another body whether they were electrified positively or negatively*.

365] It was said in Art. [331], that there seemed no reason to think that the charge of the plate D, or of any other of those glass plates was sensibly greater than it would be if the electricity was spread uniformly on their surfaces, whereas the charge of most of the plates of air was found very considerably greater than it would be on that supposition. But this is by no means inconsistent, for according to the first way of accounting for the great excess of the real charge of those plates above the computed, namely supposing that the electricity penetrates into the glass to the depth of of its thickness, the increase of its charge on account of the electricity being not spread uniformly, should be not greater than it would be if the glass was only of its real thickness, and the electricity was unable to penetrate into it at all, and therefore should not be greater than it is in a plate of air in which the thickness is of the diameter, and should therefore in all probability be quite imperceptible.

And by Prop. XXXVI. [Art. 170], the increase of charge should hardly be much, if at all, greater according to the second or third way of accounting for this phenomenon.

366] In order to try† whether the charge of coated glass is the same when hot as when cold, I made use of the apparatus in Fig, 28, where ABCba represents a short thermometer tube with a ball BCb blown at the end and another smaller ball near the top. This is filled with mercury as high as the bottom of the upper ball, and placed in an iron vessel FGMN filled with mercury as high as FN. Consequently the ball BCb was coated as a Leyden vial, the mercury within it forming the inside coating, and that in the vessel FGMN the outer one.

In trying it, I set the vessel FGMN on the wooden bars of the machine represented in Fig. 20, near the end NP, and dipt a small iron wire bound round the wire Mm into the mercury within the tube, so as to make a communication between the wire Mm and the inside coating, the outside coating, or the mercury in FGMN, being made to communicate with the ground.

† [Art. 556, March 21, 1773. See also Arts. 548, 549, 680.]

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It was heated by a lamp placed under FGMN, and its charge was frequently tried while heating by comparing with a sliding coated plate placed on the other end of the wooden bars.

When it was sufficiently heated, the lamp was taken away, and the charge frequently tried in the same manner while cooling, a thermometer being dipt every now and then into the mercury in FGMN to find its heat.

367] As it was apprehended that the electricity might spread further on the surface of the glass while hot than while cold, a paper coating DBbd was fastened on the tube, so that as the outside coating was made to extend as far as Dd, that is three or four inches above the mercury in FGMN, where the tube was very little heated, and as the inside coating reached still higher, that is to the bottom of the upper ball, no sensible error could proceed from thence.

The use of the upper ball was to prevent the mercury within the tube from overflowing when hot.

368] By a mean between the experiments made while the ball was heating and while cooling, its charge answering to the different degrees of heat was as follows.

369] At 295° the electricity passed through the glass pretty freely, but at 305° much faster. It appears, therefore, that the charge of glass is considerably greater when heated to such a degree as to suffer the electricity to pass through than when cold, but that its charge does not begin to be sensibly increased till it is heated to a considerable degree*.

370] On the charges of plates of several different sorts of glass, and also of plates of some other substances which do not conduct electricity, charged in the manner of Leyden vials.

* [Note 26.]

The result of the experiments I made on this subject is contained in the two following tables:

372]

The coatings of all these plates were circular.

In computing the charge of the glass plates, the diameter of the coating was corrected on account of the spreading of the electricity as in the fourth column, the electricity being supposed to spread 07 of an inch if the thickness is 21 and 09 if the thickness is 08, and so on in proportion in other thicknesses. But no correction is made in computing the charges of the other plates, as I was uncertain how much to allow.