598 Mr. A. M. Tyndall on the 



7 



2*8 centimetres. This sudden change in ~ was always 



accompanied by a speck of light on the plate ; this is no 

 doubt the source of a back discharge which effectually re- 

 duces the value of p. The effect was observed in both positive 

 and negative discharge. Such a phenomenon will occur, it' 

 at any region on the plate the back discharge accidentally 

 increases ; the lines of force from the point will then con- 

 verge towards that region and concentrate the current there, 

 thus tending to increase the back discharge still further, and 

 so intensify the concentration of the lines. In the earlier 

 work on hydrogen when, in negative discharge, this back 

 discharge was known to be present, very marked effects of 

 this kind were observed (see Phil. Mag. xix. p. 455). 



This instability apparently sets in when the field at the 

 plate reaches some critical value, since the necessary distance 

 between point and plate decreases as the current increases. 

 For values of z less than that from to B, it may be assumed 

 that the back discharge is general over the surface of the 

 plate, but increases with increasing field and current. 



Discharge in Hydrogen. 



Franck* has suggested that gases may be arranged in the 

 following order according to the magnitude of the affinity 

 which their molecules possess for negative electrons :— 

 chlorine, nitric oxide, oxygen, hydrogen, nitrogen, argon, 

 and helium. According to him chlorine and oxygen mole- 

 cules, for instance, have a strong affinity for negative ions, 

 but the gases at the other end of the series have comparatively 

 little. Thus he has shown that when argon and nitrogen have 

 had all electropositive impurities such as oxygen and chlorine 

 eliminated irom them, the velocities of the negative ions rise 

 to very high values, even at atmospheric pressure. For 

 instance, in pure argon the velocity of a negative ion was 

 206*3 cms. per volt cm., and in pure nitrogen 80-145 cms.; 

 but traces of oxygen reduced these to normal values 1*70 

 and 1*84 respectively. The velocity of the positive ion was 

 normal throughout ; thus in pure nitrogen it was 1'27 and 

 in impure nitrogen 1*30. 



Previous to this Prof. A. P. Chattock and the author (Phil. 

 Mag. xix. 1910) found effects in the wind-pressure work in 

 hydrogen which may be similarly explained. These experi- 

 ments, however, were complicated by the presence of an 

 unknown amount of back discharge from the plate, so that, 

 in negative discharge in pure hydrogen, the direction of the 



* Verh. d. D. Fhys. Gesell. xii. pp. 291 & 613 (1910). 



