96 PLANT COMPOUNDS 



to that peculiar class of compounds called colloids. Solutions 

 of proteins are levorotatory. One of the commonest tests 

 for a protein is to dissolve it in concentrated nitric acid 

 which gives a yellow color, turning to orange on the addition 

 of ammonium hydroxide. This is known as the xantho- 

 proteic reaction— or "yellow protein" reaction. Chemists 

 are familiar with the fact that strong nitric acid stains the 

 skin vellow. This is due to the action of nitric acid on the 

 proteins of the skin. 



Some of the proteins in solution are precipitated unaltered 

 by saturating the solution with sodium chloride, ammonium 

 sulphate, or magnesium sulphate, a process called "salting 

 out;" others are thrown down in a changed form by metallic 

 salts; while still others are precipitated as insoluble salts 

 by tannin. No single reaction is common to all proteins. 

 According to their properties proteins have been classified 

 into some eighteen groups which serve to distinguish them 

 and aid in their studv. 



85. Alkaloids. — These are plant compounds containing 

 nitrogen and possessing strongly basic properties. They 

 differ from the other organic bases, like amines and amides, 

 in being more complex in structure (the exact formula is 

 unknown in most cases) and more basic in reaction. They 

 differ from the proteins in being less complex. Their most 

 characteristic property is their very powerful physiological 

 action on animals. They are strong medicines or strong 

 poisons. As a rule, they are colorless or white, crystalline 

 solids, and contain oxygen in addition to carbon, hydrogen, 

 and nitrogen. Nicotine is an exception being a liquid and 

 containing no oxygen. Most of the alkaloids are only slightly 

 soluble in alcohol. They dissolve in acids with the formation 

 of salts. From their solutions they are as a rule precipitated 

 by tannin, phosphomolybdic acid, and some other reagents. 

 In the plant they occur in the bark of the stem or root, in 

 seeds, and in the fruit rind. Their function is not definitely 

 known, being considered by some authorities as end products 

 of metabolism — waste compounds stored where they are 

 most easily removed; other authorities, however, claim that 

 alkaloids are intermediate or transitory compounds necessary 



