that they would never become equal to unity though the distance of the plates was ever so small in respect of their diameter, and I should think, or rather I imagine, would never be less than 1·1, so that it seems as if the charge of a plate of air was rather greater in proportion to that of the globe than it ought to be, and I believe nearly in the proportion of 11 to 10*.

348] The reason of this, I imagine, is as follows. It seems reasonable to conclude from the theory that when a globe or any other shaped body is connected by a wire to a charged Leyden vial, and thereby electrified, the quantity of redundant fluid in the globe will bear a less proportion to that on the positive side of the jar than it would do if they could be connected by a canal of incompressible fluid†, but in all probability when a plate of air is connected in like manner to the Leyden vial, the quantity of redundant fluid on its positive side will bear nearly the same proportion to that in the vial that it would do if they were connected by a canal of incompressible fluid, and consequently the charge of the plate of air in these experiments ought to bear a greater proportion to that of the globe than if they had been connected to the vial by which they were electrified by canals of incompressible fluid.

349] It was said in Art. 339 that the charges of the glass plates were rather more than eight times greater than they ought to be by the theory, if the electric fluid did not penetrate to any sensible depth into the glass. Though this is what I did not expect before I made the experiment, yet it will agree very well with the theory if we suppose that the electricity, instead of entering into the glass to an extremely small depth, as I thought most likely when I wrote the second part of this work ‡, is in reality able to enter into the glass to the depth of of the whole thickness of the glass, that is, to such a depth that the space into which it can not penetrate is only of the thickness of the glass, as in that case it is evident that the charge should be as great as it would be if the thickness of the glass was only of its real thickness, and the electricity was unable to penetrate into it at all.

350] There is also a way of accounting for it without suppos

* [Art. 670.]

+ This seems likely from Appendix, Coroll. 5 [Art. 184].

[Refers to Art. 132.]

ing the electricity to enter to any sensible depth into the glass, by supposing that the electricity at a certain depth within the glass is moveable, or can move freely from one side of the glass to the other.

Thus, in Fig. 25, let ABDE be a section of the glass plate perpendicular to its plane, suppose that the electricity from withFig. 25.

B

16

B

D

out can penetrate freely into the glass as far as the line ab or ed but not further, suppose too that within the spaces abßa and edde the electric fluid is immoveable, but that within the space aßde it is moveable, or is able to move freely from the line aß to de. Then will the charge of the plate be just the same as on the former supposition, provided the distances az and ee are each

thickness of the plate*.

of the

351] But I think the most probable supposition is that there are a great number of spaces within the thickness of the glass in which the fluid is alternately moveable and immoveable.

Fig. 26.

B

8

E

'D

Thus let ABDE (Fig. 26) represent a section of the plate of glass as before, and let the glass be divided into a great number of spaces by the parallel lines ab, aß, ed, ed, &c., and suppose that in the two outermost spaces ABba and EDde the fluid is moveable, that in the two next spaces abßa and edde it is immoveable, and

* The only reason why I suppose the electric fluid to be able to enter into the glass from without as far as the lines ab and ed is that Dr Franklin has shewn that the charge resides chiefly in the plate of glass and not in the coating, and consequently that the electricity is able to penetrate into the glass to a certain depth. Otherwise it would have done as well if we had supposed the fluid to be immoveable in the whole spaces ABẞa and EDde, and that the distance Aa and Ee are each of AE.

that in the two next spaces it is moveable, and so on. The charge will be the same as before, supposing the sum of the thickness of the spaces in which the electricity is immoveable to be of the whole thickness of the glass, as it is shewn that the charge of such a plate will be the same as that of a plate in which the electricity is entirely immoveable, whose thickness is equal to the sum of the thicknesses of those spaces in which we supposed the fluid immoveable*.

