30 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XI.) 



cular piece of shell-lac a decided superiority over air in allowing or causing the act 

 of induction, it proved the growing necessity of a more close and rigid examination 

 of the whole question. 



1267. The shell-lac was of the best quality, and had been carefully selected and 

 cleaned ; but as the action of any conducting particles in it would tend, virtually, to 

 diminish the quantity or thickness of the dielectric used, and produce effects as if 

 the two inducing surfaces of the conductors in that apparatus were nearer together 

 than in the one with air only, 1 prepared another shell-lac hemisphere, of which the 

 material had been dissolved in strong spirit of wine, the solution filtered, and then 

 carefully evaporated. This is not an easy operation, for it is difficult to drive off the 

 last portions of alcohol without injuring the lac by the heat applied ; and unless they 

 be dissipated, the substance left conducts too well to be used in these experiments. I 

 prepared two hemispheres this way, one of them unexceptionable ; and with it I re- 

 peated the former experiments with all precautions. The results were exactly of the 

 same kind ; the following expressions for the capacity of the shell-lac apparatus, 

 whether it were app. i. or ii., being given directly by the experiments 1*46, 1*50, 1*52, 

 1*51 ; the. average of these and several others being very nearly 1*5. 



1 268. As a final check upon the general conclusion, I then actually brought the 

 surfaces of the air apparatus, corresponding to the place of the shell-lac in its appa- 

 ratus, nearer together, by putting a metallic lining into the lower hemisphere of the 

 one not containing the lac (1213.). The distance of the metal surface from the carrier 

 ball was in this way diminished from 0*62 of an inch to 0*435 of an inch, whilst the 

 interval occupied by the lac in the other apparatus remained 0*62 of an inch as before. 

 Notwithstanding this change, the lac apparatus showed its former superiority ; and 

 whether it or the air apparatus was charged first, the capacity of the lac apparatus to 

 the air apparatus was by the experimental results as 1*45 to 1. 



1 269. From all the experiments I have made, and their constant results, I cannot 

 resist the conclusion that shell-lac does exhibit a case of specific inductive capacity. 

 I have tried to check the trials in every way, and if not remove, at least estimate, 

 every source of error. That the final result is not due to common conduction is 

 shown by the capability of the apparatus to retain the communicated charge ; that 

 it is not due to the conductive power of inclosed small particles, by which they could 

 acquire a polarized condition as conductors, is shown by the effects of the shell-lac 

 purified by alcohol ; and, that it is not due to any influence of the charged state, 

 formerly described (1250.), first absorbing and then evolving electricity, is indicated 

 by the instantaneous assumption and discharge of those portions of the power which 

 are concerned in the phenomena, that effect occurring in these cases, as in all others 

 of ordinary induction by charged conductors. The latter argument is the more 

 striking in the case where the air apparatus is employed to divide the charge with 

 the lac apparatus, for it obtains its portion of electricity in an instant, and yet is 

 charged far above the mean. 



