148 Dr. Priestley’s Experiments and Ob/ervations on 
in the above-mentioned procefs was no proper condituent part 
of the air, but only fuch as had been diffufed through it, and 
in home manner attached to it, and kept fu (pended in it, and 
therefore might be fcparated from it, without decompofing the 
air; 011 refuming thefe experiments, I ufed every precaution I 
could think of to detach all water from the air on which I 
operated. I11 order to this, I kept it confined by mercury, 
together with a quantity of fixed ammoniac , which imbibes 
water more readily, if not in greater quantity, than quick- 
lime, or any other known fubdance. 
In this more accurate method of making the experiment, I 
was gradually led to difeover the acid, which had efcaped my 
obfervation before. But I am not certain that I fhould have 
found it even now, if I had not been aided by the fagacity of 
Mr. Keir, who was always of opinion, that fome acid mufi 
be the produce of this experiment, or rather that the produce 
would be fomething which would become acid by expofure to 
the open air. 
I began with making the explofions in the fame glafs veffel 
from which the mixture of air had difplaced the mercury with 
which it had been filled ; when I found, as I have obferved in 
my lad publication, the whole of the veffel was filled with a 
denfe fmoke, which fettled into a black coating of all the 
in fide of the veflel, and which appeared, as before, to be mer- 
cury ; becoming white by expofure to the air. For fome time 
I perceived no appearance of water : but placing the vefiel at a 
proper didance from a fire, I found about a quarter of a grain 
colle&ed on the oppofite fide ; when, as the veffel contained 
four ounce meafures of air, the water produced ought to have 
been at lead a grain. 
The 
