ELECTRICITY. 



25 



In carbonic acid gas, the spark is white 

 and vivid ; in hydrogen gas, it is faint 

 and red. 



(90.) It should be recollected, in 

 making these experiments, that in pro- 

 portion as the medium is more rare, its 

 conducting power increases, and a 

 smaller intensity of electricity is re- 

 quired for the production of light. In 

 the ordinary vacuum produced by the 

 air-pump, the passage of electricity is 

 rendered sensible by streams or co- 

 lumns of diffused liii'nt occasionally 

 varying in their breadth and intensity, 

 and exhibiting movements which give 

 them a marked resemblance to the co- 

 ruscations of the Aurora Borealis. After 

 rarefying the air contained in a glass 

 jar, about one foot long and eight inches 

 in diameter, to the 500th part, Mr. 

 Smeatofi placed the jar upon a lathe, 

 and caused it to revolve rapidly, whilst 

 at the same time he rubbed it with his 

 hand. A considerable quantity of lam- 

 bent flame appeared under his hand, 

 variegated with all the colours of the 

 rainbow. The light was steady ; but 

 every part of it was constantly changing 

 colours. When a very perfect vacuum 

 is made in a glass cylinder covered with 

 a brass plate, the electric stream will 

 pass between it and the plate of the re- 

 ceiver of the air-pump, in a continued 

 stream of the same size throughout its 

 whole length. If a Torricellian vacuum 

 be formed in the upper portion of a long 

 bent glass, tube filled with mercury, and 

 inverted, by placing the legs of the bent 

 tube in separate "basins of mercury, 

 when electricity is transmitted through 

 the tube, light is seen to pervade the 

 vacuum in a continued arch of lam- 

 bent flame, without the least diver- 

 gency. 



(91.) It was natural to suppose, be- 

 fore sufficient consideration had been 

 bestowed upon the subject, that the 

 light which appears during the passage 

 of electricity, was actually the electric 

 fluid itself, which, at some certain 

 degree of accumulation, was in itself 

 luminous ; and such was the notion en- 

 tertained by the early electricians. But 

 since we know that common atmos- 

 pheric air becomes luminous by violent 

 compression, and we must also pre- 

 sume that electricity exerts a very sud- 

 den and powerful pressure upon the 

 air by its passage through that resisting 

 medium, we are certainly justified in 

 drawing the inference that the same 

 phenomena proceed in both cases from 



the same cause. Biot has adopted this 

 opinion, which appears to be more con- 

 sonant with philosophical views of the 

 subject than any other : for it is certain 

 that the whole of the electrical light 

 that appears is not more than what may 

 proceed from the mechanical compres- 

 sion of the air, the vapours, and other 

 constituents of the medium through 

 which the passage of the electricity is 

 effected. 



(92.) The sound which accompanies 

 these various modes of transference is 

 subject to corresponding modifications, 

 dependent likewise, no doubt, upon the 

 degree and the suddenness of the im- 

 pulses given to the air. The full, short, 

 and undivided spark is attended with a 

 loud explosion; the more lengthened 

 spark, with a sharper snap, which be- 

 comes more broken and rattling in pro- 

 portion to the distance it has to traverse. 

 The luminous streams produced by a 

 succession of minute sparks are scarcely 

 productive of noise, but are accompa- 

 nied only by a faint rustling sound, like 

 that of a stream of wind through a nar- 

 row chink. 



(93.) A peculiar odour has some- 

 times been perceived in the neighbour- 

 hood of an electrical machine which has 

 been briskly worked, so as to emit for 

 some time a great number of sparks ; 

 and it has been thought to resemble 

 that of phosphorus. This is also pro- 

 bably owing to some unknown chemical 

 decomposition effected by the electricity 

 during its passage through the air. 



(94.) We have already had occasion 

 to remark the great increase of inten- 

 sity which the electric fluid acquires at 

 the extremity of all elongated parts of 

 conducting bodies ; and the indefinite 

 augmentation of this intensity which 

 takes place at the apex of all projecting 

 points. This high intensity will neces- 

 sarily be accompanied with a powerful 

 tendency in the fluid to escape ; a cir- 

 cumstance which furnishes a natural 

 and exact explanation of the rapid dis- 

 sipation of electricity which takes plac.e 

 from all bodies of a slender and pointed 

 form. 



The following experiments illustrate 

 these positions. Let the insulated con- 

 ductor of a machine be furnished with 

 a pair of pith-balls, suspended by a fine 

 wire, and charged with either species of 

 electricity ; the divergence of the balls 

 will indicate the presence and degree of 

 this electricity. If a metallic rod with 

 a ball at one end be held in the hand, 



