374 KECENT PROGRESS IN PHYSICS. 



"■ On using a smaller ball, the general brush, was smaller, and the 

 sound, though weaker, more continuous. On resolving the brush into 

 its elementary parts as before these were found to occur at much 

 shorter intervals. 



" Employing a wire with a round end, the brush was still smaller., 

 but, as before, separable into successive discharges. The sound, though 

 feebler, was higher in pitch, being a distinct musical note." 



The sound is in fact due to the recurrence of the noise of each 

 separate discharge, and these happening at intervals nearly equal, 

 under ordinary circumstances, cause a definite note to be head, whose E 

 pitch rises with the increased rapidity and regularity of the discharge. 



" By using wires with finer terminations, smaller brushes were 

 obtained, until they could hardly be distinguished as brushes. But 

 as long as sound was heard the discharge could be ascertained by the 

 eye to be intermitting ; and when the sound ceased the light became 

 continuous as a glow." 



To those not accustomed to use the eye in the above-described man- 

 ner, Wheatstone's apparatus with the revolving mirror is recom- 

 mended. Another excellent process for analyzing the brush is to 

 produce it on the end of a rod, held in the hand opposite to the prime 

 conductor, and then move the rod rapidly from side to side, whilst the 

 eye remains still.— (1428— 1423.) 



§ 83. The brush in various gases. — The experiments on the brush in 

 various gases Faraday made with brass rods, about one quarter of ati 

 inch tliick, and whose rounded ends were placed opposite each other 

 in a glass globe of seven inches diameter, containing the gas. One 

 of these rods was connected with the prime conductor, the other with 

 the ground. — (Pog. Ann., XL VII, 553.) 



" Air. Fine positive brushes are easily obtained in air, at common 

 pressures, possessing the well known purplish light. When the air 

 is rarefied the ramifications are very long, filling the globe; the light 

 is greatly increased and is of a beautiful purple color, with an occa- 

 sional rose tint in it. 



" Oxygen. At common pressures the brush is very close and com- 

 pressed, and of a dull whitish color. In rarefied oxygen the form and 

 appearance are better ; the color somewhat purplish, but all the char- 

 acters very poor compared to those in air." 



. "Nitrogen gives brushes with great facility at the positive surface, far 

 beyond any other gas." " They are almost always fine in form, light, 

 and color, and in rarefied nitrogen are mao-nificent. They surpass 

 the discharges in any other gas as to the quantity of light evolved." 



" Hydrogen, at common pressures, gives a better brush than oxygen, 

 but does not equal nitrogen ; the color was greenish gray. In rare- 

 fied hydrogen the ramifications were very fine in form and distinctness, 

 but pale in color, with a soft and velvety appearance, and not at all 

 equal to those in nitrogen. In the rarest state of the gas the color 

 was a pale gray green." 



"Coal gas. The brushes were rather difficult to produce." "They 

 were short and strong, generally of a greenish color." "In rare 

 coal gas the brush forms were better, but the light very poor and the 

 color gray." 



