(S'j-4 Prof. Barkla and Mr. Philpot : Ionization hi Gases and 



boundary of the square terminal, and left through the opposite 

 carbon plate. The only portion of the terminal exposed to 

 the radiation was the mesh work of threads, — too fine to pro- 

 duce an appreciable secondary radiation. The corpuscular 

 radiation emerging from the carbon plates into the gas was 

 then small, less in fact than the corpuscular radiation leaving 

 the gas and entering the carbon except when hydrogen was 

 the ionized gas, or in some cases when the pressure of the 

 gas was low. 



The ionization produced in the gas by the beam of Rontgen 

 radiation was observed first when the carbon plates were 

 uncovered inside, and afterwards when they were both covered 

 inside witli three gold leaves about 10 ~ 5 cm. in thickness. 

 (A second electroscope was used to standardize the intensity 

 of the homogeneous Rontgen radiation.) The difference 

 between the tvTo ionizations observed in the ionization- 

 chamber described was clue to the much greater intensity of 

 corpuscular radiation entering the contained gas from the gold 

 than from the carbon. It may be readily shown from the 

 observations of Barkla and Sadler on secondary X-radiation, 

 that the X-radiation s from the gold produced a negligible 

 ionization in a chamber so short as that used. This has also 

 been shown by direct experiment by Kleeman * for a longer 

 ionization-chamber. The loss of secondary radiations of all 

 types from the gas into the walls of the chamber was of 

 course the same in each case, and for our present purpose 

 need not be considered. The return of tertiary X-radiations 

 excited by these secondaries in the carbon or gold walls of 

 the chamber may also be neglected because tertiary radiation 

 of the X-type from either carbon or gold was certainly far 

 too weak to be appreciable. We have, how T ever, to consider 

 the possibility of different amounts of energy of corpuscular 

 radiation being returned by the gases into the ends and by 

 the ends into the gases when the ionizations in two gases were 

 compared. Though accurate data giving the amounts returned 

 thus are not available, examination of the different possi- 

 bilities shows that the error would be small in the cases inves- 

 tigated, and a close examination of the results themselves 

 supports this. We hope by using ends of very different 

 reflecting power to be able to reduce the possible error, if 

 this is at all appreciable. 



The difference between the two ionizations when a par- 

 ticular primary X-radiation was used was thus due to the 

 excess of corpuscular radiation escaping from gold over that 

 from carbon, when these were traversed by that primary 



* Loc. cit. 



