Christianity, centring upon an ideally perfect Personality, has to shape men towards an increasingly fuller consciousness of the ultimate truths of God, man and the universe. Its career and the stages leading up to it can be placed, as has been seen, upon the background of history and religion. But while the line of de velopment can be clearly traced back, its future course cannot be easily foreshadowed. Christianity is based upon a single book, or rather a collection of books (see BIBLE) covering the cen turies during which there were the profoundest developments of which we know, and upon which the Bible is the only direct source of knowledge. Entirely characteristic is the utterly un compromising recognition that God is no respecter of persons or peoples, but that the Divine purpose in all its workings is not arbitrary. Certain awe-inspiring ideas of God and man were realized, and have proved capable of continuous reinterpretation; but the real significance of the great religious truths has yet to be restated in the light of modern knowledge. (S. A. C.) JET, a substance which seems to be a peculiar kind of lignite or anthracite; often cut and polished for ornaments. (Fr. jais, Ger. Gagat.) The word "jet" probably comes, through O.Fr.
jaiet, (from the classical gagates, a word which was derived, ac cording to Pliny, from Gagas, in Lycia, where jet, or a similar substance, was originally found). Jet was used in Britain in pre historic times; many round barrows of the bronze age have yielded jet beads, buttons, rings, armlets and other ornaments. The abun dance of jet in Britain is alluded to by Gaius Iulius Solinus (fl. 3rd century) and jet ornaments are found with Roman relics in Britain. Probably the supply was obtained from the coast of Yorkshire, especially near Whitby, where nodules of jet were f or merly picked up on the shore. Caedmon refers to this jet, and at a later date it was used for rosary beads by the monks of Whitby Abbey.
The Whitby jet occurs in irregular masses, often of lenticular shape, embedded in hard shales known as jet-rock and belonging to that division of the Upper Lias which is termed the zone of Ammonites serpentinus. Microscopic examination of jet occa
sionally reveals the structure of coniferous wood, which A. C. Seward has shown to be araucarian. Probably masses of wood were brought down by a river, and drifted out to sea, where they sank and were buried in a deposit of fine mud which eventually hardened into shale. Under pressure, perhaps assisted by heat, and with exclusion of air, the wood suffered a peculiar kind of dec om.
position, probably modified by the presence of salt water, as sug gested by Percy E. Spielmann. Scales of fish and other fossils of the jet-rock are frequently impregnated with bituminous prod ucts, which may replace the original tissues. Drops of liquid bitumen occur in the cavities of some fossils, whilst inflammable gas is not uncommon in the jet-workings, and petroleum may be detected by its smell. Iron pyrites is often associated with the jet.
Formerly sufficient jet was found in loose pieces on the shore, set free by the disintegration of the cliffs, or washed up from a submarine source. When this supply became insufficient, the rock was attacked by the jet-workers; ultimately the workings took the form of true mines, levels being driven into the shales not only at their outcrop in the cliffs but in some of the inland dales of the Yorkshire moorlands, such as Eskdale. The best jet has a uniform black colour, and is hard, compact and homogeneous in texture, breaking with a conchoidal fracture. It must be tough enough to be readily carved or turned on the lathe, and sufficiently compact in texture to receive a high polish. The final polish was formerly given by means of rouge, which produces a beautiful velvety sur face, but rotten-stone and lampblack are often employed instead. The softer kinds, not capable of being freely worked, are known as bastard jet. A soft jet is obtained from the estuarine series of the Lower Oolites of Yorkshire.
See P. E. Spielmann, "On the Origin of Jet," Chemical News (Dec. 14, 1906) ; C. Fox-Strangways, "The Jurassic Rocks of Britain, vol. i., Yorkshire," Mem. Geol. Surv. (1892) ; J. A. Bower, "Whitby Jet and its Manufacture," Journ. Soc. Arts (1874, vol. xxii. p. 8o).