17 ^ Arialyfis of the Air, 
celTion of cold air muft give a brisker motion 
to the Fire, than the like fucceffion of hot . 
air ; And fiich colder and more condenfed . 
air will alfo (as Sir Ifaac Newton ohktwcSy 
qu. II.) by its greater weight check the 
afeent of the vapours and exhalations of the 
Fire, more than a warmer lighter air. So 
that between the aclion and re-adion of the | 
air and fulphur of the fuel, and of the colder ' 
and denfer cirumambient air, which ratifies ! 
much upon entering the Fire, the heat of \ 
the Fire is greatly increafed. 
This continual fupply of frefii air to the ! 
fuel feems hence alfo very neceflary for 
keeping a Fire alive ; becaufe it is found, that 
a Brimjlone Match will not take Fire in 
avacuumy but only boil and fmoakj nor will ' 
Nitre incorporated into Brown "Baper then 
detonizc, except here and there a Angle grain; 
that part only of the Baper turning black f, 
on which the focus of the burning glafs fallsi | 
nor would they burn when a half exhaufted | ■ 
receiver with fumes in it was filled with j 
frefh air added to thofe fumes: In which ■ , 
cafe it is plain, that a good quantity of the !<,; 
fuppofed vivifying fpirit of air muft enter :i. 
the receiver with the frefh air, and confe- | . 
quently *1 
