28(5 DIELECTRICS. SECT. XXVIII. 



two, having now opposite electricities, will attract each other. 

 The attraction between electrified and tinelectrified substances is 

 a consequence of the altered state of their molecules. Induction 

 depends upon the facility with which the equilibrium of the 

 neutral body can be overcome, a facility which is proportional to 

 its conducting power. Consequently the attraction exerted by 

 an electrified substance upon another substance previously 

 neutral will be much more energetic if the latter be a conductor 

 than if it be a non-conductor. 



It is clear that one body cannot act upon another at a 

 distance without some means of communication. Dr. Faraday 

 has proved that the intervening non-conducting substance or 

 dielectric has a great influence upon induction. Thus the 

 inductive force is greater when sulphur is interposed between 

 the two bodies than when shellac is the dielectric, and greater 

 when shellac is the dielectric than glass, &c. Professor Matteucci 

 has proved by the following experiment that the intervening sub- 

 stance is itself polarized by induction. A number of plates of 

 mica in contact were placed between two plates of metal, one 

 of which was electrified, so that the whole was charged like a 

 Leyden jar. On separating the plates with insulating handles, 

 each plate of mica was electrified ; one side of it was positive and 

 the other negative, showing decidedly a polarization by induction 

 throughout the whole intervening non-conducting substance ; and 

 thus, although the interposed substance or dielectric is incapable 

 of conducting the electrical force from one body to the other, 

 it becomes by induction capable of transmitting it. In the 

 atmosphere induction is transmitted by that of the intervening 

 strata of air. It is true that induction takes place through 

 the most perfect vacuum we can make, but there always remains 

 some highly elastic air ; and even if air could be altogether 

 excluded, the ethereal medium cannot, and it must be capable of 

 induction, since, however attenuated, it must consist of material 

 atoms, otherwise it would be a nonentity. 



The law of electrical attraction and repulsion has been 

 determined by suspending a needle of gum-lac horizontally by a 

 silk fibre, the needle carrying at one end a piece of electrified 

 gold leaf. A globe in the same or opposite electrical state when 

 presented to the gold leaf will repel or attract it, and will there- 

 fore cause the needle to vibrate more or less rapidly according 



