Eleven years later, on the anniversary of the fall of the Bastile in Paris, a fanatical mob, knowing Priestley's sympathies with the French revolutionists, attacked his house and chapel, burning both and destroying a great number of valuable papers and scientific instruments. Priestley and his family escaped violence by flight, but his most cherished possessions were destroyed; and three years later he quitted England forever, removing to the United States, whose struggle for liberty he had championed. The last ten years of his life were spent at Northumberland, Pennsylvania, where he continued his scientific researches. Early in his scientific career Priestley began investigations upon the "fixed air" of Dr. Black, and, oddly enough, he was stimulated to this by the same thing that had influenced Black--that is, his residence in the immediate neighborhood of a brewery. It was during the course of a series of experiments on this and other gases that he made his greatest discovery, that of oxygen, or "dephlogisticated air," as he called it. The story of this important discovery is probably best told in Priestley's own words: "There are, I believe, very few maxims in philosophy that have laid firmer hold upon the mind than that air, meaning atmospheric air, is a simple elementary substance, indestructible and unalterable, at least as much so as water is supposed to be. In the course of my inquiries I was, however, soon satisfied that atmospheric air is not an unalterable thing; for that, according to my first hypothesis, the phlogiston with which it becomes loaded from bodies burning in it, and the animals breathing it, and various other chemical processes, so far alters and depraves it as to render it altogether unfit for inflammation, respiration, and other purposes to which it is subservient; and I had discovered that agitation in the water, the process of vegetation, and probably other natural processes, restore it to its original purity....