On the Electric Discharge in Rarefied Gases. 235 



tions of all the ramifications of the discharge recognizable b} r 

 the eye. This phosphorescent drawing shows moreover a 

 marvellous number of finer ramifications, which the eve is 

 not able to trace in the discharge itself which produces them. 

 As this phosphorescent light produced by the discharge is 

 much brighter than the light directly emitted by the discharge, 

 we are probably right in supposing that the phosphorescence 

 in the fine ramifications represents parts of the discharge 

 whose light is too weak for their direct observation, in the 

 study of which, therefore, the production of phosphorescence 

 renders useful aid. I hope to be able to show, later on, that 

 the study of such branching brush-discharges is absolutely 

 necessary to a more intimate knowledge of the fear i^o^rjv 

 electric spark (so-called) and of forked lightning. 



The green phosphorescence produced in finely-powdered 

 glass by such discharges was recognizable as the pressure was 

 gradually reduced, when the pressure reached 50 millims.; 

 and with pulverized calc-spar, it showed itself in a magnificent 

 orange-red pattern, even at atmospheric pressure. As the 

 pressure of the gas diminishes, the brightness of the phospho- 

 rescent light increases, and at the same time the figures extend 

 over greater surface and increase in fineness and richness of 

 detail. Instead of exciting this phosphorescence by connecting 

 both electrodes with the induction-coil and touching the outer 

 surface by a conductor, they may be produced even in greater 

 perfection by disconnecting the wire from the electrode not 

 surrounded by the powder, and bringing it, instead of the con- 

 ducting body, into contact with the outer surface of the glass 

 in the neighbourhood of the powder. We observe in this case 

 that the luminous figures possess a different habit according to 

 the polarity of the touching wire. These are amongst the 

 most beautiful of the luminous phenomena produced by elec- 

 tricity. 



The second mode of phosphorescence is less striking in the 

 form in which it appears, but leads to important conclusions 

 on the nature of the so-called positive light. Whilst the 

 kathode-light radiates in inflexibly straight lines, it appears 

 as if the positive light always consisted of bundles of more 

 flexible rays following each bend of the discharge-tube — if 

 indeed we employ the word ray at all for so great an enfeeble- 

 ment of the properties of negative light. 



This view is decisively opposed by experiments which I 

 have made during the last year, at least for the positive light 

 produced in gas of very small density. When the positive light 

 fills a highly exhausted tube which is bent at any point (with- 

 out alteration of section, see woodcut) we observe as follows : — 



S2 



