446 PREPARATION OF FULMINATE OF SILVER. dipped in concentrated sulphuric or nitric acid. The electric spark of course explodes it. Cap composition.-The explosion of the fulminate of mercury is so violent and rapid that it is necessary to moderate it for percussion-caps. For this purpose it is mixed with nitrate or chlorate of potash, the oxidising property of these salts possibly causing them to be preferred to any merely inactive substances, since it would tend to increase the temperature of the flash by burning the carbonic oxide into carbonic acid, and would thus ensure the ignition of the cartridge. For military caps, in this country, chlorate of potash is always mixed with the fulminate, and powdered glass is sometimes added to increase the sensibility of the mixture to explosion by percussion. Sulphide of antimony is sometimes substituted for powdered glass, apparently for the purpose of lengthening the flash by taking advantage of the powerful oxidising action of chlorate of potash upon that compound (p. 157). Since the composition is very liable to explode under friction, it is made in small quantities at a time, and without contact with any hard substance. After a little of the composition has been introduced into the cap, it is made to adhere and waterproofed by a drop of solution of shell-lac in spirit of wine. If a thin train of fulminate of mercury be laid upon a plate, and covered, except a little at one end, with gunpowder, it will be found on touching the fulminate with a hot wire, that its explosion scatters the gunpowder, but does not inflame it. On repeating the experiment with a mixture of 10 grains of the fulminate and 15 grains of chlorate of potash, made upon paper with a card, the explosion will be found to inflame the gunpowder. By sprinkling a thin layer of the fulminate upon a glass plate, and firing it with a hot wire, the separated mercury may be made to coat the glass, so as to give it all the appearance of a looking-glass. Although the effect produced by the explosion of fulminate of mercury is very violent in its immediate neighbourhood, it is very slightly felt at a distance, and the sudden expansion of the gas will burst fire-arms, because it does not allow time for overcoming the inertia of the ball, though, if the barrel escape destruction, the projectile effect of the fulminate is found inferior to that of powder. The fulminate of mercury is generally contaminated with oxalate of mercury (HgO. CO), which is one of the secondary products formed. during its preparation. Fulminate of silver is prepared by a process very similar to that for fulminate of mercury, but since its explosive properties are far more violent, it is not advisable to prepare so large a quantity. 10 grains of pure silver are dissolved, at a gentle heat, in 70 minims of ordinary concentrated nitric acid (sp. gr. 1·42) and 50 minims of water. As soon as the silver is dissolved, the heat is removed, and 200 minims of alcohol (sp. gr. 0-87) are added. If the action does not commence after a short time, a very gentle heat may be applied until effervescence begins, when the fulminate of silver will be deposited in minute needles, and may be further treated as in the case of fulminate of mercury.* When dry, the fulminate of silver must be handled with the greatest caution, since it is exploded far more easily than the mercury salt; it should be kept in small quantities wrapped up separately in paper, and placed in a card-board box. * If the nitric acid and alcohol are not of the exact strength here prescribed, it may be somewhat difficult to start the action unless two or three drops of red nitric acid (containing nitrous acid) are added. Standard silver (containing copper) may be used for preparing the fulminate. CHEMICAL CONSTITUTION OF THE FULMINATES. 447 Nothing harder than paper should be employed in manipulating it. The violence of its explosion renders it useless for percussion caps, but it is employed in detonating crackers. Fulminate of silver is sparingly soluble in cold water, but dissolves in 36 parts of boiling water. If a minute particle of fulminate of silver be placed upon a piece of quartz, and gently pressed with the angle of another piece, it will explode with a flash and smart report. A throw-down detonating cracker may be made by screwing up a particle of the fulminate of silver in a piece of thin paper, with some fragments obtained by crushing a common quartz pebble. The explosion of fulminate of silver may be compared with that of the mercury salt by heating equal quantities upon thin copper or platinum foil, when the fulminate of mercury will explode with a slight puff, and will not injure the foil, but that of silver will give a loud crack and rend a hole in the metal. If a particle of fulminate of silver be placed upon a glass plate and touched with a glass rod dipped in