Dielectric influence of temperature \ 8 1 



364] I could not find that there was any difference in the proportion 

 which the charge of a glass plate bore to that of 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 T 7 jj 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 ac- 

 counting for this phenomenon. 



366] In order to tryf 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 com- 

 munication between the wire Mm and the inside coating, the outside coating, 

 or the mercury in FGMN, being made to communicate with the ground. 



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 



* [Art. 463.] f [Art. 556, March 21, 1773. See also Arts. 548, 549, 680.] 



