30 THE ELEMENTS OF PLANT FOOD 



Carbon Dioxide (CO 2 ). This substance is often 

 called carbonic acid gas. It is one of the constituents 



FIG. 7. Making Charcoal. 



of air, of which it forms about four parts in ten thousand. 

 It occurs combined in all carbonates, the most abundant 

 of which is limestone (CaCO 3 ). Millions of tons of 

 CO-> are thrown into the air yearly by the burning of 

 carbonaceous matter, the breathing of animals, by 

 fermentation, and by the decay of animal and vegetable 

 matter. Almost any acid will act on any carbonate 