oil of vitriol, it will detonate and leave a deposit of silver upon the glass. When fulminate of silver is dissolved in warm ammonia, the solution deposits, on cooling, crystals of a double fulminate of silver and ammonia, AgO.NH. HO.C.NO,, which is even more violently explosive, and is dangerous while still moist. On adding chloride of potassium in excess to fulminate of silver, only half the silver is removed as chloride, and the double fulminate of silver and potassium, AgO, KO, CN,O,, may be crystallised from the solution. By the careful addition of nitric acid, the KO may be removed from this salt, and the acid fulminate of silver, AgO. HO. C,NO,, obtained, which is easily soluble in boiling water, and crystallises out on cooling; by boiling with oxide of silver, it is converted into the neutral fulminate. Various other fulminates and double fulminates have been obtained. They are all more or less explosive. Chemical constitution of the fulminates.-The fact of the existence of double fulminates and acid fulminates renders it necessary to write the empirical formula of fulminate of silver, for example, Ag,C,N,O,, instead of AgC.NO,, in order to show that half of the silver is capable of being exchanged for another metal or for hydrogen. It will be seen that this formula would also represent two equivalents of cyanate of silver (AgO. C,NO), but the properties of this salt are entirely different from those of the fulminate. That a strong connection exists, however, between the fulminates and the cyanogen-compounds, is shown by several reactions. Thus, if fulminate of mercury be heated with hydrochloric acid, it is dissolved with evolution of a powerful odour of hydrocyanic acid, whilst mercuric chloride and oxalate, with hydrochlorate of ammonia, remain in the solution. Again, if an excess of fulminate of silver be acted on by hydrosulphuric acid, cyanic acid may be obtained in solution, and becomes converted into hydrosulphocyanic acid, when the hydrosulphuric acid is in excess. By decomposing the double fulminate of copper and ammonia (CuO. NH. HO. C,N,O,) with hydrosulphuric acid, there are produced hydrosulphocyanic acid and urea, the latter having the same composition as cyanate of ammonia- CuO.NH,. HO.C ̧N ̧0, + 3HS = CuS + 2HO + H. C.NS2+ C,H,N,O,. Hydrosulphocyanic acid. Urea. These reactions have induced many chemists to regard the fulminates as compounds of the metallic oxides with an acid having the composition Cy,O,, intermediate in composition between the hypothetical anhydrous cyanic acid (CyO) and the hypothetical anhydrous cyanuric acid (Cỵ,0 ̧), but neither the anhydrous nor the hydrated fulminic acid has yet been obtained in a separate form. This view of the constitution of the fulminates, however, has the recommendation of simplicity, and enables the greater number of their reactions to be easily explained. Fulminate of mercury dissolves when boiled with solution of chloride of potassium, and the solution, when evaporated, yields crystals of fulminurate or isocyanurate of potash, KO.CN,H,O,, which has the same percentage composition as acid cyanurate of potash, KO.2HO. Cy,O,, but the acid contained in fulminurate of potash forms only one series of salts, and is therefore monobasic. The fulminurates are feebly explosive. The production of fulminuric acid from the hypothetical hydrate of fulminic acid may be represented by the equation 2(2HO.C.N2O2) + 2HO= 2CO2+ NH3 + HO.C2N2H2O ̧ . PRODUCTS OF THE DESTRUCTIVE DISTILLATION OF COAL. 332. Much of the extraordinary progress made by chemistry during the last half century must be attributed to the introduction and great extension of the manufacture of coal-gas. No other branch of manufacture has brought into notice so many compounds not previously obtained from any other source; and, above all, offering, at first sight, so very little promise of utility, as to press urgently upon the chemist the necessity for submitting them to investigation. Although many important additions to chemical knowledge have resulted from the labours of those who have engaged in devising the best methods of obtaining the coal-gas itself in the state best fitted for consumption, far more benefit has accrued to the science from investigations into the nature of the secondary products of the manufacture, the removal of which was the object to be attained in the purification of the gas. Of the compounds of carbon and hydrogen, very little was known pre viously to the introduction of coal-gas; and although the liquid hydrocarbons composing coal-naphtha were originally obtained from other sources, the investigation of their chemical properties has been greatly promoted by the facility with which they may be obtained in large quantities from that liquid. The most important of these hydrocarbons, benzole or benzine, was originally procured from benzoic acid; but