largely used in the form of sheets, and is employed as a protecting covering for iron. The sheets of iron are plunged into melted zinc, covered with sal-ammoniac, which keeps the surface of the zinc free from oxide, and allows the two metals to unite. zinc is said to be galvanized. alloy of zinc, copper, and nickel.

Iron thus coated with
German silver is an

82. IRON SALTS.—The oxides of iron are four in number: (1) ferrous oxide, FeO, from which the green or ferrous salts are derived; (2) ferric oxide, Fe2O, yielding the yellow ferric salts; (3) the magnetic, or black oxide, Fe3O4, which does not form any definite salts; (4) ferric acid, H2FeO4, a weak acid, forming colored salts with potassium. The ferrous oxide colors glass green, and gives the peculiar tint to common bottleglass. The most important of the ferrous salts are,

(1) Ferrous Sulphate, FeSO. This soluble salt, sometimes called green vitriol, is obtained by dissolving (1) metallic iron, or (2) ferrous sulphide, in sulphuric acid, or by the slow oxidation of iron pyrites, FeS2.

(1) Fe+ H2SO1=FeSO4 + H2

(2) FeS+H2SO1=FeSO+H2S.

The solution thus obtained yields, on evaporation, large green crystals of the salt. It is largely used in the manufacture of several black dyes, and is one of the constituents of writing ink. Like ferrous oxide, this salt easily takes up oxygen, producing a new salt called ferric sulphate.

(2) Ferrous acetate, formed by the action of acetic acid on iron, is largely used in dyeing and calico-printing.

Of the ferric salts, the chloride, Fe,Cl, is the most

important; it forms in brilliant red crystals when chlorine gas is passed over heated metallic iron.

83. LEAD SALTS.-Three compounds of lead and O are known: (1) Litharge, or plumbous oxide, PbO, a straw-colored powder obtained by heating lead in a current of air. It is used in painting and in glassmaking, and with acids forms the important lead salts. (2) Plumbic oxide, PbO. (3) Red oxide, or red lead, a compound of the two last oxides, having the composition, 2 PbO, PbO2. It is obtained by exposing litharge to the air at a moderate red heat, oxygen being absorbed. Red lead is used as a paint and in glass-making.

is

Of the soluble salts of lead, the nitrate, Pb2NO, is the most important. This compound is obtained by dissolving the oxide, the carbonate, or metallic lead in warm nitric acid. Plumbic acetate, or sugar of lead, is also a soluble salt. Almost all the other lead salts are insoluble in water. Plumbic carbonate, or white lead, PbCO3, a substance much used in the arts as a paint. The salt may be obtained in the pure state by precipitating a cold solution of the nitrate with an alkaline carbonate, when it falls down as a white powder. For preparing the salt in quantity, two plans are employed: the one similar in principle to that described for the pure salt; and the other an old and interesting process, known as the Dutch method. In this process thin sheets of lead are rolled into a coil, and each coil placed in an earthen pot containing a small quantity of crude vinegar, which, however, does not come in contact with the lead. Several hundred of these jars and coils are packed on a floor in a bed of stable manure or spent tan-bark, and then covered with boards, while a second layer of pots similarly charged is placed above, and this is continued until the building is filled. After remaining thus for several weeks,

the coils are taken out, when the greater part of the lead is found to be converted into white carbonate. It appears that a lead acetate is first formed, and that the acetic acid is gradually driven out from its combination by the carbonic acid evolved from the putrefying organic matter, and thus enabled to unite with another portion of the lead lying underneath that which was first attacked.

Plumbic sulphide, or galena, PbS, is found native, and is the chief ore of the metal. It is prepared as a black precipitate by passing sulphuretted hydrogen gas through a solution of a lead salt. Galena has a bright bluish-white metallic lustre. Plumbic sulphate PbSO,, is a white insoluble salt, which is found native, and is prepared artificially by adding sulphuric acid to a soluble lead salt. Plumbic chloride, PbCl, is prepared by adding muriatic acid to a strong solution of plumbic nitrate, when a crystalline precipitate of plumbic chloride is formed. It dissolves in about thirty parts of boiling water, separating out in shining needles on cooling. Plumbic iodide, PbI2, is precipitated in the form of splendid yellow spangles, when hot solutions of potassic iodide and plumbic nitrate are mixed and allowed to cool. Plumbic chromate, PbCrO4, is a yellow insoluble salt, generally known as chrome yellow, and much used as a paint.

