136 



Introduction to the Study of Science 



with water, carbon dioxicl has a great share in shaping the crust 

 of the earth and bringing into circulation substances that 

 millions of years ago were buried deep in the earth's crust. 

 After its action upon rocks beneath the surface, it escapes to 

 the air, ready to begin another cycle of the same kind. For 

 carbon dioxid not only unites freely with water, but just as 

 freely escapes from it. 



The carbon cycle. In this manner not only in life, but in 

 death and decay, and in all the processes of nature, carbon 



dioxid is being produced and 

 released in the air. The dia- 

 gram (Fig. 38) presents 

 graphically the chief stages 

 in the great cycle of this im- 

 portant substance. Air is 

 taken up by plants, carbon 

 dioxid is decomposed, oxygen 

 and other gases liberated to 

 the atmosphere, and the car- 

 bon assimilated with other 

 substances in forming sugars, 

 starch, other food materials, 



FIG. 38. The carbon cycle. Trace and plant tissues. Animals 



eat plants or plant products 



its progress from air to air, and show by 

 what agencies the cycle is made possible. 



and thus get the carbon com- 

 pounds they need for making tissue and producing energy. 

 Some of the carbon is oxidized and the carbon dioxid returned 

 by the lungs to the air. Animals and many plants die and 

 decay, a process carried out by bacteria and molds, in which 

 carbon is oxidized and set free as carbon dioxid. Again it is 

 absorbed by plants and so on, cycle following cycle eternally. 



Formation of coal. The importance of carbon dioxid to 

 plants to-day (pages 515 ff.) is suggestive when we consider the 

 luxuriant growths of vegetation that were necessary to form 

 the immense stores of coal and the reservoirs of natural gas 



