the light from the green conferva in water, but was doubtful as to the nature of the conferva until the following winter, when, with the help of William Bewley [q. v.] and others, he found it to be vegetable, and then extended his researches to other plants, but did not publish them till 1781. Meanwhile John Ingenhousz [q. v.] had published the main facts in 1779. Priestley accused him of plagiarism in 1800, after exonerating him from all suspicion in 1787 (Doctrine of Phlogiston established, pp. 80 sq). Priestley showed that the oxygen given off is due to the presence of gas in the water, and, also with the help of Bewley (Experiments on Natural Philosophy, i. 335 sq.), and in opposition to Ingenhousz, that the ‘seeds’ (spores) of the conferva come from the air, or pre-exist in the water (ib. ii. 17, 33), and are not spontaneously generated. He made numerous minor experiments of varying value on the effect of gases on plants.
In 1781 he decomposed ammonia by means of the electric spark; the experiments were interpreted later by Berthollet. In the same year Priestley, continuing with John Warltire of Birmingham certain observations of the latter on the burning of hydrogen in 1777, made experiments which led to the synthesis of nitric acid and water by Cavendish, and the interpretation of Cavendish's experiments by Lavoisier. Priestley and Warltire noticed that when hydrogen and air or oxygen are exploded, by means of an electric spark, a dew is formed; and Priestley had previously shown that when a spark is passed in air an acid is formed (Experiments … on … Air, i. 183 sq.). Cavendish repeated the experiments quantitatively in the summer of 1781, and told Priestley verbally of the formation of water without loss of weight when hydrogen and oxygen are exploded. Priestley in 1783, before Cavendish's paper was published, repeated the information to James Watt, who suggested to him that water was not an element, but a compound of dephlogisticated air and phlogiston. Hence arose a controversy on the relative claims of Watt and Cavendish with regard to priority, which Priestley might have settled, but did not. The repetition of Cavendish's experiments on a large scale in France, and Lavoisier's experiments on the action of steam on iron, made him waver for a moment in his adherence to the old theory. He had, in 1783, made the important discovery that ‘calces’ are reduced to the metallic state by heating in hydrogen, but failed to notice the water formed. In 1785, however, he made an admirable series of quantitative experiments on the oxidation of iron and the reduction of the oxide by hydrogen, with formation of water; but, in spite of this, under the influence of Watt (Phil. Trans. 1785, pp. 279–89), he finally rejected the Lavoisierian doctrine. He concluded later that water was already contained in all gases, and that the acid formed in the Cavendish experiments was the essential product of what he viewed as the ‘decomposition of dephlogisticated and inflammable air.’ In 1786 he published a series of experiments on ‘various kinds of inflammable air,’ under which name he included hydrogen, carbon monoxide, and various inflammable vapours; though he was aware that these had distinct properties, he often confused them. In the same year he published a further statement of his general theoretical views (Experiments on Natural Philosophy, iii. 400). In the condensed edition of his works, published in 1790, he described interesting experiments on the thermal conductibility of gases, which he found to be much the greatest in the case of hydrogen. In 1793 he published his ‘Experiments on the Generation of Air from Water,’ with a dedication to the Lunar Society, in which he explains the reasons for his rupture with the Royal Society, and with a reprint of the only paper contributed to their ‘Philosophical Transactions’ and not included in his own works—the ‘Experiments relating to the Decomposition of Inflammable and Dephlogisticated Air’ (Phil. Trans. 1791, p. 213).
In 1796 Priestley published his ‘Considerations on … Phlogiston.’ This, addressed to ‘the surviving answerers of Mr. Kirwan,’ was promptly replied to by Pierre Auguste Adet, the eminent chemist, then French ambassador to the United States. Priestley rejoined in a second edition of his work, to which Berthollet and Fourcroy replied (Annales de Chimie, vol. xxvi.). The controversy, which relates chiefly to the composition of water, and to the existence of oxygen in ‘finery cinder’ (magnetic oxide of iron), on which the new theories partly depended, was continued, mainly in America.
In 1798, evidently through forgetfulness (Med. Repository, ii. 254, v. 264), Priestley published, as if they were new, experiments on the combustion of the diamond, well known through numerous researches of Cadet, Lavoisier, and others, at least fifteen years previously. Priestley's objections to the explanation of certain experiments on the action of charcoal on steam and on metallic oxides (a stumbling-block to him since 1785) were well founded. They led William Cruickshank to discover that Priestley and his opponents alike had failed to recognise the existence of carbonic oxide as a distinct