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



superficial density extending to a distance from the surface in- 

 comparably smaller than the distance to which the original 

 disturbance affected the atoms. Consequently the excess of the 

 molecular attraction above the molecular repulsion, to which that 

 disturbance gave rise, will continue so long as the condensation 

 of the air is undisturbed, and thus the body will remain in a 

 state of negative electricity. 



6. Similarly, if the friction disturbs the atoms in such a 

 manner that the superficial molecular repulsion is in excess, the 

 surrounding air is repelled from the electrified body, and the 

 parts contiguous to the surface are rarefied. At the same time, 

 the atomic repulsion of the body being equalized, by an extremely 

 small change of the superficial density, to the repulsion of this 

 rarefied air, the excess of the molecular repulsion above the 

 molecular attraction remains, and the body continues in a state 

 of positive electricity. 



7. It is evident that the air can operate in this manner only 

 to a limited extent, depending on its density, and that the 

 amount of the permanent electricity, of which any substance is 

 capable, is limited by this circumstance. The electricity will be 

 permanent in proportion as the air adheres to the body without 

 being replaced by other air. For this reason dry air is more 

 favourable to the permanence of electricity than air containing 

 vapour of water, which being only connected with the air me- 

 chanically, and not a constituent part of it, is liable to relative 

 displacement by extraneous causes. 



8. As the excess of molecular attraction, or of molecular re- 

 pulsion, acts through considerable spaces, the resulting con- 

 densation or rarefaction of the air at any point of the surface of 

 the electrified body, depends on the curvature of the surface at 

 that point, being less as the curvature is greater. This explains 

 the escape of electricity at sharp points, where on account of the 

 great curvature of the surface, the sum of the molecular attrac- 

 tions or repulsions is too feeble to produce a degree of condensa- 

 tion or rarefaction sufficient to maintain the electric state. 



9. When two substances are rubbed against each other, the 

 external force applied to cause the friction, by putting the con- 

 tiguous superficial atoms of both into states of disturbance, gives 

 rise to mutual molecular actions, which in fact determine the new 

 positions that the atoms assume. Now, whatever be the resultant 

 of these actions on a given atom, it is evident that those atoms 

 of one of the bodies which are most nearly contiguous to atoms 

 of the other, must be acted upon by resulting forces very nearly 

 of the same magnitude and in the same direction as the forces 

 which act upon the latter. Hence, if for one of the substances the 

 resulting force be from within its boundary to without, for the 



