80 THEORY of 
the glafs, repelling and throwing off an equal quantity of 
the electric fluid from the oppofite furface, through the 
non-electric coating in contact with it; nor can any charge 
be given to either fide without a proportional difcharge 
from the other. In like manner, when an uncoated tube 
is excited by friction, a quantity of the fluid, equal to that 
which is excited and condenfed upon the outer furface, is 
thrown out from the inner, provided there is any fub- 
ftance within in a capacity to receive and abforb it, with- 
out which no excitation can take place. ‘A glafs tube, 
“ out of which the air is exhaufted, difcovers no figns of 
“ electricity outwards*,”’ there being no fubftance within 
capable of receiving and abforbing the fluid from the in~ 
ner furface, which though repelled from it inwards dur- 
ing the operation, yet returns to it again inftantly upon a 
ceflation of the ation of the rubber without. But upon 
a readmiflion of air the excitation is eafy, and is attended 
with the ufual effets. Air then, which is the only fub- 
ftance admitted (excepting perhaps a few ftraggling va- 
pors which float in it) receives and abforbs a fufficient 
quantity of the ele€tric fluid from the inner furface to per- 
mit an excitation of the tube which contains it. But as 
we have feen that air, when condenfed within, prevents 
the vifible effets of an excitation, equally with a total va- 
cuity, we may adopt the idea of dotor Franklin, mutatis 
mutandis, and conlcude that *¢ what the fponge is to water 
“ the fame is ar to the eletric fluid:”” At leaft that this ca- 
pacity of air if leffened by condenfation in a manner, not 
indeed perfetly fimilar, but, fomewhat analogous to that in 
which the capacity of a fponge to receive and retain water 
is leflened by compreflion. Agreeably to which idea, the 
condenfed air within the tube, having its electric capacity 
filled and even crowded with the electric matter, will re= 
ceive none from the inner furface, which, on the contrary, 
is thereby prevented from being forced out of it, without 
which 
* Prieftley’s hiftory of electricity, page 550. 
