Disintegration of the Aluminium Cathode. 349 



tube. If too much gas got into the discharge-tube, a portion 

 was frozen out by means of the charcoal-liquid-air tube. 



Fin-. 1. 



Charcoal tube 



In the early work an induction coil was used to excite 

 the discharge, while later the discharge was sent through 

 the tube by means of a battery of small accumulator cells 

 giving a voltage of about 2400 volts. Whenever the elec- 

 trodes were contaminated in any way by use, new ones were 

 introduced in the tubes. 



Disintegration in Chlorine.— -The chlorine was prepared by 

 placing a small amount of gold chloride, Au01 3 , in the annex 

 tube, and gently heating the salt by holding a lighted match 

 beneath the bulb of the tube. First the water of crystalliza- 

 tion was driven off, and this was removed by the phosphorus 

 pentoxide and the charcoal-liquid-air tube. Gold chloride is 

 said to decompose at about 120° C, at atmospheric pressure, 

 while under the very low pressure of the discharge-tube, it 

 must decompose at a still lower temperature. At any rate, 

 a very gentle heat was all that was required to set free the 

 small amount of chlorine needed. This amount of chlorine 

 did not entirely remove the green fluorescence of the X-ray 

 vacuum staoe. The exact pressure was not measured, as 

 the letting in of chlorine into the McLeod gauge would have 

 contaminated the mercury. It seems fair to assume from 

 the °Teen fluorescence and the width of the cathode dark 

 space*, that the pressure in the tube was much below the 

 pressure of O'Ol mm. 