it would have been impossible for it to have fulfilled its present useful purposes but for the circumstance that it is obtained in abundance as a secondary product in the manufacture of coal-gas; for, leaving out of consideration the various uses to which benzole itself is devoted, it yields the nitrobenzole, so much used in perfumery, and from this we obtain aniline, from which many of the most beautiful dyes are now prepared. The naphthaline found so abundantly in coal-tar possesses à peculiar interest, as having formed the subject of the beautiful researches by which Laurent was led to propose the doctrine of substitution, which has since thrown so much light upon the constitution of organic substances. We are also especially indebted to coal-tar for our acquaintance with the very interesting and rapidly-extending class of volatile alkalies, of which the above-mentioned aniline is the chief representative, and for phenic or carbolic acid, from which are derived the large number of substances composing the phenyle-series. MANUFACTURE OF COAL-GAS. 449 The retorts in which the distillation of coal is effected are made either of cast-iron or stoneware, generally having the form of a flattened cylinder, and arranged in sets of three or five, heated by the same coal fire (fig. 267). The charge for each retort is about two bushels, and is thrown on to the red-hot floor of the retort, as soon as the coke from the previous distillation has been raked out; the mouth of the retort is then closed with an iron plate luted with clay. An iron pipe rises from the upper side of the front of the retort projecting from the furnace, and is curved round at the upper extremity, which passes into the side of a much wider tube, called the hydraulic main, running above the furnaces, at right angles to the retorts, and receiving the tubes from all of them. This tube is always kept half full of the tar and water which condense from the gas, and below the surface of this liquid the delivery tubes from the retorts are allowed to dip, so that although the gas can bubble freely through the liquid as it issues from the retort, none can return through the tube whilst the retort is open for the introduction of a fresh charge. The aqueous portion of the liquid deposited in the hydraulic main is known as the ammoniacal liquor, from its consisting chiefly of a solution of various salts of ammonia, the chief of which is the sesquicarbonate; sulphide, cyanide, and sulphocyanide of ammonium are also found in it. From the hydraulic main the gas passes into the condenser, which is composed of a series of bent iron tubes kept cool either by the large surface which they expose to the air, or sometimes by a stream of cold water. In these are deposited, in addition to water, any of the volatile hydrocarbons and salts of ammonia which may have escaped condensation in the hydraulic main. Even in the condenser the removal of the ammoniacal salts is not complete, so that it is usually necessary to pass the gas through a scrubber or case containing fragments of coke, over which a stream of water is allowed to trickle in order to absorb the remaining ammoniacal vapours. 450 PURIFICATION OF COAL-GAS. The tar which condenses in the hydraulic main is a very complex mixture, of which the following are some of the leading components The gas is now passed through the lime-purifier, which is an iron box with shelves, on which dry slaked lime is placed in order to absorb the carbonic acid and sulphuretted hydrogen, and the last portions of ammonia are removed by passing the gas through dilute sulphuric acid. A great many other methods have been devised for the purification of the gas from sulphuretted hydrogen, but none appears to be so efficacious and economical as that which consists in passing the gas over a mixture of sulphate of iron (green vitriol or copperas), slaked lime, and sawdust (which is employed to prevent the other materials from caking together). The lime decomposes the sulphate of iron, forming sulphate of lime and hydrated oxide of iron 3 = FeO. HO + CaO. SOg. FeO.SO, + CaO. HỌ The action of air upon the mixture soon converts the oxide into sesquioxide of iron, which absorbs the sulphuretted hydrogen and the hydrocyanic acid, producing with the former sulphide of iron, and with the latter Prussian blue, or some similar compound. The sulphate of lime existing in this purifying mixture is useful in absorbing any vapour of carbonate of ammonia from the gas, forming sulphate of ammonia and carbonate of lime.t * Benzole, originally derived from benzoic acid; toluole, from balsam of tolu; xylole, found among the products from wood (¿úλov); isocumole, isomeric with cumole, obtained from oil of cummin. † Sesquioxide of iron itself, derived from various natural and artificial sources, is also employed for the purification of coal-gas. |