THE CHEMICAL BASIS OF LIFE 235 



OXYGEN 



Oxygen is a transparent, odorless gas which unites 

 readily with a large number of elements and compounds. 

 It is one of the most abundant elements, as it occurs 

 in water, and forms, in combination with various minerals, 

 about one-half of the earth's crust. Air is composed of 

 about one part of oxygen to four of nitrogen together with 

 small quantities of water vapor and other gases. The 

 oxygen of the air is simply mixed with nitrogen and not 

 chemically combined with it. Oxygen in a pure state 

 may be obtained by the decomposition of water by the 

 electric current or by heating various substances that 

 contain it in chemical combination. In its pure state 

 it acts very vigorously upon many substances that it 

 attacks but feebly in the air. Thus a steel watch spring 

 will burn in pure oxygen and a glowing match thrust 

 into pure oxygen will quickly burst into flame. Most 

 of what is called combustion or burning is the combination 

 of substances with oxygen. When wood and coal are 

 burned they combine with the oxygen of the air giving 

 rise mainly to carbon dioxide and water. The combina- 

 tion of substances with oxygen is called oxidation, a 

 process which may be a rapid chemical change such as 

 takes place in the explosion that occurs when oxygen 

 and hydrogen are mixed and ignited, or a very slow one 

 such as the gradual rusting of iron. Oxidation plays an 

 essential role in the living body. Oxygen occurs in all 

 living tissues, and it is found in all foods. 



CARBON 



Carbon is a solid devoid of taste or odor. It may be 

 seen in almost pure form in charcoal. When burned it 

 combines with oxygen to form a gas, carbon dioxide. 



