278 ELECTRICAL INTENSITY. SECT. XXVIII. 



pears that the resistance of the air to the passage of the 

 electric fluid is proportional to the square of its density, 

 but that the action of electricity on distant bodies by in- 

 duction is quite independent 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 equilibrio, 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 center 

 to the surface. In an oblong spheroid, the intensity or 

 thickness 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 elon- 

 gated, 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 cylindrical or 

 prismatic form, and the more so in proportion 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. Not- 

 withstanding 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 attractive force generated by induction in 

 external bodies, rather than from an original concentra- 

 tion 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 propor- 

 tion to the obstacles opposed to the freedom of its 

 course. The heat produced by the electric shock is 

 intense, fusing metals, and even volatilizing substances, 



