Dr Barkla, Phenomena of X-Ray Transmission. 257 



Phenomena of X-Ray Transmission. (Preliminary Paper.) By 

 Charles G. Barkla, M.A., D.Sc. (Communicated by Professor 

 Sir J. J. Thomson.)* 



[Read 17 May 1909.] 



The principal phenomena accompanying the transmission of 

 X-rays through matter may be classified under three heads — 

 Absorption, Ionization, and Secondary Radiation. 



Much that has been written on these subjects has been in the 

 form of disconnected papers, in which little attempt has been made 

 to collect and classify known experimental facts, or to give an 

 explanation of observed phenomena in terms of laws previously 

 discovered. As a consequence X-ray phenomena have been made 

 to appear much more complicated than they actually are. 



In the following paper it is intended to give, as briefly as 

 possible and without entering into experimental details, a state- 

 ment of facts already published, an account of the results of 

 further experiments, and a survey of the whole subject of X-ray 

 transmission, showing the connection between the various phe- 

 nomena and accounting for apparent anomalies. As the phenomena 

 of secondary radiation form the basis of classification they will be 

 discussed first. 



Secondary X-Radiation. 



Secondary X-rays of two distinct types have been found to be 

 emitted by substances subject to X-rays. Of these the scattered 

 X-radiation — one of the same penetrating power as the primary — 

 has invariably been found to be emitted during the transmission 

 of a primary beam through matter. 



All the experiments that have been made on this radiation 

 have strikingly verified the theory of scattering, as given by 

 Sir J. J, Thomson -j-, many of the results being foretold as an 

 immediate consequence of this theory. 



The second type of secondary X-radiation emitted by many 

 elements, and probably by all, is a homogeneous radiation 

 characteristic simply of the element emitting it. Unlike the 

 scattered radiation this is distributed uniformly in all directions, 

 and gives no evidence of polarization in a primary beam where 

 such polarization exists. This radiation was shown by Barkla and 



* The expenses of these researches were partially covered by a Government 

 Grant through the Royal Society. 



t Conduction of Electricity throuyh Gases. 



