“I had no sooner produced this new kind of air but I was eager to see the effect it would have on _water_, and to produce the stony crust formed by their union, as described by Mr Scheele; and I was not disappointed in my expectations. The moment the water came into contact with this air the surface of it became white and opaque by a _stony film_.... Few philosophical experiments exhibit a more pleasing appearance than this, which can only be made by first producing the air confined by quicksilver, and then admitting a large body of water to it. Most persons to whom I have shown the experiment have been exceedingly struck with it.... The union of this acid air and water may also be exhibited in another manner, which to some persons makes a still more striking experiment, _viz._, by admitting the air, as fast as it is generated, to a large body of water resting on quicksilver.... It is, then, very pleasing to observe that the moment any bubble of air, after passing through the quicksilver, reaches the water, it is instantly, as it were, converted into a stone; but continuing hollow for a short space of time, generally rises to the top of the water.... I have met with few persons who are soon weary of looking at it; and some could sit by it almost a whole hour, and be agreeably amused all the time.” Priestley’s attempts to explain the real nature of the _fluor acid air_ were, as may be expected, not very happy. “These appearances I explain by supposing that the vitriolic acid, in uniting with the spar, is in part volatilised by means of some phlogiston contained in it, so as to form a vitriolic acid air; and there is also combined with this air a portion of the solid earthy part of the spar, which continues in a state of solution till, coming into contact with the water, the fluid unites with the acid, and the earth is precipitated.” The third volume of the work was published in the early part of 1777, with a dedication to Lord Stanhope. It opens, as usual, with the characteristically discursive preface, extending to thirty pages, in which the author apologises for the character of much in the volume. He is constrained to admit that numerous as his _facts_ are, “few of them will appear so brilliant in the eye of the _general scholar_” as in either of the two former volumes, although he trusts they will “be thought no less valuable by philosophers and chemists.” Priestley, it would seem, was conscious that he was beginning, as the phrase goes, “to write himself out.”