Researches in Electricity : Eleventh Series, 209 



flint-glass, and spermaceti, and with these, corresponding results 

 were obtained. These results, the author considers, cannot but be 

 admitted as arguments against the received theory of induction, 

 and in favour of that which he has put forth. 



In the course of these experimental researches, some effects due 

 to conduction, which had not been anticipated, and which were si- 

 milar to the residual charge in the Leyden jar, had been obtained 

 with such bodies as glass, lac, sulphur, &c. If the inductive appa- 

 ratus, fitted with a hemispherical cup of shell-lac, after having re- 

 mained charged for fifteen or twenty minutes, was suddenly and per- 

 fectly discharged, and then left to itself, it would gradually recover 

 every sensible charge ; the electricity which thus returned from an 

 apparently latent to a sensible state being always of the same kind 

 as that given by the charge. This return charge is attributed to an 

 actual penetration by conduction of the charge to some distance 

 within the dielectric at each of its two surfaces, and several experi- 

 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 guarded 

 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 : — I 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 un-insulated, A will then induce equally towards B and C. If 

 in this position of the plates, some other 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 dielectrics interposed between them ? 



The experiments 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 

 would occur. Entertaining these doubts of the conclusions deduci- 

 ble 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 purpose in view, he chose shell-lac as the 

 substance best calculated to serve as 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 two spheres ; and their charges 



Phil. Mag, S. 3. Vol. 12. No. 73. Feb, 1838. 2 A 



