7. HYDROFLUORIC ACID.

The direct estimation of hydrofluoric acid is uniformly effected by weighing the FLUORIDE OF CALCIUM.

Fluoride of calcium forms a gelatinous precipitate, difficult of edulcoration. If it is digested with ammonia, previously to filtration, it becomes more dense and less gelatinous. It is wholly insoluble in water, and likewise in aqueous solutions of the alkalies; it is barely soluble in dilute, but more readily in concentrated hydrochloric acid. When acted upon by sulphuric acid, it is decomposed, and, its elements transposing with those of the decomposing acid, gypsum and hydrofluoric acid are formed. Fluoride of calcium is unalterable in the air and at a red heat; it fuses at a very intense red heat.

The direct estimation of carbonic acid is usually effected by weighing the carbonate of lime. For the properties of this substance, vide § 47.

9. SILICIC ACID.

Silicic acid is uniformly weighed in its insoluble modification. The insoluble modification is artificially prepared by evaporating a solution of the soluble modification of silicic acid in water or in any volatile acid, (with the exception of hydrofluoric acid); in this process we obtain, at first, the silicic acid as a gelatinous hydrate, which, upon further evaporation to dryness and subsequent exsiccation, loses its water of hydration, and becomes converted into the insoluble modification. This forms a white powder, insoluble in water and in acids; it is soluble in potass

ley, and likewise in solutions of the fixede arbonated alkalies. It is perfectly unalterable in the air and at a red heat, and requires the very highest degree of heat for its fusion. It does not alter vegetable colors.

Hydrochloric acid is almost uniformly determined in the form of CHLORIDE OF SILVER. For the properties of this substance, vide § 56.

2. HYDROBROMIC ACID.

Hydrobromic acid is uniformly determined as bromide of

silver.

BROMIDE OF SILVER, prepared in the humid way, is a yellowish-white precipitate; it is wholly insoluble in water and in nitric acid, but tolerably soluble in ammonia; it dissolves in hot solution of sal ammoniac, but very sparingly in solution of nitrate of ammonia. When acted upon by chlorine, no matter whether in the dry or in the humid way, it is decomposed, chloride of silver is formed, and bromine separated. Exposed to light, it turns gradually grey, and finally black. Exposed to the action of heat, it fuses into a fluid of a reddish color which, upon cooling, solidifies, forming a yellow mass of horn-like appearance.

When brought into contact with zinc and water, bromide of silver is decomposed; a spongy mass of metallic silver subsides, and the solution contains protobromide of zinc.

Hydriodic acid is usually determined in the form of IODIDE OF SILVER, and sometimes in that of PROTIODIDE OF PALLADIUM.

a. Iodide of silver, produced in the humid way, is a bright yellow precipitate, insoluble in water and in dilute nitric acid, and barely soluble in ammonia. It is decomposed by chlorine, both in the dry and in the humid way. Hot concentrated nitric and sulphuric acid convert it, but with difficulty, into the corresponding nitrate and sulphate of silver, with expulsion of the iodine. Iodide of silver acquires a black color when exposed to light. When heated, it fuses without decomposition into a reddish fluid which, upon cooling, solidifies, forming a yellow mass that may be cut with a knife. When brought into contact with zinc and water, it is decomposed; protiodide of zinc is formed, and metallic silver separates.

b. The protiodide of palladium, produced by precipitating solution of the iodide of one of the alkali metals, with protochloride of palladium, is a deep brown-black flocculent mass. This precipitate does not dissolve in water; it is slightly soluble in saline solutions, (solution of chloride of sodium, chlo

ride of magnesium, chloride of calcium, &c.,) it is insoluble in dilute hydrochloric acid. It is unalterable in the air; when dried simply in the air, it contains 1 equivalent of water-505 per cent. Dried at a high temperature, (158° to 176°,) or in vacuo, it loses this water completely, without suffering any loss of iodine. Dried at 2120, a trace of iodine escapes, and finally if the precipitate of protiodide of palladium is exposed to a heat of from 572° to 752°, all its iodine is completely expelled. It may be washed with hot water without losing its iodine.

The direct estimation of this acid is uniformly effected by weighing the CYANIDE OF SILVER. For the properties of this substance, vide § 56.

5. HYDROSULPHURIC ACID.

Sulphuretted hydrogen.

The forms into which sulphuretted hydrogen, or the sulphur in metallic sulphurets, are converted for the purpose of quantitative estimation, are SULPHARSENIOUS ACID, and SULPHATE

BARYTES.

a. Sulpharsenious acid, vide § 66.

b. Sulphate of Barytes, vide § 45.

OF

ACIDS OF THE THIRD GROUP.

$ 69.

1. NITRIC ACID, and 2. CHLORIC ACID.

These two acids are determined invariably in an indirect way.

We have had occasion already, in the preceding paragraphs, to treat of those compounds which serve for their indirect quantitative estimation.

SECTION IV.

In the preceding section, we have examined the composition and properties of the various appropriate forms and combinations in which substances are to be separated from others, or into which they are to be converted, for the purpose of determining their absolute and relative weight and proportion. We have now to consider the means of converting substances into these appropriate forms and combinations.

For the sake of greater clearness and simplicity, we will divide this part of the work into two sections, confining ourselves, in the first, to the exposition of the various methods applied to effect the quantitative estimation of substances, and deferring to the next section the consideration of the means best adapted for the separation of substances from one another.

QUANTITATIVE DETERMINATION OF SUBSTANCES.

§ 70.

We have to deal here exclusively with compounds consisting of one base and one acid, or of one metal and one metalloid.

In the quantitative estimation of substances, we have to study two points, viz., first, the most appropriate manner of dissolving the analysed substance, either in its isolated state, or in its various combinations,―ITS SOLUTION; and secondly, the methods best adapted to convert the analysed substance into a ponderable form,-ITS ACTUAL DETERMINATION OF WEIGHT.