from quite di ute solutions. But if the addition of the precipitant is preceded by that of chloride of ammonium and ammonia, a white crystalline precipitate of BASIC PHOSPHATE OF MAGNESIA AND AMMONIA (2 Mg O, N H, O, P O,+12 aq.) will separate, even from very dilute solutions of magnesia; its separation may be greatly promoted and accelerated by vigorous stirring with a glass rod: even should the solution be so extremely dilute as to forbid the formation of a precipitate, yet the lines of direction in which the glass rod has moved along the side of the vessel will, after the lapse of some time, appear distinctly as white streaks. Water and solutions of salts of ammonia dissolve the precipitate but very slightly; but it is readily soluble in acids, even in acetic acid. In water containing ammonia it is as good as insoluble.

In

8. Oxalate of ammonia produces no precipitate in highly dilute solutions of magnesia; in less dilute solutions no precipitate is formed at first, but after some time crystalline crusts of various oxalates of ammonia and magnesia make their appearance. highly concentrated solutions oxalate of ammonia very speedily produces precipitates of. oxalate of magnesia (2 Mg O, C, O+4 aq.), which contain small quantities of the above-named double salts. Chloride of ammonium, especially in presence of free ammonia, interferes with the formation of these precipitates; but, as a rule, it does not absolutely prevent it.

9. Sulphuric acid and hydrofluosilicic acid do not precipitate salts of magnesia.

10. If magnesia, or a salt of magnesia, is moistened with water, heated to redness on a charcoal support, then moistened with 1 drop of solution of nitrate of protoxide of cobalt, and again heated at first to gentle redness, ultimately to intense redness, in the oxidation flame, a pinkish mass is obtained, the color of which becomes distinctly apparent only upon cooling, but is never very intense. Alkalies, alkaline earths, and heavy metallic oxides prevent the reaction. 11. Salts of magnesia impart no coloration to the flame.

§ 102.

Recapitulation and remarks.-The difficult solubility of the hydrate of magnesia, the ready solubility of the sulphate, and the disposition of salts of magnesia to form double salts with ammoniacal compounds, are the three principal points in which magnesia differs from the other alkaline earths. To detect magnesia we always first remove the baryta, strontia, and lime, if these bodies happen to be present.

This is accomplished most conveniently (because the latter bases are at the same time procured in the form best adapted for further examination) by precipitating with carbonate of ammonia with addition of some caustic ammonia and chloride of ammonium, and

gently heating. If the solutions are moderately dilute and the precipitate is shortly brought upon a filter, it consists of carbonates of baryta, strontia, and lime, while all the magnesia remains in solution and is found in the filtrate. Since, however, chloride of ammonium dissolves carbonate of baryta and even carbonate of lime to some, though but a slight extent, small quantities of these bases pass into the filtrate, and when but traces of them are present they may remain entirely in solution.

In exact investigations it is therefore advisable to divide the filtrate into three portions, in one of which any dissolved baryta is detected by dilute sulphuric acid, while the second is tested for lime by means of oxalate of ammonia. In case these reagents produce no turbidity, even after the lapse of some time, the third portion is examined for magnesia by means of phosphate of soda. If, on the other hand, a precipitate appears in either of the first two portions, it is filtered off after it has completely subsided, and the fi'trate is tested for magnesia. If both the first and second portions are turbid they are mixed together, after some time are filtered, and the filtrate is tested with phosphate of soda. To prove that a precipitate produced by oxa'ate of ammonia is really oxalate of lime, and not an oxalate of ammonia and magnesia, it is dissolved in a little hydrochloric acid, and a few drops of dilute sulphuric acid and some alcohol are added, when sulphate of lime should separate, if lime were present.

