300 ATMOSPHERIC ELECTRICITY. 



fore, that if there is a disengagement of electricity by the action 

 of the ignited tinder on the conductor of Volta's electrometer, this 

 electricity is too feeble sensibly to affect the results which are obtained 

 by this instrument in the researches on atmospheric electricity. 



But how does an ignited body increase the divergence of the electro- 

 meter if the electricity disengaged must not be attributed to an action 

 of the flame on the metal ? We do not know what explanation Volta 

 gave to this phenomena, but M. Gay Lussac* says : " The smoke 

 which passes off from the tinder ignited at the point by which the 

 electrometers, designed to ascertain atmospheric electricity, are armed, 

 may exhibit an image of an electrified cloud. This smoke acts as 

 as a conductor, which, as it expands, collects the electricity of the air, 

 and renders it more sensible by accumulating it at the surface ; but 

 the ignited tinder acts also in another way by heating the air and 

 thus rendering it a conductor." We shall see farther on that an 

 electrometer becomes charged by induction and not by the contact 

 of the surrounding air ; consequently that its straws or leaves may 

 diverge as well negatively as positively in the same stratum of air. 

 We may conclude from this that the smoke produced by the ignited 

 tinder does not act as a conductor to accumulate in the electrometer 

 the electricity of the surrounding air, for then it would be necessary 

 for the instrument to take a permanent and not a transient charge. 

 We believe rather, with M. Gay Lussac, that the effect produced is from 

 the conductibility which the air acquires by heat ; for we see that the in- 

 tensity of the electric tension, observed in the electrometer, depends on 

 the effect of the conductibility of the air, this latter favoring the elec- 

 tric radiation of the instrument. § Besides, M. Gay Lussac has demon- 

 strated by a very simple experiment that the air, the temperature of 

 which is raised, becomes a conductor of electricity. He observed that 

 an electrometer on being charged quickly loses its electricity when red 

 hot charcoal or the flame of an insulated candle is brought near it, while 

 it preserves its charge if the charcoal has not been heated. Thus may 

 be explained the fact observed by M. Mattencci,t that the discharges 

 of a musket, fired at the height of the flame of electrometer of Volta, 

 occasion a very strong divergence in the straws of that instrument. 



By means of series of straw electrometers rendered capable of com- 

 parison by the method indicated by Volta, Schiiblerl made numerous 

 observations, the special object of which was to determine the pe- 

 riodical variations of atmospheric electricity. These instruments 

 were provided with metallic stems three feet long, terminating in 

 coils, furnished with a combustible substance ; and, like Volta, he 

 took for the unit of his scale 0.5 of a line. Schiibler preferred Volta's 

 electrometer to that of Bennet, with gold leaf, because with the former 

 the valuation by degrees could be made with more exactness, because 

 it preserved longer the electricity which had been communicated to it, 

 and also because it is more convenient to observe comparatively the de- 



«■ Ann. de Ch. et de Phys., torn. VIII, p. 68, 181S. 



fBiblioth. Univers. Sc. et Arts, torn. LI, p. 352, 1832. 



j Journ. de Scliweigger, torn. Ill, p. 123, torn. VIII, p. 22, and torn. XIX, p. 11, 1811, 

 1813, and 1817. 



§The term radiation is used in this article to denote the escape of electricity from the 

 point of the electrometer by the partial conduction of the air. 



