THE; PLANT AND THE: ATMOSPHERE: 37 



absorb the carbon dioxide. The potash bulb is weighed before 

 and after the experiment, and the gain in weight is carbon dioxide. 

 The carbon dioxide may also be absorbed by soda-lime, or by 

 barium hydroxide. Water lost from the absorbing tube is col- 

 lected in a small tube containing calcium chloride, and weighed 

 with the absorbing tube. 



While the percentage of carbon dioxide in the air is small, and 

 is continually depleted by plants during the day, yet the total 

 quantity is large. The income and outgo of the carbon dioxide of 

 the air appear so nearly to balance that no great variation in the 

 amount takes place. The following are the chief processes which 

 restore carbon dioxide to the atmosphere : - 



(1) The respiration of animals. Animals absorb oxygen and 

 give off carbon dioxide. The oxidation of organic material de- 

 rived from food or body substances produces the carbon dioxide. 



(2) Combustion. All processes of combustion of organic ma- 

 terials produce carbon dioxide. 



(3) Fermentation and decay. These are changes which occur 

 in organic materials, and are usually accompanied by production 

 of carbon dioxide. 



(4) Decomposition of calcium bicarbonate by shell fish. The 

 calcium bicarbonate dissolved in the sea water is decomposed, 

 setting carbon dioxide free, and the calcium carbonate is used by 

 the animal to form its shell. 



Ca(HCO 3 ) 2 = CaCO 3 + H 2 O + CO 2 . 

 This carbon dioxide was originally derived from the air. 



(5) Dissociation of carbonates by heat, as in the burning of 

 lime. This is a matter of small importance, especially as the 

 lime takes up the carbon dioxide again sooner or later. 



CaCO 3 = CaO + CO, 2 



Carbon dioxide is also emitted from some volcanoes, deep 

 springs, and other subterranean sources. 



Quantity Present. Country air contains on an average 0.029 

 per cent, carbon dioxide, or, in round numbers, 3 volumes to 

 10,000 volumes of air. City air contains larger quantities. Angus 



