1456. Air.—­Fine positive brushes are easily obtained in air at common pressures, and possess the well-known purplish light.  When the air is rarefied, the ramifications are very long, filling the globe (1477.); the light is greatly increased, and is of a beautiful purple colour, with an occasional rose tint in it.

1457. Oxygen.—­At common pressures, the brush is very close and compressed, and of a dull whitish colour.  In rarefied oxygen, the form and appearance are better, the colour somewhat purplish, but all the characters very poor compared to those in air.

1458. Nitrogen gives brushes with great facility at the positive surface, far beyond any other gas I have tried:  they are almost always fine in form, light, and colour, and in rarefied nitrogen, are magnificent.  They surpass the discharges in any other gas as to the quantity of light evolved.

1459. Hydrogen, at common pressures, gave a better brush than oxygen, but did not equal nitrogen; the colour was greenish gray.  In rarefied hydrogen, the ramifications were very fine in form and distinctness, but pale in colour, with a soft and velvety appearance, and not at all equal to those in nitrogen.  In the rarest state of the gas, the colour of the light was a pale gray green.

1460. Coal gas.—­The brushes were rather difficult to produce, the contrast with nitrogen being great in this respect.  They were short and strong, generally of a greenish colour, and possessing much of the spark character:  for, occurring on both the positive and negative terminations, often when there was a dark interval of some length between the two brushes, still the quick, sharp sound of the spark was produced, as if the discharge had been sudden through this gas, and partaking, in that respect, of the character of a spark.  In rare coal gas, the brush forms were better, but the light very poor and the colour gray.

1461. Carbonic acid gas produces a very poor brush at common pressures, as regards either size, light, or colour; and this is probably connected with the tendency which this gas has to discharge the electricity as a spark (1422.).  In rarefied carbonic acid, the brush is better in form, but weak as to light, being of a dull greenish or purplish line, varying with the pressure and other circumstances.

1462. Muriatic 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 when 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 (1455.) from the machine was used, still I could only with difficulty obtain a brush, and that very short, though I used rods with rounded terminations