58 Electric and Magnetic Science 



force near a conductor is proportional to the surface-density of 

 electrification. 



Since the overthrow of the doctrine of electric effluvia by 

 Aepinus, the aim of electricians had been to establish their 

 science upon the foundation of a law of action at a distance, 

 resembling that which had led to such triumphs in Celestial 

 Mechanics. When the law first stated by Priestley was at 

 length decisively established by Coulomb, its simplicity and 

 beauty gave rise to a general feeling of complete trust in it as 

 the best attainable conception of electrostatic phenomena. 

 The result was that attention was almost exclusively focused 

 on action-at-a-distance theories, until the time, long afterwards,, 

 when Faraday led natural philosophers back to the right' 

 path. 



Coulomb rendered great services to magnetic theory. It was 

 he who in 1777, by simple mechanical reasoning, completed 

 the overthrow of the hypothesis of vortices.* He also, in the 

 second of the Memoirs already quoted,f confirmed Michell's 

 law, according to which the particles of the magnetic fluids 

 attract or repel each other with forces proportional to the 

 inverse square of the distance. Coulomb, however, went beyond 

 this, and endeavoured to account for the fact that the two 

 magnetic fluids, unlike the two electric fluids, cannot be 

 obtained separately; for when a magnet is broken into 

 two pieces, one containing its north and the other its south 

 pole, it is found that each piece is an independent magnet 

 possessing two poles of its own, so that it is impossible 

 to obtain a north or south pole in a state of isolation. 

 Coulomb explained this by supposing^ that the mag- 

 netic fluids are permanently imprisoned within the molecules 

 of magnetic bodies, so as to be incapable of crossing from 

 one molecule to the next ; each molecule therefore under all 

 circumstances contains as much of the boreal as of the 



* Mem. presences par divers Savans, ix (1780), p. 165. 



t Mem de 1'Acad., 1785, p. 593. Gauss finally established the law by a 

 much more refined method. 



J In his Seventh Memoir, Mem, de 1'Acad., 1789, p. 488. 



