_Properties._--Perchloride of iron may be obtained in the solid form by heating iron wire in excess of chlorine; it condenses in the shape of brilliant and iridescent brown crystals, which are volatile, and dissolve in water, the solution being acid to test-paper. It is also soluble in alcohol, forming the _tinctura ferri sesquichloridi_ of the Pharmacopoeia. Commercial perchloride of iron ordinarily contains an excess of hydrochloric acid. Litmus. Litmus is a vegetable substance, prepared from various _lichens_, which are principally collected on rocks adjoining the sea. The coloring matter is extracted by a peculiar process, and afterwards made up into a paste with chalk, plaster of Paris, &c. Litmus occurs in commerce in the form of small cubes, of a fine violet color. In using it for the preparation of test-papers, it is digested in hot water, and sheets of porous paper are soaked in the blue liquid so formed. The red papers are prepared at first in the same manner, but afterwards placed in water which has been rendered faintly acid with sulphuric or hydrochloric acid. Mercury, Bichloride of. 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_. Milk. 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.