128 THE TEMPERATURE OF THE SOIL [chap. 



inches during an April day with a bright sun and a 

 strong drying wind. The land was a light loam of 

 a grey-brown colour when dry; it had been culti- 

 vated, rolled, and the surface hoed over before the 

 thermometers were inserted ; on plot I the bare 

 ground was left untouched, plot 2 received a dressing 

 of soot until the surface was black, plot 3 was 

 similarly whitened over with lime. It will be seen 

 that the covering of soot warmed the soil until at 3 

 P.M., when the maximum temperature was attained, 

 the difference was 2-4°; this superiority is also re- 

 tained during the later cooling stages; even at 9 P.M. 

 the blackened soil was still 2-5° warmer than the bare 

 ground. The whitening with lime had caused so con- 

 siderable a reflection of the radiant heat that the soil 

 beneath was always 2 to 3 cooler than the bare ground. 

 In carrying out this experiment it is necessary to use no 

 more lime or soot than will distinctly colour the soil ; 

 the results will be disturbed if an excess of either loose 

 powder acts as a mulch. 



Specific Heat. 



The specific heat of the substances of which soil 

 is composed is comparatively low, ranging from 01 

 to 02, z>., only from one-tenth to one-fifth as much 

 heat will be necessary to raise the temperature of 1 

 lb. of dry soil by i°, as would be required to produce 

 the same rise of temperature in an equal weight of 

 water. The humus possesses the greatest specific heat 

 and the sand the least ; against this must be set off the 

 fact that the densities of these soil constituents vary 

 in the opposite sense, so that the amounts of heat 

 required to bring about a given rise of temperature to 

 a certain depth in different soils are more nearly equal. 

 The specific heats are, however, small in every case 



