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tinguished accordingly as solar heat, terrestrial heat and chemi- 

 cal heat. The chemical heat derived from decaying organic 

 matter especially in porous soils, is very considerable ; but as 

 this heat is evolved very slowly it has little perceptible effect 

 on plant-life. Owing to the internal heat of the earth, there 

 is very little change of temperature due to surface radiation, 

 between day and night below a depth of 4 feet from the sur- 

 face in warm countries. In cold countries, below a depth of 

 75 to 80 feet the temperature is constant, i.e. not affected by 

 radiation at night and solar heat by day. The mean annual 

 temperature of the surface soil is slightly over that of the 

 air ; but moist clay soils are colder than the atmosphere 

 above them, as the continual evaporation going on from them 

 renders a large amount of heat in them latent. Water as- 

 cending by capillary action from the subsoil and taking the 

 place of that evaporated from the surface soil, keeps the 

 surface soil always cold. 



92. Specific heat. The less the specific heat of the soil 

 the more rapidly is it heated. The specific heat of soils 

 compared to that of water varies from '2 to -5 for equal 

 volumes and from '16 to '3 for equal weights. Sand has 

 greater specific heat than clay. Actual capacity of soil for 

 heat, however, is largely dependant on its capacity for water 

 as water has 4 or 5 times the specific heat of soils. Quartz- 

 sand becomes heated to the highest temperature, and white 

 chalk soil to the least temperature under the same solar in- 

 fluence. The coolness of lime soils is therefore of great 

 advantage in warm climates, and the advantage of Kankar 

 beds can be viewed from this point also. Moist clay soils which 

 are considered very objectionable from the temperature point 

 of view in cold countries should from the same point of view 

 be looked upon as highly advantageous for this climate. 



93. Radiation. -Radiation also affects temperature. 

 Smooth and polished surfaces which reflect heat most per- 

 fectly, absorb and radiate it least readily, The radiation from 



