SEPARATION OF MANGANESE FROM OTHER METALS. 647 again redissolved in hydrochloric acid; chloride of ammonium must be added, and then a mixture of ammonia and carbonate of ammonium in excess, by which the manganese will be held in solution; and if strontium, calcium, or barium be present, they will remain undissolved in the form of carbonates, which must be collected on a filter, weighed, and deducted from the weight of the oxide previously obtained. Separation from Zinc, Cadmium, Cobalt, and Nickel.—The solution is mixed with acetate of potassium in excess, to convert the metals into acetate, then sulphuretted hydrogen is transmitted; the manganese remains in solution, whilst the other metals are precipitated as sulphides if the solution is only faintly acid. If cadmium alone is present, the addition of acetate of potassium is unnecessary. Separation from Iron, Chromium, Uranium, Aluminum, and Glucinum. This is readily effected after converting the iron into a ferric salt and diluting the solution largely with water, by digesting it upon finely levigated carbonate of barium. Manganese alone remains in the liquid, the other oxides being displaced by baryta. The excess of barium is removed by sulphuric acid, and the manganese precipitated by carbonate of sodium. Manganese is connected by isomorphous relations with a great number of the elementary bodies. Its protoxide is isomorphous with the oxides of the magnesian group: its sesquioxide is isomorphous with alumina and the sesquioxides of iron and chromium. The manganates are isomorphous with the sulphates, and the permanganates with the perchlorates. CHAPTER XVII. GROUP VI.-CERTAIN METALS WHICH FORM ACIDS WITH OXYGEN. IN this list columbium and tantalum are omitted, because so little is known of them. The foregoing list of metals capable of yielding acids with oxygen is divisible into three natural families, which have little in common. (See pp. 13 and 340.) § I. TIN: Sn1=118, or Sn=59. Sp. Gr. 7·292; Fusing-point, 442°. (808) This beautiful metal is one of those which have been longest known to man, as it is mentioned in the Books of Moses. Tin, however, is met with in but few localities. Its only ore of importance is the binoxide, or tin-stone, which occurs crystallized in prisms isomorphous with those of rutile. It is usually found in veins, running through primitive rocks of porphyry, granite, or clay-slate, and is generally mingled with the sulphides and arsenides of copper and iron, and frequently also with wolfram. The most celebrated tin mines are those of Cornwall, which were worked before the Roman invasion; they furnish annually upwards of 6000 tons of the metal. The mines of Malacca also yield a very pure tin: the metal is likewise obtained to a smaller extent from Mexico. The tin-veins in Cornwall are frequently associated with those of copper, and they run almost invariably east and west. The tin ore is often met with in alluvial soils, whither it has been carried from its original position by the action of water. In this case the ore occurs in detached, rounded masses, and is very pure, constituting what is termed stream tin. The position of the veins is frequently traced by following the stream EXTRACTION OF METALLIC TIN. 649 towards its source, up to the point where the ore ceases to be found; a careful examination of the vicinity generally leads to the discovery of the vein. (809) Extraction of Metallic Tin.-In order to extract the metal from the ore, it is subjected to a series of operations, some of which are of a mechanical and others of a chemical character. They may be classified thus: 1.-Stamping and washing, to remove the earthy and lighter portions. 2.-Roasting, to decompose the pyrites and get rid of the arsenic and sulphur. 3.-Washing, to dissolve out sulphate of copper, and carry off the oxide of iron. 4.-Reduction, by which the tin is separated from the oxygen and the gangue or earthy matter. 5.-Refining, or liquation, and boiling with green wood. 1. The purer portions of the ore are first picked out by hand; the residue, consisting chiefly of tin-stone, with the earthy impurities of the matrix, mixed with arsenical copper and iron pyrites, passes to the stamping mill, where it is reduced to a coarse powder. This powder is then buddled and washed (529), to remove the lighter impurities. 2.-The heavier portion, however, still retains a considerable quantity of arsenical iron and copper pyrites. The next operation is intended to get rid of these substances; with this view the washed ore is roasted in a reverberatory furnace until the arsenic and a good deal of the sulphur are expelled, and the ore becomes converted into yellowish-brown powder; this process usually lasts about twelve hours. During this roasting, frequent stirring is necessary in order to expose fresh surfaces freely to the air. By this means the iron pyrites is decomposed, and is converted into sulphurous anhydride and peroxide of iron; the arsenic is expelled as arsenious anhydride, and the greater part of the sulphide of copper is converted into sulphate of copper; this conversion is completed by exposing the mass in a moistened state to the air for some days. 3. The sulphate of copper is then dissolved out by lixiviation; after which the principal part of the peroxide of iron, as it is much lighter than the oxide of tin, is got rid of by washing. 4.-The washed ore is now ready for reduction.* In order *When much wolfram is contained in the ore it is sometimes fused with carbonate of sodium before proceeding to the reduction; the tungstic acid is thus removed in the form of tungstate of sodium, which is extracted by water, and is sometimes employed in calico-printing as a mordant: it has lately also been proposed to apply it to muslin dresses to prevent them from burning with flame, should they happen to take fire (618). 