142 ANNUAL OF SCIENTIFIC DISCOVERY. 



air itself but to the remaining traces of the combustible vapor, which dis- 

 appear if some drops of concentrated sulphuric acid are placed inside the 

 globe. Then the electrical light presents these characteristics : the negative 

 ball and the rod which supports it are enveloped by a luminous aureola, 

 which seem to be formed of several layers, which are all of a blue color, but 

 of different shades ; the positive ball, and a certain portion of its supporting- 

 rod, are also enveloped by a luminous, brilliant, rosy, very thin and cotton- 

 like layer; and between the two balls is a sort of cloud of continued, dif- 

 fused, red-colored light, in the form of a candle's flame. When the electric 

 light is produced in a space exclusively occupied by essential vapors, the 

 negative aureola presents the same characteristics as when in air, but the 

 different layers which form it are of a dimmer color. There is no luminous 

 layer upon the positive bah 1 , and between the two balls there is a sheaf of 

 light, wide-spread, white, and finely stratified. When the medium is a mix- 

 ture of air and the vapor of essence of turpentine, the aspects of the light 

 vary with the proportions of the mixture ; when the air is in great excess, 

 the appearance is nearly the same as in pure air, but the luminous sheaf is 

 divided into six or seven irregular forms of a lively red, which are sometimes 

 nearly a centimeter thick ; they have a very short existence. In a few 

 seconds they are succeeded by that cloudy and diffuse light which charac- 

 terizes the air exempt from vapors ; and, commonly, if the current is inter- 

 rupted some minutes, the red strata momentarily re-appear ; when the essence 

 of turpentine is in great excess, the luminous appearances are the same as in 

 the case when the essence is alone ; the strata, however, are red and pur- 

 plish. When the current has acted for some time, the red color gradually 

 disappears, and is replaced by that dim color, proper to the vapor of essence ; 

 a temporary interruption of the current does not make the red color re- 

 appear. These two last phenomena are easily explained, if it be granted, as 

 M. Gaugain contends, the red strata arise from the combustion of the essence ; 

 when the combustion is ended, the red strata are replaced by the diffused 

 light which characterizes air exempt from vapor, or by the white strata 

 which mark the vapor of essences. This assumption, M. Gaugain clearly 

 shows by experiment to be founded on the truth. But why is not the effect 

 uniform ? why, by the side of a brilliant, is there an obscure section ? M. 

 Gaugain says, because the first effect of electrical forces is to separate ma- 

 terially the gaseous medium into sections of different natures. 



i 



CAUSES OP CHAIN LIGHTNING. 



In a paper recently communicated to the Royal Society, Mr. Grove stated, 

 and proved by experiment, that the effects of rarefaction upon gases, either 

 produced by the air-pump or by heat, tend to render discharges of electricity 

 more facile, and to enable them to pass across much larger spaces than would 

 otherwise be the case. So strikingly was this evidenced with flame, that 

 when the flame of a spirit-lamp was held near one of the terminal points of 

 a coil-apparatus, the terminals being separated to a distance far beyond that 

 at which the? spark would pass in cold air, the spark darted to and along the 



