EXAMPLES OF IHE FOUR METHODS. 295 



In the case, therefore, of the electric machine, it appears that the accumula- 

 tion of electricity in an insulated conductor is always accompanied by the 

 excitement of the contrary electricity in the surrounding atmosphere, and 

 in every conductor placed near the former conductor. It does not seem 

 possible, in this case, to produce one electricity by itself. 



Let us now examine all the other instances which we can obtain, resem- 

 bling this instance in the given consequent, namely, the evolution of an op- 

 posite electricity in the neighborhood of an electrified body. As one re- 

 markable instance we have the Leyden jar; and after the splendid experi- 

 ments of Faraday in complete and final establishment of the substantial 

 identity of magnetism and electricity, we may cite the magnet, both the 

 natural and the electro-magnet, in neither of which it is possible to produce 

 one kind of electricity by itself, or to charge one pole without charging an 

 opposite pole with the contrary electricity at the same time. We can not 

 have a magnet with one pole : if we break a natural loadstone into a thou- 

 sand pieces, each piece will have its two oppositely electrified poles com- 

 plete within itself. In the voltaic circuit, again, we can not have one cur- 

 rent without its opposite. In the ordinary electric machine, the glass cyl- 

 inder 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 consequent, the excitement of the op- 

 posite electric state in some other body or bodies. It seems to follow that 

 the two facts are invariably connected, and that the excitement of electric- 

 ity in any body has for one of its necessary conditions the isossibility of 

 a simultaneous excitement of the opposite electricity in some neighbor- 

 ing body. 



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 Meth- 

 od 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 conduct- 

 ing surfaces of equal extent, and parallel to each other through the whole 

 of that extent, with a non-conducting substance such as glass between them. 

 When one side of the jar is charged positively, the other is charged nega- 

 tively, 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 dis- 

 charged at the same time. A conductor held to the positive side can not con- 

 vey away any electricity 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 ordinarily 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 primaiy 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 neighboring body which is to 

 be oppositely electrified is the surrounding atmosphere, or any body casu- 

 ally larought near to the conductor ; and as these are generally much in- 