650 EXTRACTION OF METALLIC TIN. to attain this object it is mixed with from one-fifth to one-eighth its weight of powdered anthracite or of charcoal, and with a small proportion of lime to facilitate the fusion of the siliceous gangue, which still remains mingled with the ore. The mixture having been rendered damp, for the purpose of preventing the finer particles from being carried away by the current of air, is introduced into the reducing furnace. This is a reverberatory furnace with a low arch or crown. The charge having been placed upon the hearth, the doors are closed up and the heat is gradually raised for five or six hours; the binoxide of tin is thus reduced by the carbon, before the temperature rises high enough to cause the oxide to fuse with the silica, with which it would form an enamel difficult of reduction. Towards the end of the operation the heat is raised until it becomes very intense; the slags are thus rendered fluid, and the reduced metal subsides to the bottom, and is allowed to run off into cast-iron pans, from which it is ladled off into moulds; but the ingots thus obtained are by no means pure. 5. They are therefore next submitted to a process of liquation, which consists in heating the ingots to incipient fusion, upon the bed of a reverberatory furnace: the purer tin, being the more fusible portion, gradually melts out and leaves an alloy, which has a higher melting-point. This less fusible portion, when remelted, forms the inferior variety called block tin. The tin which has run out of the ingots is drawn off into a second pan in which the metal is gently heated, being kept in a state of fusion by a fire underneath; here it is agitated briskly by thrusting into the mass stakes soaked in water; the steam thus produced, as it bubbles up through the molten metal, carries the dust, slag, and other mechanical impurities to the surface. After this treatment has been continued for about three hours the metal is allowed to remain undisturbed for a couple of hours; it is then skimmed, ladled out, and cast into ingots for the market. The portion contained in the upper half of the pan is the purest, as owing to the low density of tin, and its tendency to separate from its alloys, it rises to the surface. The finest quality of the metal is frequently heated a second time to a temperature a little short of its melting-point; at this high temperature it becomes brittle, and, if allowed to fall from a height, it breaks into irregular prismatic fragments, which are known as dropped or grain tin. The splitting of the mass into these fragments is a rude guarantee of the purity of the metal, since impure tin does not become brittle in this manner. On the continent the stream tin is frequently reduced in small blast furnaces termed by the French fourneaux à manche; the fuel PROPERTIES AND USES OF TIN. 651 used in this case is charcoal. The tin which is imported from Banca is almost chemically pure. English tin usually contains small quantities of arsenic, copper, iron, and lead, and often traces of gold. When required in a state of perfect purity, the metal may be obtained by means of voltaic action. For this purpose a concentrated solution of crude tin in hydrochloric acid may be placed in a beaker, and water cautiously poured in without disturbing the dense solution below. If a bar of tin be plunged into the liquid, beautiful prismatic crystals of pure tin are gradually deposited on the bar at the point of junction between the metallic solution and the water. (810) Properties.-Tin is a white metal with a tinge of yellow, and a high metallic lustre. It is rather soft, and is very malleable, but is deficient in tenacity. At a temperature of about 212° its ductility is considerable, and it may then be easily drawn into wire. In a laminated state it is well known as tinfoil. If a bar of tin be bent, it emits a creaking sound, a property which it possesses in common with cadmium; if bent several times in succession backwards and forwards, it becomes sensibly hot at the point of flexure. These effects depend upon a mechanical alteration of the relative position of its molecules, and their mutual friction. Tin, when handled, communicates a peculiar odour to the fingers. It is a tolerably good conductor both of heat and electricity. It fuses at 442°, according to Crichton (or 451°, Person), but is not sensibly volatilized in the furnace. It may be obtained in crystals by slow cooling after fusion. Tin is but slowly tarnished by exposure to the air and moisture at ordinary temperatures, but if heated to redness in a current of steam, or if exposed to the air at a high temperature, it becomes rapidly converted into the binoxide and burns with a brilliant white light. Nitric acid of sp. gr. 152 does not attack tin, but if diluted to 13 it acts upon it violently, and produces an insoluble hydrated binoxide of the metal known as metastannic acid; at the same time, owing to the decomposition of water, a considerable quantity of ammonia is formed, which enters into combination with the excess of nitric acid. Strong hydrochloric acid, when heated upon tin, dissolves it gradually with extrication of hydrogen. Aqua regia, if not too concentrated, dissolves the metal and converts it into the perchloride. Diluted sulphuric acid is without action on the metal in the cold; but if the concentrated acid be boiled upon it, the tin becomes converted into sulphate of the peroxide, and globules of sulphur are separated, while sulphurous anhydride escapes: the tin appears in this |