Prussiate of Potash, (NC) β Fe.K 4 + 3H 2 O (syn. ferrocyanide of potassium; Germ. Blutlaτιgensalz).— This salt is being produced industrially from animal refuse (hide and horn clippings, old shoes, blood solids, &c. ), carbonate of potash, and iron filings or borings as raw materials. The carbonate of potash is fused at a red heat in an iron pear-shaped vessel suspended within a furnace, or on the cupel-shaped sole of a reverberatory furnace, and the animal matter, which should be as dry as possible, is then introduced in instalments along with the iron. The fusion is continued as long as inflammable gases are going off; then the still fluid mass is ladled out and allowed to cool, when it hardens into a black stone-like body known to the manufacturer as “metal.” When the broken-up metal is digested with water in an iron vessel prussiate of potash passes into solution, while a black residue of charcoal, metallic iron, sulphide of iron, &c., remains. The clarified solution, after sufficient concentration in the heat, deposits on cooling part of its prussiate in lemon-yellow quadratic crystals (generally truncated octahedra), which are purified by recrystallization. The last mother-liquors furnish an impure green salt, which is added to a fresh fuse and so utilized.

In former times it was believed that the prussiate was produced during the fusion process, and in the subsequent process of lixiviation simply passed into solution, until Liebig showed that this view was untenable. The fuse cannot contain ready-formed prussiate, because this salt at a red heat breaks up with formation of a residue of carbide of iron and cyanide of potassium. The metal in fact when treated with dilute alcohol gives up to it plain cyanide of potassium, and the fully exhausted residue yields no prussiate on treatment with water. The prussiate accordingly must be produced during the process of lixiviation by the action of the cyanide of potassium on some ferrous compound in the metal. Liebig thought that it was partly the metallic iron, partly the sulphide of iron present in the metal, which effected the conversion. According to more recent researches a double sulphide, K 2 S + Fe 2 S 3 , which is always produced during the fusion (from the reagents proper and the sulphur of the organic matter and that of the sulphate of potash present in the carbonate as an impurity), plays this important part. The double sulphide by the action of water breaks up into alkaline sulphide, sulphide of iron (FeS), and sulphur. This sulphide of iron is of a peculiar kind; it does what ordinary FeS does not effect, readily at least: it converts the cyanide into prussiate, thus, 6NC. K + FeS = K 2 S + (NC) β Fe.K 4 , while the eliminated sulphur of the original Fe 2 S 3 unites with another part of the cyanide of potassium into sulphocyanate, S + NCK = SNC.K, which latter salt is thus unavoidably produced as a (rather inconvenient) byeproduct. Pure prussiate of potash has the specific gravity 1*83; it is permanent in the air. It loses its water, part at 60° C., the rest at 100° C., but very slowly. The anhydrous salt is a white powder. The crystals dissolve in four parts of cold and in two parts of boiling water. It is insoluble in, and not dehydrated by, alcohol.

Prussiate of potash has the composition of a double salt, Fe(NC) 2 +4KNO, but the idea that it contains these two binary cyanides is entirely at variance with its reactions. Cyanide of potassium is readily decomposed by even the feeblest acids, and to some extent even by water, with elimination of hydrocyanic acid, and on this account perhaps is intensely poisonous. A solution of the prussiate remains absolutely unchanged on evaporation, and the action on it even of strong acids in the cold results in the formation of the hydrogen salt, (NC) β FeH 4 , which is decomposed, it is true, but only when the mixture is heated, with evolution of hydrocyanic acid. It is not poisonous. Its solution when mixed with nitrate of silver does not give a precipitate of cyanide of silver, NC. Ag, and a solution of the two nitrates, but yields a unitary precipitate of the composition (NC) 6 Fe.Ag 4 , which contains all the iron; only nitrate of potassium passes into solution. Other heavy metallic salts behave similarly. On the strength of these considerations chemists, following the lead of Liebig, view prussiate as a binary compound of potassium, K 4 , with a complex radical, N β C β Fe, ‘ ‘ ferrocyanogen.”

