Leaves and Their Work 135 



bon dioxide out of the account altogether. For except 

 in confined spaces, and under special circumstances, one 

 part in twenty-five thousand is all the carbon dioxide that 

 the air contains, so vast is the space through which the 

 gas is distributed. There is just enough carbon dioxide in 

 the air to furnish twenty-eight tons to every acre all over 

 the globe — twenty-eight tons of gas, or eight tons of car- 

 bon! 



But an acre of beech-forest would use up the whole of 

 this allov/ance in about eight years; and it would not last 

 an acre of bananas much more than one year. 



All plants do not, it is true, use up carbon at these 

 rates; but it is evident that the supply needs pretty con- 

 stant renewing. And it is renewed day by day, hour by 

 hour; nor, so long as animals breathe, and fires burn, and 

 vegetable matter decays, is there any danger that the sup- 

 ply will run short. 



Whenever carbon unites with oxygen it is what we call 

 burned, and carbon dioxide is produced. The carbon dis- 

 appears, but it is not destroyed — it has only been made 

 invisible by combining with oxygen. Whenever, there- 

 fore, animal or vegetable matter decays, the carbon which 

 it contains is slowly burned, and the gas passes off into the 

 air as it forms, unless prevented, as it is, in a great meas- 

 ure, when produced underground. 



Again, when animals or plants breathe, the oxygen 

 which they inhale unites with and burns part of the car- 

 bon 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 more slowly than any warm- 



