CARBOHYDRATES AND RELATED BODIES. 45 



starch. The reaction is employed on the large scale in the 

 manufacture of British gum which is used in the preparation 

 of size and paste for various purposes. 



Ex. Heat about 10 gm. of starch in a porcelain dish on a sand-bath to 

 a temperature short of the point where it begins to scorch. It is neces- 

 sary to stir well all the time, and continue the heat ten minutes after the 

 starch has become uniformly yellowish brown. Then allow the dish to 

 cool, add water and boil thoroughly, which brings part of the product into 

 solution. When sufficiently diluted this solution can be filtered. The fil- 

 trate is precipitated by alcohol. The addition of a few drops of iodine 

 solution to the aqueous liquid gives rise to a reddish color characteristic 

 of dextrin. 



The chief uses of starch have been referred to in other con- 

 nections. Much is directly employed as food and large quantities 

 are converted into glucose as shown above. The production 

 of various kinds of dextrin and British gum is also extremely 

 important and consumes enormous quantities of starch. In 

 the form in which it occurs in nature, that is mixed with 

 other substances in small amount, starch is the most abundant 

 of our foodstuffs, and the one consumed in largest amount. 

 Great interest therefore attaches to the reactions by which 

 this starch is made soluble or digested as a step in its assim- 

 ilation. The discussion of this fundamental point will be 

 left, however, for a following chapter, when the theory of 

 digestive operations can be explained as a whole. 



In certain plants a variety of starch called inulin occurs. It 

 is best obtained from tubers of the dahlia, and is interesting 

 from the fact that by hydrolysis it yields fructose instead of 

 glucose. It differs from the ordinary starch in yielding a 

 true solution with hot water, and in giving a yellow instead 

 of a blue color with iodine. 



GLYCOGEN, or animal starch. This product, which is 

 formed in the liver, is related in many ways, both chemically 

 and physiologically, to common starch. In some respects it 

 resembles also the simple sugars, from which it is indeed 

 derived, and may be said to stand between them and vegetable 

 starch. It is readily soluble in water, giving, however, an 



