8 INTRODUCTORY. 



distribution, the superficial density at this point, is the ratio of the 

 charge of an element of surface taken about this point to the extent 

 of the element. 



On the hypothesis of fluids, it must be admitted that the layer of 

 electricity on the surface has a certain thickness, and that it penetrates 

 to a certain depth, which may be extremely small, in the conductor, 

 or in the dielectric which surrounds it. As the thickness of this 

 layer cannot be determined by experiment, holding to the same order 

 of considerations, we may either suppose the density to vary with 

 constant thickness, or suppose the density constant with variable 

 thickness ; in this case, the expressions electric density and electric 

 thickness at a point are equivalent. 



It will be seen that, apart from any idea of fluid, the expressions ot 

 electric density in volume, or superficial density, have a purely mathe- 

 matical or experimental meaning quite apart from any hypothesis. 



