EXAMPLES OF THE FOUR METHODS. 455 



As the two contrary electricities can only be produced 

 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 

 considerable quantity, by the contrivance of having two con 

 ducting surfaces of equal extent, and parallel to each other 

 through the whole of that extent, with a non-conducting sub 

 stance 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 Agree 

 ment. Now it is impossible to discharge one of the coatings 

 unless the other can be discharged at the same time. A con 

 ductor held to the positive side cannot convey away any elec 

 tricity unless an equal quantity be allowed to pass from the 

 negative side : if one coating be perfectly insulated, the charge 

 is safe. The dissipation of one must proceed pari passu with 

 that of the other. 



The law thus strongly indicated admits of corroboration 

 by the Method of Concomitant Variations. The Leyden jar 

 is capable of receiving a much higher charge than can ordi 

 narily be given to the conductor of an electrical machine. 

 Now in the case of the Leyden jar, the metallic surface which 

 receives the induced electricity is a conductor exactly similar 

 to that which receives the primary charge, and is therefore as 

 susceptible of receiving and retaining the one electricity, as 

 the opposite surface of receiving and retaining the other ; but 

 in the machine, the neighbouring body which is to be op 

 positely electrified is the surrounding atmosphere, or any body 

 casually brought near to the conductor ; and as these are gene 

 rally much inferior in their capacity of becoming electrified, to 

 the conductor itself, their limited power imposes a correspond 

 ing limit to the capacity of the conductor for being charged. 

 As the capacity of the neighbouring body for supporting the 

 opposition increases, a higher charge becomes possible : and 

 to this appears to be owing the great superiority of the Leyden 

 jar. 



