Acetic acid is also produced on a large scale by heating _wood_ in close vessels; a substance distils over which is acetic acid contaminated with empyreumatic and tarry matter; it is termed pyroligneous acid, and is much used in commerce. The most concentrated acetic acid may be obtained by neutralizing common vinegar with carbonate of soda and crystallizing out the acetate of soda so formed; this acetate of soda is then distilled with sulphuric acid, which removes the soda and liberates acetic acid: the acetic acid being volatile, distils over, and may be condensed. _Properties of Acetic Acid._--The strongest acid contains only a single atom of water; it is sold under the name of "glacial acetic acid," so called from its property of solidifying at a moderately low temperature. At about 50° the crystals melt, and form a limpid liquid of pungent odor and a density nearly corresponding to that of water; the specific gravity of acetic acid, however, is no test of its real strength, which can only be estimated by analysis. The commercial glacial acetic acid is often diluted with water, which may be suspected if it does not solidify during the cold winter months. Sulphurous and hydrochloric acids are also common impurities. They are injurious in photographic processes from their property of precipitating nitrate of silver. To detect them proceed as follows:--dissolve a small crystal of nitrate of silver in a few drops of water, and add to it about half a drachm of the glacial acid; the mixture should remain quite clear even when exposed to the light. Hydrochloric and sulphurous acids produce a white deposit of chloride or sulphite of silver; and if _aldehyde_ or volatile tarry matter be present in the acetic acid, the mixture with nitrate of silver, although clear at first, becomes discolored by the action of light. Many photographers employ a cheaper form of acetic acid, sold by druggists as "Beaufoy's" acid;[A] it should be of the strength of the acetic acid fortiss. of the London Pharmacopoeia, containing 30 per cent, real acid, and must be tested for sulphuric acid (see sulphuric acid), and also by mixing with nitrate of silver. [Footnote A: In this country the practitioner uses the article sold in market as "Acetic Acid, No. 8."--S. D. H.] Acetate of Silver. (_See_ Silver, Acetate of.) Albumen. Albumen is an organic principle, found both in the animal and vegetable kingdom. Its properties are best studied in the _white of egg_, which is a very pure form of albumen. Albumen is capable of existing in two states; in one of which it is soluble, in the other insoluble in water. The aqueous solution of the soluble variety gives a slightly alkaline reaction to test-paper; it is somewhat thick and glutinous, but becomes more fluid on the addition of a small quantity of an alkali, such as potash or ammonia. Soluble albumen may be converted into the insoluble form in the following ways:--