Since we have in this Experiment a full Proof of 
the brisk Agitation and Effervefcencc which arifes 
from the Mixture of frefh Air with Air that is im- 
pregnated with fulphureous Vapours, which arife 
from feveral mineral Subftanccs, cfpecially from the 
‘Pyrites, which abounds in many Parts of the Earth ; 
may we not with good Reafon conclude, that the 
irkfome Heat, which we feel in what is called a 
dole fultry Temperature of the Air, is occalioned 
by the inteftine Motion between the Air and the 
fulphureous Vapours, which are exhaled from the 
Earth? Which EfFervcfcence ceafes, as foon as the 
Vapours are equably and uniformly mixed in the 
Air; as happens alio in the Effervefcences and Fer- 
ments of other Liquors. The common Obfervation 
therefore, that Lightning cools the Air, feems to be 
founded on good Reafon s that being the utmoft 
and laft Effort of this EfFervcfcence. 
May we not hence alfo, with good Probability, 
conclude, that the firft Kindling of Lightning is ef- 
fected by the fudden Mixture of the pure ferene 
Air above the Clouds, with the fulphureous Va- 
pours, which are fometimes raifed in Plenty, im- 
mediately below the Clouds? The mod dreadful 
Thunders being ufually when the Air is very black 
with Clouds ; it rarely thundering without Clouds : 
Clouds ferving, in this Cafe, like the above-men* 
tion’d inverted Glaffes, as a Partition between the 
pure and fulphureous Airs: Which mud therefore, 
upon their fudden Admixture thro 1 the Interfaces 
of’ the Clouds, make (like the two Airs in the Glafs) 
a more violent EfFervcfcence, than if thofe Airs had, 
without the Intervention of the Clouds, more gra- 
dually 
