COMPOSITION OF MATTER 5 



ment is the simplest form in which matter can exist. 

 All substances found in nature, as plant and animal bod- 

 ies, rocks and soils, are composed of compounds which, 

 in turn, are composed of elements. There are about 74 

 of these elementary forms of matter, although only about 

 1 8 take any important part directly or indirectly, as far 

 as is known, in either plant or animal life processes. 

 There are a few substances found in nature in elementary 

 form, as iron, copper, gold, and sulfur, but most of the 

 elements are in combination with others, forming com- 

 pounds. 



8. Compounds. The substances found most abun- 

 dantly in nature are compounds. A compound is formed 

 by the chemical union of two or more elements. All 

 compounds are made up of a definite amount, by weight, 

 of separate elements which unite according to the laws of 

 chemical combination. Water, for example, is a com- 

 pound made up of two elements, hydrogen and oxygen. 

 Sugar is a compound made up of three elements, hydrogen, 

 carbon, and oxygen. When elements unite to form a 

 compound, the elements lose their identity and the com- 

 pound that is produced has entirely different and distinct 

 chemical and physical properties from those of the ele- 

 ments of which it is composed. 



9. Chemical Affinity. The force or power which 

 causes elements to combine to form compounds is called 

 chemical affinity, and about this comparatively little is 

 known. Whenever a compound is separated into its ele- 

 ments, chemical affinity or the force which holds the ele- 

 ments together is overcome. When elements com- 



