Mercury

Mercury (symbol Hg, atomic weight = 200), in chemistry, a metallic element which is easily distinguished from all others by its being liquid at even the lowest temperatures naturally occurring in moderate climates. To this -exceptional property it owes the synonyms of quicksilver in English (with the Germans Quecksilber is the only recognized name) and of hydrargyrum (from iibcop, water, and iipyupos, silver) in Graeco-Latin. This metal does not appear to have been known to the ancient Jews, nor is it mentioned by the earlier Greek writers. Theophrastus (about 300 B.C.) mentions it as prepared from cinnabar by treatment with copper and vinegar; Dioscorides obtained it from the same mineral with the aid of iron, employing at the same time a primitive distillation apparatus. With the alchemists it was a substance of great consequence. Its appearance commended it as a substance for investigation; many of its compounds, especially corrosive sublimate and calomel, were studied, and improved methods for extracting and purifying the metal were devised. Being ignorant of its susceptibility of freezing into a compact solid, they did not recognize it as a true metal, and yet, on the authority of Geber, they held that mercury (meaning the predominating element in this metal) enters into the composition of all metals, and is the very cause of their metallicity (see Element). When, about the beginning of the 16th century, chemistry and scientific medicine came to merge into one, this same mysterious element of " mercury " played a great part in the theories of pathology; and the metal,, h.

in the free as in certain combined states, came to be looked upon as a powerful medicinal agent.

Occurrence

Mercury occurs in nature chiefly in the form of a red sulphide, HgS, called cinnabar (q.v.), which, as a rule, is accompanied by more or less of the reguline metal - the latter being probably derived from the former by some secondary reaction. The most important mercury mines in Europe are those of Almaden in Spain and of Idria in Illyria; and in America those of California and Texas. Deposits also occur in Russia, the Bavarian palatinate, in Hungary, Italy, Transylvania, Bohemia, Mexico, Peru and in some other countries.

Mercury occurs in formations of all ages from the Archean to the Quaternary, and. it has been found in both sedimentary and eruptive rocks of the most varied character, e.g. conglomerates, sandstones, shales, limestones, quartzites, slates, serpentines, crystalline schists, and eruptive rocks from the most acid to the most basic. It appears that nearly all known deposits occur along lines of;'continental uplift, where active shearing of the formations has occurred. Large deposits are seldom found in eruptive rocks, but generally near such formations or near active or extinct hot springs. The deposits are of many types, simple fissure veins being less usual than compound, reticulated, or linked veins. Segregations and impregnations are very common. The form of the deposit seems to depend chiefly on the physical properties and structure of the enclosing rocks and the nature of the fissure systems that result from their disturbance. The principal ore is cinnabar, though metacinnabarite and native mercury are often abundant; the selenide (tiemannite), chloride, and iodide are rare. Of the associated heavy minerals, pyrite (or marcasite) is almost universal, and chalcopyrite, tetrahedrite, blende and realgar are frequent. Many deposits contain traces of gold and silver, and some deposits, as the Mercur in Utah, are more valuable for their gold than their mercury content. The usual gangue-forming minerals are quartz, dolomite, calcite, barite, fluorspar and various zeolites. Some form of bituminous matter is one of the most universal and intimate associates of cinnabar. Formerly quicksilver deposits were supposed to be formed by sublimation, but from a careful study of the California occurrences S. B. Christy was convinced as early as 1875 that this was unlikely, and that deposition from hot alkaline sulphide solutions was more probable. By treating the black mercuric sulphide with such solutions, hot and under pressure, he succeeded in producing artificial cinnabar and metacinnabarite. He also showed that the mineral water at the New Almaden mines, when charged with sulphydric acid and heated under pressure, was capable of effecting the same change, and that this method of production agreed better with all the facts than the sublimation theory. (See " Genesis of Cinnabar Deposits," Amer. Jour. Science, xvii. 453.) The investigations of Dr G. F. Becker on the " Quicksilver Deposits of the Pacific " (U.S. Geol. Survey, Mon. xiii., 1888) established the correctness of these views beyond doubt.

