504 



Dr. ft. S. "Willows on the 



induction-coil, produced similar changes. For example*, 

 they found that on first passing the discharge through tubes 

 supplied by Geissler, the appearance of the luminosity was 

 totally altered after the current had been passing for a few 

 minutes only, and hence they had to discontinue the use of 

 tubes sealed off from the pump. 



More recently it is known by all who have had experience 

 with ftontgen-ray bulbs, that a tube giving out soft rays, of 

 little use in surgical applications, may, by continuous use, be 

 made to emit rays of the most penetrating kind, and that 

 finally the tube requires heating in order to admit of the 

 discharge passing at all. When this stage is reached in the 

 life of a bulb, the only way to restore it to its original 

 condition is to let in fresh gas and re-pump it. 



By an examination of the spectrum of the discharge. 

 Hutchins t found that if a trace of foreign gas or vapour be 

 admitted into a tube in which the pressure is very low, the 

 spectral lines of this gas disappeared after a few seconds, 

 leaving only the hydrogen lines visible. The latter are 

 no doubt due to the water-vapour, from which it is so difficult 

 to free the tube. 



While working on the striated positive column by the help 

 of a large battery of small storage-cells, I noticed that the 

 pressure was liable to undergo variation if the current was 

 kept on for many minutes, and that if the tube had been in 

 use for some time there was always an apparent absorption 

 of gas. 



Prof. J. J. Thomson informed me that he had carried out 

 some experiments on the subject with an induction-coil as the 

 source of current, and suggested that I should continue the 

 investigation, especially as a battery lends itself much better 

 to quantitative results than the intermittent and irregular 

 supply from a coil. Qualitatively the results are the same 

 whether a coil or a battery be used. 



The experiments have had for their object £he investigation 

 of the conditions influencing the absorption of gas in a 

 Crookes tube, and if possible to throw light on its desti- 

 nation. 



1. The apparatus used was that shown in fig. I. 

 A Toepier pump could be put in communication with the 

 remainder of the apparatus by a tap B. Next comes the 

 tube N to be experimented on. C is a large bulb containing 

 P 2 5 ; it can be put in communication with the exterior by 

 means of a three-way tap F, and with N through two tap< 



* Phil. Trans. Part i. 1878, p. 155. 



+ Hutclrius, Amer. J burn, of Science, vol. vii. 1893, p. 61. 



