240 IXDUCTION. 



though it. never can amount to that of the Method of Diffevence prop- 

 erly so called, may approach indefifiitely near to it. 



No similar defect of completeness in proof will be found i'n the 

 following Original investigation, for which I am indebted to ISIr. Alex- 

 ander "Bain, at present Lecturer on Moral Philosophy in Marischal 

 CoUeo^e, Aberdeen ; one of the men from whom science and philoso- 

 phy have most to hope, and Avho has permitted me to lay his extensive 

 knowledgeof every department of physical inquiry freely under con- 

 tribution, for the purpose of exemplifying and illustrating the doctrines 

 ofthis work..^ 



§■ 3. Let the object be to ascertain the law of what is termed hiduccd 

 electricity ; to find under what conditions any electrified body, whether 

 positively or negatively electrified, gives rise to a contrary electric 

 state in some other body adjacent to it. 



The most familial- exemplification oPthe phenomenon to be investi- 

 gated, is the following. Around the prime conductors of an electrical 

 machine, the atmosphere to some distance, or any conducting surface 

 suspended in. that atmosj)here, is ibund to be in an electric condition 

 opposite to that of the prime conductoi- itself Near and around the 

 positive prime conductor there is a negative electricity, and near and 

 around the negative- prime conductor there is positive electricity. 

 When pith balls are brought near to either of the conductors, they 

 become electiified with the opposite electricity to it ; either receiving 

 a share from the alrfeady electrified atmosphere by conduction, or 

 acted upon by the direct inductive influence of, the conductor itself: 

 they are then attracted by the conductor to which they are in opposi- 

 tion; or, if withdrawn in their electrified state, they will be attracted 

 by any other oppositely dafarged body. In like Inanner the hand, if 

 brought near enough to the coiiductor, receives or gives an electric 

 discharge ; now we have no evidence that a charged conductor can be 

 suddenly discharged unless by the approach of a body oppositely elec- 

 trified. In the case, therefore, of the electrical' machine, it apj^ears 

 that the accumulation of electricity in an insulated conductor is always 

 accompanied by the- excitement of , the contrary electricity in the sur- 

 rounding atmosphere, and in exery 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, 

 resembling this instance in the given consequent, namely, the evolution 

 of an opposite electricity in the neighborhood of an electi'ified body. 

 As one remai'kable instance we have the Leyden jar ; and after the 

 splendid experiments 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 is it possible to produce one kind of electricity by itself, or to 

 chai"*e one pole without charging an opposite pole with the contrary 

 electricity at the same time; We cannot have a magnet with one 

 pole : if we break a natural loadstone into a thoasand pieces, each 

 piece will have its two oppositely electrified poles complete within 

 itself In the voltaic circuit, again, we cannot have one cunent with- 

 out its opposite. In the ordinary electric anachine, the glass cylinder 

 or plate, aild the rubber, acquire opposite electricities. 



