376 RECENT PROGKESS IN PHYSICS, 



a brush formed on it, extending to the negative ball ; and when 

 still nearer, the brush ceased and bright sparks passed." 



As we have already seen, § 80, the spark discharge passes into the 

 brush at far less distances if the surface on which the discharge begins 

 (the small ball or the rounded end of a rod) is negative, than if it is 

 positive ; but on going further into the succession of charges we find 

 that the positive brush passes into glow long before the negative. 



" A metal rod 0.3 of an inch in diameter, with a rounded end pro- 

 brush. It was ascertained, both by sight and sound, that the succesisve 

 jecting into the air, was charged negatively and gave a short noisy 

 discharges were very rapid in their recurrence, six or seven times more 

 numerous than when the rod was charged positively to an equal 

 degree." 



' ' When the rod was positive it was easy, by working the machine 

 a little quicker, to replace the brush by a glow, but when it was nega- 

 tive no efforts could produce this change." — (1468.) 



"A point opposite the negative brush exhibited a star, and, as it was 

 approximated, caused the size and sound of the brush to diminish, and 

 at last to cease, leaving the negative end silent and dark, yet effective 

 as to discharge." — (1469.) 



" When the round end of a smaller wire was advanced towards the 

 negative brush, it (becoming positive by induction) exhibited the quiet 

 glow at eight inches distance, the negative brush continuing. When 

 nearer, the pitch of the sound of the negative brush rose, indicating 

 quicker intermittances ; still nearer the positive end threw off ramifi- 

 cation and distinct brushes, at the same time the negative brush con- 

 tracted in its lateral direction and collected together, giving a peculiar, 

 narrow, longish brush, in shape like a hair pencil ; the two brushes 

 existing at once, but were very different in their form and appearance, 

 and especially in the more rapid recurrence of the negative discharges 

 than of the positive. On using a smaller positive- wire for the same 

 experiment the glow first appeared in it and then the brush, and 

 the two at one distance became exceedingly alike in appearance." 

 (1470.) 



"In air the superiority of the positive brush is well known. In 

 nit7'ogen it is as great or even greater than in air. In hydrogen the 

 positive brush loses a part of its superiority, not being so good as in 

 nitrogen or air, whilst the negative brush does not seem injured. 

 In oxygen the positive brush is compressed and poor, whilst the nega- 

 tive did not become less ; the two were so alike that the eye frequently 

 could not tell one from the other. In coal gas the brushes are 

 difficult of production ;" " and the positive not much superior to the 

 negative, either at common or low pressure. In carbonic acid this 

 approximation of character also occurred. In muriatic acid gas the 

 positive brush was very little better than the negative." — (1476.) 



§ 86. Glow discharge. — The glow " seems to depend upon a quick 

 and almost continuous charging of the air close to and in contact with 

 the conductor." — (Faraday's Researches, 1526.) Faraday was never 

 able to separate it into visible intermitting elementary discharges. 

 The glow is produced by — 



1st. Diminution of the charging surface. — At the end of a metal rod 



