3. _By the action of metallic salts._--Many of the salts of the metals coagulate Albumen completely. Nitrate of Silver does so; also the Bichloride of Mercury. Ammoniacal Oxide of Silver however does not coagulate Albumen. The white precipitate formed on mixing Albumen with Nitrate of Silver is a chemical compound of the animal matter with Protoxide of Silver. This substance, which has been termed Albuminate of Silver, is soluble in Ammonia and Hyposulphite of Soda; but after exposure to light, or heating in a current of Hydrogen gas, it assumes a brick-red colour, being probably reduced to the condition of an organic compound of a _Suboxide_ of Silver. It is then almost insoluble in Ammonia, but enough dissolves to tinge the liquid wine-red. The _red coloration_ of solution of Nitrate of Silver employed in sensitizing the Albuminized photographic paper is probably produced by the same compound, although, often referred to the presence of Sulphuret of Silver. Albumen also combines with Lime and Baryta. When Chloride of Barium is used with Albumen, a white precipitate of this kind usually forms. _Chemical composition of Albumen._--Albumen belongs to the _Nitrogenous_ class of organic substances (see page 325). It also contains small quantities of Sulphur and Phosphorus. ALCOHOL. Symbol, C{4}H{6}O{2}. Atomic weight, 46. Alcohol is obtained by the careful distillation of any spirituous or fermented liquor. If wine or beer be placed in a retort, and heat applied, the Alcohol, being more volatile than water, rises first, and is condensed in an appropriate receiver; a portion of the vapour of water however passes over with the Alcohol, and dilutes it to a certain extent, forming what is termed "Spirits of Wine." Much of this water may be removed by redistillation from Carbonate of Potash, in the manner described at page 196 of this work; but in order to render the Alcohol thoroughly _anhydrous_, it is necessary to employ _Quicklime_, which possesses a still greater attraction for water. An equal weight of this powdered lime is mixed with strong Alcohol of ·823, and the two are distilled together. _Properties of Alcohol._--Pure anhydrous Alcohol is a limpid liquid, of an agreeable odour and pungent taste; sp. gr. at 60°, ·794. It absorbs vapour of water, and becomes diluted by exposure to damp air; boils at 173° Fahr. It has never been frozen. Alcohol distilled from Carbonate of Potash has a specific gravity of ·815 to ·823, and contains 90 to 93 per cent, of real spirit. The specific gravity of ordinary rectified Spirits of Wine is usually about ·840, and it contains 80 to 83 per cent, of absolute Alcohol. AMMONIA. Symbol, NH{3} or NH{4}O. Atomic weight, 17.