14 Capacities of bodies of different substances 



seems as unexceptionable as it would have been to have tried them by 

 themselves, supposing it had been possible to have done so. 



The last mentioned substance was quite fluid, but had sufficient 

 tenacity to prevent its flowing immediately to the lowest part of the 

 plate. In those substances in which the thickness of the coating is not 

 set down it was not measured, but the thickness was small. 



271] All these things were supported on the pillars of baked wood 

 and waxed glass described at [Art. 255]. The panes of glass were laid on 

 these pillars with their coated sides uppermost, so that the wires Rr and 

 Dd fell on their coated sides. As many of the substances used were but 

 imperfect conductors of electricity, I fastened bits of tinfoil about an 

 inch square on the places on which the wires Rr and Dd touched the 

 plate in order to make the electric fluid spread more readily over it, and 

 I satisfied myself beforehand that with this precaution they conducted 

 readily enough for my purpose, as I found by discharging a Leyden vial, 

 and making these substances part of the circuit. 



272] It appears from these experiments that the charge of a thick 

 plate is greater than that of a thin one of the same base, as might be 

 guessed from the theory*, and it seems to be equal to that of a very thin 

 one whose side exceeds that of the thick one by about i$ of its thickness. 

 Let us therefore increase the mean side of each of these plates by i^ of 

 its thickness, where that quantity is worth regarding, and alter the charge 

 found by experiment in the ratio of 12 inches to the side thus increased, 

 which will give us the charge of a plate of the same materials and shape 

 whose increased side is 12 inches, when the charge of each substance will 

 stand as in the last column of the preceding Table. These numbers do 

 not differ from each other by more than what may fairly be supposed 

 owing to the error of the experiment, and therefore I think we may 

 conclude firstly, that the charge of a body of a given shape and size is 

 the same whatever materials it consists of, and, though the experiment 

 was tried only with square plates, yet I think there can be no doubt but 



* [Note 20, p. 409.] 



