

AND AGRICULTURE. 



29 



openings, or mouths, spread all over their surface, 

 but especially on their under side. Plants take 

 in carbonic acid only during the day ; during the 

 night they give it off. 



Twenty-two pounds of carbonic acid gas con- 

 tain six pounds of carbon and sixteen of oxygen ; 

 and this is readily proved by burning charcoal in 

 oxygen. (See Appendix, 16.) 



Fig. 12. Plants only retain 



the carbon contained 

 in the carbonic acid; 

 they give the oxygen 

 back again to the air. 

 We can prove this very 

 easily. Fill a tum- 

 bler with clear water ; 

 s put into it a few fresh 

 green leaves ; then in- 

 vert the tumbler in a saucer, so that it shall 

 remain full. Place it in the sun, and bubbles 

 of oxygen may be seen arising from the leaves, 

 occupying the upper part of the tumbler. 



Questions. Do the leaves take in carbonic acid all the 

 time ? Of what is carbonic acid composed ? Can you prove 

 this ? Do plants retain all the carbonic acid ? How can 

 you prove they give back the oxygen ? 



3* 



