THE PLANT AND THE AIR. 23 



We have said that the quantity of carbonic acid gas in the 

 air is very small ; there are only three parts in every ten thou- 

 sand parts by volume. The air, or atmosphere, is made up 

 almost entirely of nitrogen and oxygen, mixed together, not 

 united, in the proportion of about four to one ; that is, in every 

 one hundred volumes of air there are nearly eighty parts of 

 nitrogen to a little over twenty parts of oxygen. In addition, 

 there are very small quantities of other gases, such as ammonia, 

 but we need not refer to these here. The facts now to be 

 fixed in the memory are that the plant, through the leaf, does 

 not take up the nitrogen and oxygen which are in such large 

 quantities, but does take up carbon from the carbonic acid gas 

 which exists in such small quantities, and from this carbon, 

 along with the elements of water, it builds up the larger portion 

 of its entire structure. How it does this is largely a mystery. 

 CONCLUSIONS : 



1. Besides the water and the mineral matter of the plant, 

 which come in through the roots, there are in plants large 

 quantities of such substances as starch, sugar, oil, and gluten. 



2. All of these substances contain caibon. 



3. This carbon comes from the carbonic acid gas of the air. 



4. Animals breathe in oxygen and breathe out carbonic acid 

 gas through their lungs ; plants take in carbonic acid gas and 

 give off oxygen through their leaves. 



