1 62 Leaves and their Work 



not so much as a blade of grass could exist until the 

 supply were renewed. 



Let us put it in another way. The amount is large 

 in itself, but it is enormously diluted— so much diluted, 

 indeed, as to be hardly reckoned at all ! That is, in 

 speaking of the air, we commonly say that it consists 

 of about four-fifths nitrogen and ^ne-fifth oxygen, 

 leaving the carbon - dioxide out of the account 

 altogether. For, except in confined spaces, and under 

 special circumstances, one part in twenty-five thou- 

 sand is all the carbon-dioxide that the air contains, 

 so vast is the space through which the gas is dis- 

 tributed. 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 

 carbon ! 



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

 of this allowance 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 

 constant renewing. And it h 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 supply will run short. 



Whenever carbon unites with oxygen it is what we 

 call burnt, and carbon-dioxide is produced. The 

 carbon disappears, but it is not destroyed — it has only 

 been made invisible by combining with oxygen. When- 

 ever, therefore, animal or vegetable matter decays, the 

 carbon which it contains is slowly burnt, and the gas 

 passes off into the air as it forms, unless prevented, 

 as it is, in a great measure, when produced underground. 



