« ZurückWeiter »
76. LEAD.- Lead does not occur free in nature. It is obtained from galena, or plumbic sulphide, PbS. galena is roasted in a reverberatory furnace, with the addition of a small quantity of lime to form a fusible slag with any silicious mineral matter present in the ore. By the action of the air, a portion of the sulphide is oxidized to sulphate, while in another portion the sulphur burns off, and plumbic oxide is left behind. After a time, the air is excluded, and the heat of the furnace raised, and the remaining sulphur burns at the expense of the oxygen in the oxide already formed, leaving the lead free.
Lead is a bluish-white metal, and so soft that it may be scratched with the nail. It is quite ductile and malleable, but has little tenacity or elasticity. It melts at 617°, and at a higher temperature volatilizes, though not in quantity sufficient to be distilled.
The surface of the metal remains bright in dry air, but it soon becomes tarnished in moist air, owing to the formation of a film of oxide; and this oxidation proceeds rapidly in presence of a small quantity of weak acid, such as carbonic or acetic. In pure water freed from air, lead also preserves its lustre ; but, if air be present, plumbic oxide is formed, and as this dissolves slowly in the water, a fresh portion of metal is exposed for oxidation. This solvent action of water upon lead is a matter of much importance, owing to the common use of lead waterpipes, and the peculiarly poisonous action of lead compounds upon the system when taken even in minute quantities for a length of time. The small quantity of certain salts contained in all spring and river waters exerts an important influence on the action of lead. Thus waters containing nitrates or chlorides are liable to contamination with lead, while those hard waters containing sulphates or carbonates may generally be brought into contact with lead without danger, as a thin deposit
of sulphate or carbonate is formed, which preserves the metal from further action. If the water contains much free cațbonic acid, it should not be allowed to come into contact with lead, as the carbonate dissolves in water containing this substance. The presence of lead in water may easily be demonstrated by acidulating the water, passing a current of hydric sulphide (sulphuretted hydrogen) through a deep column of the liquid, and noticing whether it becomes tinged of a brown color, from the formation of plumbic sulphide.
Lead pipes lined with tin have been recommended for water-pipes; but if the lining is not absolutely perfect, they are more dangerous than ordinary lead pipes. If from defective soldering, or cracks, or breaks in the lining, or from corrosive action, the water comes in contact with the lead, a galvanic action begins, and the lead is more rapidly oxidized than it would be if not joined with the tin.
There is the same danger in the use of the metallic double-lined ice-pitchers. The lining is often made of dissimilar metals, and the parts are joined by a solder containing lead. The thin film of silver is soon worn off upon the interior; galvanic action then promotes corrosive action, and the water becomes poisonous.
On the whole, the best way of protecting lead-pipes from oxidation, is by coating them with sulphide of lead, which is insoluble in water. This can be done by dissolving one pound of potassic sulphide (sulphide of potassium) in two gallons of water, and letting the solution remain in the pipe twelve hours, or until the whole inside is thoroughly blackened. This preventive process is not perfect, but it is very nearly so.
77. COPPER.- Copper is an important metal, largely used in the arts. It has been known from very
early times, as it occurs native in the metallic state, and is moreover easily reduced from its ores. per is found in enormous quantity near Lake Superior, in North America, and other localities. The following ores are the most important: (1) a compound of copper, sulphur, and iron, known as copper pyrites, Cu,S+ Fe Sz; (2) the cuprous sulphide, Cu,S; (3) the carbonate or malachite; and (4) the red or cuprous oxide, Cu,O. Copper is obtained on a large scale from the carbonate or oxide, by reducing these ores, together with carbon and some silica, in a blast-furnace.
Metallic copper has a peculiar deep red color, which is best seen when a ray of light is several times reflected from a bright surface of the metal; it is very malleable, ductile, and tenacious; it melts at a red heat, and is slightly volatile at a white heat, giving a green tint to a flame of hydrogen gas which is passed over it; and it is one of the best conductors of heat and electricity. Copper does not oxidize, either in pure dry or moist air, at ordinary temperatures, but if heated to redness in the air, it is soon converted into cupric oxide.
