THE CHEMICAL ELEMENTS 19 



19. Iodine. — Iodine is distributed in nature only in 

 minute quantities. It is apparently absent in some plants, 

 but is found in minute quantities in others. Nevertheless 

 it appears to exercise important fimctions in animal life. 



20. Potassium. — ^Familiar compounds of this element 

 are the potassium carbonate leached from wood-ashes, 

 saleratus, and the caustic potash of the market. This 

 element is found in the flesh of all animals, mostly as the 

 phosphate, and is abundantly, supplied for the purposes 

 of nutrition in the ordinary combinations of natural 

 feeding-stuffs. 



21. Sodium. — ^This is the basal element of common 

 salt, a compound which is fiu-nished to domestic animals 

 in a liberal supply. This is the only sodium compound 

 which it is necessary to consider. Sodium plays an impor- 

 tant part in the digestion of food as it is the basis of cer- 

 tain bile salts and is, concerned in other ways in the 

 digestive processes. (See Par. 151.) 



22. Calcium. — ^The most commonly known compound 

 of this element is lime, which is calcium united with 

 oxygen. Large masses of lime-rock, or calcium carbonate, 

 exist in many parts of the earth's surface. AH soils con- 

 tain lime compounds. Its universal presence in plant 

 tissues and in the milk of all animals in most instances 

 assures a sufficient supply to meet the demands of animal 

 life. The growing animal makes a generous use of lime 

 because, in union with the phosphoric acid, it is the chief 

 building-material of the bony framework. A deficiency 

 of food lime causes an abnormal development of the bony 

 structure. Lime is especially important for poultry, for 

 egg-shells are mostly a lime compoimd. (See Par. 55.) 



23. Iron. — ^The common properties of iron are familiar 

 to everyone. Iron rust and iron ore are oxides of this 



