41 4« Mr. Faraday's Researches in ElectricitT). {Series XL) 



the air in the one direction, or through the shell-lac of the 

 plate in the other ; provided the second surface of the plate 

 had not, by contact with conductors, the action of dust, or 

 any other means, become charged (1203.). Its solid con- 

 dition enabled it to retain the excited particles in a perma- 

 nent position, but that appeared to be all ; for these particles 

 acted just as freely through the shell-lac on one side as 

 through the air on the other. The same general experiment 

 was made by attaching a disc of tin foil to one side of the 

 shell-lac plate, and electrifying it, and the results were the 

 same. Scarcely any other solid substance than shell-lac and 

 sulphur, and no liquid substance that I have tried^ will bear 

 this examination. Glass in its ordinary state utterly fails; 

 yet it was essentially necessary to obtain this prior degree of 

 perfection in the dielectric used, before any further progress 

 could be made in the principal investigation. 



1256. Shell-lac a7id air were compared in the first place. 

 For this purpose a thick hemispherical cup of shell-lac was 

 introduced into the lower hemisphere of one of the inductive 

 apparatus (1187, &c.), so as nearly to fill the lower half of 

 the space o, o (fig. 1.) between it and the inner ball; and then 

 charges were divided in the manner already described (1198. 

 1207.), each apparatus being used in turn to receive the first 

 charge before its division by the other. As the apparatus 

 were known to have equal inductive power when air was in 

 both (1209. 1211.), any differences resulting from the intro- 

 duction of the shell-lac would show a peculiar action in it, 

 and if unequivocally referable to a specific inductive influence, 

 would establish the point sought to be sustained. I have al- 

 ready referred to the precautions necessary in making the ex- 

 periments (1199, &c.); and with respect to the error which 

 might be introduced by the assumption of the peculiar state, 

 it was guarded against as far as possible in the first place, by 

 operating quickly (124-8.), and afterwards by using that di- 

 electric as glass or sulphur, which assumed the peculiar state 

 most slowly, and in the least degree (1239. 1241.). 



1257. The shell-lac hemisphere was put into app. i., and 

 app. ii. left filled with air. The results of an experiment in 

 which the charge through air was divided and reduced by the 

 shell-lac app. were as follows : 



App. i. Lac. App. ii. Air. 



Balls 255°. 

 0° . . . . 



.... 304° 

 . . . ; 297 



