Mercury, or QUICKSILVER (sym. Hg; atomic weight = 200; sp. gr. 13.6), one of the so-called noble metals, remarkable as being the only metal that is fluid at ordinary temperatures. It is of a silvery white colour, with a striking metallic lustre. When pure it runs in small spherical drops over smooth surfaces; but when not perfectly pure the drops assume an elongated or tailed form, and often leave a gray stain on the surface of glass or porcelain. Moreover, the pure metal, when shaken with air, presents no change upon its surface; while if impure it becomes covered with a gray film. It is slightly volatile at ordinary temperatures, and at 662° F. it boils, and forms a colourless vapour of sp. gr. 6.976. Hence it is capable of being distilled; and the fact of its being somewhat volatile at ordinary temperatures helps to explain its pernicious effects upon those whose trades require them to come much in contact with it—as, for example, the makers of barometers, looking-glasses, &c. At a temperature of -39° F. it freezes, when it contracts considerably, and becomes malleable. In consequence of the uniform rate at which it expands when heated, from considerably below 0° to above 300°, it is employed in the construction of the mercurial thermometer.
All mercurial compounds are either volatilised or decomposed by heat; and when heated with carbonate of soda they yield metallic mercury. Native or virgin quicksilver only occurs in small quantity, usually in cavities of mercurial ores. Of these ores by far the most important is Cinnabar (q.v.). There are two means of obtaining the metal from the cinnabar: the ore may be burned in a furnace, in which case the sulphur is given off as sulphurous acid, and the mercury is collected in a condensing chamber; or the ore may be distilled with some substance capable of combining with the sulphur—as, for example, with slaked lime or iron filings. The mercury as imported is usually almost chemically pure. If the presence of other metals is suspected, it may be pressed through leather, redistilled, and then digested for a few days in dilute cold nitric acid, which exerts little action on the mercury if more oxidisable metals are present; or better, in a solution of mercuric nitrate, which deposits mercury and takes up the more oxidisable metals. The mercury, after being washed with water, is chemically pure.
Mercury is first spoken of by Theophrastus (3d century B.C.); the name hydrargyros (whence comes the symbol Hg) dates from Dioscorides. Greeks and Phoenicians procured cinnabar from Almaden in Spain. After the discovery of the New World, the mercury of Peru was famous. California now produces the great bulk of the mercury of commerce, and most of it comes from the New Almaden mine. The total produce of California was 60,851 flasks (of 76½ lb. each) in 1881; in 1888 it had sunk to 33,250 flasks (value $1,413,125).
There are two oxides of mercury, the black suboxide, Hg2O, and the red oxide, HgO. Both of these lose all their oxygen when heated, and form salts with acids. The black suboxide, although a powerful base, is very unstable when isolated, being readily converted by gentle warmth, or even by mere exposure to light, into red oxide and the metal: Hg2O = HgO + Hg. The most important of its salts is the nitrate, Hg2(NO3)2 + 2Aq, from whose watery solution ammonia throws down a black precipitate known in pharmacy as Mercurius solubilis Hahnenmanni, from its discoverer, and consisting essentially of the black suboxide with some ammonia and nitric acid, which are apparently in combination. Of the red oxide the most important salts are the nitrate, Hg(NO3)2 + 8Aq; the sulphate, HgSO4, which is employed in the manufacture of corrosive sublimate; and the basic sulphate, HgSO4·2HgO, which is of a yellow colour, and is known as Turpeth Mineral.
The haloid salts of mercury correspond in their composition to the oxides. Of the most important of these—the chlorides—there are the subchloride, Hg2Cl2, well known as Calomel (q.v.), and the chloride, HgCl2, or corrosive sublimate.
The chloride, HgCl2, when crystallised from a watery solution occurs in long white glistening prisms; but when obtained by sublimation it occurs in white transparent heavy masses, which have a crystalline fracture, and chink with a peculiar metallic sound against the sides of the bottle in which they are contained. This salt melts at 509° F., and volatilises unchanged at about 570°. It has an acrid metallic taste. It is soluble in sixteen parts of cold, and in less than three parts of boiling water, and dissolves very freely in alcohol and in ether. Corrosive sublimate enters into combination with the alkaline chlorides, forming numerous distinct compounds. (A double chloride of ammonium and mercury, represented by the formula CH4NCl·HgCl2 + Aq, has been long known as sal alenbroth.) It combines with oxide of mercury in various proportions, forming a class of compounds of great interest in theoretical chemistry, termed oxychlorides of mercury. On adding a solution of corrosive sublimate to a solution of ammonia in excess, a compound, which from its physical characters is termed white precipitate, is thrown down, the composition of which is Hg2N2H6Cl2. Chloride of mercury coagulates albumen, and combines with the albuminous tissues generally, forming sparingly soluble compounds. Hence, in cases of poisoning with the salt, the white of raw eggs is the best antidote; and for the same reason corrosive sublimate is a powerful antiseptic, and is employed to preserve anatomical preparations.
