Deflexion of ot Particles through Large Angles. 611 



■error. The experiments, therefore, prove that the number 

 of ot particles scattered in a definite direction varies as 

 cosec 4 cf>/2. 



Variation with Thickness of Material, 



In investigating the variation o£ scattering with thickness 

 of material, it seemed necessary to use a homogeneous source 

 of ot particles, for according to the theory the effect of the 

 change of velocity with increasing thickness will be very 

 appreciable for ot particles of low velocity. In the experi- 

 ments on " compound scattering " by one of us, a source was 

 used consisting of Ra deposited from radium emanation 

 in situ in a small conical tube fitted with a mica window, the 

 emanation being withdrawn when measurements were taken 

 by expanding into a large volume connected to it. In our 

 first experiments we used such a source, but the observations 

 eventually showed it to be unsuitable. After expansion 

 some emanation remains clinging to the walls of the glass 

 tube. This emanation and the Ra A associated with it gives 

 ot particles of considerably lower velocity than the ot particles 

 of Ra C, and although the number of ot particles so con- 

 tributed was of the order of only a few per cent, of the 

 number from the Ra C, yet owing to the fact that the 

 amount of scattering increases very rapidly 

 with decreasing velocity, the disturbances 

 caused by the slower ot particles were so large 

 as to render the source unsuitable for the 

 present work. 



The source finally adopted was prepared as 

 shown in fig. 2. About 80 millicuries of 

 radium emanation were very highly purified 

 and pressed into the conical end of the glass 

 tube T of about 1 mm. internal diameter. 

 After the emanation had remained in position 

 for a sufficient time to attain equilibrium 

 with Ra 0, it was expanded into a bulb below, 

 and a small part of the capillary tube was 

 drawn off at b. About 1 mm. of the end of 

 the tube which was coated with the Ra C was 

 then cut off (at a) and freed from occluded 

 emanation by washing with alcohol and by 

 heating. The resulting source of RaC was 

 used in the experiments, and with due care 

 its decay was found to be in agreement with 

 theory, at least for the first 80 minutes. 

 The arrangement used for the comparison of the scattering 



2T2 



F\S. 2. 



