24 ELEMENTS OF AGRICULTURE 



above the earth that the little light they give off is not 

 visible. Few meteors ever reach the earth. 



20. Composition of the Air. — Chemists tell us that 

 air is a mixture, consisting principally of two gases, 

 nitrogen and oxygen, but containing in addition to 

 these two main elements small quantities of several 

 other gases, and even some liquids and solids. A gas 

 called carbonic acid, or, more properly, carbon dioxide, 

 another gas, ammonia, and water vapor are three im- 

 portant gases found in small quantities in the air. 

 Water in the rain clouds and mist make up the liquid 

 portion of the air, and dust particles and bacteria make 

 up the solid portions. Now, what are these various 

 substances — nitrogen, oxygen, ammonia, etc.? They 

 are what chemists call elements and compounds. 



21. Elements. — Nitrogen and oxygen are elements. 

 They are called elements because they cannot be 

 divided into anything else. Iron is an element; it may 

 be divided into the finest possible pieces, but it is still 

 iron. There is no known way of dividing iron into any- 

 thing but iron, and this is true of nitrogen and oxygen. 

 All substances that cannot be divided into anything 

 else are called elements. There are something over 70 

 elements known, but we shall be concerned in this book 

 with not more than 16 or 17 of them. 



22. Compounds. — Two or more elements may unite 

 with each other +o form substances entirely different in 

 appearance or behavior from either of the two elements 

 themselves. Substances made up of two or more ele- 

 ments are called compounds. Water is a compound, 

 for it can be divided into two gases, oxygen and hydro- 



