monium, and in sulphuret of potassium; it dissolves readily in hot concentrated hydrochloric acid. Heated, with exclusion of air, it loses its water of hydration, and becomes converted into anhydrous sulphuret of tin; when it is exposed to the continued action of a gentle heat, it becomes transformed into sulphurous acid, which escapes, and peroxide of tin,, which remains behind.

c. Hydrated bisulphuret of tin is produced as a bright yellow precipitate, which, upon drying, assumes a darker tint. It is insoluble in water, but dissolves slightly in water impregnated with sulphuretted hydrogen; it dissolves with difficulty in ammonia, but readily in potass and alkaline sulphurets, and likewise in hot concentrated hydrochloric acid. When heated, with exclusion of air, it loses its water of hydration, and at the same time, according to the greater or less degree of heat applied, one-half or one equivalent of sulphur, becoming converted either into sesquisulphuret, or into sulphuret of tin; when heated very slowly, with free access of air, it becomes converted into peroxide of tin, whilst sulphurous acid escapes.

§ 66.

6. ARSENIOUS ACID; and 7. ARSENIC ACID.

Arsenic is weighed either in the form of ARSENIATE OF LEAD, or in that of SULPHARSENIOUS ACID.

a. Arseniate of lead, in a pure state, is a white powder, which begins to agglutinate into a semi-fused mass, when exposed to a gentle red-heat, at the same time transitorily acquiring a yellow tint; it fuses when exposed to a higher degree of heat. When strongly ignited, it suffers a slight diminution of weight, losing a small proportion of arsenic acid, which escapes as arsenious acid and oxygen. In analysis we have never occasion to operate upon the pure arseniate of lead, but upon a mixture of it with free oxide of lead.

b. Sulpharsenious acid forms a precipitate of a rich yellow color; it is insoluble in water, and but very sparingly soluble in

sulphuretted hydrogen water. When boiled with water, or when kept in contact with water during several days, it undergoes a very superficial decomposition: a trace of arsenious acid dissolves in the water, and a minute proportion of sulphuretted hydrogen is disengaged. This does not interfere, however, with the washing of the precipitate. The precipitate may be dried at 212°, without undergoing decomposition; all the water which it contains is expelled at that temperature. When exposed to a stronger degree of heat, sulpharsenious acid transitorily assumes a brown red color, fuses, and finally rises in vapor, without suffering decomposition. It dissolves readily in alkalies and alkaline sulphurets, and likewise in aqua regia, but it is barely soluble in concentrated boiling hydrochloric acid. When acted upon by

red fuming nitric acid, it is transformed into arsenic acid and sulphuric acid.

B. FORMS AND COMBINATIONS, IN WHICH INDIVIDUAL ACIDS ARE SEPARATED, FOR THE PURPOSE OF DETERMINING THEIR WEIGHT.

ACIDS OF THE FIRST GROUP.

§ 67.

1. ARSENIOUS ACID and ARSENIC ACID-vide § 66.

2. CHROMIC ACID.

Chromic acid is weighed either in the form of OXIDE OF CHROMIUM, or in that of CHROMate of lead.

a. Oxide of chromium.—Vide § 50 for the properties of this oxide.

b. Chromate of lead forms a precipitate of an intensely yellow color; this precipitate is insoluble in water and acetic acid, and barely soluble in dilute nitric acid, but readily so in potass ley. When chromate of lead is boiled with concentrated hydrochloric acid, it is readily decomposed, and its elements transposing with those of the decomposing acid, chloride of lead and protochloride of chromium are formed. (Addition of alcohol tends to promote this double decomposition.) Chromate of lead is unalterable in the air; it may be dried perfectly at 212°. When exposed to the action of heat, it transitorily acquires a red brown tint; it fuses at a red-heat; when heated beyond its point of fusion, it loses oxygen, and becomes transformed into a mixture of oxide of chromium and semichromate of lead. When heated in contact with organic substances, it readily yields up oxygen to the latter.

Sulphuric acid is uniformly determined in the form of SULPHATE OF BARYTES, for the properties of which, vide § 45.

4. PHOSPHORIC ACID.

Phosphoric acid may be weighed as PHOSPHATE OF LEAD, PYRO

PHOSPHATE OF MAGNESIA, BASIC PERPHOSPHATE OF IRON, PHOSPHATE and PYROPHOSPHATE OF SILVER.

a. The phosphate of lead, ordinarily obtained in the course of analysis, is not quite pure, but intermixed with free oxide of lead. The basic phosphate of lead, (PO,, 3 Pb O,) when in a pure state, exists as a white powder, insoluble in water and in

acetic acid, and equally so in ammonia, but readily soluble in nitric acid; when exposed to the action of heat, it fuses without undergoing decomposition.

b. Pyrophosphate of magnesia.-Vide § 48.

c. Basic phosphate of peroxide of iron.-The white or yellowish white precipitate which is thrown down from solutions of phosphoric acid upon the addition of perchloride of iron in excess, in conjunction with that of an alkaline acetate is of variable composition, and therefore unfit to serve for the direct determination of the phosphoric acid. But when a solution containing peroxide of iron, and phosphoric acid, the latter in excess, is acted upon by an acetated alkali, a precipitate of constant composition is formed, exhibiting the same outward appearance with that obtained in the former instance. The composition of this precipitate, after ignition, is

This precipitate is insoluble in water, but readily soluble in ammonia, and likewise in acids; it is slightly soluble in peracetate of iron. Upon boiling the latter solution, all the phosphoric acid precipitates as hyperbasic phosphate of peroxide of iron, in conjunction with the basic peracetate of iron. The hyperbasic phosphate of peroxide of iron is invariably obtained (mixed with free hydrated peroxide of iron) when a solution containing phosphoric acid and peroxide of iron, the latter in excess, is precipitated with ammonia. This precipitate is wholly insoluble in water, and in ammonia.

d. Tri-basic phosphate of silver is a powder of a rich yellow color; it is insoluble in water, but readily soluble in nitric acid, and likewise in ammonia;-it is of difficult solubility in ammoniacal salts. It is unalterable in the air. When ignited,

it acquires transitorily a red-brown color; when strongly ignited, it fuses without decomposition.

e. Pyrophosphate of silver is a white powder, unalterable and insoluble in water, even upon ebullition, but readily soluble in ammonia, and likewise in nitric acid. It is unalterable in the air; at a heat, below ignition, it fuses without decomposition, forming a liquid of a dark brown color, which, upon cooling, solidifies into a white radiated mass.

Borate of lead is the only form which interests us here. Borate of lead, in a pure state, is a white powder, of difficult solubility in water. Exposed to the action of heat, it fuses into a clear glass, without suffering decomposition. In the course of analysis, borate of lead is never obtained in a pure state, but invariably mixed with oxide of lead.

6. OXALIC ACID.

Oxalic acid is usually precipitated in the form of OXALATE OF LIME, and its proportion is generally inferred from the carbonate of lime remaining when the oxalate is heated to incipient redness, by a spirit-lamp. For the properties of oxalate of lime and of carbonate of lime, vide § 47.

L