there does not seem to be any difference which can be depended on with certainty. The experiment indeed would have been more satisfactory if the piece of ground glass and the pieces with which it was compared had been all made out of the same pot. But as it would have been difficult procuring such pieces, and as I have found very little difference in the specific gravity of different pieces of crown glass, and as I am informed it is all made at the same glass house, I did not take that precaution. 379] Let two or more flat plates of different non-conducting substances, as Aab B, Bbc C and CcdD, (Fig. 29) be placed close together and coated in the manner of a single plate with the coatings Ee and Ff. Let the charge of the plate AabB, supposing it placed by itself and coated in the usual manner, be equal to that of a plate of glass whose thickness is A and whose coatings are of the same size as those of AabB. In like manner let the charge of Bbc C be equal to that of a plate of the same glass whose thickness is equal to B, and let that of CcdD equal that of one whose thickness is C. of ac E Fig. 29. A B C a с Then whichever of the three ways counting for the excess of the real charge of glass plates above the computed we prefer, it is a necessary consequence of our theory that the charge of this compound plate AadD should be equal to that of a single plate of glass whose thickness equals A + B + C, and whose coatings are of the same size as Ee and Ff. 380] In like manner if two or more plates of the same kind of glass are placed together and coated as above, the charge of this compound plate should be equal to that of a single plate of the same glass whose thickness is equal to that of all the plates together. This appears from the following experiments to be the case, for 1st. I took the three plates of glass A, B and C, and laid them on one another, having first taken off their old coatings and coated the outside surfaces as in fig. 29 with circles of tinfoil 6.6 inches in diameter. The charge of this compound plate was found to be to that of the three plates D, E and F together as 944 to 1. The sum of the thicknesses of A, B and C together is 6309, and the computed charge of a plate of that thickness with coatings 6.6 in. diameter is to that of D, E and F together, allowing in the same manner as in [Art. 328] for the instantaneous spreading of the electricity, as 94 to one. So that the charge of this compound plate is exactly the same that it ought to be according to the foregoing rule. 381] 2ndly, I made a plate of a mixture of rosin and bees waxt, about 8 inches square and somewhat more than 12 thick, and coated it with circles 661 in. diameter. Its charge was found to be to that of the plates K, D and E together as 56 to 55, and therefore should be equal to that of a plate of glass of the same kind as K whose thickness is 345 and the diameter of whose coatings is the same as those of the rosin plate, namely 661 inches. This plate was then inclosed between the glass plates B and H+, the coatings being first taken off, and the outside surfaces of B and H coated with circles 6.6 inches in diameter. Its charge was found to be to that of K as 7.56 to 8. According to the foregoing rule, its charge should be the same as that of a plate of glass of the same kind as B 634 of an inch thick with coatings 66 inches in diameter, and should therefore be to that of K as 7:34 to 8, which is very nearly the same that it was actually found to be, 382] On the charges of such Leyden vials as do not consist of flat plates of glass. These experiments were made with hollow cylindrical pieces of glass, open at both ends, and coated both within and without with pieces of tinfoil surrounding the cylinder in the form of a ring, the breadth of the ring being everywhere the same, and the inside and outside coatings being of the same breadth, and placed exactly opposite to each other. Only as the inside diameter of the two +[Arts. 548, 678.] * [Arts. 534, 544, 546, 677.] [Arts. 552, 679.] thermometer tubes was too small to admit of being coated in this manner, they were filled with mercury by way of inside coating. The thickness of the glass was found by suspending the cylinder by one end from a pair of scales with its axis in a vertical position, and the lower part immersed in a vessel of water, and finding the alteration of the weight of the cylinder according as a greater or less portion of it was under water*. 383] The result of the experiments is contained in the following tablet. The lengths of the coating here set down are the real lengths. But in computing the charges of the white jar and cylinder and the three green cylinders, these lengths were increased on account of the spreading of the electricity according to the same supposition as was used in computing the charges of the flat plates. But in computing the charges of the thermometer tubes no correction was made, as I was uncertain how much to allow, but as the length of their coatings is so great, this can hardly make any sensible error. 384] It should seem from these experiments as if the proportion of the real to the computed charge was rather less in a cylinder in which the thickness of the glass is of the semidiameter than in one in which it is only, and most likely rather less in that than in a flat plate, but then it seems to be not much less in a cylinder in which the inside diameter is many times less than * [Art. 594.] + [See Art. 676, and Note 28.] the outside, that is, in which the thickness of the glass is almost equal to the outside semidiameter, than it is in the first mentioned cylinder. Nothing certain, however, can be inferred as to this point, as in all probability the four pieces of flint glass used in these experiments and the two flat pieces used in [Art. 370] did not consist exactly of the same kind of glass, as indeed appears from their specific gravities. 385] The three green cylinders, indeed, were all made at the same time and out of the same pot, so that it seems difficult to suppose that there should be any difference of that kind between them*. But then I had no flat plates to compare them with. On the whole, I think we may with tolerable certainty infer that the ratio of the real to the computed charge is not very different from what it is in flat plates, whatever is the proportion which the thickness of the glass bears to the diameter of the cylinder, though it seems to be not exactly the same. Though it seems not likely that there should be any difference in the nature of the glass of which the three green cylinders consisted, yet I am not sure that there was not, for the inside of the glass, that is, that part which was nearest to the inside surface, was manifestly more opaque and of a different colour from the outside, and the separation between these two sorts of glass appeared well defined, so that the cylinder seemed to consist of two different coats of glass lying one over the other. The distinction was the most visible in those cylinders which consisted of the thickest glass and in the thickest part of those cylinders. The specific gravities, however, do not indicate any difference in the nature of the glass. What was the reason of the above-mentioned appearance I cannot tell. WHETHER THE FORCE WITH WHICH TWO BODIES REPEL IS AS THE SQUARE OF THE REDUNDANT 386] If two bodies, A and B, placed near to each other, are both connected to the same overcharged Leyden jar, and the force with which this jar is electrified is varied, everything else remaining unaltered, the force with which A and B repel each other ought by the theory to be as the square of the quantity of redundant fluid in the jar, supposing the distance of the bodies A and B to remain unaltered. For the quantity of redundant fluid in A is directly as the quantity of redundant fluid in the jar, and therefore the force with which each particle of redundant fluid in B is repelled by A is also directly as the quantity of redundant fluid in the jar, and therefore as the number of particles of redundant fluid in B is also as the quantity of redundant fluid in the jar, the force with which B is repelled by A is as the square of the quantity of redundant fluid in the jar. 387] In order to try whether this was the case, I made use of the following apparatus †. CD (Fig. 31) is a wooden rod 43 inches long, covered with tinfoil and supported horizontally by non-conductors. At the end * [Title supplied from Cavendishes Index to his experiments, Art. 563.] + [Arts. 563, 567, also Art. 525.] |