11. A phosphate of soda and ammonia bead saturated with oxide of copper, mixed with a substance containing bromine, and then ignited in the inner blowpipe flame, colors the flame BLUE, inclining to GREEN, more particularly at the edges (Berzelius).

§ 154.

C. HYDRIODIC ACID (HI).

1. Iodine is a solid, soft body, of a peculiarly disagreeable odor. It is generally seen in the form of black, shining, crystalline scales. It fuses at a gentle heat; at a somewhat higher temperature, it is converted into iodine vapor, which has a beautiful violet-blue color, and condenses upon cooling to a black sublimate. It is very sparingly soluble in water, but readily in alcohol and ether; the aqueous solution is of a lightbrown, the alcoholic and ethereal solutions are a deep red-brown color. Iodine destroys vegetable colors only slowly and imperfectly; it stains the skin brown; with starch it forms a compound of an intensely deep blue color. This compound is formed invariably when iodine vapor or a solution containing free iodine comes in contact with starch, best with starch-paste. It is decomposed by alkalies, and also by chlorine and bromine.

2. Hydriodic acid gas resembles hydrochloric and hydrobromic acid gas; it dissolves copiously in water. The colorless hydrated hydriodic acid turns speedily to a reddish-brown when in contact with the air, owing to the formation of water, and a solution of iodine in hydriodic acid.

3. The iodides also correspond in many respects with the chlorides. Of the iodides of the heavy metals, however, many more are insoluble in water than is the case with the corresponding chlorides. Many iodides

have characteristic colors.

4. Nitrate of silver produces in aqueous solutions of hydriodic acid and of iodides yellowish-white precipitates of IODIDE OF SILVER (Ag I), which blacken on exposure to light; these precipitates are insoluble in dilute nitric acid, and very sparingly soluble in ammonia, but dissolve readily in cyanide of potassium.

5. Protochloride of palladium and nitrate of protoxide of palladium produce even in very dilute solutions of hydriodic acid or metallic iodides а brownish-black precipitate of PROTIODIDE OF PALLADIUM, which dissolves to a trifling extent in saline solutions (solution of chloride of sodium, chloride of magnesium, &c.), but is insoluble or nearly so in dilute cold hydrochloric and nitric acids.

6. A solution of 1 part of sulphate of oxide of copper and 2 parts of sulphate of protoxide of iron throws down from neutral aqueous solutions of the iodides SUBIODIDE OF COPPER (Cu, I), in the form of a dirtywhite precipitate. The addition of ammonia promotes the complete precipitation of the iodine. Chlorides and bromides are not precipitated by this reagent.

7. Pure nitric acid, free from nitrous acid, or iodides only when acting upon them in its larly when aided by the application of heat. nitric acid decompose hydriodic acid and facility even in the most dilute solutions.

decomposes hydriodic acid concentrated forın, particuBut nitrous acid and hypoiodides with the greatest Colorless solutions of iodides

therefore acquire immediately a brownish-red color, upon addition of some red-fuming nitric acid, or a mixture of this with concentrated sulphuric acid or, better still, a solution of hyponitric acid in hydrated sulphuric acid or nitrite of potassa and some sulphuric or hydrochloric acid. From more concentrated solutions the iodine separates under these circumstances in the form of small black plates or scales, whilst nitric oxide gas and iodine vapors escape.

8. As the blue coloration of iodide of starch remains still visible in much more highly dilute solutions than the yellow color of solutions of iodine in water, the delicacy of the reaction just now described (7) is considerably heightened by mixing the fluid to be tested for iodine first with some thin, clear starch-paste, then adding one or two drops of dilute sulphuric acid, to make the fluid acid, and adding finally one or the other of the reagents given in 7. Of the solution of hyponitric acid in sulphuric acid a single drop on a glass rod suffices to produce the reaction most distinctly. I can therefore strongly recommend this reagent, which was first proposed by Otto. Red fuming nitric acid must be added in somewhat larger quantity, to call forth the reaction in its highest intensity; this reagent therefore is not well adapted to detect very minute quantities of iodine. The reaction with nitrite of potassa also is very delicate. The fluid to be tested is mixed with dilute sulphuric acid or with hydrochloric acid to distinctly acid reaction, and a drop or two of a concentrated solution of nitrite of potassa is then added. In cases where the quantity of iodine present is very minute, the fluid turns reddish, instead of blue. An excess of the fluid containing nitrous acid or hyponitric acid does not materially impair the delicacy of the reaction. As iodide of starch dissolves in hot water to a colorless liquid, the fluids must of necessity be cold; the colder they are, the more delicate the reaction.

9. Chlorine gas and chlorine water decompose compounds of iodine also, setting the iodine free; but if the chlorine is applied in excess, the liberated iodine combines with it to colorless chloride of iodine. A dilute solution of a metallic iodide, mixed with starch-paste, acquires therefore upon addition of a little chlorine water at once a blue tint, but becomes colorless again upon addition of more chlorine water. As it is therefore difficult not to exceed the proper limit, especially where the quantity of iodine present is only small, chlorine water is not well adapted for the detection of minute quantities of iodinė.

