The next genus of the metallic poisons includes the preparations of mercury. Some of these are hardly less important than the arsenical compounds. They act with equal energy, produce the same violent symptoms, and cause death with the same rapidity. They have therefore been often given with a criminal intent; and have thus become the subject of inquiry upon trials. In another respect, too, they claim the regard of the medical jurist: their effects on the body, when insidiously introduced in the practice of the arts in which mercury is used, form a branch of that department of medical police, which treats of the influence of trades on the health.
Section I.—Of the Chemical History and Tests for the preparations of Mercury.
Mercury is a fluid metal, exceedingly brilliant, of a silver-white colour, and of the specific gravity 13·568.
When heated to about 660° F. it sublimes, and on cooling it condenses unchanged. If this experiment is made in a small glass tube, the metal forms a white ring of brilliant globules, which may be made to coalesce into a single large one. In this way its physical properties may be recognised, though the quantity is exceedingly minute.
Two oxides of this metal, a protoxide and peroxide, exist in combination with acids. A bluish-gray or grayish-black protoxide is separated from the salts of the protoxide by the fixed alkalis. The peroxide has an orange-red colour, and is the common red precipitate of the apothecary. Mercury unites with sulphur in two proportions. The proto-sulphuret, which is black, is formed from the salts of the protoxide by the action of sulphuretted-hydrogen: the bisulphuret is the well known pigment, cinnabar or vermilion. Mercury likewise unites with chlorine in two proportions, forming an insoluble protochloride and a soluble bichloride, the former calomel, the latter corrosive sublimate. It likewise unites with cyanogen. Mercury also unites in the state of protoxide and peroxide with the acids. Several compound salts are known to the chemist, but few occur in commerce or the arts.
Among the compounds resulting from the action of this metal with other substances, those which require notice in a toxicological treatise are the following:—1. The binoxide or red precipitate; 2. The bisulphuret or vermilion; 3. The protochloride or calomel; 4. The bichloride or corrosive sublimate; 5. The sulphate or Turbith mineral; 6. The bicyanide or prussiate of mercury; and 7. The nitrates of mercury. Its other compounds are of little consequence to the toxicologist.
1. Of Red Precipitate.
Red precipitate, when well prepared, is in the form of fine powder or small, brilliant, heavy scales of a scarlet or orange colour. It consists of 101 mercury and 8 oxygen. It is insoluble in water.
It is easily distinguished from all other substances by the action of heat. If a little of it is heated in a small glass tube, it becomes dark brown, and on cooling recovers its original colour. But if the heat be raised higher, metallic globules are sublimed, and oxygen gas is disengaged. The escape of oxygen may be ascertained by plunging to the bottom a small bit of burning wood, when the combustion will be observed to be enlivened.