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A R A A R A which proved them to arise from the mixture of the pencils of light reflected at the two neighbouring surfaces. He pressed a lens of glass against a plate of metal, in which case the central spot is white or black when light polarised perpendicularly to the plane of incidence is reflected at an angle greater or less than the polarising angle for glass; and the rings vanish altogether at the polarising angle results which Airy subsequently proved conformable to the undulatory theory. To Arago is also due the discovery of the power of rotatory polarisation exhibited by quartz. He found that a beam of polarised light transmitted along the axis of a crystal of quartz becomes depolarised, and produces colours varying with the thickness of the crystal. If the polarised light be homogeneous, and the analysing plate be placed athwart this polarised beam so as to reflect none of it, the interposing of a quartz crystal will turn round the polarised beam in its passage, so that when it falls on the analysing plate it is no longer athwart it, and the light is partly reflected. The researches in this field were taken up by Biot, Fresnel, Her- schel, Seebeck, Airy, and others; and the phenomenon was first explained by Airy to be due to the fact that along the axis of a quartz crystal the beam consists of two circu larly polarised portions, while in other directions it consists of two elliptically polarised portions. In some substances the rotation is to the right hand, and in others to the left. Last of all, among the many contributions of Arago to the support of the uudulatory hypothesis, comes the experi- mentum 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. Wheatstone s experiment, in 1835, of measuring the velocity of the electric discharge had sug gested the idea to Arago ; and in 1838 he communicated to the Academy the details of his apparatus. Owing to the great care required in the carrying out of the project, and to the interruptions to his labours by the events of the revolution, it was the spring of 1850 before he was ready to put his idea to the test. But, as fate would have it, just at that time his eye-sight suddenly gave way. With a melan choly resignation he consoled himself with having merely laid down the problem and having indicated the means of its solution, and with the hope that other experimenters would follow in his steps and add a new proof in favour of the undulatory system. Arago lived to see his wishes realised in the beautiful experiments of Fizeau and Foucault, which, with improvements of detail in the apparatus, were based on the plan proposed by him. These, as all the scientific world knows, confirmed the deductions from the wave theory, and established the retardation of the velo city of light in denser media. With this ended those brilliant labours which have in no small degree contributed to render the arguments in favour of the undulatory doc trine all but unanswerable. Arago s (Euvrcsvfcre published after his death under the direction of M. J. A. Banal, in 17 vols., 8vo, 1854 to 1862; also separately his Astronomic populaire, in 4 vols. ; Notices biographiques, in 3 vols. ; Notices sdentifiques, in 5 vols. ; Voyages sdentifiques, 1 vol. ; Memoires scicntifiques, in 2 vols. ; Melanges (Miscellanies), in 1 vol. ; and Tables analytiquts et documents importants (with por trait), 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, iloye 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. E, 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 2fen, translated by Smyth, Powell, and Grant, 8vo, London, 1857. ARAGON, a captaincy general, or, as it is usually called by the Spaniards, a kingdom, of Spain, situated between lat. 40 and 42 51 N., and between long. 2 10 W. and 1 45 E., is bounded on the N. by the Pyrenees, which separate it from France, E. by Catalonia and Valencia, S. by Valencia, and W. by Navarre and the two Castiles. It has an area of 14,726 English square miles, and a population of 925,773 calculated for 1867 on the basis of the census of 1860. It is divided by the river Ebro, which flows through it in a south-easterly direction, into two nearly equal parts, known as Trans-ibero and Cis- ibero. The north is occupied by a portion of the Pyrenees, which here attain in Monte Perdido, or as it is variously called Mont Perdu, Las Tres Sorores, almost their highest altitude (11,430 feet); and in the south and west are various sierras of considerable elevation. The whole sur face, indeed, is very irregular. The large and fertile plain in the middle, bordering on the Ebro, is broken by offshoots from the north and south, while the mountain ranges, on the other hand, are interrupted by numerous valleys of great beauty. The central pass for carriages over the Pyrenees is on the line between Saragossa and Pau, and bears the name of Port de Confranc. 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, the Essera, the No guera Ribagorzana, and the Noguera Pallaresa the last two belonging to Catalonia ; and from the south the Xalon and Xiloca and the Guadalupe. The Imperial Canal of Aragon, which was commenced by Charles V. in 1529, but remained unfinished for nearly two hundred years, extends from Tudela almost to Sastago, a distance of 80 miles ; it has a depth of 9 feet, and an average breadth of 69, and is navigable for vessels of about 80 tons burthen. The Pioyal 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 N.N.E. of Albarracin is the remarkable fountain called Cella, 3700 feet above the sea, which forms the source of the Xiloca; and between this river and the Sierra Molina is an extensive lake called Gallo- canta, covering about 6000 acres. The climate of Aragon is, as might be supposed, of a very varied character, rigor ous in the north and among the sierras, but mild and even oppressive in the south, especially towards the Medi terranean. Its vegetable products are necessarily affected by a similar variety. The hills are clothed in many places with oak, pine, and beech, which, if utilised, would furnish a good supply of timber. The flora is exceedingly rich, but has been slightly explored. Wheat, maize, rice, wine,, oil, flax, and hemp, of fine quality, are grown in con siderable quantities ; as well as saffron, madder, liquorice, sumach, and a variety of fruits. Swine and sheep are reared in considerable numbers, the merino wool supplied by the latter being one of the chief productions of the province ; but little attention is paid to other descriptions of cattle. The manufactures are comparatively insignifi cant, being in great measure supplementary to the agricul tural and pastoral pursuits of the people. The silk manufacture has declined, and woollen and linen cloth is only produced in small quantities. The mineral resources of the country consist of iron, lead, copper, sulphur, cobalt, asphalt, lignite, alum, rocksalt, jet, marble, and jasper, which are all found in abundance, but they are very inadequately wrought. Aragon is divided into three pro vinces, HUESCA, TERTJEL, and SARAGOSSA, accounts of which will be found under these headings. The capital is Saragossa, and the other towns of importance are Jaca, Huesca, and Barbastro to the north of the Ebro, and Tara- zona, Calatayud, Daroca, Calamocha, Albarracin, Montalban,

Teruel, and Alcaniz to the south. Saragossa is an arcli-