Prof. Challis on a Theory of the Force of Electricity. 285 



other it must be from without to within ; and if in one case the 

 superficial density is made less than it is in the neutral state, in 

 the other it is made greater. Thus the fact that two bodies 

 take by friction opposite electricities, is in accordance with this 

 theory. The circumstances which determine one to take the 

 positive electricity and the other the negative, depend most pro- 

 bably on differences of atomic constitution and arrangement. 

 But this is a question distinct from the present inquiry, which 

 is concerned only with the kind of agency by which electric 

 phenomena are produced. 



10. A good conductor of electricity may, in the language of 

 the theory, be defined to be a substance which has the property 

 of transmitting along its surface the effect of a disturbance made 

 at any point of the same, so that the law of superficial gradation 

 of density which the disturbance induces obtains equally over 

 the whole of the surface. In bad conductors and non-conductors 

 the change of the state of superficial density is transmitted 

 slowly, or not at all, and the gradation of density may at the 

 same instant be different at different points of the surface. 

 What the constitutional difference may be between a conductor 

 and a non-conductor, it does not belong to the present theory 

 to inquire. 



11. The tenacity with which the electric state is maintained, 

 depends not only on the condition of the air, but also on the 

 form of the surface of the electrified body. We have already 

 seen that the condensation or rarefaction of the air contiguous 

 to the body, which in fact keeps it in the electric state, is greatest 

 where the curvature is least, and that it is very small at sharp 

 points. This explains what has been called the accumulation of 

 electricity at points of great curvature, which, according to this 

 theory, is only a less degree of retention of the electricity at 

 such points, and a greater tendency to be affected by the ap- 

 proach of a disturbing cause. 



12. The uniform distribution of electricity over the surface of 

 a good conductor may be modified by circumstances, the consi- 

 deration of which leads to the theory of electricity by induction. 

 Conceive a good conductor, whose form, for the sake of simpli- 

 city, we will suppose to be spherical, to be brought within the 

 influence of another sphere in a state of negative electricity. As, 

 according to what has been said, the dynamic action of the latter 

 attaches to itself the surrounding air, and the extent of its influ- 

 ence, as is known by experiment, may be very considerable, it 

 may be supposed to act attractively also on the other sphere, or 

 at least on the superficial atoms that are turned towards it. I 

 do not gather from experiments, that electric action can pass 

 freely through the substances of bodies as the force of gravity 