	Spain.	United	Russia.	i Austria-	Italy.	Mexico.	Total (Metric
		States.		Hungary.			Tons).
1901	754	1031	368	558	278	128	3120
1902	1425	1208	416	556	259	191	4056
1903	914	1288	362	567	314	188	3633
1904	1020	1192	393	581	357	1901	3733
1905	800	10 43	318	564	370	1901	3285

Production

At one time the world's supply of mercury was almost entirely derived from the Almaden and Idrian mines; but now the greater proportion is produced in California and Texas, where cinnabar was used by the Indians as a pigment, and first turned to metallurgical purpose in 1845 by Castellero. In the United States mercury has also been found in Utah, Nevada, Oregon and Arizona. In the 16th century the Almaden and Idrian mines were practically the only producers of this metal; statistics of Almaden dating from 1564 and of Idria since 1525 are given in B. Neumann, Die Metalle (1904). Spain produced 1151 metric tons in 1870, and in 1889 its maximum of 1975 tons; since then it has, on the whole, been decreasing. The Austria-Hungary output steadily increased to about 550-600 tons at which it appears to remain. In 1887 Russia produced 64 tons, and has steadily improved. The United States output was over 1000 tons, in 1871, and declined to 800-900 in the period 1889-1892; it has since increased and surpassed the supply from Spain. The following table gives the production in various countries for selected years: 1 Estimated.

Mercury is transported in steel bottles closed by a screw stopper; the Almaden and Idrian bottles contain 76 lb; and until the 1st of June 1904, the Californian bottles contained 762 lb of mercury; they now hold 75 lb. From the smaller works the metal is sometimes sent out in sheepskin bags holding 55 lb of mercury.

Metallurgy

Chemically speaking, the extraction of mercury from its ores is a simple matter. Metallic mercury is easily volatilized, and separated from the gangue, at temperatures far below redness, and cinnabar at a red heat is readily reduced to the metallic state by the action of iron or lime or atmospheric oxygen, the sulphur being eliminated, in the first case as iron sulphide, in the second as calcium sulphide and sulphate, in the third as sulphur dioxide. A close iron retort would at first suggest itself as the proper kind of apparatus for carrying out these operations, and this idea was, at one time, acted upon in a few small establishments - for instance, in that of Zweibrucken in the Palatinate, where lime was used as a decomposing agent; but the method has now been discarded. In all the large works the decomposition of the cinnabar is effected by the direct exposure of the ore to the oxidizing flame of a furnace, and the mercury vapour, which gets diffused through an immense mass of combustion gases, is recovered in more or less imperfect condensers.

With the exception of the massive deposits of Almaden in Spain and a few of those in California and Idria, cinnabar occurs in forms so disseminated as to make its mining very expensive. Rude handsorting of the ores is usually practised. Wet concentration has not been successful, because it necessitates ore crushing and extensive slime losses of the brittle cinnabar. As a rule low-grade ores can be roasted directly with less loss and expense. At Almaden in Spain the ores average from 5 to 7%, but in other parts of the world much poorer ores have to be treated. In California, in spite of the high cost of labour, improved furnaces enable ores containing not more than 2% to be mined and roasted at a profit.

The furnaces originally used at Almaden and Idria differ only in the condensing plant. The roasting was carried out in internally fired, vertical shafts of brickwork, and, at Almaden, the vapours were led through a series of bottles named aludels, so arranged that the neck of one entered the sole of the next; and at Idria the vapours were led into large brickwork chambers lined with cement, and there condensed. The aludel furnace, which was designed in 1633 by Lopez Saavedra Barba in Huancavelica, Peru (where cinnabar was discovered in 1566), and introduced at Almaden in 1646 by Bustamente, by whose name it is sometimes known, has now been entirely given up. The Idrian furnace was designed in 1787 by von Leithner; it was introduced at Almaden in "Boo by Larranaga, and used side by side with the aludel furnace. The crude mercury is purified by straining through dense linen or chamois leather bags.