Amongst the most important tests for this substance, which is not unfrequently used as a poison, may be mentioned (1) iodide of potassium, which, when added to a crystal or to a watery solution of chloride of mercury, gives rise to the formation of a bright scarlet iodide of mercury. (2) The galvanic test, which may be applied in various ways, of which the simplest is the 'guinea and key test,' devised by Wollaston. He placed a drop of the fluid suspected to contain corrosive sublimate on a guinea, and simultaneously touched it and the surface of the guinea with an iron key; metallic mercury was deposited on the gold in a bright silvery stain. (3) Precipitation on copper, and reduction. To apply this test we acidulate the suspected fluid with a few drops of hydrochloric acid, and introduce a little fine copper gauze, which soon becomes coated with mercury. On heating the gauze in a reduction tube the mercury is obtained in well-defined globules.
With iodine and bromine mercury forms two iodides and bromides, corresponding in composition to the chlorides. Both the iodides are used in medicine; the bromides are of no practical importance. The subiodide, , is a green powder formed by triturating 5 parts of iodine with 8 of mercury, and is of far less interest than the iodide, , which is most simply obtained by precipitating a solution of corrosive sublimate by a solution of iodide of potassium. The precipitate is at first salmon-coloured, but soon changes into a brilliant scarlet crystalline deposit.
Sulphur forms two compounds with mercury—viz. a subsulphide, , a black powder of little importance, and a sulphide, , which occurs naturally as Cinnabar (q.v.). Sulphide of mercury is thrown down as a black precipitate by passing sulphuretted hydrogen through a solution of a persalt of mercury (corrosive sublimate, for example). When dried and sublimed in vessels from which the air is excluded, it assumes its ordinary red colour. The well-known pigment vermilion is sulphide of mercury, and is sometimes obtained from pure cinnabar, but is more frequently an artificial product.
Mercury unites with most metals to form Amalgams (q.v.), several of which are employed in the arts. Of the numerous organic compounds of mercury it is unnecessary to mention more than the Fulminate (q.v.) and the cyanide, , which may be prepared by dissolving the red oxide of mercury in hydrocyanic acid, and is the best source from which to obtain cyanogen.
The uses of mercury are so numerous that a very brief allusion to the most important of these must suffice. It is employed extensively in the extraction of gold and silver from their ores by the process of amalgamation. Its amalgams have been largely employed in the processes of silvering and gilding, and some (as those of copper and cadmium) are employed by the dentist for stopping teeth. It is indispensable in the construction of philosophical instruments, and in the laboratory in the form of the mercurial bath, &c. It is the source of the valuable pigment vermilion. The use of its chloride in anatomical preparations has been already noticed; it is similarly found that wood, cordage, and canvas, if soaked in a solution of this salt (1 part to 60 or 80 of water), are better able to resist decay when exposed to the combined destructive influence of air and moisture.
MEDICINAL USES OF MERCURY AND MERCURIALS.
—Metallic mercury is used in medicine in a state of very fine division in the form of gray powder, blue pill, mercurial ointment, and other preparations. It is also used as mercurous and mercuric oxides and salts. As with other metals, the mercurial preparations have a local action and an action after absorption into the blood. The intensity of the local action varies, however, with the individual preparation; the persalts being soluble in water, and hence capable of precipitating albumen at once, are very irritating, while the mercurous salts and uncombined mercury, being insoluble in water, exert an effect only in so far as they are dissolved in the secretions. These differences in local action have a very great influence in determining their applications in medicine.
Locally, the various ointments, liniments, and plasters are used as stimulants, astringents, antiseptics, and parasiticides, the persalts are used as antiseptic lotions, while the protosalts are little employed locally, except Calomel (q.v.) in powder.
Absorption of mercurial preparations from the intestinal canal or skin takes place very readily, and in an hour or less the mercury may be detected in most of the secretions. After absorption into the blood all the preparations have the same action. In minute doses they act as alternatives, improving nutrition. In larger doses, such as are ordinarily used, they also exert profound alterative effects; but care must be exercised in their administration, otherwise symptoms of chronic poisoning are apt to ensue. These consist in excessive salivation, inflammation of the mouth and gums, and dyspepsia, while in severer cases caries of bone, nervous symptoms, a watery condition of the blood, albuminuria, cachexia, and other serious complications may occur. The mercurial preparations are given internally in syphilis, in serous inflammations, and in dropsy as diuretics. Certain of them, such as gray powder, blue pill, and calomel, are used as purgatives and as intestinal antiseptics.
The doses of the different preparations vary greatly, those of the persalts being very much smaller than in the case of the other preparations. Some persons are peculiarly susceptible to the action of mercurials, and show symptoms of chronic poisoning after very small amounts.
With regard to acute mercurial poisoning, this is due to irritation of the intestinal canal, and is only seen with the soluble salts when taken in overdoses. The perchloride (corrosive sublimate) has been most frequently employed for the purpose of poisoning. The symptoms come on immediately, with a burning pain in the throat and violent pain in the abdomen, with severe vomiting and purging. There is always a good deal of collapse. Albumen, in the form of white of egg, is the best antidote.