CHAPTER XV 



THE AIR 



Questions. 1. What has breathing to do with life? 2. How long can 

 one go without breathing ? 3. Are all parts of the air necessary for life ? 

 4. Do all animals need the same amount of air ? 5. How do fishes 

 breathe ? 6. Have whales lungs or do they breathe like fishes ? 



134. Air and life. The atmosphere has approximately the 

 composition shown by the diagram in Fig. 82. When air is 

 shut off, we suffocate, as in drowning. Now, what is the con- 

 nection between air and being alive ? This question has already 



Nitrogen 78jJ + 



Carbon dioxid, less than 0.1 5S 



Fig. 82. Composition of the air 



The air consists of several gases. The exact proportions of these gases are constantly 

 changing. The most important of the gases are oxygen and carbon dioxid, but nitro- 

 gen is the most abundant 



been answered in part (see section 76) by the statement that the 

 energy of protoplasm, in all its activities, comes from the burn- 

 ing, or oxidation, of materials derived from food. The food is 

 not burned directly, like the gasoline in an engine ; it first under- 

 goes many changes (digestion, assimilation) and becomes a 

 part of the living protoplasm. Nor is the oxidation, or burn- 

 ing, like the familiar flame : it takes place only in the presence 

 of water, whereas the fires with which we are familiar cannot 

 be kept up under water. The nearest thing to the oxidation in 

 protoplasm that is familiar is the rusting, or oxidizing, of iron, 



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