40 



ments are adduced in support of this view. With shell-lac and sper- 

 maceti the return charge was considerable ; with glass and sulphur 

 it was much less ; but with air, no decided effect of the kind could 

 be obtained. As this was an effect which might interfere with the 

 results, in the method the author adopted for deciding: the question 

 of specific inductive capacity, and as time was requisite for this pe- 

 netration of the charge, its influence on these results was p'uarded 

 against by allowing, between the successive operations, as little 

 time as possible for this peculiar action to arise. 



The author thus states the question of specific inductive capacity 

 which he had proposed to investigate : — Suppose A an electrified 

 plate of metal suspended in the air, and B and C two exactly similar 

 plates, placed parallel to and on each side of A., at equal distances, 

 and uninsulated; A will then induce equally towards B and C. If 

 in this position of the plates, some otiier dielectric than air, as shell- 

 lac, be introduced between A and C, will the induction between them 

 remain the same ; or will the relation of C and B to A be altered 

 by the difference of the dielectiics interposed between them ? 



The experiment of Coulomb, from which it appeared that a wire 

 surrounded by shell-lac took exactly the same quantity of electricity 

 from a charged body, as the same body took in air, seemed to the 

 author to be no proof of the truth of the assumption, that, under 

 such variation of the circumstances as he had supposed, no change 

 wouldoccur. Entertaining these doubts as to the conclusions deducible 

 from Coulomb's result, he had the apparatus previously described 

 constructed, as being well adapted for this investigation. After re- 

 jecting glass, resin, wax, naphtha, oil of turpentine, and other sub- 

 stances, as unfit for the pui-pose in ^iew, he chose shell-lac as the sub- 

 stance best calculated to serve a- an experimental test of the question. 



For the purpose of comparing the inductive capacities of shell-lac 

 and air, a hemispherical cup of shell-lac was introduced into the lower 

 hemisphere of one of the inductive apparatus, so as to nearly fill the 

 lower half of the space between the t^'o spheres : and their charges 

 were divided in the manner already described; each apparatus being 

 used in turn to receive the first charge, before its division with the 

 other. As the two instruments were known to have equal inductive 

 powers when aii- was contained in both, any deficiencies resulting 

 from the introduction of the shell-lac would show a peculiar action 

 in it, and, if unequivocally referable to a specific inductive influence, 

 would establish the point in question. 



The air apparatus being charged, and its disposable charge being 

 290°, this charge was divided between the two. After the division 

 the charge in the lac apparatus was 113°, and in the air apparatus 

 114°. From this it appears, that whilst by the division the induction 

 through the air lost 176°, that through lac gained only 113°. As- 

 suming that this difference depends entirely on the gi'eater facility 

 possessed bv shell-lac of allowing or causing inductive action through 

 its substance than that possessed by air, then the capacity for electric 

 induction would be inversely as the respective loss and gain ; and as- 



