HYDROCYANIC ACID-CYANIDES. 297 hot tube it is wholly converted into hydrocyanic acid and water; H1NCHO2 = HЄN + 2 H‚. If equal measures of concentrated hydrocyanic and hydrochloric acids be mixed together, formic acid and ammonia are reproduced. Hydrocyanic acid, mixed with a peculiar essential oil, is obtained by distillation from the kernels of the bitter almond, and from those of many varieties of stone fruit: it is also present in the water which is distilled off the leaves of the laurel, the peach, and some other shrubs: the juice of the tapioca plant (Jatropha manihot) likewise contains it, and it is also formed under various circumstances during the oxidation and decomposition of some kinds of nitrogenized substances. The preparation of diluted hydrocyanic acid by distillation of ferrocyanide of potassium with diluted sulphuric acid has been already noticed, but as the acid is now frequently employed in medicine, it is highly important, on account of its energetic action, to be able to insure its preparation of an uniform strength. This is easily attained by the process of a former Pharmacopoeia, which directs 48 grains of cyanide of silver to be suspended in an ounce of water, and to be decomposed by 394 grains of hydrochloric acid, decanting the clear liquid from the chloride of silver; this acid contains 2 per cent. of the anhydrous acid. The acid when dilute is less prone to decomposition than when concentrated, especially if a little free sulphuric acid be present: but it should always be excluded from the light. This acid is extremely volatile, and if a bottle containing the diluted acid be left open for a few hours it will be found to have suffered a very material reduction in strength; indeed the mere opening and closing the bottle in dispensing the medicine always reduces its strength. When subjected to distillation a large quantity is usually lost, and the greater portion of the acid comes over in the first fourth of the distillate. What is called Scheele's acid varies greatly in strength, owing to the difficulty of condensing the acid vapour. It is directed to be prepared by mixing 10 parts of ferrocyanide of potassium with 3'75 of oil of vitriol previously diluted with 40 parts of water, and distilling over till 10 parts are collected. It seldom contains more than 5 per cent. of the acid, and the proportion is often considerably less. (503) Cyanides.-The cyanides of the alkaline metals are freely soluble in water. Many of the cyanides of the heavier metals are insoluble in water, but most of them are decomposed with evolution of hydrocyanic acid when boiled with hydrochloric acid; those of silver and mercury, when heated to redness, yield 298 TESTS FOR HYDROCYANIC ACID. cyanogen gas. Solutions of mercurous salts give a grey precipitate of the reduced metal; but the mercuric salts give no precipitate with the cyanides. Most of the cyanides which are insoluble in water may be dissolved by means of a solution of the cyanides of the metals of the alkalies, or of the alkaline earths: in such cases double cyanides are generally formed. Liebig has given a ready method for the exact determination of the strength of a solution of hydrocyanic acid, founded upon the solubility of these double cyanides:-The acid to be tested is supersaturated with a solution of caustic potash, and a standard solution of nitrate of silver (17 grains of nitrate in 1000 measured grains of water) is gradually added, agitating the mixture after each addition; as soon as the precipitate is no longer redissolved, the number of divisions of nitrate added must be read off. 17 grains of the nitrate of silver represent 5'4 grains of hydrocyanic acid. The reaction is the following:: The presence of chlorides does not interfere with the application of the test. Sulphate of copper may be substituted for the nitrate of silver, if the hydrocyanic solution be rendered alkaline with ammonia instead of with potash. The reaction is complete as soon as the liquid acquires a slight blue tinge: 14'5 grains of the crystallized sulphate represent 5'4 grains of hydrocyanic acid. The presence of the soluble cyanides, or of hydrocyanic acid in solution, may be determined by the following tests: 1. With nitrate of silver a white, curdy precipitate, which does not blacken by exposure to light, is formed; it is nearly insoluble in cold nitric acid; when heated to redness it gives off the inflammable vapour of cyanogen.