eters, and to form an alloy with tin for coating the backs of mirrors. It is also used largely in the extraction of gold and silver by the process of amalgamation. The alloys of mercury are called amalgams.

428. Mercuric Oxide, HgO.-This substance, commonly known as red oxide of mercury, or red precipitate, may be formed by heating metallic mercury up to 600° F., with free access of air. A still higher heat decomposes it, liberating the oxygen, and reducing the mercury to the metallic state. This oxide furnishes a ready source of oxygen gas, being the compound from which oxygen was first obtained by Priestley, and by which Lavoisier proved the composition of air.

429. Mercuric Chloride, HgCl, (Corrosive Sublimate). -This compound is prepared by mixing mercuric sulphate, HgSO, with an equal weight of common salt, and applying heat to the mixture. It is soluble in water and in alcohol. Its solution has a metallic, acrid taste, and an acid reaction. It is a deadly poison, and accidents have occurred from its substitution for calomel. The proper antidote for it is white of egg, which forms with it an insoluble inert compound. This substance is used in a process for preserving wood, by impregnation with its solution, which is termed kyanizing.

430. Mercurous Chloride (Calomel), Hg,Cl. - This compound is found native as "horn quicksilver." It is prepared by triturating mercuric chloride, HgCl, with mercury, or is precipitated whenever solutions of any mercurous compounds and a soluble chloride are mixed together. Sublimed calomel is a crystalline powder, white or dirty white in color; very heavy, tasteless, and inodorous. Calomel is decomposed by light. It has been very extensively used in medicine, and is much less poisonous than "corrosive sublimate."

431. Mercuric Sulphide (Cinnabar), HgS, occurs in large beds at Almaden, in Spain, and is also found in

extensive deposits in California. It is produced in considerable quantity by artificial means, and sold as a pigment under the name of vermilion.

CHAPTER XIX.

THE CALCIUM GROUP-CALCIUM, STRONTIUM, BARIUM, LEAD, § 1. Calcium and its Compounds.

CALCIUM.-Symbol, Ca. Atomic Weight, 40; Quantivalence, II. and IV.; Specific Gravity, 1.57.

432. History.-Calcium is one of the most abundant constituents of the crust of the earth. It occurs in the extensive layers of limestone, marble, and chalk, as a carbonate. In gypsum and alabaster it is found as a sulphate, while as a phosphate it forms an important constituent of the bones of animals. The metal itself is rare, and is prepared by passing a galvanic current through fused calcic chloride. It is a light-yellow metal, somewhat harder than lead, very malleable, melts at a red heat, and oxidizes in the air.

433. Calcic Oxide, CaO (Lime).-This well-known substance does not occur in Nature, but is prepared by burning calcic carbonate, limestone (CaCO,), in large masses in kilns. The carbonic dioxide is driven off by the heat, and a white, stony substance remains, called quick-lime, or caustic lime. One ton of good limestone yields 11 cwt. of lime. When this is exposed to the air it first rapidly imbibes moisture and crumbles to powder. This gradually absorbs carbonic dioxide, and, becoming less and less caustic, regains the neutral condition of the carbonate.

434. Calcic Hydrate. When water is poured upon quicklime it absorbs it (every 28 lbs. of lime taking nine

pounds of water), swells to thrice its original bulk, crumbles to a fine white powder, and is converted into calcic hydrate, CaH2O. This process is called slacking, and sufficient heat is often produced by the chemical action to ignite wood. Lime-water is a saturated, transparent solution of calcic hydrate in water. Cream or milk of lime is

a thick mixture of the hydrate with water, such as is used in whitewashing. In tanneries the hides are immersed in milk of lime, which partially decomposes them, so that the hair may be easily removed. Calcic hydrate exhibits the properties of a strong alkali, decomposing organic tissues and saturating the strongest acids. It is more soluble in cold than in hot water. Hence, when cold saturated limewater is boiled, a portion of the hydrate is deposited. Slacked lime is extensively used in chemical manufactures, and as a fertilizer. Its value as a fertilizer is due to the property which it has of decomposing organic and inorganic constituents of soil.

