sulphurized by it. Sulphuretted-hydrogen is sometimes employed for the production of " oxidized" silver.

Sulphurous Acid.

Is gaseous, colorless, and possesses an acid taste. Its odor is penetrating and suffocating, and is that produced by burning sulphur in the open air. One volume of water, at ordinary temperatures, dissolves about thirtythree volumes of the gas, and acquires its characteristic properties.

Sulphurous acid decolorizes and bleaches many organic substances. It reduces many metallic salts by appropri ating their oxygen; it may therefore be employed, possi bly, more advantageously than sulphuretted-hydrogen, for metallization by means of nitrate of silver or chloride of gold. It is employed in the preparation of the sulphites and bisulphites, which we shall study further on.

It is obtained by burning sulphur in the air, from which it takes the two equivalents of oxygen necessary for its composition; or by decomposing, in a closed vessel

Fig. 167.

(Fig. 167), concentrated sulphuric acid, by means of some substance possessing a great affinity for oxygen. Sulphur, charcoal, dry sawdust, copper, mercury, etc., may be used for the purpose, although copper is generally preferred.

This metal, finely divided (chips, filings, or turnings), is rapidly corroded, produces a pure gas, and leaves a residuum of sulphate of copper useful for galvanoplastic operators.

Sulphuric Acid (poisonous).

(Oil of vitriol.)

This compound, the most powerful and useful acid in the arts, may exist in the solid or anhydrous state. It is always used in a more or less hydrated form. Its name of oil of vitriol comes from its oily consistency, and from the green vitriol (sulphate of iron) from which it wasformerly obtained by distillation in closed vessels. It is now produced in large quantities, by oxidizing in lead chambers moist sulphurous acid by the vapors of nitric acid.

Commercial sulphuric acid generally marks 66° of the hydrometer of Baumé. It is colorless, dense, flowing like oil, with feeble odor, and strong acid taste. It decomposes nearly all salts, and extracts water from most substances which contain its elements. It attacks and blackens organic substances, becoming itself more or less dark in color thereby. The particles of dust flying in the air and falling into it are sufficient to produce this phenomenon.

Poured into a concentrated solution of hydrochloric acid, sulphuric acid absorbs the water with production of heat, and the acid first named is driven off with effervescence. It displaces all the other acids from their combinations, and forms very stable compounds with metallic oxides.

We employ it in our art at nearly every stage: for cleansing, for dipping, in the preparation of many products, in batteries, etc.

Tannic Acid.

(Tannin.)

This substance, which should not be confounded with gallic acid, although it produces this last by fermentation and other chemical reactions, appears to be already formed in gall-nuts, which are supposed to be the result of the puncture of certain insects upon oak leaves. Tannic acid exists also in many vegetables, and especially in the bark of certain trees, such as the oak, hemlock, chestnut, elm, etc.

This acid is generally prepared by digesting powdered gall-nuts, at a temperature of about 85° Fah., in commercial ether, in a closed vessel. After about eight days, the settled liquid, which is quite syrupy, is decanted and spread upon many dishes, which are put into a stove. The ether is evaporated, and the nearly pure tannic acid remains in the form of uncrystallized scales, which are light, thin, yellowish, and with nacreous lustre. It is purified by solution in boiling water, which, on cooling, allows it to deposit in the shape of needle-like crystals.

Tannic acid possesses the singular property of coagulating, and rendering insoluble, certain gums, gluten, and gelatine especially, the latter being transformed into leather, which will not putrefy.

It will be remembered that in the article on moulding with gelatine, we indicated the employment of a small proportion of this acid for preventing the softening and solvent action of the baths. Gallic acid cannot take the place of tannic acid for this purpose.

Alloys.

These are combinations, or mixtures, effected by the fusion of two or more different metals in definite propor

tions. The hydroplastic operator employs them so constantly that it is important that he be acquainted with the composition of the most usual alloys, and that he learn the preparation of several of them, which, like the fusible alloys of Darcet, will often be serviceable.

All the metals are melted together in a crucible, stirred with an iron rod, and cast. Various proportions of mercury are added to these alloys, when great fusibility is desired.