DISRUPTIVE DISCHARGE — SPARK IN DIFFERENT GASES. 109 



square inches of charged glass, the discharge is complete. Hereafter we shall find 

 the influence of this effect in the formation of brushes (1 435.) ; and it is not impossible 

 that we may trace it producing the jagged spark and the forked lightning. 



1421. The characters of the electric spark in different gases vary, and the varia- 

 tion may be due simply to the eff'ect of the heat evolved at the moment. But it may 

 also be due to that specific relation of the particles and the electric forces which I 

 have assumed as the basis of a theory of induction ; the facts do not oppose such a 

 view ; and in that view, the variation strengthens the argument for molecular action, 

 as it would seem to show the influence of the latter in every part of the electrical 

 effect (1423. 1454.). 



1422. The appearances of the sparks in different gases have often been observed 

 and recorded*, but I think it not out of place to notice briefly the following results; 

 they were obtained with balls of brass, (platina surfaces would have been better,) and 

 at common pressures. In a/r, the sparks have that intense light and bluish colour 

 which are so well known, and often have faint or dark parts in their course, when the 

 quantity of electricity passing is not great. In nitrogen, they are very beautiful, having 

 the same general appearance as in air, but have decidedly more colour of a bluish 

 or purple character, and I thought were remarkably sonorous. In oxygen, the sparks 

 were whiter than in air or nitrogen, and I think not so brilliant. In hydrogen, they 

 had a very fine crimson colour, not due to its rarity, for the character passed away 

 as the atmosphere was rarefied (1459.)-)-. Very little sound was produced in this 

 gas ; but that is a consequence of its physical condition ;{. In carbonic acid gas, 

 the colour was similar to that of the spark in air, but with a little green in it : the 

 sparks were remarkably irregular in form, more so than in common air: they could 

 also, under similar circumstances as to size of ball, &c., be obtained much longer 

 than in air, the gas showing a singular readiness to pass the discharge in the 

 form of spark. In muriatic acid gas, the spark was nearly white : it was always 

 bright throughout, never presenting those dark parts which happen in air, nitrogen, 

 and some other gases. The gas was dry, and during the whole experiment the sur- 

 face of the glass globe within remained quite dry and bright. In coal gas, the spark 

 was sometiuies green, sometimes red, and occasionally one part was green and another 

 red. Black parts also occur very suddenly in the line of the spark, i. e. they are not 

 connected by any dull part with bright portions, but the two seem to join directly 

 one with the other. 



1423. These varieties of character impress my mind with a feeling, that they are 

 due to a direct relation of the electric powers to the particles of the dielectric through 

 which the discharge occurs, and are not the mere results of a casual ignition or a 



* See Van Marum's description of the Teylerian machine, vol. i. p. 112., and vol. ii. p. 196.; also Ency. 

 Britan., vol, vi.. Article Electricity, pp. 505, 507. 



t Van Marum says they are about four times as large in hydrogen as in air, vol. i. p. 122. 



4 LESX.IB. 



