THUNDER STORMS. “81 
which none can be forced into or condenfed upon the out- 
er furface, fo as to exhibit any figns of electricity; as ob- 
ferved before, 
IJ. I fhall endeavour to prove that the eleCtric capacity 
of air is increafed by heat. 
This alfo appears probable, at leaft, from the above cited 
experiments of doftor Prieffley ; for after the air in his 
tube had had this capacity fo far diminifhed by condenfa- 
tion as not to permit an excitation without, that. capacity, 
together with the confequent excitability of the tube, was 
reftored by the aétion of heat upon the included air. ‘ Re- 
“ peating my attempts (fays he) to excite the tube above 
«mentioned, I found that, after very hard rubbing, it be- 
“ gan to aca little, and that its virtue increafed with the 
“labour. Thinking it might be the warmth which pro- 
“ duced this effect, I held the tube to the fire and found 
“ that when it was pretty hot, it would a& almoft as well 
“as when it contained no more than its ufual quantity 
COk aii. ¢ 
In page 553, doctor Priefley tells us that fome of his 
electrical friends were of opinion, “ that the reafon why 
** a tube with condenfed air in it cannot be excited is, that 
“¢ the denfe air within prevents the electric fluid from be- 
‘¢ ing forced out of the infide of the tube, without which 
* none can be forced into the outfide; and that heating 
«* the tube makes the air within lefs electrical.’”’ That is, 
as I conceive their meaning, puts it in a capacity to re- 
ceive and abforb more of the electric fluid than it could 
otherwife do in that condenfed ftate. ‘The doctor indeed 
queries by way of objeCtion to the foregoing folution,— 
“ How upon this principle can a folid ftick of glafs be ex- 
“ cited ?” To which I would anfwer, that poflibly, when 
a folid ftick of glafs is excited, as much of the ele¢tric fluid 
may be drawn out of one fide of it as is thrown into, or con- 
denfed upon the other; if fo, although it may fhew equal 
figns of electricity on both fides, yet one fide will be ina 
potitive 
* Page 551. 
