PROGRESS OF ELLOTRICAL THEORY. 203 



introduced by Dufay ; 4 of t\vo electricities each repelling itself and 

 attracting the other. If we suppose that there is only one fluid, 

 which repels itself and attracts all other matter, we obtain, in many 

 cases, the same general results as if we suppose two fluids ; thus, if an 

 electrized body, overcharged with the single fluid, act upon a ball, it 

 drives the electric fluid in the ball to the further side by its repulsion, 

 ami then attracts the ball by attracting the matter of the ball more 

 than it repels the fluid which is upon the ball. If we suppose two 

 fluids, the positively electrized body draws the negative fluid to tho 

 nearer side of the ball, repels the positive fluid to the opposite side, 

 and attracts the ball on the whole, because the attracted fluid is nearer 

 than that which is repelled. The verification of either of these hypo- 

 theses, and the determination of their details, depended necessarily 

 upon experiment and calculation. It was under the hypothesis of a 

 single fluid that this trial was first properly made. JEpinus of Peters- 

 burg published, in lYoO, his Tentamen Theories Electricitatis ct Mcy- 

 netismi ; in which he traces mathematically the consequences of the 

 hypothesis of an electric fluid, attracting all other matter, but repel- 

 ling itself; the law of force of this repulsion and attraction he did 

 not pretend to assign precisely, confining himself to the supposition 

 that the mutual force of the particles increases as the distance de- 

 creases. But it was found, that in order to make this theory tena- 

 ble, an additional supposition was required, namely, that the par- 

 ticles of bodies repel each other as much as they attract the elec- 

 tric fluid. 6 For if two bodies, A and B, be in their natural electri- 

 cal condition, they neither attract nor repel each other. Now, in this 

 case, the fluid in A attracts the matter in B and repels the fluid in B 

 with equal energy, and thus no tendency to motion results from the 

 fluid in A ; and if we further suppose that the matter in A attracts 

 the fluid in B and repels the matter in B with equal energy, we have 

 the resulting mutual inactivity of the two bodies explained ; but with- 

 out the latter supposition, there would be a mutual attraction : or* we 

 may put the truth more simply thus; two negatively electrized bodies 

 repel each other ; if negative electrization were merely the abstraction 

 of the fluid which is the repulsive element, this result could not follow 

 except there were a repulsion in the bodies themselves, independent 

 of the fluid. And thus JEpinus found himself compelled to assume 

 this mutual repulsion of material particles ; he had, in fact, the a'.tcr- 



4 Mem. A. P. 1733, p. 467. 6 Robison, vol. iv. p. 18. 



