EXAMPLES OF THE FOUR METHODS. 487 



sitely electrified poles complete within itself. In the 

 voltaic circuit, again, we cannot have one current 

 without its opposite. In the ordinary electric machine, 

 the glass cylinder or plate, and the rubber, acquire 

 opposite electricities. 



From all these instances, treated by the Method 

 of Agreement, a general law appears to result. The 

 instances embrace all the known modes in which a 

 body can become charged with electricity ; and in all 

 of them there is found, as a concomitant or con- 

 sequent, the excitement of the opposite electric state 

 in some other body or bodies. It seems to follow 

 that the two facts are invariably connected, and that 

 the excitement of electricity in any body has for one 

 of its necessary conditions the possibility of a simul- 

 taneous excitement of the opposite electricity in some 

 neighbouring body. 



As the two contrary electricities can only be pro- 

 duced together, so they can only cease together. 

 This may be shown by an application of the Method 

 of Difference to the example of the Leyden jar. It 

 needs scarcely be here remarked that in the Leyden 

 jar, electricity can be accumulated and retained in con- 

 siderable quantity, by the contrivance of having two 

 conducting surfaces of equal extent, and parallel to each 

 other through the whole of that extent, with a non-con- 

 ducting substance such as glass between them. When 

 one side of the jar is charged positively, the other is 

 charged negatively, and it was by virtue of this fact 

 that the Leyden jar served just now as an instance in 

 our employment of the Method of Agreement. Now 

 it is impossible to discharge one of the coatings unless 

 the other can be discharged at the same time. A 

 conductor held to the positive side cannot convey 



