prior to the Introduction of the Potentials. 59 



austral fluid, and magnetization consists simply in a separation 

 of the two fluids to opposite ends of each molecule. Such 

 a hypothesis evidently accounts for the impossibility of 

 separating the two fluids to opposite ends of a body of finite 

 size. The same idea, here introduced for the first time, has 

 since been applied with success in other departments of 

 electrical philosophy. 



In spite of the advances which have been recounted, 

 the mathematical development of electric and magnetic theory 

 was scarcely begun at the close of the eighteenth century ; and 

 many erroneous notions were still widely entertained. In a 

 Eeport* which was presented to the French Academy in 1800, 

 it was assumed that the mutual repulsion of the particles of 

 electricity on the surface of a body is balanced by the 

 resistance of the surrounding air; and for long afterwards 

 the electric force outside a charged conductor was confused 

 with a supposed additional pressure in the atmosphere. 



Electrostatical theory was, however, suddenly advanced to 

 quite a mature state of development by Simeon Denis Poisson 

 (b. 1781, d. 1840), in a memoir which was read to the French 

 Academy in 1812.f As the opening sentences show, he accepted 

 the conceptions of the two-fluid theory. 



" The theory of electricity which is most generally accepted," 

 he says, " is that which attributes the phenomena to two 

 different fluids, which are contained in all material bodies. 

 It is supposed that molecules of the same fluid repel each 

 other and attract the molecules of the other fluid ; these 

 forces of attraction and repulsion obey the law of the inverse 

 square of the distance ; and at the same distance the attractive 

 power is equal to the repellent power; whence it follows 

 that, when all the parts of a body contain equal quantities 

 of the two fluids, the latter do not exert any influence on 

 the fluids contained in neighbouring bodies, and consequently 

 no electrical effects are discernible. This equal and uniform 



* On Yolla's discoveries. 



t Mem. de Plnstitut, 1811, Part i., p. 1, Part ii., p. 163. 



