288 APPENDIX. 
Heat which we feel in what is called a clofe ful- 
try Temperature of Air, is occafioned by the 
inteftine Motion, between the Air and the ful- 
phureous mineral Vapours which are exhaled 
from the Earth > Which EfFervefcence ceafes 
as foon as thefe Vapours are equably and uni- 
formly mixed with the Air, as happens alfo in 
the EfFervefcences of ocher Liquors : Thus it 
is found that all other Fluids and even Metals 
in Fufion do equally mix and diffufe their feveral 
component Parts. The common Obfervation 
therefore that Lightening cools the Air feems 
to be founded on good Reafon, that being the 
utmoft and laft Effort of this EfFervefcence. 
12. May we not alfo with fome Probability 
of Reafon hence conjedure, that the firfl 
ignition or kindling of Lightening is elFefted 
by the fudden Mixture of the pure ferene Air 
above the Clouds, with the fulphureous Va- 
pours which are fometimes raifed in Plenty, 
immediately below them ; the moft dreadful 
Thunders being ufually when the Air is very 
'black with Clouds, it rarely thundering with- 
"out Clouds? The Clouds ferving in this Cafe, 
like the inverted Glafs ^ z, Fig, 34. as a Parti- 
tion between the pure and fulphureous Airs, 
which mufl therefore, upon their fudden Ad- 
mixture, 
