again. This test is the most delicate of all; it will indicate the presence of sesquioxide of iron even in fluids which are so highly dilute that every other reagent fails to produce the slightest visible alteration. The red coloration may in such cases be detected most distinctly by resting the test-tube upon a sheet of white paper, and looking through it from the top.

9. Carbonate of baryta precipitates even in the cold all the iron as a basic salt mixed with hydrate of sesquioxide.

10. The reactions before the blowpipe are the same as with the protoxide.

$111.

Recapitulation and remarks. On observing the behavior of the oxides of the fourth group with solution of potassa, it would appear that the separation of the oxide of zinc, which is soluble in an excess of this reagent, might be readily effected by its means; however, in the actual experiment, we find that in the presence of sesquioxide of iron, protoxide of cobalt, &c., some of the oxide of zinc precipitates with these oxides; and if only a small quantity of oxide of zinc is present, it frequently occurs that no trace of this metal can be detected in the alkaline filtrate.

Again, from the behavior of the different oxides with chloride of ammonium and an excess of ammonia, one would conclude that the separation of sesquioxide of iron from the protoxides of cobalt, nickel, and manganese, and from oxide of zinc, might be readily effected by these agents. But this method also applied to the mixed oxides, is inaccurate, since greater or smaller portions of the other oxides will always precipitate with the sesquioxide of iron; and it may therefore happen that, in this process, small quantities of cobalt, manganese, &c., altogether escape detection.

It is far safer therefore to separate the other oxides of the fourth group from sesquioxide of iron by carbonate of baryta, as in that case the iron is precipitated free from oxide of zinc and protoxides of manganese and nickel, and mixed only with a very trifling quantity of protoxide of cobalt.

Protoxide of manganese may conveniently be separated from the protoxides of cobalt and nickel and from oxide of zinc, by treating the precipitated sulphides with moderately dilute acetic acid, which dissolves the sulphide of manganese, leaving the other sulphides unacted on. If the acetic acid solution is now mixed with solution of potassa, the least trace of a precipitate will be sufficient to recognise the manganese before the blowpipe with carbonate of soda.

If the sulphides left undissolved by acetic acid are now treated with very dilute hydrochloric acid, the sulphide of zinc dissolves, leaving almost the whole of the sulphides of cobalt and nickel behind. If the fluid is then boiled, to expel the hydrosulphuric acid, and afterwards treated with solution of potassa or soda in excess, the zinc is sure to be detected in the filtrate by hydrosulphuric acid.

Cobalt may mostly be readily and safely detected in presence of nickel by the reaction with borax in the inner flame of the blowpipe. The detection of nickel in presence of cobalt is a less easy task, which may, however, be effected with great accuracy by three different methods. The first method is to add to the solution containing the two metals nitrate of

potassa in not too small proportion, then acetic acid to strongly acid reaction, and to let the mixture stand for at least several hours in a moderately warm place, when the cobalt will separate as nitrate of sesquioxide of cobalt and potassa; the nickel may then be readily precipitated from the filtrate by soda or sulphide of ammonium. The second method is to saturate with chlorine the very dilute solution of the two metals in hydrochloric acid, having the acid slightly in excess; add carbonate of baryta in excess, and let the fluid stand twenty-four hours. The cobalt is entirely precipitated in this process as black sesquioxide, whilst the nickel remains in solution, and may, after the removal of the baryta by sulphuric acid, be thrown down with solution of soda. The third method is based upon the application of cyanide of potassium. Both cyanide of nickel and cyanide of cobalt dissolve in cyanide of potassium; but cyanide of nickel is precipitated from this solution by acids, whilst cyanide of cobalt is not precipitated if the solution contains free hydrocyanic acid, and has been exposed to the action of heat.* This reaction, i. e., the formation of a precipitate upon adding hydrochloric acid or dilute sulphuric acid to the solution of the two cyanides in cyanide of potassium, prepared under the conditions stated, indicates the presence of nickel. Whether the precipitate be cyanide of nickel or cobalticyanide of nickel, is quite immaterial as far as the detection of nickel is concerned: all we have to bear in mind is simply this, that no precipitate will form if cobalt alone is present in the solution, since cobalticyanide of potassium is not decomposed by hydrochloric acid. In order to get at the composition of the various precipitates which are formed by hydrochloric acid in solutions of the mixed cyanides of nickel and cobalt, and to comprehend the general process of their formation, we have to assume and consider three special and distinct cases, differing from one another in the relative proportions of nickel and cobalt which the solutions under examination respectively contain.

