The Effect of the Electric Discharge on Water Vapour, 139 
dimensions were such that if at any time it was necessary to open the 
apparatus {e.g., to change the X-ray tube) the mercury might be made 
to fill the tube to such a height that the pressure in the apparatus might 
Fig. 2. 
rise to one atmosphere without the mercury falling below J, so that by 
merely raising E sufficiently, W remained cut off and the water vapour 
unmixed with air. 
The arrangement was connected to the remainder of the apparatus 
through two mercury traps separated by 80 cm. of 3-mm. bore-tubing 
