ORGANIC AND INORGANIC MATTER 9 



so makes it less active. Nitrogen forms less than three 

 per cent of the body of plants and animals. 



5. Calcium., sulphur, phosphorus, iron, and potassium are 

 the other important elements found in living things. None 

 of these elements forms as much as one per cent of the body 

 of plants or animals. 



Chemical Compounds. — All these chemical elements com- 

 bine with each other to form definite substances called 

 chemical compounds, which we can see and handle. Oxy- 

 gen and nitrogen mixed together make up about ninety- 

 nine per cent of the atmosphere ; hydrogen and oxygen 

 unite to form water ; carbon, hydrogen, and oxygen unite 

 to form starch and sugar. 



The union of oxygen with any other substance produces 

 heat or energy. This uniting is called oxidation. When 

 oxygen unites with carbon in our bodies, carbonic acid gas 

 (carbon dioxide) is formed and heat is produced. The 

 production of heat is one of the most important of the 

 changes that take place in living things. 



Physical and Chemical Change. — If a solid piece of ice is 

 melted, it becomes liquid water. If the liquid water is 

 boiled, it becomes steam, vapor, or gas. If the steam is 

 condensed, it becomes water, which in turn may again be 

 frozen into ice. Any change in a substance which does 

 not alter the material of which it is composed is called a 

 physical change. 



On the other hand, when oxygen unites with wood, the 

 wood burns, giving off heat and smoke, and asli remains. 

 But this ash cannot be united with heat and smoke to form 

 the original wood. Such a change as is seen in the burn- 

 ing of wood is called a chemical change. 



Organic and Inorganic Matter. — It is customary to separate 

 chemical compounds which are made in living things from 

 those which are made outside the bodies of plants and 



