﻿424 Sir E. Rutherford and Dr. J. Chad wick on the 



brass tube T of 3 cm. diameter. The end of this tube was 

 provided with a hole 7*5 mm. in diameter, closed by a silver 

 foil of 3*75 cm. air equivalent. The zinc sulphide screen S 

 was fixed on the face of the vessel leaving a slot in which 

 absorbing screens could be inserted. The apparatus was 

 placed between the poles of an electromagnet to reduce the 

 luminosity produced in the screen by the /3 rays. 



The source R was a silver foil of 4'15 cm. stopping-power 

 coated on one side only with the active deposit of radium. 

 Its initial 7-ray activity was in most experiments equiva- 

 lent to about 30 mg. Ra. The inactive side of the silver 

 foil faced towards the ZnS screen. The distance of the source 

 from the screen was generally about 3*5 cm., but could be 

 varied, and its position read off on a scale. The elements 

 to be examined could in most cases only be obtained in 

 the combined state. The powdered compound was heated 

 in vacuo, and a film prepared by dusting on to a foil smeared 

 with alcohol. The screen thus prepared was placed immedi- 

 ately behind the source. As in our previous experiments, a 

 stream of dry oxygen was circulated through the apparatus. 



In all cases, except that of nitrogen, the maximum range 

 of the particles emitted in the backward direction was 

 greater than the range of free hydrogen particles, so that no 

 complication arises from the presence of hydrogen in the 

 silver foils or other materials in the path of the a particles. 



In the case of nitrogen, however, as our previous experi- 

 ments had shown, the range of the backward particles is 

 much less than that of free hydrogen particles, and it was 

 consequently necessary to allow for the " natural" effect, i. e. 

 for the H particles arising from hydrogen contamination of 

 the source and screens in the path of the a rays. It was 

 found inconvenient to use gaseous nitrogen for these experi- 

 ments, and a suitable screen was prepared by sifting a thin 

 layer of powdered paracyanogen, CyST x , on to a gold foil. 

 The scintillations observed on the ZnS screen when the film 

 of paracyanogen was placed against the source were due to 

 the " natural " particles from the source and screens, together 

 with those which came from the nitrogen in the paracyanogen. 

 On taking away the film of C Z N X the natural particles alone 

 were counted. In some experiments a film of paraffin wax 

 was placed against the source. The natural effect remained 

 the same, showing that even if the film of paracyanogen 

 contained a large amount of hydrogen the number of free H 

 particles scattered to the ZnS screen by the walls of the 

 vessel was negligible. 



Figure 3 shows the type of results obtained in these 



