440

EMERALD.

e\ iron rod by which the table e is joined to its support e'.

f f, uprights of the cloth-carrying frame.

f' f', horizontal sides of the same frame.

G, four roll beams.

g", the piece of cloth.

g", the strings, which serve to stretch the cloth laterally.

EMERALD (Emerav.de, Fr.; Smaragd , Germ.) is a precious stone of a beautifulgreen color; valued next to diamond, and in the same rank as oriental ruby and sapphire.It occurs in prisms with a regular hexagonal base; sp. grav. 2‘7; scratches quartzwith difficulty; is scratched by topaz; fusible at the blowpipe into a frothy bead; theprecipitate afforded by ammonia, from its solution, is soluble, in a great measure, in car-bonate of ammonia. Its analysis is given very variously by different chemists. It con-tains about 14 per cent, of glucina, which is its characteristic constituent; along with68 of silica, 16 of alumina, a very little lime and iron. The beautiful emerald of Peru is found in a clay schist mixed with some calcareous matter. A stone of 4 grains weightis said to be worth from 4 l. to 5Z.; one of 8 grains, 101.; one of 15 grains, being fine, isworth 60/.; one of 24 grains fetched, at the sale of M. de Dree’s cabinet, 2400 francs,or nearly 100Z.

The beryl is analogous in composition to the emerald, and is employed (when of thecommon opaque kind, found near Limoges ) by chemists for procuring the earthglucina.

EMERY. This mineral was long regarded as an ore of iron; and was called byHaiiy fer oxide qnartzifere. It is very abundant in the island of Naxos, at cape Emeri,whence it is imported in large quantities. It occurs also in the islands of Jersey andGuernsey , at Almaden, in Poland , Saxony, Sweden , Persia , &c. Its color varies fromred brown to dark brown ; its specific era city is about 4*000; it is so hard as to scratchquartz and many precious stones. By Mr. Tenant’s analysis it consists of alumina,80; silica, 3 ; iron, 4. Another inferior kind yielded 32 of iron, and only 50 ofalumina.

The alumina of emery is believed to be aggregated to the same degree of hardness asin corundum or adamantine spar; which is one of the hardest minerals known. Emeryis extensively employed for grinding metals, glass, &c.; for which purpose it is reducedto powders of different degrees of fineness, by grinding and elutrialion. When sotreated, it is sold under the name of flour of emery, or washed emery.

EMPYREUMA, means the offensive smell produced by fire applied to organicmatters, chiefly vegetable, in close vessels. Thus, empyreumatic vinegar is obtainedby distilling wood at a red heat, and empyreumatic oil from many animal substances mthe same way.

ENAMELS ( Emanx , Fr.; Schmelzglas, Gefro.) are varieties of glass, generally opaqueand colored, always formed by the combination of different metallic oxydes, to whichcertain fixed fusible salts are added, such as the borates, fluates, and phosphates.

The simplest enamel, and the one which serves as a basis to most of the others, isobtained by calcining first of all a mixture of lead and tin, in proportions varying fro®15 to 50 parts of tin for 100 of lead. The middle term appears to be the most suitablefor the greater number of enamels; and this alloy has such an affinity for oxygen, thait may be calcined with the greatest ease in a flat cast-iron pot, and at a temperature noabove a cherry red, provided the dose of tin is not too great. The oxyde is drawn onthe sides of the melted metal according as it is generated, new pieces of the alloy ® eI1 j?thrown in from time to time, till enough of the powder be obtained. Great care ougto be taken that no metallic particles be left in the oxyde, and that the calcining heat _as low as is barely sufficient; for a strong fire frits the powder, and obstructs its suquent comminution. The powder when cold is ground in a proper mill, levigated '' ^water, and elutriated, as will be described under Red had. In this state of finenesspurity, it is called calcine, or flux, and it is mixed with silicious sand and some alk® 0 (matter or sea-salt. The most ordinary proportions are, 4 of sand, 1 of sea-salt, an ,calcine. Chaptal states that he has obtained a very fine product from 100 parts o ^cine, made by calcining equal parts of lead and tin, 100 parts of ground flint, anparts of pure subcarbonate of potash. In either case, the mixture is put into a c , r1 ' i, esor laid simply on a stratum of sand, quicklime spontaneously slaked, or w°° gtplaced under a pottery or porcelain kiln. This mass undergoes a semi-vitrifica i ’ a jeven a complete fusion on its surface. It is this kind of frit which serves as a ra‘to almost every enamel; and by varying the proportions of the ingredient, more {lUmore opaque, or whiter enamels are obtained. The first of these qualities depenthe quantity of sand or flux, and the other two on that of the tin. , v _ ar e

The sea-salt employed as a flux may be replaced either by salt of tartar, ojpotash, or by soda; but each of these fluxes gives peculiar qualities to the ename • ^ ^

Most authors who have written on the preparation of enamels, insist a g re