The distance of the two circular coatings of tinfoil was measured by the same instrument with which I measured the thickness of the plates of glass, and may be depended on to the 1000th or at least to the 500th part of an inch*.

342] In this manner I made the experiment with the plates at four different distances, namely 910, 420, 288 and 256, and when I had made a sufficient number of trials with the plates at each distance, I took off these circular coatings and put on smaller, namely of 6.35 inches diameter, and tried the experiment as before with the plates at 259 inches distance. The result of the experiments is given in the following table:

It is plain that some allowance ought to be made in these trials for the spreading of the electricity on the surface of the glass. In the above table I have supposed it to spread 05 of an inch, but the effect is so small that it is of very little signification whether that allowance is made or not.

344] In my former paper [Art. 134] I expressed a doubt whether the air contained between the two plates in this experiment is overcharged on one side and undercharged on the other, as is the case with the plate of glass in the Leyden vial, or whether the redundant and deficient fluid is lodged only in the plates, and that the air between them serves only to prevent the electricity from running from one plate to the other, but the following experiment shows that the latter opinion is true.

I placed the two brass plates on the machine (Fig. 20), and tried their charge as before, except that, after having charged the plates, I immediately lifted up the upper plate by a silk string so as to separate it two or three inches from the lower one, and let it * [See Art. 459, "Bird's instrument," and "dividing machine," Art. 517. 594, 595.]

[See Arts. 669, 519.]

[Arts. 511, 516, Dec. 18, 26, 1772.]

Also

down again in its place before I found its charge by making the communication between Bb and Dd and between Aa and Ee.

The way I did this was that as soon as I had let down the wire Cc on Aa and Bb, and thereby charged the plates, I lifted it up again half way so as to take away the communication between Cc and the upper plate &c., but did not lift it quite up, so as to make the communication between Bb and Dd, and between Aa and Ee, till after I had separated the upper plate from the lower, and put it back in its place.

I could not perceive any sensible difference in the charge, whether I lifted up the upper plate in the above-mentioned manner, or whether I tried its charge without lifting it up.

345] It is plain that in lifting up the upper plate from the lower and letting it down again, the greatest part of the air contained between the two plates must be dissipated and mixed with the other air of the room, so that if the air contained between the two plates was overcharged on one side and undercharged on the other, the charge must have been very much diminished by lifting up the upper plate and letting it down again, whereas, as I said before, it was not sensibly diminished.

I think we may conclude, therefore, that redundant and deficient fluid is lodged only in the plates, and that the air between them serves only to prevent the electricity from running from one plate to the other.

346] As this is the case, the charge of these plates ought, according to the theory, to be equal to that of a globe whose diameter equals the square of the radius of the plate or circular coating divided by twice their distance, that is, to their computed charge, provided the electricity is spread uniformly on the surface of the plates, and therefore in reality the numbers in the last column but one ought to be rather greater than in the last but two, and moreover the less the distance of the plates is in proportion to the diameter of the coating, the less should be the proportion in which those numbers differed, and if the distance is infinitely small in proportion to the diameter, the proportion in which those numbers differ, should also be infinitely small.

347] This will appear by inspecting the table to be the case, only it seems from the manner in which the numbers decrease,

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.

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.

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, e§, &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 aA, 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 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.]