The most important improvements in the metallurgy of mercury are the introduction of furnaces for treating coarse ores, and the replacement of the old discontinuous furnaces by those which work continuously. The most successful of these continuous furnaces was a modification of Count Rumford's continuous limekiln. This furnace was introduced at New Almaden by J. B. Randol, the author of many improvements in the metallurgy of mercury. The success of the continuous coarse-ore furnace at New Almaden led Randol to attempt the continuous treatment of fine ores also, and the Huettner and Scott continuous fine-ore furnace, which was the result of these experiments solved the problem completely. It contains several vertical shafts in which the descending ore is retarded at will by inclined shelving, which causes it to be exposed to the flames as long as may be necessary to roast it thoroughly. The time of treatment is determined by the rapidity with which the roasted ore is withdrawn at the bottom. Several similar furnaces are in use, as the Knox and Osborne, the Livermore and the Cormak-Spirek. The fumes from the roasting furnaces are received in masonry chambers, usually provided with water-cooled pipes; from these they pass through earthenware pipes, and finally through others of wood and glass. Not all the yield is in liquid mercury; much of it is entangled in masses of soot that cover the condenser walls, and this is only recovered after much labour.

The conditions for effective condensation are: (1) The furnace gases should be well oxidized, to avoid the production of an excess of soot. Gas firing would meet this requirement better than the use of wood or coal. (2) The volume of permanent gases passing through the furnace should be reduced to a minimum consistently with complete oxidation. (3) The cross-section of the condensers should be sufficient to reduce the velocity of the escaping gases, and the surface large enough for cooling and for the adhesion of condensed mercury. The latter requirement is best provided for by hanging wooden aprons in the path of the cooled gases. (4) The temperature of the escaping gases should not exceed 15° to 20° C., but cooling below this temperature would not give any adequate return for the expense. Cooling by water is quicker, but more expensive than by air. Water sprays, acting directly on the fumes, have not given good results, on account of the difficulty of recovering " floured " quicksilver from the water. (5) The use of an artificial inward draught is absolutely necessary to control the operation of the furnaces and condensers and to avoid the salivation of the workmen. (6) The condenser should be easily and quickly cleaned during the operation of the furnace. (7) Both furnaces and condensers should have inclined iron plates in their foundations to prevent the infiltration of mercury. (8) There is a great need of some substance for the construction of quicksilver condensers which shall be strong enough to be made thin, be a good conductor of heat, and resistant to abrasion and the alternate action of heat and cold. It should also resist the action of mercury and warm dilute sulphuric acid, and be not too expensive.

Quicksilver is best removed from the " soot," not by pressure, but by the opposite treatment. A machine in use for this purpose at New Almaden, devised by Colonel von Leicht, consists of an iron bowl, perforated at the bottom, in which revolves a vertical shaft carrying a propeller blade which tosses the soot (mixed with wood ashes and a little coal oil) into the air, so that the entangled mercury is free to run out through the bottom of the bowl. The residue from which no more mercury can be extracted mechanically is returned to the roasting furnace.

The losses of treatment are: (I) Furnace loss, which is easily reduced to nothing, and (2) condenser loss, which can never be zero. The latter consists of mercury lost as vapour and as mist, and its minimum amount is determined not by the richness of the ore but by the volume of escaping gases, their velocity and temperature. The percentage of loss will be higher with a poor than a rich ore. On a 3% ore the losses need not exceed 3 or 4% ore content. On a % ore they will run from 5 to 10%. But in poorly arranged plants under bad management they may easily be doubled or even trebled. The Huettner and Scott fine-ore furnace costs with condensers in California about $30,000, and roasts from 30 to 45 tons of ore (from 22 in. to dust) in 24 hours at a cost of from $i to $0.62 per ton.

