172 Rev. P. J. Kirkby on Union of Hydrogen and Oxygen 



dried in a vessel containing pentoxide of phosphorus, and 

 were then admitted to a glass bell -jar which contained the 

 parallel plates between which the current was to be passed. 



The bell-jar was connected by glass tubing to an ordinary 

 manometer and to two McLeod gauges which combined to 

 measure pressures below 40 mm. It was also connected to a 

 large mercury-pump. But the tube leading to the latter 

 could be closed by a stopcock ; for it was obviously important 

 to confine the hydrogen and oxygen within as small a space 

 as possible, with a view to determine the more accurately the 

 falls of pressure resulting from their partial union. 



The arrangement within the bell-jar is shown in fig. 1, 

 which represents a section of it by a plane through its axis. 



Fio-. 1. 



The -dark parts represent ebonite pieces : the light, metallic 

 pieces, except the stopper which was glass. The figure shows 

 how by turning the stopper the screw, whose pitch was 1 mm., 

 could be turned and the upper plate raised or lowered thereby, 

 and how the distance between the plates could be most accu- 

 rately determined by the vertical scale and by the graduated 

 drum of ebonite whose rim almost touched the scale. The 

 possibility of a leak between the stopper and the neck of the 

 jar was obviated by pouring mercury into the cup above 

 the neck, as shown in the figure. 



