rays which varied 

 considerably in penetrating power. The most penetrating 

 radiation dealt with was seventy times as penetrating as the 

 most absorl able. Tlie absorption of these radiations by Al, 

 together with the substances which emitted them, is shown 

 in Table T. X is defined by the equation I = I e~ Ax during 



transmission through aluminium, - is tabulated because it 



is a constant for a given radiation and given absorbing- 

 substance whatever the density of that absorbing substance. 

 In studying the results of experiments on ionization it is 

 necessary to consider the ionizing effects of : — 



(a) Radiations from the walls of the ionization chamber : 



(1) Scattered X-rays — (one type of secondary X-rays); 



(2) Transformed X-rays — (secondary X-rays character- 



istic of the radiating elements) ; 



(3) Corpuscular rays — (secondary rays consisting of 



ejected electrons). 



* Communicated by the Author. The expenses of this research have 

 been partiully covered by a Government Grant through the Royal 



Society. 



