1891.] liquid electrodes in vacuum tubes. 227 



These observers found that the illumination of the cathode 

 surface by ultra-violet light, which in general has at high gaseous 

 pressures a remarkable effect in facilitating the discharge, ceases 

 to have any certain effect at low pressures when the presence 

 of phosphorescence shows the existence of molecular streams. 



Their explanation is that ultra-violet light promotes the dis- 

 charge by setting up at the cathode surface, apparently in the 

 ether, vibrations of great rapidity such as would be produced 

 by heating the cathode to a high temperature — in general a 

 most efficacious way of promoting discharge — , but that the mole- 

 cular streams are the consequence or the concomitant of the 

 production of such rapid vibrations by some independent cause 

 which they do not specify. Thus the function which the ultra- 

 violet light performs at high pressures, is already fully provided 

 for at low pressures. 



The point I more specially wish to bring out is this. At 

 high pressures whatever tends to increase the violence of the 

 negative discharge — e.g. an air spark in the circuit on the cathode 

 side of the tube — tends to set up the molecular streams. There 

 seems, however, strong grounds for believing that the production 

 of these streams actually facilitates the discharge, rendering it 

 less violent and disruptive than it otherwise would be, especially 

 at very low pressures. 



As regards the nature of the discharge itself various views 

 are entertained, to some of which reference is necessary to explain 

 what follows. In the opinion of Professors J. J. Thomson 1 , 

 Schuster 2 and others the ordinary vacuum tube discharge is 

 in a way electrolytic. The molecules of gas become dissociated 

 and the atoms act as carriers of positive and negative electricity. 

 The heating of the cathode or anode, as the case may be, or the 

 presence of a heated wire, influences which have been shown by 

 Hittorf 3 , Elster and Geitel 4 and others to promote discharge 

 in a wonderful way, operate on this theory by dissociating the 

 gas and so furnishing atoms ready to respond at once to the 

 directive action of the external source of electricity. At a 

 luminous part of the discharge there is a production of heat 

 owing to the coming together of oppositely charged atoms, at a 

 dark part such coalitions are rare. Other writers, e.g. Lehmann 5 

 and E. Wiedemann 6 , regard the discharge as usually of two co- 



1 Phil. Mag. Vol. xv., 1883, pp. 427—434; Aug. 1890, pp. 129—140; March 

 1891, pp. 149—171, etc. 



2 Proceedings of the Roijal Society, Vol. xxxvir., 1884, pp. 317 — 339, and Vol. 

 XLifc, 1887, pp. 371—9. 



3 Wied. Ann. 21, 1884, pp. 106—139. 



4 Wied. Ann. 37, 1889, pp. 315—329, and Ann. 38, 1889, pp. 27—39, etc. 



5 Molekularphysik, Bd. n. pp. 220 et seq. 



6 Wied. Ann. 35, 1888, p. 256. 



VOL. VII. PT. IV. 18 



