Boil 1 oz of coarsely powdered turmeric root in half a pint of water for half an hour, and strain: dip paper in the liquid, and dry it. [It is rendered brown by alkalies, and also by boraeic acid and borates.] Test Solutions, etc. [The vegetable preparations are here placed first.]
Digest red cabbage with rectified spirit in a warm place for a few days; strain, distil off most of the spirit, and evaporate what remains to the consistence of syrup. It will keep for years. When required for use, dilute it with a little water; or the concentrated infusion directed above for the paper may have a little spirit added to it. [If the cabbage leaves be well dried, they may be kept in a close vessel for use, and a strong infusion made when wanted.]
Faraday directs one or more red cabbages to be cut up in strips, and boiling water poured on them, and a little dilute sulphuric acid (equal to 1/2 oz. of oil of vitriol to a large cabbage) to be added, and the whole kept hot for an hour or two in a copper or earthen vessel. It is then strained, the cabbage infused in a little more water and acid, and the mixed infusion evaporated to one third its first bulk, allowed to settle, and put into bottles. When required for use, the acid is neutralized by caustic potash or soda. Another plan is to dry the leaves at 120°; and when required for use to make a strong infusion, adding a drop of sulphuric acid, to neutralize the strained infusion with marble, filter, and add a little spirit, if required to be kept.
This is made as directed above for litmus paper. Or an ounce of powdered litmus may be triturated with 6 oz. of boiling water, digested near the fire for an hour, and mixed, when cool, with 2 oz. of spirit. Or digest 1 oz. of powdered litmus in a pint of proof spirit for 7 days. If required red, a few drops of acetic acid are added to either of these. The next day decant the clear liquor. Dr. Pereira directs 1 part of litmus to 25 of water. When made very strong, it must be diluted when used.
Fresh powdered blue galls 1 oz., proof spirit 8 oz.; digest in a close vessel for a week, and filter. A watery infusion of galls may be made in the same proportion with boiling water for immediate use. Pettenkofer directs 1 oz. of powdered galls to be infused in 3 or 4 oz. of boiling water for several hours, and 2 oz. of salt added. After filtration, it retains its transparency and power of precipitating gelatin for years. [This is used to detect iron, with the persalts of which it produces a bluish-black precipitate; for gelatin, which it precipitates in brownish-white flocks; and several of the organic alkaloids.]