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



conductors) of causing discharge with more or less readiness and quickness from the 

 machine*. 



1455. The following are a few of the effects I observed in different gases at the 

 positively charged surfaces, and with atmospheres varying in their pressure. The 

 general effect of rarefaction was the same for all the gases : at first, sparks passed ; 

 these gradually were converted into brushes, which became larger and more distinct 

 in their ramifications, until, upon further rarefaction, the latter began to collapse and 

 draw in upon each other, till they formed a stream across from conductor to con- 

 ductor : then a few lateral streams shot out towards the glass of the vessel from the 

 conductors ; these became thick, flossy, and soft in appearance, and were succeeded 

 by the full constant glow which covered the discharging wire. The phenomena 

 varied with the size of the vessel (1477-)? ^^e degree of rarefaction, and the dis- 

 charge of electricity from the machine. When the latter was in successive sparks, 

 they were most beautiful, the effect of a spark from a small machine being equal 

 to, and often surpassing, that produced by the constant discharge of a far more power- 

 ful one. 



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 ramifica- 

 tions 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 grey. In rarefied hydrogen, the ramifi- 

 cations 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. 



1 460. 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 



* Exception must, of course, be made of those cases where the root of the brush, becoming a spark, 

 causes a little diffusion or even decomposition of the matter there, and so gains more or less of a particular 

 colour at that part. 



