Facing is usually easily detected by the microscope. A portion of the leaf is mounted as an opaque object, the coloring matter appearing in small dots.

Prussian blue. - This substance is easily detected by means of the microscope. Shake the leaves in a glass cylinder with water and examine the detached particles with the microscope. If the coloring matter sought is present, transparent particles of a brilliant blue may be seen. Prussian blue may often be identitied by the microscope on the leaf mounted as an opaque object. The particles detached as above may be examined chemically as follows: Treat with hot sodium hydroxide solution, acidulate with acetic acid, and add ferric chloride. If Prussian blue was present in the facing the characteristic blue precipitate will be formed. The powdered tea leaf may be examined by the chemical method, but it is advisable to remove the tannin by precipitation with gelatin3 solution and filtration through powdered kaolin, after acidulating with acetic acid. The color of Prussian blue is discharged by sodium or potassium hydroxide.

Indigo. - Under the microscope indigo appears of a greenish blue. Its color is not discharged by sodium hydroxide, a distinction from Prussian blue. Indigo forms a deep blue solution with sulphuric acid.

Turmeric. - Turmeric is identified by means of the microscope. According to Hassell (op. cit., 143) turmeric consists of characteristic yellow cells of a rounded form which are tilled with peculiar shaped starch granules. On the addition of an alkali the cells turn brown, swell up, and the outlines of the starch granules become visible.

Plumbago. - The microscope is employed in the detection of plumbago. A thin slice of the tea leaf will exhibit numerous bright particles if plumbago facing has been used.

Gypsum, soapstone, etc. - These substances, employed with the coloring matter in facing teas, may be separated by shaking the leaves in a cylinder with water. The sediment is examined by the usual qualitative methods for these substances.

1 Food, A. H. Hassall, pp. 122 to129.

2 Bulletin No. 7, Imperial College of Agriculture, p. 28.

3 See method for estimation of tannin, p. 890,