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 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.
In 1778, he pursued his experiments on the property of vegetables growing in the light, to renovate impure air, and on the use of vegetation in this part of the economy of nature. Chemistry is also indebted to him for the method of decomposing metallic oxides by means of hydrogen gas, and for noticing that this gas has the property of dissolving iron. He observed also that lime is less soluble in hot than cold water; and that when a solution of lime in cold water is heated, part of the lime is deposited.
In the first volume of his work on air (p. 278), Dr. Priestley has anticipated the idea of Dr. Arnott and Sir J. F. W. Herschel, that electricity, acting on the brain and nerves, may excite muscular action.
Dr. Henry, in the memoir already quoted, has remarked, that facts are to be met with in various parts of Dr. Priestley’s works that might have given him a hint of the law, since unfolded by the sagacity of M. Gay-Lussac, “that gaseous substances combine in definite volumes.” From the same memoir we extract the following observations, in conclusion of this short account of Dr. Priestley’s scientific labours:—“He greatly enlarged our knowledge of the important class of metals, and traced out many of their most interesting relations to oxygen and to acids. He unfolded, and illustrated by simple and beautiful experiments, distinct views of combustion; of the respiration of animals, both of the inferior and higher classes; of the changes produced in organized bodies by putrefaction, and of the causes that accelerate or retard that process; of the importance of azote as the characteristic ingredient of animal substances, observable by the action of dilute nitric acid on muscle and tendon; of the functions and economy of living vegetables; and of the relations and subserviency which exist between the animal and vegetable kingdoms.”
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ARIOSTO.
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