78 THEORY or 
«¢ the furface of the earth, and that it buries itfelf there*.” 
But, with deference to the judgment of that unwearied 
friend to fcience, I apprehend that fuch an infulation is 
hardly confiftent with that diftribution of conductors, 
efpecially of water, which provident nature has made 
through all parts of our globe; the higheft mountains be- 
ing furnithed with imternal fprings and fountains, and 
watered externally by rivulets, which derive their origin 
from condenfing mifts or melting fnows upon their fum- 
mits: While the furface of the earth in general, not ex- 
cepting the moft fandy deferts, affords fupplies of water 
to thofe who will be at the pains of digging for it. If then 
the vapors which conftitute the cloud are, of themfelves, 
incapable of furnifhing fuch-quantities of electric matter as 
are neceflary for the repeated difcharges in a fevere thun- 
der ftorm, as fignior Beccaria thinks they are, and as feems 
to me indubitable; and if the infulations of large portions 
of the furface or exterior parts of the earth, which are ab- 
folutely neceffary to fupport Beccaria’s hypothefis, cannot 
take place ; which, how they can in our terraqueous mafs, 
is dificult to conceive, confiftently with the Aztherto dif- 
covered properties of the electric fluid: We mutt feek for 
fome other fubftance in nature which may be capable of 
affording thofe reiterated fupplies, of that powerful element 
which are ufually exhibited in a thunder ftorm. This I 
prefume, we fhall find in the atmofphere over our heads; 
not in the vapors which float therein, but in the azr itfelf 
which fuftains them. 
Air is by eleCtricians juftly clafled with e/eé?ric fubftan- 
ces, as it poflefles the fame general properties in common 
with others of that denomination, particular inftances of 
which may occur in the following pages ; wherein I fhall 
endeavour to prove, 
I. That the electric capacity of air is leffened by con- 
denfation. 
Il, That ¢hzs capacity is increafed by heat. 
Premifing 
* Prieftley, page 335. 
