408 Dr. Barkla and Mr. Sadler on Secondary 



that the difference in the rates of decay as measured in the 

 two vessels is due to the slowly moving ft rays emitted hy 

 radium B. 



To further test this point, the rays were made to pass 

 between the pole- pieces of a small electromagnet before 

 entering the vessel A. On exciting the magnet, the slowly 

 moving ft rays were deflected and the balance was disturbed, 

 the ionization in the vessel A at once falling below that in B. 

 This fact confirms the conclusion given above. 



Physical Laboratory, 



The University, Manchester. 



XXXVII. Secondary X-Rays and the Atomic Weight of Nickel, 

 By Charles G. Barkla, M. A., D.Sc, Lecturer in Advanced 

 Electricity, and G. A. Sadler, M.Sc, Demonstrator in 

 Physics, University of Liverpool* . 



IN a paper on Secondary Rontgen Radiation f, one of the 

 authors suggested a method of determining atomic 

 weights by means of experiments on the more penetrating 

 secondary rays — (those which traversed several centimetres of 

 air and thin sheets of paper and aluminium foil) — emitted by 

 elements in the form of thick plates when subject to #-rays. 

 It was found by graphically plotting as ordinates the per- 

 centage absorptions % by aluminium of the secondary rays 

 proceeding from various elements, and as abscissas the atomic 

 weights of the radiators, a periodic curve was obtained, in 

 many portions of which the gradient was so great that atomic 

 weights could be obtained by interpolation with considerable 

 accuracy. The curve shown on page 820 f was that obtained 

 by preliminary experiments, and for reasons given was not 

 regarded as accurate in detail, the main characteristics alone 

 being discussed. 



It was seen that from chromium to selenium the relation 

 between the absorption and the atomic weight of the radiator 



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

 been partially covered by a Government Grant through the Royal 

 Society.— C. G. B. 



t C. G. Barkla, Phil. Mag. pp. 812-828, June 1906. 



\ As in previous papers, by " percentage absorption," is meant the 

 percentage diminution in the ionization produced by an .r-ray beam in its 

 passage through an electroscope by placing absorbing plates in its path 

 before it falls on the electroscope. This, of course, is only strictly per- 

 centage absorption of energy when the beam is homogeneous. 