Purification

Commercial mercury, as a rule, only needs to be forced through chamois leather or allowed to run though a very fine hole to become fit for all ordinary applications; but the metal, having the power of dissolving most other metals, is very liable to get contaminated, and requires then to be purified. For this purpose many chemical methods have been proposed; the commonest consist in allowing the metal to fall in a very fine stream through a column of a mixture of nitric acid and mercurous nitrate, or of sulphuric acid, or of potassium bichromate and sulphuric acid; the metal being subsequently dried and filtered through a perforated paper filter. The only really exhaustive method is distillation in a vacuum out of a glass apparatus. Many forms of apparatus have been devised to effect this. Recent researches have shown, however, that the metal so obtained is not chemically pure, there being found in the distillate traces of other metals. Absolutely pure mercury does not at all adhere to any surface which does not consist of a metal soluble in mercury. Hence the least quantity of it, when placed on a sheet of paper, forms a neatly rounded-off globule, which retains its form on being rolled about, and, when subtlivided, breaks up into a number of equally perfect globules, which tend to coalesce when sufficiently near to each other. The presence in it of the minutest trace of lead or tin causes it to " draw tails." A very impure metal may adhere even to glass, and in a glass vessel, instead of the normal convex, form an irregular flat meniscus.

Properties

Pure mercury is a freely flowing liquid, which does not wet objects placed in it, and has a silvery white colour and perfect metallic lustre; in very thin layers it transmits a bluish-violet light. It freezes at about - 39 0 C. (Mallet gives - 3 8.8 5 Hutchins,. - 39.44) with contraction, and the formation of a white, very ductile and malleable mass, easily cut with a knife, and exhibiting crystals belonging to the cubic system. When heated the metal expands very uniformly, and vaporizes at about 360°; the volatility is generally increased by the presence of impurities; its high expansion and the wide range of temperature over which it is fluid render it especially valuable as a thermometric fluid (see Thermometry). The vapour is colourless, and its density points to the conclusion that the molecules are monatomic. Its specific gravity at o° is 13.5959, i.e. it is about half as heavy again as copper volume for volume, a quarter as heavy again as lead, and nearly twice as heavy as zinc; this property is turned to account in the construction of barometers and air-pumps. Its specific heat is about 0.0333 (see Calorimetry); its electrical conductivity is involved in the definition of the ohm (see Electric conduction); and its thermal conductivity is about two thirds that of silver.

Pure mercury remains unchanged in dry air, oxygen, nitrous oxide, carbon dioxide, ammonia and some other gases at ordinary temperatures; hence its application for collecting and measuring gases. In damp air it slowly becomes coated with a film of mercurous oxide; and when heated for some time in air or oxygen it becomes transformed into the red mercuric oxide, which decomposes into mercury and oxygen when heated to a higher temperature; this reaction is of great historical importance, since it led to the discovery of oxygen at the hands of Priestley and Scheele. The halogen elements and sulphur combine directly with the metal. Mercury is unattacked by dilute sulphuric acid; the strong acid, however, dissolves it ors heating with the formation of sulphur dioxide and mercurous or mercuric sulphate according as mercury is in excess or not. Hydrochloric acid has no action. Dilute nitric acid readily attacks it, mercurous nitrate being formed in the cold with excess of mercury, mercuric nitrate with excess of acid, or with strong acid, in the warm. The metal dissolves in solutions containing chlorine or bromine, and consequently in aqua regia.

Mercury readily dissolves many metals to form a class of compounds termed amalgams, which have considerable applications in the arts.

Compounds of Mercury. Mercury forms two well-defined series of salts - the mercurous salts derived from the oxide Hg 2 0, and the mercuric salts from the oxide HgO; the existence of these salts can hardly be inseparably connected with a variable valency, i.e. that mercury is monovalent in mercurous, and divalent in mercuric compounds, for according to Baker mercurous chloride or calomel (q.v.) has the formula Hg2Cl2.

Mercurous Oxide, Hg 2 0, is an unstable dark-brown powder formed when caustic potash acts on calomel; it is decomposed by light or on trituration into mercury and mercuric oxide Mercuric oxide, HgO, occurs in two forms: it is obtained as a bright-red crystalline powder (also known as " red precipitate," or as mercurius praecipitatus per se) by heating the metal in air, or by calcining the nitrate, and as an orange-yellow powder by precipitating a solution of a mercuric salt with potash; the difference is probably one of subdivision. The yellow form is the most reactive and is transformed into the red when heated to 400°. If the red oxide be heated it becomes black, regaining its colour on cooling, and on further heating to 630° it decomposes into mercury and oxygen. It is slightly soluble in water, to which it imparts an alkaline reaction and strongly metallic taste. A peroxide is obtained as a brown solid from mercury and slightly acid 30% hydrogen peroxide at low temperatures.

