appointment to a professorship of mathematics in the Royal Military College at Marlow, in Buckinghamshire (afterwards removed to Sandhurst), with which establishment he remained until his retirement from public service. This was the most active period of his life, for while fulfilling assiduously the duties of his professorship he continued unremittingly his scientific studies. His earliest writings were three memoirs, which he communicated in the years 1796, 1799, and 1802, to the Royal Society of Edinburgh. The first of these was entitled, 'A New Series for the Rectification of the Ellipse;' the second, 'A New Method of Resolving Cubic Equations;' and the third, 'A New and Universal Solution of Kepler's Problem;' all of them evincing great analytical skill, as well as originality of thought. Mr. Ivory contributed fifteen papers to 'The Transactions of the Royal Society of London,' nearly all of them relating to physical astronomy, and every one containing mathematical investigations of the most refined nature. The first, published in the 'Transactions of 1809,' and entitled, 'On the Attractions of Homogeneous Ellipsoids,' is his most celebrated paper, in which he completely and definitely resolved the problem of attraction for every class of ellipsoidal bodies. Many of Ivory's remaining contributions, ranging through a period of nearly thirty years, related to the subject of the attraction of spheroids and the theory of the figure of the Earth, and some of them are considered masterpieces of anylitical skill. One of the last subjects which occupied his attention was the possible equilibrium of a spheroid with three unequal axes when revolving about one of the axes, a fact which Jacobi had discovered. This Ivory demonstrates in the volume for 1838 of the 'Philosophical Transactions.' The volumes in 1823 and 1838, contain Ivory's two papers on the Theory of Atmospheric Refraction,' a subject which, next to the Theory of Attractions, engaged most seriously his attention on account of its great importance in astronomy and the curious mathematical difficulties which its investigation presents. For each of these papers he was awarded the Royal medal by the Society. Of all his contributions to the Transactions,' only one is purely mathematical; this is contained in the volume for 1831, and is entitled, 'On the Theory of Elliptic Transcendants.' Besides these contributions to the Royal Society, Ivory wrote several papers in the Philosophical Magazine of 1821-27; in Maseres's Scriptores Logarithmici;' in Leybourne's 'Mathematical Repository; and in the Supplement to the sixth edition of the Encyclopædia Britannica. In the beginning of 1819 Ivory, finding that his health began to decline under the great exertions which he made in carrying on his scientific researches, and performing his duties as professor, resigned his professorship at Sandhurst and retired into private life. In consideration, however, of his great merit, the pension due for the full period of service required by the regulations was granted to him, although that period had not been completed.

After his retirement, Ivory devoted himself entirely to his scientific researches, living in or near London until his death. In 1814 he had received the Copley medal for his communications to the Royal Society; in 1815 he became a Fellow of the same society. He was also an honorary fellow of the Royal Society of Edinburgh; an honorary member of the Royal Irish Academy and of the Cambridge Philosophical Society; a corresponding member of the Institute of France, of the Royal Academy of Sciences at Berlin, and of the Royal Society of Göttingen.

In the year 1831, in consideration of the great talent displayed in his investigations, Ivory was recommended by Lord Brougham, whom he had known in early life, to the notice of the King (Wm. IV.), who, with the Hanoverian Guelphic Order of Knighthood, gave him an annual pension of 300%., which he enjoyed during the rest of his life; and in 1839 he received the degree of Doctor in Laws from the University of St. Andrews.

Mr. Ivory attained the age of seventy-seven before his death; he was essentially a self-taught mathematician, and spent most of his leisure in retirement. He fathomed in private the profoundest writings of the most learned continental mathematicians, and at a period when few Englishmen were able to understand those difficult works; he even added to their value by many original contributions, and must always be remembered with special interest when the singular destitution of higher mathematical talent, which had reigned in this country for so long a period before his time, is considered.English Cyclopædia. London, 1856.- Encyclopædia Britannica. Eighth Edition.

