Scattering of at Particles by Gases. 293 



to make the hole as small as possible. In the apparatus 

 finally adopted it measured 70/4, but it can be made smaller 

 still. 



(6) is the plate holder. It is made of two bra<s strips 

 bent and soldered together as shown. Three little catches 

 c, c, c serve to keep the plate P in position. The three legs 

 of the holder are of such a length that when the holder is 

 pushed into G (fig. 2), the distance between the hole F and 

 the photographic film is always exactly 3 cm. After the 

 holder with the plate fixed in position has been introduced, 

 the open end of C is closed by the slate G. 



(c) represents in longitudinal section a holder for the active 

 source S. It consists of an ebonite piece E in which a brass 

 tube fits closely and, in turn, carries a small ebonite cylinder e. 

 The latter is perforated by a narrow canal just wide enough 

 to admit the active needle. The whole apparatus is made to 

 fit like a cap on the outside of 0'. The needle is fixed in the 

 small ebonite rod e in such a position that the active tip is 

 just within the end of the brass tube. After the active needle 

 has been fixed in position, the brass tube should project several 

 millimetres within the hollow of the outer ebonite E (that is, 

 as shown in the diagram). When E is pushed over C, the 

 brass tube will be pressed back to the necessary extent. 



This elaborate form of holder for the active source was 

 necessary in order to ensure that, the needle may be fixed in 

 position with the active point exactly opposite the hole F, and 

 almost in contact with the mica window, within a few seconds.. 

 A small paper cap was always put over the active end so that 

 the point only projects to about the same extent through a 

 hole in the cap. It will be seen the active source is placed 

 outside the gas-chamber. The emanation which is known to 

 cling to the active point for a considerable time is thus kept 

 away from the photographic plate. The needle-point was 

 activated by exposing it to about 30 millicuries of radium 

 emanation for the necessary period of time. It was inserted 

 in the holder a quarter of an hour after withdrawal from the 

 emanation-tube. The method of carrying out the experiment 

 is explained by the arrangement of apparatus already 

 discussed. As a rule, the plates were developed imme- 

 diately after exposure ; occasionally an unexposed plate 

 was also developed along with the exposed plate to serve as 

 a control. 



Examination of the Plates. 



A naked-eye examination of the photographic plates pre- 

 pared with any particular gas at different pressures showed 

 that while with a pressure of a few millimetres the impression 