Mercurous and mercuric chlorides, known respectively as calomel (q.v.) and corrosive sublimate (q.v.), are two of the most important salts of mercury. Mercurous bromide, Hg 2 Br 2, is a yellowish-white powder, insoluble in water. Mercuric bromide, HgBr 2, forms white crystals, sparingly soluble in cold water, readily in hot, and prepared by the direct union of its components. Mercurous iodide, Hg 2 1 21 is a yellowish-green powder obtained by heating its components to about 250°, or by triturating them with a little alcohol; it is also obtained by precipitating a solution of mercurous nitrate with potassium iodide. It is blackened by exposure to light. Mercuric iodide, Hgl 2, exists in two crystalline forms. By mixing solutions of mercuric chloride and potassium iodide under a microscope, yellow rhombic plates are seen to be formed which are transformed very quickly into scarlet quadratic octahedra. On heating to about 126° the red form is transformed into the yellow modification; on cooling the reverse gradually occurs, and immediately if the yellow iodide be touched. Mercuric iodide is insoluble in water, but soluble in absolute alcohol; and also in potassium iodide solution, with the formation of K 2 HgI 4, which may be obtained in lemon-yellow crystals. A strongly alkaline solution of this salt is known as Nessler's reagent, and is specially used for determining traces of ammonia (see below). Mercuric iodide dissolves in other iodide solutions to form similar compounds; these solutions are characterized by their exceptionally high specific gravity, and hence are employed in density determinations (see Density). It also forms many other double salts. Oxidation with strong nitric acid gives the iodate, Hg(IOs)2. An iodide, Hg213, intermediate between mercurous and mercuric iodides, is obtained as a yellow insoluble powder by precipitating mercurous nitrate with a solution of iodine in potassium iodide. Mercurous fluoride, Hg 2 F 2, and mercuric fluoride, HgF 2, are unstable substances obtained from the corresponding oxide and hydrofluoric acid.

Mercurous Nitrate, Hg 2 (NO 3) 2.2H 2 O, is obtained as a white crystalline salt soluble in water by dissolving the metal in cold dilute nitric acid; if the metal be in excess a basic salt Hg 2 (NO 3) 2.2HG20.3H 2 0 is obtained. Several other basic salts are known. By adding ammonia to a solution of mercurous nitrate a black precipitate of variable composition, known in pharmacy as mercurius solubilis Hahnemanni, is obtained.

Mercuric Nitrate

By dissolving mercuric oxide in strong nitric acid there is obtained a thick liquid which will not crystallize, and which gives on the addition of strong nitric acid a white precipitate of 2Hg(N03)2. H 2 O. Water decomposes it to give basic salts of variable composition. By dissolving the oxide in dilute nitric acid, the basic salt Hg(N03)2. HgO. H 2 O, crystallizing in needles, is obtained.

Mercurous Sulphide, Hg 2 S, is an unstable black powder obtained by acting with sulphuretted hydrogen, diluted with carbon dioxide, on calomel at -10°. It decomposes into mercuric sulphide and mercury at 0°. Mercuric sulphide, HgS, is one of the most important mercury compounds; it is the principal ore, occurring in nature as the mineral cinnabar (q.v.), and is extensively used as a pigment, vermilion. It is obtained as a black powder by triturating mercury with sulphur, the compound thus formed being known in pharmacy as Aethiops mineralis, and also by precipitating a mercuric salt with sulphuretted hydrogen. It is only slightly acted upon by nitric acid; it dissolves in aqua regia; chlorine gives a yellow compound, 2HgS. HgC1 2; and it dissolves in potassium sulphide solutions to form double salts of variable composition.

