Priestley's rings) when a discharge takes place on a metallic surface. He showed great insight by pointing out the need for the measure of electric resistance, and proposed a method for measuring what is now called 'impedance,' which at the time was not distinguished from resistance (Phil. Trans. 1769, p. 63). In February 1770 (ib. 1770, p. 192) he investigated the 'lateral explosion' produced in the discharge of a Leyden jar, and showed that it is of an oscillatory nature, thus anticipating in part recent discoveries on this subject, especially those of Dr. Oliver Lodge (The Electrician, 1888, vol. xxi. pp. 234, 276, 302). In 1772 he corresponded with Volta at Como ; and received a commission from Leopold, grand duke of Tuscany (afterwards the Emperor Leopold II), for an electrical machine, which was made under his direction by Edward Nairne [q.v.]
But after 1770 Priestley practically abandoned the study of electricity for that of chemistry, to which he had been led incidentally. He had attended a course of chemical lectures given in Warrington Academy by Dr. Turner of Liverpool. But he admitted that he 'knew very little of chemistry at this time,' and even attributed his success to the ignorance which forced him to devise apparatus and processes of his own (Memoirs, i. 61). Much later he declared himself 'no professed chemist.' It was precisely to this ignorance of chemical history and practice that was due his lasting incapacity to analyse experiments thoroughly, and to push them to their logical conclusion. He began his chemical work by attacking the problem of combustion, the solution of which created the science of modern chemistry (Phil. Trans. 1770, p. 211). He was led to study gases by watching the process of fermentation in a brewery next to his house ; and in March 1772 he read his first paper, 'On different Kinds of Air.' It was inspired by the work of Stephen Hales [q.v.], of Joseph Black [q. v.], and of Cavendish.
Despite its many wrong conclusions, and its records of unsatisfactory experiments, this essay marked an epoch in the history of the science. In the first place, Priestley set forth improvements in the methods of collecting gases, and especially the use of mercury in the pneumatic trough, which enabled him to deal for the first time with gases soluble in water. He announced the discovery of marine acid air (hydrochloric acid) and nitrous air (nitric oxide), and showed the feasibility of substituting the latter for living mice as a means of measuring the goodness of air, a suggestion which led, in the hands of Fontana, Landriani, Cavendish, and others, to exact eudiometry. He showed that in air exposed over water, one-fifth disappears in processes of combustion, respiration, and putrefaction, and that plants restore air vitiated by these processes ; and that no known gas conducted electricity. The paper also contained a proposal to saturate water with carbonic acid under either atmospheric or increased pressure, which has led to the creation of the mineral-water industry. Of this means of making 'Pyrmont water' (which he described in a pamphlet in June 1777), he wrote : 'I can make better than you import, and what cost you five shillings will not cost me a penny. I might have turned quack' (Memoirs, i. 177). Certain experiments on this part of his work were made for Priestley by William Hey [q. v.] Priestley likewise described the preparation of pure nitrogen, a gas to which he gave the vague name of < phlogisticated air,' only recognising it later as a distinct species. Daniel Rutherford [q. v.] simultaneously and independently obtained a like result, which he first described in 'De Aere fixo' (p. 16), dated 12 Sept. 1772. In the same dissertation Priestley noted, without comment, that he had produced two other gases, which were subsequently recognised as new, and were designated respectively carbonic oxide and nitrous oxide, and that he had disengaged from nitre a gas which further examination would have proved to be identical with the as yet undiscovered oxygen. The paper was awarded the Copley medal of the Royal Society (30 Nov. 1773), and was at once abstracted at length by Lavoisier (Œuvres, i. 512, 621) and criticised by him. Henceforward Lavoisier acted as a sieve to separate the inaccurate work and conclusions of Priestley from the accurate.
There followed in 1772 Priestley's 'History of ... Light.' His knowledge of mathematics was insufficient to enable him to produce anything more than a clear but unoriginal narrative, and with its publication he abandoned his scheme of writing a general scientific history, owing to the financial failure of the work. He wrote to Canton (18 Nov. 1771), 'If I do work for nothing, it shall be on theological subjects.' In the 'History of Light' (pp. 390 sq.) be announced his adherence to Boscowich's theory of points of force (see supra). After 1772 Priestley decided, with the approbation of the president, Sir John Pringle, not to present his papers to the Royal Society, but to publish them separately, and from 1774 to 1786 he published six successive volumes of researches on air and kindred subjects (condensed into three volumes in 1790), occasionally contributing shorter accounts of