Career
In 1802, Gay-Lussac discovered and published the law giving the effect of temperature upon the volume of gases. This law had previously been discovered, although not published, by Jacques Alexandre CésarCesar Charles, and it is generally referred to as Charles' law. At the request of the French Academy, in 1804 Gay-Lussac made two balloon ascensions--one to 13,120 feet (3,987 meters), the other to 23,012 feet (7,014 meters)--to study the effect of altitude on terrestrial magnetism. From his observations he concluded that the magnetic effect remains constant at all attainable altitudes. In 1805, in collaboration with Alexander von Humboldt, he determined that the combining ratio of hydrogen and oxygen in water was 2 parts hydrogen to 1 part oxygen. This led, after further experimentation, to the statement of his law of combining volumes, which was presented in a paper read to the SociétéSociete Philomathique in 1808: "When gases react, their volumes bear a simple ratio to one another and to the volume of the product, if gaseous."
Gay-Lussac performed a series of experiments with chlorine in 1809. He at first disagreed with but eventually acceded to Humphry Davy's claim that chlorine was an element and not an oxygenated compound. Gay-Lussac's later work with cyanogen compounds, notably prussic acid, did much to discredit Antoine Lavoisier's oxygen-acid theory and establish the hydrogen-acid concept. He also served as a consultant to France's growing chemical industry during this period and made significant contributions to the manufacturing processes of gunpowder, sulfuric acid, and oxalic acid. In 1829, while in the capacity of assayer to the mint, he developed a superior method of silver assaying using a standard solution of common salt.