THUNDER STORMS. “9 
Premifing that by azr I here intend ¢ha¢ fluid in its com- 
‘mon comprefled ftate with us near the furface of the earth; 
and by its electric capacity, that ftate of it which difpofes 
it, under any circumftances whatever, “ to attract, abforb 
and retain,” what doctor Franklin calls its natural quantity, 
er the quantity which is zatura/ to it in that ftate. 
I. I fhall endeavour to prove that the electric capacity 
of air is leffened by condenfation. 
That a change of denfity in air produces alfo a change 
in its electric capacity (as above defined), follows from fome 
experiments of monfieur de Faye and dogtor Prieffley, the 
former of whom found, upon repeated trials, that no elec= 
tricity could be excited by the friGion of a glafs tube in 
which the air was condenfed*. The doctor, repeating the 
experiments with fome variation, found, that when one 
additional atmofphere was forced into the tube, the elec- 
tricity excited by rubbing it was fearcely difcernable. 
Now, though the effect was a fufpenfion of the operation 
of the excited tube wzthout, the caufe was evidently the 
condenfed ftate of the air wzthzm; which may be accounted 
for if we confider, that although it is certain from many 
experiments that glafs is abfolutely impermeable to the 
electric fluid, infomuch that it cannot force its way through 
a pane of glafs, or the fides of a phial, without breaking 
the glafs, as was the cafe in thofe fpontaneous difcharges 
of feveral of the jars in the ele&trical battery mentioned by 
doctor Prieftley} ; yet it is as certain, that this impermea- 
bility of the glafs to the fluid itfelf, is no obftru€tion to 
the operation of that repellent power upon which the vi-- 
fible effe&s of this element feem principally to depend; 
which power undeniably acts from one fide of the glafs,, 
through the very fubftance of it, upon the fame fluid on 
the other fide, provided there be any other fubftance on 
that fide capable of receiving it when thus repelled. 
This is the cafe in the Leyden experiment in every form 
in. which it can be made; the charge given to one fide of 
the 
* Page 50. +. Page. 489, 
