1891.] liquid electrodes in vacuum tubes. 225 



glass of the tube sets up this luminosity, to be gas molecules 

 charged at the cathode and projected with great velocity at 

 right angles to its surface. According to Goldstein and others 

 the direction of projection varies to some extent from the 

 normal. Professor Crookes 1 appears to have originally thought 

 these molecular streams peculiar to very low vacua and indicative 

 of a fourth state of matter, bearing to the gaseous state some- 

 what the same relation as it bears to the liquid. Messrs Spottis- 

 woode and Moulton 2 have, however, shown that phosphorescence 

 can be produced at quite high pressures provided the intensity 

 of the negative discharge be sufficiently increased. 



These latter observers 3 have treated with great fulness other 

 less conspicuous phosphorescent effects. They found that under 

 certain conditions a portion of the wall of a vacuum tube touched 

 by the finger or having an earthed conductor in its neighbour- 

 hood acts as a sort of secondary cathode, setting up phosphor- 

 escence on the opposite side of the tube. They also found that 

 at very low pressures the positive discharge when in the form 

 of a hazy luminous column, occupying in general only a portion 

 of the tube's cross-section, creates a sort of demand for negative 

 electricity which may be supplied by a discharge proceeding from 

 the walls of the tube in directions at right angles to its length 

 and creating phosphorescence. Further when the positive column 

 is cut at an angle by the wall of the tube, as at a sharp bend, 

 phosphorescence appears whose position is as if it were due to 

 the impact of molecules travelling along the positive column 

 in the direction from cathode to anode. This latter phenomenon 

 has been more exhaustively treated by Goldstein 4 . He found 

 that at very low pressures in a tube bent at right angles any 

 number of times, there is at every bend between the cathode 

 and anode where the positive column extends, a patch of phos- 

 phorescence, situated as if due to rays travelling from the cathode 

 to the anode. Supposing an electrode at A in a tube AD at 

 right angles to a tube BDE, and that the latter tube is closed 

 at the end B, while a third tube at right angles to it leads 

 from E to an electrode G, then according to Goldstein, if I in- 

 terpret him correctly, there is phosphorescence at E whether 

 the cathode be A or C, but none at B unless C is cathode. 



Goldstein 5 also describes the production of phosphorescence 

 at positions which could not be reached by molecular streams 

 travelling in straight lines from the cathode, which he apparently 



1 Phil. Trans. 1879, pp. 142, 143, 163, 164, etc. 



2 Phil. Trans. 1880, pp. 582—6, etc. 



3 1. c, pp. 602—6, 616—7, 620, etc. 



4 Wied. Ann. 12, 1881, pp. 104—9, and figs. 16 and 17, Taf. i. 



5 Wied. Ann. 15, 1882, pp. 246—254, 



