DISRUFflVE DISCHARGE — BRUSH IN DIFFERENT GASES. 119 



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 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 re- 

 ■gards 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 car- 

 bonic acid, the brush is better in form, but weak as to light, being of a dull greenish 

 or purplish hue, 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 again. 

 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 (about 0*25 of inch in diameter) which had before given 

 them most freely in air and nitrogen. During the time of this difficulty with the 

 muriatic gas, magnificent brushes were passing off* from different parts of the machine 

 into the surrounding air. On rarefying the gas, the formation of the brush was faci- 

 litated, but it was generally 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 ramifications were obtained of a pale bluish colour, utterly unlike those in 

 nitrogen. 



1463. In all the gases, the diff*erent forms of disruptive discharge may be linked 

 together and gradually traced from one extreme to the other, i. e. from the spark 

 to the glow (1405.), or, it may be, to a still further condition to be called dark 

 discharge ; but it is, nevertheless, very surprising to see what a specific character 

 each keeps whilst under the predominance of the general law. Thus, in muriatic 

 acid, the brush is very difficult to obtain, and there comes in its place almost a dark 

 discharge, partaking of the readiness of the spark action. Moreover, in muriatic acid, 

 I have never observed the spark with any dark interval in it. In nitrogen, the spark 

 readily changes its character into that of brush. In carbonic acid gas, there seems 

 to be a facility to occasion spark discharge, whilst yet that gas is unlike nitrogen in 

 the facility of the latter to form brushes, and unlike muriatic acid in its own facility 

 to continue the spark. These differences add further force, first to the observations 

 already made respecting the spark in various gases (1422. 1423.), and then, to the 

 proofs deducible from it, of the relation of the electrical forces to the particles of 

 matter. 



1464. The peculiar characters of nitrogen in relation to the electric discharge 

 (1422. 1458.) must, evidently, have an important influence over the form and even 

 the occurrence of lightning. Being that gas which most readily produces corusca- 



