Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/538

Rh 526 CHEMISTRY [MAGNESIUM GROUP. hydroxides of the alkali metals; it separates from water in crystals of the composition Ba(OH) 2, 8H 2 ; it is not decomposed even by prolonged ignition. Strontium hydroxide, which also crystallizes with 8 molecules of water, is much less soluble than the barium compound ; it is not decomposed by ignition. Calcium hydroxide, Ca(OH).,, however, requires about 700 parts of cold water to dissolve it, the crystals of strontium hydroxide requiring only 50 parts, and is decomposed into calcium oxide and water by prolonged ignition. When barium hydroxide solution is added to hydrogen dioxide a hydrated barium peroxide, Ba0. 2, 6H.,O, is precipitated in crystalline scales ; the cor responding strontium and calcium compounds, SrO 2 ,8H 2 O and Ca0 2, 8H 2 O, may be prepared in a similar manner, and are converted into the anhydrous oxides by heating to 100 C. Barium and strontium carbonates are practically in soluble in water, and are not decomposed on ignition ; calcium carbonate, CaCO 3, is only very slightly soluble in water, and is slowly converted into calcium oxide and carbon dioxide on ignition. In this respect lithium carbonate, Li 2 C0 3, closely resembles calcium carbonate, as it is only sparingly soluble in water, and decomposes on ignition ; whereas the carbonates of the other alkali metals are very soluble, and are not decomposed by heat. Calcium sulphate, CaSO 4, crystallizes with two molecules of water, and is slightly soluble in water ; but stronti urn and barium sulphates form anhydrous crystals, and are practically insoluble in water. Similarly, calcium nitrate crystallizes in deliquescent, very soluble monoclinic prisms of the composition Ca(NO 3 ).,, 4H 2 O, and an apparently isomorphous salt of similar composition, but efflorescent, may be obtained by crystallizing strontium nitrate at a low temperature ; but from a hot solution strontium nitrate separates in anhydrous octahedra isomorphous with barium nitrate, which crystallizes without water. The chlorides of calcium and strontium are deliquescent crystalline sub stances, very soluble in water, and also soluble in alcohol ; barium chloride is very much less soluble in water, and is insoluble in alcohol, and does not deliquesce. The chlorides of barium, strontium, and calcium furnish perfectly neutral solutions. Barium salts communicate an apple-green colour to flame, strontium salts a brilliant crimson, and calcium salts an orange-red ; the spectra of the three elements are comparatively simple. GLUCINUM MAGNESIUM ZINO CADMIUM MERCURY. Name. Symbol. At. wt. Sp. gv. At TOl. F. P. C. B. P. C. Electric conduc tivity. Glucinum G 9-3 2-1 4-4 Magnesium Mg 23-9 1-74 13-7 J 25-47 at 17 Zinc Zn 64-9 7-15 9-1 412 1040 { 29-02 ( at Cadmium Cd 111-6 8-65 12-9 228 860 I 2372 { at Mercury Hg 199-8 13-56 147 -38-8 357 1-63 1 at 23 Glucinum, or beryllium, obtained by the action of sodium on its chloride, is a white, malleable metal fusible below the melting-point of silver ; it does not burn in air or oxygen, but becomes coated with a layer of oxide which seems to protect it from further change. When heated in an atmosphere of chlorine it is converted into the chloride GC1 2 . It does not decompose water even when heated to whiteness. It is easily dissolved by dilute hydrochloric and sulphuric acids ; but nitric acid, even when concen trated, has little action on it. It is readily dissolved by a solution of potassium hydroxide. Glucinum hydroxide, G(OH) 2, separates as a white bulky precipitate on the addition of an alkaline hydroxide to a solution of a glucinum salt ; it exhibits pronounced acid properties, dissolving in solutions of the alkalies and even in those of alkaline carbonates. Glucinum oxide, GO, is a light, very bulky, white powder ; when strongly heated it volatilizes without fusing. Glucinum chloride^ GC1 2, crystallizes in silky needles ; it is very deliquescent, and dissolves in water with a hissing noise and considerable rise of temperature. Glucinum hydroxide readily dissolves in acids, but the ignited oxide slowly ; the glucinum salts are mostly soluble, and have a sweet, slightly astringent taste. Basic glucinum salts are obtained with great facility ; thus by digestion with the hydroxide, glucinum nitrate, G(NO 3 ) 2, is converted into the basic nitrate, G 3 N 2 O 8, and the basic sulphate, G 3 SO C , may in a similar manner be obtained from the normal sulphate, GSO 4. Glucinum sulphate, GSO 4, 4H 2 O, is extremely soluble in water ; it forms a double salt with potassium sulphate of the composition GK 2 (S0 4 ) 2 , 2H 2 O. Magnesium, Zinc, Cadmium. Magnesium is a white, malleable ductile metal about as hard as calcspar ; it melts and volatilizes nearly as readily as zinc. It readily decom poses water below its boiling-point, but not in the cold, unless associated with a less positive metal such as copper. It is easily dissolved by dilute acids. Zinc is a hard, white metal with a shade of blue ; when pure it is very malleable, but it is usually brittle owing to the presence of impurities such as lead and iron. Com mercial zinc, however, becomes malleable at 100-150 C., and may then be rolled ; at 210 C. it again becomes brittle, and may easily be reduced to powder at that temperature. Zinc decomposes vapour of water at a temperature below dull redness, and readily, even at ordinary temperatures, when it is associated with coper or other less positive metals. The pure metal dissolves very slowly in dilute acids, but ordinary zinc dissolves quickly, because the lead contained in it acts as an electro-negative element. Zinc dissolves in aqueous alkalies with evolution of hydrogen. Cadmium is a bluish-white metal like zinc ; it is soft, though harder and more tenacious than tin, and is very malleable and ductile. When heated to about 80 C. it becomes very brittle, and may be powdered with facility. It appears to decompose water at a high temperature, and dissolves when heated with slightly diluted hydrochloric or sulphuric acid, but its best solvent is dilute nitric acid. Magnesium is usually prepared by heating its chloride with sodium ; zinc and cadmium are readily obtained by reducing their oxides with charcoal. Magnesium, zinc, and cadmium are capable of being polished, and then assume a bright metallic lustre ; they retain their lustre in dry air, but when exposed to moist air a film of oxide forms on the surface which protects them from further oxidation. Magnesium is the most positive metal of the three, as it precipitates zinc and cadmium from their salts, and zinc is more positive than cadmium. When heated in chlorine, or in the vapour of bromine or of iodine, they burn brilliantly, forming corresponding chlorides, bromides, or iodides, such as MgCl 2, ZnBr 2 , CdI 2 ; if moistened with water, zinc and cadmium readily combine with the halogens at the ordinary temperature. They are easily combustible in air or oxygen ; the light emitted by burning magnesium is intensely brilliant and very rich in actinic rays ; zinc also burns with a brilliant flame. They are thus converted into the oxides MgO, ZnO, CdO ; magnesium and zinc oxides are white, but the latter becomes lemon yellow when heated ; cadmium oxide is yellowish brown. These oxides are unalterable in the fire ; they dissolve readily in acids, forming corresponding