10. If a solution in which iodine has been set free by nitrous acid, chlorine, &c., is mixed with ether, and the mixture shaken, the ether dissolves the liberated iodine, and acquires thereby a reddish-brown or yellow color. The color imparted to the ether by iodine is much more intense than that imparted to that fluid by an equal quantity of bromine. If the solution is mixed with some chloroform or bisulphide of carbon, and the mixture shaken, leaving a few drops of the reagent undissolved, these will subside to the bottom of the fluid, exhibiting a lighter or darker red color. This reaction also is very delicate.

11. If metallic iodides are heated with concentrated sulphuric acid, or with sulphuric acid and binoxide of manganese, or with sulphuric acid and chromate of potassa, iodine separates, which may be known by the color of its vapor and, in the case of very minute quantities, also by its action upon a slip of paper coated with starch-paste.

12. The iodides which are insoluble in water and nitric acid comport

themselves upon fusion with carbonate of soda and potassa in the same manner as the corresponding chlorides.

13. A phosphate of soda and ammonia bead, saturated with oxide of copper, when mixed with a substance containing iodine, and ignited in the inner blowpipe flame, imparts an intense GREEN color to the flame.

§ 155.

d. HYDROCYANIC ACID (H Cy).

1. Cyanogen is a colorless gas of a peculiar, penetrating odor; it burns with a crimson flame, and is pretty soluble in water.

2. Hydrocyanic acid is a colorless, volatile, inflammable liquid, the odor of which resembles that of bitter almonds; it is miscible with water in all proportions; in the pure state it speedily suffers decomposition. It is extremely poisonous.

3. The cyanides with alkalies and alkaline earths are soluble in water; the solutions smell of hydrocyanic acid. They are readily decomposed by acids, even by carbonic acid; but ignition fails to decompose them, if the access of air is precluded. When fused with oxides of lead, copper, antimony, tin, &c., the cyanides reduce these oxides, and are converted into cyanates. Only a few of the cyanides with heavy metals are soluble in water; all of them are decomposed upon ignition, the cyanides of the noble metals being converted into cyanogen gas and metal, the cyanides of the other heavy metals into nitrogen gas and metallic carbides. Many of the cyanides with heavy metals are not decomposed by dilute oxygen acids, and only with difficulty by concentrated nitric acid. But hydrochloric acid and hydrosulphuric acid decompose most of them readily and completely.

4. The cyanides have a great tendency to combine with each other; hence most of the cyanides of the heavy metals dissolve in cyanide of potassium. The resulting compounds are either:

a. True double salts, compounds of the second class, e.g., K Cy + Ni Cy. From solutions of such double salts, acids, by decomposing the cyanide of potassium, precipitate the metallic cyanide which was combined with it. Or they are:

b. Simple haloid salts, compounds of the first class, in which a metal, e. g., potassium, is combined with a compound radical consisting of cyanogen and another metal (iron, cobalt, manganese, chromium). Compounds of this kind are the ferro- and ferricyanide of potassium, K, Cy, Fe or K, Cfy, and K, Cy, Fe, or K, Cfdy, cobalticyanide of potassium, K, Cy Co,, &c. From solutions of compounds of this nature dilute acids do not separate metallic cyanides in the cold. If the potassium is replaced by hydrogen, peculiar hydracids are formed, which must not be confounded with hydrocyanic acid.

2

3

We will now first consider the reaction of hydrocyanic acid and the simple cyanides, then, in an appendix to this paragraph, those of hydroferro- and hydroferricyanic acid.

5. Nitrate of silver produces in solutions of free hydrocyanic acid and of cyanides of the alkali metals white precipitates of CYANIDE OF SILVER (Ag Cy), which are readily soluble in cyanide of potassium, dissolve with some difficulty in ammonia, and are insoluble in nitric acid; these pre

cipitates are decomposed upon ignition, leaving metallic silver with some paracyanide of silver.

6. If solution of sulphate of protoxide of iron which has been for some time in contact with the air is added to a solution of free hydrocyanic acid, no alteration takes place; but if solution of potassa or soda is now added, a bluish-green precipitate forms, which consists of a mixture of Prussian-blue (Fe, Cfy,), and hydrate of protosesquioxide of iron. Upon now adding hydrochloric acid (best after previous application of heat), the hydrate of protosesquioxide of iron dissolves, whilst the PRUSSIANBLUE remains undissolved. If only a very minute quantity of hydrocyanic acid is present, the fluid simply appears green after the addition of the hydrochloric acid, and it is only after long standing that a trifling blue precipitate separates from it. The same reactions are observed when sulphate of protoxide of iron is added to the solution of an alkaline cyanide.

7. If a liquid containing a small quantity of hydrocyanic acid or cyanide of potassium is mixed with a small quantity (one or two drops) of yellow sulphide of ammonium and a trace of ammonia, and the mixture warmed in a porcelain dish until it has become colorless, sulphocyanide of ammonium is formed, and the fluid, after being acidified with hydrochloric acid, acquires a blood-red tint upon addition of sesquichloride of iron (Liebig). This reaction is exceedingly delicate. The following formula expresses the transformation of hydrocyanic acid into sulphocyanide of ammonium: NH, S, + 2 (N H,, O) + 2 H Cy = 2 (N H1, Cy S2) + NH, S + 2H O. If an acetate is present, the reaction takes place only upon addition of more hydrochloric acid.

