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his work to the 'Philosophical Transactions.' The first volume records the discoveries of alkaline air (ammonia gas) and dephlogisticated nitrous air (nitrous oxide), and the synthesis of sal-ammoniac, as well as (p. 258) his first general view of the then current hypothesis of Becher and Stahl that fire is a decomposition, in which phlogiston is separated from all burning bodies. Priestley adopted modifications of detail in this view under the compulsion of facts and the influence of Richard Kirwan [q. v.] and Cavendish. At various periods he identified phlogiston with electricity and with hydrogen (Phil. Trans. 1785, p. 280). But his whole scientific energies from this time forward were devoted to the upholding of the phlogistic theory, which his own experiments (and their completion by Cavendish) by a strange fate were destined, in the hands of Lavoisier, completely to overturn.

On 1 Aug. 1774, at Lansdowne House, Priestley obtained what was to him a new gas from mercurius calcinatus per se, in which a candle burnt vigorously, but he remained to support respiration, as well as combustion, better, and called it 'dephlogisticated air.' From its property of yielding acid compounds this gas was named oxygen by Lavoisier at a later date. As it both came from the atmosphere and could also be produced by heating certain metallic nitrates, Priestley concluded that the air is not an element, but 'consists of the nitrous [nitric] acid and earth, with so much phlogiston as is necessary to its elasticity ' (Experiments … on … Air, ii. 55), a mistaken opinion which he modified, but did not improve, in 1779 (Experiments and Observations on Natural Philosophy, L 192). Priestley's great discovery of oxygen contained the germ of the modern science of chemistry, but, owing to his blind faith in the phlogistic theory, the significance of the discovery was lost upon him.

Priestley made the first public announcement of his discovery of oxygen in a letter to Sir John Pringle, dated 15 March 1775, which was read to the Royal Society on 25 May. But while in Paris, in October 1774, Priestley, according to his own account, spoke of the experiments he had already performed, and of those he meant to perform, in relation to the new gas (Experiments … on … Air, Nov. 1775, ii. 320). Fifteen years later in the 1790 edition of 'Experiments on Air' (vol. ii. 108) Priestley declared specifically that he told Lavoisier of his experiments during this visit to Paris. There is no doubt that immediately after that date Lavoisier made oxygen for himself, and in the May following published the first of a long series of memoirs, in which he used his experiments to explain the constitution of the air, combustion and respiration, and to give an experimental interpretation of the Greek idea of the conservation of matter, thus founding chemistry on a new basis. Priestley refused to accept Lavoisier's sagacious views. The centenary of Priestley's discovery of oxygen was celebrated in Birmingham and in Northumberland, Pennsylvania, on 1 Aug. 1874, but there is some divergence of opinion as to who is entitled to the full credit of the original discovery. Although Priestley was 'in possession of 'the gas 'before November 1771' (Experiments on Natural Philosophy, i. 194), it is admitted that Karl Wilhelm Scheele, the great Swedish chemist, working quite independently, first recognised it as a distinct species 'before 1773' (Nordenskjöld and Thorpe), but Scheele did not publish his researches until after Priestley. Lavoisier's claim to subsequent but independent discovery, for which his own statement is the only evidence, offers greater difficulty. Lavoisier was possibly among the first chemists to whom Priestley's discovery was communicated before its public announcement. Priestley made no definite charge of plagiarism when Lavoisier published his memoir in May 1775. When, in 1790, Priestley first asserted that he had himself told Lavoisier of his discovery in October 1774, Lavoisier made no reply. Lavoisier died in 1794, and it was not until 1800, after twenty-five years had elapsed since the discovery, and memory was failing him, that Priestley made Lavoisier's pretensions a matter of complaint (Doctrine of Phlogiston established, 1800, p. 88).

In November 1774 Priestley discovered vitriolic acid air (sulphur dioxide), and before November 1775, continuing an investigation by Scheele (Kopp), fluor acid air (silicon tetrafluoride). This completes the list of Priestley's great discoveries of gases (nine in all), of which only three species had been recognised before he began his researches.

Priestley's memoir on respiration, read in January 1776 (Phil. Trans, p. 226), in which he regards respiration as 'a true phlogistic process,' was not original in idea, but was acknowledged by Lavoisier as the starting-point of his own work on the subject (Œuvres, ii. 174), published in the next year. In the spring of 1778 Priestley returned to the important researches on vegetable physiology of 1772, and discovered oxygen in the bladders of seaweed. In June and the following months he found that this gas is given off in