412 Prof. E. Wiedemann on the Behaviour of Gases 



An experiment to determine whether there was any differ- 

 ence in time between the appearance of the violet light and of 

 the green gave no reliable result — partly, no doubt, because of 

 the feebleness of the light at c. 



4. I have, further, made experiments to determine whether 

 the kathode-rays propagate themselves with a measurable ve- 

 locity; and employed for this purpose a discharge-tube 1 metre 

 long, provided with a disk electrode at one end, which at a 

 sufficient exhaustion sent ihe kathode-rays through the entire 

 tube. When the tube was viewed in a revolving mirror, the 

 phosphorescence appeared at the same instant at both ends of 

 the tube. 



5. In order to examine the influence of the shape of the 

 electrodes on the number of discharges, and the potential ne- 

 cessary to produce them, the following experiments were made. 



The following pieces of apparatus were connected together 

 by fusion onto a forked tube : — 1st, the apparatus fig. 11 from 

 a to m ; secondly, the apparatus fig. 10 ; thirdly, a discharge- 

 tube similar to fig. 11, except that at b there was a disk parallel 

 to a. The whole was exhausted, so that when the disks were 

 negative the whole tube shone brightly with green light. In 

 the revolving mirror the following results were obtained: — 



(a) With the apparatus fig. 10, no difference whether the 

 positive or the negative electrode was put to earth ; the green 

 band of light appeared almost entirely continuous. 



(b) With the apparatus with the two disks the same is the 

 case. 



(c) With the apparatus fig. 11, when the disk is negative 

 no discontinuity is observed in the revolving mirror; but if it 

 is positive, then the appearances represented in fig. 13 are 

 observed. They consist of single, sharp, very bright images 

 of the tube, e. g. b and d; at the side of which is seen a much 

 feebler preceding discharge, a a, cc, sharply bounded at its 

 commencement and of nearly equal intensity throughout. If 

 the velocity of rotation of the electrical machine be increased, 

 the appearance is not altered. If the outside of the tube be 

 connected with the earth, the broad strips are resolved into 

 separate images. The appearance is probably to be explained 

 by the charge taken by the tube. At very small pressures, 

 therefore, not only the number of the discharges, but the whole 

 mechanism depends to a great extent on the form of the elec- 

 trodes. 



6. In order to measure the influence of the kathode-rays 

 radiating from the negative electrode on the intensity of the 

 current, a mica disk provided with a hinge was so placed be- 

 tween two disk electrodes opposed to each other at a distance 



