SPECIFIC INDUCTIVE CAPACITY OP GLASS AND SULPHUR. 31 



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, 0, of the apparatus containing it, through which the induction is sus- 

 tained ; 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 comparison 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 possi- 

 bility 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 air, 

 they were allowed to pass as possible errors of experiment ; and the glass was con- 

 sidered 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 0"62 of an inch between the inducting me- 

 tallic surfaces. It was covered with a film of shell-lac as the former was, (1272.) 

 and being made very warm, was introduced into the apparatus, also warmed, 

 and experiments made with it as in the former instances (1257- &c.). The general 

 results were the same as with shell-lac, i. e. glass surpassed air in its power of favour- 

 ing induction through it. The two best results as respected the state of the apparatus 

 for retention of charge, &c., gave, when the air apparatus was charged first 1-336, 

 and when the glass apparatus was charged first 1*45, as the specific inductive capa- 

 city for glass, both being without correction. The average of nine results, four with 

 the glass apparatus first charged, and five with the air apparatus first charged, gave 

 1*38 as the power of the glass apparatus; 1*22 and 1*46 being the minimum and 

 maximum numbers with all the errors of experiment upon them. In all the experi- 

 ments the glass apparatus took up its inductive charge instantly, and lost it as 

 readily ; and during the short time of each experiment, acquired the peculiar state 

 in a small degree only, so that the influence of this state, and also of conduction upon 

 the results, must have been small. 



1274. Allowing specific inductive capacity to be proved and active in this case, and 



