Class III.—All the granules are truncated at one end. This class includes sago, tapioca and arum, several drugs and cinnamon and cassia. Class IV.—In this class all the granules are angular in form and it includes oats, tacca, rice, pepper and ipecacuanha. =200. Preparation of Starches for Microscopical Examination.=—The approximately pure starches of commerce may be prepared for microscopic examination by rubbing them up with water and mounting some of the suspended particles by one of the methods to be described below. In grains, seeds and nuts the starch is separated by grinding with water and working through fine linen. The starch which is worked through is allowed to subside, again beaten up with water if necessary and the process continued until the grains are separated sufficiently for microscopic examination. A little potash or soda lye may be used, if necessary, to separate the granules from albuminous and other adhering matter. The analyst should have a collection of samples of all common starches of known origin for purposes of comparison. The granules are mounted for examination by plain light in a medium of glycerol and camphor water. When polarized light is used the mounting should be in Canada balsam.[166] The reader can find excellent photomicrographs of the more common starches in Griffith’s book.[167] =201. Appearance in Balsam with Polarized Light.=—Mounted in balsam the starches are scarcely visible under any form of illumination with ordinary light, the index of refraction of the granules and the balsam being so nearly alike. When, however, polarized light is used the effect is a striking one. It is very easy to distinguish all the characteristics, except the rings, the center of the cross being at the nucleus of the granule. With the selenite plate a play of colors is produced, which is peculiar to some of the starches and forms the basis of Blyth’s classification. =202. Description Of Typical Starches.=—The more commonly occurring starches are described by Richardson as they appear under the microscope magnified about 350 diameters.[168] The illustrations, with the exception of the cassava starch, and the maize starch accompanying it were drawn by the late Dr. Geo. Marx from photographs made by Richardson in this laboratory. The two samples excepted were photographed for the author by Dr. G. L. Spencer. _Maranta Starch._—Of the same type as the potato starch are the various arrowroots, the only one of which commonly met with in this country being the Bermuda, the starch of the rhizome of _Maranta arundinacea_, and the starch of turmeric.