“For my own part, I will frankly acknowledge that at the commencement of the experiments recited in this section I was so far from having formed any hypothesis that led to the discoveries I made in pursuing them that they would have appeared very improbable to me had I been told of them; and when the decisive facts did at length obtrude themselves upon my notice it was very slowly, and with great hesitation, that I yielded to the evidence of my senses. And yet, when I reconsider the matter, and compare my last discoveries relating to the constitution of the atmosphere with the first, I see the closest and the easiest connection in the world between them, so as to wonder that I should not have been led immediately from the one to the other. That this was not the case I attribute to the force of prejudice which, unknown to ourselves, biases not only our _judgments_, properly so 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 entirely change, and often hardly modify, our persuasions; 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.” He then points out that there are few maxims in philosophy that have laid firmer hold upon the mind than that air, meaning atmospherical air ... is a _simple elementary substance_, indestructible and unalterable, at least as much so as water was supposed to be. Priestley, in the course of his inquiries, was soon satisfied that atmospherical air was not an unalterable thing; that bodies burning in it, and animals breathing it and various other chemical processes, so far alter and deprive it as to render it altogether unfit for the purposes to which it is subservient; and he had discovered methods, particularly the process of vegetation, which tended to restore it to its original purity. “But,” he says, “I own I had no idea of the possibility of going any further in this way and thereby procuring air purer than the best common air.” As this paper is one of the classics of chemistry, as well as the chief corner-stone in the monument which Priestley erected to himself, it is necessary to examine it, as well as certain other papers which grew immediately out of it, in some degree of detail. After a reference to a hypothesis of the origin and constitution of the atmosphere which occurs among the “Queries, Speculations and Hints” above referred to, and which is on a par with much in Priestley’s speculations, he proceeds to relate the circumstances which more immediately led to the most important of all his discoveries. It was the accident of possessing a burning lens of “considerable force,” for want of which he could not possibly make many of the experiments that he had projected.