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Rh in all probability, constituted an essential feature in the Scottish forts. Except on the hypothesis of buttresses of a similar kind, it is impossible to explain the vast quantities of loose stones which are found both inside and outside many of the vitrified walls.

The method by which the fusion of such extensive fortifications was produced has excited much conjecture. Williams maintained that the builders found out, either during the process of smelting bog ore, or whilst offering sacrifices, the power of fire in vitrifying stone, and that they utilized this method to cement and strengthen their defences. This view has been keenly controverted, and it has been suggested that the vitrified summits were not forts but the craters of extinct volcanoes, an hypothesis long since abandoned; that they are not so much forts as vitrified sites, and that the vitrescence was produced by fires lighted during times of invasion, or in religious celebrations; and, lastly, that if they were forts they must originally have been built of wood and stone, and that their present appearance is due to their being set on fire by a besieging enemy. The theory of Williams has, with modifications, been accepted by the principal authorities. It is supported by the following facts:—

(1) The idea of strengthening walls by means of fire is not singular, or confined to a distinct race or area, as is proved by the burnt-earth enclosure of Aztalan, in Wisconsin, and the vitrified stone monuments of the Mississippi valley. (2) Many of the Primary rocks, particularly the schists, gneisses and traps, which contain large quantities of potash and soda, can be readily fused in the open air by means of wood fires—the alkali of the wood serving in some measure as a flux. (3) The walls are chiefly vitrified at the weakest points, the naturally inaccessible parts being unvitrified. (4) When the forts have been placed on materials practically infusible, as on the quartzose conglomerates of the Old Red Sandstone, as at Craig Phadraic, and on the limestones of Dun Mac Uisneachain, pieces of fusible rocks have been selected and carried to the top from a considerable distance. (5) The vitrified walls of the Scottish forts are invariably formed of small stones which could be easily acted upon by fire, whereas the outer ramparts, which are not vitrified, are built of large blocks. (6) Many of the continental forts are so constructed that the fire must have been applied internally, and at the time when the structure was being erected. (7) Daubrée, in an analysis which he made on vitrified materials taken from four French forts, and which he submitted to the Academy of Paris in February 1881, found the presence of natron in such great abundance that he inferred that sea-salt was used to facilitate fusion. (8) In Scandinavia, where there are hundreds of ordinary forts, and where for centuries a system of signal fires was enforced by law, no trace of vitrifaction has yet been detected.

A great antiquity has been assigned to vitrified forts, without sufficient proof. Articles of bronze and iron have been found in the Scottish forts, while in Puy de Gaudy a Roman tile has been discovered soldered to a piece of vitrified rock. In a few of the German forts Professor Virchow found some of the logs used as fuel in vitrifying the walls, and he concluded from the evenness of their cut surfaces that iron and not stone implements must have been used. These results indicate that these structures were possibly in use as late as the early centuries of the Christian era. It has been suggested that they were built as refuges against the Norsemen. Much in the situation and character of the forts favours this supposition. This is especially the case with reference to the Scottish forts. Here the vitrified summits are invariably so selected that they not only command what were the favourite landing-places of the vikings, but are the best natural defences against attacks made from the direction of the seacoast. In Saxony and Lusatia the forts are known as Schwedenburgen, and in the Highlands of Scotland as the fortresses of the Feinne—designations which also seem to point to an origin dating back to the times of the vikings.

—John Williams, An Account of some Remarkable Ancient Ruins (1777); A. Fraser Tytler. ''Edin. Phil. Trans.'' vol. ii.; Sir George Mackenzie, Observations on Vitrified Forts: Hibbert, Arch. Scot. vol. iv.; J. MacCulloch, Highlands and Western Islands (1824), vol. i.; Hugh Miller, Rambles of a Geologist (1858), chap. ix.; Sir Daniel Wilson, Archaeology and Prehistoric Annals of Scotland (1851), vol. ii.; J. H. Burton, History of Scotland (1867), vol. i.; R. Angus Smith, Loch Etive and the Sons of Uisneach (1879);