Mercurous Sulphate, Hg2S04, is a white, sparingly soluble, crystalline substance obtained by adding sodium sulphate to a solution of mercurous nitrate. Mercuric sulphate, HgSO 4, is a white, soluble salt obtained by dissolving mercury in hot strong sulphuric acid; on digestion with water, it decomposes into a basic salt HgSO 4.2HgO known as turbith or turpeth mineral, and into an acid salt, HgSO 4.2S03.

Mercury Phosphide, Hg 3 P 2, is obtained as brilliant red, hexagonal crystals by heating mercury with phosphorus iodide to 300° and removing the mercuric iodide simultaneously formed by means of potassium iodide solution. Mercurous phosphate, Hg 3 PO 4, and mercuric phosphate, Hg3(P04)2, are obtained as white precipitates by adding sodium phosphate to solutions of mercurous and mercuric nitrates respectively.

Mercurammonium Compounds

By the action of ammonia and ammonium salts mercury compounds yield a number of substances, many of which have long been used in medicine. By the action of dry ammonia on calomel mercuroso-ammonium chloride, NH 3 HgC1, is obtained; aqueous ammonia on calomel gives dimercuroso-ammonium chloride, NH 2 Hg 2 C1. By adding ammonia to a solution of mercuric chloride, mercurammonium chloride, known in pharmacy as " infusible white precipitate," NH2HgC1, is obtained; " fusible white precipitate " is mercuro-diammonium chloride, Hg(NH 3 C1) 2, and is obtained by adding a solution of mercuric chloride to hot solutions of ammonium chloride and ammonia so long as the precipitate first formed redissolves; the substance separates out on cooling. By precipitating a strongly alkaline solution of mercuric iodide in potassium iodide (Nessler's solution) there is obtained a yellow precipitate of NH 2 Hg 2 OI; this reaction is the most delicate test for ammonia, a yellow coloration being given by minute traces. By passing dry ammonia over precipitated mercuric oxide at 130°, a nitride N 2 Hg 3 is obtained. The oxide and ammonia solution gives the stable and basic mercurhydroxylamine, NHg 2 OH. The constitution of these compounds has been especially studied by K. A. Hofmann and E. C. Marburg (Zeit. Anorg. Chem. 23, p. 126); these chemists formulate " infusible precipitate " as Hg(NH 2)Cl, " fusible precipitate " as Hg(NH3C1)2 " Millon's base " as (HO. Hg) 2 :NH 2 OH, thus postulating three distinct types of compounds, (1) amidochlorides; (2) amines; (3) substituted ammonium derivatives.

Analysis

Mercury compounds, when heated in a closed tube with sodium carbonate, yield a grey to black sublimate of metallic mercury, which readily unites to form visible globules. The metal is precipitated from solutions by digestion with bright copper-foil, a coating being formed on the copper, which becomes silvery on rubbing, and disappears when the quicksilvered copper is heated in a sublimation tube.

Solutions of mercurous salts with hydrochloric acid give a white precipitate of calomel, which becomes jet-black on treatment with ammonia. Stannous chloride, in its twofold capacity as a chloride and a reducing agent, precipitates both mercurous and mercuric solutions, at first as calomel, and on addition of an excess of reagent the precipitate becomes grey through conversion into finely-divided quicksilver. Sulphuretted hydrogen, when added very gradually to an acid mercuric solution, gives at first an almost white precipitate, which, on addition of more and more reagent, assumes successively a yellow, orange and at last jet-black colour. The black precipitate is HgS, which is identified by its great heaviness, and by being insoluble in boiling nitric and in boiling hydrochloric acid. A mixture of the two (aqua regia) dissolves it as chloride.

" Mercurous " mercury is quantitatively estimated by precipitating as calomel and weighing the precipitate on a tared filter at 100°. The metal may also be estimated by distillation in a closed tube with lime, the metal being collected and weighed, or by precipitating the solution with an excess of stannous chloride. More convenient is the method of precipitating as sulphide by an excess of sulphuretted hydrogen, and weighing the precipitate on a tared filter; or by means of a Gooch crucible.