The precipitate by carbonate of ammonia may be examined for baryta, strontia, and lime, as follows:

It is dissolved, after washing, in dilute hydrochloric acid. To a small portion of the liquid, solution of sulphate of lime is added; an immediate precipitate is proof of the presence of BARYTA. The rest of the hydrochloric solution is evaporated to dryness and the residue is treated with absolute alcohol (§ 19). Chloride of barium mostly remains undissolved while the chlorides of strontium and calcium enter into solution. The alcoholic solution is mixed with an equal volume of water and some drops of hydrofluosilicic acid; the last traces of baryta are thus thrown down if the mixture is allowed to stand for several hours. To the alcoholic filtrate, sulphuric acid is added which precipitates from it strontia and lime. The precipitated sulphates are collected on a filter, washed with weak alcohol [4 volumes of alcohol of 92 per cent. mixed with 5 volumes of water], and converted into carbonates by boiling with solution of carbonate of soda. The carbonates thus obtained are well washed with water, dissolved in a little Lydrochloric acid, the solution is evaporated to dryness, and the residue is taken up by a little water. The aqueous solution is filtered, if necessary, and divided into two portions. To one of these, solution of sulphate of lime is added; a precipitate formed after some time, it may be

a long time, indicates STRONTIA. The other portion is treated with solution of sulphate of potassa, heated to boiling and filtered, to remove strontia. The filtrate from the sulphate of strontia is tested with oxalate of ammonia for LIME. In precipitating strontia by sulphate of potassa, a portion of the lime may also be separated if it is present in large quantity, but enough always remains in the filtrate to detect with certainty.

The best way of detecting the alkaline earths, when in the form of phosphates, is to decompose these latter by means of sesquichloride of iron with the addition of acetate of soda (§ 145). The oxalates of the alkaline earths are converted into carbonates by ignition, preparatory to the detection of the individual earths which they contain.

A mixture of the sulphates of the alkaline earths is first washed with small quantities of boiling water. All the sulphate of magnesia and a small quantity of sulphate of lime thus pass into solution. The residue, which must be finely pulverized, is digested, as Rose recommends, for 12 hours in a cold solution of carbonate of ammonia, or is boiled 10 minutes in a solution of one part of carbonate and three parts of sulphate of potassa, thrown on a filter, thoroughly washed, and then treated with dilute hydrochloric acid, which dissolves the carbonates of lime and strontia, and likewise a slight trace of baryta (Fresenius); but leaves behind sulphate of baryta. The latter can be decomposed by fusion with carbonate of soda. The solutions thus obtained are to be further tested as previously directed.

By aid of the spectroscope, baryta, strontia, and lime may be detected, even when occurring together, far more easily than by following the highly instructive but somewhat tedious processes of the humid method. The substance under examination is brought into the flame of the gas-lamp, either directly or after moistening with hydrochloric acid.

[Minute quantities of baryta and strontia give no spectral lines in presence of much lime. Engelbach directs that the mixed carbonates be strongly ignited by means of a blast-lamp for a few minutes, whereby, in presence of carbonate of lime, baryta and strontia readily become caustic. The mass is extracted with a little distilled water, the solution evaporated to dryness with hydrochloric acid and the residue examined in the spectroscope.]

The earths just named may also be severally recognized in their mixtures, without the spectroscope, by observing the coloration they impart to the flame. The substance is repeatedly moistened with sulphuric acid and cautiously dried. It is then brought into the zone of fusion of the gas flame. After any alkalies that may be present in the mixture have volatilized, the baryta coloration appears by itself. (§ 98, 10.)

After it has entirely disappeared, and the substance on moistening with hydrochloric acid gives, at the moment when the latter evaporates, no more a flame, that seen through the green glass appears blue-green, the substance is again moistened with hydrochloric acid, and its flame examined by help of the green glass (§100, 8), for lime, and with the blue glass for strontia.—Merz.

§ 103.

THIRD GROUP.

Of common occurrence: ALUMINA, SESQUIOXIDE OF CHROMIUM. Of rare occurrence: GLUCINA, THORIA, ZIRCONIA, YTTRIA, TERBIA, ERBIA, OXIDES OF CERIUM, LANTHANUM, DIDYMIUM; TITANIC, TANTALIC, AND HYPONIOBIC ACIDS:

Properties of the group.-The oxides of the third group are insoluble in water, both in the pure state and as hydrates. Their sulphides cannot be produced in the humid way. Hydrosulphuric acid therefore fails to precipitate their solutions. From salts in which the oxides of the third group play the part of base,* sulphide of ammonium as well as ammonia, throws down the hydrated oxides. This deportment with sulphide of ammonium distinguishes the oxides of the third from those of the two preceding groups.

