it is bright, hard, malleable, ductile, difficultly fusible; it does not oxidize in the air at common temperatures, but it oxidizes slowly upon ignition; it is attracted by the magnet, and may itself become magnetic. It slowly dissolves in hydrochloric acid and dilute sulphuric acid upon the application of heat, the solution being attended with evolution of hydrogen gas. It dissolves readily in nitric acid.

2. Protoxide of nickel is a gray, its hydrate a green powder. Both the protoxide and its hydrate are unalterable in the air, and readily soluble in hydrochloric, nitric, and sulphuric acids.

3. Most of the salts of protoxide of nickel are yellow in the anhydrous, green in the hydrated state; their solutions are of a light green color. The soluble neutral salts slightly redden litmus paper, and are decomposed at a red heat.

4. Hydrosulphuric acid does not precipitate acid solutions of salts of nickel; it fails also to precipitate neutral solutions of salts of nickel with the stronger acids, or it precipitates them only very imperfectly.

5. Sulphide of ammonium produces in neutral, and hydrosulphuric acid in alkaline solutions of salts of protoxide of nickel, a black precipitate of hydrated SULPHIDE OF NICKEL (Ni S), which is not altogether insoluble in sulphide of ammonium, especially if containing free ammonia; the fluid from which the precipitate has been thrown down exhibits therefore usually a brownish color. Sulphide of nickel dissolves with great difficulty in hydrochloric acid, but readily in nitrohydrochloric acid upon application of heat.

6. Potassa and soda produce a light green precipitate of HYDRATE OF PROTOXIDE OF NICKEL (Ni O, H O), which is insoluble in an excess of the precipitants, and unalterable in the air. Carbonate of ammonia dissolves this precipitate, when filtered and washed, to a greenish-blue fluid, from which potassa or soda reprecipitates the nickel as an apple-green hydrate of protoxide of nickel.

7. Ammonia added in small quantity to solutions of protoxide of nickel produces in them a trifling greenish turbidity; upon further addition of the reagent this redissolves readily to a blue fluid containing a compound of protoxide of nickel and ammonia. Potassa and soda precipitate from this solution hydrate of protoxide of nickel. Solutions containing salts of ammonia or free acid are not rendered turbid by ammonia.

8. Cyanide of potassium produces a yellowish-green precipitate of CYANIDE OF NICKEL (Ni Cy), which redissolves readily in an excess of the precipitant as a double cyanide of nickel and potassium (Ni Cy + K Cy); the solution is brownish-yellow. If sulphuric acid or hydrochloric acid is added to this solution, the cyanide of potassium is decomposed, and the cyanide of nickel reprecipitated; the latter substance is very difficultly soluble in an excess of these acids in the cold, but more readily upon boiling.

9. Carbonate of baryta does not precipitate protoxide of nickel from its solutions, upon digestion in the cold.

10. Nitrite of potassa, used in conjunction with acetic acid, fails to precipitate even concentrated solutions of nickel.

11. Borax and phosphate of soda and ammonia dissolve compounds of protoxide of nickel in the outer flame of the blowpipe to clear beads; the bead produced with borax is violet whilst hot, reddish-brown when cold; the bead produced with the phosphate of soda and ammonia is reddish, inclining to brown whilst hot, but turns yellow or reddish-yellow upon cool

ing. The bead which phosphate of soda and ammonia forms with salts of protoxide of nickel remains unaltered in the inner flame of the blowpipe, but that formed with borax turns gray and turbid from reduced metallic nickel. Upon continued exposure to the blowpipe flame the particles of nickel unite, but without fusing to a grain, and the bead becomes colorless. § 108.

d. PROTOXIDE OF COBALT (Co O).

1. Metallic cobalt is reddish-gray, slightly lustrous, pretty hard, slightly malleable, ductile, and difficultly fusible; it does not oxidize in the air at the common temperature, and oxidizes only slowly at a red heat; with acids it comports itself like nickel.

