24 HISTORY OF ELECTRICITY. 



fluid. If we suppose two fluids, the positively 

 electrized body draws the negative fluid to the 

 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 hypotheses, 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. ^Epinus 

 of Petersburg published, in 1759, his Tentamen 

 Theorice Electricitatis et Magnetism* ; in which he 

 traces mathematically the consequences of the hy- 

 pothesis of an electric fluid, attracting all other 

 matter, but repelling itself; the law of force of this 

 repulsion and attraction he did not pretend to 

 assign precisely, confining himself to the supposi- 

 tion that the mutual force of the particles increases 

 as the distance decreases. But it was found, that 

 in order to make this theory tenable, an additional 

 supposition was required, namely, that the particles 

 of bodies repel each other as much as they attract 

 the electric fluid 5 . For if two bodies, A and B, be 

 in their natural electrical 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 ten- 

 dency to motion results from the fluid in A ; and 

 if we further suppose that the matter in A attracts 

 s Robison, vol. iv. p. 18. 



