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GAY and visited Africa in 1828. He devoted himself to the study of Arabic MSS., and on his return was attached to the foreign office as interpreter. Having married an English lady, whom he met at Algiers, he resided for some time in England, and distinguished himself by contributions on subjects connected with Arabic literature to the Penny Cyclopædia, the Edinburgh Review, and the Biographical Dictionary of the Useful Knowledge Society. A valuable article on Arabic MSS. in the Westminster for 1834, is from his pen. He is also the author of a "History of the Mohammedan Dynasties of Spain," translated from the Arabic chronicles, and enriched with most valuable notes, London, 1843. Gayangos also contributed to the illustrated work of Gowry and Owen Jones on the Alhambra (in the Crystal Palace), a "Historical Notice of the Kings of Granada." In his own language he has also written a preliminary discourse and descriptive catalogue of the books of chivalry in Ariban's collection. While in England he afforded substantial aid, which has been gracefully acknowledged, to the historians Prescott and Ticknor, and subsequently, in conjunction with Signer Vedia, he published a translation into Spanish of Ticknor's History of Spanish Literature, with notes and appendices of considerable value. In 1843 he was recalled to Spain, and became professor of Arabic in the university of Madrid, an office which, we believe, he still holds. He is also a member of the Academy of History, and editor of its memoirs.—F. M. W.  GAY-LUSSAC,, was born December 6, 1778, at St. Léonard, in the department of Haute-Vienne, where his father was procureur-royal. His father, Antoine Gay, called himself Gay-Lussac from an estate belonging to him in the neighbourhood of St. Léonard, in order to distinguish himself from other persons bearing the same name. He at first was anxious that his son should devote himself to the study of the classics, but this was prevented by the outbreak of the French revolution. In 1795 Gay-Lussac went to Paris to prepare himself for the examination for admission into the école polytechnique; and on December 27th, 1797, having passed his examination successfully, he entered that institution, where he remained to the end of 1800. He then entered as pupil (élève ingénieur) in the école des Ponts-et-Chaussées. Berthollet, who as professor of chemistry at the école polytechnique, had become acquainted with Gay-Lussac, and had made several experiments with him, now appointed him demonstrator to his class, and shortly afterwards employed him at his estate at Arcueil, where he was working at his Statique Chimique. Gay-Lussac now devoted himself entirely to the pursuit of physics and chemistry, which had lately received a fresh impulse from the labours of Dalton (1801). In 1805 he became a member of the consulting committee of arts and manufactures; and in 1808 he married. In 1809 he was elected professor of chemistry at the école polytechnique, and also obtained the chair of physics at the Sorbonne, which he filled from 1808 to 1832, He was chosen professor of general chemistry at the jardin des plantes in 1832. In 1829 he became a member of the council for improving gunpowder and saltpetre, and was attached to the mint. In 1831 he was chosen deputy for the first time, and was afterwards several times re-elected; and in 1839 was created a peer. He died of atrophy of the heart. May 9, 1850, at Paris. Gay-Lussac at first principally occupied himself with physical investigation, and afterwards more especially with chemistry, while the latter part of his life was chiefly devoted to practical applications of science and to politics. He edited the Annales de Chimie el de Physique, conjointly with Arago. His first work on the expansion of gases and vapours, was published in 1801 (Ann. Chim. xl. 137), and was followed by papers on the tension of vapours, evaporation, &c. He next turned his attention to the investigation and perfection of thermometric, barometric, and hygrometric instruments, and to capillary attraction. In 1804 he applied to M. Chaptal, minister of the interior, for aid in a balloon ascent which he contemplated. A balloon was supplied at the expense of the government; and accompanied by Biot, Gay-Lussac ascended to the height of 4000 metres (13,122 feet). Their object was to ascertain whether the terrestrial magnetism ceased out of contact with the earth. They found that it suffered no perceptible diminution, and that the atmospheric electricity increased as they rose, and was always negative. In the same year Gay-Lussac undertook a second ascent, and reached the hitherto-unattained height of 7000 metres (nearly 4½ miles); confirming his previous