2. Protoxide of cobalt is an olive-green, its hydrate a pale red powder. Both dissolve readily in hydrochloric, nitric, and sulphuric acids.

3. The salts of protoxide of cobalt, containing water of crystallization, are red, the anhydrous salts mostly blue. The moderately concentrated solutions appear of a light red color, which they retain even though considerably diluted. The soluble neutral salts redden litmus paper slightly, and are decomposed at a red heat; sulphate of protoxide of cobalt alone can bear a moderate red heat without suffering decomposition. When a solution of chloride of cobalt is evaporated, the light red color changes towards the end of the operation to blue; addition of water restores the red color.

4. Hydrosulphuric acid does not precipitate acid solutions of cobalt ; it also fails to precipitate neutral solutions of salts of protoxide of cobalt with the stronger acids, or it precipitates them only very imperfectly.

5. Sulphide of ammonium precipitates from neutral, and hydrosulphuric acid from alkaline solutions of salts of protoxide of cobalt, the whole of the metal as black hydrated SULPHIDE OF COBALT (Co S). This substance is insoluble in alkalies and sulphide of ammonium, very difficultly soluble in hydrochloric acid, but readily so in nitrohydrochloric acid upon application of heat.

6. Potassa and soda produce in solutions of cobalt BLUE precipitates of basic salts of cobalt, which turn GREEN upon exposure to the air, owing to the absorption of oxygen; upon boiling, they are converted into pale red hydrate of protoxide of cobalt, which contains alkali, and generally appears rather discolored from an admixture of sesquioxide formed in the process. These precipitates are insoluble in solutions of potassa and soda; but neutral carbonate of ammonia dissolves them completely to intensely violet-red fluids, in which a somewhat larger proportion of potassa or soda produces a blue precipitate, the fluid still retaining its violet color.

7. Ammonia produces the same precipitate as potassa, but this redissolves in an excess of the ammonia to a reddish-brown fluid, from which solution of potassa or soda throws down part of the cobalt as a blue basic salt. Ammonia fails to precipitate solutions of protoxide of cobalt containing salts of ammonia or free acid.

8. Addition of cyanide of potassium to a solution of cobalt gives rise to the formation of a brownish-white precipitate of PROTOCYANIDE OF COBALT (Co Cy), which dissolves readily as a double cyanide of cobalt and potassium in an excess of solution of cyanide of potassium. Acids precipitate from this solution cyanide of cobalt. But if the solution is

boiled with cyanide of potassium in excess, in presence of free hydrocyanic acid (liberated by addition of one or two drops of hydrochloric acid), a double compound of sesquicyanide of cobalt and cyanide of potassium (K,, Co, Cy, K, Ckdy) is formed, in the solution of which acids produce no precipitate.

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9. Carbonate of baryta does not precipitate solution of protoxide of cobalt in the cold.

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10. If nitrite of potassa is added in not too small proportion to a solution of protoxide of cobalt, then acetic acid to strongly acid reaction, and the mixture put in a moderately warm place, all the cobalt separates, from concentrated solutions immediately or very soon, from dilute solutions after some time, as NITRITE OF SESQUIOXIDE OF COBALT AND POTASSA (Co, 0,, 3 K 0, 5 NO,, 2 H O), in form of a crystalline precipitate of a beautiful yellow color. The mode in which this precipitate forms may be seen from the following equation : 2 (Co O, SO) + 6 (KO, NO1) + A = KO, A + 2 KO, SO, + Co, O,, 3 K 0, 5 NO, NO,. The precipitate is only sparingly soluble in pure water, and altogether insoluble in saline solutions and in alcohol. When boiled with water, it dissolves, though not copiously, to a red fluid, which remains clear upon cooling, and from which alkalies throw down hydrate of protoxide of cobalt (Fischer, Aug. Stromeyer). This excellent reaction enables us to distinguish nickel from cobalt. It is always necessary to concentrate the solution of protoxide of cobalt to a considerable extent before adding the nitrite of potassa.

