Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/43

Rh MERCURY 33 with considerable contraction, into a compact mass of regular octahedra, which can be cut with a knife and be flattened under the hammer. The specific gravity of the frozen metal is 14 39 ; that of the liquid metal at C. is 13&quot;f)95 (water of 4C. = 1). Under 7GO mm. pressure it boils at 357 3 C. (675 l Fahr.) (Regnault). At very low temperatures it seems to be absolutely devoid of volatility (Faraday); but from - 13 C. upwards (Regnault) it exhibits an appreciable vapour tension. The following table gives the tensions &quot;p&quot; in millimetres of mercury of C., for a series of centigrade temperatures &quot; t,&quot; according to Regnault : 1= 10 20 50 100 150 200 ^=02 -03 04 -11 -75 4-27 19 90 t= 250&quot; 300 350 400 450* 500 p = 75-75 242-1 663 2 1588 3384 6520 According to the same authority, its average coefficient of expansion k per degree C. is as follows : 0-1 00 C. 0-200 C. 0-300C. =0001815 -0001841 0001866 or 1/5510 1/5432 1/5359 Its specific heat in the liquid state is 03332 ; that of the frozen metal (between -78 and -40 C.) is 0319 (Regnault). Its electric conductivity is -$- r of that of pure silver (Matthiesen). Its conductive power for heat is greater than that of water, and is proved (by Herwig) to be perfectly constant from 40 to 160 C. Its vapour density (air of the same temperature and pressure =1) is 6 976 (Dumas), or 100 93 for hydrogen =1. Hence its molecular weight (H 2 = 2) is 201 - 86. The atomic weight, by chemical methods, was found = 200 (Erdmann and Marchand) ; hence mercury-vapour molecules consist of single atoms. Mercury does not appreciably absorb any chemically inert gas. Mercury is in constant requisition in the laboratory. It is used for the collecting and measuring of gases, in the construction of thermometers, barometers, and manometers, for the determination of the capacity of vessels, and many other purposes. In medicine it serves for the preparation of mercurial ointment and of &quot; hydrargyrum cum creta &quot; (the chief component of &quot; blue pills &quot;) ; both are obtained by diligently triturating the metal with certain proportions of grease and chalk respectively until it is &quot; deadened,&quot; i.e., subdivided into invisibly small globules (see below). Alloys. Mercury readily unites directly with all metals (except iron and platinum) into what are called amalgams. In some cases the union takes place with considerable evolution of heat and large modification of the mean pro perties of the components. Thus, for instance, sodium when rubbed up with mercury unites with it with deflagra tion and formation of an alloy which, if it contains more than 2 per cent, of sodium, is hard and brittle, although sodium is as soft as wax and mercury a liquid. Liquid amalgams of gold and silver are employed for gilding and silvering objects of copper, bronze, or other base metal. The amalgam is spread out on the surface of the object by means of a brush, and the mercury then driven off by the application of heat, when a polishable, firmly adhering film of the noble metal remains. Copper amalgam containing from 25 to 33 per cent, of the solid metal, when worked in a mortar at 100 C., becomes highly plastic, but on standing in the cold for ten or twelve hours becomes hard and crystalline. Hence it is used for the stuffing of teeth. A certain amalgam of cadmium is similarly employed. Oxides. There are two oxides of mercury, namely, an oxide, Hg.,0, called mercurous, and another, HgO, called mercuric oxide. The latter can be produced directly by keeping the metal for a long time in air at a temperature somewhat below its boiling point, when the oxide is gradually formed as a red powdery solid has long been known as &quot;red precipitate,&quot; or solid. This as mercurius prxcipitatus per se. Priestley made the important discovery that the &quot; precipitate &quot; when heated to dull redness is reduced to metal, with evolution of what has since been known as oxygen gas ; but it was reserved for Lavoisier to correctly interpret this experiment, and thus to establish our present views on the constitution of atmospheric air. The oxide is easily prepared by heating any nitrate of the metal as long as nitrous fumes are seen to go olf (when it remains as a scaly mass, black when hot, red after cooling), or else by precipitating the solution of a mercuric salt with excess of caustic potash or soda, when it comes down as an amorphous yellow precipitate, which