79 



A Comparison of Some Forms of electric 

 Radiation. 



By W. H. Bragg, M.A., F.R.S., Elder Professor of Mathe- 

 matics and Physics, the University of Adelaide. 



[Read May 7, 1907.] 



We are now aware of the existence of a number of differ- 

 ent types of radiation, each of which is able to ionise a gas, to 

 act on a photographic plate, and to excite phosphorescence in 

 certain materials. Of these the a and canal rays consist of 

 positively charged particles of atomic magnitude ; the cathode 

 and /i rays are negative rays, and consist of electrons ; the X 

 and 7 rays are supposed to be ether pulses; and ultraviolet 

 light co.nsists of short ether waves. The S rays stand by 

 themselves, for, though they consist of negative electrons like 

 the cathode and ji rays, they have so small a velocity that they 

 possess no appreciable ionising powers 



The present jDaper contains an attempt to find whether 

 there is anything to be learnt from a comparison of the pro- 

 perties of the various rays. 



It apjDears to me to be a first deduction from such a 

 comparison that in all cases the bulk of the ionisation which 

 the rays effect is of the same character, and consists in the 

 displacement of slow-moving electrons, or 8 rays, from the 

 atoms of the gas or other substance which they traverse. Let 

 us consider the various rays in turn. 



In the case of the cathode rays this principle has been 

 clearly established by Lenard in the course of his long series 

 of beautiful experiments. He has shown that cathode ra3^s of 

 the most varied speeds, impinging on bodies of various kinds, 

 or traversing different gases, cause the liberation of slow-speed 

 electrons from the atoms of the solid or gas. The speed of 

 the electrons is in every case that due to the fall through less 

 than ten volts. This is in no way a contradiction of the fact 

 that cathode rays of high speed are also liberated from a 

 solid surface struck by primary cathode rays; or from the 

 atoms of a gas through which the primary rays pass. But, 

 whether these high-speed secondary rays are scattered pri- 

 mary rays, or are true secondary rays, they must in their 

 turn produce electrons of slow speed in the gas through 

 which they pass ; and so, directly or indirectly, by primary or 

 secondary or tertiary or rays still more transformed, eventually 



