14 Mr. Faraday and "Dr. P. Riess on the Action of 



ginable nianuer for all cases here concerned. It presumes that 

 the three electric strata — upon the surface of the globe P, upon 

 the face n, and the face p — act independently of each other, in- 

 ducing upon the globe N. If we denote with /(P), the inductive 

 effect of the globe P on the globe N, with —f{n) that of n (the 

 sign — says that the effect is contrary to that of P), and with 

 f{p) the inductive effect of the face jo, the theory asserts that in 

 all cases the final effect on N is dependent on the amount of the 

 sum /(P)— /('O +/{]>)) ^11^^ leaves it to the calculation to say if 

 the sum is greater or less than /(P). In the sole case where 

 f(p) vanishes, that is to say, when the intermediate metallic 

 plate has been touched or is uninsulated, it can be said without 

 computation that the sum of inductive effects of globe and plate 

 is less than the effect of the globe alone. 



After having experimentally shown, that with conducting 

 intermediate plates the induction can be strengthened as well as 

 weakened, and with non-conducting plates weakened as well as 

 strengthened, I ventured to advance the opinion, that the action 

 of plates of whatever nature have the same cause, viz. the arrange- 

 ment of the electricities of opposite kind upon the surfaces of 

 the plates. I examined roughly (if necessary, it could be made 

 very accurately) the arrangement of the electricities upon a 

 metallic disc, and I concluded that the electricities are arranged 

 in a similar manner (not the same) upon a non-conducting disc. 

 I concede that this conclusion is not unobjectionable, but I main- 

 tain that it must be made necessarily at first, and cannot be 

 abandoned unless it is proved false. The simple fact, that a 

 non-conducting body is attracted by an electrified body, shows 

 clearly that the non-conducting matter as well as the conducting 

 is instantaneously provided by induction with both electricities. 



As a more direct proof of this induction upon insulators, with- 

 out an essential connexion with the subject I treated of, I have 

 described an experiment which occurred to me and which I had 

 nowhere found. To this supplementary experiment belongs your 

 third remark, to which I proceed. A shell-lac disc is quickly 

 moved once from above to below between a flame and the globe 

 of the conductor of an electric machine (not "to and fro;" I 

 have indicated this error of translation to Prof. Tyndall in a 

 letter dated 19th June)*. The anterior face of the disc is found 

 to be strongly negative. You agree with me, that without the 

 flame both faces of the disc have been instantaneously provided 



polaritj' of the particles which I assume as the cause? Or again, how can 

 coniluction and insulation, considered as contingent causes, give as their 

 result the same distribution of force? — M. F. 



* I would obsene that I am not responsible for the translation, which, 

 however, on the whole, appears to be excellent. — J. T. 



