heated to intense redness, it undergoes partial decomposition, losing part of its acid, after which it is no longer perfectly soluble in water. By means of a blowpipe-lamp, it is tolerably easy to expel the whole of the sulphuric acid from small quantities of sulphate of magnesia (Experiment No. 30). Ignited with chloride of ammonium, sulphate of magnesia is not decomposed.

b. Basic phosphate of magnesia and ammonia is a white crystalline powder. It dissolves, at the common temperature, in 15293 parts of cold water (Experiment No. 31). In water containing ammonia, it is much more difficultly soluble-one part of the salt requiring 45000 parts of the solvent (Experiment No. 32). Chloride of ammonium slightly increases its solubility (Experiments No. 34 and 35). Presence of alkaline phosphates exercises no influence in this respect. The basic phosphate of magnesia and ammonia dissolves readily in acids, even in acetic acid. Its composition is expressed by the formula

10 of the 12 equivalents of water escape at 212° F., the remaining 2, together with the ammonia, at a red heat, leaving 2 Mg O, PO,. The change of the phosphoric to pyrophosphoric acid, is indicated by a vivid incandescence of the whole mass.

Ammonia precipitates phosphate of magnesia and ammonia from its solution in dilute hydrochloric or dilute nitric acid, to the extent corresponding with the degree of solubility of that salt in water impregnated with ammonia, or with ammonia and an ammoniacal salt. Weber (Pogg. 73, p. 152) having questioned the correctness of this fact, I was induced to test it again by experiment (No. 33) which, however, yielded exactly the same results as before.

c. Pyrophosphate of magnesia presents the appearance of a white mass, often slightly inclining to gray. It is barely soluble in water, but readily so in hydrochloric acid, and in nitric acid. It remains unaltered in the air, and even at a red heat; at a very intense red heat it fuses unaltered. It leaves the color of moist turmeric, and of reddened litmus paper unchanged.

If we dissolve pyrophosphate of magnesia in hydrochloric or nitric acid, add water to the solution, boil for some time, and then precipitate with ammonia in excess, we obtain a precipitate of phosphate of maguesia and ammonia which, after ignition, gives less weight of 2 Mg O, PO,, than corresponds with the amount of substance used. Weber gives the loss as from 13 to 2.3 per cent. My own experiments (No. 36) confirm this statement, and point out the circumstances under which the loss is the least considerable (compare also § 135). By long-continued fusion, with carbonate of potassa and soda, pyrophosphate of magnesia is completely decomposed, the phosphoric acid being reconverted into the tribasic state. If, therefore, we treat the fused mass with hydrochloric acid, add water and ammonia, and heat the precipitate to redness, we re-obtain the whole quantity of the salt used.

d. Pure magnesia is a white, light, loose powder. It dissolves in 55368 parts of cold, and in the same proportion of boiling water (Experiment No. 37). Its aqueous solution has a very slightly alkaline reaction. Magnesia dissolves in hydrochloric acid, and in other acids, without evolution of gas. Exposed to the air, it slowly absorbs carbonic acid and water. Magnesia is highly infusible, remaining unaltered at a strong red heat, and fusing superficially only at the very highest temperature.

Alumina is usually precipitated as HYDRATE OF ALUMINA, and always weighed in the pure state.

a. Hydrate of alumina, recently precipitated, is a gelatinous mass, which invariably retains a minute proportion of the acid with which the alumina was combined previous to precipitation, as well as of the alkali which has served as the precipitant; it is freed with difficulty from these admixtures by repeated washing.

Hydrate of alumina is insoluble in pure water; but it readily dissolves in solutions of soda or potassa, and in ethylamine (Sonnenschein); it is sparingly soluble in caustic ammonia, and altogether insoluble in carbonate of ammonia; presence of ammoniacal salts greatly diminishes its solubility in caustic ammonia (Experiment No. 38). The correctness of this statement of mine in the first edition of the present work, has been amply confirmed since by Malaguti and Durocher ("Ann. de Chim. et de Phys.," 3 Ser. 16, 421); and also by experiments made by my former assistant, Mr. J. Fuchs. The former chemists (Malaguti and Durocher) state also that, when a solution of alumina is precipitated with sulphide of ammonium, the fluid may be filtered off five minutes after, without a trace of alumina in it. Fuchs did not find this to be the case (Experiment No. 39).

