510 LECTURE LIII. 



electrified body ; and the parts thus electrified may afterwards be distin- 

 guished from the rest, by the attraction which they exert on any small 

 particles of dust or powder projected near them ; the manner in which 

 the particles arrange themselves on the surface, indicating also in some 

 cases the species of electricity, whether positive or negative, that has 

 been employed ; positive electricity producing an appearance somewhat 

 resembling feathers ; and negative electricity an arrangement more like 

 spots. The inequality in the distribution of the electric fluid in a noncon- 

 ductor may remain for some hours, or even some days, continually 

 diminishing till it becomes imperceptible. 



These are the fundamental properties of the electric fluid, and of the 

 different kinds of matter as connected with that fluid. We are next to 

 examine its distribution, and the attractive and repulsive effects exhibited 

 by it, under different forms. Supposing a quantity of redundant fluid to 

 exist in a spherical conducting body, it will be almost wholly collected into 

 a minute space contiguous to the surface, while the internal parts remain 

 but little overcharged. For we may neglect the actions of the portion of 

 fluid which is only occupied in saturating the matter, and also the effect of 

 the matter thus neutralised, since the redundant fluid is repelled as much 

 by the one as it is attracted by the other : and we need only to consider 

 the mutual actions of the particles of this superfluous fluid on each other. 

 It may then be shown, in the same manner as it is demonstrated of the 

 force of gravitation, that all the spherical strata which are remoter from 

 the centre than any given particle, will have the whole of their action on it 

 annihilated by the balance of their forces, and that the effective repulsion 

 of the interior strata will be the same, as if they were all collected in the 

 centre. This repulsion will, therefore, impel the particles of the fluid 

 towards the surface, as long as it exists, and nothing will impede the 

 condensation of the redundant fluid there, until it is exhausted from the 

 neighbourhood of the centre. In the same manner it may be shown, that 

 if there be a deficiency of fluid, it will be only in the external parts, the 

 central parts remaining always in a state of neutrality : and since the 

 quantity of electric fluid taken away from a body, in any common experi- 

 ment, bears but a very small proportion to the whole that it contains, the 

 deficiency will also be found in a very small portion of the sphere, next to 

 its surface. And if, instead of being spherical, the body be of any 

 other form, the effects of electricity will still be principally confined to 

 its surface. This proposition was very satisfactorily investigated by Mr. 

 Cavendish ;* and it was afterwards more fully shown, by Dr. Gray'st 

 experiments, that the capacities of different bodies for receiving electricity, 

 depend much more on the quantity of their surfaces, than on their solid 

 contents : thus, the conductor of an electrical machine will contain very 

 nearly or quite as much electricity if hollow as if solid. 



If two spheres be united by a cylindrical conducting substance of small 

 dimensions, there will be an equilibrium, when the actions of the redundant 

 fluid in the spheres, on the whole fluid in the cylinder, are equal ; that is, 

 when both the spheres have their surfaces electrified in an equal degree : 



* Ph. Tr. 177G. See also ibid. 1771, p. 584. f Ibid. 1788, p. 121. 



