It has been necessary to enter at some length into the nature of the phlogistic theory, because the discoveries of the time were expressed in its language. The fire-air or vital air of Mayow was termed dephlogisticated air; i.e. air wholly deprived of the power of burning, or air more capable of supporting combustion than ordinary air; while airs capable of burning were supposed to be more or less highly charged with phlogiston; indeed, at one time, it was imagined that hydrogen was phlogiston itself.
It is to Joseph Black that the discovery of carbon dioxide, that constituent of air first to be definitely recognised, if we except Mayow’s early work, is generally ascribed. But we must remember that it had been prepared by Becher and by Hales, and had been doubtless obtained in an impure state by many others. It will be seen that Black’s work was so complete, and established the identity of this gas in so definite a manner, that his right to be named as its true discoverer can hardly be questioned.
Black was born near Bordeaux in 1728. His father, a wine-merchant, was originally a native of Belfast, being descended from a Scottish family which had been settled there for some time. When twelve years of age. Black returned to Belfast, and received his education in the local grammar-school, afterwards proceeding to the University of Glasgow in 1746, at the age of eighteen. He was a pupil of Dr. Cullen, then Lecturer on Chemistry at the College there, who is mentioned by Professor Thomas Thomson, in his History of Chemistry, as an excellent and instructive lecturer. Black intended to choose the career of medicine, and he indeed practised occasionally as a medical man during the greater part of his life.
JOSEPH BLACK.
He began his medical studies in Edinburgh in the year 1751, and in 1755 he published, as his thesis for the degree of M.D., the work which has rendered his name famous. It appears that as early as 1752 he had been occupied with investigations on quicklime, which was then attracting attention as a remedy for urinary calculi. Opinion was divided regarding its virtue. In a manuscript copy of notes of Black’s lectures, which the author is so fortunate as to possess, he mentions that his attention was directed to the subject through the rival views of Drs. Alston and Whytt. It was not long before he proved that, in opposition to the commonly received notion, quicklime had gained nothing from the fire in which it was made, but that the limestone used for its preparation had lost nearly half its weight in becoming caustic. He also attempted successfully to trap the escaping gas, and again placed it in presence of lime, confining it over water. Instead of any escape of material when the lime became mild, “nothing escapes—the cup rises considerably by absorbing air.” And in his notes, a few pages farther on, he compares the loss of weight undergone by limestone on being calcined, with its loss on being dissolved in muriatic acid. These experiments appear from his journal to have been made before November 1752.
His thesis was not published, however, until 1755. Immediately after, in 1756, he succeeded Dr. Cullen as Professor in Glasgow, where he remained until 1766. During these ten years he began and made great progress with his well-known researches on the heat of fusion of ice, and the heat of vaporisation of water, or, as he termed them, the “latent heats” of water and of steam. In 1766, Dr. Cullen was appointed Professor of Medicine in the University of Edinburgh, and Black again succeeded him as Professor of Chemistry. There he lectured until 1797, when he retired from public life; he died as peacefully as he had lived, in 1799, in the seventy-first year of his age. Thomson, who relates these particulars, was one of his last students; he writes:—“I never listened to any lectures with so much pleasure as to his; and it was the elegant simplicity of his manner, the perfect clearness of his statements, and the vast quantity of information which he contrived in this way to communicate, that delighted me.... His illustrations were just sufficient to answer completely the object in view, and no more.”
Black’s original thesis for his degree was entitled Experiments upon Magnesia Alba, Quicklime, and other Alcaline Substances. It was published in 1755, and several times reprinted. It is now to be had in a convenient form as one of the “Alembic Club Reprints.”
It was the custom in those days to administer alkalies as a remedy for urinary calculi; and about the year 1750 lime-water was tried as a substitute. Opinion was divided as regarded its efficacy; and it was with the view of preparing a better remedy that Black undertook researches on magnesia alba. Black prepared magnesia from “bittern,” which remains in the pans after the crystallisation of salt from sea-water, and also from Epsom salts, “which is evidently composed of magnesia and the vitriolic acid.” The magnesia is thrown down from the sulphate as carbonate, by the addition of pearl ashes, at the temperature of ebullition, the soluble product being “vitriolated tartar,” or potassium sulphate. He describes how “magnesia is quickly dissolved with effervescence or explosion of air, by the acids of vitriol, nitre, and of common salt, and by distilled vinegar,” and gives an account of the properties of the sulphate, nitrate, chloride, and acetate. He subsequently heated this magnesia, and found that it lost “a remarkable proportion of its weight in the fire,” and his “attempts were directed to the investigation of this volatile part.” The residue in the retort did not effervesce on the addition of acids; hence the volatile part had been driven away by the heat. “Chemists have often observed, in their distillations, that part of the body has vanished from their senses, notwithstanding the utmost care to retain it; and they have always found, upon further inquiry, that subtile part to be air, which, having been imprisoned in the body, under a solid form, was set free, and rendered fluid and elastic by the fire. We may safely conclude that the volatile matter lost in the calcination of magnesia is mostly air; and hence the calcined magnesia does not emit air, or make an effervescence when mixed with acids.”