138 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XIII.) 



the same action in the same place, whereas the ramification is due to a momentary, 

 independent and intermitting- action of the same kind. 



Dark Discharge. 



1544. I will now notice a very remarkable circumstance in the luminous discharge 

 accompanied by negative glow, which may, perhaps, be correctly traced hereafter 

 into discharges of much higher intensity. Two brass rods, 0*3 of an inch in diameter, 

 entering a glass globe on opposite sides, had their ends brought into contact, and 

 the air about them very much rarefied. A discharge of electricity from the machine 

 was then made through them, and whilst that was continued the ends were separated 

 from each other. At the moment of separation a continuous glow came over the 

 end of the negative rod, the positive termination remaining quite dark. As the di- 

 stance was increased, a purple stream or haze appeared on the end of the positive 

 rod, and proceeded directly outwards towards the negative rod ; elongating as the 

 interval was enlarged, but never joining the negative glow, there being always a short 

 dark space between. This space, of about -j-Vth or V^th of an inch, was apparently 

 invariable in its extent and its position, relative to the negative rod ; nor did the ne- 

 gative glow vary. Whether the negative end were inductric or inducteous, the same 

 effect was produced. It was strange to see the positive purple haze diminish or 

 lengthen as the ends were separated, and yet this dark space and the negative glow 

 remain unaltered (fig. 19.). 



1545. Two balls were then used in a large air pump receiver, and the air rarefied. 

 The usual transitions in the character of the discharge took place ; but whenever the 

 luminous stream, which appears after the spark and the brush have ceased, was itself 

 changed into glow at the balls, the dark space occurred, and that whether the one or 

 the other ball was made inductric, or positive, or negative. 



1546. Sometimes when the negative ball was large, the machine in powerful action, 

 and the rarefaction high, the ball would be covered over half its surface with glow, 

 and then, upon a hasty observation, would seem to exhibit no dark space : but this 

 was a deception, arising from the overlapping of the convex termination of the nega- 

 tive glow and the concave termination of the positive stream. More careful obser- 

 vation and experiment have convinced me, that when the negative glow occurs it never 

 visibly touches the luminous part of the positive discharge, but that the dark space 

 is always there. 



1547. This singular separation of the positive and negative discharge, as far as 

 concerns their luminous character, under circumstances which one would have 

 thought very favourable to their coalescence, is probably connected with their differ- 

 ences when in the form of brush, and is perhaps even dependent on the same 