Hydrocyanic Acid, NC.H.—This acid is prepared most conveniently from prussiate of potash. Wöhler recommends the following method. Ten parts of powdered prussiate are placed in a retort, the neck of which is turned upwards, and a (cooled down) mixture of seven parts of oil of vitriol and fourteen parts of water is then added. If the aqueous acid is wanted, the exit-end of the retort is joined on direct to a Liebig’s condenser, which must be kept very cool by a current of cold water. If the anhydrous acid is desired, two wide-necked bottles (or two large U-tubes) charged with fused chloride of calcium and kept at 30“ C. by immersion in a water bath of this temperature, must be inserted between the retort and condenser. In this case more particularly it is indispensable to provide for a most efficient condensation of the vapours; the exitend of the condenser should be provided with an adapter going down to near the bottom of the receiver, which must be surrounded by a freezing mixture. The temperature of the latter, of course, must not be allowed to fall to the freezing-point of the distillate. The retort is heated by means of a sand bath and a brisk distillation maintained until the residue begins to dry up. The result of the reaction is in accordance with the assumption that the dilute vitriol, in the first instance, converts the prussiate, one-half into (NC) 6 Fe.H 4 , the other into (NC) β Fe.K 2 H 2 , and that through the effect of the heat these two bodies decompose each other into {(NC) 6 Fe} K 2 Fe, which remains in the residue as a precipitate, and (NC) β H 6 =6NCH, which distils over. Real NCH is a colourless liquid of 0 ’ 6967 specific gravity at 18° C., which freezes at -15° C. (Gay-Lussac) into a white fibrous solid. According to Schulz the acid, if really pure, remains liquid at - 37° C. It boils at 26° '5 C.; at 4° ’ 5 its vapour-tension already amounts to half an atmosphere. The vapour is inflammable and burns into carbonic acid, water, and nitrogen. The acid mixes with water in all proportions, with contraction and yet absorption of heat. The solution behaves on distillation like a mere mechanical mixture of its two components. Prussic acid has a very peculiar powerful smell; more characteristic still is a kind of choking action which even the highly attenuated vapour exerts on the larynx. Prussic acid is fearfully poisonous; a few drops of even the ordinary pharmaceutical preparation (of 2 per cent.) are sufficient to kill a large dog. It acts with characteristic promptitude, especially when inhaled as a vapour. Even a relatively large dose, if it has once found its way into the stomach without producing a fatal effect, is said to do relatively little harm there.

Prussic acid is characteristically prone to suffer “spontaneous decomposition.” Whether the pure anhydrous acid really is, in the strictest sense of the word, still requires to be found out; the ordinary preparation, when kept in a close bottle, soon turns brown and turbid from ‘ ‘ azulmic " acid, a substance of complex constitution. Other things are formed at the same time. The pure aqueous acid is liable to similar changes; in its case formiate of ammonia always forms the predominant product. This change is easily understood—

NC. H + 2H 2 O = NH 3 + H. COOH. Ammonia. Formic acid.

A strong aqueous prussic acid, when mixed with fuming hydrochloric acid, is soon converted into a magma of crystals of salammoniac, with formation of formic acid, which remains dissolved. And yet, most singularly, the addition to the preparation of a small proportion of hydrochloric or sulphuric acid is the best means for preventing, or at least greatly retarding, its spontaneous change in the very same direction. Aqueous prussic acid acts only very feebly (if at all) on blue litmus; it combines with aqueous caustic alkalis but does not decompose their carbonates; nor does it act upon the generality of insoluble basic metallic oxides or hydrates; mercuric oxide and oxide of silver form noteworthy exceptions to this rule.

Cyanogen, (NC) 2 ∙—When dry mercuric cyanide is heated it breaks up, below redness, into mercury and cyanogen gas; part of the latter, however, always suffers polymerization into a solid called “.paracyanogen,” and presumed to consist of molecules (NC) S . Cyanogen gas is colourless; it has the specific gravity demanded by its formula. It possesses a peculiar odour and has a characteristic irritating effect on the eyes and mucous membranes of the nose. It is poisonous. By strong pressure it is condensible into a liquid which freezes at — 34 o- 4 C., and has the following vapour-tensions P at the temperatures t stated— [table]

At ordinary temperatures water dissolves about 4 '5 times, alcohol about 23 times its volume of the gas. The solutions are liable to (very complex) spontaneous decomposition. The list of products includes oxalate of ammonia and urea. Cyanogen burns with a characteristically beautiful peach-blossom coloured flame into carbonic acid and nitrogen. This gas cyanogen, as already stated, is to cyanides what chlorine gas Cl 2 is to chlorides, but it is well to remember that the analogy, though perfect in regard to the corresponding formulae, does not, as a rule, extend to the conditions of formation of the bodies represented. Thus cyanogen does not unite with hydrogen into prιtssic acid, nor does it combine with ordinary metals in the chlorine fashion. When passed over heated potassium, it is true, it combines with it into cyanide; and caustic potash-ley absorbs it λvith formation of cyanide and cyanate (NCO.K), just as chlorine yields chloride and hypochlorite KC10; but this is about the sum-total of the analogies in action. Yet metallic cyanides of all kinds can be produced indirectly.

