This salt has long been described as a bichloride of mercury, consisting of two atoms of chlorine and one of mercury; but it is actually composed of one equivalent each of mercury and chlorine, and should be known as the protochloride of mercury, a name very commonly given to calomel, which is properly a subchloride, consisting of two atoms of mercury and one of chlorine. This distinction is the more important to be made, as both these chlorides are powerful medicines, and a mistake in administering one for the other might be attended with the most serious consequences. Corrosive sublimate was known at an early period to the Chinese, and was spoken of by Geber about A. D. 800. It is obtained in cakes of small translucent and colorless prisms, clustering together, which are soluble in water, and more readily in alcohol. The specific gravity is variously given from 5.14 to 6.5. The salt fuses at 509°, and boils at 563°, being converted into a colorless vapor of density 9.42. Corrosive sublimate has an acrid and caustic taste, somewhat styptic, and shows an acid reaction in reddening litmus paper.

Taken internally, it acts as a violent poison, corroding the parts with which it comes in contact, and producing violent irritation, intense pain in the bowels and stomach, with vomiting and diarrhoea. It is neutralized and its effects mitigated by administering albuminous matters, as the white of eggs, or gluten in the form of flour, or caseous matters, as the curd of milk. It should be remembered, however, that the compounds formed by these substances with corrosive sublimate, although much less active than that salt alone, are only relatively insoluble, and should be got rid of by emetics or the stomach pump as soon as possible. Corrosive sublimate is decernposed by protosulphuret of iron, iron filings, and Peruvian bark, which may consequently be used as antidotes. The salt is obtained in an experimental way by heating a globule of mercury in an iron spoon, and plunging it into a bottle of chlorine gas; the metal takes fire and burns, producing the chloride. It may also be obtained by dissolving the red oxide of mercury in hot hydrochloric acid; as the solution cools, the chloride crystallizes.

In the large way, it is usually prepared by heating in a retort the protosulphate with a mixture of sea salt; the protochloride sublimes, and is deposited upon cold surfaces, and sulphate of soda remains in the retort. The process of the United States Pharmacopoeia is to boil 2 lbs. of mercury with 3 lbs. of sulphuric acid, until a white dry mass is left. This when cold is to be rubbed with chloride of sodium in an earthenware mortar, and then distilled. Corrosive sublimate has strong antiseptic properties, owing in part to its forming insoluble compounds with nitrogenous substances. A mere trace of it in a bucket of water, it is stated, serves to preserve for a long time meat that has been placed in it for a few hours. The common use of the substance for this purpose would, however, be dangerous. The method of preserving wood from decay introduced by Mr. Kyan consists of filling its pores with a solution of this salt. - In medicine, corrosive sublimate is given in small doses by the mouth to obtain the constitutional effects of mercury upon the human system. It has the advantage of being less liable to salivate than most other mercurial preparations. It has been used hy-podermically for the same purposes, but is liable when thus used to cause severe local irritation.

Externally it is a parasiticide and irritant. It is used as an antiseptic for the preservation of anatomical specimens, which should be cautiously handled lest the hands become inflamed by contact with it, or absorption take place from an abraded surface. Corrosive sublimate, like all mercurial preparations, is much less employed in medicine than formerly. When secondary syphilis occurs in persons of robust health, undoubted advantage may be derived from its appropriate administration, but its action should be watched by a competent physician. It is more likely to. do harm than good in the tertiary forms of the same disease. Glandular swellings and chronic inflammatory affections of mucous surfaces, especially of the throat, ear, and uterus, are sometimes benefited by the judicious use of corrosive sublimate. Persons of delicate health or feeble constitution should not be subjected to its influence, nor should it be allowed to vitiate the blood or disturb the digestion. It may be given in pill or solution. The dose varies from 1/50 to 1/8 gr., which may be given three times a day.

The stomach tolerates it better after food than before. (See Mercury).