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Rh i.e. Pannonia and Noricum. Dalmatia was partially reconquered by Justinian in 536, but after 565 it was devastated by the Avars, and throughout the century bands of Slavonic invaders had been gradually establishing themselves in Illyria, where, unlike the earlier barbarian conquerors, they formed permanent settlements. Between 600 and 650 the main body of the immigrants occupied Illyria (see : History; and ). It consisted of Croats and Serbs, two groups of tribes who spoke a single language and were so closely related that the origin of the distinction between them is obscure. The Croats settled in the western half of Illyria, the Serbs in the eastern; thus the former came gradually under the influence of Italy and Roman Catholicism, the latter under the influence of Byzantium and the Greek Church. Hence the distinction between them became a marked difference of civilization and creed, which has always tended to keep the Illyrian Slavs politically disunited.

The Croats and Serbs rapidly absorbed most of the Latinized Illyrians. But the wealthy and powerful city-states on the coast were strong enough to maintain their independence and their distinctively Italian character. Other Roman provincials took refuge in the mountains of the interior; these Mavrovlachi, as they were called (see : Population; and ), preserved their language and nationality for many centuries. The Illyrian tribes which had withstood the attraction of Roman civilization remained unconquered among the mountains of Albania and were never Slavonized. With these exceptions Illyria became entirely Serbo-Croatian in population, language and culture.

The name of Illyria had by this time disappeared from history. In literature it was preserved, and the scene of Shakespeare’s comedy, Twelfth Night, is laid in Illyria. Politically the name was revived in 1809, when the name Illyrian Provinces was given to Carniola, Dalmatia, Istria, Fiume, Görz and Gradisca, and Trieste, with parts of Carinthia and Croatia; these territories were ceded by Austria to Italy at the peace of Schönnbrun (14th Oct. 1809). The Illyrian Provinces were occupied by French troops and governed in the interest of Napoleon; the republic of Ragusa was annexed to them in 1811, but about the end of 1813 the French occupation ceased to be effective and the provinces reverted to Austria. The kingdom of Illyria, which was constituted in 1816 out of the crown-lands of Carinthia, Carniola, Istria, Görz and Gradisca, and Trieste, formed until 1849 a kingdom of the Austrian crown. For the political propaganda known as Illyrism, see : History.

—In addition to the authorities quoted above, see G. Zippel, Die römische Herrschaft in Illyrien bis auf Augustus (Leipzig, 1877); P. O. Bahn, Der Ursprung der römischen Provinz Illyrien (Grimma, 1876); J. Marquardt, Römische Staatsverwaltung, i. (1881), p. 295; E. A. Freeman, “The Illyrian Emperors and their Land” (Historical Essays, series 3, 1879); C. Patsch in Pauly-Wissowa’s Realencyklopädie, iv. pt. 2 (1901); Th. Mommsen, The Provinces of the Roman Empire (ed. F. Haverfield, 1909).

ILMENAU, a town and summer resort of Germany, in the grand-duchy of Saxe-Weimar, at the north foot of the Thuringian Forest, on the river Ilm, 30 m. by rail south of Erfurt. Pop. (1905) 11,222. The town, which stands picturesquely among wooded hills, is much frequented by visitors in the summer. It was a favourite resort of Goethe, who wrote here his Iphigenie, and often stayed at Gabelbach in the neighbourhood. It has a grand-ducal palace, a Roman Catholic and two Evangelical churches, a sanatorium for nervous disorders, and several educational establishments. Its chief manufactures are glass and porcelain, toys, gloves and chemicals, and the town has tanneries and saw-mills. Formerly a part of the county of Henneberg, Ilmenau came in 1631 into the possession of electoral Saxony, afterwards passing to Saxe-Weimar.

See R. Springer, Die klassischen Stätten von Jena und Ilmenau (Berlin, 1869); Pasig, Goethe und Ilmenau (2nd ed., Weimar, 1902); and Fils, Bad Ilmenau und seine Umgebung (Hildburghausen, 1886).