observations, and making new ones on the regular decrease of pressure, temperature, and moisture in the air. He also ascertained that the air has the same composition at the greatest height as at the surface of the earth. The results of his research on the co-efficient of expansion of the permanent gases, though more accurate than those of Dalton, were not perfectly correct, but from the improved method of determination introduced, were of the highest importance. In 1804 he was chosen a member of the Society of Arcueil, which had been formed by La Place and Berthollet, and consisted of only nine members. Between 1804 and 1805 he made some eudiometric researches with Humboldt. He discovered that 100 volumes of oxygen combine with as nearly as possible 200 volumes of hydrogen to form water; and this led him to the conclusion, that the relation was 1 : 2, and that a simple law analogous to that discovered by Dalton for combination by weight also governed combination by volume. In 1808 he ventured to publish his results, laying down the law that "when two gases combine, their volumes bear a simple numerical relation to one another, and the volume of their common product in the gaseous form likewise bears a simple relation to the sum of volumes of the gases which have entered into the combination." In 1809 he published a paper on the density of various vapours, giving for their determination a new method which is still in use; and in 1816, a paper on the expansion of liquids. In his research on iodine he gave the first example of the employment of the law of combination by volume, so as to conclude by induction the vapour-density of bodies in cases where this is unknown, or the body cannot actually be converted into vapour. In this manner he calculated the vapour-density of iodine, and his results have since been confirmed by direct experiment. He then determined the vapour-density of carbon from the composition of carbonic acid, supposing that one volume of this gas is composed of one volume carbon vapour and one volume oxygen, without condensation. He also showed, that when two gases unite in various proportions, the volume of the first remaining constant, the volume of the second is a simple multiple of the first, as is the case with the oxides of nitrogen; and also, that the volume of the products of such combinations, in the gaseous state, bears a simple relation to the sum of the volumes of the constituents, being either equal to their sum, or one half or one third of the sum, &c. In 1811 he published in conjunction with Thénard, with whom a great many of his researches were made, "Recherches physico-chimiques sur la pile," a large battery having been constructed for the purpose by the Emperor Napoleon. With Thénard he succeeded in preparing sodium and potassium in considerable quantities by reducing their oxides with red-hot iron. They were then enabled to try the action of these metals on various other bodies, and made various important discoveries, reducing boron from boracic acid, &c. They also occupied themselves with the constitution of hydrochloric acid, but were led into error by their adherence to the theory of Lavoisier, that all acids contained oxygen. Davy in the meanwhile discovered the element chlorine; they found chloride of phosphorus, and discovered that a mixture of chlorine and hydrogen explodes in sunshine. In 1813 Gay-Lussac read a short paper on the new substance lately discovered by Courtois in kelp, and described it as a simple substance under the name of iodine. He showed its analogy to chlorine, and prepared hydriodic acid corresponding to hydrochloric acid. He made various researches on the compounds of sulphur, and investigated hyposulphuric acid; he also discovered peroxide of barium. In 1824 he discovered and investigated fulminic acid, in conjunction with Liebig. Gay-Lussac also made numerous discoveries in organic chemistry, and first prepared pure liquid hydrocyanic (prussic) acid. In 1815 he published his celebrated work on cyanogen (azotase de carbone), and showed that prussic acid is the hydracid of cyanogen, which may be regarded as a compound radical playing the part of a simple body. With Thénard he greatly improved the method of organic analysis, carrying out the idea of burning the body with oxygen (first from chlorate of potash, for where oxide of copper was afterwards substituted), and estimating the carbonic acid and water produced. He also made special researches on fermentation, etherification, racemic acid, and alcohol. Gay-Lussac is the author of practical methods of analyzing gunpowder; of chlorimetry, and alkalimetry; of the silver test in the wet way; and he wrote on the manufacture of sulphuric acid, glass, and chloride of lime.—C. E. L. 