J. Anderson, Scotland in Pagan Times (1886); C. MacLagan, The Hill Forts of Ancient Scotland; Thomas Aitken, ''Trans. Inverness'' Scientific Soc. vol. i.; Charles Proctor, Chemical Analysis of Vitrified Stones from Tap o' Noth and Dunideer (Huntly Field Club); various papers in ''Proceedings of Soc. Antiq. Scot. (since 1903 The Scottish Historical Review) and Proceedings of Royal Irish Academy''; R. Munro, Prehistoric Scotland (1899); G. Chalmers, Caledonia (new ed., 7 vols., Paisley, 1887–94); Murray's Handbook to Scotland (1903 ed.); Leonhard, Archiv für Mineralogie, vol. i.; Virchow, ''Ztschr. für Ethnologie'', vols. iii. and iv.; Schaaffhausen, Verhandlungen ''der deutsch. anthrop. Gesellschaft (1881); Köhl, Verhand. d.'' ''deutsch. anthrop. Gesellschaft (1883); Thuot, La Forteresse vitrifiée'' du Puy de Gaudy, &c.; De Nadaillac, Les Premiers Hommes, vol. i.; ''Mémoires de la Soc. Antiq. de France'', vol. xxxviii.; Hildebrand, De forhistoriska folken i Europa (Stockholm, 1880); Behla, Die vorgeschichtlichen Rundwälle im östlichen Deutschland (Berlin, 1888); Oppermann and Schuchhardt, Atlas vorgeschichtlicher Befestigungen in Niedersachen (Hanover, 1888-98); Zschiesche, Die vorgeschichtlichen Burgen und Wälle im Thüringer Zentralbecken (Halle, 1889); Bug, Schlesische Heidenschanzen (Grottkau, 1890); Gohausen, Die Befestigungsweisen der Vorzeit und des Mittelalters (Wiesbaden, 1898).

 VITRIOL, a name given to sulphuric acid and to certain sulphates. Oil of vitriol is concentrated sulphuric acid. Blue or Roman vitriol is copper sulphate; green vitriol, ferrous sulphate (copperas); white vitriol, zinc sulphate; and vitriol of Mars is a basic iron sulphate.  VITRUVIUS, Roman architect and engineer, author of a celebrated work on architecture. Nothing is known concerning him except what can be gathered from his own writings. Owing to the discovery of inscriptions relating to the Gens Vitruvia at Formiae in Campania (Mola di Gaeta), it has been suggested that he was a native of that city, and he has been less reasonably connected with Verona on the strength of an existing arch of the 3rd century, which is inscribed with the name of a later architect of the same family name—“Lucius Vitruvius Cerdo, a freedman of Lucius.” From Vitruvius himself we learn that he was appointed, in the reign of Augustus, together with three others, a superintendent of balistae and other military engines, a post which, he says, he owed to the friendly influence of the emperor's sister, probably Octavia (De Architectura, i. pref.). In another passage (v. i) he describes a basilica and adjacent aedes Augusti, of which he was the architect. From viii. 3 it has been supposed that he had served in Africa in the time of Julius Caesar, probably as a military engineer, but the words hardly bear this interpretation. He speaks of himself as being low in stature, and at the time of his writing bowed down by age and ill-health (ii. pref.). He appears to have enjoyed no great reputation as an architect, and, with philosophic contentment, records that he possessed but little fortune. Though a great student of Greek philosophy and science, he was unpractised in literature, and his style is very involved and obscure. To a great extent the theoretical and historical parts of his work are compiled from earlier Greek authors, of whom he gives a list at i. 1 and viii. 3. The practical portions, on the contrary, are evidently the result of his own professional experience, and are written with much sagacity, and in a far clearer style than the more pedantic chapters, in which he gives the somewhat fanciful theories of the Greeks. Some sections of the latter, especially those on the connexion between music and architecture, the scale of harmonic proportions, and the Greek use of bronze vases to reverberate and strengthen the actors' voices in the theatre, are now almost wholly unintelligible. Vitruvius's name is mentioned by Frontinus in his work on the aqueducts of Rome; and most of what Pliny says (Hist. Nat. xxxv. and xxxvi.) about methods of wall-painting, the preparation of the stucco surface, and other practical details in building is taken almost word for word from Vitruvius, especially from vi. i, though without any acknowledgment of the source.

The treatise De Architectura Libri Decem is dedicated to Augustus. Lost for a long time, it was rediscovered in the 15th century at St Gall; the oldest existing MS. dates from the 10th century. From the early Renaissance down to a comparatively recent time the influence of this treatise has been remarkably great. Throughout the period of the classical revival