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



dark discharge need not, of necessity, occur. But I incline to the former opinion, 

 and think, that the diminution in size of the negative brush, as the positive glow 

 comes on to the end of the opposed wire, is in favour of that view. 



1551. Using rarefied air as the dielectric, it is very easy to obtain luminous pheno- 

 mena as brushes, or glow, upon both conducting balls or terminations, whilst the 

 interval is dark, and that, when the action is so momentary that I think we cannot 

 refer to currents as affecting discharge across the dark part. Thus if two balls, 

 about an inch in diameter, and 4 or more inches apart, have the air rarefied about 

 them, and are then interposed in the course of discharge, an interrupted or spark 

 current being produced at the machine *, each termination may be made to show lu- 

 minous phenomena, whilst more or less of the interval is quite dark. The discharge 

 will pass as suddenly as a retarded spark (295. 334.), i. e. in an interval of time al- 

 most inappreciably small, and in such a case, I think it must have passed across 

 the dark part as true disruptive discharge, and not by convection. 



1552. Hence I conclude that dark disruptive discharge may occur (1547. 1550.) ; 

 and also, that, in the luminous brush, the visible ramifications may not show the full 

 extent of the disruptive discharge (1444. 1452.), but that each may have a dark out- 

 side, enveloping, as it were, every part through which the discharge extends. It is 

 probable, even, that there are such things as dark discharges analogous in form to 

 the brush and the spark, but not luminous in any part (1445.). 



1553. The occurrence of dark discharge in any case shows at how low a tension 

 disruptive discharge may occur (1548.), and indicates that the light of the ulti- 

 mate brush or spark is in no relation to the intensity required (1368. 1370.). So to 

 speak, the discharge begins in darkness, and the light is a mere consequence of 

 the quantity which, after discharge has commenced, flows to that spot and there finds 

 its most facile passage (1418. 1435.). As an illustration of the growth generally of 

 discharge, I may remark that, in the experiments on the transition in oxygen of the 

 discharge from spark to brush (1518.), every spark was immediately preceded by a 

 short brush. 



1554. The phenomena relative to dark discharge in other gases, though difflering 

 in certain characters from those in air, confirm the conclusions drawn above. The 

 two rounded terminations (1544.) (fig. 19.), were placed in muriatic acid gas (1445. 

 1463.) at the pressure of 6*5 inches of mercury, and a continuous machine current of 

 electricity sent through the apparatus : bright sparks occurred until the interval was 

 about or above an inch, when they were replaced by squat brushy intermitting glows 

 upon both terminations, with a dark part between. When the current at the ma- 

 chine was in spark, then each spark caused a discharge across the muriatic acid gas, 

 which, with a certain interval, was bright ; with a larger interval, was straight across 



* By spark current I mean one passing in a series of spark between the conductor of the machine and the 

 apparatus : by a continuous current one that passes through metallic conductors, and in that respect without 

 interruption at the same place. 



