they would have appeared very improbable to me had I been told of them; and when the decifive facts did at length obtrude themselves upon my notice, it was very flowly, and with great hesitation, that I yielded to the evidence of my fenfes. And yet, when I re-consider the matter, and compare my last discoveries relating to the conftitution of the atmosphere with the firft, I fee the clofeft and the eafieft connexion in the world between them, fo as to wonder that I fhould not have been led immediately from the one to the other. That this was not the cafe, I attribute to the force of prejudice, which, unknown to ourselves, biaffes not only our judgments, properly fo called, but even the perceptions of our senses: for we may take a maxim so strongly for granted, that the plainest evidence of sense will not intirely change, and often hardly modify our perfuafions; and the more ingenious a man is, the more effectually he is entangled in his errors; his ingenuity only helping him to deceive himself, by evading the force of truth.

There are, I believe, very few maxims in philofophy that have laid firmer hold upon the mind, than that air, meaning atmospherical air (free from various foreign matters, which were always fuppofed to be diffolved, and intermixed with it) is a fimple elementary fubftance, indestruc

indeftructible, and unalterable, at least as much fo as water is fuppofed to be. In the courfe of my inquiries, I was, however, foon fatisfied that atmofpherical air is not an unalterable thing; for that the phlogifton with which it becomes loaded from bodies burning in it, and animals breathing it, and various other chemical proceffes, fo far alters and depraves it, as to render it altogether unfit for inflammation, refpiration, and other purposes to which it is fubfervient; and I had difco;* vered that agitation in water, the process of vegetation, and probably other natural proceffes, by taking out the fuperfluous phlogifton, reftore it to its original purity. But I own I had no idea of the poffibility of going any farther in this way, and thereby procuring air purer than the best common air. I might, indeed, have naturally, imagined that fuch would be air that should contain lefs phlogiston than the air of the atmosphere; but I had no idea that fuch a compofition was poffible.

It will be seen in my last publication, that, from the experiments which I made on the marine acid air, I was led to conclude, that common air confifted of fome acid (and I naturally inclined to the acid that I was then operating upon) and phlogifton; because the union of this acid vapour and phlogiston made

inflammable

inflammable air; and inflammable air, by agitation in water, ceases to be inflammable, and becomes refpirable. And though I could never make it quite fo good as common air, I thought it very probable that vegetation, in more favourable circumstances than any in which I could apply it, or fome other natural process, might render it more pure.

Upon this, which no person can say was an improbable fuppofition, was founded my conjecture, of volcanos having given birth to the atmosphere of this planet, fupplying it with a permanent air, firft inflammable, then deprived of its inflammability by agitation in water, and farther purified by vegetation.

Several of the known phenomena of the nitrous acid might have led me to think, that this was more proper for the conftitution of the atmosphere than the marine acid: but my thoughts had got into a different train, and nothing but a series of obfervations, which I shall now distinctly relate, compelled me to adopt another hypothefis, and brought me, in a way of which I had then no idea, to the folution of the great problem, which my reader will perceive I have had in view ever fince my discovery that the atmospherical air is alterable, and therefore that it is not an ele

mentary

mentary fubftance, but a compofition viz. what this compofition is, or what is the thing that we breathe, and how is it to be made from its constituent principles.

At the time of my former publication, I was not poffeffed of a burning lens of any confiderable force; and for want of one, I could not poffibly make many of the experiments that I had projected, and which, in theory, appeared very promising. I had, indeed, a mirror of force fufficient for my purpose. But the nature of this instrument is fuch, that it cannot be applied, with effect, except upon substances that are capable of being fufpended, or resting on a very flender support. It cannot be directed at all upon any substance in the form of powder, nor hardly upon any thing that requires to be put into a veffel of quickfilver; which appears to me to be the most accurate method of extracting air from a great variety of substances, as was explained in the Introduction to this volume. But having afterwards procured a lens of twelve inches diameter, and twenty inches focal diftance, I proceeded with great alacrity to examine, by the help of it, what kind of air a great variety of fubftances, natural and factitious, would yield, putting them into the veffels represented fig. a, which I filled with quickD filver,

filver, and kept inverted in a bason of the fame. Mr. Warltire, a good chymift, and lecturer in natural philofophy, happening to be at that time in Calne, I explained my views to him, and was furnished by him with many fubftances, which I could not otherwife have procured.

With this apparatus, after a variety of other experiments, an account of which will be found in its proper place, on the 1st of Auguft, 1774, I endeavoured to extract air from mercurius calcinatus per fe; and I prefently found that, by means of this lens, air was expelled from it very readily. Having got about three or four times as much as the bulk of my materials, I admitted water to it, and found that it was not imbibed by it. But what furprized me more than I can well exprefs, was, that a candle burned in this air with a remarkably vigorous flame, very much like that enlarged flame with which a candle. burns in nitrous air, exposed to iron or liver of fulphur; but as I had got nothing like this remarkable appearance from any kind of air befides this particular modification of nitrous air, and I knew no nitrous acid was ufed in the preparation of mercurius calcinatus, I was utterly at a lofs how to account for it.

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