SHOT, xxvin.] ELECTRICAL TENSION. 317 



proportional to the square of its density, but that the action 

 of electricity on distant bodies by induction is quite inde- 

 pendent of atmospheric pressure, and is the same in vacuo 

 as in air. 



The power of retaining electricity depends also upon the 

 shape of the body. It is most easily retained by a sphere, 

 next to that by a spheroid, but it readily escapes from a 

 point ; and a pointed object receives it with most facility. 

 It appears, from analysis, that electricity, when in equi- 

 librio, spreads itself in a thin stratum over the surface of 

 a sphere, in consequence of the repulsion of its particles, 

 which force is directed from the centre to the surface. In 

 an oblong spheroid, the intensity or thicMbess of the stratum 

 of electricity at the extremities of the two axes is exactly 

 in the proportion of the axes themselves ; hence, when the 

 ellipsoid is much elongated, the electricity becomes very 

 feeble at the equator, and powerful at the poles. A still 

 greater difference in the intensities takes place in bodies of 

 a cylindrical or prismatic form, and the more so in propor- 

 tion as their length exceeds their breadth ; therefore, the 

 electrical intensity is very powerful at a point where nearly 

 the whole electricity in the body is concentrated. Notwith- 

 standing these analytical results, it is doubted whether the 

 disposition of electrified bodies to discharge their electricity 

 from points or edges may not arise from the superior at- 

 tractive force generated by induction in external bodies, 

 rather than from an original concentration of the electric 

 fluid in these parts. 



A perfect conductor is not mechanically affected by the 

 passage of electricity, if it be of sufficient size to carry off 

 the whole ; but it is shivered to pieces in an instant if it be 

 too small to carry off the charge : this also happens to a bad 

 conductor. In that case the physical change is generally a 

 separation of the particles, though it may occasionally be 

 attributed to chemical action, or expansion from the heat 

 evolved during the passage of the fluid ; but all these effects 

 are in proportion to the obstacles opposed to the freedom 



