Prof. Challis on a Theory of the Force of Electricity. 289 



body electrified by induction there exists that gradation of den- 

 sity here supposed, and the condition required for generating a 

 secondary current is consequently fulfilled. A body inductively 

 electrified acts reciprocally on that by which its electricity was 

 induced, and thus both are in a condition proper for generating 

 secondary currents. If the electricities on the adjacent sides are 

 of opposite kinds, the two currents are in the same direction, 

 either towards the right hand or towards the left. And as the 

 velocity of the streams diminishes rapidly with distance from 

 the surfaces of the bodies by divergence and lateral Spreading, 

 there will be a position on the shortest distance between them at 

 which the conspiring streams will produce a maximum of velo- 

 city, and consequently a minimum of pressure. Towards this 

 point, therefore, there will be a tendency of both bodies to move 

 by the action of the pressure of the medium on the individual 

 atoms, and thus a reason is given by the theory for the attraction 

 towards each other of bodies oppositely electrified. 



20. If the electricities on the adjacent sides of the bodies be 

 of the same kind, the currents will be in opposite directions, either 

 from or towards the space between. In both cases there will be 

 a minimum of velocity, and consequently a maximum of pres- 

 sure somewhere in the shortest line joining the bodies, because 

 there must be a point at which the difference of the opposite 

 velocities is least. The maximum of condensation at this point 

 is produced by the meeting of the currents when they flow 

 towards it, and by the confluence of the adjacent fluid w T hen they 

 flow from it. Thus in both cases the pressure of the fluid will 

 tend from the intermediate position, and will separate the bodies 

 by pressure on their individual atoms. In this manner the 

 theory accounts for the observed mutual repulsion of two sub- 

 stances having the same electricities, 



21. As the efficacy of these currents must be in proportion to 

 the molecular attractions and repulsions before treated of, to 

 which they owe their origin, and as these forces vary inversely as 

 the square of the distance, it follows that the effective force of 

 the currents varies with the distance according to the same law. 

 This result accords with experiment. 



22. The above explanations give an account of motions of 



electrified bodies the same in kind as those observed. Whether 



the agency be sufficient in degree, cannot be readily determined. 



On this point it may be remarked, first, that the pressures do 



not act partially on the bodies, but, as in the case of gravity, on 



every atom, and for this reason they are the more likely to be of 



sufficient motive power; and again, the moving force of the 



dV 

 pressure on a given atom, being proportional to V-p depends 



