trace of organic substance is sufficient to impart to it a greenish or yellowish tinge, as if it were colored by an iron salt.

The electro-plater employs it for preparing the chlorides of certain metals, such as silver, zinc, etc.; it enters into the composition of aqua-regia; and the common salt, added to certain cleansing acids, depends for its utility on the formation of a small quantity of hydrochloric acid. It is prepared by introducing into a glass flask (Fig. 165) common salt and an excess of commercial sulphuric

gas is col

acid. A gentle heat is gradually applied, and the lected and absorbed in distilled water contained in a series of tubulated flasks. These receivers should be kept constantly cooled by a stream of water, or by ice, as otherwise the elevation of temperature, caused by the combination of the water and acid, would prevent the liquid from becoming thoroughly saturated. The first bottle contains. but little water, and is intended to arrest the impurities mechanically carried by the gas. The generating vessel after the operation contains acid sulphate of sodium.

Aqua-Regia (poisonous).

(Nitro-muriatic acid.)

When nitric and hydrochloric acids are mixed, there are formed, besides water, free chlorine and hyponitric acid. When heated this liquid disengages yellowish fumes. This mixture is called aqua-regia because it dissolves gold, which was formerly named the king of metals.

Chlorine alone will also dissolve gold, and as the resulting salt is the same as that obtained by its treatment with aqua-regia, we should infer that the dissolving action of aqua-regia is due to the chlorine it contains, and which it liberates in the nascent state. Moreover, aquaregia dissolves all the metals, and produces salts which are chlorides, and not nitrites or hyponitrates.

The composition of aqua-regia is varied, according to the substances which we desire to dissolve. For gold, we mix 1 part of nitric acid of 40° Baumé, with 2 parts of hydrochloric acid of 22° Baumé; for platinum, 3 of nitric acid to 5 of hydrochloric acid.

Aqua-regia is employed in the preparation of the chlorides of copper, platinum, gold, etc.

Hydrofluoric Acid (very poisonous).

(Fluoric acid.)

This acid is gaseous when freed from its combinations, but condenses, by cooling, into a colorless liquid possess

Fig. 166.

บ

ing a penetrating smell and strongly acid taste. It attacks glass and silicates, and combines with their silica to form a new acid called hydrofluosilicic or fluosilicic acid.

Hydrofluoric acid is prepared by decomposing in a retort of

lead or platinum (Fig. 166), a paste of fluoride of calcium (fluor-spar) with sulphuric acid. The various joints of the retort are carefully luted with clay or plaster of Paris, and the receiver is made of a bent tube of one of these metals, immersed in a mixture of 3 parts of broken ice and 2 of common salt, or, more simply, into ice alone. The end of the receiver is perforated with a very small hole only, in order to aid the condensation by a moderate pressure. A gentle heat is applied at the bottom of the

retort.

This acid is kept in lead bottles which are but slightly acted upon, or in flasks of platinum or gutta-percha, upon which it has no action whatever.

We must carefully avoid any contact with the vapors of hydrofluoric acid, otherwise, after a few hours, even the less sensitive parts of the skin will be covered with painful blisters. The same caution applies also to inhaling its vapors.

This acid is employed for depolishing glass, and other vitreous substances, and thus to prepare a ground more easily metallized. The parts which should not be corroded are protected by a coat of wax.

When a level surface is to be operated upon, a pane of glass for instance, the easiest process consists in covering it entirely with wax, and drawing the pattern with a pointed tool, which should penetrate to the glass. The object is then placed upon a shallow dish of lead containing the mixture for the production of the acid. At ordinary temperatures, or more rapidly with the aid of feeble heat, the vapors of the acid attack the glass where it has been uncovered. On the other hand, if large surfaces are to be depolished, the acid vapors are produced in a leaden box of suitable size, and perfectly luted. The objects may also be laid in a trough of lead or gutta percha filled

with a more or less diluted solution of hydrofluoric acid. This last process is employed for the production of patterns, hollow or in relief, upon "flashed" glass, that is, glass having a surface coat of differently colored glass. Glass pearls or beads, treated in the same manner, acquire a beautiful dead lustre.

Stearic Acid.

(Stearine.)

We may consider all fatty substances of animal origin, and tallow particularly, as composed of a base called glycerine, which is united with oleic, margaric, and stearic acids. The last one is the most important for us, since we take moulds with it, and render impervious certain substances which otherwise would be penetrated and injured by the sulphate of copper of the galvanoplastic baths.

This acid is white, nearly without taste and smell, and more or less greasy to the touch according as it is more or less completely deprived of margaric and oleic acids. Its fracture is rarely crystalline, but often granular. It melts at a temperature from 158° to 167° Fah. to a clear liquid, which again solidifies by cooling. It is this property which renders stearic acid valuable for taking casts. If it be too greasy it sticks to the pattern, and especially to plaster of Paris coated with plumbago; in this case, it should be mixed with a certain proportion of wax or sper maceti. When too dry, on the other hand, it contracts considerably in cooling, often breaks, and the galvanoplastic deposits have a crystalline surface. This defect is corrected by the addition of tallow or olive oil.

Heated to 535 to 575° Fah., stearic acid is partly charred, becomes brown, and gives off certain volatile pro

ducts; it is then too dry, and much less valuable for our purposes.

The manufacturers of stearine candles extract this acid from tallow.

Sulphuretted-Hydrogen. Sulphydric Acid (poisonous). (Hydrosulphuric Acid.)

This acid is gaseous, but may be dissolved in water, which absorbs two or three times its own volume of it at the ordinary temperature, and which then acquires the same properties as the gas itself.

Sulphuretted-hydrogen is colorless; its smell is fetid, and resembles that of rotten eggs. It is slightly acid, and reddens blue litmus paper, but the primitive color reappears on heating. It precipitates many of the metals from their solutions as sulphides, of various colors-white, black, yellow, etc. Some of these sulphides appear to be good conductors of electricity.

Sulphuretted-hydrogen, which is exceedingly poisonous, may be obtained by the reaction of hydrochloric acid, or diluted sulphuric acid, upon many sulphides, such as those of antimony, iron, barium, strontium, etc. The gas is collected under receivers filled with mercury, or is dissolved in water with the aid of a Woulf's apparatus, such as that described in the manufacture of hydrochloric acid. The distilled water employed should be deprived of air by ebullition, otherwise the solution will be milky from the partial decomposition of the acid, the hydrogen of which unites with the absorbed oxygen, while the sulphur is separated.

this

We should carefully avoid bringing in contact with gas, not only metallic salts, and gilt or silvered articles, but likewise pure gold and silver, which are rapidly