32 THE CHBMISTEY OF THE FARM 



to the greatest depth, while soils retaining more water 

 will warm to a smaller depth. The specific heat of 

 wet peat does not differ greatly from that of its own 

 bulk of water. 



The depth to which a soil will be heated depends, 

 however, partly on the conductive power of its con- 

 stituents. Quartz has the greatest power of conducting 

 heat of any soil constituent. Air is the worst con- 

 ductor of heat present in the soil. A dry soil, in fine 

 powder, is thus a very poor conductor of heat, each 

 particle of :2oil being surrounded by air. Consolidation 

 improves the conductivity, and the presence of stones 

 has a still greater effect. Wetting the soil doubles the 

 conductivity of quartz sand, chalk, or clay, by dis- 

 placing the air. We see, therefore, that a dry, 

 pulverulent, loose soil will get very hot at the surface 

 when exposed to the sun, but the heat will penetrate 

 to a small depth. A solid, stony soil, especially when 

 wet, is the one in which heat will pass most easily to 

 the subsoil. The suitability of gravelly soils for early 

 spring crops has been already noticed. 



The presence or absence of much water is the con- 

 dition which chiefly determines the cold or warm 

 character of a soil. We have already noticed the high 

 specific heat of water, in consequence of which the 

 same amount of sunshine will warm a wet soil far less 

 than a dry one. A still more potent reason for the 

 coldness of wet soils is, however, the loss of heat durhig 

 evaporation. If one pint of water is evaporated from 

 97 pints, the 96 pints remaining will have fallen 10** 

 Fahr. in temperature, or this amount of heat must 

 have been supplied from some external source. Un- 

 drained meadows, and heavy clays, are thus cold soils, 



