70 



Dr. S. J. Plimpton on the Recombination oj 



pumped out as before. This process was repeated at least 

 three times. 



Great care was taken in every case to make the gas or 

 vapour dust-free, by passing it through several inches of 

 glass-wool and then allowing it to settle for several hours. 



The gases were dried by passing slowly through a tube 

 containing phosphorus pentoxide, and the vapours were 

 bubbled through sulphuric acid when necessary and were 

 always passed through a calcium-chloride drying-tube. 



Description of Apparatus. 



The general arrangement of apparatus is shown in fig. 1. 

 The ionization chamber K is lined with aluminium and pro- 

 vided with two parallel plate electrodes A and B of alu- 

 minium, the latter being surrounded by a guard-ring C 



Fig. 1. 



which is connected with the walls of the chamber. The 

 insulated electrode B may be connected with an electrometer 

 by means of a key D. The beam of X-rays is restricted by 

 adjustable lead screens, MM' and NN', so that it does not 

 fall on the electrodes. A pendulum Ii is used for auto- 

 matically giving the flash of X-rays by opening the key S 

 and after a definite interval of time, applying a potential to 

 the electrode A by opening the switch F, which may be 

 placed at any position along the path of the pendulum. 



In order to obtain X-ray flashes of sufficient intensity it 



