VHEUUM 



J NEON 1002*5?, 



^ XENON 



The air consists of at least 

 seven distinct gases. Nitro- 

 gen and oxygen together 

 make up about 99 per cent 

 of the total. Although the 

 proportions of these gases 

 are constantly changing, the 

 turbulence of the air mixes 

 them so thoroughly that sam- 

 ples taken in different places 

 vary but little. In addition to 

 these gases, the air contains 

 varying portions of water and 

 dust. So far as life is con- 

 cerned, the most important 

 parts of the air are oxygen, 

 carbon dioxide, and water. 

 Nitrogen is an essential part 

 of all living matter, but very 

 few organisms can get it di- 

 rectly from the atmosphere 



COMPOSITION OF DRY AIR 



The nearest thing to the oxidation of protoplasm that is famiHar to most of 

 us is the rusting, or oxidizing, of iron, which also takes place in water. 



Air and Energy^ We may compare the oxidation of food in living pro- 

 toplasm with the burning of fuel. When we burn coal, which consists chiefly 

 of the element carbon, oxygen of the air combines with the carbon, forming 

 carbon dioxide and liberating heat: 



C + O2- 



carbon oxygen 



• CO2 (and heat) 



carbon dioxide 



Wood is composed chiefly of cellulose, an insoluble material consisting of 

 carbon, hydrogen and oxygen, in the same proportions as they are found in 

 a simple sugar. When wood burns, heat is liberated, and water is given off, 

 as well as carbon dioxide. 



Familiar Aires give off heat and light. Oxidation in protoplasm also re- 

 sults in heat and other forms of energy. When glucose, a kind of sugar, is 

 oxidized in protoplasm, energy is liberated, and carbon dioxide and water 

 pass off as waste substances: 



CeHisOe + 6 O2 — >" 6 CO2 + 6 H2O (and release of energy) 



glucose 



oxygen 



carbon dioxide 



water vapor 



In an engine the oxidation takes place in the firebox or in the cylinder. 

 In a living plant or animal oxidation takes place in every living cell. 



iSee Nos. 1-5, pp. 93-94. 

 84 



