ATMOSPHERIC ELECTRICITY. 363 



presence of iron in tlie rain water which falls from thunder cloud.s, 

 the odor and vapor of sulphur that the lightning almost always 

 gives out where it breaks forth, appear to establish the conclusion that 

 the atmosphere contains, at least as high as the region of tho tlmnder 

 clouds, iron, sulphur, and other substances. [It is more probable that 

 these substances are carried immediately up from the earth by the force 

 of the discharge. — J. H ] The existence of the deposits, of which 

 we have spoken above, proves that lightning conveys these bodies. 

 Besides it is known that the ordinary electric spark may also convey 

 ponderable substances in a state of great tenuity. It may happen then 

 that the lightning, in passing through the air, may induce a com- 

 mencement of the union of the different substances which it contains^ 

 and that the electricity concentrated there by their conductibility may 

 appear under a form more or less globular. Nev/ experiments and new 

 observations, however, can only assign the proper value which this 

 opinion should have in science. 



C H A P T E R II . 



OF THUNDEK. 



As is the case with every electric explosion, the flash of lightning 

 is accoDjpanied by a noise more or less intense of longer or shorter 

 duration, to which is given the name of thunder. This noise varies 

 with the distance of the observer from the place where the electrical 

 discharge takes place ; if it is very near, the flash is almost immediately 

 followed by successive and rapid detonations, but at a little distance 

 these detonations are replaced by a sort of reverberation^ which lasts for 

 several seconds, and which is frequently intermingled with violent ex- 

 plosions. Physicists generally are agreed in considering the thunder as 

 the result of the sudden re-entrance of the air into an empty space, thus 

 likening the phenomenon to what happens in the experiment known as 

 the burst bladder, but they differ in opinion as to the manner in 

 which this void is produced. x\ccording to some, the flash of lightning 

 is always attended with the sudden formation of rain, and the vapor 

 contained in a large space being suddenly condensed, it must form in 

 the atmosphere, almost instantaneously, a void into which the strata of 

 surrounding air rush by virtue of their elastic force. This rapid motion 

 will produce a loud noise, and as the portions of the atmosphere 

 which have filled the void formed by the condensation of vapor leave 

 in their turn an unoccupied space, new strata of air will rush into 

 this latter, producing a nev/ report, and so on. 



It is conceived, also, that the intensity of the explosions must become 

 weakened, since, on the one hand, the cause which produces them 

 loses its intensity, and, on the other, because they take place at dis- 

 tances more or less remote from the observer. To this we may reduce 

 in substance tiie opinions advanced by De Luc,* Girtanner,t Mayer, 

 and Monge.j; 



*^ Idues 8ur la M^te'orologie, torn. II, page 150. Paris, ]787. 



t Dictionnaire de Gehler, torn. I, pages 5G8 and 5G9. 



X Encyclopedic Moderne, torn. XXII, page 319. Brussels, 1832. 



