a clergyman and a chemist, and in 1768 he went to Leeds, and was settled as pastor over a large congregation. He happened to reside near a brewery, and "accidentally observed that the beer, during its fermentation in the vats, gave forth a remarkable aerial substance. The flame of a lighted stick immersed in it was at once extinguished, and the smoke floating on the top of the stratum showed that it was very heavy, a result which was perfectly confirmed by the observation that, invisible and intangible as it was, it could be poured from vessel to vessel like water; and in the vats, in which it originally occurred, it would overflow their edges, and descend to the floor, along which it would run like a stream, its course being readily tracked by the expedient of putting a lighted stick into it, and observing the extinction of the flame. Moreover, he found that it would dissolve in water, for, if dishes of that liquid were placed where it had access, an agreeably acidulous and sparkling fluid, soda-water, was formed; and, that the agent which brought all these results about possessed a physiological potency, was proved by the fatal fact, too often known in such manufactories, that, if by accident it was breathed, death at once took place."
This substance was then called "fixed air," and is familiarly known as carbonic-acid gas. It is now exactly a hundred years since Priestley published a pamphlet "On impregnating Water with Fixed Air," and a year later he received the Copley medal from the Royal Society for his "Observations on the Different Kinds of Air." In the year 1774, he made the splendid discovery of oxygen, and, in allusion to its power as the sustainer of life, he applied to it the epithet "vital air." When it is remembered that this wonderful substance is the active element of the atmosphere, and essential to the existence and activity of the entire living world; that it enters largely into the composition of all the natural objects around us, forming three-fourths the weight of all living things, half the weight of the rocky strata, and eight-ninths of the oceans; and, moreover, that it is an element of great chemical energy, and is involved in nearly every transformation of matter in the laboratory of Nature, and in the processes of the arts, we shall be prepared to comprehend the significance of its discovery. It has given us a new chemistry and a new physiology, and it probably carries the mind of man deeper into the order of Nature than any other single scientific revelation ever made.
But the great discoverer had his troubles. He carried his independence and power of thought into theology and politics, and his life of course became a turbulent battle with sects and parties. In relation to this part of Priestley's career, Dr. J. W. Draper has well remarked: We must not impute it to mental weakness, but rather to a pursuit of the truth, that in succession he passed through many phases of religious belief, and four different sects, the Presbyterian, Arminian, Arian, and Unitarian, received him as a votary. This is not the occasion nor the place to explain the causes which led him to this course. It is only for us to judge of so great a man with charity. But, imbued as he was with a deep religious sentiment, and feeling that even the most exalted objects of this life are not to be compared with the importance of another world, he regarded his philosophical pursuits as a very secondary affair, and gave much of his time and talent to controversial theology. He seems to have come to the conclusion that it was incumbent on him to make a religious war. As his biographer says, 'Atheists, Deists, Jews, Arians, Quakers, Methodists, Calvinists, Catholics, Episcopalians, had alike to combat him.' In more than a hundred volumes which he printed, each of these found an adversary of such force and 24
