370 ATMOSPHERIC ELECTRICITY. 



tlie places where they were fixed ; raises and transfers to a dis- 

 tance masses of great weight, and even has been known to tear from 

 its foundation and transport a whole wall composed of 7,000 bricks, 

 which would weigh 26 tons. These mechanical effects have been 

 differently accounted for by physicists. M. Pouillet* thinks that 

 they cannot be explained by the ordinary laws of electric attrac- 

 tions ; but that they depend on a difterence in the decomposition of 

 the natural electricities of bodies, according as this takes place by 

 slow or by sudden action. In the first case the conductibility is suffi- 

 cient for allowing the displacement of the electric fluids, and they have 

 time to arrange themselves at the surface, where they exercise against 

 the air a pressure which is capable of retaining the body in posi- 

 tion. In the second case, all the atoms of the mass undergo simul 

 taneously and suddenly a decomposition of their natural electricities ; 

 they are acted upon with so much violence that the arrangement 

 required by the laws of equilibrium has not time to be perfected, and 

 the masses are thus acted upon by forces incomparably greater than 

 those which tend to retain them in tlieir position. 



M. Aragof has given to these phenomena of transportation pro- 

 duced by the lightning an entirely different explanation. He admits 

 that the lightning in its passage through bodies which contain water 

 developes steam, the expansive force of which is exerted in every direc- 

 tion ; so that, if we conceive of moisture being in the pores or cells of 

 a free-stone which the lightning strikes, the sudden development of 

 the vapor will break it, and its fragments will be projected to a dis- 

 tance in every direction. Under the same circumstances the rapid 

 transformation into vapor of water mixed with the stratum of the 

 ground on which the foundations of a house rests will be sufficient, 

 according to M. Arago, to raise up the building in a single mass and 

 transport it to a distance. This philosopher refers, to the same cause, 

 the singular dividing into splinters which wood undergoes when 

 lightning passes through it. It divides it lengthwise into a multi- 

 tude of slender laths or very fine splinters, and in green Avood 

 separates the bark from the trunk; the wood, leaves, and bark are oftea 

 completely scorched, in consequence of the dispersion of the water, 

 without presenting any trace of combustion. 



It is probable that the expansive force of the vapor produced by the 

 passage of the electricity through wood may be the cause of the phe- 

 nomena which is presented when lightning strikes it, since water 

 which has only reached the temperature of 260° possesses an elastic 

 force of 45 atmospheres ; and since the heat developed by lightning in 

 j)assing through metallic wires is sufficient to melt them, it must 

 raise the temperature of the small quantities of water which it meets 

 in its passage to a high degree. | But if the ingenious explanation of 



* Ele'mons de Phyfiique Expcrimentule et de Meteorologie, torn. I, p. 613. 



t Annuaire pour 1838, page ■!(13. 



'I la advancing tlie same opinion, M. Beoiuerel thought that we might explain thcse- 

 phenomena by supposing that the electric discharge meets less resistance in running 

 through the woody ti'orcs in the direction of lengtli than across thein ; this, according 

 to him, shows an expansive action, and, consequent!)', a tendency to cleave in the form of 

 laths. Traite de I'Electricite et du Magnctisme, torn. VI, Itre partie, page 107. 



