The washings of the flour are allowed to stand for some time in a conical-shaped vessel. As soon as the amylaceous matter has entirely settled to the bottom of the vessel, the greater portion of the water is decanted, and the residual mass brought upon a small filter and allowed to dry. The residue is then examined for potato and rice starch. _Potato starch._ The grains of potato starch are much larger than those of wheaten starch. If a portion of the residue mentioned above is crushed in an agate mortar, the granules of potato starch present are ruptured, and their contents liberated; the wheaten starch remaining unaltered. The mass is then taken up with water, and the fluid filtered. If potato starch be present, the filtrate will acquire a blue color upon addition of an aqueous solution of iodine; otherwise, a yellow or violet-rose coloration is produced. It is necessary to avoid crushing the residue for too long a time, as the granules of wheaten starch would also become ruptured by prolonged comminution. Besides the difference presented by potato starch in the size of the granules in comparison to those of wheaten starch, the former swell to ten or fifteen times the volume of the latter, when treated with a solution of potassa: wheaten starch granules are not affected by the treatment, if the solution used does not contain more than 2 per cent. of the salt. The results obtained by the above operation should be confirmed by a microscopic examination. A portion of the residue is moistened with solution of iodine, then carefully dried, and placed on the slide of a microscope. The mass is next moistened with a solution containing 2 per cent. of potassa, and examined. The addition of iodine causes the potato starch granules to acquire a blue color, and renders their shape and volume more easily perceptible; thus allowing the two varieties of starch to be readily distinguished. Fig. 13 represents the relative size of the granules as observed under the microscope.[Q] [Q] It may be added, as a distinguishing property, that granules of potato starch, when viewed in polarized light by aid of a Nicol's prism, present a well-defined black cross, corresponding to the hilum; wheaten-starch fails to exhibit this phenomenon.--_Trans._ [Illustration: Fig. 13.] The presence of potato starch in bread is also detected by crushing a small portion of the sample under examination on the glass, and then adding a few drops of the alkaline solution. _Rice and Corn._--If rice or corn meal have been mixed with the flour, angular and translucent fragments (Fig. 14) are observed in the microscopic examination. Corn meal acquires a yellow color, if treated with dilute potassa solution. [Illustration: Fig. 14.] MISCELLANEOUS TESTS.