500 Lord BIythswood and Mr. H. S. Allen on 



1 all* in liquid air. In this case 420 grams of liquid were 

 evaporated, but the amount required to cover the bulb at the 

 outset was only 570 grams. 



§ 6. In order to obtain a still higher vacuum in a large 

 discharge-tube without the use of a pump, two charcoal 

 receptacles were employed. At first No. 1 was cooled in liquid 

 air, whilst No. 2 was strongly heated to drive off' as much gas 

 as possible. Then Ko. 1 was sealed off' with the blowpipe, and 

 No. 2 was cooled. One hour was devoted to each operation. 

 After the X-ray tube was sealed off and had been allowed to 

 cool, it was found that the discharge from a large induction- 

 coil would pass across a five-inch spark-gap rather than go 

 through the tube. 



§ 7. These experiments show that it is practically possible 

 to apply Dewar's method to the exhaustion of large receivers, 

 and that this method requires but a moderate amount of liquid 

 air. It is particularly useful when it is desirable to avoid 

 the presence of mercury vapour in the vacuum-tube, as in 

 Geissler tubes for spectroscopic analysis. It is possible to 

 dispense with drying-tubes, though this procedure is not 

 recommended in general as the charcoal is rendered less 

 efficient for subsequent work. The method is a rapid one, 

 and a further recommendation to the amateur glass-blower is 

 the simplicity of the apparatus required. 



Experiments to Determine the Amount and Rate of 

 Absorption of Air by Charcoal cooled to the 

 Temperature of Liquid Air. 



(i.) Absorption by Charcoal in Presence of an Excess of Air, 



§ 8. A number of experiments were carried out to find the 

 absorption by charcoal in the presence of a volume of air 

 greater than the charcoal could absorb when cooled to the 

 temperature of liquid air. 



A small quantity (2 grams) of charcoal was placed in a glass 

 bulb in communication with a barometer-tube. Glass vessels 

 of known volume could be attached to the apparatus in order 

 to determine the effect of altering the initial volume of air 

 experimented upon. 



Let v denote the volume of the bulb which is cooled to the 

 temperature of liquid air, 

 Y denote the volume of the connecting tubes and the 

 attached vessel. 



Initially we have a volume V + v at pressure I* and tem- 

 perature T. Suppose that when the small bulb is cooled to a 

 temperature t, without any charcoal in it ; the pressure in the 

 ap| aratus is reduced to p 1 '. 



