Boltwood — Radio-activity of Uranium Minerals. 273 



held together by a spring clamp after the lower half had been 

 swung back into position. The charged electrode was a 

 circular aluminium plate 7'5 cm in diameter and was 9"5 cm from 

 the bottom of the chamber. The vertical brass rod holding 

 the electrode passed through a sealing-wax plug in the top of 

 the chamber and carried on its top a brass plate and gold-leaf 

 five centimeters in length and eight millimeters in width. 

 The insulating sealing-wax plug was surrounded by a brass 



guard-ring which was kept permanently attached to the charg- 

 ing battery, consisting of 190 small storage cells. The gold- 

 leaf was protected by a metal case with small mica-covered 

 windows. The insulated electrode could be charged at will by 

 touching the back of the plate carrying the gold-leaf with a 

 fine wire inserted through a short glass tube in the side of the 

 metal case. The guard ring and charging wire were connected 

 through a water resistance with the negative terminal of the 

 charging battery. The positive pole of the battery and the 

 case of the electroscope were connected to earth. The move- 

 ment of the gold-leaf was observed through a microscope rigidly 

 mounted in front of the electroscope. The type of microscope 

 used has already been described* and the practice was always 

 followed of noting the time required for the passage of the 



*This Journal, xviii, 99, 1904. 



