Arago’s fame as an experimenter and discoverer rests mainly on his contributions to magnetism and still more to optics. He found that a magnetic needle, made to oscillate over nonferruginous surfaces, such as water, glass, copper, &c., falls more rapidly in the extent of its oscillations according as it is more or less approached to the surface. This discovery, which gained him the Copley medal of the Royal Society in 1825, 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 we owe to him not only important optical discoveries of his own, but 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. Shortly after the beginning of the 19th century the labours of these three philosophers were shaping the modern doctrine of the undulatory theory of light. Fresnel’s arguments in favour of that theory found little favour with 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. This was the foundation of an intimate friendship between Arago and Fresnel, and of a determination to carry on together further researches in this subject, which led to the enunciation of the fundamental laws of the polarization of light known by their names (see [Polarization]). As a result of this work Arago constructed a polariscope, which he used for some interesting observations 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 the spring of 1850 before he was ready to put his idea to the test; and then his eyesight suddenly gave way. Before his death, however, the retardation of light in denser media was demonstrated by the experiments of H.L. Fizeau and J.B.L. Foucault, which, with improvements in detail, were based on the plan proposed by him.
Arago’s Œuvres were published after his death under the direction of J.A. Barral, in 17 vols., 8vo, 1854-1862; also separately his Astronomie populaire, in 4 vols.; Notices biographiques, in 3 vols.; Notices scientifiques, in 5 vols.; Voyages scientifiques, in 1 vol.; Mémoires scientifiques, in 2 vols.; Mélanges, in 1 vol.; and Tables analytiques et documents importants (with portrait), in 1 vol. English translations of the following portions of his works have appeared:—Treatise on Comets, by C. Gold, C.B. (London, 1833); also translated by Smyth and Grant (London, 1861); Hist. éloge of James Watt, by James Muirhead (London, 1839); also translated, with notes, by Lord Brougham; Popular Lectures on Astronomy, by Walter Kelly and Rev. L. Tomlinson (London, 1854); also translated by Dr W.H. Smyth and Prof. R. Grant, 2 vols. (London, 1855); Arago’s Autobiography, translated by the Rev. Baden Powell (London, 1855, 1858); Arago’s Meteorological Essays, with introduction by Humboldt, translated under the superintendence of Colonel Sabine (London, 1855), and Arago’s Biographies of Scientific Men, translated by Smyth, Powell and Grant, 8vo (London, 1857).
ARAGON, or Arragon (in Span. Aragón), a captaincy-general, and formerly a kingdom of Spain; bounded on the N. by the Pyrenees, which separate it from France, on the E. by Catalonia and Valencia, S. by Valencia, and W. by the two Castiles and Navarre. Pop. (1900) 912,711; area, 18,294 sq. m. Aragon was divided in 1833 into the provinces of Huesca, Teruel and Saragossa; an account of its modern condition is therefore given under these names, which have not, however, superseded the older designation in popular usage.
Aragon consists of a central plain, edged by mountain ranges. On the south, east and west, these ranges, though wild and rugged, are of no great elevation, but on the north the Pyrenees attain their greatest altitude in the peaks of Aneto (11,168 ft.) and Monte Perdido (10,998 ft.)—also known as Las Tres Sorores, and, in French, as Mont Perdu. The central pass over the Pyrenees is the Port de Canfranc, on the line between Saragossa and Pau. Aragon is divided by the river Ebro (q.v.), which flows through it in a south-easterly direction, into two nearly equal parts, known as Trans-ibero and Cis-ibero. The Ebro is the principal river, and receives from the north, in its passage through the province, the Arba, the Gallego and the united waters of the Cinca, Esera, Noguera Ribagorzana, Noguera Pallaresa and Segre—the last three belonging to Catalonia. From the south it receives the Jalon and Jiloca (or Xalon and Xiloca) and the Guadalope. The Imperial Canal of Aragon, which was begun by the emperor Charles V. in 1529, but remained unfinished for nearly two hundred years, extends from Tudela to El Burgo de Ebro, a distance of 80 m.; it has a depth of 9 ft., and an average breadth of 69, and is navigable for vessels of about 80 tons. The Royal Canal of Tauste, which lies along the north side of the Ebro, was cut for purposes of irrigation, and gives fertility to the district. Two leagues north-north-east of Albarracin is the remarkable fountain called Cella, 3700 ft. above the sea, which forms the source of the Jiloca; and between this river and the Sierra Molina is an extensive lake called Gallocanta, covering about 6000 acres. The climate is characterized by extreme heat in the summer and cold in the winter; among the mountains the snowfall is heavy, and thunderstorms are frequent, but there is comparatively little rain.
