PHYSICAL PROPERTIES OF SOILS 113 



Specific Gravity of Soils. The specific gravity of a body is the 

 weight of the body divided by the weight of an equal volume of 

 water. Suppose we place 10 grams of soil in a bottle of known 

 weight which holds exactly 25 cubic centimeters, and weigh. 

 Then fill the bottle exactly full of water, so that it contains no 

 air, and weigh again. The gain in weight subtracted from the 

 weight of 25 cubic centimeters of water gives the weight of water 

 displaced by 10 grams soil. Ten divided by this gives the specific 

 gravity of the soil. 



In estimating the specific gravity of a body, we must allow for 

 the space occupied by air. If the soil could be fused into a solid 

 mass without chemical change, the weight of i cubic centimeter 

 expressed in grams would be the specific gravity. 



The specific gravity of some soil materials is given in the 

 following table: 



SPECIFIC GRAVITY OF SOIL MATERIALS. / 



/ 



Quartz 2.6 Water i.o 



Felspar 2.5-2.8 Humus 1.2-1.5 



Limestone 2.6-2.8 Mica 2.8-3.2 



Granite 2.6-2.7 Hornblende 2.9-3.4 



Clay 2.5 Talc 2.6-2.7 



SPECIFIC GRAVITY OF SOILS. 



Clay soil 2.65 



Sandy soil 2. 67 



Fine soil 2.71 



Humus soil 2.53 



Apparent Specific Gravity. The apparent specific gravity of 

 a soil is the weight of a given volume of the soil, including air 

 spaces, compared with the weight of an equal volume of water. 



Apparent specific gravity depends upon the true specific gravity 

 of the soil particles, and the amount of air spaces in the soil. The 

 former is constant, the latter is variable, as it depends upon the 

 size and shape of the soil particles and the treatment to which the 

 soil has been subjected. A cultivated soil contains more air 

 spaces than the same soil in pasture. 



The apparent specific gravity will vary, then, with the treat- 

 ment to which the soil has been subjected, if it is determined in 



