Mephitico,” also published in 1772, mentioned a few of its properties without giving it any name. As Dr. Priestley’s papers were read before the Royal Society so early as in March, it is not improbable that he was the first discoverer of the gas in question. In 1774 appeared the first of three volumes, entitled “Experiments and Observations on different kinds of Air;” and these were followed by three more, entitled “Experiments and Observations relating to various Branches of Natural Philosophy, with a continuation of the Observations on Air:” the last of these was published in 1786. This work contains a series of experiments, unrivalled for their number, novelty, and importance. Dr. Priestley’s greatest discovery, that of oxygen gas, which he called dephlogisticated air, was made on the 1st of August, 1774, and announced in the Philosophical Transactions for 1775. This gas he first procured from red oxide of mercury, and afterwards from red oxide of lead, and several other substances. In 1776 Dr. Priestley’s Observations on Respiration were read before the Royal Society. In these he showed that atmospheric air, during inspiration, was diminished in quantity, and deteriorated in quality, by the action of the blood upon it through the blood-vessels of the lungs. He also proved that gases have the power of acting through bladders, and one of his latest papers was on this curious subject: it appeared in the fifth volume of the American Philosophical Transactions, and seems to have been completely overlooked by later experimenters on the same subject. Another of his early and important observations related to the permanent mixture of gases of different densities, in cases in which they do not combine; and he cited this circumstance to account for the perfect mixture of the two gases which form the atmosphere, and which are well known to be of different densities. In addition to oxygen gas, already mentioned, Dr. Priestley also discovered muriatic acid gas, sulphurous acid gas, fluoric acid gas, nitrous oxide gas, ammoniacal gas, and carbonic oxide gas; but he entirely mistook the nature of the last-mentioned body. He also showed that muriatic acid gas and ammoniacal gas, when mixed, condense into solid sal ammoniac. He must also have obtained chlorine gas, but it escaped his notice, because, being received over mercury, it quickly combined with it. Hydrogen gas and carbonic acid gas were known before his time; but his experiments upon them greatly extended our acquaintance with their properties. Nitrous gas, barely discovered by Dr. Hales, was first investigated by Priestley, and applied by him to eudiometry, a most important branch of chemical science originating with himself.