CALCIUM. CHAPTER XXVIII. CALCIUM-STRONTIUM-BARIUM-LEAD. 467 CALCIUM. 546. This metal is a constituent of several of the commonest and most important minerals; it forms a very considerable portion (perhaps as much as one-sixteenth) of the solid crust of the earth. Before considering the properties of the metal itself, let us examine some of its familiar compounds. 547. Carbonate of Calcium (CaCO1) occurs in nature in many different forms, called by a great variety of names, among which may be mentioned limestone, chalk, marble, calc-spar, and coral. There are whole ranges of mountains composed almost entirely of limestone, while in many extensive tracts of country the soil is calcareous and reposes upon limestone rocks. The shells of shellfish are almost entirely composed of it, and it is an important constituent of dolomite, marl, and many other rocks and minerals. It is formed artificially, as has been seen (Exp. 168), when carbonic acid is brought into contact with lime-water; but it is noteworthy that carbonic acid will not unite with the anhydrous oxide of calcium (quicklime). Carbonate of calcium, though tasteless, is slightly soluble in water, and the solution exhibits a faint alkaline reaction; it is, however, rather freely soluble in water charged with carbonic acid (§ 403). Exp. 275.-Place in a test-tube 20 or 30 drops of lime-water, and as much pure water; immerse in the mixture the delivery-tube of a bottle from which carbonic acid gas is being evolved (Exp. 171). Carbonate of calcium will be thrown down at first; but after a while, as the water in the test-tube becomes saturated with carbonic acid, the precipitated carbonate will redissolve, and there will be obtained a perfectly clear solution, which, in spite of the large proportion of carbonic acid contained in it, has a decided alkaline reaction. By boiling the solution, so that a portion of its carbonic acid may be expelled, the carbonate of calcium can be again precipitated. So, too, if the liquid 468 CALCAREOUS PETRIFACTIONS. be left exposed to the air, it will gradually give off carbonic acid, and become turbid from deposition of carbonate of calcium. The phenomena illustrated in this experiment often occur in nature. In many districts where limestone is abundant, the well- and riverwaters are highly charged with carbonate of calcium held dissolved by carbonic acid; the water is thus made "hard" (see § 560), and is, comparatively speaking, unfit for washing and for many other purposes. When employed as a source of steam-power, such waters deposit carbonate of calcium as an incrustation upon the sides of the boilers as fast as the excess of carbonic acid is expelled by boiling. This scale, or incrustation, forms a more or less coherent coating upon the inner surface of the boiler, and, being a very poor conductor of heat, it greatly interferes with the heating of the water; the scale keeps the water away from the iron sides of the boiler, and the metal, being thus unduly heated, is rapidly oxidized, or "burnt out," as the fireman correctly states it. The formation of calcareous petrifactions, of stalactites and stalagmites, of the stones called tufa and travertine, and of many deposits of crystallized carbonate of calcium, is directly referable to the escape of carbonic acid from calcareous waters. Whenever water, charged with carbonate of calcium, flows out from the earth into the open air, or trickles into hollows or caverns within the earth, carbonic acid is given off in the gaseous state, and carbonate of calcium is deposited. Stalactites are the pendent masses, like icicles, which hang from the roofs of caverns and the walls of cellars, bridges, and like covered ways; stalagmites are the opposite masses which grow up out of the drops of water which fall from the stalactites above them, before all the dissolved carbonate has been deposited. The waters of some mineral springs are so highly charged with carbonate of calcium, that, on being exposed to the air, they quickly deposit a considerable quantity of it upon any solid substance with which they come in contact. In case such waters flow over pieces of wood or other organic matter, the form of the wood will be preserved in the cast or "petrifaction," long after the wood itself has decayed and disappeared. Where such deposits are formed upon a scale so large as to be of geological importance, as is the case in some of the volcanic districts of Italy, the rock formed is called tufa when porous, and travertine if compact. 548. Carbonate of calcium dissolves also in aqueous solutions of several of the salts of ammonium, such as the chloride, nitrate, and sulphate, especially if it has only recently been precipitated and is still moist and incoherent. Exp. 276.-Through 2 or 3 c. c. of lime-water, contained in a test CARBONATE OF CALCIUM NOT INSOLUBLE. 469 tube, blow, by means of a glass tube, a quantity of air from the lungs; to the milky liquid obtained, add, drop by drop, a cold, saturated aqueous solution of chloride of ammonium, until the cloudiness in the lime-water has disappeared-that is, until the carbonate of calcium has all been dissolved. Exp. 277.-Place a drop or two of a solution of chloride of calcium in a test-tube, pour upon it several drops of a strong solution of chloride of ammonium; shake the mixture, and then add to it a few drops of a solution of carbonate of ammonium, and also a few drops of ammonia-water. If enough chloride of ammonium has been added to the liquid, no precipitate will be formed in it, though, in the absence of chloride of ammonium, a precipitate will at once be produced on mixing the other ingredients. A precipitate may, however, always be obtained by boiling the mixed solutions, unless a large excess of chloride of ammonium be present, or unless the chloride-of-calcium solution be very dilute. By repeating this experiment under varied conditions, taking note, in each case, of the number of drops of the solutions of chloride of ammonium, chloride of calcium, and of water employed, and methodically increasing or diminishing each of these, the student will quickly perceive the real significance of the solvent power of the ammoniacal salt, and will appreciate the fact that, in testing for small quantities of either lime or carbonic acid, it is necessary for the analyst to exclude ammonium-salts from his solutions as far as may be practicable. When boiled with solutions of the salts of ammonium (with choride of ammonium for example), carbonate of calcium is rapidly decomposed and dissolved, carbonate of ammonium being given off, while the chloride (or some other salt) of calcium remains in solution. 