Analyfis of the Air. 
That a Fire which is fupplied with a hot 
air will not bum fo briskly as a Fire which 
is fed by a cool air is evident from hence; 
that when the Sun fhines on a Fire, and there- 
by too much ratifies the ambient air, that 
Fire will not burn well, nor will a fmall 
Fire burn fo well near a large one as at 
fotne diftance from it. And e contra , it is a 
common obfcrvation, that in very cold frofty 
weather. Fires burn moil briskly 5 the reafon 
of which feems to be this, that the elaftick 
expanlion of the cold condenfed air to a 
ratified ftate, when it enters the Fire, is much 
brisker than that of an air already rarified 
in a good meafure by heat, before it enters 
the Fire ; and consequently a continued fuc- 
ceffion of cold air muft give a brisker motion 
to the Fire, than the like fucceffion of hot 
air : And fuch colder and more condenfed 
air will alfo (as Sir Ifaac Newton obferves 
qu. 11.) by its greater weight check the 
afcent of the vapours and exhalations of the 
Fire, more than a warmer lighter air. So 
that between the a&ion and re-a£tion of the 
air and fulphur of the fuel, and of the colder 
and dcnfer circumambient air, which rarifies 
much upon entring the Fire, the heat of 
the Fire is greatly increafed. 
This 
