Prof. Challis on a Theory of the Force of Electricity. 283 



sion and the superficial molecular attraction would be destroyed 

 by any extraneous action, such as the friction of one substance 

 against another, by which the relative positions of the atoms 

 would be changed, and the normal state of the superficial degrada- 

 tion of density be disturbed. Such disturbance would produce 

 the electric state. If the result of the disturbance is to cause 

 the molecular attraction to be in excess of the molecular repul- 

 sion, the electricity is of a certain kind, different from that in 

 which the molecular repulsion is in excess. The theory thus 

 accounts for the existence of two kinds of electricity, and for the 

 fact that electricity is confined to the surfaces of bodies. 



4. It seems evident that, according as the superficial mole- 

 cular attraction is greater or less than the superficial molecular 

 repulsion, the superficial density is greater or less than that in 

 the neutral state. It does not, however, readily appear to which 

 of the two kinds of electricity the denser, and to which the rarer, 

 condition of the superficial strata applies. But since experiment 

 indicates that when two substances are electrified by friction, 

 that which is the more yielding, or more susceptible of mole- 

 cular displacement, is negatively electrified, it may perhaps be 

 inferred that in the case of negative electricity the superficial 

 density is greater than in the neutral state, and the molecular 

 attraction in excess ; and in that of positive electricity, the super- 

 ficial density is less than in the neutral state, and the molecular 

 repulsion in excess. 



5. Upon the cessation of the friction it might be expected 

 that the superficial atoms would quickly return to the neutral 

 positions, and the normal gradation of density be resumed. 

 What, then, is the reason that the electric state of a substance 

 is found by experiment to continue with little abatement long 

 after the exciting cause has ceased to act ? The answer to this 

 question involves the consideration of the part which the air 

 performs in electrical phenomena, which, as experience shows, 

 is so essential that no theory of electricity can be complete which 

 does not give an account of it. The explanation which the pre- 

 sent theory offers is as follows. When the law of density of the 

 superficial strata has been altered by the disturbing action of 

 friction, and the equality of the molecular attraction and repul- 

 sion is destroyed, so that, for instance, the former exceeds the 

 latter, immediately the surrounding air is attracted towards the 

 electrified body, and condensed on its surface, and at the same 

 instant an equilibrium is established at the surface between the 

 repulsion of the condensed air and the atomic repulsion of the 

 body. Considering the rapid change of the atomic repulsion 

 with distance, and the small sphere of its activity, this equaliza- 

 tion of the two repulsions may be the result of a change of the 



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