366 Mr. J. Crosby Chapman on 



with which the corpuscles are ejected varies directly with the 

 penetrating power of the exciting radiations. Suppose in 

 this experiment it had been possible to use a very soft 



radiation, say — in aluminium = 60, the corpuscles ejected 



r 



by this primary would have had relatively low velocities, and 

 would be rapidly stopped, and few would be able to emerge 

 from the gold leaf, so that the total effect would have been 

 that practically all the corpuscular radiation would have 

 been absorbed in the gold leaf, whether radiator I. or 

 radiator II. was used. In this case the experiment as a test 

 of the two theories breaks down. If now instead of a soft 

 radiation the hard primary beam is employed, it can be 

 shown that as large a fraction as 70 per cent, of the total 

 corpuscular energy produced in a gold leaf escapes from the 

 metal. 



As the only assumption underlying the experiment depends 

 on the fact that a reasonable fraction of corpuscles which 

 have still sufficient velocity to excite the characteristic 

 radiation from the gold shall escape, it was thought advisable 

 to determine this fraction experimentally, though the pene- 

 trating power of the corpuscular radiation could easily have 

 been calculated from the figures of Beatty or Sadler. 



Corpuscular Radiation produced in Gold by the 

 Tin Radiation. 



Tin serves as a convenient secondary radiator, for it emits 

 in moderate quantity a very hard radiation the absorption 



coefficient I — ) of which in aluminium =1'5, a hardness of 



the order of the primary beam used in this experiment (see 

 Table I. column 4). The object of this part of the research 

 was to determine what fraction of the total energy of the 

 corpuscles produced in a gold leaf by tin radiation is able to 

 escape from the metal itself. 



This was accomplished in the following manner : — An ioni- 

 zation-chamber (1 cm. thick), kindly lent me by Mr. Philpot, 

 was fitted with an electrode made of fine aluminium wire 

 mounted on an aluminium frame, and this electrode was 

 connected to an electroscope. The back and front of this 

 chamber were made of carbon. Initially when rays passed 

 through the chamber, the ionization, which was small, was 

 almost wholly due to the ionization of the air by the tin 



