410 Prof. E. Wiedemann on the Behaviour of Gases 



Experiments on the Discharge in very high Vacua. 



1. Beitlinger and Urbanitsky*, Spottiswoodef, and others 

 have found that on approaching the finger to, or on touching 

 a highly exhausted Geissler's tube, the discharge is deflected. 

 I have further shown J that this phenomenon occurs especially 

 with the positive discharge, and that sometimes the inner and 

 sometimes the outer surface of the tube phosphoresces with 

 green light. 



By new experiments I have established that the point of the 

 tube put into contact with the ground behaves in every respect 

 like a negative electrode ; for 



{a) a faint reddish light shows itself on the inner face of 

 the tube-wall, the spectrum of which is exactly similar to that 

 of the red glow which forms about the negative electrode; 



(b) the discharge given off from the wall by a positive or 

 insulated electrode throws images on the opposite wall, which 

 are displaced on the approach of a magnet. 



(c) if the apparatus (fig. 10) be employed, and a be con- 

 nected with the positive pole of the machine, b with the nega- 

 tive pole, and if the wall be touched at d, then at /there is 

 formed a magnified shadow of b, in the same way as one ne- 

 gative electrode throws a shadow of another. The properties 

 and forms of these shadows have been thoroughly studied by 

 Goldstein §. 



The discharge sometimes loses the property of sensitiveness 

 in a very remarkable manner, as Spottiswoode has observed. 

 An experiment which I have made shows this very plainly. 

 The tube used was 5 metres long and 5 millims. wide. It was 

 covered with a spiral strip of tinfoil for a distance of about 

 half a metre. If the discharge was rendered sensitive by in- 

 terpolating air-sparks, and the tinfoil strip connected with the 

 ground, there was produced on the glass wall opposite to it a 

 similar spiral of phosphorescent light, which became con- 

 tinually feebler from the positive electrode towards the nega- 

 tive, and at last disappeared. Also at points more than two 

 metres from the point of contact there was hardly any green 

 light produced by approach of the finger. It was as if the dis- 

 charge had changed its form of motion, and lost the power of 

 producing phosphorescence. 



It is to be noticed that the green light of the glass does 

 not appear most brilliantly in the parts of the tube traversed 



* Reitlinger and Urbanitzky, Beibl. i. p. 416 (1877). 

 t Spottiswoode, Beibl. iii. p. 643 (1879). 

 X E. Wiedemann, Wied. Ann. ix. p. 157 (1880). 



§ Goldstein, Berl. Ber. p. 284 (1876), and JEine neue Form elect?'ischer 

 Abstossung (Berlin, T. Springer, 1880), 



