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



forces, that which is described as emanating from individual 

 atoms, will be called atomic repulsion) the second, or that ema- 

 nating from the smaller aggregation of atoms, will be called 

 molecular repulsion ; and that emanating from the larger aggre- 

 gation, molecular attraction. The last two are the forces which 

 will chiefly come under consideration in the following theory of 

 electricity. 



2. In the ordinary or neutral state of any substance, the 

 sphere of activity of each of these forces appears to be extremely 

 small. Hence if a plane be conceived to pass through the posi- 

 tion of an atom situated in the interior of a uniform medium, 

 not very close to its boundary, the forces of each kind, ema- 

 nating from the atoms on opposite sides of the plane, just 

 counteract each other, and the atom is thus maintained in a 

 position of stable equilibrium. But an atom situated at the 

 boundary of the substance is held in equilibrium by the equal 

 and opposite actions of the attractive and repulsive forces. As 

 the sphere of activity of the attraction was shown in the theory 

 to be much greater than that of repulsion, and to vary much 

 more slowly with the distance, the equality of the attraction and 

 repulsion on a superficial atom may be supposed to be the con- 

 sequence of a rapid degradation of the density of the substance 

 towards the bounding surface. The effect of this would be, to 

 diminish the resulting repulsion on a superficial atom from 

 within to without, so as to reduce it to an equality with the 

 resulting attraction from without to within, which is less affected 

 by the gradation of density at the surface, on account of its 

 larger sphere of activity. It is possible that the diminution of 

 density may be such that at the boundary the atomic repulsion 

 becomes of very small magnitude, as is the case, according to 

 the theory, in the interior of aeriform bodies, by reason of the 

 comparatively large intervals between their atoms. Supposing, 

 however, the body to be surrounded by air, the atomic repulsion 

 must be such as would suffice to counteract the pressure of the 

 air. The molecular repulsion would then be counteracted by the 

 molecular attraction, excepting so far as a slight excess must be 

 attributed to the latter very close to the surface, in order to 

 account for the phamoincna of capillary attraction. As these 

 forces both vary with the distance from their origins, according 

 to the law of the inverse square, their resultants would neutralize 

 each other at all sensible distances from the surface of the sub- 

 stance from whose, atoms they emanate. In this manner the 

 theory accounts for the fact that the molecular forces of bodies 

 have in general no perceptible effect at sensible distances from 

 the boundaries. 



3. Now this equality between the superficial molecular repul- 