Pharmacology and Therapeutics The use of mercury as a therapeutic agent is of comparatively recent date. To the Greeks and Romans its value was unknown, and the Arabian physicians only used it for skin affections. It was not till the middle of the 16th century that the special properties of mercury were fully appreciated, but since that time the metal has continued to hold a high though fluctuating value as a medicine. At first the metal in a finely divided state or in vapour was used; but very soon its various compounds were found to be endowed with powers even greater than those of the metal itself, and with the discovery of new compounds the number of mercurial medicines has largely increased.

The British Pharmacopeia contains some twenty-five mercurial preparations, including those of calomel (q.v.). Only the useful preparations will be mentioned here. Free mercury is contained in Hydrargyrum cum Creta, or " grey powder," which consists of one part of mercury to two of prepared chalk. The power of this valuable and widely used preparation varies somewhat with its age, as old specimens contain some mercuric oxide, which makes them more active. The dose is 1-5 gr., and the preparation is usually employed for children. The Pilula Hydrargyri, or " blue pill," contains one part of mercury in three, and the dose is 4-8 gr. It is usually employed for adults. There are also five preparations of free mercury for external use. Of these the most useful is the Unguentum Hydrargyri, " or blue ointment," which contains one part of mercury in two. Weaker ointments are also prepared from the red and the yellow forms of mercuric oxide. The perchloride of mercury or corrosive sublimate is therapeutically the most important salt of mercury. The dose is s l y1 1 6 gr. It is incompatible with alkalies, alkaline carbonates, potassium iodide, albumen and many other substances, and should therefore be prescribed alone. It is decomposed by impure water, and distilled water is therefore used in making the Liquor Hydrargyri Perchloridi, in which form it is usually prescribed. This contains half a grain of the perchloride to the fluid ounce and its dose is 30-60 minims. The perchloride is also compounded with lime-water to form the Lotio Hydrargyri Flava, or " yellow wash," which contains two grains of the salt to the fluid ounce. Mercuric iodide is an equally potent salt and has come into wide use of late years. It has the same dose as the perchloride and is largely prescribed in the Liquor Arsenii et Hydrargyri lodidi, or Donovan's solution, which contains i % of arsenious iodide and I % of mercuric iodide, the dose being 5-20 minims. An ointment widely used is prepared from the mercurammonium chloride (Unguentum Hydrargyri ammoniatum) of which it contains one part in ten. It is known as " white precipitate ointment." In discussing the pharmacology of mercury and its compounds, it is of the first importance to observe that metallic mercury is inert as such, and that the same may practically be said of mercurous salts generally. Both mercury itself and mercurous salts tend to be converted in the body into mercuric salts, to which the action is due. When metallic mercury is triturated or exposed to air it is partly oxidized, the first stage of its transformation to an active condition being thus reached.

Metallic mercury can be absorbed by the skin, passing in minute globules through the ducts of the sweat-glands. The mercury contained in " blue ointment " is certainly thus absorbed, actually circulating in the blood in a very different form, as described below. There is no local action on the skin. The mercuric salts, and especially the chloride and iodide, are probably the most powerful of all known antiseptics. One part of the perchloride in 500,000 will prevent the growth of anthrax bacilli, and one part in 2000 - the strength commonly employed in surgery - kills all known bacteria. The action is apparently specific and not due to the fact that perchloride of mercury precipitates albumen, including the albuminous bodies of bacteria, for the iodide is still more powerful as a germicide, though it does not coagulate albumen. These salts cannot be employed for sterilizing metallic instruments, which they tarnish. As these drugs are essentially poisons they must be used with the greatest care in surgical practice, and as they are particularly deleterious to the secreting structure of the kidney they must not be employed as antiseptics in diseases where renal inflammation is already present or probable. They are therefore contra-indicated for application to the throat in scarlet-fever or to the uterus in eclampsia. The stronger mercurial ointments kill cutaneous parasites and also possess some degree of antipruritic action, especially when the cause of the itching is somewhat obscure. Mercuric salts, when in strong solution, are caustic. It is important to observe that the volatility of metallic mercury and many of its compounds causes their absorption by the lungs even when no such effect is intended to follow their external application. This fact explains the occurrence of chronic mercurial poisoning in certain trades.

