Ultra- Violet Light from the Mercury Arc. 



395 



The vacuum was obtained as follows : — The apparatus is 

 exhausted by a Toepler pump down to a pressure of about' 

 c 05 mm., then filled with pure oxygen and re-exhausted. 



Fig.L 



uiutvn&U r^**^>*™** 



Meanwhile both the large charcoal tubes C\ and C 2 were 

 heated. The charcoal tube Ci is then cooled by liquid air, 

 while C 2 is surrounded by a heating jacket. After four or 

 five hours, during which the arc is repeatedly struck to get 

 rid of any gases in the mercury, the communication between 

 the charcoal tubes is cut off and liquid air is brought up 

 around C 2 . After the liquid air has been in position for 

 about three hours, readings are taken. To diminish as far 

 as possible the pressure of mercury vapour at the top of the 

 tube, a water-jacket surrounds the middle portion and the 

 upper part is kept cool by C0 2 snow. A discharge tube 

 (about 20 cm. by 4 cm.) serves to indicate the state of the 

 vacuum. When consistent readings were obtained the alter- 

 native spark-gap was always greater than 6 or 7 inches. 



2*D 2 



