[ ] 
nments were not repeated, as I wifh they might be 
done, in pure fixed air, extracted from chalk by 
means of oil of vitriol. 
For every purpofe, in which it was necefiary that 
the fixed air fhould be as unmixed as pofiible, I 
generally made it by pouring oil of vitriol upon chalk 
and water, catching it in a bladder, fattened to the 
neck of the phial, in which they were contained, 
taking care to prefs out all the common air, and alfo 
the firft, and fometimes the fecond, produce of fixed 
air 3 and alio, by agitation, making it as quickly as 
I poffibly could. At other times, I made it pafs 
from the phial in which it was generated through a 
glafs tube, without the intervention of any bladder, 
which, as I found by experience, will not long make 
a fufficient feparation between feveral kinds of air and 
common air. 
I had once thought that the readied: method of 
procuring fixed air, and' in fufficient purity, would 
be by the fimple procefs of burning chalk, or 
pounded lime-ftone in a gun-barrel, making it pafs 
through the ttem of a tobacco-pipe, or a glafs tube 
carefully luted to the orifice of it j and in this man- 
ner I find that air is produced in great plenty ; but, 
upon examining it, I found, to my very great furprize, 
that little more than one half of it was fixed air, 
capable of being abforbed by water j and that the 
rett was inflammable, fometimes very weakly, but 
fometimes pretty highly fo. Whence this inflam- 
mability proceeds, I am not able to determine, the 
lime or chalk not being fuppofed to contain any 
other than fixed air. I conjecture, however, that it 
mutt proceed from the iron, and the feparation of it 
from 
