2 , 6 Dr. Priestley’s Experiments relating to the 
thing more than one part of dephlogifticated air, in a copper 
vefl'el which holds thirty-feven ounces of water ; and a little 
more rauft have have remained in the vefl'el, which I could 
not get out by draining or fhaking it. It is moft evident, there- 
fore, that the acid neceffary to diflolve fo much copper muft 
have come from the union of the dephlogifticated and inflam- 
mable air, becaufe there was nothing elfe in the vefl'el. The 
inflammable air was procured from iron by means of fteam. 
This very pure dephlogifticated air I firft imagined could only 
be got by the procefs in which I obferved (Experiments on Air, 
Vol ll.p. 170.) that I once before procured it, though I then fup- 
pofed the extraordinary refult to be accidental ; becaufe in other 
circumftances I have fometimes had it very pure when I could 
not fucceed in a fecond attempt of the fame kind. It was by 
heating the yellow produd of the folution of mercury in fpirit 
of nitre, without fufifering the red precipitate into which it is 
converted by heat to come into contaft with the external 
air, from which I thought it probable that it might attrad 
fome phlogifton. Afterwards, however, I found that this 
circumftance makes no difference whatever; and that the air fo 
procured appeared to be purer, arofe from the greater purity of 
the nitrous air which I made ufe of as a teft, and which I got 
from mercury, and not from copper, the nitrous air from 
whichIfindto.be much lefs pure. For trying the dephlo- 
gifticated air yielded by fome red precipitate which had been 
prepared many months by the nitrous air from mercury, it 
appeared to be as pure as that which was procured in the man- 
ner above defcribed. 
That the dephlogifticated air which I now made ufe of was 
fufficiently pure for my purpofe, appeared from mixing one 
meafure of it with two of nitrous air, when the whole quan- 
, t[t y 
