238 Mr Kaye , The Selective Absorption of Rontgen Rays. 
was fastened a piece of soft iron, and by means of a small electro- 
magnet outside, it proved very easy to move the car along the rails 
and bring any metal desired under the beam of cathode rays. 
In every case the metal of the anticathode had its surface 
cleaned and polished. This was usually done first with the finest 
flour emery and finally with jeweller’s rouge. The lead plate had 
its surface renewed by planing. 
With the first batch of metals introduced, a small Willemite 
screen took the place of one of the metals. This was prepared by 
dusting finely crushed Willemite (natural silicate of zinc) on a 
small piece of glass plate previously moistened with a weak solu- 
tion of water-glass (silicate of soda), and then drying over a small 
flame. Such a screen does not impair the vacuum by giving off 
gas when a discharge plays on it. The cathode rays, when they 
struck the screen, produced a bright green elliptical patch of 
phosphorescence, which served as a test of whether the rays would 
be truly directed on the anticathode metals which were in line 
with the screen on the car. 
The whole anticathode system was earthed and joined outside 
by a wire to the anode. The discharge was generated by a 6-inch 
Apps Induction Coil. In parallel with the tube was an adjustable 
spark gap between two brass spheres 254 cms. in diameter. 
These were kept well cleaned and polished, and their distance 
apart for the first spark gave a rough measure of the mean 
potential difference between the terminals of the tube. 
A Topler pump was used to produce the vacua, and a 
McLeod gauge gave a measure of the pressures. With such a 
large mass of metal in the tube it was to be expected that a con- 
siderable quantity of gas would be evolved when the pressure was 
reduced, and indeed these expectations were not belied, for it took 
some weeks of pretty steady pumping with the discharge running 
before any readings could be taken. At one stage one could not 
keep pace with the evolution of gas, and it was difficult, with the 
discharge running, to get below a pressure in the neighbourhood 
of mm. 
Such pressures are very favourable to cathodic disintegration, 
and aluminium deposits were in this way formed on the anti- 
cathode. It may be worth noting that the evolution of gas was 
hastened by reversing the discharge, and thus using the anti- 
cathode as cathode. The plan has moreover the advantage of 
splashing off on to the surrounding glass any cathodic deposit 
which may have been formed on the anticathode. The formation 
of these disintegration deposits, it was found, was almost if not 
entirely prevented by earthing the anticathode system and with 
it the anode. 
At this point it is interesting to note the behaviour of the lead 
