The identification of the various starches is accomplished by means of the microscope. Starch possesses an organised structure which, fortunately, differs in different plants. Besides varying in size, the granules develope in a different manner and form from centres of growth, and therefore exhibit characteristic conditions and positions. These distinctions, together with their effect upon polarised light, are of great utility in the determination of the source of any particular starch. For this purpose, it is necessary to become familiar with the distinctive microscopical appearance of each individual starch. A collection of those most usually met with should be made, and, after careful study, preserved in a dried state for comparative purposes. Polarised light is a very useful adjunct in the examination of starch granules. In the microscopical investigation, a minute portion of the sample is placed upon the glass slide and well moistened with a solution of 1 part glycerine in 2 parts of water; it is then protected by a thin glass cover, which is put on with gentle pressure. The appearance of various starches, under polarised light, is seen in Plate IX., where the cross lies at the hilum or nucleus of the granule and the form and relative size is visible in outline. This plate, and Plates VII. and XII. are copied, with permission, from Bulletin No. 11 of the Chemical Division of the U.S. Department of Agriculture. The original negatives (made by Clifford Richardson) were used, but the auto-types are presented in a somewhat modified form. PLATE IX. [Illustration: Potato × 145.] [Illustration: Maize × 145.] [Illustration: Wheat × 145.] [Illustration: Rice × 450.] [Illustration: Bean × 145.] [Illustration: Pea × 145.] STARCHES. FOOTNOTES: [50] ‘Bread Analysis.’ [51] Wanklyn applies his ammonia process (see p. 205), to the estimation of albuminoids in vegetable substances. In this manner he obtained the following percentages of ammonia from various flours:--Rice, 0·62; maize and malt, 1·03; wheat and barley, 1·10; rye, 1·45; pea, 2·30. [52] ‘An Investigation of the Composition of American Wheat and Corn.’ United States Department of Agriculture, 1883. [53] ‘Die Menschlichen Nahrungs- und Genussmittel’ p. 420. Berlin, 1883. BAKERS’ CHEMICALS. The substances employed for the artificial production of porosity in bread, as already mentioned, are sodium bicarbonate (now termed “saleratus”), potassium bitartrate, tartaric acid, and calcium diphosphate, the various mixtures of these compounds being known as baking powders. Some of the above chemicals are not always used in the pure state, and, in addition to this source of contamination, baking powders are often excessively diluted with flour or starch, and seriously adulterated with alum.