549. Carbonate of calcium is remarkable not only for the very great diversity of external appearance which is presented by its several massive and amorphous varieties, but it is likewise found in a greater variety of regular crystalline forms than any other substance; more than 150 native varieties of it have been observed by mineralogists. As calc-spar, it occurs in rhombohedrons and other derivative forms of the sixth or hexagonal system (§ 191); but it is found also as the mineral arragonite, in forms of the trimetric system, and is consequently dimorphous. The two forms of carbonate of calcium, calc-spar and arragonite, present many differences in their physical properties. Some specimens of calc-spar, called Iceland spar, are perfectly transparent and colorless, and exhibit to a remarkable degree the 470 CALC-SPAR AND ARRAGONITE. phenomena of double refraction. Transparent crystals of arragonite exhibit also the phenomena of double refraction; but arragonite has two axes of double refraction, calc-spar only one. Crystals of calc-spar are cleavable parallel to the faces of the rhombohedron which is the primary form of the mineral, and masses of it may often be broken up into more or less perfect rhombohedrons. Arragonite, on the contrary, presents two directions of distinct cleavage, parallel to the faces of a right rhombic prism. The fractures of the two minerals are therefore quite unlike. The specific gravity of calc-spar ranges from 2-7 to 2.75, while the specific gravity of arragonite is generally between 2.9 and 3.3. Arragonite is considerably harder than calc-spar, but its specific heat (0·1966) is less. When carbonate of calcium crystallizes from hot solutions it takes the form of arragonite, but from cold solutions it crystallizes as calc-spar. In like manner the precipitate formed by mixing boiling solutions of chloride of calcium and carbonate of ammonium is seen under the microscope to consist of acicular crystals of arragonite, while the precipitate obtained from cold solutions of the same salts is amorphous. In either case, however, if the moist precipitate be left to itself for some time in the cold, it will gradually assume the rhombohedral form of calc-spar, no matter whether it was at first acicular or amorphous. In all its varieties carbonate of calcium is readily attacked by acids, even if they be dilute; the action is attended with effervescence, owing to the expulsion of carbonic acid and the escape of this gas through the liquid : 2 CaO,CO, + 2HC1 = CaCl, + CO2+ H20. Limestone is readily distinguished by this reaction from other rocks. 550. Oxide of Calcium (CaO).—On being heated, carbonate of calcium begins to give off carbonic acid at a low red heat, as has been seen in Exp. 170, and at full redness is completely resolved into oxide of calcium, commonly called quicklime, and carbonic acid. Exp. 278.-Place a small fragment of marble upon a piece of charcoal and heat it strongly in the blowpipe-flame during several minutes. Or throw a lump of limestone upon an anthracite fire, and leave it there for half an hour or more. In either case, it will be found, upon examination, that the calcined product has lost the property of effervescing with acids; that it weighs less than the original limestone, and that it exhibits a distinct alkaline reaction when placed on wet testpaper. For use in the arts, limestone is burned in special furnaces, of peculiar construction, called lime-kilns, some of which are so arranged that they may be kept in operation for years without intermission. When carbonate of calcium, instead of being heated merely in quiescent air, is heated in a current of air, or of any other gas, such as steam for example, it will give off all its carbonic acid very easily. It has been found in practice that limestone fresh from the quarry can be more readily burned than that which has been long dug out of the ground and has so lost its natural moisture; in damp weather, moreover, the burning is said to go on more satisfactorily than when the atmosphere is dry. If carbonate of calcium be ignited in a tube of iron, or other metal, closed hermetically, so that no carbonic acid can escape from the tube, the carbonate disengages carbonic acid until the pressure of the confined gas becomes so great as to arrest the further decomposition of the carbonate. Under these conditions, the temperature may be raised high enough to fuse the undecomposed carbonate; the cooled mass often presents the appearance of fine-grained marble. If the tube in which the experiment has been performed be very slowly cooled, the carbonic acid will be reabsorbed. Of the anhydrous oxide of calcium little need here be said. It is infusible at the most intense heat at present at our command, and is therefore used for making crucibles in which the most refractory metals are melted by the aid of the compound blowpipe. It has no power to unite with dry carbonic acid at ordinary temperatures, but when exposed at very high temperatures to an atmosphere of carbonic acid possessing a certain tension, some of the gas is absorbed. It unites with water very energetically, and the product of this union combines readily with carbonic acid. When lumps of quicklime are exposed to the air they slowly absorb both water and carbonic acid, and after a while fall to powder. This powder is known as air-slaked lime; its composi |