Joseph Louis Gay-Lussac, a French chemist, born at St. Leonard, Limousin,- Dec. 6, 1778, died in Paris, May 9, 1850. He was educated at the polytechnic school of Paris, then called Lecole centrale des travaux publics, where he attracted the notice of Berthollet, and was employed by him for a short time in the laboratory of the government chemical works at Arcueil. He then returned to the polytechnic school as assistant professor. Observations made on balloon ascensions having led to the supposition that the magnetic force diminishes at great elevations above the surface of the earth, Gay-Lussac and Biot were commissioned by the institute to make experiments with reference to it. Two ascensions were made, the first, Aug. 23, 1804, by both, and the second, Sept. 15, by Gay-Lussac alone. In the latter he reached the extraordinary height of about 23,000 feet. These ascensions were the first made for exclusively scientific purposes. Their results were not conclusive, but interesting observations were made upon the decrement of temperature with the increase of elevation, and upon the uniformity in the composition of the atmosphere at all heights.

On Oct. 1 of the same year Alexander von Humboldt and Gay-Lussac submitted to the French academy a joint paper upon the combination of gases, especially of oxygen and hydrogen, which attracted much attention and marks an era in the progress of chemical science. In 1805-'6 Gay-Lussac prosecuted in company with Humboldt scientific inquiries in France, Switzerland, Germany, and Italy. They were present at Vesuvius when there was an eruption and an earthquake. Their observations on terrestrial magnetism were published in the Memoires de la societe dArcueil. Gay-Lussac began in 1807 to investigate the expansion of the air and gases under increased temperatures, and established the law that when free from moisture they all dilate uniformly and to equal amounts for all equal increments of temperature, at least between zero and 100° C. He also showed that the gases combine in simple proportions of their volumes, and that the contraction sometimes experienced when several of them are compounded is always an exact simple fraction, usually one half, one third, or one quarter of their joint bulk.

Sir Humphry Davy having shown by means of the voltaic pile that potassium and sodium are not simple substances, as had previously been supposed, and having decomposed them by the same means, Gay-Lussac and Thenard obtained potassium and sodium in greater proportions even than they had been obtained by Davy himself. They also developed the compound character of boracic and fluoric acids, introduced new methods of analyzing organic substances by their combination with chlorate of potash, and elucidated the composition of many of these compounds. The results of their investigation were given in Recherches physico-chimiques sur la pile, sur les alcools, etc. (2 vols. 8vo, 1811). Gay-Lussac afterward made original researches of great value into the newly discovered elementary substances of iodine and cyanogen, into Prussian blue, chloric and hydrosulphuric acids, the theory of vapors, capillary attraction, and other subjects. Their results were published in the Annales de chimie et de physique, which he edited with Arago, and in other publications. In 1816 he invented the siphon barometer, since modified by Bunsen, by whose name it is best known.

He also invented instruments for estimating the quantities of alcohol, chlorine, and alkali present in solutions, known severally as the alcoholometer, chlorometer, and alkalimeter. In 1832 he gave up the professorship at the Sorbonne, to which he had been appointed in 1809, and accepted that of general chemistry at the jardin des plantes. As an expounder of science he was distinguished for the clearness of his explanations. In 1831 he was chosen by the electors of his native town member of the chamber of deputies; and in 1839 he was. made a peer of France.