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



does, so that the opposite side of the sphere in the present 

 instance could be immediately acted upon by the first sphere. 

 The fact rather seems to be that a partial disturbance of the 

 normal atomic condition of a body, at whatever part it takes 

 place, is transmitted through the intervention of its proper 

 molecular forces to all other parts, so that the relative positions 

 of the atoms both in the interior and contiguous to the bound- 

 aries are in some degree changed. This remark may be illus- 

 trated as follows. Suppose a cylindrical rod to be held by the 

 hand near its middle point, and to be moved in the direction of 

 its length. The force which causes the motion is applied at 

 certain parts only ; but if we admit that the rod is composed of 

 discrete atoms, the disturbance immediately impressed gives rise 

 to molecular action which extends throughout the substance of 

 the rod, causing each atom to move in the same direction and 

 through the same space as the rod itself. If we consider an 

 atom at the preceding extremity of the rod, it is evident that its 

 motion must be due to an excess of molecular repulsion from 

 within to without above molecular attraction in the contrary 

 direction, so that, according to the theory, this end would be in 

 a state corresponding to positive electricity. At the other ex- 

 tremity the motion of an atom must be caused by an excess of 

 molecular attraction from without to within above molecular 

 repulsion in the contrary direction, and the superficial condition 

 would correspond to that of negative electricity. At the same 

 time there must be a gradual increase of density from the pre- 

 ceding to the following end, to account for the movements of the 

 interior atoms by molecular action, there being no such action 

 in the interior if the density be uniform. 



13. Now it may be presumed that the electrified sphere, in 

 the case before us, by acting partially on the other sphere, that 

 is, by attracting only the superficial parts which are turned 

 towards it, produces effects on the whole sphere analogous to 

 those above described. By attracting towards itself the atoms 

 on which it immediately acts, it conspires with the repulsion 

 from within to without of the other sphere, and thus produces 

 an apparent excess of repulsion, and consequently a positive 

 state of electricity. Thus the electricity directly induced is con- 

 trary to that of the electrified sphere. At the same time, since 

 all the atoms of the sphere submitted to the electric influence 

 must tend to move in the direction of the partial impulse, and 

 since those of its atoms which are not immediately acted upon 

 can only move by its proper molecular action, it follows, just as 

 in the case given above for illustration, that there must be a 

 gradual increase of density from the nearer to the further side 

 of the sphere to produce internal molecular action in the direc- 