Hydrate of alumina, recently precipitated, dissolves readily in hydrochloric or nitric acid; but after filtration, or after having remained for some time in the fluid from which it has been precipitated, it dissolves in these acids with much greater difficulty, and only after long digestion. Hydrate of alumina shrinks considerably upon desiccation, and, when dry, becomes a hard, transparent, yellowish, or a white, earthy mass. When heated to redness, it loses its water, and this loss is frequently attended with slight decrepitation, and invariably with considerable diminution of bulk.

b. Alumina, prepared by heating the hydrate to a moderate degree of redness, is a loose and soft mass; but upon the application of a very intense degree of redness, it concretes into small, hard lumps. At the most intense white heat, it fuses to a colorless glass. Ignited alumina is dissolved by dilute acids with very great difficulty; in fuming hydrochloric acid, it dissolves upon long-continued digestion in a warm place, slowly, but completely. Ignition in a current of hydrogen gas leaves it unaltered. By fusion with bisulphate of potassa, it is rendered soluble, the residue dissolving readily in hydrochloric acid and water. When alumina is fused at a very high temperature, in conjunction with ten times the quantity of carbonate of soda, aluminate of soda is formed, which is soluble in water (R. Richter). Placed upon moist reddened litmus paper, pure alumina does not change the color to blue. Upon igniting alumina with chloride of ammonium, chloride of alumi nium escapes; but the process fails to effect complete volatilization of the alumina (H. Rose).

Sesquioxide of chromium is usually precipitated as HYDRATED SESQUIOXIDE OF CHROMIUM, and always weighed in the pure state.

a. Hydrated sesquioxide of chromium, recently precipitated, is a greenish-gray, gelatinous mass, insoluble in water; it dissolves readily, in the cold, in solutions of potassa or soda, to a dark-green fluid; it dissolves, also in the cold, but rather sparingly, in solution of ammonia, to a bright violet red fluid. In acids it dissolves readily, imparting a darkgreen tint to the fluid. Presence of chloride of ammonium exercises no influence upon the solubility of hydrated sesquioxide of chromium in ammonia. Boiling effects the complete separation of the sesquioxide from its solutions in potassa, soda, or ammonia (Experiment No. 40). Dried hydrated sesquioxide of chromium is a greenish-blue powder; it loses its water of hydration at a gentle red heat.

b. Sesquioxide of chromium, produced by heating the hydrate to dull redness, is a dark green powder; upon the application of a higher degree of heat, it assumes a lighter tint, but suffers no diminution of weight; the transition from the darker to the lighter tint is marked by a vivid incandescence of the powder. The feebly ignited sesquioxide is difficultly soluble in hydrochloric acid, and the strongly ignited sesquioxide is altogether insoluble in that acid. Mixed with chloride of ammonium, and exposed to a red heat, sesquioxide of chromium remains unaltered; it suffers no alteration when ignited in a current of hydrogen gas.

BASES OF THE FOURTH GROUP.

§ 77.

1. OXIDE OF ZINC.

Zinc is always weighed in the form of oxide; it is, however, usually precipitated as BASIC CARBONATE OF ZINC, or as SULPHIDE OF ZINC.

a. Basic carbonate of zinc, recently precipitated, is a white, flocculent mass, nearly insoluble in water-(one part requiring 44600 parts-Experiment No. 42)—but readily soluble in solutions of potassa, ammonia, carbonate of ammonia, and in acids. The solutions of zinc in soda or potassa, if concentrated, are not altered by boiling; but if dilute, nearly all the oxide of zinc present is thrown down, as a white precipitate. From the solutions of ammonia also, especially if they are dilute, oxide of zinc separates upon boiling. When a neutral solution of zinc is precipitated with carbonate of soda or carbonate of potassa, carbonic acid is evolved, since the precipitate formed is not Zn O, CO,, but consists of a mixture of hydrated oxide of zinc with carbonate of zinc, in varying proportions, according to the degree of concentration of the solution, and to the mode of precipitation. Owing to the presence and action of this carbonic acid, part of the oxide of zinc remains in solution; the cold filtered fluid yields, therefore, a precipitate when acted upon by sulphide of ammonium.

