Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/318

300 L A P L A P

fragile or soft stones disks of hardwood are employed. Precious stones are cut in forms known as brilliants and roses, and the several parts are designated as shown in fig. 7 (side-elevations). Turquoise, opal, cats-eye, car buncle, asteria, and a few other stones are cut en cabochon. Prior to engraving on a stone, the polish is removed with emery from the surface to be engraved, and the device marked on it with a brass point; the outline is then sharply incised, and the work continued by means of small drills, the diamond point, &c.

Fig. 8.

Within the last few years a great advance has been made in our knowledge of the minute structure and mineral con stitution of rocks by cutting and grinding small slices of them so thin that they readily transmit light, and can then be examined under the microscope, and the optical proper ties of their constituent minerals conveniently studied. Sections suitable for this purpose may be prepared by grinding thin flakes or splinters of a rock or mineral upon a cast-iron plate smeared with emery powder and water. The emery employed for the first grinding should not be very fine, that of medium grain being best suited for the purpose. The fragment is pressed by the fingers against the slab, and ground uniformly over all parts of the plate with a circular motion. When a flat surface is obtained, the fragment should be carefully washed from all traces of the emery mud, and a finer face should be imparted to it by a second grinding with the finest flour-emery and water, smeared upon a slab of plate-glass or a smoothly-planed brass slab. When thoroughly cleaned, the smooth face of the chip is warmed and cemented to a small piece of plate- glass with Canada balsam (fig 8). The older and drier the balsam the better it answers this purpose. A little should be placed on the piece of glass and warmed until it liquefies (it must not boil). The smooth surface of the stone is then laid upon the balsam and pressed tightly against the glass; when the balsam has hardened, the grinding process is renewed, the piece of glass serving as a handle, and the flour-emery should generally be employed as soon as the fragment is thin enough to transmit light. When finished, the glass and section are cleaned, the glass is warmed, and the section is pushed off with a blunt needle or wire into a saucer of turpentine, which should be gently warmed, and all traces of dirt should 1)3 removed with a camel's hair brush. The section is then lifted from its bath by means of a needle and allowed to subside gently upon a drop of fluid Canada balsam placed on a clean glass slip which has been previously warmed. A thin covering-glass is then slightly warmed and placed over the preparation, care being taken not to include any air-bubbles. The process of grinding sections by hand is necessarily a rather slow one, and, although in the finishing it cannot well be superseded by other methods, still the rough grinding may be much more expeditiously done by means of various machines which have been devised for this purpose. Some of these are worked by hand, others by a treadle. Among the latter, the apparatus devised by Mr J. B. Jordan, and manufactured by Messrs Cotton & Johnstone of Grafton Street, Soho,^[1] and that made by Fuess of Berlin are those in most general use. These machines are provided with slitting disks for cutting thin slices with diamond dust. This saves much grinding, but presents some difficulties to the novice. The grinding laps with which the machines are supplied are generally cast in lead or pewter, while occasionally prepared corundum disks are employed, and disks of hard wood are now and then used for imparting a final polish. (F. R.)

LAPIS LAZULI, a mineral possessing usually a fine blue colour, whence it is much prized for decorative purposes. From the large number of Egyptian ornaments in this material, which have been preserved from the time of the Pharaohs, it is evident that it was a favourite stone with the ancient Egyptians. A few Assyrian seal-cylinders in lapis lazuli are also known. It appears to have been the Greek sapphire; thus, Theophrastus describes the (Greek characters) as sprinkled with gold-dust, a description which is utterly inappropriate to any variety of our modern sapphire, but which applies with much force to the lapis lazuli, inasmuch as this stone frequently contains disseminated particles of iron pyrites, which by their colour and lustre may readily be mistaken for such a metal. In like manner, Pliny refers to one variety of the sapphirus as being spotted with gold. An allusion to the same quality is perhaps found in Job xxviii. 6.

It is but rarely that the lapis lazuli exhibits anything approaching to distinct crystallization. Usually it occurs in compact masses, which break with an uneven fracture; but occasionally it presents an imperfect cleavage parallel to the faces of a rhombic dodecahedron, and still more rarely offers distinct faces of this form. Its specific gravity is about 2 - 5, and its degree of hardness between 5 and 6; it therefore scratches glass, and is capable of receiving a fair polish. Although the colour is generally a fine azure, or rich Berlin blue, some varieties exhibit violet, green, or even red tints, or are altogether colourless. The mineral is always opaque, with only slight translucency at the edges. Microscopic sections reveal a want of homogeneity in the constitution of the substance, bluish particles being disseminated through a white matrix.

The lapis lazuli is a silicate of aluminium, calcium, and sodium; but the published analyses are rather discordant. All agree, how ever, in recording the presence of sulphur, and it is generally sup posed that this element exists as a sulphide of iron and sodium, and that it is upon the presence of such a compound that the blue colour depends. The following is an analysis of the South Ameri can variety: silica, 457; alumina, 25 34 ; soda, 10 f>5 ; potash, 1 35; lime, 7 48; ferric oxide, 1 30 ; sulphur, 3 96; and sulphuric acid, 4 32. By the action of hydrochloric acid the mineral is de composed, with separation of gelatinous silica and evolution of sul phuretted hydrogen. Before the blowpipe it fuses readily, with loss of colour.

The lapis lazuli is usually found in crystalline limestone or in gneissose rocks, but its occurrence is confined to very limited localities. It is found in Persia, Tartary, Tibet, and China, and in the neighbourhood of Lake Baikal in Siberia. Fine masses occur in the Andes of Chili and Peru. In Europe it has been found at Ditro in Transylvania, and in the ejected blocks of Monte Somma. In addition to its use as an ornamental stone, the lapis lazuli was formerly employed, to a large extent, in the preparation of the beautiful blue pigment called ultramarine. For this purpose the mineral was ground, calcined, and carefully levigated in water. Of late years, however, artificial ultramarine has been prepared which claims to be of equal beauty and permanency with the natural pigment. Artists, however, still regard the natural colour as superior.

LAPITHÆ, a mythic race whose contest with the Centaurs is one of the most famous events in Greek mythology and one of the most favourite subjects of Greek art. The home of the legend is the district round Mount Pelion in Thessaly; it is not found in the other places where the Centaur legend has its home – Pholoe in Arcadia, and the river Evenus in Ætolia. It is impossible to write of the Lapithaa without including also their adversaries the Centaurs and the great battle at the marriage of Pirithous and Deidamia. The outlines of the legend have already been given under CENTAUR ; here we shall merely attempt to distinguish between earlier and later elements in the myth, and thus trace its growth. By the Greek sculptors of the school of Phidias the battle of Lapithce with Centaurs was conceived as a struggle between mankind and mischievous monsters, and symbolical of the great conflict between Greeks and Persians. There can be no

1. Described and figured in The Study of Rocks (Longman's Text-Books of Science).