182 THE CARBON, NITROGEN, AND PHOSPHORUS CYCLES 



absence of bacteria is incompatible, with life on this earth, or, as 

 stated by Pasteur, "they are the important, almost the only, agents 

 of universal hygiene. They clear away more quickly than the dogs 

 of Constantinople or the wild beasts of the desert, the remains of 

 all that has had life; they protect the living against the dead; they 

 do more; if there are still living beings, if, since the hundreds of 

 centuries the world has been inhabited, life continues, it is to them 

 we owe it." 



The Carbon Cycle. Carbon occurs free in the earth as coal to the 

 extent of over 500 billion tons. Chemically combined, it is found 

 in far larger quantities in limestone, chalk, marble, and dolomite- 

 rocks which form such a considerable portion of the surface of the 

 earth. According to Pettenkofer, a man weighing 154 pounds 

 contains 26.4 pounds of carbon; no less than 257 million tons' weight 

 of it is, therefore, stored up in the bodies of men and women living 

 upon the earth at the present time, to say nothing of the far greater 

 quantities occurring in the tissues of trees, plants, and lower animals. 



Carbon dioxid occurs in the atmosphere to the extent of three 

 parts in 10,000. This is the equivalent of 600 billion tons of carbon. 

 Moreover, the ocean is a vast reservoir of carbon dioxid, which is 

 partly in solution and partly combined. Between the surface of 

 the sea and the atmosphere there is a continual interchange, each at 

 times losing and at times gaining the gas. 



Carbon dioxid is being added to the air from several sources: 

 the combustion of fuel, the respiration of animals, and the decay of 

 organic matter. It is also being evolved in enormous quantities 

 from mineral springs and volcanoes. Krogh estimates that the 

 annual consumption of coal adds yearly to the atmosphere about 

 one-thousandth of its present content in carbon dioxid. Were 

 there no factors offsetting this increase in atmospheric carbon dioxid 

 animal life would soon become extinct. 



On the other hand, there are two large factors at work removing 

 carbon from the atmosphere first, the decomposition of carbon 

 dioxid by plants w r ith the liberation of oxygen, and second, the 

 consumption of carbon dioxid in the weathering of rocks. No 

 precise valuation can be given to either of these factors, although 

 various writers have attempted to estimate their magnitude. Cook 

 computes that leaf action alone more than compensates for the 

 production of carbon dioxid. Chamberlain estimates that the 

 amount of carbon dioxid annually withdrawn from the atmosphere 

 is 1,620,000,000 tons, and that the greater part of this is taken up 

 by the weathering of mineral. This is continually being returned 

 to the atmosphere by the factors considered in the preceding chapter. 

 There are then two compensating sets of factors decay, respiration, 

 and combustion liberating carbon; plant growth and rock weather- 

 ing fixing it. These balance each other, thereby completing 