78. Alloys of Copper.– Copper combines with several of the metals to form what are called alloys.
Brass is an alloy containing about two-thirds of copper and one-third of zinc. It is harder than copper, and can be more easily worked. The addition of from one to two per cent of lead improves the quality of brass for most purposes. The yellow metal, used for the sheathing of ships, contains sixty per cent of copper. Bronze, gun-metal, bell-metal, and speculum-metal are alloys of copper and tin in varying quantities. They are all remarkable for the property of being hard and brittle when slowly cooled, but of becoming soft and malleable if, when red-hot, they are cooled suddenly by dipping into cold water.
79. TIN. — The ores of tin although this metal has been known from very early times - occur in but few localities, and the metallic tin is not found in nature. The chief European sources of tin are the Cornish mines, where it is found as tin-stone, Snog. It was probably from these mines that the Phænicians and Romans obtained all the tin which they employed in the manufacture of bronze. Tin-stone is also met with in Malacca and Borneo and Mexico. In order to prepare the metal, the tin-stone is crushed and washed to remove mechanically the lighter portions of rock with which it is mixed, and the purified ore is then placed in a reverberatory furnace, with anthracite or charcoal, and a small quantity of lime. The oxide is thus reduced, and the liquid metal, together with the slag, consisting of calcic silicate, falls to the lower part of the furnace. The blocks of tin, still impure, are then refined by gradually melting oat the pure tin, leaving an impure alloy behind.
Tin has a white color resembling that of silver; it is soft, malleable, and ductile, but has little tenacity. When bent, pure tin emits a peculiar crackling sound. It melts at 442°, and is not sensibly volatile. Tin does not lose its lustre on exposure to the air, whether dry or moist, at ordinary temperatures; but if strongly heated, it takes fire, and a white powder of stannic oxide is formed.
Owing to its brilliancy, and its power of resisting ordinary atmospheric changes, tin is largely used for coating iron, copper, and other metals, which are more abundant and more easily oxidized. Tin plate, or sheet tin, as it is called, is iron thus coated with tin. The thin sheets of iron are thoroughly cleaned with sulphuric acid, and then immersed in melted tin for an hour or so. Copper is tinned by brushing the melted tin over its surface, which must first be made perfectly clean.
Tin is sometimes used for water-pipes; and there is a
popular impression that it is never acted upon by water. In certain localities, however, it oxidizes rapidly, and is soon rendered worthless. The safety of tin pipes, as compared with lead, does not consist in their exemption from corrosive action, but in the harmlessness of the resultant oxides and carbonates.
80. Alloys of Tin.— Several alloys of tin, besides those already mentioned (78), are employed in the arts. Bria tannia metal is an alloy of equal parts of brass, tin, antimony, and bismuth. The best pewter is an alloy of four parts of tin to one of lead. Common solder contains equal parts of tin and lead, and is more fusible than lead.
81. ZINC. — Zinc is an abundant and useful metal, closely resembling magnesium in its chemical character, but it is much more easily extracted from its ores. The chief ores of zinc are the sulphide or blende, the carbonate or calamine, and the red oxide. In order to extract the metal, the powdered ore is roasted, to convert the sulphide or carbonate into oxide; the roasted ore is then mixed with fine coal or charcoal, and strongly heated in crucibles or retorts of peculiar shape; the zincic oxide is reduced by the carbon, carbonic oxide gas comes off, and the metallic zinc distils over, and is easily condensed.
Zinc is a bluish-white metal, with a crystalline structure. It is brittle at the ordinary temperature, but when heated to between 200° and 300°, it may be rolled out or hammered with ease. If more strongly heated, it is again brittle, and may be broken up in a mortar. Zinc melts at 773°, and at a bright red heat it begins to boil, and volatilizes; or if air be present, it takes fire, and burns with a greenish flame, forming zincic oxide. Zinc is not acted upon by moist or dry air, and hence it is