NEPENTHES, an Egyptian drug spoken of by Homer in the Odyssey (iv. 221). Generally in the form "nepenthe" the name is given to any drug producing exhilaration and also occa sionally to the herb or plant from which such a drug is produced. It is also applied to an interesting genus of plants, comprising 6o species, chiefly East Indian, known as the "pitcher-plants" on account of the formation of the leaves. Numerous varieties and artificial hybrids of several species, especially N. ampullaria and N. Raffiesiana, are grown as curiosities in hothouses. NEPHELINE, a rock-forming mineral consisting of sodium, potassium and aluminium silicate, with the approximate formula Its crystals belong to the hexagonal system, and usually have the form of a short six-sided prism terminated by the basal plane. The hardness is 5.5. The specific gravity (2.6), the low index of refraction and the feeble double refraction are nearly the same as in quartz; but since in nepheline the sign of the double refraction is negative, while in quartz it is positive, the two minerals can be distinguished under the microscope. An important determinative character of nepheline is the ease with which it is decomposed by hydrochloric acid, with separation of gelatinous silica (which may be readily stained by colouring matters) and cubes of salt. A clear crystal of nepheline when immersed in acid becomes for this reason cloudy; hence the name, from Gr. veckiXn, a cloud.
Although in naturally occurring nepheline sodium and potas sium are always present in approximately the atomic ratio 3 : I, artificially prepared crystals have the composition, NaAlSiO4; the corresponding potassium compound, KA1SiO,, which is the mineral kaliophilite, has also been prepared artificially. It has therefore been suggested that the orthosilicate formula, (Na,K)AlSiO,, represents the true composition of nepheline, and that the excess of silica is due to the presence of albite, leucite, or silica molecules in "solid solution" in the mineral.
The mineral is specially liable to alteration, and in the labora tory various substitution products of nepheline have been pre pared. In nature it is frequently altered to zeolites (especially natrolite), sodalite, kaolin, or compact muscovite. Two varieties are distinguished, differing in external appearance and mode of occurrence. "Glassy nepheline" has the form of small, colourless, transparent crystals and grains with a vitreous lustre. It is characteristic of the later volcanic rocks rich in alkalis, such as phonolite, nepheline-basalt, leucite-basalt, etc., and also of certain dike-rocks, such as tinguaite. The best crystals are those which occur with mica, sanidine, garnet, etc., in the crystal-lined cavities
of the ejected blocks of Monte Somma, Vesuvius. The other variety, known as elaeolite, occurs as large, rough crystals, or more of ten as irregular masses, which have a greasy lustre and are opaque, or at most translucent, with a reddish, greenish, brownish or grey colour. It forms an essential constituent of certain alkaline plutonic rocks of the nepheline-syenite series, which are typically developed in southern Norway.
The colour and greasy lustre of elaeolite (Gr. gXatov, oil, and XOLos, stone; Ger. Fettstein) are due to the presence of numerous microscopic enclosures of other minerals, possibly augite or horn blende. These enclosures sometimes give rise to a chatoyant effect like that of cat's-eye and cymophane; and elaeolite when of a good green or red colour and showing a distinct band of light is sometimes cut as a gem-stone with a convex surface.
or a holocrystalline plutonic rock which consists largely of nepheline and alkali felspar. The rocks are mostly pale coloured, grey or pink, and in general appearance they are not unlike granites, but dark green varieties are also known. They do not contain quartz, as that mineral and nepheline are mutually exclusive. From ordinary syenites they are distinguished not only by the presence of nepheline but also by the occurrence of many other minerals rich in alkalis or in rare earths. Orthoclase and albite are the principal felspars; usually they are intergrown to form perthite. In some rocks the potash felspar, in others the soda felspar predominates. Soda-lime felspar such as oligoclase and andesine are rare or entirely absent. Fresh clear microcline is very char acteristic of some types of nepheline-syenite. Sodalite, colourless and transparent in the slides, but frequently pale blue in the hand specimens, is the principal felspathoid mineral in addition to nepheline. As a rule these two crystallize before felspar, but they may occur in perthitic intergrowth with it. The com monest ferro-magnesian mineral is pale green augite, which may be surrounded by rims of dark-green, pleochroic soda-augite (aegirine). The latter forms long flat prisms or bundles of radiating needles. A dark reddish-brown biotite is very common in some of these rocks and a white mica, probably not muscovite but lepidolite, is occasionally present. The hornblende may be brown, brownish-green, blue or blue-black, belonging as a rule to the varieties which contain soda ; it is often intergrown with the pyroxene or enclosed in it. The dark-brown triclinic horn blende aenigmatite occurs also in these rocks. Olivine is rare, but may be found in some basic forms of nepheline-syenite.