Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/539

MERCURY.] CHEMISTRY 527 salts, e.g. : MgO + H 2 S0 4 = MgSO 4 + H 2 O. Magnesium oxide, when mixed with water, gradually combines with it, and forms the hydroxide Mg(OH) 2 , but no sensible develop ment of heat occurs during the hydration. Cadmium oxide also unites with water. The hydroxides of magnesium, zinc, and cadmium separate as white amorphous precipitates, insoluble in water, on the addition of alkalies to solutions of the salts of these metals. Magnesium hydroxide alone exhibits a faint alkaline reaction ; zinc hydroxide readily dissolves in alkalies ; they are all dissolved by solutions of ammonium salts ; they are easily resolved by heat into water and the oxide. Cadmium also furnishes a suboxide, Cd 2 O, which is a green powder ; on treatment with acids it is resolved into metallic cadmium and the oxide CdO ; in other words, a salt of the oxide CdO is produced, and cadmium remains. The chlorides of these metals, like glucinum chloride, are volatile deliquescent solids ; magnesium and zinc chlorides are extremely soluble in water, and their solutions are strongly acid, but cadmium chloride is less soluble ; their dissolution in water is attended with development of heat, thus : MgCl 2 , Aq= 35,920 units of heat. ZnCl 2 ,A<i= 15,630 A(i = 3,010 Their bromides and iodides are also soluble in water. The chlorides of magnesium, zinc, and cadmium form crystalline compounds with the chlorides of many other metals, the tendency to combine with other chlorides being especi ally marked in the case of cadmium chloride. Their sulphates are soluble in water, especially those of magnesium and zinc, which are isomorphous ; the sulphates of the three metals form isomorphous double salts with potassium sulphate. The composition of their sulphates and double sulphates with potassium is as follows : MgS0 4 , 7H 2 ZnS0 4 ,7H 2 CdS0 4 , 4H 2 MgK 2 (S0 4 ) 2 , 6H 2 ZnK 2 (S0 4 ) 2 , 6H 2 CdK 2 (S0 4 ) 2 , 6H 2 Zinc, like glucinum, manifests a great tendency to form basic sulphates. The carbonates of magnesium, zinc, and cadmium are white and insoluble in water ; they are without difficulty decomposed by heat into the oxide and carbon dioxide. Glucinum sulphide is soluble in water without decom position, but is decomposed by dilute acids ; magnesium sulphide is converted by water into magnesium hydroxide with evolution of hydrogen sulphide : MgS + 2 H 2 = Mg(OH) 2 + H 2 S ; zinc sulphide is insoluble in water, but is dissolved and decomposed by dilute mineral acids ; lastly, cadmium sulphide is insoluble in water and dissolves with difficulty even in boiling dilute hydrochloric acid, though it is readily decomposed by concentrated hydrochloric acid even at ordinary temperatures. Cadmium sulphide is a brilliant yellow substance, the "remaining sulphides are white. Mercury. This element is the only metal that is liquid at common temperatures, with the exception, possibly, of caesium, and the recently discovered element gallium. The latter element, indeed, appears to be closely allied to zinc in properties, but is less positive, and is probably a member of the aluminium group ; its specific gravity is 5 95. Mercury occurs native chiefly in the form of sulphide, from which it is separated either by distillation with slaked lime (calcium hydroxide), or by burning off the sulphur. It is a highly lustrous metal, and in the solid state is malleable. It slowly combines with the halogens at ordinary tempera tures, and also with sulphur if triturated with it ; it does not tarnish, even in moist air, but slowly absorbs oxygen when heated to about 400 C., being converted into the red oxide HgO ; this oxide is decomposed on iguition. Hydrochloric acid, whether cold or hot, is without action on mercury, but it is slowly dissolved by hydriodic acid ; concentrated nitric acid dissolves it readily ; it is also dissolved by hot concentrated sulphuric acid. It has no action upon water at any temperature. It is capable of uniting with most metals, forming liquid or solid com pounds called amalgams ; the solid amalgams appear for the most part to be of definite composition, but the two metals are held together by very feeble affinities. Mercury unites with the halogens, with oxygen, and with sulphur in two proportions, forming the two series distin guished as mercurous and mercuric compounds ; thus : Hg 2 Cl s Hg,O Hg. 2 S Mercurous chloride. Mercurous oxide. Mercurous sulphide. HgCl. 2 Mercuric chloride. HgO Mercuric oxide. HgS Mercuric sulphide. The compounds of mercury with halogens, and mercury compounds generally, are difficultly soluble, or insoluble, in water. But the compounds with halogens dissolve readily in solutions of the haloid compounds of the alkali metals in consequence of the formation of double salts ; the haloid compounds of mercury, in fact, exhibit a more marked tendency than those of any other metal of the group to form double salts with the haloid compounds of other metals, cadmium being most nearly allied to mercury in this respect ; they unite also with the haloid acids, forming the compounds HHgCl 3 , HHgBr 3 , and HHgI 3 . Mercurous chloride or calomel, Hg 2 Cl 2 , is a white crys talline solid, insoluble in water ; it blackens slowly on exposure to light, and is decomposed by heat into metallic mercury and mercuric chloride, HgCl 2 . Mercuric chloride or corrosive sublimate is also white and crystalline, but soluble in water ; it boils without decomposition at 295 C. ; the density of its vapour corresponds with the formula HgCl 2 . By the action of ammonia on the chlorides of mer cury, one-half the chlorine is removed and displaced by the monad radicle amidogen : Hg 2 Cl 2 + 2NH 3 = Hg.,Cl(NH 2 ) + NH 4 C1 ; the lower chloride is thus converted in to black mercurous amidochloride, the higher chloride furnishing mercuric amidochloride or white precipitate, HgCl(iSrH 2 ). Mercury hydroxides are unknown, the chlorides of mer cury being at once converted into corresponding oxides by the action of alkalies ; but when mercury is associated with positive hydrocarbon radicles, it forms powerfully basic hydroxides such as Hg(C 2 H 5 )OH, which is a colourless oil. Mercuroua oxide, Hg. 2 O, is a black sub stance which by mere exposure to light, or a very gentle heat, is converted into the metal and the red oxide. Mercuric oxide, HgO, when prepared by heating mercury in oxgyen, forms red crystalline scales, but it is precipitated as a yellow powder on the addition of an alkali to a solu tion of mercuric chloride. These two forms of the oxide differ even in their chemical behaviour, and are perhaps polymeric ; thus, when the yellow oxide is boiled with a solution of potassium dichromate a basic mercuric chromate, Hg 3 Cr0 6 , is formed, but the crystallized oxide forms a more basic salt, viz., Hg 4 Cr0 7 , under similar circumstances. Mercuric oxide is dissolved by acids, forming mercuric salts such as mercuric nitrate, Hg(XO 3 ) 2 ; by digesting solutions of these salts with mercury they are converted into corre sponding mercurous salts such as mercurous nitrate Hg 2 (NO 3 ) 2 . But mercuric oxide also exhibits feeble acid properties ; thus it forms the compound K 2 Hg0 3 when dissolved in fused potassium hydroxide. Basic mercuric salts are also very readily produced ; mercuric sulphate, HgS0 4 , for example, is decomposed by water into a soluble acid salt and the insoluble basic sulphate Hg 3 SO 6 .

Mercuric sulphide, HgS, is thrown down as a black