. . . . 0 215 . . . .

204 . . . .

Charge divided.

. . . . 118 118 . . . .

. . . . 0 after being discharged.

0 . . . . after being discharged.

1261. Here 204 must be the utmost of the divisible charge. The app. i. and app. ii. present 118 as their respective forces; both now much _above_ the half of the first force, or 102, whereas in the former case they were below it. The lac app. i. has lost only 86, yet it has given to the air app. ii. 118, so that the lac still appears much to surpa.s.s the air, the capacity of the lac app. i. to the air app. ii. being as 1.37 to 1.

1262. The difference of 1.55 and 1.37 as the expression of the capacity for the induction of sh.e.l.l-lac seems considerable, but is in reality very admissible under the circ.u.mstances, for both are in error in _contrary directions_. Thus in the last experiment the charge fell from 215 to 204 by the joint effects of dissipation and absorption (1192. 1250.), during the time which elapsed in the electrometer operations, between the applications of the carrier ball required to give those two results. Nearly an equal time must have elapsed between the application of the carrier which gave the 204 result, and the division of the charge between the two apparatus; and as the fall in force progressively decreases in amount (1192.), if in this case it be taken at 6 only, it will reduce the whole transferable charge at the time of division to 198 instead of 204; this diminishes the loss of the sh.e.l.l-lac charge to 80 instead of 86; and then the expression of specific capacity for it is increased, and, instead of 1.37, is 1.47 times that of air.

1263. Applying the same correction to the former experiment in which air was _first_ charged, the result is of the _contrary_ kind. No sh.e.l.l-lac hemisphere was then in the apparatus, and therefore the loss would be princ.i.p.ally from dissipation, and not from absorption: hence it would be nearer to the degree of loss shown by the numbers 304 and 297, and being a.s.sumed as 6 would reduce the divisible charge to 284. In that case the air would have lost 170, and communicated only 113 to the sh.e.l.l-lac; and the relative specific capacity of the latter would appear to be 1.50, which is very little indeed removed from 1.47, the expression given by the second experiment when corrected in the same way.

1264. The sh.e.l.l-lac was then removed from app. i. and put into app. ii. and the experiments of division again made. I give the results, because I think the importance of the point justifies and even requires them.

App. i. Air. App. ii. Lac.

b.a.l.l.s 200.

. . . . 0.

286 . . . .

283 . . . .

Charge divided.

. . . . 110 109 . . . .

. . . . 0.25 after discharge.

Trace . . . . after discharge.

Here app. i. retained 109, having lost 174 in communicating 110 to app.

ii.; and the capacity of the air app. is to the lac app., therefore, as 1 to 1.58. If the divided charge be corrected for an a.s.sumed loss of only 3, being the amount of previous loss in the same time, it will make the capacity of the sh.e.l.l-lac app. 1.55 only.

1265. Then app. ii. was charged, and the charge divided thus:

App. i. Air. App. ii. Lac, 0 . . . .

. . . . 250 . . . . 251 Charge divided.

146 . . . .

. . . . 149 a little . . . . after discharge.

. . . . a little after discharge.

Here app. i. acquired a charge of 146, while app. ii. lost only 102 in communicating that amount of force; the capacities being, therefore, to each other as 1 to 1.43. If the whole transferable charge be corrected for a loss of 4 previous to division, it gives the expression of l.49 for the capacity of the sh.e.l.l-lac apparatus.

1266. These four expressions of 1.47, 1.50, 1.55, and 1.49 for the power of the sh.e.l.l-lac apparatus, through the different variations of the experiment, are very near to each other; the average is close upon 1.5, which may hereafter be used as the expression of the result. It is a very important result; and, showing for this particular piece of sh.e.l.l-lac a decided superiority over air in allowing or causing the act of induction, it proved the growing necessity of a more close and rigid examination of the whole question.

1267. The sh.e.l.l-lac was of the best quality, and had been carefully selected and cleaned; but as the action of any conducting particles in it would tend, virtually, to diminish the quant.i.ty or thickness of the dielectric used, and produce effects as if the two inducing surfaces of the conductors in that apparatus were nearer together than in the one with air only, I prepared another sh.e.l.l-lac hemisphere, of which the material had been dissolved in strong spirit of wine, the solution filtered, and then carefully evaporated. This is not an easy operation, for it is difficult to drive off the last portions of alcohol without injuring the lac by the heat applied; and unless they be dissipated, the substance left conducts too well to be used in these experiments. I prepared two hemispheres this way, one of them unexceptionable; and with it I repeated the former experiments with all precautions. The results were exactly of the same kind; the following expressions for the capacity of the sh.e.l.l-lac apparatus, whether it were app. i. or ii., being given directly by the experiments, 1.46, 1.50, 1.52, 1.51; the average of these and several others being very nearly 1.5.

1268. As a final check upon the general conclusion, I then actually brought the surfaces of the air apparatus, corresponding to the place of the sh.e.l.l-lac in its apparatus, nearer together, by putting a metallic lining into the lower hemisphere of the one not containing the lac (1213.). The distance of the metal surface from the carrier ball was in this way diminished from 0.62 of an inch to 0.435 of an inch, whilst the interval occupied by the lac in the other apparatus remained O.62 of an inch as before. Notwithstanding this change, the lac apparatus showed its former superiority; and whether it or the air apparatus was charged first, the capacity of the lac apparatus to the air apparatus was by the experimental results as 1.45 to 1.

1269. From all the experiments I have made, and their constant results, I cannot resist the conclusion that sh.e.l.l-lac does exhibit a case of _specific inductive capacity_. I have tried to check the trials in every way, and if not remove, at least estimate, every source of error. That the final result is not due to common conduction is shown by the capability of the apparatus to retain the communicated charge; that it is not due to the conductive power of inclosed small particles, by which they could acquire a polarized condition as conductors, is shown by the effects of the sh.e.l.l-lac purified by alcohol; and, that it is not due to any influence of the charged state, formerly described (1250.), first absorbing and then evolving electricity, is indicated by the _instantaneous_ a.s.sumption and discharge of those portions of the power which are concerned in the phenomena, that instantaneous 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 apparatus, 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 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.