PRINCIPLES OF AGRICULTURAL CHEMISTRY 



Carbon Dioxide. If a vessel of clear lime water is exposed to 

 the air, the lime water after a time becomes cloudy, and a white 

 precipitate or a crust of calcium carbonate is formed. The 

 formation of this precipitate is a test for carbon dioxide, and it is 

 due in this case to the carbon dioxide of the air : 



Ca(OH) 



CO 2 = CaCO 3 



HO. 



Since plants can be grown to full maturity in water containing 

 certain mineral salts, and the plant contains much more carbon 

 than the seed, while the water does not contain any, it follow r s 

 that the plant must have taken carbon from the air. 



The following is an example of an experiment showing that 

 plants can assimulate carbon dioxide in the presence of light. 

 Boussingault placed a sprig of leaves in a vessel containing 86.5 cc. 

 of a mixture of oxygen and carbon dioxide and exposed it to the 

 sun. After nine hours exposure, the gas was measured and sub- 

 jected to analysis. The results of the experiment follow: 



This shows that the leaves absorbed carbon dioxide and re- 

 placed it with a nearly equal volume of oxygen. 



Another experiment consists in enclosing the plant in an air- 

 tight vessel, through which air is passed. A known amount of 

 carbon dioxide is added to the air, and the carbon dioxide in the 

 air which passes out is determined. The quantity which dis- 

 appears is absorbed by the plant. Numerous experiments give 

 the same results. 7'he green leaves of plants absorb carbon 

 dioxide in the presence of light, and replace it with an equal or 

 nearly equal volume of oxygen. 



The amount of carbon dioxide in the air can be estimated by 

 drawing a known volume of air first through calcium chloride to 

 remove water, and then through a solution of caustic potash to 



