# 1911 Encyclopædia Britannica/Dulong, Pierre Louis

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​DULONG, PIERRE LOUIS (1785–1838), French chemist and physicist, was born at Rouen on the 12th (or 13th) of February 1785. He began as a doctor in one of the poorest districts of Paris, but soon abandoned medicine for scientific research. After acting as assistant to Berthollet, he became successively professor of chemistry at the faculty of sciences and the normal and veterinary schools at Alfort, and then (1820) professor of physics at the École Polytechnique, of which he was appointed director in 1830. He died in Paris on the 18th (or 19th) of July 1838. His earliest work was chemical in character. In 1811 he discovered chloride of nitrogen; during his experiments serious explosions occurred twice, and he lost one eye, besides sustaining severe injuries to his hand. He also investigated the oxygen compounds of phosphorus and nitrogen, and was ​one of the first to hold the hydrogen theory of acids. In 1815, in conjunction with Alexis Thérèse Petit (1791–1820), the professor of physics at the École Polytechnique, he made careful comparisons between the mercury and the air thermometer. The first published research (1816) dealt with the dilatation of solids, liquids and gases and with the exact measurement of temperature, and it was followed by another in 1818 on the measurement of temperature and the communication of heat, which was crowned by the French Academy. In a third, “On some important points in the theory of heat” (1819), they stated that the specific heats of thirteen solid elements which they had investigated were nearly proportional to their atomic weights—a fact otherwise expressed in the “law of Dulong and Petit” that the atoms of simple substances have equal capacities for heat. Subsequent papers by Dulong were concerned with “New determinations of the proportions of water and the density of certain elastic fluids” (1820, with Berzelius); the property possessed by certain metals of facilitating the combination of gases (1823 with Thénard); the refracting powers of gases (1826); and the specific heats of gases (1829). In 1830 he published a research, undertaken with Arago for the academy of sciences, on the elastic force of steam at high temperatures. For the purposes of this determination he set up a continuous column of mercury, constructed with 13 sections of glass tube each 2 metres long and 5 mm. in diameter, in the tower of the old church of St Geneviève in the Collège Henri IV. The apparatus was first used to investigate the variation in the volume of air with pressure, and the conclusion was that up to twenty-seven atmospheres, the highest pressure attained in the experiments, Boyle’s law holds good. In regard to steam, the old tower was so shaky that it was considered unwise to risk the effects of an explosion, and therefore the mercury column was removed bodily to a court in the observatory. The original intention was to push the experiments to a pressure equivalent to thirty atmospheres, but owing to the signs of failure exhibited by the boiler the limit actually reached was twenty-four atmospheres, at which pressure the thermometers indicated a temperature of about 224°C. In his last paper, published posthumously in 1838, Dulong gave an account of experiments made to determine the heat disengaged in the combination of various simple and compound bodies, together with a description of the calorimeter he employed.