Cyanide of Potassium, NC.K.—An aqueous mixture of the quantities NCH and KHO no doubt contains this salt, but it smells of the acid, and on evaporation behaves more like a mixture of the two congeners than in any other way. An exhaustive union can be brought about by passing NCH vapour into an alcoholic solution of KHO; the salt NC.K then comes down as a crystalline precipitate, which must be washed with alcohol and dried, cold, over vitriol. A more convenient method is to dehydrate yellow prussiate and then decompose it by heating it to redness in an iron crucible. The Fe(NC) 2 part of the salt breaks up into cyanogen and nitrogen, which go off, and a carboniferous finely-divided iron, which remains, with cyanide of potassium, which at that temperature is a thin fluid. Yet the iron sometimes refuses to settle with sufficient promptitude to enable one to decant off the bulk even of the fused cyanide. According to private information received by the writer a French manufacturer uses a certain kind of very porous fireclay as an efficient filtering medium.

The ordinary “cyanide of potassium” of trade is not strictly that at all, but at best a mixture of the real salt with cyanate. It is produced by fusing a mixture of eight parts of anhydrous prussiate and three parts of anhydrous carbonate of potash, allowing the reaction

(NC) 6 Fe. K 4 + K 2 CO 3 = CO 2 + Fe + 5NCK + K. NCO

Cyanate to complete itself and the iron to settle, and decanting off the clear fuse. The product goes by the name of “Liebig’s cyanide,” but the process was really invented by Rodgers.

Fused cyanide of potassium assumes on cooling the form of a milky white stone-like solid. It fuses readily at a red heat, and at a white heat volatilizes without decomposition, provided that it is under the influence of heat alone; in the presence of air it gradually passes into cyanate; when heated in steam it is converted into carbonate of potash with evolution of ammonia, carbonic oxide, and hydrogen. When heated to redness with any of the more easily reducible metallic oxides it reduces them to the metallic state, while it passes itself into cyanate. It also reduces the corresponding sulphides with formation of sulphocyanate; for example, Pb(S or 0)+ NCK = Pb +NC(O or S)K. Hence its frequent application in blowpipe analysis. When heated with chlorates or nitrates it reduces them with violent explosion. The aqueous solution of the salt has a strongly alkaline reaction; it smells of hydrocyanic acid and is readily decomposed by even such feeble acids as acetic or carbonic. It readily dissolves precipitated chloride, bromide, and iodide of silver; this is the basis of its application in photography. Large quantities of the salt are used in electroplating.

Other Binary Cyanides.— Of these only a few can be noticed here. (1) Cyanide of sodium is very similar to the potassium salt. The same remark, in a more limited sense, holds for the cyanides of barium, strontium, and calcium. (2) Cyanide of ammonium (NC.NH 4 ) forms crystals volatile at 36° C. and smelling of ammonia and hydrocyanic acid. The solution in water decomposes spontaneously, pretty much like that of the free acid. But the anhydrous vapour by itself stands a high temperature, as is proved by the fact that it is produced largely when ammonia is passed over red-hot charcoal, C + 2NH 3 = H 2 + NCH. NH 3 . (3) Mercuric cyanide, Hg(NC) 2 , forms very readily when mercuric oxide is dissolved in aqueous prussic acid. The solution on evaporation and cooling deposits crystals soluble in eight parts of cold water. This salt is not at all decomposed, even when heated, by water, nor appreciably by dilute sulphuric or nitric acid; boiling hydrochloric acid eliminates the NC as hydrocyanic acid; sulphuretted hydrogen acts similarly in the cold. It gives no precipitate with nitrate of silver, nor is it changed visibly by caustic alkalis. It readily unites not only with other cyanides but also with a multitude of other salts into crystallizable double salts. Mercurous cyanide, Hg 2 (NC) 2 , seems to have no existence. When it is attempted to produce it by double decompositions, the mixture Hg + (NC) 2 Hg comes forth instead of the compound Hg 2 (NC) 2 ∙ (4) Heavy metallic cyanides are mostly insoluble in water, and the general method for their preparation is to decompose a solution of the respective sulphate, chloride, &c., with one of cyanide of potassium. The most important general property of these bodies is that they readily dissolve in solution of cyanide of potassium with formation of double cyanides, which in their capacity as double salts all exhibit, in a higher or lower degree, those anomalies which were fully explained above (see “prussiate of potash ”). These “metallo-cyanides,” as we will call them, being all, unlike plain cyanide of potassium, very stable in opposition to water and aqueous alkalis, are readily produced from almost any compound of the respective metallic radical—some from the metal itself— by treatment with solution of cyanide of potassium. In all we have said “potassium” may be taken as including sodium and in a limited sense ammonium, but the potassium compounds are best known, and we accordingly in the following section confine ourselves to these.

