i *4$ ] 
or a foot in depth, within which any kind of fub^ 
ftance may be very conveniently placed j and though 
it mud be continually mixing with the common air* 
and is far from being perfectly pure, yet there is a 
conftant fupply from the fermenting liquor, and it is 
pure enough for many purpofes. 
A perfon, who is quite a Arranger to the properties 
of this kind of air, would be agreeably amufed with 
extinguifhing lighted candles, or chips of wood in it, 
as it lies upon the furface of the fermenting liquor j 
for the fmoke readily unites with this kind of air, 
probably by means of the water which it contains ; 
fo that very little or none of the fmoke will efcape 
into the open air, which is incumbent upon it. It 
is remarkable, that the upper furface of this fmoke, 
floating in the fixed air, is fmooth, and well defined ; 
whereas the lower furface is exceedingly ragged, fe- 
veral parts hanging down to a confiderable diflance 
within the body of the fixed air, and fometimes in 
the form of balls, connected to the upper ftratum by 
flender threads, as if they were fufpended. The 
fmoke is alfo apt to form itfelf into broad flakes, 
parallel to the furface of the liquor, and at different 
diftances from it, exadtly like clouds. Thefe ap- 
pearances will fometimes continue above an hour, 
with very little variation. When this fixed air is 
very ftrong, the fmoke of a fmall quantity of gun- 
powder fired in it will be wholly retained by it, no 
part efcaping into the common air. 
Making an agitation in this air, the furface of it, 
which ftill continues to be exactly defined, is thrown 
into the form of waves, which it is very amufing to 
look upon ; and if, by this agitation, any of the fixed 
