34 M E R C U K Y The writer is unable to say whether "Wohler s calomel has ever found its way anywhere into medicinal practice. The Iodides. The mercuric salt HgI 2 is produced in two ways, viz., first by mixing the two elementary components intimately and subjecting the mixture to sublimation, and secondly by pre cipitating corrosive sublimate solution with its exact equivalent of iodide of potassium. In the first case the salt is obtained in yellow crystals, which, on the slightest touch with a solid body, assume and then permanently retain a rich scarlet colour. The precipita tion process at once yields the scarlet salt. The salt is insoluble in water, but soluble in alcohol and in iodide of potassium solution. The mercurous salt Hg. 2 I 2 is obtained by precipitating mercurous nitrate with iodide of potassium as a dirty -green powder insoluble in water. Both iodides are used medicinally. The Sulphides. Mercurous sulphide, Hg 2 S, does not seem to exist. The mercuric salt, HgS, exists in two modifications, of which one is amorphous and has a black colour, while the other is crystalline and red. The black one is obtained by precipitation of solutions of mercuric salts with excess of sulphuretted hydrogen, or by direct synthesis. The right proportions of mercury and flowers of sulphur are rubbed together in a mortar until the whole is converted into a jet-black uniform powder. This preparation (the azthiops mineralis of the pharmaceutist), however, is apt to be contaminated with uncombined sulphur and mercury. Application of a gentle heat causes exhaustive combination. The red sulphide, HgS, occurs in nature as cinnabar, and can be produced artificially from the black. The artificial preparation, known as vermilion, is highly valued as the most brilliant, stable, and innocuous of all mineral red pigments. Vermilion can be produced from the black sulphide in two ways, viz., first by sublimation, and secondly by treatment of it with fixed alkaline sulphide solution. According to Brunner, 100 parts of mercury are mixed intimately with 38 parts of flowers of sulphur, and the rethiops is digested, with constant agitation, in a solution of 25 parts of potash in 150 parts of water at 45 C. (the water lost by evaporation being constantly replaced), until the preparation has come up to its maximum of fire and brilliancy, which takes a good many hours. Purely sublimed vermilion has a comparatively dull colour, and must be manipulated with alkaline sulphide solution to give it the necessary fire. The action of the alkaline sulphide consists probably in this, that it dissolves succes sive instalments of the amorphous preparation and redeposits them in the crystalline form. Mercuric Derivatives of Ammonia. (1) Eecently precipitated oxide HgO is digested, cold, in carbonic-acid-free ammonia, and the mixture allowed to stand for a few days. The liquor is then decanted off, and the precipitate washed with alcohol and then with ether, and dried over sulphuric acid. The product is a yellow solid base ("Millon s base ) of the composition N 2 H 6 + 4 HgO + H 2 = N 2 Hg 4 . 2H 2 + 2K,0 . It is insoluble in alcohol and in ether, and requires 13,000 parts of cold water for its solution. It readily unites with all acids, forming salts, which, as a rule, are insoluble in water. Hence all ordinary salt solutions, when shaken with the base, are decomposed with elimination of the base of the salt. Thus, for instance, even such salts as alkaline nitrates, chlorides, or sulphates are decom posed with formation of solutions of caustic alkali. (2) A body N 2 Hg 4 I 2 + 2H 2 0, i.e., of the composition of the iodide corresponding to the oxide in (1), is produced as a brown precipitate when ammonia or an ammonia salt is added to a solution of mercuric iodide in iodide of potassium mixed with large excess of caustic potash or soda (" Nessler s reagent"). In very dilute solutions of ammonia Nessler s reagent produces only a brown or yellow color ation, which, however, is so intense that ^iniVrinjth of ammonia in about 50 cubic centimetres of liquid becomes clearly visible. (3) The chloride NH 2 Hg.Cl of the "ammonium" NH 2 Hg is produced as an insoluble white precipitate when ammonia is added to a solution of corrosive sublimate. This substance is known in medicine as infusible white precipitate, in contradistinction to (4). (4) The fusible white precipitate was at one time supposed to be identical with (3), and is obtained by boiling it with sal-ammoniac solution. Its composition is NH 2 HgCl + NH 4 C1 = N 2 H 6 . Hg . C1 2 . Analysis. Any ordinary solid mercury compound" when heated in a sublimation tube with carbonate of soda, yields a sublimate of metallic mercury, which, if necessary, needs only to be scraped together with a wooden spill to unite into visible globules. From any mercury-salt solution the metal is precipitated by digestion with a piece of bright copper-foil. The precipitated mercury forms a coat ing on the copper, which becomes silvery on being rubbed with blotting paper. When the quicksilvered copper is heated in a sub limation tube, it reassumes its red colour with formation of a sub limate of mercury. Solutions of mercurous salts with hydrochloric acid give a white precipitate of calomel, which, after filtration, is easily identified by its becoming jet-black on treatment with ammonia. From mercuric solutions hydrochloric acid precipitates nothing; but stannous chloride, in its twofold capacity as a chloride and a reducing agent, yields a precipitate of calomel. 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 precipi tate, which, on addition of more and more reagent, assumes suc cessively a yellow, orange, and at last jet-black colour. The black precipitate is HgS, which is identified by its great heaviness, and by its being insoluble in boiling nitric and in boiling hydro chloric acid. A mixture of the two (aqua regia) dissolves it as chloride. (W. D.) Therapeutics of Mercury. The use of mercury as a therapeutic agent is of com paratively 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 con tinued 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 preparations now in use may be thus classified. (1) Of the preparations containing metallic mercury the chief members are blue pill (pilula hydrargyri), grey powder (hydrargyrum cum creta), and blue ointment (unguentum hydrargyri). The first consists of mercury, liquorice root, and confection of roses, the second of mercury and chalk, the third of mercury, suet, and lard. The power of the three preparations seems to depend on the fine state of subdivision of the mercury they contain ; mercury in its ordinary liquid state seems devoid of medicinal properties. It is thought by some that the fine subdivision of the metal leads to the formation of a little oxide, and that the efficacy of the preparations in part depends on this. (2) Three oxides of mercury are employed in medicine, the red, from which is made red precipitate ointment (unguen tum hydrargyri oxydi rubri), the yellow, an allotropic form of the red, and the black oxide. The yellow and black oxides suspended in lime water form respectively yellow and black wash (lotio flava and lotio nigra). (3) The chlorides of mercury form a very important group : calomel (hydrargyri subchloridum) is a white heavy powder ; corrosive sublimate (hydrargyri perchloridum) is a heavy crystalline substance. (4) Two iodides are used medicinally ; they are known from their colour as the green and red iodides. (5) Nitrate of mercury enters into the composition of a powerful caustic known as the acid nitrate of mercury. It is also the active principle of citrine ointment (unguentum hydrargyri nitratis). (6) In this class only ammoniated mercury and its ointment commonly known as vhite precipitate ointment, are contained. Of the many compounds not included in the above classifica tion the oleate and albuminate are the most important. Mercurial preparations are largely employed as external appli cations. Several of them are potent agents for the destruction of the lower forms of animal life, and hence are employed to destroy parasites having their habitat in skin, hair, and clothing. The white and red precipitate ointments are specially effective in the destruction of pediculi, and blue ointment is occasionally used for the same purpose. Corrosive sublimate is, however, the most energetic of the mercurial parasiticides, and recent observations seem to show that it is superior to almost all other substances as a germ destroyer. It is sometimes used to get rid of ringworm. It should be remembered that corrosive sublimate is a powerful irritant to the skin, and also an active poison. Acid nitrate of mercury is a caustic, and by it warts and small growths are sometimes removed ; it is also one of the caustics occa sionally applied to prevent the spread of lupus. In skin diseases mercurial preparations are largely used, especially in some forms of eczema. A few grains of the red oxide or of ammoniated mercury in an ounce of zinc ointment are often found
of great service in this ailment ; citrine ointment is also useful.