RECENT PROGRESS IN PHYSICS. 375 



'■' Carbonic acid produces a very poor brush at common pressures." 

 *' In rarefied carbonic acid the brush is better in form, but weak as to 

 light, being of a dull greenish or purplish hue." 



" 3Iuriatic acid gas. It is very difficult to obtain the brush in this 

 gas at common pressures. On gradually increasing the distance of 

 the rounded ends the sparks suddenly ceased ^^hen the interval was 

 about an inch, and the discharge, which was still through the gas in 

 the globe, was silent and dark. Occasionally, a very short brush could, 

 for a few moments, be obtained, but it quickly disappeared. Even 

 when the intermitting spark current from the machine was used a 

 brush wa-s obtained with difficulty, and that very short ;" "in the 

 mean time, magnificent brushes were passing off from different parts of 

 the machine into the surrounding air. On rarefying the gas the forma- 

 tion of the brush was facilitated, but it was yet of a low, squat form, 

 very poor in light, and very similar on both the positive and negative 

 surfaces." "On rarefying the gas still more a few large ramifica- 

 tions were obtained, of a pale bluish color, utterly unlike those in 

 nitrogen. ' '-—(1456—1462.) 



§84. Brush in denser media. — Electrical brushes are produced, not 

 only in air and gases, but in far denser media. Faraday procured it 

 in oil of turpentine, (1452,) " from the end of a wire going through a 

 glass tube into the fluid, contained in a metal vessel. The brush was 

 small, and yqxj difficult to obtain; the ramifications were simple, 

 and stretched out from each other, diverging very much. The light 

 was exceedingly feeble, a perfectly dark room being required for its 

 observation. When a few solid particles, as of dust or silk, were in 

 the liquid, the brush was produced with much greater facility." 



§ 85. Difference of the positive and negative brush discharge. — On this 

 subject I extract the following remarks by Faraday : 



" When the brush discharge is observed in air, at the positive and 

 negative surfaces, there is a very remarkable difference. ' The differ- 

 ence in question used to be expressed in former times by saying that 

 " a point charged positively gave brushes into the air, whilst the same 

 point charged negatively gave a star." This is true only of bad con- 

 ductors, or of metallic conductors charged intermittingly. If metallic 

 points project freely into the air the positive and negative light upon 

 them differ very little in appearance." 



These phenomena vary exceedingly under different circumstances, 

 as Faraday shows : 



" If a metallic wire, with a rounded termination in free air, be used 

 to produce the brushy discharge, then the brushes obtained when the 

 wire is charged negatively are very pour and small by comparison with 

 those produced when the charge is positive. Or if a large metal ball, 

 connected with the electrical machine, be charged positively, and a 

 fine uninsulated point be gradually brought towards it, a star appears 

 on the point when at a considerable distance, which, though it becomes 

 brighter, does not change its form of a star until it is close up to the 

 ball ; whereas, if the ball be charged negatively, the point, at a con- 

 siderable distance, has a star on it as before ; but when brought nearer, 