But if the solution is precipitated boiling, and kept at that temperature for some time, the precipitation of the zinc is complete to the extent that the filtrate is not rendered turbid by the addition of sulphide of ammonium; still, if the filtrate, mixed with sulphide of ammonium, be allowed to stand at rest for many hours, minute and almost imponderable flakes of sulphide of zinc will separate from the fluid. The precipitate of carbonate of zinc, obtained in the manner just described, may be completely freed from all admixture of alkali, by washing with hot water. If ammoniacal salts be present, every trace of ammonia must be expelled first, before the complete* precipitation of the zinc can be effected. If the solution of a zinc salt is mixed with carbonate of potassa or soda in excess, the mixture evaporated to dryness, at a gentle heat, and the residue treated with cold water, a perceptible proportion of the zinc is obtained in solution as carbonate of zinc and potassa or soda; but if the mixture is evaporated to dryness, at boiling heat, and the residue treated with hot water, the whole of the zinc, with the exception of an extremely minute proportion, as we have already had occasion to observe, is obtained as carbonate of zinc.

The dried basic carbonate of zinc is a fine, white, loose powder; exposure to a red heat converts it into oxide of zinc.

b. Oxide of zinc, produced from the carbonate by the application of a red heat, is a white light powder, with a slightly yellow tint When heated, it acquires a yellow color, which disappears again on cooling. Upon exposing oxide of zinc, mixed with charcoal, to a red heat, carbonic oxide gas and zinc fumes escape. By igniting the oxide in a strong and rapid current of hydrogen gas, metallic zinc is produced; whilst by igniting it in a feeble current of hydrogen gas, crys

* That is to say, complete within the limits indicated just now, so that sulphide of an monium occasions no turbidity in the filtrate, although it may produce subsequently and after long standing some slight and almost imponderable flakes of sulphide of zinc.

tallized oxide of zinc is obtained (St. Claire Deville). Oxide of zinc is insoluble in water. Placed on moist turmeric paper, it does not change the color to brown. In acids, oxide of zinc dissolves readily, and without evolution of gas. When oxide of zinc is heated to redness with chloride of ammonium, fused chloride of zinc is produced, which volatilizes with very great difficulty, if the air is excluded; but readily and completely, with free access of air, and with chloride of ammonium fumes (H. Rose).

c. Sulphide of zinc, recently precipitated, is a white, loose mass (Zn S, H O), insoluble in water, in caustic alkalies, alkaline carbonates, and alkaline sulphides. It dissolves readily and completely in hydrochloric acid and in nitric acid, but only very sparingly in acetic acid. When dried, the precipitated sulphide of zinc is a white powder; at 212° F. it loses half, and at a red heat the whole of its water. During the latter process some sulphuretted hydrogen escapes, and the remaining sulphide of zinc contains an admixture of oxide of zinc. By roasting in the air, and intense ignition of the residue, small quantities of sulphide of zinc may be readily converted into the oxide.

$ 78.

2. PROTOXIDE OF MANGANESE.

Manganese is weighed either as PROTOSESQUIOXIDE OF MANGANESE (red oxide of manganese)-Mn O + Mn ̧ O ̧) = Mn,O,—or as SULPHATE OF PROTOXIDE OF MANGANESE. For the purpose of converting it into the first form, it is precipitated as CARBONATE OF PROTOXIDE OF MANGANESE, HYDRATED FROTOXIDE OF MANGANESE, BINOXIDE OF MANGANESE, or SULPHIDE OF MANGANESE.

a. Carbonate of protoxide of manganese, recently precipitated, is a white, flocculent mass, nearly insoluble in pure water, but somewhat more soluble in water impregnated with carbonic acid. Presence of carbonate of soda or potassa does not increase its solubility. Recently precipitated carbonate of protoxide of manganese dissolves pretty readily in solution of chloride of ammonium; it is owing to this property that a solution of protoxide of manganese cannot be completely precipitated by carbonate of potassa or soda, in presence of chloride of ammonium (or some other ammoniacal salt), until the latter is completely decomposed. If the precipitate, while still moist, is exposed to the air, or washed with water impregnated with air, it slowly assumes a dirty brownish-white color, part of it becoming converted into hydrated protosesquioxide of manganese. If the precipitate is dried removed from the contact of air, it forms a delicate white powder, persistent in the air [2 (Mn O, CO,) + aq.]; but when dried with free access of air, the powder is of a more or less dirty white color. When heated to redness, with access of air, this powder first turns black, and changes subsequently to brown protosesquioxide of manganese. However, this conversion takes some time, and must never be held to be completed