exposure to the air, forming baric hydrate. Baric chloride dihydrate (BaCl,+2H,O) is interesting chiefly as the usual test for sulphuric acid, with which it gives a dense, white, insoluble precipitate of baric sulphate. It has been employed in medicine. Baric sulphate, or heavy spar, occurs in large quantities, and when ground is extensively consumed under the name of barytes, in the adulteration of paints.

§ 3. Lead and its Compounds.

LEAD.--Symbol, Pb. (Plumbum). Atomic Weight, 207; Quantivalence, II. and IV.; Specific Gravity, 11.44.

441. Lead. This useful and common metal is of doubtful native occurrence, but is obtained from various ores, of which the mineral galena, a plumbic sulphide, is the most important. Lead is a soft, blue metal, easily scratched by the nail, and leaving a stain when rubbed upon paper. It is highly malleable, but not very ductile. In the air a film of oxide rapidly forms on its surface, which protects it from further corrosion. It melts at about 330° C., and on solidifying contracts to such an extent as to render it unfit for castings. Lead is much used in the manufacture of pipe for conducting drinking-water to the different parts of dwellings.

If lead is exposed to the combined action of pure water and air, plumbic hydrate is formed on the exposed surface, which is dissolved by the water with which it is in contact. This solution of plumbic hydrate absorbs carbonic dioxide with formation of plumbic carbonate, a highly-poisonous compound. The presence of chlorides or nitrates assists this corroding action, while it is retarded by the sulphates, phosphates, or carbonates. Hydro-calcic carbonate, a salt found in many spring-waters, also prevents this corrosion by depositing a coating on the exposed surface. As all lead-salts are poisonous, it is not safe to use water which

has been kept in cisterns lined with lead, or which has been conveyed through lead pipes, unless it has been carefully ascertained that the water contains such foreign matters as will prevent its action upon the metal. Lead in the presence of air and moisture is acted upon by feeble acids. Hence the use of vessels made of lead should be carefully avoided in the culinary department. This metal is extensively used in the arts, both alone and alloyed with other metals. An alloy prepared by mixing 2 parts of arsenic with 100 parts of lead is employed in the manufacture of shot.

442. Plumbic Monoxide, Pb"O.- This substance is found native as lead-ochre, a yellow massive mineral of crystalline structure. It is obtained on a large scale by heating lead to a point a little below redness, or in the process of cupellation. The former product is known as massicot, the latter as litharge. Plumbic monoxide is met with in several isomeric modifications, as a yellow or red crystalline substance, or as an amorphous powder. At a red-heat plumbic monoxide melts to a clear, dark-red liquid. In water it is slightly soluble with formation of lead hydrate. Acids dissolve it readily, giving rise to plumbic salts. It is much used in glass-making, and in glazing earthen-ware. Triplumbic tetroxide (Pb,O,), (minium or red lead), occurs native, and is formed when plumbic monoxide is for some time exposed to a low red heat in contact with air. It is extensively used as a pigment, and in the manufacture of flint-glass.

443. Plumbic Carbonate, PbCO,, White Lead. This salt it found beautifully crystallized in Nature, but it is largely manufactured as a paint. It is produced in several ways, but the following, which is known as the Dutch method, is considered the best: Thin sheets of lead, rolled up into loose scrolls, are placed in earthen pots with weak vinegar or acetic acid. Thousands of these pots, fitted with lead covers and closely packed, are then buried in

spent tan-bark. The acetic acid corrodes the metal, forming a superficial coating of plumbic acetate, and the carbon dioxide set free by the decomposing vegetable matter decomposes the acetate with formation of plumbic carbonate and free acetic acid. The acetic acid attacks more metal, which is again converted into carbonate; and thus, with a small charge of vinegar, the operation is continued a long time, and a large quantity of lead changed. White lead is extensively adulterated with baric sulphate; it may be detected by adding nitric acid, which dissolves the lead, leaving the baric sulphate as an insoluble residue.

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444. Plumbic Acetate, Pb (C,H,O,),. This important salt of lead is easily procured by dissolving plumbic monoxide (PbO) in acetic acid. It receives its common name sugar of lead" from its sweet taste, and its general likeness, in appearance, to loaf-sugar. It is exceedingly poiThe soluble salts of lead are most of them colorless, and redden litmus-paper. Metallic lead is easily precipitated from solutions of its salts by means of iron or zinc.

sonous.

