[ 649 ] 
part II. 
CoHtalnitig a cotnparifon of the foregoing theory ‘with 
experiment. 
§ I . It appears from experiment, that fome bodies 
fufFer the eleiSlrlc fluid to pafs with great read!- 
nefs between their pores ; while others will not 
fufFer it to do fo without great difficulty ; and fome 
hardly fufFer it to do fo at all. The firfl; fort of 
bodies are called condudlors, the others non-con- 
fludors. What this difference in bodies is owing 
to I do not pretend to explain. 
It is evident that the eledtric fluid in non-con- 
dudlors may be confidered as moveable, or anfwers 
to the definition given of that term in p. 588. As 
to the fluid contained in non-condudling fubflances, 
though it does not abfolutely anfwer to the defi- 
nition of immoveable, as it is not abfolutely con^* 
fined from moving, but only does fo with great 
difficulty ; yet it may in mofl: cafes be looked upon 
as fuch without fenfible error. 
Air does in fome meafure permit the eledlric 
fluid to pafs through it ; though, if it is dry, it 
lets it pafs but very flovvly, and not without diffi- 
culty ; it is therefore to be called a non-condudlor. 
It appears that condudlors would readily fufFer 
the fluid to run in and out of them, were it not 
for the air which furrounds them: for if the end 
of a condudlor is inferted into a vacuum, the fluid 
runs in and out of it with perfect readinefs ; but 
VoL. LX I. 4 O when 
