3 22 Dr. Gray’s Obfervations on the 
“ furface, but inafmuch as an equal dofe of natural fire can 
“ quit the other furface*.” 
Thefe aflertions are, I apprehend, dlredtly contrary to what 
really happens. lnftead of which, 1 believe, we mav fafely 
aflert, that giafs, and every other known fubftance, may have 
its natural quantity of eledtric fluid either increafed or dimi- 
nifhed to a certain limited degree; which degree bears no pro- 
portion to the quantity of matter contained in a body, but is 
[cater is paribus ) in proportion to the extent of its furface. 
This law, which is, perhaps, without exception, may be 
confidered as one of the fundamental laws of eledtricity, and 
one upon which many of its principal phenomena depend. 
At prefent I fhall only confider it fo far as it is the caufe of 
what is commonly called the charge of a coated jar. 
Suppofe fuch a jar infulated, and connected by its knob to 
the prime condudtor of an eledtric machine ; if then the ma- 
chine be put in adtion, a certain quantity of eledtric fluid 
(agreeable to the above-mentioned law) is added to the natural 
quantity belonging to the inner furface of the jar. After 
which, if the finger, or any other conducting fubftance, be 
prefented to the outer coating of the jar, a quantity of eledtric 
fluid, nearly equal to that thrown in, comes from it. But 
this departure of eledtric fluid from the outfide of the jar, 
cannot be (as Dr. Franklin fuppofes it) the caufe which per- 
mits the addition of fluid to the infide, but is merely the con- 
fequence of the adtion of that fuperfluous quantity which was 
thrown in. And the operator may, if he pleafes, inflead of 
taking eledtric fluid from the outfide of the jar, take out again 
(by touching the knob) nearly the whole of what he had 
* Treatife upon Artificial Ele&ricity, Eng. Tranf. fe&. 181. 
thrown 
