Specific Indiictixie Capacity of Glass. 419 



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

 electricity, is indicated by the i^istantaneous assumption and 

 discharge of those portions of the power which are concerned 

 in the phasnomena, 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 ap- 

 paratus, for it obtains its portion of electricity in an instant, 

 and yet is charged far above the mean. 



1270. Admitting for the present the general fact sought to 

 be proved; then 1*5, though it expresses the capacity of the 

 apparatus containing the hemisphere of shell- lac, by no means 

 expresses the relation of lac to air. The lac only occupies 

 one half of the space o, o, of the apparatus containing it, 

 through which the induction is sustained; the rest is filled 

 with air, as in the other apparatus ; and if the effect of the 

 two upper halves of the globes be abstracted, then the com- 

 parison of the shell-lac powers in the lower half of the one, 

 with the power of the air in the lower half of the other, will 

 be as 2 : 1 ; and even this must be less than the truth, for the 

 induction of the upper part of the apparatus, i. e. of the wire 

 and ball B (fig. 1.) to external objects, must be the same in 

 both, and considerably diminish the difference dependent 

 upon, and really producible by, the influence of the shell-lac 

 within. 



1271. Glass. — I next worked with glass as the dielectric. 

 It involved the possibility of conduction on its surface, but it 

 excluded the idea of conducting particles within its substance 

 (1267.) other than those of its own mass. Besides this it does 

 not assume the charged state (1239.) so readily, or to such 

 an extent as shell-lac. 



1272. A thin hemispherical cup of glass being made hot 

 was covered with a coat of shell-lac dissolved in alcohol, and 

 after being dried for many hours in a hot place, was put into 

 the apparatus and experimented with. It exhibited effects so 

 slight, that, though they were in the direction indicating a 

 superiority of glass over aii', they were allowed to pass as 

 possible errors of experiment; and the glass was considered 

 as producing no sensible effect. 



1273. I then procured a thick flint glass hemispherical cup 

 resembling that of shell-lac (1239.), but not filling up the 

 space o, o, so well. Its average thickness was 0'4< of an inch, 

 there being an additional thickness of air, averaging 0-22 of 

 an inch, to make up the whole space of ()*62 of an inch be- 

 tween the inducting metallic surfaces. It was covered with 

 a film of shell-lac as the former was, (1272.) and being made 



2E 2 