11. Borax dissolves compounds of cobalt both in the inner and outer flame of the blowpipe, giving clear beads of a magnificent BLUE color, which appear violet by candle-light, and almost black if the cobalt is present in considerable proportion. This test is as delicate as it is characteristic. Phosphate of soda and ammonia manifests with salts of cobalt before the blowpipe an analogous but less delicate reaction.

$109.

e. PROTOXIDE OF IRON (Fe O).

1. Metallic iron in the pure state has a light whitish gray color (iron containing carbon is more or less gray); the metal is hard, lustrous, malleable, ductile, exceedingly difficult to fuse, and is attracted by the magnet. In contact with air and moisture a coating of rust (hydrate of sesquioxide of iron) forms on its surface; upon ignition in the air, a coating of black protosesquioxide. Hydrochloric acid and dilute sulphuric acid dissolve iron, with evolution of hydrogen gas; if the iron contains carbon, the hydrogen is mixed with carbide of hydrogen. The solutions contain protoxide. Dilute nitric acid dissolves iron in the cold to nitrate of protoxide, with evolution of nitrous oxide; at a high temperature, to nitrate of sesquioxide, with evolution of nitric oxide; if the iron contains carbon, some carbonic acid is also evolved, and there is left undissolved a brown substance resembling humus, which is soluble in alkalies.

2. Protoxide of iron is a black powder; its hydrate is a white powder, which in the moist state absorbs oxygen and speedily acquires a grayish-green, and ultimately a brownish-red color. Both the protoxide and its hydrate are readily dissolved by hydrochloric, sulphuric, and nitric acids.

3. The salts of protoxide of iron have in the anhydrous state a white, in the hydrated state a greenish color; their solutions appear colored only when concentrated. Exposed to the air, they absorb oxygen and are converted into salts of the protosesquioxide. The soluble neutral salts redden litmus paper, and are decomposed at a red heat.

4. Acid solutions of salts of protoxide of iron are not precipitated by hydrosulphuric acid; neutral solutions of salts of protoxide of iron with weak acids are precipitated by this reagent at the most but very incompletely; the precipitates are of a black color.

5. Sulphide of ammonium precipitates from neutral, and hydrosulphuric acid from alkaline solutions of salts of protoxide of iron, the whole of the metal as black hydrated PROTOSULPHIDE OF IRON (Fe S), which is insoluble in alkalies and sulphides of the alkali metals, but dissolves readily in hydrochloric and nitric acids: this black precipitate turns reddish-brown in the air by oxidation. To highly dilute solutions of protoxide of iron, addition of sulphide of ammonium imparts a green color, and it is only after some time that the protosulphide of iron separates as a black precipitate.

6. Potassa and ammonia produce a precipitate of HYDRATE OF PROTOXIDE OF IRON (Fe O, H O), which in the first moment looks almost white, but acquires after a very short time a dirty green, and ultimately a reddish-brown color, owing to absorption of oxygen from the air. Presence of salts of ammonia prevents the precipitation by potassa partly, and that by ammonia altogether. If alkaline solutions of protoxide of iron thus obtained by the agency of salts of ammonia are exposed to the air, hydrate of sesquioxide of iron precipitates.

7. Ferrocyanide of potassium produces in solutions of protoxide of iron a bluish-white precipitate of FERROCYANIDE OF POTASSIUM AND IRON (K, Fe,, Cfy,), which, by absorption of oxygen from the air, speedily acquires a blue color. Nitric acid or chlorine converts it immediately into Prussian blue, 3 (K, Fe,, Cfy,) + 4 Cl = 3 K Cl + FeCl + 2 (Fe, Cfy).

8. Ferricyanide of potassium produces a magnificently blue precipitate of FERRICYANIDE OF IRON (Fe, Cfdy). This precipitate does not differ in color from Prussian blue. It is insoluble in hydrochloric acid, but is readily decomposed by potassa. In highly dilute solutions of salts of protoxide of iron the reagent produces simply a deep blue-green coloration.

