A. G.

Priestley's Scientific Work.—It is as a man of science, and chiefly as a chemist, the 'discoverer' of oxygen, that Priestley is most generally remembered ; and except for certain references to religion in the prefaces to his 'Experiments ... on ... Air,' his scientific work has little connection with his other occupations. His fuller interest in science dates from 1758, when he bought a few scientific books, a small air-pump, an electric machine, and other instruments, with the help of which he made experiments for his pupils at Nantwich, as well as for his own amusement and that of his friends (Phil. Trans. 1770, p. 192). The delight in pretty experiments finds constant expression throughout his work. Although his preference for science over literature appears, in 1761, in his 'English Grammar' (p. 62), and in the introduction to the 'Chart on Biography,' Priestley seems to have been long prevented by an unusual diffidence from attacking the subject on his own account. This diffidence was removed during his visit to London in January 1766, when he met Richard Price (1723-1791) [q. v.], Sir William Watson, M.D. [q.v.], John Canton [q.v.], and Benjamin Franklin (1706-1790). Franklin encouraged him to undertake the 'History of Electricity,' which Priestley intended as part of a general history of experimental philosophy. The book drew him 'into a large field of original experiments,' and on the strength of these he was elected F.R.S. on 12 June 1766, on the proposition of Watson, Franklin, Canton, and Price. With the last three men he maintained a scientific correspondence till death. Franklin and Canton corrected the proofs of the 'History,' which was printed in 1767, within twelve months of its inception. Priestley's electrical work is mostly sound, and much of it brilliant ; it shows him at his best, although the discoveries contained therein are of less importance in the history of science than his later discoveries in chemistry. The 'History of Electricity' supplies an excellent account of previous work both treated historically and summarised systematically, and his own reflections and experiments described in a 'simple, exact, and artless style' borrowed, as he admits, from Stephen Gray [q.v.]; the style contrasts with the excessive fluency of much of his purely literary work. In the second part Priestley enounces his views on scientific method (Hist, of Electricity, 3rd edit. ii. preface), which he derived from Locke and possibly in part from Condillac. The object of science is 'to comprehend things clearly, and to comprise as much knowledge as possible in the smallest compass ; 'hypotheses are useful only in order to ascertain facts, and must not be valued for their own sake. At this time Priestley, adhering to his principles, and showing a critical power that was not equally conspicuous in his later work, declined to adopt either of the two contending fluid theories, and suggested to Canton on 12 Nov. 1767 (quoted in Chemical News, 14 May 1869) that electrification may be only a modification of the body electrified ; but he afterwards identified 'the electric matter' with phlogiston (Experiments . . . on . . . Air, i. 186). In his 'History' he anticipated Henry Cavendish [q.v.] and Charles Augustin de Coulomb in the important suggestion that the law of electric attraction is that of the inverse square, deducing this from an experiment suggested by Franklin. He found that an electrified body is discharged by the proximity of flame, that charcoal, blacklead, and red-hot glass are conductors ; and satisfactorily explained the formation of rings (since known as