[ *59 ] 
from the calx may be promoted by that fmall quan- 
tity of oil of vitriol, which I am informed is con- 
tained in chalk, i not in lime-ftone alfo. But it is 
an objection to this hypothecs, that the inflammable 
air produced in this manner burns blue, and not at 
all like that which is produced from iron, or any other 
metal, by means of an acid. It has alfo the fmeil 
of that kind of inflammable air which is produced 
from vegetable fubftances. Befides, oil of vitriol 
without water, will not diffolve iron ; nor can inflam- 
mable air be got from it, unlefs the acid be confl- 
derably diluted ; and when I mixed brimflone with 
the chalk, neither the quality nor the quantity of the 
air was changed by it. Indeed no air, or permanently 
elaftic vapour, can be got from brimflone, or any 
oil. 
In the method in which I generally made the 
fixed air, and indeed always, unlefs the contrary be 
particularly mentioned, viz. by diluted oil of vitriol 
and chalk, I found by experiment that it was as pure 
as Mr. Cavendiih made it. For after it had pafled 
through a large body of water , in fmall bubbles, ftill 
or part only was not abforbed by water. In 
order to try this as expeditioufly as poffible, I kept 
pouring the air from one glafs veflel into another;, 
immerled in a quantity of cold water, in which 
manner I found by experience, that almoft any 
quantity may be reduced as far as pofiible in little 
more than a quarter of an hour. 
At the fame time that I was trying the purity of 
my fixed air, I had the curiofity to endeavour to 
nfcertain whether that part of it which is not mif- 
fcible in water, be equally difl'ufed through the whole 
mafs 3 
