prior to the Introduction of the Potentials. 49 



electric fluid. That this applies even to air they proved by 

 constructing a machine analogous to the Leyden jar, in which, 

 however, air took the place of glass as the medium between 

 two oppositely charged surfaces. The success of this experi- 

 ment led Aepinus to deny altogether the existence of electric 

 effluvia surrounding charged bodies :* a position which he 

 regarded as strengthened by Franklin's observation, that the 

 electric field in the neighbourhood of an excited body is not 

 destroyed when the adjacent air is blown away. The electric 

 fluid must therefore be supposed not to extend beyond the 

 excited bodies themselves. The experiment of Gray, to which 

 we have already referred, showed that it does not penetrate 

 far into their substance; and thus it became necessary to 

 suppose that the electric fluid, in its state of rest, is con- 

 fined to thin layers on the surfaces of the excited bodies. 

 This being granted, the attractions and repulsions observed 

 between the bodies compel us to believe that electricity acts 

 at a distance across the intervening air. 



Since two vitreously charged bodies repel each other, the 

 force between two particles of the electric fluid must (on 

 Franklin's one-fluid theory, which Aepinus adopted) be 

 repulsive : and since there is 'an attraction between oppositely 

 charged bodies, the force between electricity and ordinary 

 matter must be attractive. These assumptions had been made, 

 as we have seen, by Franklin; but in order to account for 

 the repulsion between two resinously charged bodies, Aepinus 

 introduced a new supposition namely, that the particles 

 of ordinary matter repel each other. This, at first, startled 

 his contemporaries; but, as he pointed out, the "unelectrified" 

 matter with which we are acquainted is really matter saturated 

 with its natural quantity of the electric fluid, and the forces 

 due to the matter and fluid balance each other ; or perhaps, 

 as he suggested, a slight want of equality between these 

 forces might give, as a residual, the force of gravitation. 



Assuming that the attractive and repellent forces increase as " 



* This was also maint.iined about the same time by Giacomo Battista Beet-aria 

 of Turin (b. 1716, d. 1781;. 



E 