-2. If to the liquid a slight excess of potash be added, and then a mixture of ferrous and ferric sulphate, a precipitate of hydrated ferric and ferrous oxide is occasioned, which when treated with excess of hydrochloric acid leaves Prussian blue. The reaction, omitting the water of hydration of the oxide, may be thus represented; 18 KCy + 2 Fe¿Ð ̧, 3 FeÐ + 18 HCl = 18 KCl + 9 H2O + Fe̟,Fe, Cy18. This Fe2O test may be modified by heating gently the suspected mixture with sulphuric acid, and suspending in the flask or retort for a few minutes, a piece of paper moistened with a solution of potash; on dropping a weak solution of the mixed sulphates of iron upon the paper, and immersing it in diluted sulphuric acid, hydrocyanic acid may be recognised by the formation of Prussian blue when very minute traces only are present.-3. Let the liquid be 3' 2 TESTS FOR HYDROCYANIC ACID-CYANIC ACID. 299 acidulated with a few drops of hydrochloric acid, place it in a watch-glass, and let a second watch-glass be inverted over it, moistened with a drop of a solution of sulphuretted hydrosulphate of sulphide of ammonium (containing bisulphide of ammonium); after a few minutes let the upper watch-glass be removed, and the liquid be evaporated to dryness by steam heat; 2H1NS + HCy H1NCYS + H,NHS; let the dry residue be treated with a drop of a weak solution of ferric chloride; a red sulphocyanide of iron is formed under these circumstances. = The cyanides of iron, cobalt, chromium, platinum, and some other metals, form, with the cyanides of the alkalies and the earths, compounds of a peculiar character, in which the presence of the iron, or the cobalt, &c., cannot be detected by the usual tests for these metals. Some of these compounds are of considerable importance, and will be noticed at a future point. (504) CYANIC ACID : HCyO, or HO,CyO=43.—If cyanogen gas be passed into an alkaline solution, a change ensues something analogous to that which occurs when chlorine is similarly treated, cyanide and cyanate of the metal being produced; but the cyanic acid contains a smaller proportion of oxygen than chloric acid does; Cy2+2 KHQ=KCy+KCy++ H2→. If carbonate of potassium be heated in cyanogen gas, a mixture of cyanide and cyanate of potassium is formed, whilst carbonic anhydride is set free: K ̧€Ð ̧+Cy2=KCy+KCy✪+€Ð ̧. Cyanate of potassium, however, is better prepared in a state of purity, by fusing the cyanide of potassium in a crucible, and adding litharge (oxide of lead) in small quantities, till the oxide ceases to be decomposed; KCy+Pb0=KCy++Pb. The cyanate is easily separated from the reduced lead and the excess of oxide of lead, which, from their superior density, sink through the melted mass to the bottom. Another method of preparing the cyanate of potassium consists in heating an intimate mixture of 2 parts of thoroughly dried ferrocyanide of potassium with 1 part of finely powdered anhydrous black oxide of manganese: the mixture is placed upon a sheet-iron plate and heated to dull redness, and the mass is kept constantly stirred. The oxidation of the cyanide is effected partly at the expense of the oxygen in the oxide of manganese, partly of that in the atmosphere. When the combustion has ceased, cyanate of potassium may be dissolved out of the residue with hot alcohol, from which it crystallizes in deliquescent plates as the solution cools. Cyanate of potassium, if kept dry, may be preserved without 300 CYANIC ACID-CYAMELID. change; but so unstable is the cyanic acid when uncombined with a fixed base, that on attempting to separate it from the cyanate of potassium by the addition of sulphuric or any other strong acid, traces of it only are obtained: a brisk effervescence ensues, each atom of the cyanic acid assimilates the elements of water, and is almost entirely resolved into 1 atom of ammonia, which remains in combination with the acid employed in decomposing the cyanate, and 1 atom of carbonic anhydride, which escapes with effer 3 31 3 Cyanic acid may be otherwise procured: viz., by distilling cyanuric acid (H ̧¤ ̧Ñ ̧Ð ̧), which is a crystallizable acid substance, I atom of which contains exactly the same elements as 3 atoms of cyanic acid. When this compound is sealed up in a bent