Page:Popular Science Monthly Volume 25.djvu/222

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212
THE POPULAR SCIENCE MONTHLY.

through the agency of organic growth. From a black and amorphous matter we have made to issue crystalline substances of every shade of color—reds, saffrons, greens, violets, and blues—alizarine, the same substance as tints the flowers of the madder, and that wonderful aniline, colorless as the ray of light before it has been resolved by the prism, but containing in posse, like the same ray, all the colors of the rainbow. What do we know of stone-coal, the origin of so many marvels and refractory to all analysis? Nothing, except that it has lived.—Translated for the Popular Science Monthly from the Revue des Deux Mondes.

 
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THE CHEMISTRY OF COOKERY.
By W. MATTIEU WILLIAMS.
XXX.

THE changes which occur when starch-granules are subjected to the action of water, at a temperature of 140°, have been described. If the heat is raised to the boiling-point, and the boiling continues, the gelatinous mass becomes thicker and thicker; and if there are more than fifty parts of water to one of starch a separation takes place, the starch settling down with its fifty parts of water, the excess of water standing above it. Carefully-dried starch may be heated to above 300° without becoming soluble, but at 400° a remarkable change commences. The same occurs to ordinary commercial starch at 820°, the difference evidently depending on the water retained by it. If the heat is continued a little beyond this it is converted into dextrin, otherwise named "British gum," "gommeline," "starch-gum," and "Alsace gum," from its resemblance to gum-arabic, for which it is now very extensively substituted. Solutions of this in bottles are sold in the stationers' shops under various names for desk uses.

The remarkable feature of this conversion of starch into dextrin is that it is accompanied by no change of chemical composition. Starch is composed of six equivalents of carbon, ten of hydrogen, and five of oxygen—C6H10O5, i. e., six of carbon and five of water or its elements. Dextrin has exactly the same composition; so also has gum-arabic when purified. But their properties differ considerably. Starch, as everybody knows, when dried, is white, and opaque and pulverent; dextrin, similarly dried, is transparent and brittle; gum-arabic the same. If a piece of starch, or a solution of starch, is touched by a solution of iodine, it becomes blue almost to blackness, if the solution is strong; no such change occurs when the iodine solution is added to dextrin or gum. A solution of dextrin when mixed with potash changes to a rich blue color when a little sulphate of cop-