THE ALPHABET OF ALL LIVING THINGS 15 



SODIUM, POTASSIUM, AND CALCIUM 



Our list of elements important to organic life will end with three 

 similar ones sodium, potassium, and calcium. These are light, 

 metallic substances which burn when put in water and are there- 

 fore very dangerous to handle. Potassium compounds must be 

 in the soil if plants are to thrive, while sodium and calcium com- 

 pounds are necessary for the blood and skeleton of animals. 



Nitrogen, sulphur, phosphorus, iron, sodium, potassium, and 

 calcium are all obtained from their mineral compounds in the soil; 

 animals use salt (a sodium compound) directly, while they get 

 the other elements from plant foods. Plants in turn obtain them 

 from the soil. 



By themselves, all these elements are inorganic substances, but 

 in the wonderful process of assimilation, plants and animals can 

 combine them to form the living stuff of which their tissues are 

 made. On the other hand, by the processes of oxidation, death, 

 and decay, the complex organic compounds are broken up into 

 simpler forms, and return to the soil or air as inorganic compounds 

 or elements, to be used over again by organic things. 



Here is an estimate of the composition of the human body, 

 which may give an idea of the comparative amounts of the different 

 elements in animal tissue. 



