SOURCES OF SOIL HEAT 145 



to what extent the temperature of the surface soil is affected 

 by the internal heat of the earth. The average temperature 

 of the surface soil is in England about 1 F. higher than 

 the temperature of the air above the surface, and the average 

 temperature of the subsoil becomes slowly higher as we 

 descend. Since, however, the air at the surface of the earth 

 derives almost the whole of its heat from contact with the 

 earth, and is itself cooled by mixture with the air of the 

 upper regions of the atmosphere, the surface of the earth 

 should be a little warmer than the air even if the whole 

 of its heat was derived from solar radiation. The heat 

 derived from the interior of the earth will be equally dis- 

 tributed through all the seasons of the year ; it will probably 

 differ in amount in different places, owing to the greater or 

 less thickness and conductivity of the earth' scrust. 



We have already seen, p. 62, that a thoroughly dried soil 

 rises somewhat in temperature when moistened, especially 

 if it is rich in colloid constituents. A still greater rise in 

 temperature is observed when a soil condenses water from 

 the air. 



Under special circumstances, considerable quantities of heat 

 may be generated by chemical action. It has been well said 

 that when a log of wood decays in the forest it produces as 

 much heat as when burnt in a furnace. If the products 

 water and carbonic acid are the same under both conditions, 

 the heat produced must also be the same. The heat evolved 

 is, however, in one case spread over many years, and in the 

 other case is probably concentrated into one hour. The rise 

 of temperature is thus in the first instance imperceptible, 

 and in the latter very great ; the total quantity of heat being 

 the same in each case. 



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