The catechol-tannins, dissolved in water, yield a precipitate when bromine-water is added till the solution smells strongly of it. The precipitate is occasionally crystalline, but generally amorphous, and of yellowish or brownish colour. When the infusion of tannin is very weak, the precipitate is sometimes only slight, or forms slowly. Pyrogallol-tannins give no precipitate with bromine-water. Another reaction, which is generally characteristic of catechol-tannins, is that if concentrated sulphuric acid is added to a single drop of the infusion in a test-tube, a dark red, or crimson ring is formed at the junction of the two liquids, and on dilution with water, the solution is generally pink. Pyrogallol-tannins on the other hand give a yellow, or at most a dark brown ring, which dilutes to a yellowish solution. This reaction is of great delicacy, which may be further increased by the use of an alcoholic instead of an aqueous extract. It is often given also by the non-tannin residue of catechol-tannins which is left after treatment with hide-powder, in which case it is probably due to the presence of catechins allied to the tannins. With ferric salts (preferably a solution of iron-alum), pyrogallol-tannins give blue-blacks, while catechol-tannins generally give greenish blacks, though the reaction is apt to be rendered uncertain by the presence of colouring matters, or perhaps in some cases by the constitution of the tannin. Thus aqueous infusions of common oak-bark (_Quercus robur_) give a decidedly blue black with iron, though the tannin is a catechol one, and the purified tannin gives a green-black. Most of the barks of American oaks, such as _Q. prinus_, give green-blacks without purification. The Australian mimosas generally give dull purple-blacks with iron-salts, though they all contain catechol-tannins. The iron test was first proposed by Stenhouse as a means of classification. Trimble has shown that while the purified pyrogallol-tannins only contain about 52 per cent. of carbon, the catechol-tannins have about 60 per cent.[150] [150] ‘The Tannins,’ ii. p. 131.