is free of combined water. Mercurous oxide, a black solid, can be obtained only indirectly, by the decom position of mercurous salts with fixed caustic alkalies. Both oxides are insoluble in water, but dissolve in certain, and combine with all, aqueous acids with formation of mercury salts and elimination of water. Thus, for instance, Hg.O + 2HN0 3 = H.,0 + Hg 2 (NO :t ), , Mercurous nitrate HgO + 2HN0 3 = H 2 + Hg(N 3 ),. Mercuric nitrate. The Nitrates. When metallic mercury is set aside with its own weight of nitric acid of 1 2 specific gravity, at ordinary tempera tures, the normal mercurous salt Hg.,(N0 3 ) 2 is gradually produced, and after a day or two is found to have separated out in colourless crystals. These are soluble (somewhat sparingly) in water acidu lated with nitric acid, but are decomposed by the action of pure water, with formation of difficultly soluble basic salts. &quot;When this salt (or the metal itself) is treated with excess of nitric acid it is oxidized into mercuric nitrate Hg(N0 3 ) 2, a white crystalline salt, readily soluble in water without decomposition. The Sulphates. Cold aqueous sulphuric acid does not act upon mercury, but the hot concentrated acid converts it first into mercurous and then into mercuric sulphate, with evolution of sulphurous acid. Hg a S0 4 + 2H 2 S0 4 = 2H 2 + S0 2 + 2HgS0 4. Both salts form white crystalline magmas. The mercurous salt is difficultly soluble in water, and consequently producible by precipitation of the nitrate with sulphuric acid. The mercuric salt, when treated with water, is decomposed with formation of a yellow insoluble basic salt, which has long been known as turpcthum mine- rale. Its composition is S0 3. 3HgO when produced by excess of hot water. Mercuric sulphate is of importance chiefly as forming the basis for the manufacture of the two chlorides. The Chlorides. These are both extensively used medicinal agents. The mercuric salt, HgCl 2, known in medicine as corrosive sub limate, is prepared by mixing the sulphate intimately with common salt, and subjecting the mixture to sublimation, a little binoxide of manganese being added to oxidize the mercurous salt, which is generally present as an impurity. The process is conducted in a glass flask buried in a hot sand-bath. When the decomposition is accomplished, the sand is removed from the upper half of the flask and the temperature raised so that the chloride HgCl 2 produced sublimes up and condenses in the upper part as a &quot;sublimate.&quot; The salt, as thus produced, forms compact crystalline crusts, which, when heated, melt into a limpid liquid before volatilizing. It is soluble in water, 100 parts of which at 10, 20, 100 dissolve 6 57, 7 39, 54 parts of salt. Corrosive sublimate dissolves in 3 parts of alcohol and in 4 parts of ether. This salt, on account of its solubility in water, is a deadly poison. Mercurous chloride, Hg 2 Cl.,, better known as &quot;calomel&quot; (from /caAos, fair, and /j.tas, black, because it becomes dead-black when treated with ammonia, mer curic chloride yielding a white product), is prepared by mixing corrosive sublimate with the proper proportion of metallic mercury (HgCl 2 : Hg) or mercuric sulphate with salt and mercury in the proportions of HgS0 4 : Hg : 2NaCl, and subjecting the mixture to sublimation in glass flasks. The salt Hg 2 Cl 2 is thus obtained in the form of white, opaque, crystalline crusts, which, when heated, volatilize, without previously melting, into a mixture of HgCl 2 and Hg vapour, which, on cooling, recombine into calomel. For medicinal purposes the sublimate is reduced to an impalpable powder, washed with water to remove any corrosive sublimate that may be present, and dried. Being insoluble in water, it acts far less violently on the organism than mercuric chloride does. Its action, no doubt, is due to its very gradual conversion in the stomach into mercury and corrosive sublimate. Finely divided calomel can be produced, without trouble, by the precipitation of a solution of mercurous nitrate with hydrochloric acid or common salt ; but this preparation is liable to be contaminated with mercurous nitrate, and, even when pure, has been found to act far more violently than ordinary calomel does. Hence its use is not tolerated by the pharmacopoeias. According to Vohler a mercurous chloride, more nearly equivalent to the sublimed article, is produc ible by heat ing corrosive sublimate solution with sulphurous acid 2HgCl, + H 2 SO, + H.,0 - II 2 S0 4 + 2HC1 + Hg,Cl 2.