318 Mr. L. L. Campbell on 



disintegrated badly when an electric discharge was sent 

 through tubes containing the above gases in a rarefied state. 



c © © 



For the spectroscopic study o£ the monatoinic gases Baly * 

 has called attention to a special form of aluminium cathode 

 that claims to lessen the disintegration during the electric 

 discharge. 



In gases other than the monatomic ones, the aluminium 

 cathode was supposed not to disintegrate, or the amount oi: 

 disintegration was considered to be negligible. 



In the autumn of 1913, while bombarding AgCl with 

 cathode rays from an aluminium cathode, Prof. J. J. Thom- 

 son found that the discharge-tube became heavily coated 

 with a metallic mirror, which on examination proved to be 

 aluminium. It was supposed that the cathode rays had 

 decomposed the AgCl into Ag and CI, and that in some way 

 the CI had been instrumental in bringing about the dis- 

 integration of the aluminium cathode. 

 © 



The following preliminary investigation was undertaken 

 to see if the aluminium cathode did disintegrate in the 

 presence of chlorine, and of other gases not included under 

 the list of monatoinic gases. 



©■ 



General Method of Procedure. 

 The discharge-tubes first used were of the type shown in 

 fig. 1. The length of the tubes was about 30 cm., the 

 diameter of the spherical bulb G cm., the diameter of the 

 larger cylindrical portion 2*5 cm., while that of the smaller 

 end portions was 1 cm. The anode consisted of a small 

 fused mass of aluminium, while the cathode was made of 

 an aluminium wire 2 mm. in diameter and 4 cm. long, to one 

 end of which was fastened an aluminium disk 1*0 mm. thick 

 and 1*5 cm. in diameter. To the discharge- tube w r as con- 

 nected an annex tube, X (fig. 1), in which could be placed 

 easily decomposable salts for supplying the desired gases to 

 the discharge-tube. The discharge-tube was connected to a 

 water-pump, to a mercury pump, McLeod gauge, drying- 

 tube, and charcoal tube for liquid air. The tube was 

 exhausted by means of the pumps, and the electric discharge 

 from an induction coil was allowed to pass through the tube 

 for a considerable time to drive out the major portion of 

 the gases occluded in the electrodes. A pressure of about 

 O'Ol mm. was obtained with the mercury pump, then the 

 pressure was reduced to the X-ray stage by the application 

 of liquid air to the charcoal tube. Then a small amount of 

 gas or vapour was let into the discharge-tube from the annex 



* Baly's ' Spectroscopy,' 2nd edition, p. -126 (1912). 