9. Sulphocyanide of potassium does not alter solutions of protoxide of iron free from sesquioxide.

10. Carbonate of baryta does not precipitate solutions of protoxide of iron in the cold.

11. Borax dissolves salts of protoxide of iron in the oxidizing flame, giving beads varying in color from YELLOW to DARK RED; when cold, the beads vary from colorless to dark yellow. In the inner flame the beads change to bottle-green, owing to the reduction of the newly-formed sesquioxide to protosesquioxide. Phosphate of soda and ammonia manifests a similar deportment with the salts of protoxide of iron; the beads produced with this reagent lose their color upon cooling still more completely than is the case with those produced with borax; the signs of the ensuing reduction in the reducing flame are also less marked.

$ 110.

f. SESQUIOXIDE OF IRON (Fe, O.).

1. The native crystallized sesquioxide of iron is steel-gray; the native as well as the artificially prepared sesquioxide of iron gives upon trituration a brownish-red powder; the color of hydrate of sesquioxide of iron is more inclined to reddish-brown. Both the sesquioxide and its hydrate dissolve in hydrochloric, nitric, and sulphuric acids; the hydrate dissolves readily in these acids, but the anhydrous sesquioxide dissolves with greater difficulty, and completely only after long exposure

to heat.

2. The neutral anhydrous salts of sesquioxide of iron are nearly white; the basic salts are yellow or reddish-brown. The color of the solution is brownish-yellow, and becomes reddish-yellow upon the application of heat. The soluble neutral salts redden litmus paper, and are decomposed by heat.

3. Hydrosulphuric acid produces in neutral and acid solutions of salts of sesquioxide of iron a milky white turbidity, proceeding from separated SULPHUR. This reaction is caused by a mutual decomposition of the sesquioxide of iron and the hydrosulphuric acid, in which the former is deprived of one-third of its oxygen, and thus reduced to the state of protoxide; the oxygen forms water with the hydrogen of the hydrosulphuric acid, and the liberated sulphur separates. Solution of hydrosulphuric acid, rapidly added to neutral solutions, imparts to the fluid a transitory blackening. In solution of neutral acetate of sesquioxide of iron it produces a permanent precipitate of sulphide of iron.

4. Sulphide of ammonium precipitates from neutral, and hydrosulphuric acid from alkaline solutions of salts of sesquioxide of iron, the whole of the metal as black hydrated PROTOSULPHIDE OF IRON (Fe S). This precipitation is preceded by the reduction of the sesquioxide to protoxide. In very dilute solutions the reagent produces only a blackish-green coloration. The minutely divided protosulphide of iron subsides in such cases only after long standing. Protosulphide of iron, as already stated (§ 109, 5), is insoluble in alkalies and alkaline sulphides, but dissolves readily in hydrochloric and nitric acids.

5. Potassa and ammonia produce bulky reddish-brown precipitates of HYDRATE OF SESQUIOXIDE OF IRON (F, O,, H O), which are insoluble in an excess of the precipitant as well as in salts of ammonia. In presence of sesquioxide of chromium, an excess of solution of potassa or ammonia will dissolve part of the sesquioxide of iron along with the sesquioxide of chromium. Generally, however, a portion of both oxides remains undissolved; and, moreover, the dissolved oxides speedily separate again from the solution.

6. Ferrocyanide of potassium produces even in highly dilute solutions a magnificently blue precipitate of FERROCYANIDE OF IRON, or Prussian blue (Fe, Cfy,), which is insoluble in hydrochloric acid, but is decomposed by potassa, with separation of hydrate of sesquioxide of iron.

7. Ferricyanide of potassium deepens the color of solutions of salts of sesquioxide of iron to reddish-brown ; but it fails to produce a precipitate. 8. Sulphocyanide of potassium imparts to neutral or slightly acid solutions of salts of sesquioxide of iron a most intense blood-red color, arising from the formation of a soluble SULPHOCYANIDE OF IRON. Addition of acetate of soda destroys this color, hydrochloric acid restores it