50 THE SOIL. III. 



''apparent specific gravities" or the weight of a given volume 

 of dry soil or powdered material compared with that of the 

 same volume of water are much lower, since there are air 

 spaces between the particles of the soil or powder 



The apparent specific gravity of powdered quartz is found to 

 be 1-449, of clay 1-011, and of humus 0'335. 



The apparent specific gravity of a good arable soil will 

 usually be about 1-2, its real specific gravity being about 2'5. 



Sand is thus the heaviest of the main constituents and 

 humus the lightest. 



Sand confers friability, power of draining quickly, and good 

 conductivity for heat upon a soil ; it also has a low specific 

 heat, consequently is soon rendered hot and cools quickly. 



The specific heats of various soil constituents are given in 

 the following table : 



Equal weights. Equal volumes. 



Water ... ... 1-000 1-000 



Humus ... ... -477 -587 



Clay ... ... -233 -568 



Calcium carbonate ... "206 -561 



Quartz ... ... -189 -499 



The average specific heat of a dry soil is -20 to '25 (equal 

 weights) . 



The relative conductivities for heat are as follows : 



Dry and light. Wet. 



Quartz powder ... 100 201-7 



Peat ' ... 90-7 94-3 



Clay ... 90-7 155-6 



Chalk ... 85-2 153-2 



The conductivity, however, becomes greater with an increase 

 in the coarseness and compactness of the material, as well as 

 with an increasing amount of moisture, since the bad conduc- 

 tivity of all powders is due mainly to air spaces. 



The amount of water held by the constituents varies greatly ,. 

 thus lOOpts. by weight 



of sand were found to hold 25pts. of water 

 of clay ... ... 70 



of fine calcium carbonate 85 ,, ,, 



of humus ... 181 ,, 



