1273.  I then procured a thick hemispherical flint glass 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 inductive metallic 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 favouring 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 capacity 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 experiments the glass apparatus took up its inductive charge instantly, and lost it as readily (1269.); 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 1.38 as the expression for the glass apparatus, then the specific inductive capacity of flint glass will be above 1.76, not forgetting that this expression is for a piece of glass of such thickness as to occupy not quite two-thirds of the space through which the induction is sustained (1253. 1273.).

* * * * *

1275. Sulphur.—­The same hemisphere of this substance was used in app. ii. as was formerly referred to (1242.).  The experiments were well made, i.e. the sulphur itself was free from charge both before and after each experiment, and no action from the stem appeared (1203. 1232.), so that no correction was required on that account.  The following are the results when the air apparatus was first charged and divided: 

App. i.  Air, App. ii.  Sulphur. 
        Balls 280 deg..

    0 deg. . . . .
        . . . . 0 deg.
  438 . . . .
  434 . . . . 
      Charge divided.
        . . . . 162
  164 . . . .
        . . . . 160
  162 . . . .
        . . . . 0 after discharge.
    0 . . . . after discharge.

Here app. i. retained 164 deg., having lost 276 deg. in communicating 162 deg. to app. ii., and the capacity of the air apparatus is to that of the sulphur apparatus as 1 to 1.66.