226 COMMON CHEMICAL PROCESSES 



oxidation. Iron rust, the most common of metallic 

 oxides, is formed by the union of iron with oxygen, but 

 this is always greatly helped by water, even if only the 

 moisture in the air. 



265. Oxidation in Animal Bodies We breathe air 



into the lungs for the oxygen that it contains. Carbon is 

 taken into the body in the food that is eaten (258), 

 and is found all over the system. In the lungs O is 

 separated from the air and is carried by the blood to all 

 parts of the body. There it unites with the C which is 

 already in those parts, and forms CO 2 . This chemical 

 union of the C with O gives off energy, as does any 

 chemical union ; the energy is used by the body, partly 

 as heat to keep us warm, and partly as muscular energy 

 so that all parts may move and do their work. 



The CO 2 that is formed is carried to the lungs, where 

 it leaves the body in the air that is breathed out. Plants 

 take air into their leaves, separate the C from the O of 

 the CO 2 , use the carbon in making starch, and give out 

 pure oxygen to the air again. 



266. Decay. Many substances, particularly of plant 

 and animal matter, will decay after a time unless cared 

 for in some special way. The signs of decay are many : 

 the body is usually soft and easily crumbles ;, it is gen- 

 erally much smaller in size than before ; and often an 

 odor is given off. The smaller size is due to the fact 

 that a large proportion of any animal or plant matter is 

 of gaseous elements ; these of course pass off when they 

 are set free by decay. The odor is caused by gases that 

 are formed ; one of the most common of these gases is 