352] It must be observed that in those spaces in which we supposed the fluid to be moveable, as in the space ABba for example, though the fluid is able to move freely from the plane Ab to ab, that is, though it moves freely in the direction Aa or a▲, or in a direction perpendicular to the plane of the plate, yet it must not [be] able to move lengthways, or from A to B, for if it could, and one end of the plate AE was electrified, some fluid would instantly flow from AE to BD, and make that end overcharged, which is well known not to be the case. The same thing must be observed also with regard to the two former ways of explaining this phenomenon.

353] The chief reason which induces me to prefer the latter way of accounting for it is that in the two former ways the thickness of the spaces in which the fluid is moveable must necessarily be very considerable. In thick glass, for example, in a plate of the same thickness as D, it must be not less than 18 of an inch in the first way of explaining it, and in the second way it must be still greater. Now if the electric fluid is able to move through so great a space in the direction AE, it seems extraordinary that it should not be able to move in the direction AB, whereas in the latter way of accounting for it the thickness of the spaces in which the electricity is moveable may be supposed infinitely small, and consequently the distance through which the electricity moves in the direction AE also infinitely small.

354] Another thing which inclines me to this way of accounting for it is that there seems some analogy between this and the power by which a particle of light is alternately attracted and repelled many times in its approach towards the surface of any refracting or reflecting medium. See Mr Michell's explanation

[Prop. xxxv. Art. 169, and Note 15.]

of the fits of easy reflection and transmission in Priestley's Optics, page 309.

355] To whichever of these causes it is owing that the charges of these plates are so much greater than they should be if the electric fluid was unable to enter into the plate, it was reasonable to expect that the greater the force with which the plate was electrified, the greater should be the depth to which the electric fluid penetrates into the glass, or the greater should be the thickness of the spaces in which we supposed the fluid to be moveable, and consequently in comparing the charge of the plate D with the circle of 36 inches diameter, or with any other body, the greater the force with which they are electrified the greater proportion should the charge of the glass plate bear to that of the circle.

356] I therefore compared the charge of the plate D with that of the circle of 36 inches with electricity of two different degrees of strength, namely the same which I made use of in [Art. 329], in trying whether the distance to which the electricity spread on the surface of glass was different according to the strength of the electricity.

The way in which I compared their charges was just the same that I made use of in comparing the rosin plate with the tin circles in [Art. 337]. The event was that I could not perceive that the proportion which their charges bore to each other with the stronger degree of electricity was sensibly different from what they did with the weaker.

357] But it must be remembered that it seemed from the experiment related in [Art. 329], that the electricity spread of an inch further on the surface of the glass with the stronger degree of electricity than with the weaker. The difference of charge owing to this difference in the spreading of the electricity is part of the whole, so that it seems that if the electricity had been prevented from spreading on the surface of the glass, the proportion of the charge of the glass plate to that of the tin circle would have been less with the stronger degree of electricity than with the weaker, and that nearly in the proportion of 16 to 17.

* [Arts. 547, 551, 553, also Arts. 451, 463, 526, 535, 538, 664.]

358] I also made an experiment to determine whether the charge of a coated plate of glass bore the same proportion to that of another body when the electricity was very weak as when it was of the usual strength*.

For this purpose I first found what proportion the charge of a tin cylinder 15 feet long and 17 inches in circumference bore to that of the two plates D and E together when the electricity was very weak. This I did in the manner represented in Fig. 27,

Fig. 27.

B

where AB is the tin cylinder supported horizontally by non-conductors. DC is a brass wire 37 inches long and about inch in diameter supported also horizontally by non-conductors, the end C being in contact with the cylinder, and a pair of fine pith balls being suspended from the other end D. FE is a piece of wire communicating with the prime conductor, and between it and DC is suspended by a silk string the wire W in a vertical situation.

359] The cylinder AB, and consequently the wire DC, were first electrified negatively to such a degree as to make the pith balls separate to the distance of one diameter of the balls. The prime conductor and wire FE being then charged to the usual degree, as shewn by the usual electrometer hung down from it, one end of the wire W was brought in contact with E so as to be electrified by it, and was then immediately removed and *[Arts. 539, 666.]