Any. gua 
aL ‘Mr. Cavespisu’s Anfwer to 
There is an experiment mentioned by Mr. Ktrwaw which, © 
though it cannot be confidered as an argument in favour of the 
-generation of fixed air, as he only fuppofes, without any proof, 
that fixed air is produced in it, does yet deferve to be taken 
notice of as a curious experiment. Itis, that, if nitrous and 
‘common air be mixed over dr y quickfilver, the common air is 
not at all diminifhed, ‘that is, the bulk of the mixture will be 
not lefs than that ef the common air employed, until water is 
admitted, and the mixture agitated for a few minutes. .The 
veafon-of this in all probability is, that part of the phlogifts 
cated nitrous acid, into which the nitrous air is converted; re- 
‘mains in the ftate of vapour until condenfed by the addition of 
water. A proof that this is the real cafe is, that, inthis man- 
‘ner of performing the experiment, the red fumes produced on 
mixing the airs remain vifible fot fome hours, but immediately 
‘difappear on the addition of water and agitation. : 
The moft material experiment alledged by Mr. Kirwan is 
sone of Dr. PriEsTLEY’s, in which he obtained fixed air from 
a mixture of red precipitate and iron filings. This at firft 
feems really a, {trong argument in favour of the generation of 
‘fixed air; for though plumbago, which is known to confift 
‘chiefly of that fubftance, has lately been found to be contained ~ 
in iron, yet one would not have expected it to be decom= 
pounded by the red precipitate, efpecially when the quantity of 
pure iron in the filings was much more than fufficient to fupply 
the precipitate with phlogifton. The following experiment, 
however, fhews that it was really decompounded; and that’ 
the fixed air obtained was not generated, but only a by 
means of this decompofition. 
500 grains of red precipitate mixed with 1ooo of iron iia i 
yielded, by the affiftance of heat, 7800 grain meafures of fixed 
alr, 
