21.2 THE POPULAR SCIENCE MONTHLY. 



tbrough 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 sub- 

 stance 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 iii 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. — Trans- 

 lated for the Popular Science Monthly from the Revue des Deux 

 Mondes. 



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THE CHEMISTEY OF COOKEKY. 



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 de- 

 scribed. 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 remark- 

 able change commences. The same occurs to ordinary commercial 

 starch at 820°, the difference evidently depending on the water re- 

 tained by it. If the heat is continued a little beyond this it is con- 

 verted 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 hydi'ogen, and 

 five of oxygen — CgHj^O^^, 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 stronsr ; no such change occurs when the iodine solu- 

 tion is added to dextrin or e;um. A solution of dextrin when mixed 

 with potash changes to a x'ich blue color when a little sulphate of cop- 



