32 CHEMISTRY, GEOLOGY 



Now, if you remember that plants drink in carbon- 

 ic acid and water, you will see that they can form 

 all these very unlike substances therefrom ; and, 

 as the leaves require only carbon and water to 

 form the woody fibre and starch of which they 

 are composed, they give off the oxygen of the 

 carbonic acid because they cannot make any use 

 of it. (See Appendix, 19.) 



Notwithstanding plants drink in so much car- 

 bonic acid from the air, they can never exhaust 

 it, because new supplies are being continually 

 thrown into it from other sources, which are 

 three in number : 



1. From the breathing of animals; for all 

 animals throw a small portion of carbonic acid 

 into the air, every time they breathe. 



2. From the combustion of wood, coal, can- 

 dles, oil, etc. ; for all these things contain carbon, 

 which, as they burn in the air, form carbonic 

 acid, as the charcoal did when burned in oxygen. 



3. From the decay of vegetables and roots ; 

 for decay is only a slow kind of combustion, 



Questions. Can all these be formed from the food the 

 leaves drink in ? Why do the leaves give out the oxygen of 

 the carbonic acid ? Why do not plants exhaust the air 

 of all its carbonic acid ? From whence does the air obtain 

 carbonic acid ? 



