CARBOHYDRATES 53 



(a) Xanthoproteic te><t. Proteid boiled in nitric acid is 

 dissoh't'd and the solution colored yellow by the action of 

 the acid. If to this n;ixture is added enough ammonia to 

 neutralize the acid the color turns a deep orange. 



(b) Millon's test. Proteid, brought slowly to boiling 

 with a solution of mercury in nitric acid, becomes rose 

 red. The nitric acid solution of the mercury is called 

 Millon's reagent. 



(c) Biuret test. If a cold solution of proteid is first 

 made alkaline, by the addition of caustic soda, and then a 

 few drops of a very dilute solution of copper sulphate are 

 added, a violet color appears which deepens in color with 

 boiling. 



Any one of these tests applied to a food which contains 

 proteid will reveal its presence. Of the three, the xantho- 

 proteic is best for common use. 



Albuminoids. — These substances, an example of which 

 is the gelatine found in bones, are closely allied to proteids 

 in composition. This resemblance to proteid is confined 

 to their composition and the presence of nitrogen as they 

 vary from proteid, not only in failing to respond to proteid 

 tests, but also in their action. In this group is included 

 Iceratin which forms horn, hair, feathers, nails, and hoofs; 

 elastin, which forms the elastic fibers of connective tissue; 

 and certain bodies called enzymes which we shall inves- 

 tigate later. The different forms vary so much in charac- 

 teristics that it is impossible to name any one feature 

 that is distinctive. Of the common foods, soups and 

 soup stock made from bone marrow are richest in this 

 nutrient. 



Carbohydrates. — Under this name are included the 

 starches and sugars. These are compounds of carbon. 



