20 MANUAL OF GENERAL AGRICULTURE. 



in which Sp.=specific gravity, Wt. S.=weight of sand or 

 soil, and Wt. W.=weight of water displaced. Calculate 

 the specific gravity of clay and loan soils.* 



Questions: 1. What is specific gravity? 2. Why 

 is it necessary to use water-free soils? 3. How does the 

 amount of humus affect the specific gravity? 



The specific gravity of the material which forms the 

 great bulk of most soils is about 2.6. But the soil is not a 

 solid mass. It is composed of spherical particles which 

 touch each other at different points. About 50% of a cul- 

 tivated soil is air-space. Hence this air space reduces the 

 weight of a volume of soil much below the specific grav- 

 ity of its constituents. In this experiment we really found 

 the specific gravity of the constituents, which is termed 

 "real specific gravity." In the following experiment we 

 are to determine the specific gravity, including air-space, 

 which is termed "apparent specific gravity." 



19. DETERMINATION OF APPARENT SPECIFIC 

 GRAVITY OF SOILS. 



Materials: Same as in last experiment. 

 In this experiment we are to determine the ratio of 

 unit weight to unit volume of different soils. 



Pour into a dry cylinder 10 cc. water-free sand. 

 Weigh the sand. Having already determined the weight 

 of 10 cc. water calculate the apparent specific gravity 

 according to the following formula : 



V. S. 

 Sp.= 



y. wt. w. 



Wt. S.=weight of soil (i.e., weight of 10 cc. soil), V. