THE DISCOVERY OF “DEPHLOGISTICATED AIR”
BY PRIESTLEY AND BY SCHEELE— THE OVERTHROW
OF THE PHLOGISTIC THEORY BY LAVOISIER

We have seen that Stephen Hales must have prepared oxygen, among the numerous gases and mixtures of gases which he extracted from various substances; for, among the many materials which he heated, one was red-lead. The red-lead of that day, however, must have contained carbonate, because, as we shall see, Priestley always obtained a mixture of oxygen and carbon dioxide from that source. In the account of his researches, Hales only incidentally mentions the collection of gas from minium; and he appears to have made no experiments with the object of ascertaining its properties.

The discovery of oxygen was made nearly simultaneously by Priestley and Scheele, though it appears from the recent publication of Scheele’s laboratory notes by Baron Nordenskjöld that Scheele had in reality anticipated Priestley by about two years. His researches, however, were not published until a year after Priestley had given to the world an account of his experiments. Priestley had no theory to defend; his experiments were undertaken in an almost haphazard manner, probably as a relaxation. “For my own part,” he says,[4] “I will frankly acknowledge that, at the commencement of the experiments recited in this section, I was so far from having formed any hypothesis that led to the discoveries made in pursuing them, that they would have appeared very improbable to me had I been told of them; and when the decisive facts did at length obtrude themselves upon my notice, it was very slowly, and with great hesitation, that I yielded to the evidence of my senses.” On the other hand, Scheele was engaged in forming a theory of the nature of fire. He writes:[5]— “I perceived the necessity of a knowledge of fire, because without this it is impossible to make any experiment; and without fire or heat, it is impossible to utilise the action of any solvent. I began, therefore, to dismiss from my mind all explanations of fire, and undertook a series of experiments in order to gain as full knowledge as possible of these lovely phenomena. I ere long found, however, that it was not possible to form any correct opinion concerning the appearances which fire exhibits, without a knowledge of the air. After a series of experiments, I saw that air really is concerned in the mixture termed fire, and that it is a constituent of flame and sparks. I learned, moreover, that such a treatise on fire as this could not be compiled with thoroughness without also taking air into consideration.”

Scheele’s views concerning fire need not be mentioned here; but his researches on air are so methodical and so complete as to command our entire admiration. They remind us of those of Mayow, and had the latter lived a little longer, they would not improbably have been carried out by him. Since, however, Priestley had the advantage of priority of publication, we shall commence with an account of his researches.

Joseph Priestley was born in 1733 at Fieldheads, about six miles from Leeds. His father, a maker and dresser of woollen cloth, lost his wife when his son Joseph was about six years of age; and being poor, his sister, Mrs. Keighley, offered to bring up the boy. The early associations of the lad were closely connected with dissent; and after some time spent at a public school in the neighbourhood, he was sent, in 1752, to the Academy at Daventry, in which he was trained for the ministry. There he gained some knowledge of mechanics and metaphysics, and also acquired some acquaintance with Chaldee, Syriac, and Arabic, besides being a competent French and German scholar. After leaving the Academy, he settled at Needham in Suffolk, as assistant in a small meeting-house, where his income was not over £30 a year. His views were, however, too liberal for his hearers; and after some years he moved to Nantwich in Cheshire, where he preached and also taught a school. Here his income was improved, though still miserably small; yet he managed to buy some books, a small air-pump, and an electrical machine. He subsequently removed to Warrington, being employed there in teaching and in literary work; among his writings was a History of Electricity, which first brought him into notice, and which procured for him the degree of LL.D. of Edinburgh, thus giving him a right to the title of Doctor, by which he was always afterwards known. At Warrington, too, he married. We next find him being asked in 1767 to take the pastorship of Millhill Chapel at Leeds, a call which he accepted. The chapel was next door to a brewery, and this circumstance first induced him to take up the subject of the chemistry of gases, which has made his name famous. Here too he published his History of Discoveries relative to Light and Colours. After six years spent at Leeds, he became librarian to the Earl of Shelburne (afterwards Marquis of Lansdowne) and travelled with him on the Continent. While with Lord Shelburne he published the first three volumes of Experiments on Air, and carried out investigations which were recorded in a fourth volume, published after his removal to Birmingham. After some years spent in this way, he was pensioned off, and settled as minister of a meeting-house in Birmingham, where he employed his time partly in theological controversy, and partly in prosecuting researches in chemistry. He published during this period another three volumes giving a description of his experiments on air, and communicated several papers to the Philosophical Transactions of the Royal Society, of which he had been made a Fellow. Towards the year 1790 he was so unfortunate as to attack Burke’s book on the French Revolution; and this had the effect of rousing popular opinion against him, more especially that of the local clergy, whose political views he had frequently opposed. During the riots which took place at Birmingham in 1791, his house was burned, and he was obliged to escape to London under an assumed name. After some years spent in the charge of a meeting-house at Hackney, he left England for America. His opinions, though by no means uncommon at the present day, were so antagonistic to those of his English contemporaries that he was cut by his Fellows of the Royal Society, and he therefore resigned his Fellowship. And this feeling was in no way lessened by the action of the French Government of the time, which made him a Citizen of the Republic, and even chose him as a member of their Legislative Assembly. Arriving in America in 1795, he was well received, and settled at Northumberland, not far from Philadelphia. There he died in 1804.

JOSEPH PRIESTLEY.

In Priestley’s work on gases he employed the form of apparatus which had been used by Mayow a century before. Such apparatus is indeed generally used now: the flasks with bent delivery-tubes, the Wolff’s bottles with two necks, and the pneumatic trough filled with water or mercury were his chief utensils. By means of such apparatus, gases can be collected in a state of comparative purity: they can be easily transferred from one vessel to another, and substances which it is desired to submit to their action can be readily introduced. Scheele, on the other hand, employed less convenient methods: his gases were generally collected in bladders, and their transference to bottles must have been attended with the introduction of atmospheric air. Scheele’s method was to allow a certain amount of gas to escape from the generating flask in order to expel air; an empty bladder was then tied over the neck, and the gas entered the bladder. When he wished to transfer the gas to a bottle, the bladder was tied at some distance from the neck, and its loose open end was secured by a string round the neck of a bottle full of water. The string confining the gas was then untied, and the bottle was inverted; the water ran into the bladder and was replaced by gas. A cork was also enclosed in the bladder, and it was possible to push this cork into the neck of the bottle and re-tie the string which confined the gas; and then, by loosing the string which secured the bottle to the bladder, the full bottle could be conveyed away. This process is obviously a clumsy one, although in Scheele’s hands it yielded splendid results; and the methods which Priestley had borrowed from Mayow have attested their superiority by their survival.