CHAPTER X X.

MAGNESIUM GROUP-MAGNESIUM, ZINC, CADMIUM.

§ 1. Magnesium and its Compounds.

MAGNESIUM.--Symbol, Mg. Atomic Weight, 24; Quantivalence, II.; Specific Gravity, 1.74.

445. History and Occurrence. This metal was first obtained by Davy, in 1808. It does not occur native, but may be obtained by decomposing magnesic chloride by metallic sodium. Magnesium is a white or bluish-gray crystalline metal; malleable and ductile, melts at a moderate red-heat, and volatilizes at higher temperatures.

Heated in the air it burns with a dazzling bluish-white light, and is on this account much used for signaling, and, as a source of artificial light in photography.

446. Magnesic Oxide, MgO, Magnesia.—This compound is obtained by strongly heating magnesic carbonate. It is a white, light powder, with feeble alkaline properties, very sparingly soluble in water, but dissolving readily in acids. It is found native as the mineral periclase. It is used principally in medicine as a mild aperient and antacid. Magnesic Sulphate, Mg SO+7H2O (Epsom Salts), is a common ingredient of mineral waters, and takes its name from the circumstance of its being contained in great quantities in the springs near Epsom, in England. The commercial supply is chiefly derived from sea-water, by decomposing the magnesic compounds with lime, and then adding sulphuric acid. It may also be obtained from magnesian limestone. It is soluble in water, has a bitter, saline taste, and is used in medicine as a cathartic and an antidote to various poisons. It has also been used as a fertilizer.

§ 2. Zinc and Cadmium.

ZINC.-Symbol, Zn. Atomic Weight, 65; Quantivalence, II.; Molecular Weight, 65; Molecular Volume, 2; Specific Gravity, 7.0.

447. History and Occurrence.-This element is not found native, but is obtained on a very extensive scale by the decomposition of certain ores, among which zincic sulphide or “blende" (ZnS), zincic carbonate (ZnCO2), zincic oxide (ZnO), and a zincic silicate, are the most important. It is a brilliant, bluish-white metal. At common temperatures it is brittle, but, when heated from 212° to 300° F., it may be rolled out into thin sheets, and retains its malleability when cold. At 400° it again becomes quite brittle; at 770° it melts, and at a red heat volatilizes. When strongly heated in the air it takes fire, burning with a whitish-green flame and production of zincic oxide. Zinc

soon tarnishes in a moist atmosphere, forming a thin film of oxide, which resists further change. This property renders it useful for a variety of purposes, such as for gaspipes, gutters, roofing, and for galvanizing iron, thus preventing it from oxidation. It is also used in the preparation of hydrogen gas.

448. Zincic Oxide, ZnO.-This compound is found when zine is burned with free access of air. It is a fine white powder, familiarly known as zinc white. It is largely used as a paint. Zincic Chloride, ZnCl2, may be prepared by distilling an intimate mixture of zincic sulphate and sodic chloride. Zincic chloride is a whitish-gray translucent substance, soft like wax, and of 2.7 spec. grav. It melts easily and distills at a red heat; it is deliquescent, dissolves easily in water and alcohol; has a burning taste, and is poisonous. It is used in various chemical manufactures. Wood, impregnated with a crude solution of zincic chloride known under the name of "Sir William Burnett's Fluid," is effectually preserved from decay, this process being called Burnettizing. Zincic sulphate, Zn SO,+7H2O (White Vitriol), may be prepared either by roasting zincic sulphide, or by the action of sulphuric acid on metallic zinc. It strongly resembles magnesic sulphate, and is used in medicine, and in certain operations of calicoprinting.

CADMIUM.-Symbol, Cd. Atomic Weight, 112; Quantivalence, II.; Molecular Weight, 112; Molecular Volume, 2; Specific Gravity, 8.6.

449. Cadmium.-This metal does not occur native, but may be obtained from ores of zinc, and from some of the secondary products of zinc-manufacture. It is a bluishwhite, strongly lustrous metal, tarnishing in the air. It is soft, flexible, malleable, and ductile, melts at 315° C., is volatile, and crystallizes from the fused state, in regular octahedrons. In the air at higher temperatures it burns, cadmic oxide (CdO) being formed.