_Products of the Decomposition of Tannins by Heat._--Pyrogallol, _pyrogallic acid_, C_{6}H_{6}O_{3}, has a bitter, but not sour taste, and feebly reddens litmus, but the addition of the smallest trace of alkali gives it an alkaline reaction. It is poisonous, 2 gr. having killed a dog. It is soluble in less than 3 parts of cold water, and still more freely in hot. It is also soluble in alcohol, ether, acetone, ethyl acetate, and glycerin, but not in absolute chloroform, or petroleum spirit. It fuses at 268° F. (131° C.) (_Etti_), and sublimes at about 410° F. (210° C.). With pure ferrous sulphate it gives a white precipitate, which redissolves to a fine blue liquid in presence of the least trace of ferric salt. Mineral acids change this to red, and the blue tint is restored by cautious neutralisation with ammonia, and is not destroyed, but sometimes rendered greenish by excess of acetic, and other organic acids. Any excess of ammonia produces an amethyst-red, and acetic acid restores the blue. Its solution is turned brown by traces of nitrous acid. With lime-water it produces a beautiful but evanescent purple, rapidly turning brown. In presence of alkalies it absorbs oxygen from the air with great avidity, turning orange, brown, and black. Pyrogallol does not precipitate gelatin. Its solution rapidly reduces permanganate, Fehling's solution, and salts of gold, silver,[F] mercury, and platinum. It precipitates copper and lead acetates, and with ammoniacal cupric sulphate it gives an intense purple-brown coloration. Gum arabic, saliva, and various other organic matters cause solutions of pyrogallol exposed to the air to absorb oxygen, by which purpurogallin is formed and separates in small yellow capillary crystals. If 0·2 per cent. of pyrogallol be added to a 1 per cent. solution of gum arabic, it becomes yellow in a few hours, and purpurogallin separates in hairlike crystals, which continue to increase for some months. It these crystals are freed from pyrogallol by washing with water, and a trace of alkali is added, they dissolve with an intense blue colour. Purpurogallol is also formed by oxidation with silver nitrate, potash permanganate, and many other reagents. Pyrogallol forms compounds with aldehydes; with formaldehyde, a body which reacts like a tannin and precipitates gelatin is produced. If pyrogallol be heated with hydrochloric acid, and aldehyde, chloral, or acetone, a red substance is produced. The less volatile portions of crude beech-tar creasote contains ethers of pyrogallol, methyl-pyrogallol, and propyl-pyrogallol, from which these bodies may be obtained by the action of hydrochloric acid under pressure. Methyl- and propyl-pyrogallols differ from ordinary pyrogallol in having an atom of hydrogen replaced by the groups CH_{3} or C_{3}H_{7} respectively; and it is very probable that some tannins are derivatives of such modified pyrogallols. [Footnote F: Hence its use as a "developer" in photography.]