SPECIFIC GRAVITY 43 



from the specific gravity of its constituents, but a weight 

 very much smaller, only a portion of the cubic foot being 

 occupied by solid matter. The weight of a given volume of 

 dry soil, divided by the weight of the same volume of water, 

 is called its * apparent specific gravity.' According to Wollny, 

 the apparent specific gravity of powdered quartz is 1-449, f 

 clay i -on, of hunius 0-335 ; all these were weighed in an 

 air-dried condition. As one cubic foot of water weighs 

 62-32 lb., these specific gravities are equivalent to weights 

 of 90-3 lb., 63-0 lb., and 20-9 lb. per cubic foot. Siliceous 

 sand is thus the heaviest constituent of ordinary soils, clay 

 much lighter, and humus far lighter still. According to 

 Wiley, an ordinary arable soil, in good tilth, will have an 

 apparent specific gravity of about J-2, and will consequently 

 weigh 74-8 lb. per cubic foot.. 



The difference between the apparent specific gravities of 

 quartz sand and clay is seen to be considerably greater than 

 the difference in their true specific gravities ; this arises from 

 the fact that the extremely fine particles of clay lie more 

 loosely, and are much more difficult to pack than the large 

 heavy particles of sand. The difference between the apparent 

 and true specific gravities is still more marked in the case of 

 humus, the proportion of empty space in dry humus being far 

 greater than it is in the case of either clay or sand. 



The whole of the apparent specific gravities and weights 

 per cubic foot given above relate to dry powdered soil, shaken 

 or pressed together in a vessel of known capacity ; they 

 represent therefore the weight of soils when in a condition 

 of fine tilth, and not their weight when in a consolidated 

 condition in the field. 



King has, by boring, cut out successive cylinders of the 



