tation in water, the process of vegetation, and probably other natural processes, by taking out the superfluous phlogiston, restore it to its original purity. But I own I had no idea of the possibility of going any farther in this way, and thereby procuring air purer than the best common air. I might, indeed, have naturally imagined that such would be the air that should contain less phlogiston than the air of the atmosphere; but I had no idea that such a composition was possible.
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 consisted of some acid (and I naturally inclined to the acid that I was then operating upon) and phlogiston; because the union of this acid vapor and phlogiston made inflammable air, and inflammable air, by agitation in water, ceases to be inflammable and becomes respirable. And though I could never make it quite so good as common air, I thought it very probable that vegetation, in more favorable circumstances than any in which I could apply it, or some other natural process, might render it more pure.
Upon this, which no person can say was an improbable supposition, was founded my conjecture of volcanoes having given birth to the atmosphere of this planet, supplying it with a permanent air, first 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 constitution of the atmosphere than the marine acid; but my thoughts had got into a different train, and nothing but a series of observations, which I shall now distinctly relate, compelled me to adopt another hypothesis, and brought me, in a way of which I had then no idea, to the solution of the great problem, which my reader will perceive I had had in view ever since my discovery that the atmospherical air is alterable, and, therefore, that it is not an elementary substance, but a composition, viz., what this composition is, or what is the thing that we breathe, and how it is to be made from its constituent principles.
At the time of my former publication I was not possessed of a burning lens of any considerable force; and for want of one I could not possibly make many of the experiments that I had projected, and which, in theory, appeared very promising. I had, indeed, a mirror of force sufficient for my purpose. But the nature of this instrument is such that it cannot be applied, with effect, except upon substances that are capable of being suspended or resting on a very slender support. It cannot be directed at all upon any substance in the form of powder, nor hardly upon anything that requires to be put into a vessel of quicksilver; which a appears to me to be the most accurate method of extracting air from a great variety of substances, as was explained in the introduction
