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gone the spiritous fermentation. It is principally employed in the preparation of varnishes, being the only solvent of many resinous bodies ; also medi. cinally.

SEALING WAX.

A composition of gum-lac, melted and prepared with resins, and coloured, the best red with vermilion, and the black with ivory black, but a variety of colours are now made. There are two kinds, a hard one used for sealing letters, &c. and a soft kind designed for receiving the impressions of seals of office to charters, patents, &c. Inferior qualities are also made for common purposes.

STARCH.

A farinaceous sediment found at the bottom of vessels in which wheat has been steeped. It is often made directly from the wheat, where a better sort is required, by putting pure wheat into tubs of water and fermenting it by the heat of the sun; the water being frequently changed during the six or seven days it is thus exposed. When sufficiently softened, it is poured into canvas bags and beaten, by which the mealy parts are extracted, and the liquid flour falling into a vessel placed under to receive it, is soon

covered with a reddish fluid, which is skimmed off, and pure water added. After being briskly agitated, as the sediment subsides, the water is drained off, and afterwards the starch is made into loaves, which are dried and then cut into pieces for sale. What remains after the starch is separated from the flour is called gluten, which is a tough elastic substance, without taste, and of a dirty white colour. It appears to be very slightly soluble in cold water, and extremely nutritive. Wheat is indebted for its superiority over other grain to the quantity of gluten it contains. Sugar may be obtained from Starch, but it does not crystallize like common sugar, the grains appearing more in the form of spheres like honey. A pound of good wheat contains 10 oz. of starch, 6 dr. of gluten, and 2 dr. of sugar. Starch may also be obtained from other vegetable substances, as potatoes, tapioca, sago, &c. all of wbich owe their nutritive powers to the starch they contain. Pulverized Starch is the basis of hair powder, and it was first introduced for stiffening linen in the reign of Queen Mary. (See Arrow Root.)

SLATE, OR STEGANIA.

A fossil found in many mountains, and likewise in some counties of Britain and Ireland, near the surface of the soil, the largest masses of wbich are dug out of one of the Islands of the Hebrides, whence it is exported to England, the West Indies, &c. Of this fossil there are several varieties, principally distinguished by their colour. It is a compact stone, composed of a multitude of eveu plates, which may be easily split, and it is chiefly applied to the covering of houses, as it forms a strong and elegant roof.

SODA.

A mineral alkali, procured from a marine plant, called the Salsola Soda, growing on cliffs, which is collected and burnt by the Spaniards for the manufacture of Barilla, which name it also bears. Soda of an inferior kind is procured by burning the sea weeds on our own coast, particularly in Scotland, from which kelp is produced. It is the basis of common salt, and when pure, is called Carbonate of Soda.

SPONGE.

A soft, porous, light substance, which easily imbibes water, and this, added to its elasticity, renders it very useful. The greatest portion of what we use, is brought from Smyrna and Aleppo. It is common in the Mediterranean and other seas, and

adheres in large masses to rock, stones, and shells. It is chiefly procured by divers; and the people employed to collect it, perceiving it shrink when torn from the rocks, considered it as a symptom of sensation, and this opinion prevailed in the days of Aristotle and Pliny. Sponge was afterwards supposed to be a vegetable substance; then the production of animals, and lastly, it was concluded that Sponge was an animal plant, belonging to the order of Zoophites, (55) and that the openings of the branched tubes were the mouths by which it received nourishment. There are various species-among them the Branched Sponge, the Cox-comb Sponge, the Tow Sponge, &c.

SILK.

A fibrous production, spontaneously spun from a caterpillar, called Silkworm, a native of China, where it propagates upon the leaves of the mulberry tree.

Great numbers are bred in Italy, and attempts to rear them in Britain have been made but without success; the coldness of the climate preventing the attainment of so desirable an object. The colour of the Silk varies either from the food or climate; that brought from China being almost white, while other kinds appear of a golden hue. The first silk mill was erected on an island in the river Derwent, by John Lombe, in 1717. By corrupting some of the workmen be obtained a sight of the machinery used in Italy, as the Italians, who were long in exclusive possession of the art, prohibited admission to any foreigner. To the labours of this invaluable insect, we owe the luxury of our finer clothing, silks, crapes, gauzes, ribbons, &c. being manufactured from its production. The machinery is now principally worked by steam engines.

SILVER.

A perfect metal, and the whitest of all. It is harder and more elastic than lead, tin, or gold, but less so than copper, platina, or iron. It becomes hard by hammering, but is easily reduced to its former state by annealing (56). It is found in a native state, and also combined with other metals. It is found in some parts of Cornwall and Devon. sbire; but in very great abundance in the mines of Mexico and Peru. The surface of Silver easily becomes tarnished by exposure to the air. It is employed in coinage; in the manufacture of various articles, and sometimes in medicine. It readily combines with gold, without materially diminishing its ductility, and such alloy is generally employed in gold coinage. The methods of giving a silvery

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