472 Professor Faraday, [Feb. 12, 



the insulating space above supposed, the shell-lac could not exist 

 independently in the excited state : it would then keep its lines of 

 force altogether turned upon the body by which it had first been 

 excited, the induction between the two being sustained by their 

 reciprocal action, without which electricity could neither be excited 

 nor exist. Such are some of the consequences which follow in- 

 evitably upon the laws of static induction, combined with the law 

 of the conservation of force. 



But if this function of induction be so essential to the very exist- 

 ence of electricity in its developed or active, state, what is its 

 nature ? It acts through distance and across intervening bodies : 

 how are the space and the bodies affected ? In all actions at a dis- 

 tance it is most important to ascertain, if possible, what occurs in the 

 intervening medium, or the interposed space ; whether the investi- 

 gation ends in the establishment of a particular process for the 

 particular case, or the reference of the process to any more general 

 mode of action representing all cases of distant action. 



Induction acts across any insulating body, whether it be solid, 

 fluid, or gaseous. Common air is concerned in most inductive 

 actions, but being mobile, its particles cannot be retained in a 

 given place, position, or state, so as to allow of close examination. 

 Sulphur and shell-lac are excellent bodies as subjects of investigation, 

 the more especially as their specific inductive capacity is about 

 twice that of air ; and being solid bodies, their superficial or bound- 

 ing particles can be thrown into a given state, yet preserved in 

 their place to be examined with the purpose of showing what that 

 state is. If a round plate of metal, 9 inches in diameter, be set 

 up vertically in the air and insulated, and a like plate of good gutta- 

 percha raised on an insulating pillar be placed parallel to and about 

 9 inches from it ; then, upon exciting the gutta-percha, strong 

 induction occurs. The gutta-percha presents the inductric, the 

 copper plate the inducteous, surfaces which limit the field of induc- 

 tion, which field supplies an excellent place for experiment. The 

 gutta-percha should be excited by a piece of close broad cloth, free 

 from loose particles, and all dust, or other sources of convective 

 effects should be avoided. Plates of sulphur, about 3 or 4 inches 

 square, and 1 inch thick, may be employed as the inductive 

 medium, and these having white silk loops introduced into he 

 edges when cast, may, by the further use of white silk slings, be 

 suspended or handled with perfect facility. Some discs of stiff 

 paper, gilt on both sides, being attached at the edge to thin stems 

 of shell-lac, are thus well insulated, and serve either as metallic 

 plates or carriers. 



It is almost impossible to take a block of sulphur out of paper, 

 or from off the table without finding it electric ; if, however, a small 

 spirit-lamp flame be moved for a moment before its surface at about 

 an inch distance, it will discharge it perfectly. Being then laid on 

 the cap of the electrometer it will probably not cause divergence of 