Metallo-cyanides. —(1) Silver.— Cyanide of silver, Ag.NC, is produced as a precipitate by addition of hydrocyanic acid or cyanide of potassium to solution of nitrate of silver. The precipitate is similar in appearance to chloride of silver and, like it, insoluble in cold dilute mineral acids, but soluble in ammonia. At a red heat it is decomposed with formation of a residue of carboniferous metallic silver. Precipitated cyanide of silver, though insoluble in hydrocyanic acid, dissolves readily in cyanide of potassium with formation of argentocyanide, AgK. (NC) 2 , which is easily obtained in crystals, permanent in the air and soluble in eight parts of cold water. Chloride of silver dissolves in cyanide of potassium solution as readily as the cyanide does and with formation of the same double salt— AgCl + 2 KN C = KC1 + AgK( NC) 2 . This salt is used very largely in electro-plating. (2) Lead.— From a solution of the acetate cyanide of lead is precipitated by addition of hydrocyanic acid or cyanide of potassium. The precipitate, Pb(NC) 2 , has the exceptional property of being insoluble in cyanide of potassium. (3) Zinc.— Cyanide of zinc, Zn(NC) 2 , is obtained by addition of hydrocyanic acid to a solution of the acetate, as a white precipitate readily soluble in cyanide of potassium with formation of a double salt, ZnK 2 (NC) 4 , which forms well-defined crystals. (4) Nickel.— The cyanide, Ni(NC) 2 , is an apple-green precipitate, which is obtained by methods similar to those given under ‘ ‘ zinc.” It readily dissolves in cyanide of potassium with formation of a crystallizable salt, NiK 2 (NC) 4 + H 2 O, the solution of which is stable in air and not convertible into one of a nickelic (Ni"') compound by chlorine (compare “cobalt” infra). The potassio-cyanides of silver, zinc, and nickel as solutions are not changed visibly by caustic alkalis, but their heavy metals can be precipitated by sulphuretted hydrogen or sulphide of ammonium, as from solutions of, for instance, the chlorides. Aqueous mineral acids (in the heat at least) decompose them exhaustively with elimination of all the NC as NCH. (5) Copper.— When cyanide of potassium solution is added to one of sulphate of copper, a yellow precipitate of cupι - ic cyanide, Cu(NC) 2 , comes down; but on boiling this precipitate loses cyanogen and is converted into a white precipitate of the cuprous salt Cu(NC). This white precipitate dissolves in cyanide of potassium with formation chiefly of two crystalline double salts, viz., CuNC + 6NCK, easily soluble in water, and CuNC + NCK. The latter is decomposed by water with elimination of Cu.NC. The solution of the 6NC.K salt is not precipitated by sulphuretted hydrogen. Solutions of potassio-cyanides of cuprosum are used in electro-plating. (6) Gold. —Metallic gold dissolves in cyanide of potassium solution in the presence of air, thus—

Au + 2KNC + ⅞O=⅜K 2 O+AuK.(NC) 2 .

This auro-cyanide of potassium is used largely for electro-gilding, for which purpose it is conveniently prepared as follows. Six parts of gold are dissolved in aqua regia and the solution is precipitated by ammonia. The precipitate (an explosive compound known as “fulminating gold ”) is dissolved in a solution of six parts of cyanide of potassium, when the double salt is formed with evolution of ammonia. The salt crystallizes in rhombic octahedra, soluble in seven parts of cold water.

In the following potassio - cyanides the heavy metals cannot be detected by means of their ordinary precipitants; these salts all behave like the potassium salts of complex radicals composed of the heavy metal and all the cyanogen. (7) Cobalt.— Cyanide of potassium when added to a solution of...