a. Rays from Radium and Polonium. 



543 



and this value is in entire agreement with the value 3*98 X 10 5 

 found by Rutherford in his paper just cited. Rutherford 

 found that the power of acting on a photographic plate and 

 of producing phosphorescence ceased for the a. rays when 

 their velocity fell to 64 per cent, of the maximum, that is 



The minimum velocity which I have 



when — - is 2*55 x 10 5 . 

 e 



found above is 2*65 x 10 5 approximately, and if the scintil- 

 lating action ceases at the same time as the rays fail to affect 



mv 

 a photographic plate, this value of — would be that expected. 



Electrostatic Deflexion of u Rays from Radium. 



The general form of the apparatus used in measuring the 

 electrostatic deflexion of the a. rays from radium is like that 

 already described, and is shown in figure 3. The radium cell 



Fiar. 3. 



R, the mica window M, and the camera end E are as before. 

 The charged plates BB were 6 cm. long, 1*87 cm. wide, and 

 -i mm. thick, and were placed *506 cm. apart. They were 

 attached to and supported by the metal slit-tube by means of 

 two quartz rods A A soldered to them. The distance from 

 the end of the slit-tube to the nearest edges of the plate was 

 1*72 cm., and from the end of the slit-tube to the zinc sulphide 

 was 14" 61 cm. The main tube was of glass with side tubes 



