The Absorption of Rontgen Rays. 739 



bars. In plotting the 3$-<?%? curves for these particular bars, 

 it was possible in two cases only to distinguish the curves 

 from those for the square bars, and in these two cases the 

 difference was extremely small. 



For equal values of the ratio of I to \/A, it was found in 

 general that the self-demagnetizing factor, for bars having a 

 sectional ratio of 2 to 1, was about 93 per cent, of that for 

 bars of square section ; while for flat bars, having a sectional 

 ratio of 10 to 1, the value of the self-demagnetizing factor 

 went down to about 75 per cent, of that for bars of square 

 section. 



LXIX. The Absorption of Rontgen Rags. Bg C. Gr. Barkla, 

 21. A., D.Sc, Lecturer in Advanced Electricity, and 

 C. A. Sadler, M.Sc, Oliver Lodge Fellow, University of 

 Liverpool*. 



r PHE results of experiments that have been made by 

 J- a number of investigators on the absorption of X-rays 

 are so complicated by a variety of conditions, and frequently 

 appear so inconsistent, that few general conclusions can be 

 drawn from them. 



The heterogeneity of the beams used not only masks any 

 peculiarity in the phenomena connected with a particular 

 constituent, but makes exact comparison between the results 

 of different experimenters, and even of the same experimenter, 

 impossible. 



In addition to this, as recent investigations have shown f, 

 there are peculiarities in the absorption phenomena which are 

 intimately connected with certain phenomena of secondary 

 radiation ; and a knowledge of these is necessary in order to 

 classify and explain the former. 



Through our investigations on the secondary X-rays 

 emitted by substances subject to X-rays, we have been 

 enabled to use almost perfectly homogeneous beams, and 

 have become acquainted with the character of the secondary 

 radiation emitted by many elements. 



It has been found that each of the elements Cr, Fe, Co, Ni, 

 Cu, Zn, As, Se, Ag, when subject to a suitable primary beam 

 of X-rays, emits an almost perfectly homogeneous beam of 



* Communicated by the Authors. 



The expenses of this research have been paitially covered by a 

 Government Grant through the Royal Society. — C. G. B. 



t " Homogeneous Secondary Rontgen Radiations," Barkla & Sadler, 

 Phil. Mag. Oct. 1908, pp. 550-584. 



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