Litmus occurs in commerce in the form of small cubes of a fine violet colour. 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 by the addition of free Hydrochloric Acid, or of Chloride of Ammonium. The Protochloride of Mercury is an insoluble white powder, commonly known under the name of _Calomel_. METHYLIC ALCOHOL. This liquid, known also by the names of _wood naphtha_ and _pyroxylic spirit_, is one of the volatile products of the destructive distillation of wood. It is very volatile and limpid, with a pungent odour. By a recent excise regulation, ordinary Spirit mixed with ten per cent, of wood naphtha is sold free of duty, under the name of "Methylated Spirit." 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 shimmed 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 commonly 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 portion 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.