ive state of a liquid. I am beholden for this idea, you will
perceive, to your own very curious discovery of the true nature of Mr.
Cavendish's _marine_ vapour.

When I first repeated your experiment of impregnating water with nitrous
air, the water, I must own tasted acid; as it did in one, or perhaps two
trials afterwards; but, to my great astonishment, in all the following
experiments, though some part of the factitious air, or vapour, was
visibly absorbed by the water, I could not perceive the latter to have
acquired any sensible acidity. I at length found, however, that I could
render this same water _very_ acid, by means only of the nitrous air
already included in the phial with it. Taking the inverted phial out of
the water, I remove my finger from the mouth of it, to admit a little
of the common air, and instantly replace my finger. The redness,
effervescence, and diminution take place. Again taking off my finger,
and instantly replacing it, more common, air rushes in, and the same
phenomena recur. The process sometimes requires to be seven or eight
times repeated, before the whole of the nitrous _vapour_ (as I shall
venture to call it) is condensed into nitrous _acid_, by the successive
entrance of fresh parcels of common air after each effervescence; and
the water becomes evidently more and more acid after every such fresh
admission of the external air, which at length ceases to enter, when the
whole of the vapour has been condensed. No agitation of the water is
requisite, except a gentle motion, just sufficient to rince the sides of
the phial, in order to wash off the condensed vapour.

The acidity which you (and I likewise, at first) observed in the water
agitated with nitrous air _alone_, I account for thus. On bringing the
phial to the mouth, the common air meeting with the nitrous vapour in
the neck of the phial, condenses it, and impregnates the water with the
acid, in the very act of receiving it upon the tongue. On stopping the
mouth of the phial with my tongue for a short time and afterwards
withdrawing it a very little, to suffer the common air to rush past it
into the phial, the sensation of acidity has been sometimes intolerable:
but taking a large gulph of the water at the same time, it has been
found very slightly acid.--The following is one of the methods by which
I have given water a very strong acid impregnation, by means of a
mixture of nitrous and common air.

Into a small phial, containin