JOSEPH PRIESTLY, LL.D.

Born March 24, 1773. Died February 26, 1804.

Joseph Priestly was the son of a cloth-dresser at Burstal-Fieldhead, near Leeds. His family appear to have been in humble circumstances, and he was taken off their hands after the death of his mother by his paternal aunt, who sent him to a free school at Batley. There he learnt something of Greek, Latin, and a little Hebrew. To this he added some knowledge of other Eastern languages connected with Biblical literature; he made a considerable progress in Syriac and Chaldean, and began to learn Arabic; he also had a little instruction in mathematics, but in this science he did not make much proficiency. Indeed his whole education was exceedingly imperfect, and, excepting in Hebrew and Greek, he

never afterwards improved it by any systematic course of study, Even in chemistry, the science which he best knew, and in which he made so important a figure, he was only half-taught, so that he presents one of the memorable examples of knowledge pursued, science cultivated, and even its bounds extended, by those whose circumstances made their exertions a continued struggle against difficulties which only genius like theirs could have overcome. After studying for some years at the Dissenting Academy founded by Mr. Coward at Daventry (afterwards transferred to London), Priestly quitted Daventry and became minister of a congregation at Needham Market, in Suffolk, where his salary never exceeded thirty pounds. He had been brought up in the strictest Calvinistic principles, but he very soon abandoned these, and his tenets continued in after life to be those of the moderate Unitarians, whose leading doctrine is the proper humanity of Christ, and who confine all adoration to one Supreme Being. Priestly's religious opinions proving distasteful to his congregation at Needham Market, caused him to remove in 1758 to Nantwich, in Cheshire, where he obtained a considerable number of pupils, which greatly increased his income and enabled him by strict frugality to purchase a scanty scientific apparatus, and commence a study of natural philosophy. In 1761, Priestly removed to Warrington, where he was chosen to succeed Dr. Aitken as tutor in the belles lettres at that academy. On settling at Warrington he married the daughter of Mr. Wilkinson, an ironmaster in Wales, by whom he had several children. His literary career may be said to have commenced here, and having once begun to publish, his appeals to the press were incessant and on almost every subject. The universality and originality of his pursuits may be judged from his delivering at Warrington a course of lectures on anatomy, while his published works during the next seven or eight years comprise:-The Theory of Language and Universal Grammar,' 1762; On Oratory and Criticism,' 1777; 'On History and General Policy,' 1788; 'On the Laws and Constitution of England,' 1772; On Education,' 1765; Chart of Biography,' 1765; 'Chart of History,' 1769. During the same period appeared, in 1767, his work entitled, 'A History of Electricity,' &c., which was so well received that it went through five editions. This was followed in 1772 by a 'History of Vision.' In 1767, on account of a dispute with the Warrington trustees, Priestly removed to Leeds, where he became minister of the Mill-Hill Chapel, and wrote many controversial books and pamphlets. In after times he wrote 'Letters to a Philosophical Institution; An Answer to Gibbon;''Disquisitions on Matter and Spirit;' Corruptions of Christianity;' Early Opinions on Christ;' Familiar Letters to the Inhabitants of Birmingham;' 'Two different Histories of the Christian Church;' 'On Education;' Comparison of Heathen and Christian Philosophy; Doctrine of Necessity;' 'On the Roman Catholic Claims;' On the French Re

