Joseph Louis Gay-Lussac (6.12.1778-9.05.1850) was a French physicist and chemist and a member of the Paris Academy of Sciences (1806). He was born in Saint-Léonard. He graduated from the École Polytechnique (1800), where he worked from 1802 (professor of chemistry from 1809), and at the same time, in 1808 —32, he was professor of physics at the University of Paris. From 1832 he was professor of chemistry at the Paris Botanical Garden.
His work in physics was in the field of molecular physics and heat. He studied the expansion and the properties of gases. In 1802, independently of J. Dalton, he discovered one of the gas laws (Gay-Lussac's law), proved that the coefficient of expansion of all gases is the same and calculated it (0.00375 deg-1). In 1804 he twice made flights in a balloon (to a height of 7 km), during which he carried out a number of scientific investigations, in particular on the temperature and the humidity of the air. In 1807 he established that the temperature of air falls when it expands and rises when it is compressed (without heat exchange), and found that the heat capacity of a gas does not depend on its volume. In 1808 he established empirically that all simple gases combine with one another in simple multiple ratios (the law of combining volumes). In 1811 he developed a method of determining the density of a vapour, and studied (1816) vapour pressure. He was an adherent of the corpuscular theory of light. In chemistry he developed a method of obtaining potassium and sodium (together with L. Thénard), obtained a number of chemical compounds, in particular hydrocyanic acid and cyanogen, and laid the foundations of volumetric analysis. He invented a number of instruments (a hydrometer, an alcoholometer, a barometer, thermometers and a pump).
From 1815, together with D. Arago, he edited the journal Annales de Chimie et de Physique. A member of the St Petersburg Academy of Sciences (1829).