There are several methods of detecting starch in those cases where its presence is doubtful; and the two modes that are usually employed are polarised light and the iodide of potassium. When polarised light is employed--a subject on which we shall have something to say presently--the starch grains assume the characteristic “black-cross,” and when a plate of selenite is placed immediately beneath the slide containing the starch grains, they glow with all the colours of the rainbow. The second plan is to treat them with a very weak solution of iodine and iodide of potassium, and in this case the iodine has the effect on the starch granules of staining them blue. They are so susceptible of this reaction that when the liquid is too strong the grains actually become black from the amount of iodine which they imbibe. Nothing is easier than to procure starch granules in the highest perfection. Take a raw potato, and with a razor cut a very thin slice from its interior, the direction of the cut not being of the slightest importance. Put this delicate slice upon a slide, drop a little water upon it, cover it with a piece of thin glass, give it a good squeeze, and place it under a power of a hundred or a hundred and fifty diameters. Any part of the slice, provided that it be very thin, will then present the appearance shown in Plate III. Fig. 9, where an ordinary cell of potato is seen filled loosely with starch grains of different sizes. Around the edges of the slice a vast number of starch granules will be seen, which have been squeezed out of their cells by pressure, and are now floating freely in the water. As cold water has no perceptible effect upon starch, the grains are not altered in form by the moisture, and can be examined at leisure. III.