138 CHEMISTRY FOR AGRICULTURAL STUDENTS 



produced, showing that the dextrin has in turn been converted 

 into a new body. Now add to the boiUng liquid a drop of 

 solution of sulphate of copper, and then caustic soda or potash 

 till alkaline. Note that instead of getting black cupric oxide 

 as would be expected, red cuprous oxide is precipitated, 

 showing that a reducing agent is now present. This sub- 

 stance, into which the dextrin has been converted, is a sugar. 



The foregoing changes can be carried out by alkalies as well as by acids ; 

 by "diastase," a substance existing in the leaves and germinating seeds of 

 plants; and by " ptyalin," a substance existing in the saliva of animals. 

 Dry heat has a similar action, and dextrin is produced on the large scale 

 by merely heating starch. 



The starch of commerce is prepared from cereal grains such as wheat, 

 maize, and rice ; from the stem of the sago palm ; from arrowroot and 

 from potatoes ; by grinding or grating, mixing with water rendered alkaline 

 with caustic soda, straining the mixture from fibre and other impurities, 

 and allowing it to stand for the starch to deposit. The various starches 

 are distinguished by the size, shape, and markings of the granules. Arti- 

 chokes contain a variety of starch known as inulin. This is distinguished 

 by giving no blue colour with iodine. It is converted into fruit sugar by 

 boiling with dilute acid. 



Procure some cotton wool (nearly pure cellulose). Note 

 that it is unchanged by boiling with water, and that iodine has 

 no action upon it, but that it is soluble in strong sulphuric 

 acid forming a sulphate. To some concentrated acid in a 

 beaker kept cool by water, add cotton wool in small quantities 

 at a time, and stir till dissolved. Dilute this solution of sul- 

 phate of cellulose very largely with water, and boil. Test a 

 few drops of the resulting liquid with iodine, and, after longer 

 boiling, with sulphate of copper and then potash till alkaline. 

 Note that at first iodine gives the brown coloration charac- 

 teristic of dextrin, and that afterwards the red precipitate of 

 cuprous oxide, characteristic of a sugar, is obtained. 



The percentage composition of cellulose, starch, and dextrin, as obtained 

 by analysis, is represented by the formula CyTIioOs. The molecular 

 formula is undoubtedly a multiple of this, for the sulphate has the formula 



