514 Mr. Grove on the Electro-chemical Polarity of Gases. 



attention, may possibly be connected with these phsenomena. I 

 have observed, that in a well-exhausted receiver containing a 

 small piece of phosphorus, the discharge is throughout its course 

 striated by transverse non-luminous bands, presenting a very 

 beautiful eflPect, and a yellow deposit, which, as far as I have 

 yet examined it seems to be allotropic phosphorus, is deposited 

 on the plate of the air-pump and on the neighbouring substances ; 

 to show this effect well the needle should be positive and the 

 plate negative, and the distance between them about an inch. 



I could ddate much fuiiher on these experiments, but have 

 already trespassed perhaps too far for a postscript. Variations 

 in the form of the terminals, in the nature of the gas, vapour, or 

 gaseous mixture, in the density of the gas, in the intensity and 

 quantity of the discharge, in the nature of the plate, &c., will 

 occur to those who may feel incliued to repeat these experi- 

 ments, and if I am not over-sanguine, promise results of much 

 interest. 



Additional Note on the dark discharge, July 9, 1852. 



I find the ti'ansverse dark bands can be produced in other gases when 

 very much attenuated, probably in all; and I rather think the reason why 

 they are more easily seen in the phosphorus vapour is, that all the oxygen 

 having been consumed a better vacuum is formed. 



In addition to these bauds, and under circumstances where they are barely 

 visible, there is always seen a well-defined dark space intervening between 

 the glow surrounding the negative, and the stream of light jji-oceeding from 

 the positive terminal ; it appears iudependentlj' of the length of the dis- 

 charge, though a space of an inch is a convenient distance for exhibiting 

 the eifect well. 



This dark discharge is elaborately described by Faraday as produced by 

 the ordinary electrical machine. Experimental Researches, § 1544 et seq. 



Having in my mind the analogy of interference, it seemed to me that this 

 dark space might be due to the crossing of the lines of discharge from the 

 successive points of the needle, the knob, or plate from which the negative 

 discharge- issues. 



As the positive discharge appears to issue from a point, and not to sur- 

 round the wire, as does the negative, the position of the dark space in close 

 approximation to the negative terminal was in favom- of this view ; if cor- 

 rect, it should follow that if the terminals were coated points instead of 

 wires, knobs or plates, this dark space would not be observed, or its j)osition 

 would be changed. Experiment verified this expectation : when platinum 

 wires sealed in glass were employed and a good vacvumi formed, the line of 

 luminous discharge was continuous when the platinum points were brought 

 to a distance of half an inch. 



When these terminals are so far separated as not to give a continuous 

 line of discharge, a pencil appears on each terminal, which gradually be- 

 comes fainter and fainter towards the middle of the inteiTening space ; and 

 if the distance be great, the discharge ceases to be luminous towards the 

 middle of the intervening space, from excessive difi\ision ; but this will be 

 seen to be a veiy different eft'ect from the abrupt and well-defined dark 

 space which appears in close approximation to tlie negative terminal when 

 the coated wires are not cmjdoyed. 



