C 3 l 3 ] 
by glafs ; for in this cafe it appeared to come from 
the earth, then into the tube, and fo on to the refinous 
cylinder. Thofe bodies therefore which have no com- 
munication with the earth, and from which the refi- 
nous cylinder is fupplied, mud: be electrified minus *. 
Thefe luminous knobs (were there no other proofs) 
are a drong confirmation of the exigence of a medium 
at or on the furfaces of bodies, which, to a limited 
degree, refids or hinders the entrance and exit of the 
efedtric fluid. But we have fome other experiments 
to illuflrate the fame, and the Tourmalin does afiid us 
not a little. 
There is one thing more, which I am obliged to 
take notice of, concerning the impermeability of glad 
by electricity. Our friend, Dr. Franklin , feems to 
found a great part of his fydem on this opinion. 
“ Glafs, fays he, lias within its fubdance always 
c£ the fame quantity of eleCtrical fire, and that a very 
“ great quantity in proportion to the mafs. This 
“ quantity, proportioned to the glafs, it drongly and 
“ obdinately retains, and will have neither more 
“ nor lejs , though it will fufter a change to be made 
“ in its parts and fituationj i. e. we may take away 
<( part of it from one of its fides, provided we throw 
<c an equal quantity into the other.” 
This doCtrine I could never agree to, fo far as it re- 
lates to the impermeability of glafs, as appears by my 
letters in anfwer to the Abbe Nollet and M. le Roy , 
on their requeding to know my fentiments about it, 
* The fame cylinder made rough with emmery, and rubbed with 
flannel at one time, and with leather at another, will afford thefe 
different appearances, the firfl: caufing the minus, and the laft the 
plus appearance. 
Vol. LI. S s 
lil 
