Alpha Particles from Light Atoms. 537 



studied, 15 cm. broad, 2 cm. high, and varying in thickness 

 from 1 mm. to a thickness equivalent to 6 cm. air absorption *. 

 Tt was placed so as to make equal angles with the directions 

 of the entering and emerging particles which were counted. 



This was done so that the paths of the entering and emerging- 

 particles should be nearly equal. The line from the centre 

 of S to the centre of T was originally intended to make an 

 angle of 120° with the line from the centre of T to the 

 portion of the ZnS screen Z used for counting. The re- 

 arrangements made in distances changed this angle to L05°. 

 The box was exhausted through the tube 1) and held to 

 about b or 8 mm. of pressure, being closed on the top by a 



* In the case of Al it was n piece of Al 1 nun. thick which b\ chemical 

 analysis was shown to he 99'6 per cent. pure, the chi^f impurity being 

 silicon, which Ins a deflecting power about equal to Al. 