Within a recent geological period, central Aragon was undoubtedly submerged by the sea, and the parched chalky soil remains saturated with salt, while many of the smaller streams run brackish. As the mountains of Valencia and Catalonia effectually bar out the fertilizing moisture of the sea-winds, much of the province is a sheer wilderness, stony, ash-coloured, scarred with dry watercourses, and destitute of any vegetation except thin grass and heaths. In contrast with the splendid fertility of Valencia or the south of France, the landscape of this region, like the rest of central Spain, seems almost a continuation of the north African desert area. There are, however, extensive oak, pine and beech forests in the highlands, and many beautiful oases in the deeply sunk valleys, and along the rivers, especially beside the Ebro, which is, therefore, often called the “Nile of Aragon.” In such oases the flora is exceedingly rich. Wheat, maize, rice, oil, flax and hemp, of fine quality, are grown in considerable quantities; as well as saffron, madder, liquorice, sumach, and a variety of fruits. Merino wool is one of the chief products.
In purity of race the Aragonese are probably equal to the Castilians, to whom, rather than to the Catalans or Valencians, they are also allied in character. The dress of the women is less distinctive than that of the men, who wear a picturesque black and white costume, with knee-breeches, a brilliantly coloured sash, black hempen sandals, and a handkerchief wound round the head.
Three counties—Sobrarbe, situated near the headwaters of the Cinca, Aragon, to the west, and Ribagorza or Ribagorça, to the east—are indicated by tradition and the earliest chronicles as the cradle of the Aragonese monarchy. These districts were never wholly subdued when the Moors overran the country (711-713). Sobrarbe especially was for a time the headquarters of the Christian defence in eastern Spain. About 1035, Sancho III. the Great, ruler of the newly established kingdom of Navarre, which included the three counties above mentioned, bequeathed them to Gonzalez and Ramiro, his sons. Ramiro soon rid himself of his rival, and welded Sobrarbe, Ribagorza and Aragon into a single kingdom, which thenceforward grew rapidly in size and power and shared with Castile the chief part in the struggle against the Moors. The history of this period, which was terminated by the union of Castile and Aragon under Ferdinand and Isabella in 1479, is given, along with a full account of the very interesting constitution of Aragon, under [Spain] (q.v.). At the height of its power under James I. (1213-1276), the kingdom included Valencia, Catalonia, the Balearic Islands and the considerable territory of Montpellier in France; while Peter III. (1276-1285) added Sicily to his dominions.
The literature relating to Aragon is very extensive. See, in addition to the works cited in the article [Spain] (section History), “Les Archives d’Aragon et de Navarre,” by L. Cadier, in Bibliothèque de l’École des Chartes, 49 (Paris, 1888). Among the more important original authorities, the following may be selected:—for general history, Anales de la corona de Aragón, by G. Çurita, 3rd ed. in 7 folio volumes (Saragossa, 1668-1671; 1st ed. 1562-1580);—for ecclesiastical history, Teatro histórico de las iglesias de Aragón (Pamplona, 1770-1807); for economic history, História de la economia politica de Aragón, by I.J. de Asso y del Rio (Saragossa, 1798). For the constitution and laws of Aragon, see Orígines del Justicia de Aragón, &c., by J. Ribera Tarrago (Saragossa, 1897), and Instituciones y reyes de Aragón, by V. Balaguér (Madrid, 1896). The topography, inhabitants, art, products, &c., of the kingdom are described in a volume of the series España entitled Aragón, by J.M. Quadrado (Barcelona, 1886).