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, containing only common air, I force a quantity of nitrous air at random, out of a bladder, and instantly clap my finger on the mouth of the bottle. I then immerse the neck of it into water, a small quantity of which I suffer to enter, which squirts into it with violence; and immediately replacing my finger, remove the phial. The water contained in it is already _very_ acid, and it becomes more and more so (if a sufficient quantity of nitrous air was at first thrown in) on alternately stopping the mouth of the phial, and opening it, as often as fresh air will enter.