ILMENITE, a mineral known also as titanic iron, formerly regarded as an iron and titanium sesquioxide (Fe, Ti)2O3 isomorphous with haematite (Fe2O3), but now generally considered to be an iron titanate FeTiO3 isomorphous with pyrophanite (MnTiO3) and geikielite (MgTiO3). It crystallizes in the parallel-faced hemihedral class of the rhombohedral system, thus having the same degree of symmetry as phenacite and pyrophanite, but differing from that of haematite. The angles between the faces are very nearly the same as between the corresponding faces of haematite; but it is to be noted that the rhombohedral angle (94° 29′) of ilmenite is not intermediate between that of haematite (94° 0′) and of the artificially prepared crystals of titanium sesquioxide (92° 40′), which should be the case if the three substances were isomorphous. Analyses show wide variations in chemical composition, and there is a gradation from normal ilmenite FeTiO3 (with titanium dioxide 52.7, and ferrous oxide 47.3%) to titaniferous haematite and titaniferous magnetite. Frequently also, magnesia and manganous oxide are present in small amounts, the former reaching 16%. The formula (Fe, Mg)TiO3 is then analogous to those of geikielite and pyrophanite. Many analyses show the presence of TiO2 and (Fe, Mg)O in this ratio of 1:1, yet there is often an excess of ferric oxide to be accounted for; this may perhaps be explained by the regular intergrowth on a minute scale of ilmenite with haematite, like the intergrowth of such substances as calcite and sodium nitrate, which are similar crystallographically but not chemically.

In many of its external characters ilmenite is very similar to haematite; the crystals often have the same tabular or lamellar habit; the twin-laws are the same, giving rise to twin-lamellae and planes of parting parallel to the basal plane and the primitive rhombohedron; the colour is iron-black with a submetallic lustre; finally, the conchoidal fracture is the same in both minerals. Ilmenite has a black streak; it is opaque, but in very thin scales sometimes transparent with a clove-brown colour. It is slightly magnetic, but without polarity. The hardness is 5, and the specific gravity varies with the chemical composition from 4.3 to 5.0.

Owing to the wide variations in composition, which even yet are not properly understood, several varieties of the mineral have been distinguished by special names. Crichtonite occurs as small and brilliant crystals of acute rhombohedral habit on quartz at Le Bourg d’Oisans in Dauphiné; it agrees closely in composition with the formula FeTiO3 and has a specific gravity of 4.7. Manaccanite (or Menaccanite) is a black sandy material, first found in 1791 in a stream at Manaccan near Helston in Cornwall. Iserite, from Iserwiese in the Iser Mountains, Bohemia, is a similar sand, but containing some octahedral crystals, possibly of titaniferous magnetite. Washingtonite is found as large tabular crystals at Washington, Connecticut. Uddevallite is from Uddevalla in Sweden. Picrotitanite or picroilmenite (Gr. , “bitter”) is the name given to varieties containing a considerable amount of magnesia. Other varieties are kibdelophane, hystatite, &c. The name ilmenite, proposed by A. T. Kupffer in 1827, is after the Ilmen Mountains in the southern Urals, whence come the best crystals of the mineral. The largest crystals, sometimes as much as 16 ℔ in weight, are from Kragerö and Arendal in Norway.

Ilmenite occurs, often in association with magnetite, in gneisses and schists, sometimes forming beds of considerable extent, but of little or no economic value. It is a common accessory constituent of igneous rocks of all kinds, more especially basic rocks such as gabbro, diabase and basalt. In these rocks it occurs as platy crystals, and is frequently represented by a white, opaque alteration product known as leucoxene.

ILOILO, a town, port of entry and the capital of the province of Iloilo, Panay, Philippine Islands, at the mouth of Iloilo river, on the S.E. coast. Pop. (1903) 19,054. In 1903, after the census had been taken, the population of the town was more than doubled by the addition of the municipalities of La Paz (pop. 5724), Mandurriao (pop. 4482), Molo (pop. 8551) and Jaro (pop. 10,681); in 1908 Jaro again became a separate town. The town is built on low sandy ground, is irregularly laid out,