[44] The type BiX_{5} is represented by the pentoxide, Bi_{2}O_{5}, its metahydrate, Bi_{2}O_{5},H_{2}O, or BiHO_{3}, known as bismuthic acid, and the pyrohydrate, Bi_{2}H_{4}O_{7}. _Bismuth pentoxide_ is obtained by the prolonged passage of chlorine through a boiling solution of potassium hydroxide (sp. gr. 1·38), containing bismuth oxide in suspension; the precipitate is washed with water, with boiling nitric acid (but not for long, as otherwise the bismuthic acid is decomposed), then again with water, and finally the resultant bright red powder of the hydrate BiHO_{3} is dried at 125°. The prolonged action of nitric acid on bismuthic anhydride, Bi_{2}O_{5}, results in the formation of the compound Bi_{2}O_{4},H_{2}O, which decomposes in moist air, forming Bi_{2}O_{3}. The density of bismuthic anhydride is 5·10, of the tetroxide, Bi_{2}O_{4}, 3·60, and of bismuthic acid, BiHO_{3}, 5·75. _Pyrobismuthic acid_, Bi_{2}H_{4}O_{7}, forms a brown powder, which loses a portion of its water at 150°, and decomposes on further heating, with the evolution of oxygen and water. It is obtained by the action of potassium cyanide on a solution of bismuth nitrate. The meta-salts of bismuthic acid are known, for example KBiO_{3}. They generally occur, however, in combinations with metabismuthic acid itself. Thus André (1891) took a solution of the double salt of BiBr_{3} and KBr, treated it with bromine after adding ammonia, and obtained a red-brown precipitate, which after being washed (for several weeks) had the composition KBiO_{3},HBiO_{3} When washed with dilute nitric acid this salt gave bismuthic acid. [Illustration: FIG. 85.--Furnace used for the extraction of bismuth from its ores.]