1858.] Remarks on Static Inductmi. 471 



that any one of them becomes negative to those which precede it 

 in the list, or positive to those which follow. Thus, metal becomes 

 negative to shell-lac, and positive to sulphur ; and as either of 

 these substances can be employed in the investigation of the fun- 

 damental principles of induction, this difference is important in 

 some of the methods of examining by the electrometer their 

 temporary or permanent state. If a stick of shell-lac have a 

 flannel cap fitted to one extremity, both being unexcited, and these, 

 either separate or associated, be examined by the gold-leaf electro- 

 meter, they will show no signs of electricity. If the cap, grasped 

 by the hand, be turned round on the shell-lac with friction, but left 

 in its place, the associated substances will still show no signs of 

 electricity. If separated, each will show a strongly excited state 

 opposite to that of the other. If one be laid on the cap of the 

 electrometer (the gold leaves of which were 7 inches long and 

 H inches wide, with perfect insulation) it will show a highly 

 excited state ; if the other be gradually brought near, and finally 

 placed by the side of the first, all the electric signs will disappear, 

 to reappear when the separation is again produced. The experi- 

 ment presents a type case of excitation and induction. By the 

 friction together the opposite electricities are excited ; they then 

 exist and keep their state by mutual induction ; they are perfectly 

 equivalent to each other, and hold their existence by this definite 

 and relative equivalency ; for one electricity cannot exist by itself. 

 They show no external signs of electricity whilst the forces are 

 related only to each other, but when the two bodies in which the 

 states are located are separated, then this relation is not exclusive, 

 but by so much as the induction is diminished between the two sub- 

 stances, it is thrown in other directions ; as towards the electrometer, 

 or the walls of the room. When one is carried into a separate 

 room, or put into a vessel of conducting matter, then the excited 

 bodies become independent of each other ; each has raised up an 

 exactly equal amount of the contrary force by action terminating at 

 a distance, according to the laws of ordinary^induction. The power 

 exerted by each excited body in this distant action may be expressed 

 by the term, lines of force. These lines, or the force they represent, 

 are sustained, so long as they are contained in or pass through an 

 insulating medium. They continue, until meeting with conducting 

 matter they evolve the contrary state to that at which they originate, 

 and in the equivalent proportion, and so terminate the insulation ; 

 or failing that, they continue their course outwards. If it were 

 possible to place the excited shell-lac in the centre of an almost 

 infinite extent of insulating medium, the lines of force would be as 

 Infinitely extended from it. If the power at any section of the 

 whole of the lines of force could be compared with that at any other 

 section, they would be found equal to each other ; though one 

 section might be close to the shell-»lac, and the other at an infinite 

 distance. If there were no conducting matter at the boundaries of 



