210 Faraday. 



mutual repulsion of these lines, or pressure laterally. Where a 

 line of force ends on one of the spheres, its tension is exercised 

 on the sphere: in this way, every surface-element of each 

 sphere is pulled outwards. If the spheres were entirely 

 removed from each other's influence, the state of stress would be 

 uniform round each sphere, and the pulls on its surf ace -elements 

 would balance, giving no resultant force on the sphere. But 

 when the two spheres are brought into each other's presence, 

 the unit lines of force become somewhat more crowded together 

 on the sides of the spheres which face than on the remote sides, 

 and thus the resultant pull on either sphere tends to draw it 

 toward the other. When the spheres are at distances great 

 compared with their radii, the attraction is nearly proportional 

 to the inverse square of the distance, which is Priestley's law. 



In the following year (1838) Faraday amplified* his theory 

 of electrostatic induction, by making further use of the analogy 

 with the induction of magnetism. Fourteen years previously 

 Poisson had imaginedf an admirable model of the molecular 

 processes which accompany magnetization; and this was now 

 applied with very little change by Faraday to the case of induc- 

 tion in dielectrics. " The particles of an insulating dielectric," 

 he suggested, J " whilst under induction may be compared to a 

 series of small magnetic needles, or, more correctly still, to a 

 series of small insulated conductors. If the space round a 

 charged globe were filled with a mixture of an insulating 

 dielectric, as oil of turpentine or air, and small globular 

 conductors, as shot, the latter being at a little distance from 

 each other so as to be insulated, then these would in their 

 condition and action exactly resemble what I consider to be 

 the condition and action of the particles of the insulating 

 dielectric itself. If the globe were charged, these little con- 

 ductors would all be polar ; if the globe were discharged, they 

 would all return to their normal state, to be polarized again 

 upon the recharging of the globe/' 



That this explanation accounts for the phenomena of specific 



* Exp. Res., Series xiv. t Cf. p. 65. J Exp. Res., 1679. 



