Fire and Heat 137 



and other hydrocarbons upon which we now draw for heat and 

 power. In ordinary conditions when a tree falls in the forest, 

 it decays or oxidizes, giving up carbon dioxid and water. 

 Finally little remains but mineral ash. If the wood of the tree 

 is burned, the process is hastened, but the products of the burn- 

 ing are the same. If, however, the tree falls into the waters of 

 a swamp and lies there, the process of decay or oxidation is 

 retarded and much of the carbon is preserved. Such in sub- 

 stance are the conditions in which coal is formed. 



In very remote periods of the earth's history, millions of 

 years ago, all of the conditions were at times favorable for the 

 formation of coal. Great swamp areas existed, and the climate 

 was moist, warm, and equable with long seasons for growth. 

 The temperature and humidity of the atmosphere may have 

 been much like the condition found in a tropical swamp to-day. 

 The vast forests and jungles appropriated the carbon dioxid 

 from the air, utilized the carbon in building up plant tissue, 

 and restored the oxygen to the air. The great stores of carbon 

 in the luxuriant vegetation were not lost through decay, but 

 preserved and accumulated mass upon mass in the swamps of 

 that age. This continued for ages and ages, as the coal measures 

 indicate by their extent and depth, until different conditions 

 appeared and with these a new geological period of the earth's 

 history. In succeeding periods the accumulated masses of 

 vegetation with their carbon content were weighted down by 

 deposits of mineral materials, subjected to pressure, heat, 

 and other conditions, and gradually changed into the hydro- 

 carbons and coal. 



To-day the stored-up treasures of carbon, the products of 

 ancient vegetation, are brought forth from their hiding places 

 in the earth's crust and burned in producing heat and power. 

 Thus the carbon dioxid taken from the air in the early history 

 of our earth is being restored. In that remote period carbon 

 dioxid may have been slightly more abundant in the air than 

 it is to-day, but this does not seem probable. The balance of 



