10 THE ELEMENTS OF PLANT FOOD 



are compounds formed by the union of the elements 

 carbon, oxygen, and hydrogen. 



Mixtures. Sometimes these elements, although 

 associated intimately, do not combine. Air is a good 

 illustration of a mixture of elements which do not 

 form a single chemical compound. It is composed of 

 the elements oxygen, nitrogen, and carbon dioxide, 

 with small quantities of water vapor and several 

 other less important compounds. If oxygen and 

 nitrogen are united to form a compound, as hy- 

 drogen and oxygen are united to form water, they 

 form a substance which is different from either of the 

 elements. 



Molecules. According to the generally accepted 

 theory, all bodies are composed of very minute particles, 

 called molecules. The molecules are so small that they 

 cannot be seen separately even by the highest power 

 microscope. They are the smallest particles of matter 

 that can exist' alone and still hold the characteristics of 

 the body of which they are a part. 



A molecule of water, then, is the smallest particle of 

 water that can exist and still remain water. If it 

 became any smaller than the molecule, it would cease to 

 exist as water, and would return to the elements oxygen 

 and hydrogen, of which water is composed. 



The Atom. --The atom is one of the parts of which 

 a molecule is composed. Nearly every molecule is com- 

 posed of two or more atoms. The molecule of water 

 is composed of two atoms of hydrogen and one of 

 oxygen ; the cane sugar molecule consists of twelve 

 atoms of carbon, twenty-two atoms of hydrogen, and 

 eleven atoms of oxygen, while the molecule of oxygen 

 consists of two atoms of oxygen. 



