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 followed by "Le Perfectionnement des Thermomeètres et Baromètres," "L'Action Capillaire," "La Fusion des Vapeurs," etc.

In 1804 Gay-Lussac, in conjunction with Alexander von Humboldt, published an important memoir on the chemical and physical properties of air taken at an altitude of 23,000 feet. Ballooning in the early part of the nineteenth century was attended with the greatest risks; but he and Humboldt were intrepid naturalists, and undertook the investigation of the atmosphere at great heights—hygrometry, atmospheric electricity, aqueous tension, atmospheric pressure, etc., were some of the subjects investigated. Their memoir was published by the Académie des Sciences, and contained the announcement that hydrogen and oxygen combine to form water in the proportion of two hundred volumes of hydrogen to one hundred volumes of oxygen. It was from this discovery that Gay-Lussac was led to the most important law of volumes, which he discovered in 1809. This was his chef d'œuvre; but more anon.

Gay-Lassac investigated the nature of volcanic gases—more particularly hydrochloric acid gas—and "the vapours that rise from the fumarolles cause the sublimation of the chlorides of iron, copper, lead, and ammonium; iron glance and chloride of sodium (the latter often in large quantities) fill the cavities of recent lava streams and the fissures of the margin of the crater." In the Annales de