Prof. Challis on a Theory of the Force of Electricity . 287 



tion of the immediate impulse ; and at the boundary furthest 

 from the electrified sphere, there must, for the same reason, be 

 an excess of molecular attraction from without to within above 

 the opposite molecular repulsion. Thus the further side of the 

 sphere is electrified negatively. The opposite electricities would 

 plainly have been induced if the first sphere had been electrified 

 positively. These inferences are in accordance with well-known 

 results of experiment. 



14. The induced electricity is maintained by the presence of 

 the electrified sphere, which supplies the pulling or pushing 

 force by which the electric condition of the other sphere is gene- 

 rated. On the removal of the former, the electric state of the 

 latter would immediately vanish, for the same reason that, in the 

 case of the rod, as soon as the hand ceases moving it, the mole- 

 cular action also ceases. 



15. It has been supposed in the above reasoning that the 

 body on which the electricity is induced, is insulated by a non- 

 conductor, and is of limited extent. If it were connected by a 

 conductor with the earth, it must be considered, as far as regards 

 its electric state, to be a body of unlimited extent, and in that 

 case the argument which accounted for induced electricity on the 

 more remote parts of the same kind as that of the electrified body 

 would no longer apply. This electricity would be removed, as 

 it were, to an infinite distance, and the only induced electricity 

 would be that of the opposite kind. 



16. It is found by experiment that if a spherical shell of 

 metal, perforated at one part, be electrified, little or no electri- 

 city is found on the interior surface. This fact is accounted for 

 by the neutralization of the electricities on opposite parts of that 

 surface by induction. The same explanation applies in the case 

 of an ingenious experiment made by Faraday, in which an elec- 

 trified hollow cone, having an acute vertical angle, was turned 

 inside out by pulling a thread. The electricity was always on 

 the surface which was exterior, that on the interior surface being 

 neutralized by induction. 



17. An electric discharge takes place when on the approach of 

 two bodies oppositely electrified towards each other, the condensa- 

 tion of the air due to the negative electricity of one is neutralized 

 in the space between them by the rarefaction of the air due to the 

 positive electricity of the other. The spark is probably a conse- 

 quence of the disturbance of the aether caused by the sudden 

 return of the superficial atoms of both bodies, at the instant of 

 the discharge, to their normal positions. 



18. It might at first sight appear that the theory which 

 accounted for induced electricity would also explain the attrac- 

 tions and repulsions by which bodies in a state of induced elec- 