Single doses of mercury or its compounds have no action upon the mouth, the characteristic salivation being produced only after many doses. Their typical action on the bowel is purgative, the effect varying with the state of the mercury. So relatively inert is metallic mercury that a pound of it has been given without ill effects in cases of intestinal obstruction, which it was hoped to relieve by the mere weight of the metal. Half a grain of the perchloride, on the other hand, is a highly toxic dose. The action of mercurials on the bowel is mostly exerted on the duodenum and jejunum, though the lower part of the bowel is slightly affected. Hence a dose of mercury usually needs a saline aperient to complete its action, as in the " blue pill and black draught " of former days. Mercurials do not cause, in therapeutic doses, much increase in the intestinal secretion, the action being mainly exerted on the muscular wall of the bowel. The bile is rapidly removed from the duodenum, before any re-absorption can occur, and the bacterial action which decomposes the bile-pigment is arrested by the antiseptic power of the drug, so that the excreta are of a very dark colour. The classical experiments of William Rutherford (1839-1899), of Edinburgh, showed that calomel does not increase the amount of bile formed by the liver. Corrosive sublimate does, however, stimulate the liver to a slight degree. The value of calomel in hepatic torpor is as an excretory, not a secretory, cholagogue, the gall-bladder being stimulated to expel its stagnant contents. In large doses mercurials somewhat diminish the secretion of bile. The greater part of the mercury administered by the mouth, in whatever form, is excreted as mercuric sulphide. Prior to this decomposition the mercury exists as a complex soluble compound with sodium, chlorine and albumen. When perchloride of mercury is injected subcutaneously the sodium chloride in the blood similarly prevents the precipitation of the albuminate of mercury, and it is therefore desirable to add a little sodium chloride to the solution for injection of mercuric chloride.

Some observers assert that mercury is a haematinic, increasing, like iron, the amount of haemoglobin in the blood. Whilst this is doubtful it is certain that large doses, when continued, produce marked anaemia. The excretion of the drug is accomplished by all the secreting glands, including the breasts, if these are functioning. All the secretions of the body, except that of the peptic glands of the stomach, are stimulated, but the excretion of mercury is slow, and it is typically one of the drugs that are cumulative, like arsenic and digitalis.

Mercury is largely used in affections of the alimentary canal, and has an obscure but unquestionable value in many cases of heartdisease and arterial degeneration. But its value in syphilis (see Venereal Diseases) far outweighs all its other uses.

Toxicology

Acute poisoning by mercurials usually occurs in the case of corrosive sublimate. There is intense gastro-intestinal inflammation, with vomiting, frequent " rice-water " stools and extreme collapse. The treatment, except when the case is seen at once, is very difficult, but white-of-egg or other form of albumen is the antidote, forming an insoluble compound with the perchloride.

Chronic poisoning (hydrargyrism or mercurialism) is of great importance, since any indication of its symptoms must be closely watched for in patients who are under mercurial treatment. Usually the first symptom is slight tenderness of the teeth whilst eating, and some foetor of the breath. These symptoms become more marked and the gums become the seat of severe inflammation, being spongy, vascular and prone to bleed. The salivary glands are swollen and tender, and the saliva pours from the mouth, and may amount to pints in the course of a day. The teeth become quite loose and may fall out. The symptoms are aggravated until the tongue and mouth ulcerate, the jaw-bone necroses, haemorrhages occur in various parts of the body, and the patient dies of anaemia, septic inflammation or exhaustion. The treatment consists, besides stopping the intake of poison and relieving the symptoms, in the administration of potassium iodide in small, often repeated doses.

Bibliography. - For the history of mercury see B. Neumann, Die Metalle (1904); A. Rossing, Geschichte der Metalle (1901). The general chemistry is treated in detail in O. Dammer, Handbuch der anorganischen Chemie, and H. Moissan, Traite de chimie minerale. For the metallurgy reference may be made to Carl Schnabel, Handbook of Metallurgy, vol. ii. (1906), translated by H. Louis.