volution ;' 'On the American War;' besides twenty volumes of tracts in favour of Dissenters and their Rights. His general works fill twenty-five volumes, of which only five or six are on scientific subjects; his publications being in all 141, of which only seventeen are scientific. When residing at Leeds Priestly's house immediately adjoined a brewery, which led him to make experiments upon the fixed air copiously produced during the process of fermentation. These experiments resulted in his discovering the important fact that atmospheric air, after having been corrupted by the respiration of animals, and by the burning of inflammable bodies, is restored to salubrity by the vegetation of plants; and that, if the air is exposed to a mixture of sulphur and iron-filings, its bulk is diminished between a fourth and a fifth, and the residue is both lighter than common air and unfit to support life; this residue he termed 'phlogistic air,' afterwards called azotic or nitrogen gas. For these experiments the Copley medal was awarded to him in 1773 by the Royal Society. The following year to this, from experiments with nimium or red lead, Priestly made his great and important discovery of oxygen gas. This was followed by his discovering the gases of muriatic, sulphuric, and fluoric acids, ammonial gas and nitrous oxide gas. He also discovered the combination which nitrous gas forms suddenly with oxygen; diminishing the volume of both in proportion to that combination; and he thus invented the method of eudiometry or the ascertainment of the relative purity of different kinds of atmospheric air.

*

In considering the great merits of Priestly as an experimentalist, it must not be forgotten that he had almost to create the apparatus by which his processes were to be performed. He for the most part had to construct his instruments with his own hands, or to make unskilful workmen form them under his own immediate direction. His apparatus, however, and his contrivances for collecting, keeping, transferring gaseous bodies, and for exposing substances to their action, were simple and effectual, and they continue to be still used by chemical philosophers without any material improvement. Although Priestly was the first to discover oxygen, and thus give the basis of the true theory of combustion, he clung all his life with a wonderful pertinacity to the Phlogistic Theory, and nothing in after life would make him give it up. In 1773 Priestly accepted an invitation from Lord Shelbourne (afterwards first Marquis of Lansdowne), to fill the place of librarian and philosophic companion, with a salary of 2507., reducible to 150%. for life should he quit the employment; 401. a-year was also allowed him for the expense of

* Discovered at the same time by Dr. Rutherford of Edinburgh.

†The Phlogistic Theory explained the phenomena of combustion by supposing the existence of a hypothetical substance termed Phlogiston, the union of which with bodies made them combustible, and the disengagement of which was the occasion of combustion.

apparatus and experiments, and homes were provided for his family in the neighbourhood both of Lord Shelbourne's town and country residence. Priestly remained with the Earl of Shelbourne for six or seven years, at the end of which period, in 1780, he settled at Birmingham and became minister of a dissenting body there. While residing at Birmingham he engaged fiercely in polemical writings and discussions, particularly with Gibbon and Bishop Horseley. He also displayed a warm interest in the cause of America at the time of the quarrel with the mother-country, and likewise took an active and not very temperate part in the controversy to which the French Revolution gave rise; and, having published a 'Reply' to Burke's famous pamphlet, he was in 1791 made a citizen of the French Republic. This gave considerable offence to the inhabitants of Birmingham, an ironical and somewhat bitter pamphlet against the high church party still further excited their feelings against him; and a dinner which was given on the 14th of July, to celebrate the anniversary of the attack upon the Bastile, became the signal for a general riot. The tavern where the party were assembled was attacked, and, although Dr. Priestly was not present, his house and chapel were immediately afterwards assailed, he and his family escaped, but his house, library, and manuscripts were burnt. AIthough his losses were made up to him partially by an action at law and partially by a subscription among his friends, Priestly felt that he could no longer live at Birmingham, he therefore removed to London and succeeded his friend Dr. Price as principal of the Hackney Academy. He, however, still found himself highly unpopular and shunned even by his former associates in silence. This determined Priestly to leave England, and in the spring of 1794 he withdrew with his family to America and settled at Northumberland, in Pensylvania, where he purchased 300 acres of land. Here he remained the rest of his life, occupied in cultivating his land, in occasional preaching, and in scientific studies. He continued writing and publishing until his death, in February 1804, in the 72nd year of his age. He expired very quietly, and so easily that having put his hand to his face those who were sitting close to him did not immediately perceive his death.—Brougham's Lives of Philosophers. London and Glasgow, 1855. - Encyclopædia Britannica. Eighth Edition.