compound. This method yields pretty accurate results; that obtained for the lime is, however, always a trifle too high, at the expense of the magnesia, since it is impossible to free the sulphate of lime entirely from the solution of gypsum used in the process.

THIRD GROUP.

STRONTIA OXIDE OF CHROMIUM.

I. SEPARATION OF THE OXIDES OF THE THIRD GROUP FROM

THE ALKALIES.

$118.

a. Alumina and oxide of chromium are separated from AMMONIA by ignition; the process is conducted according to the directions given at § 115, 2. (Separation of ammonia from soda and potass.)

b. The separation of POTASS and SODA from oxide of chromium and alumina, is equally simple. The compound under examination is mixed with a certain amount of sal ammoniac, and ammonia added to alkaline reaction; the mixture is then heated, and finally filtered. The precipitate contains the oxide of chromium and the alumina in the form of hydrates. (§ 78, a., and $ 79, a.)

The filtrate which contains the alkalies is evaporated to dryness, and the residue ignited; every trace of ammoniacal salt present is removed in this manner.

II. SEPARATION OF THE OXIDES OF THE THIRD GROUP FROM THE ALKALINE EARTHS.

§ 119.

We have no method of separating oxide of chromium and alumina simultaneously from the alkaline earths; the precipitation of the former two substances may indeed be effected by am

monia in presence of sal ammoniac; but the oxide of chromium combines invariably with small portions of the alkaline earth, at the moment of its precipitation, and the results are therefore too high. We must therefore proceed first to the separation of the alumina from the alkaline earths, and secondly, to the separation of the oxide of chromium from the latter substances.

A. SEPARATION OF ALUMINA FROM THE ALKALINE EARTHS. a. Separation of alumina from the whole of the alkaline earths.

The solution of the compound under examination is mixed with chloride of ammonium, and pure ammonia (free from carbonic acid) added, until it begins to predominate slightly; heat is then applied, and the fluid subsequently filtered off from the precipitate formed, (the funnel must be kept covered carefully with a glass plate, during filtration, to prevent the access of the carbonic acid of the air);-the precipitate is collected upon the filter, and quickly washed with hot water. The filtrate contains the whole of the barytes, strontia, and lime, together with the greater part of the magnesia, the remainder being combined with the precipitated hydrate of alumina. The precipitate is dissolved in dilute hydrochloric acid; and pure potass ley added, until the precipitate of hydrate of alumina, which forms at first upon the addition of the potass ley, is completely redissolved; the solution is now heated, and subsequently filtered hot from the precipitated hydrate of magnesia, which is then thoroughly washed with hot water, and subsequently dissolved in some hydrochloric acid; the solution is added to the first filtrate, which contains already the greater part of the magnesia, and likewise the whole of the barytes, strontia, and lime. The alkaline earths are finally separated from one another according to the directions given at $117. The alumina is precipitated from the potass solution, by strongly acidifying the latter with hydrochloric acid, and adding finally ammonia. (§ 78, a.)

b. Separation of individual alkaline earths from alumina.

1. BARYTES AND STRONTIA FROM ALUMINA. The barytes and strontia are precipitated with sulphuric acid; (§§ 74, 75;) the alumina in the filtrate is precipitated with ammonia, having previously added sal ammoniac to the filtrate. (§ 78, a.) This method is preferable to that given at a, for the separation of barytes from alumina, but not for that of strontia from the latter substance.

2. LIME FROM ALUMINA.

Ammonia is added to the solution of the compound under examination until a permanent precipitate begins to form, which is redissolved by the addition of acetic acid; some acetate of ammonia is now added, and finally oxalate of ammonia in slight excess. (§ 76, 2, b, B.) The fluid is filtered off from the precipitated oxalate of lime, and the alumina in the filtrate precipitated with ammonia, after the addition of sal ammoniac (§ 78, a.)

