Prep. Fifty grains of isinglass, cut into shreds, and one fluid ounce of distilled water, are mixed and digested for half an hour with repeated shaking on a water bath, and the solution filtered through clean tow moistened with distilled water.

Use. The solution is used to distinguish between gallic and tannic acid; the former gives no precipitate with gelatine, while the latter gives a yellowish-white one; it is also employed to detect tannic acid in a decoction of elm bark, with which it should afford a precipitate.