THE PHYSICAL PROPERTIES OF SOILS 81 



chiefly upon the difference between the temperature of the 

 air and that of the soil. It is also affected by the moisture 

 and conductivity but not by the colour of the soil. 



Water radiates heat faster than the solid constituents, 

 and its presence therefore increases the radiating power 

 of the soil. As it cools at the surface the warmer water 

 underneath tends to rise by convection. This helps to 

 maintain the temperature at the surface but increases the 

 total loss of heat. The heat brought to the surface by 

 conduction has a similar effect. On the other hand, owing 

 to its greater capacity for heat, the moisture of the soil 

 diminishes the rate of cooling. 



Conduction of Heat in Soil. -- The power of soils to 

 transmit heat by conduction is small. Solid compact rocks 

 conduct heat about four or five times faster than water 

 but not nearly so fast as metallic substances. When the 

 rocks are broken down into a loose, pulverent condition, 

 as in soils, the conductivity is greatly reduced, because 

 the interspaces are filled with air of which the conductive 

 power is less than a hundredth of that of the rock sub- 

 stance. When the air is wholly or partially displaced 

 from the interspaces by water the conductivity is increased 

 because the water conducts heat more than twenty-two 

 times faster than air. The conductivity of quartz is slightly 

 greater than that of ,the other solid constituents of the 

 soil, but there is very little difference between them in this 

 respect. The presence of stones and large particles in the 

 soil favours conductivity. Compression diminishes the size 

 of the interspaces and increases the number of points of 

 contact and therefore increases the conductivity. It is by 

 conduction that heat penetrates into the soil. Dry pul- 

 verent materials become hotter at the surface but remain 

 colder underneath. The conductivity of the soil tends to 

 equalise the temperature. 



The amount of heat developed by the condensation of 



S.M. G 



