Boric acid crystallizes from water in white nacreous laminae belonging to the triclinic system; it is difficultly soluble in cold water, but dissolves readily in hot water. It is one of the "weak" acids, its dissociation constant being only 0.08169 (J. Walker, _Jour. of Chem. Soc._, 1900, lxxvii. 5), and consequently its salts are appreciably hydrolysed in aqueous solution. The free acid turns blue litmus to a claret colour. Its action upon turmeric is characteristic; a turmeric paper moistened with a solution of boric acid turns brown, the colour becoming much darker as the paper dries; while the addition of sodium or potassium hydroxide turns it almost black. Boric acid is easily soluble in alcohol, and if the vapour of the solution be inflamed it burns with a characteristic vivid green colour. The acid on being heated to 100° C. loses water and is converted into _metaboric acid_, HBO3; at 140° C., _pyroboric acid_, H2B4O7, is produced; at still higher temperatures, boron trioxide is formed. The salts of the normal or orthoboric acid in all probability do not exist; metaboric acid, however, forms several well-defined salts which are readily converted, even by carbon dioxide, into salts of pyroboric acid. That orthoboric acid is a tribasic acid is shown by the formation of ethyl orthoborate on esterification, the vapour density of which corresponds to the molecular formula B(OC2H5)3; the molecular formula of the acid must consequently be B(OH)3 or H3BO3. The metallic borates are generally obtained in the hydrated condition, and with the exception of those of the alkali metals, are insoluble in water. The most important of the borates is sodium pyroborate or borax (q.v.). Entry: BORIC