A very simple and striking method of determining whether any granule is composed of starch or not, consists in adding to it, when placed in water on a slide, a drop of solution of iodine. As soon as this touches the granule, it assumes a beautiful purple colour, the depth of tint depending upon the quantity of the iodine-solution; if this be very considerable, the granule appears almost black. The section of potato forms a very interesting object when moistened with the iodine-solution, the starch-granules becoming beautifully coloured, whilst the cell-wall remains colourless, and the protoplasm becomes yellow. The form of the starch-granules differs in different plants, so that the kind of plant from which starch has been derived may be distinguished by attention to the size, form, and structure of its starch-granules. Thus, the granules represented in Pl. I. fig. 3, which it will be noticed are all drawn under the same power, are derived from different plants,--_a_ being those of wheat-flour, in which the hilum is obscure, and the rings faint; _b_ is a granule of West Indian arrowroot, in which the hilum forms a transverse crack; _c_ is a granule of potato-starch, in which the hilum is a dot, and the rings are very distinct; _d_ represents the compound granules of the oat, the separate granules being figured below; _e_ is a granule of lentil-starch, with its long dark hilum and elegant oval concentric rings; and _f_ represents a compound and separate granule of rice-starch. It will be noticed that the granules of oat-and rice-starch are angular, as it is called. The knowledge of the peculiar forms of the starch-granules is important in a practical point of view, for it enables us to recognize them when mixed as an adulteration with other substances, and also to distinguish the different kinds of starch from each other. Thus table-mustard, as it is called, is principally composed of the cheaper wheat-or pea-flour, which is easily recognized by the structure of the starch-grains. Arrowroot is considerably dearer than potato-starch; hence in trade the latter is fraudulently sold for the former, the adulteration being detected with difficulty by the eye, but easily under the microscope. Again, rice is largely mixed with wheat-flour, as it makes inferior flour into very white bread; and this may also be readily detected under the microscope. The reader can now understand how valuable the microscope is in detecting adulterations, with a knowledge of the various forms and structures of substances, especially with the aid of a few chemical tests.