97 ; 



opposed to the motion of the electric fluid : light alone is evolved without son- I 

 sible sound, as is observed when electricity is transmitted through exhausted ; 

 tubes ; but when the aurora is developed in the lower strata of the atmosphere, ? 

 it would produce the hissing and cracking noise which appears to be heard on ; 

 some occasions. If the metallic cloud possess the conducting power in a hiah f 

 degree, the electric current may pass through it without the evolution of either / 

 light or sound : and thus the magnetic needle may be affected as it would be 

 by an aurora at a time when no aurora is visible. If any cause alters the con- 

 ductability of those columnar clouds suddenly or gradually, a sudden or gradual 

 change in the splendor of the aurora would ensue. 



According as those clouds advance over more southern countries, the direc- 

 tion of their columns being constantly parallel to the dipping needle, they take 

 gradually a more horizontal position, and consequently the strata of atmosphere 

 at their extremities become gradually less distant, and consequently more 

 nearly in a state of electrical equilibrium ; hence it follows, that as the latitude 

 diminishes, the appearance of aurora becomes more and more rare, until, in the 

 lower latitudes, where the columns are nearly parallel to the horizon, such 

 phenomena are never observed. 



This ingenious and beautiful theory still, however, requires, before its va- 

 lidity can be admitted by the rigid canons of modern physics, that the main 

 fact on which it rests should be proved : it is necessary that it should be 

 hown that such metallic clouds as are here supposed, and on the agency of 

 vhich the whole theory is based, should be accounted for. This demand is 

 accordingly answered by M. Biot. 



The magnetic pole, or its vicinity, is evidently the point from which these 

 olumnar masses of meteoric light proceed. Therefore, the extremely minute 

 ays composing these columns must issue from the earth in that region. Now 

 t is well known that that part of the globe is, and always has been, character- 

 zed by the prevalence of frequent and violent volcanic eruptions, and several 

 olcanoes have been, and still are, in activity round the place where the mag- 

 netic pole is situate. These eruptions are always accompanied by electric 

 >henomena. Thunder issues from the volcanic clouds ejected by the craters ; 

 and these clouds of volcanic dust, thus charged with electricity, are projected 

 o great heights, and carried to considerable distances through the air, carrying 

 vith them all the electricity taken from the crater. 



These vast eruptions, issuing from depths so unfathomable that they seem al- 

 most to penetrate the globe, and issuing with such violence from the gulfs by 

 vhich they are projected into the atmosphere, must necessarily produce strong 

 ertical currents of air, by which the volcanic dust will be carried to an eleva- 

 ion exceeding that of common clouds. Travellers who have visited Iceland 

 lave often seen suspended over it, during eruptions, a species of volcanic fog. 

 Such clouds are known to be of a sulphureous and metallic nature, painfully irri- 

 ating the eyes, mouth, and nostrils. Moreover, the existence of dry fogs, dif- 

 using a fetid and sulphureous odor, was ascertained in 1783, when all Europe 

 was enveloped in a fog of that description. 



To this it may be added, that more recent observations have rendered it 

 lighly probable, if not certain, that metallic matter, and more particularly iron 

 n a pure and uncombined state, is frequently precipitated from the clouds in 

 hundcr-storms. 



To the theory of M. Biot it is objected by M. Becquerel, that the existence 

 of metal in that uncombined form, in which alone it has the conducting power, 

 n volcanic eruptions, has not been proved ; that the matter ejected from vol- 

 canoes consists of vitrified substances, silicates, aluminates, and other sub- 

 stances, which are non-conductors, but that pure metal is never found. 



T 