Special Reactions of the more commonly occurring Oxides. § 104.

a. ALUMINA (Al, 0,).

1. ALUMINUM is a nearly white metal. It undergoes no oxidation on exposure to the air, and when massive scarcely oxidizes on ignition. It is very malleable and may be wrought by the file. Its sp. gr. is but 2.56. It fuses at a bright red heat. The finely divided, but not the compact metal, decomposes water at a boiling heat. Aluminum dissolves easily in hydrochloric acid as well as in hot soda-lye, with evolution of hydrogen. It is but slowly dis solved even by hot nitric acid.

2. ALUMINA is non-volatile and colorless; the HYDRATE is also colorless. Alumina dissolves in acids (particularly when dilute) slowly and with very great difficulty; in fusing bisulphate of potassa it dissolves readily to a mass soluble in water. The hydrate in an amorphous state, and recently precipitated, is readily soluble in acids; but after being left some time in the fluid

*The oxides of the third group are mostly capable of uniting with acids as well as with bases to produce saline compounds, e. g. alumina unites with potassa forming aluminate of potassa, and with sulphuric acid yields sulphate of alumina. These oxides, therefore, stand in part on the boundary between bases and acid. Those which most nearly agree in character with the latter are designated as acids

from which it was precipitated, its solubility decreases, and in the crystallized state it dissolves with very great difficulty in acids. After previous ignition with alkalies, which leads to the formation of aluminates of the alkalies, alumina is readily dissolved by acids. 3. The SALTS OF ALUMINA are mostly colorless, and non-volatile: some of them are soluble, others insoluble. Anhydrous chloride of aluminum is a yellow crystalline volatile solid. The soluble salts have a sweetish astringent taste, redden litmus paper, and lose their acids upon ignition. The insoluble salts are dissolved by hydrochloric acid, with the exception of certain native compounds. The compounds of alumina which are insoluble in hydrochloric acid are decomposed and made soluble by ignition with carbonate of soda and potassa, or bisulphate of potassa. They may also be decomposed and brought into solution by heating for two hours at a temperature of 390° to 410° Fah. with hydrochloric acid of 25 per cent. or with a mixture of three parts of hydrated sulphuric acid and 1 part of water. The substance must be in a state of fine division. The heating is performed in a sealed glass tube. -(A. Mitscherlich.)

3,

4. Soda and potassa throw down from solutions of alumina a bulky precipitate of HYDRATE OF ALUMINA (Al, O,, 3 H O), which contains alkali, and generally, also an admixture of basic salt; this precipitate re-dissolves readily and completely in an excess of the precipitant, but from this solution it is reprecipitated by addition of chloride of ammonium, even in the cold, but more completely upon application of heat (compare § 54). The presence of salts of ammonia does not prevent the precipitation by potassa or soda. 5. Ammonia also produces in solutions of alumina a precipitate of HYDRATE OF ALUMINA, containing ammonia and an admixture of basic salt; this precipitate also redissolves in a very considerable excess of the precipitant, but with difficulty only, which is the greater the larger the quantity of salts of ammonia contained in the solution. It is this deportment which accounts for the complete precipitation of hydrate of alumina from solution in potassa, by an excess of chloride of ammonium.

6. Carbonates of the alkalies throw down from solutions of alumina BASIC CARBONATE OF ALUMINA, which is insoluble or but very slightly soluble in excess of the precipitant.

7. If the solution of a salt of alumina is digested with finely pulverized carbonate of baryta, the greater part of the acid of the alumina salt combines with the baryta, the liberated carbonic acid escapes, and the alumina precipitates completely as HYDRATE mixed with BASIC SALT OF ALUMINA; even digestion in the cold suffices to produce this reaction.

8. If alumina or one of its compounds is ignited upon charcoal before the blowpipe, and afterwards moistened with a solution of