Part 37 (2/2)

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 sh.e.l.l-lac, by no means expresses the relation of lac to air.

The lac only occupies one-half of the s.p.a.ce _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 comparison of the sh.e.l.l-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. 104.) to external objects, must be the same in both, and considerably diminish the difference dependent upon, and really producible by, the influence of the sh.e.l.l-lac within.

1271. _Gla.s.s._--I next worked with gla.s.s 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 ma.s.s. Besides this it does not a.s.sume the charged state (1239.) so readily, or to such an extent, as sh.e.l.l-lac.

1272. A thin hemispherical cup of gla.s.s being made hot was covered with a coat of sh.e.l.l-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 gla.s.s over air, they were allowed to pa.s.s as possible errors of experiment; and the gla.s.s was considered as producing no sensible effect.

1273. I then procured a thick hemispherical flint gla.s.s cup resembling that of sh.e.l.l-lac (1239.), but not filling up the s.p.a.ce _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 s.p.a.ce of 0.62 of an inch between the inductive metallic surfaces. It was covered with a film of sh.e.l.l-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 sh.e.l.l-lac, i.e. gla.s.s surpa.s.sed 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 gla.s.s apparatus was charged first 1.45, as the specific inductive capacity for gla.s.s, both being without correction. The average of nine results, four with the gla.s.s apparatus first charged, and five with the air apparatus first charged, gave 1.38 as the power of the gla.s.s 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 gla.s.s 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 gla.s.s apparatus, then the specific inductive capacity of flint gla.s.s will be above 1.76, not forgetting that this expression is for a piece of gla.s.s of such thickness as to occupy not quite two-thirds of the s.p.a.ce 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.

b.a.l.l.s 280.

0 . . . .

. . . . 0 438 . . . .

434 . . . .

Charge divided.

. . . . 162 164 . . . .

. . . . 160 162 . . . .

. . . . 0 after discharge.

0 . . . . after discharge.

Here app. i. retained 164, having lost 276 in communicating 162 to app.

ii., and the capacity of the air apparatus is to that of the sulphur apparatus as 1 to 1.66.

1276. Then the sulphur apparatus was charged first, thus:

. . . . 0 0 . . . .

. . . . 395 . . . . 388 Charge divided.

237 . . . .

. . . . 238 0 . . . . after discharge.

. . . . 0 after discharge.

Here app. ii. retained 238, and gave up 150 in communicating a charge of 237 to app. i., and the capacity of the air apparatus is to that of the sulphur apparatus as 1 to 1.58. These results are very near to each other, and we may take the mean 1.62 as representing the specific inductive capacity of the sulphur apparatus; in which case the specific inductive capacity of sulphur itself as compared to air = 1 (1270.) will be about or above 2.24.

1277. This result with sulphur I consider as one of the most unexceptionable. The substance when fused was perfectly clear, pellucid, and free from particles of dirt (1267.), so that no interference of small conducting bodies confused the result. The substance when solid is an excellent insulator, and by experiment was found to take up, with great slowness, that state (1244. 1242.) which alone seemed likely to disturb the conclusion. The experiments themselves, also, were free from any need of correction. Yet notwithstanding these circ.u.mstances, so favourable to the exclusion of error, the result is a higher specific inductive capacity for sulphur than for any other body as yet tried; and though this may in part be clue to the sulphur being in a better shape, i.e. filling up more completely the s.p.a.ce _o, o_, (fig. 104.) than the cups of sh.e.l.l-lac and gla.s.s, still I feel satisfied that the experiments altogether fully prove the existence of a difference between dielectrics as to their power of favouring an inductive action through them; which difference may, for the present, be expressed by the term _specific inductive capacity_.

1278. Having thus established the point in the most favourable cases that I could antic.i.p.ate, I proceeded to examine other bodies amongst solids, liquids, and gases. These results I shall give with all convenient brevity.

1279. _Spermaceti._--A good hemisphere of spermaceti being tried as to conducting power whilst its two surfaces were still in contact with the tinfoil moulds used in forming it, was found to conduct sensibly even whilst warm. On removing it from the moulds and using it in one of the apparatus, it gave results indicating a specific inductive capacity between 1.3 and 1.6 for the apparatus containing it. But as the only mode of operation was to charge the air apparatus, and then after a quick contact with the spermaceti apparatus, ascertain what was left in the former (1281.), no great confidence can be placed in the results. They are not in opposition to the general conclusion, but cannot be brought forward as argument in favour of it.

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