substance being spontaneously formed on old, damp walls which contain lime, and which are exposed to the emanations from decomposing vegetable or animal

matters.

A natural saltpetre with sodium for base (nitrate of sodium) is found in great quantities in Peru and Chili, and constitutes an important article of commerce.

Nitrate of potassium is colorless, odorless, and has a cooling, saline taste; it is very soluble in water, and a concentrated solution deposits, on cooling, fine prismatic crystals, which are more or less translucent. Thrown upon burning coals, it fuses, and increases the combustion by parting with a portion of its oxygen. Concentrated sulphuric acid, poured upon nitrate of potassium, disengages white fumes of nitric acid; on the other hand, these vapors are yellow when a metallic powder, such as copper, iron, zinc, etc., has been mixed with the nitrate of potassium.

Distilled in closed vessels with more or less concentrated sulphuric acid, nitrate of potassium produces nitric acid, or aqua-fortis, of various degrees of concentration.

Before using the crude nitrates of sodium or potassium for the manufacture of aqua-fortis, it is advantageous to purify them by one or two crystallizations, in order to separate the chlorides, iodides, and bromides with which they are impurified and which remain in the mother liquors. We employ saltpetre for producing a dead lustre upon objects gilded by fire, and in the preparation of desilverizing baths.

Bicarbonate of Potassium.

Salæratus. (Hydrogen-potassium carbonate.)

This salt is white and colorless, and crystallizes either in tabular forms, like nitrate of silver, or in cubes, like

common salt or iodide of potassium. It is soluble in tepid water, without decomposition; but at the boiling point, it loses one-fourth of its carbonic acid and becomes a sesquicarbonate. At red heat it is transformed into simple carbonate, that is to say, it loses another fourth of carbonic acid.

Bicarbonate of potassium blues red litmus paper, and possesses little taste. Its cold solution is not precipitated by one of sulphate of magnesium, but an abundant precipitate is formed on applying heat, since, then, part of the carbonic acid is disengaged, and an insoluble sesquicarbonate of magnesium is formed.

This salt, which is employed for the preparation of certain baths for gilding by simple immersion, and for the preparation of cyanide of potassium, is obtained by conducting a stream of carbonic acid gas through a concentrated solution of carbonate of potassium, until the liquid is no longer rendered turbid by the addition of sulphate of magnesium, or nitrate of calcium. It crystallizes after a brief period of rest, being much less soluble than the simple carbonate.

Bicarbonate of Sodium.

(Hydrogen-sodium carbonate. Baking powder.)

The properties, uses, and preparation of this compound correspond with those of the bicarbonate of potassium.

Binoxalate of Potassium (poisonous).

(Salt of sorrel.)

Is a white crystalline salt, which occurs already formed in the sap of certain plants, and especially in that of the sorrel (Rumex acetosella), from which it is extracted by the simple concentration of the liquid.

This salt is acid and decrepitates in the fire, the oxalic acid which it contains decomposing into carbonic acid and

carbonic oxide, which last is disengaged. It enters into the composition of the silver paste for cold silvering by friction.

Bitartrate of Potassium.

(Hydrogen-potassium tartrate. Cream of tartar. Argol.) This salt occurs nearly pure in wine, from which it becomes separated in the shape of small white or red crystals, according to the color of the liquor. It is collected from the sides of wine casks, and purified by bone black, in which state it is known as cream of tartar. Before this purification, it is crude tartar or argol.

Bitartrate of potassium is acid, slightly soluble in water, and it decrepitates in the fire, where it blackens, disengaging a smell like that of burnt sugar. It is employed for the preparation of the whitening silver baths, for those of tin, and for the silvering paste by friction.

Biborate of Sodium.

(Sodium biborate. Borax. Tincal.)

This well-known compound occurs generally in the shape of colorless prismatic crystals. It occurs abundantly in natural deposits in the borax lakes of Nevada and elsewhere.

Borax is soluble in water, has an alkaline reaction, and a bitter saline taste. When heated, it fuses in its water of crystallization, which it loses, after which it swells up considerably. A further increase of heat causes it to fuse again, and it becomes transformed into a kind of transparent and colorless glass, which may be drawn out into long filaments,

Vitreous (fused) borax is a powerful solvent of most metallic oxides, which impart to it various colors. This property of dissolving metallic oxides renders borax very useful for soldering, brazing, or welding different metals.

A powerful acid, poured into a hot and concentrated solution of borax, separates the boracic acid in the form of scales. Borax ground with a little sulphuric acid and alcohol imparts to the flame of the latter a characteristic yellowish-green color.

The biborate of sodium is prepared by saturating with an excess of boracic acid a hot and concentrated solution of carbonate of sodium; or the natural mineral is simply purified by several crystallizations.

Borax is employed for restoring the shade of defective gildings, and for destroying the subsalts of silver formed in electro-silvering baths, and which injuriously affect the color of the electro-silver deposits.

India-Rubber.

(Caoutchouc. Gum-elastic.)

This substance is white when pure; but its color is generally brown or red, caused by the smoke of the fire employed in drying it.

India-rubber is extracted from the sap flowing from incisions made in the trunk of Ficus elastica or cahuca, a tree indigenous to Java. The milky sap, by settling, becomes divided into two layers, the lower one of which is liquid, and the other thicker and cream-like, and formed of a multitude of small globules. This portion is removed, washed with salt water, and spread in thin layers upon tables or over pear-shaped clay balls with a stick as a handle. The drying is done at the ordinary temperature, or with the aid of heat; and, in the latter case, the smoke colors the substance of the gum.

India-rubber is essentially elastic, and this property renders it very valuable for many purposes, as, for instance, moulding highly wrought patterns. The elasticity increases with the temperature; but at 250° to 260° Fah.

the gum melts, and furnishes a viscous liquid which seldom returns to its primitive consistency. If the temperature be raised still higher, the mass remains permanently liquid and sticky. Lastly, at a still greater heat, Indiarubber will burn with a sooty flame.

The combination of sulphur with India-rubber furnishes a peculiar product equally elastic, but free from the disadvantage of becoming softened and sticky by heat. This product is called vulcanized rubber, and, by modification of the treatment, may be converted into the substance known as "hard rubber," which is used for a multitude

of purposes.

Water, alcohol, and acids do not dissolve Indiarubber; on the other hand, ethers, bisulphide of carbon, essential oils, and benzole dissolve and leave it behind after the volatilization of the solvent. These solutions give us the means of obtaining very delicate moulds; but, in this case, we should only apply very thin and successive coats of the solution, otherwise the exterior surface being the first to solidify, will prevent the drying of the intermediary layers.

Chloride of Silver.

(Argentic chloride. Horn silver. Muriate of silver.) This substance, when obtained by precipitation, is pure white, but soon turns blue and then black by exposure to the light; to prevent this decomposition it should be kept in blue or opaque bottles. It fuses at a high temperature, and acquires the appearance of horn, from which it derives its name of horn silver.

When chlorine water, hydrochloric acid, or any soluble chloride is introduced into a solution of a silver salt there is immediately produced a copious precipitate of chloride of silver, which is insoluble in water and in con