435. Mortar and Cement.-Lime, mixed with sand, forms the mortar employed by builders to cement stones and bricks. To make the best mortar, the lime should be perfectly caustic and the sand sharp and cross-grained. The nature of the changes by which the mortar becomes hardened is not satisfactorily explained. It is supposed to be owing in part to the absorption of carbonic dioxide from the air by the lime, and the subsequent hardening into a calcic carbonate. In time the lime also partially combines with the silica of the sand, forming an exceedingly hard silicate of lime. Common mortar, when laid in water, not only refuses to harden, but its lime gradually becomes dissolved out and washed away. Hydraulic cement possesses the property of solidifying under water. This quality is owing to the presence of clay (aluminic silicate) in the lime of which it is composed.,

436. Bleaching-Powder. When chlorine is passed through recently-slacked lime, large quantities of the gas

are absorbed, forming the bleaching-powder of commerce. The chemical constitution of this substance is yet a matter of doubt. It is generally regarded as a mixture of calcic hypochlorite with calcic chloride, but it is not impossible that it may correspond to the graphic expression Cl-Ca-O-Cl. It is a white, sparingly soluble powder, used in great quantities for bleaching purposes. In the bleaching of cotton fabrics, the goods are first freed from all greasy impurities, and then digested in a solution of this powder. They are next dipped into very dilute sulphuric acid, by which chlorine is liberated, and exerts its bleaching power. This process requires to be repeated several times before the color is entirely discharged; after which the goods are thoroughly washed in water, in order to remove all trace of acid from the fibre of the cloth.

437. Calcic Sulphate, CaSO,.-This salt occurs native as the mineral anhydrite, and is produced artificially on a large scale by calcining powdered gypsum (CaSO ̧+2H2O), at about 250° C. Thus prepared it constitutes "plaster of Paris," and possesses the property of combining with water when made into a paste. It is used for taking casts by running the mixture into hollow moulds, and colored and mixed with glue, for producing the ornamental designs known as stucco-work. Calcic sulphate dihydrate (gypsum, alabaster), CaSO, +2H,O, occurs in many parts of the world, forming extensive rocky beds. In its pure, transparent form, it is known as selenite, and in its compact and earthy varieties as gypsum and alabaster. Gypsum is used extensively as a fertilizer.

438. Calcic Carbonate, CaCO,.-Vast deposits of this substance are distributed all over the globe in the form of limestones, marbles, chalks, marls, coralreefs, shells, etc. Numerous and extensive as are these deposits, it is conjectured that they are all of animal origin. The densest limestone and the softest chalk are found to consist of the

aggregated skeletons or shells of myriads of tribes of the lower animals, which have existed in some former period of the world's history. The formation of coral-reefs, which are sea-islands of calcic carbonate, built up from the depths of the ocean by minute aquatic animals, is an example of similar deposits now in process of formation. Calcic carbonate is decomposed by heat into calcic oxide and carbonic dioxide. Hydro-calcic carbonate, CaH,C,O.. When calcic carbonate is acted upon by water containing carbonic dioxide in solution, it dissolves with the formation of this compound. This solution, naturally formed, constitutes one of the varieties of hard water, which is generally met with in limestone districts.

439. Calcic Phosphate, Ca, (PO).-This is the earthy constituent of the bones of animals. They obtain it from the plants, and the plants in turn take it from the soil. It is found abundantly in the grains of cereals, which, as the supply is limited in the soil, rapidly exhaust it, when they are cultivated, year after year; hence the importance of restoring to the land the phosphates when they are removed by the crops.

§ 2. Strontium and Barium.

440. Strontium-Resembles calcium, both in appearance and properties. It is obtained from its chloride, and is a pale-yellow metal, of specific gravity 2.54. It does not change in dry air, but decomposes water readily, evolving hydrogen. Barium is a light-yellow metal, which rapidly oxidizes in the air, decomposes water, and has a specific gravity of 4.0. The compounds of these elements, though less widely distributed, are allied to the corresponding compounds of calcium. The nitrate of strontium is used in pyrotechny, and imparts to flame a beautiful crimson color. Baric oxide, or baryta (BaO), is a gray powder having a strong attraction for water, which it absorbs on