glass tube, one limb of which is kept cool whilst heat is applied to the cyanuric acid in the other limb, a limpid, colourless liquid distils over and is condensed. The cyanuric acid is thus wholly converted into pure cyanic acid. Cyanic acid in its turn may be converted into the cyanuric, for if cyanate of potassium be decomposed by the addition of half its equivalent of acetic acid, an acid cyanurate of potassium is formed. Cyanic acid has an extremely pungent odour, and is very volatile; its vapour attacks the eyes powerfully, and when liquid it acts as a powerful caustic if incautiously dropped upon the skin. It is, however, impossible to preserve this compound, for in the course of a few hours it changes spontaneously, with evolution of heat, into a white enamel-like mass, which is permanent in the air, insoluble in water, and destitute of acid properties. To this body the name of cyamelid has been given: it has the same centesimal composition as the hydrated cyanuric and cyanic acids. Cyamelid may also be obtained by triturating cyanate of potassium with crystallized oxalic acid and washing out the soluble oxalate of potassium. It may be converted by heat into hydrated cyanic acid; and if boiled with a solution of potash, gradually yields cyanurate of the base; oil of vitriol decomposes it into carbonate of ammonium. A solution of cyanic acid in water quickly becomes decomposed into carbonate of ammonium and urea. Urea. HЄNO + H2+ = 602+ H2N; and H¤N÷ + H2N = &H ̧Ñ‚Ð. Solutions of the soluble cyanates give white precipitates with. solutions of the salts of lead, of silver, or. of suboxide of mercury; FULMINIC ACID-ISOMERISM. 301 they yield no precipitate with solution of corrosive sublimate, or with the solutions of salts of iron or of tin. With nitrate of copper they give a greenish-brown precipitate, and with terchloride of gold a brown precipitate. (505) FULMINIC ACID.-Besides the remarkable oxides of cyanogen already mentioned, there is another acid which yields on analysis the same per-centage of its components as cyamelid, and the cyanic and cyanuric acids, though it possesses properties totally different from any of them. Its compounds explode with fearful violence. By dissolving I grain of silver in 20 grains of nitric acid diluted with about 50 of alcohol, the new compound is deposited in crystals, which, when dry, detonate with the slightest friction: they consist of (Ag,,N,,). The reaction by which this salt is produced is complicated; the nitrogen, however, is derived from the nitric acid, and the carbon from the alcohol. 2 Fulminic acid has not been obtained in an isolated form; on attempting to separate it from its salts by a more powerful acid it is resolved into hydrocyanic acid and other bodies. Allusion will again be made to the fulminates when other compounds of cyanogen are described among the products of organic chemistry. When the fulminates are boiled with a solution of chloride of sodium, or of chloride of potassium, the acid is converted into another isomeric compound termed isocyanuric or fulminuric acid. This body is but very slightly explosive. (506) Isomerism.—The properties of these oxides of cyanogen serve clearly to show that mere identity in ultimate composition is not sufficient to produce identity of chemical character or properties; they place the doctrine of isomerism (or the existence of compounds identical in ultimate composition, but different in chemical properties) in a striking point of view. Numerous other instances will occur as we pursue further the study of the different compounds, not only of cyanogen, but of other bodies, and particularly of those which form the subject of organic chemistry. There are various forms of isomerism; in some cases we have no clue to the probable differences of molecular arrangement: in others there is every reason to suppose that the arrangement of the elementary molecules is on a totally different plan in the two bodies which are compared. Hydrated cyanic acid, for instance, may be represented as hydrated oxide of cyanogen; whilst it is certain that in cyamelid, which is insoluble, and presents nothing of the acid character, the arrangement of its constituents is quite different. Isomeric compounds, the equivalent numbers of which are identical are said to be metameric. In other cases, the diffe |