84. COPPER SALTS.-Copper forms two oxides, cuprous oxide, Cu,O, and cupric oxide, CuO. Cuprous oxide imparts to glass a ruby color. Cupric oxide is soluble in acids, and forms a series of salts. The most important of the soluble salts are cupric sulphate, CuSO4 and cupric acetate, or verdigris. The former is often called blue vitriol, and is largely manufactured by dissolving copper or its oxide in sulphuric acid. It is used in calico-printing, and in making Scheele's green, or cupric arseniate, and other paints.

As many of the vegetable acids act upon copper so as to form poisonous salts, copper or brass vessels should never be used for cooking unless they are lined with tin.

85. SALTS OF ZINC.-Zinc forms but one oxide, ZnO, which is much used as a white paint. Its most important salt is zincic sulphate, or white vitriol, which is used in calico-printing.

86. SALTS OF TIN.- Tin forms two oxides, the stannous, SnO, and the stannic, SnO,. Stannous chloride, SnCl, formed by dissolving tin in muriatic acid, is termed tin salts in commerce; it is largely manufactured for the calico-printer and dyer, who use it as a mordant; that is, to fix the color in the fibre of the cloth, so that it will not wash out. Potassic stannate (stannate of potash), K,SnO, formed by boiling stannic oxide with soda, is used in the same way. Stannic chloride, SnCl4, is also used by dyers, and is prepared for this purpose by dissolving tin in cold nitro-muriatic acid. Of the sulphides of tin, SnS, stannous sulphide, and SnS,, stannic sulphide, are the most important; the former is blackishgray, and the latter a bright yellow crystalline powder, known as mosaic gold.

NOBLE METALS.

The noble metals are mercury, silver, gold, and platinum. They are so called because they do not rust, that is, combine with the oxygen of the air at ordinary temperatures.

87. MERCURY. The chief ore of mercury is the sulphide, or cinnabar, which occurs at Almaden in Spain, at Idria in Illyria, in California, and also in China and

at

Japan. The metal is easily obtained by roasting the ore, when the sulphur burns off, and the metal volatilizes, and its vapor is condensed in earthen pipes. Mercury is the only metal liquid at the ordinary temperature; it freezes 39°; in the solid state it is malleable and possesses a density of 14.4. It boils at 662°, and gives off a slight amount of vapor at the ordinary temperature. Mercury, when pure, does not tarnish in moist or dry air; but when heated above 700°, it rapidly absorbs oxygen; and it combines directly with chlorine, bromine, iodine, and sulphur. Mercury is largely used in extracting gold and silver from their ores, and for many other purposes.

Mercury dissolves many of the metals, forming amalgams. The most important of these is the amalgam of tin and mercury, used for silvering mirrors. "In order to apply it to the glass, a sheet of tinfoil is spread evenly upon a smooth slab of stone, which forms the top of a table carefully levelled, and surrounded by a groove, for the reception of the superfluous mercury. Clean mercury is poured upon the tinfoil, and spread uniformly over it with a roll of flannel; more mercury is then poured on till it forms a fluid layer of the thickness of about half acrown; the surface is cleared of impurities by passing a linen cloth lightly over it; the plate of glass is carefully dried, and its edge being made to dip below the surface of the mercury, is pushed forward cautiously; all bubbles of air are thus excluded as it glides over and adheres to the surface of the amalgam. The plate is then covered with flannel, weights are placed upon the glass, and the stone is gently inclined so as to allow the excess of mercury to drain off; at the end of twenty-four hours it is placed upon a wooden table, the inclination of which is increased from day to day until the mirror assumes a vertical position in about a month it is sufficiently drained to allow the mirror to be framed."- MILLER.