LECT. V.] THE ROOT. SOILS. 221 



chanical force only which it exerts in loosening the 

 soil at the moment of its formation. 



These decompositions, which are continually 

 going forward in soils, evolve CALORIC ; and this is 

 retained sometimes very pertinaciously, and in 

 such quantity as to be considered as a component 

 of soils. It is certain that the earth is always 

 warmer in summer than the surrounding atmo- 

 sphere ; but this is very variable near the surface ; 

 although, at a considerable depth, the temperature 

 of the ground is almost all the same both in sum- 

 mer and winter. Soils however, which contain 

 much siliceous earth, are not favourable to these 

 decompositions ; and in these, little caloric is ac- 

 cumulated. But independent of these decom- 

 positions, some soils are more readily heated than 

 others ; and portions of different soils heated to 

 the same temperature cool with various degrees 

 of rapidity : a stiff white clay soil is heated with 

 difficulty ; and, being usually very moist, cools 

 rapidly ; hence it is properly termed a cold soil : a 

 chalky soil is heated with as much difficulty, but 

 being dryer retains its heat longer ; although it 

 should be observed, that, of perfectly dry soils, 

 those that are most readily heated by the solar 

 rays likewise cool most rapidly. Sir H. Davy 

 found " that a rich black mould which contained 

 4- of vegetable matter," was heated by exposure 

 to sunshine, from 65 to 88 in an hour ; whilst 



