In the space to which we are restricted, it will be impossible to give an adequate idea of the vast importance of Dr. Priestley’s chemical discoveries: they are justly regarded as forming the basis of our knowledge of pneumatic chemistry, and indeed of the science in general; for upon one of them alone, that of oxygen gas, is founded our acquaintance with the nature of air, earth, and water, and the same discovery has served also to explain the action of fire. Dr. Priestley’s residence at Leeds was near a brewery; and his first pneumatic experiments were made on the carbonic acid gas, or _fixed air_, largely generated during fermentation. Gradually pursuing the subject, he examined various other aëriform bodies, and submitted to experiment numerous substances which were convertible into, or capable of yielding, air. These investigations led him to the discovery of new gaseous bodies, both elementary and compound. So little cultivated had been the field in which he commenced his researches, that he was under the necessity of imagining and constructing new instruments, in order to carry them on. To his inventive genius chemistry is indebted for the pneumatic trough, the method of receiving and retaining gases over mercury, and the process of combining and decomposing them by electricity. “The very implements,” Dr. Henry remarks, in his Estimate of the Philosophical Character of Dr. Priestley, “with which he was to work were, for the most part, to be invented; and of the merits of those which he did invent, it is a sufficient proof that they continue in use to this day, with no very important modification. All his contrivances for collecting, transferring, and preserving different kinds of air, and for submitting those airs to the action of solid and liquid substances, were exceedingly simple, beautiful, and effectual. They were chiefly, too, the work of his own hands, or were constructed under his directions by unskilled persons.” Dr. Priestley’s first publication on pneumatic chemistry appeared in 1772; it was called “Directions for impregnating Water with fixed Air,” &c. &c. In this work he proposed the use of a condensing engine for the purpose of causing the water to dissolve a larger quantity of the gas, and thus to prepare artificial mineral waters: this plan, it is well known, is now practised to a great extent. In the Philosophical Transactions for 1772, he announced the discovery that air, which had been vitiated by respiration or the burning of candles, was restored by the vegetation of plants; that air exposed to a mixture of sulphur and iron filings, as had previously been done by Hales, was diminished by about one-fourth or one-fifth in bulk, and that the residual air was lighter than atmospheric air, and noxious to animals. This diminished air he afterwards called phlogisticated air; it is now named azotic, or nitrogen gas. The discovery of this fluid is generally attributed to Dr. Rutherford, who, in his treatise “De Aëre