428 M. Gay-Lnssac on [Dec. 



conductor. Conductors of small diameter also conduct worse 

 than those of larger. 



The electric particles are mutually repulsive, and consequently 

 tend to separate and disperse themselves through space. They 

 have no affinity for bodies, they determine only to their surfaces, 

 where they are retained solely by the pressure of the atmosphere, 

 against which, they in their turn exert a pressure proportionate 

 at every point to the square of their number. When the latter 

 pressure exceeds the first, the electric matter escapes into the 

 air in an invisible stream, or in the form of a luminous line, 

 commonly called the electric spark. 



The stratum of electric matter on the surface of a conductor 

 is not of equal density at every point of its surface, except it be 

 a sphere. On an ellipsoid the density is greater at the extremity 

 of the great axis than on the equator, in the ratio of the great 

 axis to the smaller ; at the point of a cone it is infinite. In 

 general, on a body of any form, the density of the electric mat- 

 ter, and consequently its pressure on the air, is greater on the 

 sharpest or most curved parts, than on those that are flat or 

 round. 



The electric matter tends always to spread itself over con- 

 ductors, and to assume a state of equilibrium in them, and 

 becomes divided amongst them in proportion to their form, and 

 principally to their extent of surface. Hence, if a body that is 

 charged with the fluid be in communication with the immense 

 surface of the earth, it will retain no sensible portion of it. All 

 that is necessary, therefore, to deprive a conductor of its elec- 

 tricity, is to connect it with the moist ground. 



Of several conductors of very unequal powers the electric fluid 

 will always choose the most perfect ; but if their differences be 

 small, it will be divided amongst them in proportion to their 

 capacity for receiving it. 



A Paratonnerre * is a conductor which the electric matter of 

 the lightning prefers to the surrounding bodies, in order to reach 

 the ground, and expand itself through it : it commonly consists 

 of a bar of iron elevated on the buildings it is intended to pro- 

 tect, and descends, without any divisions or breaks in its length, 

 into water or a moist ground. An intimate connexion of the 

 paratonnerre with the ground is necessary, in order that it may 

 instantly transmit the lightning as it receives it, and thus defend 

 the surrounding objects from its attacks. When lightning 

 strikes the surface of the ground, for want of a good conductor 

 it does not spread over it, but penetrates below it till it meets 

 with a sufficient number of channels to carry it completely off. 



* I adopt the French term, as we have none in our language to express in one word, 

 a conductor of lightning, meaning thereby not merely the metallic rod, but the whole 

 apparatus complete. At least we may as well use it as parasol, parachute, parabouc, 

 &c— Tr. 





