Mercury is a liquid metal. It forms two series of compounds, viz. mercurous, in which it is univalent, e.g. Hg2Cl2; and mercuric, in which it is bivalent, e.g. HgCl2. In constitution these salts are analogous to the cuprous and cupric salts.

General Sources. - The chief source is native sulphide or cinnabar.

Metallic mercury is prepared from this by roasting it either alone or with lime or iron.

General reactions of Salts of Mercury. - They are all, either volatile, or decomposed by heat with the liberation of free mercury. The soluble salts are decomposed by stannous chloride; the mercuric salts giving first a white precipitate changing into black, and the mercurous salts a black one of finely divided mercury at once. Mercurous salts are most readily distinguished from mercuric salts by their reactions with alkaline carbonates, with ammonia, or with potassium iodide. The differences will be readily seen from the following table. The difference between the reactions of potash and ammonia with mercuric salts is noteworthy.

General Reactions Of Salts Of Mercury

Reagent

Mercurous Salts

Mercuric Salts

Stannous chloride .

Black ppt. (finely divided mercury)

White ppt., turning black (calomel changing into mercury).

Caustic soda or potash

Black ppt.

Yellow ppt. (oxide).

Carbonates of sodium or potassium

White ppt. turning black

Bed-brown ppt.

Ammonia

Black ppt.

White ppt. (double salt of mercury and ammonia).

Ammonium carbonate

White ppt. turning black

White ppt.

Potassium iodide

Greenish-yellow ppt.

Bright scarlet ppt., soluble in excess either of mercuric chloride or of potassium iodide.

General Impurities. - Other metals, especially lead, arsenic, and antimony, may be present. But there is such an enormous difference between the activity of the mercurous and the mercuric salts, that the latter form the most important impurities of the former. Corrosive sublimate, for example, is so active that a slight trace of it as an impurity in calomel might cause a medicinal dose of the latter to produce poisonous effects.

General Test. - Mercuric salts are readily soluble in alcohol, and especially in ether, and also in a solution of sodium chloride, while mercurous salts are not. The presence of mercuric compounds as an impurity in mercurous preparations can be ascertained by shaking them with ether (B.P.), or with a solution of common salt (U.S.P.), filtering, and testing the nitrate for mercury. If no mercuric salt has been present, neither the ether nor salt solution will dissolve anything, and so the test will show the absence of mercury. When ether or alcohol is used, the absence of mercury may be ascertained by evaporating it and finding that no residue remains.

General Action. - Metallic mercury, mercurous salts, and mercuric salts all have actions differing from each other as far as their local effect is concerned, but agreeing together in their general result after absorption into the system. When applied locally to the skin, mercury, either in a state of vapour or when finely subdivided in the form of ointment, will pass through the epidermis without exciting any local irritation, and be absorbed into the circulation, where it will produce the general effects of the drug. Taken in the form of vapour into the lungs, it will have a similar action. The mercurous salts are also absorbed in the same way as metallic mercury. They have a slightly more stimulating effect than it, but do not produce the same intense irritation that the mercuric salts cause. The mercuric salts unite with albumen, forming albuminates. They have little action on the epidermis, but when applied to the denuded skin, or to a mucous membrane, they precipitate the albumen, and, when used in a concentrated form, produce a slough. When swallowed, they cause the symptoms of gastro-enteritis produced by other irritant poisons, but these may be quickly succeeded by the symptoms of special mercuric poisoning from the absorption of the substance into the circulation (cf. p. 398).

The general effects on the body which are produced alike by mercury and its salts are termed mercurialism.

The first symptoms produced by mercury, however it is applied, are almost always connected with the alimentary canal, and more especially the mouth. A metallic unpleasant taste is observed in the mouth, along with a feeling of heat; the saliva is somewhat increased and the breath has a most unpleasant smell. The teeth feel sticky, as if their edges were glued together with some adhesive substance, when the patient tries to separate them; they feel as if they were longer than usual. The gums are red and swollen and tender, and chewing is painful. The tongue is covered with a thick coating, and the appetite is small. The medicinal administration of mercury is generally stopped when the gums become sore and salivation begins. In children salivation occurs with difficulty, and mercury may be discontinued when the breath becomes foetid or the previously healthy stools become green and offensive. When the administration o mercury is continued the symptoms increase; the gums become still more inflamed, their edges are covered with a white sticky substance, and they bleed on the slightest touch; the teeth become loosened in their sockets, and the salivation becomes still greater.

In still worse cases ulcers form on the gums and inside the cheeks, the tongue itself becomes swollen so that articulation becomes difficult, mastication is so painful as to be nearly impossible, the feet or of the breath is insupportable, and the saliva pours from the mouth in great quantities.

Along with these symptoms there is a certain amount of fever, which, indeed, sometimes is present before any local symptoms have appeared. There is general depression, chilliness, and even rigors, followed by a rise of temperature, a feeling of heat, thirst, loss of appetite, quick pulse, weight or pain in the epigastrium, nausea, belching, vomiting, and purging, sometimes bloody motions, or more rarely constipation. These symptoms last several days and then decrease, sweating occurring at the same time, or salivation if the fever has preceded it. Mercurial fever occurs most readily after a lengthened application of blue ointment.

When the administration of mercury is stopped the symptoms decrease, though in the case of broken-down individuals necrosis of the jaw, and even death, has occurred.

Occasionally it has happened that even healthy individuals, instead of recovering after profuse salivation, have become permanently dyspeptic.