:

The solution will accordingly, in the first case, contain one eq. of cobalticyanide of potassium (K,, Co, Cy) and 3 eq. of double cyanide of nickel and potassium 3 (Ni Cy, K Cy); now upon the addition of hydrochloric acid in excess to this solution, the double cyanide of nickel and potassium suffers decomposition, and the potassium of the cobalticyanide of potassium transposes with the nickel of the cyanide of nickel the products of this process of double decomposition and transposition are chloride of potassium, hydrocyanic acid, and cobalticyanide of nickel (Ni,, Co, Cy), which latter separates in the form of a dirty-green precipitate containing the whole of the nickel and cobalt present in the solution. In the second case we obtain equally a precipitate of cobalticyanide of nickel; but this precipitate, though containing the whole of the nickel, does not contain all the cobalt of the solution, since the excess of cobalticyanide of potassium is not decomposed. In the third case, lastly, we obtain a precipitate of cobalticyanide of nickel, which contains

* In this process the double cyanide of cobalt and potassium (K Cy, Co Cy), which forms at first, is converted by the aid of the free hydrocyanic acid and the excess of cyanide of potassium, into cobalticyanide of potassium (K,, Co, Cys) : 2 (Co Cy, K Cy) +K Cy + H Cy (Kg, Co2 Cys) + H.

the whole of the cobalt and a portion of the nickel, mixed with insoluble cyanide of nickel, which contains the rest of the nickel. The cobalticyanide of nickel has been formed here in the same manner as in the first case, whilst the cyanide of nickel owes its formation to the decomposition of the excess of the double cyanide of nickel and potassium. Hence it is evident that the presence of nickel is indispensable to the formation of a precipitate, and consequently that the production of a precipitate is the most positive proof of the presence of this metal.

Protoxide and sesquioxide of iron may be detected in presence of each other, by testing for the former with ferricyanide of potassium, for the latter with ferrocyanide of potassium or, better still, with sulphocyanide of potassium.

In conclusion, it is necessary to mention that alkalies fail to precipitate the oxides of the fourth group in presence of non-volatile organic substances (such as sugar, tartaric acid, &c.). We have already seen that the same applies to alumina.

§ 112.

SUPPLEMENT TO THE FOURTH GROUP.

SESQUIOXIDE OF URANIUM (U, 0).

Sesquioxide of uranium is brick-red, the hydrate is of a yellow color. Upon ignition both are converted into the dark blackish-green protosesquioxide. The solutions of sesquioxide of uranium in acids are yellow; hydrosulphuric acid does not alter them; sulphide of ammonium throws down after neutralization of the free acid, a dark brown precipitate of SULPHIDE OF URANIUM, which subsides slowly, and is readily soluble in acids, even in acetic acid, but does not dissolve in an excess of the precipitant. Ammonia, potassa, and soda produce yellow precipitates of sesquioxide of uranium and alkali, insoluble in excess of the precipitants. Carbonate of ammonia produces a yellow precipitate of carbonate of sesquioxide of uranium and ammonia, which dissolves readily in an excess of the precipitant. Potassa and soda throw down from the solution the whole of the sesquioxide of uranium. Carbonate of baryta completely precipitates solutions of sesquioxide of uranium, even in the cold. Ferrocyanide of potassium produces a reddish-brown precipitate: this is a very delicate test for uranium. Borax and phosphate of soda and ammonia give with sesquioxide of uranium in the inner flame of the blowpipe green beads, in the outer flame yellow beads, which upon cooling acquire a yellowish-green tint.

To separate sesquioxide of uranium from the other oxides of the fourth group, and from alumina, precipitate first with carbonate of baryta, which throws down the sesquioxide of iron, the sesquioxide of uranium, and the alumina, leaving the other oxides in solution. Dissolve the precipitate in hydrochloric acid, remove the baryta from the solution by means of sulphuric acid, filter, and add ammonia to the filtrate until a precipitate begins to form; add now a sufficient quantity of carbonate of ammonia, which has previously been once boiled, to destroy any bicarbonate of ammonia it may contain. Dilute with water and filter off the precipitate, which contains the whole of the sesquioxide of iron and alumina, the filtrate containing all the sesquioxide of

uranium.

§ 113.

FIFTH GROUP.

OXIDE OF SILVER, SUBOXIDE OF MERCURY, OXIDE OF MERCURY, OXIDE OF LEAD, TEROXIDE OF BISMUTH, OXIDE OF COPPER, OXIDE OF CADMIUM.

Properties of the group.-The sulphides corresponding to the oxides of this group are insoluble both in dilute acids and in alkaline sulphides.* The solutions of these oxides are therefore completely precipitated by hydrosulphuric acid, no matter whether their reaction be neutral, alkaline, or acid.

For the sake of greater clearness and simplicity, we divide the oxides of this group into two divisions, and distinguish,

1. OXIDES PRECIPITABLE BY HYDROCHLORIC ACID, viz.: oxide of silver, suboxide of mercury, oxide of lead.

2. OXIDES NOT PRECIPITABLE BY HYDROCHLORIC ACID, viz.: oxide of mercury, oxide of copper, teroxide of bismuth, oxide of cadmium.

