Artificial Disintegration of Light Elements. 811 



aud observations taken at six months interval have been 

 found in good agreement. In order to count as many 

 particles as possible during an experiment, two counters 

 were always used who counted alternately for a period of 

 one minute each. An additional observer made the necessary 

 adjustments and recorded the data. 



Experimental arrangement. 

 The general arrangement will be clear from fig. 1. The 



Fig. 1, 



source of a. particles R was carried on a rod passing with 

 a sliding fit through a brass stopper. This fitted tightly 

 into the brass tube T of 3 cm. diameter. The end of the 

 vessel was provided with a hole 5 mm. in diameter, which 

 was closed by a silver foil of 6' 3 cm. air equivalent. The 

 zinc-sulphide screen S was fixed on the face leaving a slot of 

 1'3 mm. depth in which absorbing screens could be inserted. 

 The apparatus was placed between the poles of a large 

 electromagnet to reduce the luminosity due to the ft rays. 



The source R in most experiments was a brass disk of 

 diameter 1 cm. coated with the active deposit of radium. Its 

 initial 7- ray activity was usually equivalent to 25 mgs. Ra. 

 The distance of the source from the screen, generally 3*5 cm., 

 could be varied and the position read off on a scale. In 

 most cases the material to be exposed to a rays was in the 

 form of .a powder, generally the oxide of the element. This 

 was heated in vacuo and a film prepared by dusting on to a 

 gold foil smeared with, alcohol. The average thickness of 

 the film was determined by weighing. The screen thus 

 prepared was placed immediately in front of the source. 

 A stream of dry oxygen, which does not give any particles. 

 was continuously passed through the apparatus to eliminate 

 the effect of the nitrogen in the air which would itself give 

 long-range particles. 



3H2 



