Page:Biographies of Scientific Men.djvu/233

 celebrated Madame de Genlis, and her teaching was planned on the principles of Rousseau.

In addition to his discoveries and researches in chemistry, Gay-Lussac was a brilliant physicist. The law of the expansion of gases was discovered by him and Charles: a gas expands rd of its volume for each degree of temperature (Centigrade) above 0° C; or, in other words, the law of Gay-Lussac states that all gases in all conditions present one coefficient of expansion —0·00367; that is, when heated from 0° to 100° they expand like air, namely, a thousand volumes of a gas measured at will occupy 1367 volumes at 100°. Regnault, in the year that Gay-Lussac died (1850), showed that Gay-Lussac's law is not entirely correct.

Gay-Lussac invented an alcoholmeter. It is used to determine the strength of spirituous liquors; that is, the proportion of pure alcohol which they contain. He also invented the syphon barometer. In this barometer both the longer and shorter limbs are closed, but the shorter one contains a capillary aperture through which the atmospheric pressure is transmitted.

Gay-Lussac invented an apparatus for ascertaining vapour densities. It involved the determination of the volume of a given weight of vapour; but this method and those of Dumas, Bunsen, and Hofmann have been superseded by that of Victor Meyer, or the air-displacement method.