Symbol, HgCl{2}. Atomic weight, 274.
This salt, also called corrosive sublimate, and sometimes chloride of mercury (the atomic weight of mercury being halved), may be formed by heating mercury in excess of chlorine, or, more economically, by subliming a mixture of persulphate of mercury and chloride of sodium.
Properties.—a very corrosive and poisonous salt, usually sold in semi-transparent, crystalline masses, or in the state of powder. Soluble in 16 parts of cold, and in 3 of hot water; more abundantly so in alcohol, and also in ether. The solubility in water may be increased almost to any extent by the addition of free hydrochloric acid.
The protochloride of mercury is an insoluble white powder, commonly known under the name of calomel.
The milk of herbivorous animals contains three principal constituents—fatty matter, caseine, and sugar; in addition to these, small quantities of the chloride of potassium, and of phosphates of lime and magnesia, are present.
The fatty matter is contained in small cells, and forms the greater part of the cream which rises to the surface of the milk on standing. Hence skimmed milk is to be preferred for photographic use.
The second constituent, caseine, is an organic principle somewhat analogous to albumen in composition and properties. Its aqueous solution however does not, like albumen, coagulate on boiling, unless an acid be present, which probably removes a small portion of alkali with which the caseine was previously combined. The substance termed "rennet," which is the dried stomach of the calf, possesses the property of coagulating caseine, but the exact mode of its action is unknown. Sherry wine is also employed to curdle milk; but brandy and other spirituous liquids, when free from acid and astringent matter, have no effect.
In all these cases a proportion of the caseine usually remains in a soluble form in the whey; but when the milk is coagulated by the addition of acids, the quantity so left is very small, and hence the use of the rennet is to be preferred, since the presence of caseine facilitates the reduction of the sensitive silver salts.