Acidity , De comp option of Water, and Phlogijlon. 3 1 9 
a quantity which, after giving pure air with a moderate heat, 
gives air that is more than half fixed air with a greater degree 
of heat. It is evident, as I have obferved before, that there 
are very different kinds of manganefe. 
In thefe Papers I have fuppofed, with M. Lavoisier and 
others, that the principle of acidity is in the dephlogiflicated 
air only ; but as the acid is always formed by the union of this 
air and the inflammable, it may, perhaps, with equal proba- 
bility be fuppofed to be in either of them, or to be a com- 
pound of them both. 
Mr. Watt defires me to mention it as his conjecture, that 
the nitrous acid is contained in the inflammable air as the acid 
of vitriol is in fulphur, the phofphoric in phofphorus, &c. ; 
and that the dephlogiflicated air does nothing more than deve- 
lope the acid. Mr. Keir, who was led to expeft that an acid 
mufl be the refult of the union of dephlogiflicated and inflam- 
mable air, becaufe fome acid is always the confequence of its 
union with other inflammable fubftances, thinks that both 
may be neceffary ingredients in it. Farther experiments may 
throw more light on the fubjedt. 
TO THE REV. DR. PRIESTLEY. 
DEAR SIR, 
I at length fubmit to your confideration the trials which 
I made upon the liquors you produced, by the firing of de- 
phlogifticated and h flammable air in clofe veflels ; and if you 
think 