3. MAGNESIA FROM ALUMINA.

Bicarbonate of potass is added to the solution of the compound under examination as long as a precipitate continues to form; (this process should be conducted in a tall flask, to avoid loss by spirting.) The fluid is then filtered off from the precipitate, which consists of hydrate of alumina mixed with potass. The filtrate contains the whole of the magnesia. The precipitate is dissolved in hydrochloric acid, sal ammoniac added to the solution, and the alumina finally precipitated with ammonia. (§ 78, a.) The magnesia in the filtrate is precipitated as basic phosphate of magnesia and ammonia. (§ 77, 2.) The results are accurate.

This method may be applied also to effect the joint separation of lime and magnesia from alumina; in this case, however, the results are accurate only when the amount of lime present is inconsiderable; since, if a large amount of lime be present, a small portion of bicarbonate of lime will invariably precipitate in conjunction with the hydrate of alumina, (owing to the difficult solubility of bicarbonate of lime in water.) To effect the joint separation of lime and magnesia from ammonia, by means of

bicarbonate of potass, it is necessary to dilute the solution considerably before the addition of this reagent.

B. SEPARATION OF OXIDE OF CHROMIUM FROM THE ALKALINE EARTHS.

The most appropriate method of separating the oxide of chromium from the whole of the alkaline earths, is to convert the oxide of chromium into chromic acid. To effect this conversion the finely levigated substance is mixed with two parts of pure carbonate of soda and two and a half parts of nitrate of potass; the mixture is introduced into a porcelain crucible and heated to fusion. The fused mass is extracted with hot water; the chromium dissolved in the form of an alkaline chromate, and is finally determined according to the directions given at § 99. The residue contains the alkaline earths in the form of carbonates, or in the caustic state (magnesia).

I need hardly mention here that BARYTES and STRONTIA may also be separated from oxide of chromium by means of sulphuric acid.

III. SEPARATION OF OXIDE OF CHROMIUM FROM ALUMINA.

$120.

The known method of separating alumina from oxide of chromium by protracted boiling of the potass solution of these two substances, (upon which the oxide of chromium precipitates a hydrate, whilst the alumina remains in solution,) yields very indifferent results, since the precipitated hydrate of oxide of chromium contains invariably an admixture of alumina. The separation of oxide of chromium from alumina is effected most completely by the method described at 119, B. (separation of oxide of chromium from the alkaline earths). The residuary alumina which remains upon extracting the fused mass with water, contains alkali; to free it from this admixture it is to be dissolved in hydrochloric acid, sal ammoniac added to the solution, and the alumina finally precipitated with ammonia. (§ 78, a.)

FOURTH GROUP.

OXIDE OF ZINC-PROTOXIDE OF MANGANESE- PROTOXIDE OF

NICKEL-PROTOXIDE OF COBALT-PROTOXIDE OF IRON-PEROXIDE OF IRON.

I. SEPARATION OF THE OXIDES OF THE FOURTH GROUP FROM THE ALKALIES.

A. FROM AMMONIA.

§ 121.

The same method is applied which serves to separate oxide of chromium and alumina from ammonia. (§ 118, a.) Should this method be inapplicable, in the case of chloride of iron and ammonium for instance, the solution is to be divided into two portions, and the metal determined in the one, the ammonia in the other portion, (the latter according to § 73, 2).

B. FROM POTASS AND SODA.

a. Separation of the whole of the oxides of the fourth group from potass and soda.

The solution is mixed with ammonia till neutral, and subsequently with pure colorless hydro-sulphuret of ammonia. The precipitated metallic sulphurets are filtered off from the fluid which contains the alkalies in solution. The filtrate is evaporated and the residue ignited, to remove the ammoniacal salts. The remaining alkalies are finally separated as directed § 115.

b. Separation of individual oxides of the fourth group from potass and soda.

1. OXIDE OF ZINC FROM POTASS AND SODA.

If the bases are combined with acetic acid, and no other acid is present, the zinc may be precipitated from the acid solution as sulphuret of zinc, and may thus be separated from the alkalies. (§ 80.)

2. PROTOXIDE OF NICKEL AND PROTOXIDE OF COBALT

FROM POTASS AND SODA.

The bases are converted into chlorides, the solution is evaporated to dryness, the residue introduced into a bulbous tube, and