126 MR. G. W. C. KAYE ON THE 



leaf (Q'000025 cm. thick), silver leaf (0'00002 cm. thick), copper leaf (0'000042 cm. 

 thick), and aluminium leaf (O'OOOOl cm. thick) were obtained from Nuremberg, and 

 were piled for screens of thickness less than about O'OOl cm. Above this thickness 

 rolled samples of metal sheet could be procured in most cases. 



The Discharge Tube. The whole anticathode system was earthed and put in 

 metallic connection with the anode and the tube T. The discharge was generated by 

 a 10-inch Cox induction coil. Across the electrodes an adjustable spark gap, consist- 

 ing of two polished brass balls 2'54 cms. in diameter, furnished a rough measure of the 

 potential difference, as given by ORGLER'S formula,* 



V = 28,000d, 



where V is the potential difference in volts, and d is the spark length in centimetres. 



A Villard rectifier in circuit prevented reversals of the discharge. The discharge 

 tube was connected to a Topler pump, a McLeod gauge, and a P 2 O 5 bulb. 



In the earlier stages of the work a good deal of trouble was caused by the slow and 

 steady evolution of gas from the considerable mass of metal in the tube, which 

 necessitated much tedious and laborious pumping and delayed the taking of readings. 



In later work a cocoanut charcoal tube which could be surrounded by liquid air 

 was mounted on the apparatus, and a barometric mercury cut-off was placed between 

 this and the discharge tube. This arrangement enabled the gases which had been 

 liberated by the discharge and absorbed by the charcoal to be pumped off at leisure. 



The metal tube T was not present in the earlier form of apparatus, and its intro- 

 duction proved of great value in two directions. Firstly, it prevented the excessive 

 metallic deposition on the walls of the tube in that region, which formerly unfitted 

 the apparatus for use. Secondly, it was employed as a temporary cathode, the other 

 electrode being the anticathode system. A tube-cathode concentrates the cathode 

 rays along its axis, so that a beam of rays was obtained intense enough to raise the 

 point of impact on the anticathode to a white heat in the case of some of the powdered 

 elements. By means of a small magnet this point of incandescence could be moved 

 about over the surface of the anticathode, and the rate of evolution of the gas in the 

 metals was so greatly increased that it did not take very long to reach a state when 

 a discharge, using the normal electrodes, caused but little alteration in the vacuum. 



After some weeks' use the bulb lost all inclination to soften and would tend to 

 harden considerably during the course of a long run : the absorbed gas would usually be 

 expelled again if the bulb were given a rest. Occasionally, too, it would happen that 

 one anticathode would harden the bulb, while another would soften it. Platinum, it 

 was noticed, almost always tended to harden the tube. After running for an hour or 

 so, however, matters would usually adjust themselves, and useful measurements could 

 be made if they were taken at definite intervals. The tendency of the bulb to harden 



* ORGLER, 'Ann. der Physik,' I., p. 159 (1900). 



