122 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XII.) 



site to it was luminous at a distance ; being nearer it did not destroy the light and 

 sound of the negative brush, but only tended to have a brush produced on itself, which, 

 at a still nearer distance, passed into a spark joining the two surfaces. 



1472. When the distinct negative and positive brushes are produced simultaneously 

 in relation to each other in air, the former almost always has a contracted form, as 

 in fig. 11., very much indeed resembling the figure which the positive brush itself has 

 when influenced by the lateral vicinity of positive parts acting by induction. Thus 

 a brush issuing from a point in the re-entering angle of a positive conductor has the 

 same compressed form (fig. 12.). 



1473. The character of the negative brush is not affected by the chemical nature 

 of the substances of the conductors (1454.), but only by their possession of the con- 

 ducting power in a greater or smaller degree. 



1474. Rarefaction of common air about a negative ball or blunt point facilitated 

 the development of the negative brush, the effect being, I think, greater than on a 

 positive brush, though great on both. Extensive ramifications could be obtained from 

 a ball or end electrified negatively to the plate of the air-pump on which the jar 

 containing it stood. 



1475. A very important variation of tHe relative forms and conditions of the posi- 

 tive and negative brush takes place on varying the dielectric in which they are pro- 

 duced. The difference is so very great that it points to a specific relation of this 

 form of discharge to the particular gas in which it takes place, and opposes the idea 

 that gases are but obstructions to the discharge, acting one like another and merely 

 in proportion to their pressure (1377-)- 



1476. In air, the superiority of the positive brush is well known (1467. 1472.). 

 In nitrogen, it is as great or even greater than in air (1458.). 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 (1459.). In oxygen, the po- 

 sitive brush is compressed and poor (1457-); whilst the negative did not sink in 

 character: the two were so alike that the eye frequently could not tell the one from 

 the other, and this similarity continued when the oxygen was gradually rarefied. 

 In coal gas the brushes are difficult of production as compared to nitrogen (1460.), 

 and the positive not much superior to the negative in its character, either at com- 

 mon or low pressures. In carbonic acid gas, this approximation of character also 

 occurred. In muriatic acid gas the positive brush was very little better than the ne- 

 gative, and both difficult to produce (1462.) as compared with the facility in nitrogen 

 or air. 



1477- These experiments were made with rods of brass about a quarter of an inch 

 thick having rounded ends, the ends being opposed in a glass globe 7 inches in dia- 

 meter, containing the gas to be experimented with. The electric machine was used 

 to communicate directly, sometimes the positive, and sometimes the negative, state, 

 to the rod in connection with it. 