5

S. Neither of the above methods will serve to effect the detection of cyanogen in cyanide of mercury. To detect cyanogen in that compound, the solution is mixed with hydrosulphuric acid sulphide of mercury precipitates, and the solution contains free hydrocyanic acid. In solid cyanide of mercury the cyanogen is most readily detected by heating in a glass tube. Compare 3.

Appendix.

a. Hydroferrocyanic acid (2 H Cfy). Hydroferrocyanic acid is soluble in water. In solutions of this acid or of soluble ferrocyanides, sesquichloride of iron produces a blue precipitate of FERROCYANIDE OF IRON (Fe, Cfy); sulphate of oxide of copper, a brownish-red precipitate of FERROCYANIDE OF COPPER (Cu, Cfy); nitrate of silver, a white precipitate of FERROCYANIDE OF SILVER (Ag, Cfy), which is insoluble in nitric acid and in ammonia, but dissolves in cyanide of potassium. Insoluble ferrocyanides of metals are decomposed by boiling with solution of soda, ferrocyanide of sodium being formed and the oxides thrown down. When heated with 3 parts of sulphate and 1 part of nitrate of ammonia, they yield sulphates of the metals contained in them, the whole of the cyanogen volatilizing in form of cyanide of ammonium and the products of its decomposition (Bolley).

b. Hydroferricyanic acid. In the aqueous solutions of hydroferricyanic acid and its salts, sesquichloride of iron produces no blue precipitate; but sulphate of protoxide of iron produces a blue precipitate of PROTOFERRICYANIDE OF IRON (3 Fe, Cfdy); sulphate of copper, a yellowish-green precipitate of FERRICYANIDE OF COPPER (3 Cu, Cfdy), which is insoluble

in hydrochloric acid; nitrate of silver, an orange-colored precipitate of FERRICYANIDE OF SILVER (3 Ag, Cfdy), which is insoluble in nitric acid, but dissolves readily in ammonia and in cyanide of potassium. The insoluble ferricyanides of metals are decomposed by boiling with solution of soda, the metallic oxides being thrown down; in the fluid filtered off from them, either ferrocyanide of sodium alone is found, or a mixture of ferro- with ferricyanide of sodium. By heating with sulphate and nitrate of ammonia the ferricyanides are decomposed the same as the ferrocyanides.

$156.

e. HYDROSULPHURIC ACID (H S).

Sulphuretted Hydrogen.

1. Sulphur is a solid, brittle, friable, tasteless body, insoluble in water. It occurs occasionally in the form of yellow or brownish crystals, or crystalline masses of a yellow or brownish color, and occasionally in that of a yellow or yellowish-white or grayish-white powder. It melts at a moderate heat; upon the application of a stronger heat it is converted into brownish-yellow vapors, which, in cold air, condense to a yellow powder, and on the sides of the vessel, to drops. Heated in the air, it burns with bluish flame to sulphurous acid, which betrays its presence in the air at once by its suffocating odor. Concentrated nitric acid, nitrohydrochloric acid, and a mixture of chlorate of potassa and hydrochloric acid dissolve sulphur gradually, with the aid of a moderate heat, and convert it into sulphuric acid; in boiling solution of soda sulphur dissolves to a yellow fluid, which contains sulphide of sodium and hyposulphite of soda; in ammonia sulphur is insoluble.

2. Hydrosulphuric acid, at the common temperature and under common atmospheric pressure, is a colorless, inflammable gas, soluble in water, and which may be readily recognised by its characteristic smell of rotten eggs; it transiently imparts a red tint to litmus paper.

3. Of the sulphides only those with alkalies and alkaline earths are soluble in water. These, and the sulphides of iron, manganese, and zinc, are decomposed by dilute mineral acids, with evolution of hydrosulphuric acid gas, which may be readily detected by its peculiar smell, and by its action upon solution of lead (see 4). The decomposition of higher sulphides is attended also with separation of sulphur in a finely-divided state; the white precipitate may be readily distinguished from similar precipitates by its deportment on heating. Part of the sulphides of the metals of the fifth and sixth groups are decomposed by concentrated and boiling hydrochloric acid, with evolution of hydrosulphuric acid gas, whilst others are not dissolved by hydrochloric acid, but by concentrated and boiling nitric acid. The compounds of sulphur with mercury resist the action of both acids, but dissolve readily in nitrohydrochloric acid. Upon the solution of sulphides in nitric acid, and in nitrohydrochloric acid, sulphuric acid is formed, and the process of solution is moreover attended, in most cases, with separation of sulphur, which is readily recognised by its color and by its deportment upon heating. Many metallic sulphides, more especially of a higher degree of sulphuration, give a sublimate of sulphur when heated in a test-tube. 4. If hydrosulphuric acid, in the gaseous state or in solution, is brought into contact with nitrate of silver or acetate of lead, black precipitates of