Phloroglucol.--_Phloroglucin_ C_{6}H_{6}O_{3} is a phenol isomeric with pyrogallol. It crystallises with 2 molecules of water, which it loses at 212° F. (100° C.). It melts at about 428° F. (220° C.), sublimes without odour, and solidifies again on cooling. It is soluble in water, alcohol, and ether, and by agitation with the latter it may be removed from its aqueous solution. It is not precipitated by any metallic salt but basic lead acetate. It is coloured deep violet red by ferric chloride. If bromine be added to its concentrated solution in water, it absorbs 3 atoms, forming tribromophloroglucol, C_{6}H_{6}Br_{3}O_{3}, which separates in crystalline needles, with evolution of heat and a very irritating odour. If a deal shaving be moistened with solution of phloroglucol, and then with strong hydrochloric acid, it soon takes a deep violet colour, from the formation of phloroglucol-vanillin with the trace of vanillin contained in all coniferous woods. Pyrocatechol gives a similar reaction, and it is stated by Etti (Chem. Soc. Jour., xliv. 60) that pyrogallol forms a similar compound; it does not, however, give the colour reaction on deal. It is extremely probable that this reaction may be used to detect phloroglucide tannins without the troublesome fusion with potash, in cases where pyrocatechol is absent. The reaction is strongly given by gambier, which is known to contain phloroglucol, but not by oak bark and valonia, though these contain protocatechuic acid. If to a dilute solution of phloroglucol a solution of aniline or toluidine nitrate be added, and then a trace of potassic nitrite, the liquid gradually becomes yellow or orange, then turbid, and finally deposits a cinnabar-red precipitate. This reaction is given by gambier, but is also produced by oak bark infusion, which is not supposed to contain phloroglucol; and gall tannin, pyrogallol, and other substances give similar but browner precipitates. Protocatechuic Acid.--C_{6}H_{3}(OH)_{2}CO.OH, one of the six isomeric dihydroxybenzoic acids of this formula (see Miller, Chem. Soc. Jour., xli. 198), crystallises in needles and plates with 1 mol. water, which it loses at 212° F. (100° C.). It melts at 228° F. (109° C.) and on further heating is decomposed into pyrocatechol and carbonic acid. It is somewhat soluble in cold water, and readily so in hot water, alcohol, and ether. It is coloured bluish green by ferric chloride, which is changed to red by alkalies. Solutions of protocatechuates give a violet coloration with ferric salts. It is precipitated by lead acetate, and reduces silver ammonio-nitrate, but not Fehling's solution (see also p. 107). Constitution of Tannins.