ARAGO, DOMINIQUE FRANcOIS JEAN 1853), French physicist, was born Feb. 26 1786, at Estagel, Perpignan, eastern Pyrenees. He was the eldest of four brothers. Jean (1788-1836) emigrated to America and became a general in the Mexican army. Jacques Etienne Victor (1799-1855) took part in L. C. de S. de Freycinet's exploring voyage in the Uranie from 1817 to 1821, and on his return to France devoted himself to journalism and the drama. The fourth brother, Etienne Vin cent 0802-92), is said to have collaborated with Balzac in the Heritiere de Birague, and from 1822 to 1847 he wrote a great number of light dramatic pieces, mostly in collaboration. A strong republican, he was obliged to leave France in 1849, but returned after the amnesty of 1859. In 1879 he was nominated director of the Luxembourg Museum.
Francois Arago was educated at the municipal school of Per pignan and at the Ecole Polytechnique, but instead of entering the army as had been intended, he became secretary to the Paris observatory. He became acquainted with Laplace, and through his influence was commissioned, with J. B. Biot, to complete the meridional measurements which had been begun by J. B. J. Delambre, and interrupted since the death of P. F. A. Mechair The two left Paris in 1806, and began operations among the mountains of Spain, but Biot returned to Paris after they had determined the latitude of Formentera.
Arago was left alone to make the geodetical connexion of Majorca with Iviza and Formentera. The expedition coincided with the French invasion of Spain, and the astronomer was involved in a series of amazing adventures. In making his escape from the Balearic islands to Algiers he was captured by a Spanish corsair, and spent three months in Spanish prisons. Released with other prisoners at the demand of the Dey of Algiers, he spent six months in Africa before he returned to Marseilles. He was in quarantine there when he received his first letter from A. von Humboldt, who became his lifelong friend. Through all his vicissitudes Arago had preserved his observations and records. Though he was still only 23 years of age he suc ceeded J. B. L. Lalande in the chair of analytical geometry at the Ecole Polytechnique, and became one of the astronomers at the Royal Observatory.
In 1830 he entered the chamber as republican deputy for Seine Inf erieure. In the chamber he advocated many important practical measures for the advancement of science and for tech nical development in railways, telegraphs, etc. In the same year he was made director of the observatory, and perpetual secretary of the Academy of Sciences. During the reign of Louis Philippe he was absorbed in his scientific work, but after the revolution of 1848 he joined the Provisional Government as minister of war and marine. Arago carried important reforms during his short tenure of office. He improved the sailors' ra tions, abolished flogging, and did away with political oaths. He also secured the abolition of negro slavery in French colonies.
In 1852 when Louis Napoleon's Government demanded an oath of allegiance from all its functionaries Arago resigned his post as astronomer at the Bureau des Longitudes, but the Prince President declined to accept his resignation. Arago died in Paris on Oct. 2 1853.
Arago's fame as an experimenter and discoverer rests mainly on his contributions to magnetism and optics. He found that a magnetic needle made to oscillate over non-ferruginous surfaces, such as water, glass, copper, etc., falls more rapidly in the extent of its oscillations according as it is more or less approached to the surface. This discovery was followed by another, that a rotating plate of copper tends to communicate its motion to a magnetic needle suspended over it ("magnetism of rotation"). Arago is also fairly entitled to be regarded as having proved the long-suspected connexion between the aurora borealis and the variations of the magnetic elements.
In optics he has the credit of stimulating the genius of A. J. Fresnel, with whose history, as well as with that of E. L. Malus and of Thomas Young, this part of his life is closely interwoven. In the beginning of the 19th century these three philosophers were shaping the modern doctrine of the undulatory theory of light. Fresnel's arguments in favour of that theory were not acceptable to Laplace, Poisson and Biot, the champions of the emission theory; but they were ardently espoused by Humboldt and by Arago, who had been appointed by the Academy to report on the paper. An intimate friendship sprang up between Arago and Fresnel. They carried on together further researches, which led to the enunciation of the fundamental laws of the polarization of light known by their names.
Arago constructed a polariscope, which he used for observa tions on the polarization of the light of the sky. To him is also due the discovery of the power of rotatory polarization exhibited by quartz, and last of all, among his many contributions to the support of the undulatory hypothesis, comes the experimentum crucis which he proposed to carry out for comparing directly the velocity of light in air and in water or glass. On the emission theory the velocity should be accelerated by an increase of density in the medium; on the wave theory, it should be retarded. In 1838 he communicated to the Academy the details of his apparatus, which utilized the revolving mirrors employed by Sir C. Wheatstone in 1835 for measuring the velocity of the electric discharge; but owing to the great care required in the carrying out of the project, and to the interruption to his labours caused by the revolution of 1848, it was not until the spring of 1850 that he was ready to put his idea to the test; and then his eye sight suddenly gave way. Before his death, however, the retar dation of light in denser media was demonstrated by the experi ments of H. L. Fizeau and J. B. L. Foucault, which, with improvements in detail, were based on the plan proposed by him.
Oeuvres were published after his death under the direction of J. A. Barral, in 17 vols., 8vo (1854-62) ; also separately his Astronomie populaire, 4 vols.; Notices biographiques, 3 vols.; Notices scientifiques, in 5 vols. Voyages scientifiques, in z vol.; Mlnioires scientifiques, in 2 vols. ; Mélanges, in z vol.; and Tables analytiques et documents importants (with portrait) in z vol. English translations of various portions of his works have appeared.