Leaves and their Work 163 



Again, when animals or plants breathe, the oxygen 

 which they inhale unites with, and burns, part of the 

 carbon of their food, and the gas is breathed back into 

 the air. The air we , inhale contains but one part in 

 twenty-five thousand of carbon-dioxide ; but the air 

 we exhale contains much more — from three to six per 

 cent. Plants, however, breathe very much mcare 

 slowly than any warm-blooded animals, and give oft 

 less carbon-dioxide in proportion. 



Whenever carbon is burnt by combining with oxygen, 

 whether in food, coal, wood, gas, oil, candle, or in 

 decaying vegetable-matter, there carbon-dioxide is 

 formed. It is being constantly poured into the air, 

 therefore, by men and animals, by the chimneys of 

 factories and houses, by volcanoes, and by the soil. 



But it is not produced in anything like equal quan- 

 tities in all parts of the world. Very little is returned 

 to the air above the ocean, and that little chiefly by 

 passing vessels ; and as there is more ocean than land 

 in the southern hemisphere, much less is produced 

 there than in the northern hemisphere, which is chiefly 

 land. 



Then, again, the eastern hemisphere is much more 

 densely populated than the western, besides having, 

 of course, many more factories, furnaces, and engines 

 of all sorts, which are constantly burning carbon. It 

 might seem not improbable, therefore, that some parts 

 of the world, such as the islands of the Pacific, should 

 be at times in danger of not having carbon-dioxide 

 enough to supply the wants of their vegetation, 

 especially when we consider that bananas, which need 

 such large quantities, form an important item in their 

 crops. 