Lead must be considered in both divisions, since the sparing solubility of its chloride might lead to confounding its oxide with suboxide of mercury and oxide of silver, without affording us, on the other hand, any means of effecting its perfect separation from the oxides of the second division.

FIRST DIVISION OF THE FIFTH GROUP: OXIDES WHICH ARE PRECIPITATED BY HYDROCHLORIC ACID.

Special Reactions.
$114.

a. OXIDE OF SILVER (Ag O).

1. Metallic silver is white, very lustrous, moderately hard, highly malleable, ductile, rather difficultly fusible. It is not oxidized by ignition in the air. Nitric acid dissolves silver readily; the metal is insoluble in dilute sulphuric acid and in hydrochloric acid.

2. Oxide of silver is a grayish-brown powder; it is not altogether insoluble in water, and dissolves readily in dilute nitric acid. It forms no hydrate. It is decomposed by heat into metallic silver and oxygen gas.

3. The salts of oxide of silver are non-volatile and colorless; most of them acquire a black tint upon exposure to light. The soluble neutral salts do not alter vegetable colors, and are decomposed at a red heat.

4. Hydrosulphuric acid and sulphide of ammonium precipitate from solutions of salts of silver black SULPHIDE OF SILVER (Ag S), which is insoluble in dilute acids, alkalies, alkaline sulphides, and cyanide of potassium. Boiling nitric acid decomposes and dissolves this precipitate readily, with separation of sulphur.

5. Potassa and soda precipitate from solutions of salts of silver the oxide of this metal in the form of a LIGHT BROWN POWDER, which is insoluble in an excess of the precipitant, but dissolves readily in ammonia.

* Consult, however, the paragraphs on oxide of copper and suboxide and oxide of mercury, as the latter remark applies only partially to them.

6. Ammonia, when added in very small quantity to neutral solutions of oxide of silver, throws down the oxide as a brown precipitate, which readily redissolves in an excess of ammonia. Acid solutions of silver are not precipitated.

7. Hydrochloric acid and soluble metallic chlorides produce in solutions of salts of silver a white, curdy precipitate of CHLORIDE OF SILVER (Ag Cl). In very dilute solutions these reagents impart simply a bluishwhite opalescent appearance to the fluid. By the action of light chloride of silver first acquires a violet tint, and ultimately turns black; it is insoluble in nitric acid, but dissolves readily in ammonia as ammonio-chloride of silver, from which double compound the chloride of silver is again separated by acids. Concentrated hydrochloric acid and concentrated solutions of chlorides of the alkali metals dissolve some chloride of silver, more particularly upon application of heat; but the dissolved chloride separates again upon dilution. Upon exposure to heat, chloride of silver fuses without decomposition, giving upon cooling a transparent horny mass.

8. If compounds of silver, mixed with carbonate of soda, are exposed on a charcoal support to the inner flame of the blowpipe, WHITE, BRILLIANT, DUCTILE METALLIC GLOBULES are obtained, unattended with incrustation of the charcoal.

§ 115.

b. SUBOXIDE OF MERCURY (Hg, 0).

1. Metallic mercury is grayish-white, lustrous, fluid at the common temperature; it solidifies at 40°, and boils at 680° Fah. It is insoluble in hydrochloric acid; in dilute cold nitric acid it dissolves to nitrate of suboxide, in more concentrated hot nitric acid to nitrate of oxide of mercury.

2. Suboxide of mercury is a black powder which is readily soluble in nitric acid, and is decomposed by the action of heat, the mercury volatilizing in the metallic state. It forms no hydrate.

3. The salts of suboxide of mercury volatilize upon ignition; most of them suffer decomposition in this process. Subchloride and subbromide of mercury volatilize unaltered. Most of the salts of suboxide of mercury are colorless. The soluble salts in the neutral state redden litmus paper. Nitrate of suboxide of mercury is decomposed by addition of much water into an insoluble basic and soluble acid salt.

4. Hydrosulphuric acid and sulphide of ammonium produce black precipitates of SUBSULPHIDE OF MERCURY (Hg, S), which are insoluble in dilute acids, sulphide of ammonium, and cyanide of potassium. Protosulphide of sodium dissolves this subsulphide to sulphide, with separation of metallic mercury. Bisulphide of sodium dissolves the subsulphide to sulphide, without separation of metallic mercury. Subsulphide of mercury is readily decomposed and dissolved by nitrohydrochloric acid, but not by boiling concentrated nitric acids.

5. Potassa and ammonia produce in solutions of salts of suboxide of mercury black precipitates, which are insoluble in an excess of the precipitants. The precipitates produced by potassa consist of SUBOXIDE OF MERCURY; whilst those produced by ammonia consist of a BASIC DOUBLE SALT OF SUBOXIDE OF MERCURY AND AMMONIA, e. g. (N H,, NO, + 2 Hg, O).

6. Hydrochloric acid and soluble metallic chlorides precipitate from solutions of salts of suboxide of mercury SUBCHLORIDE OF MERCURY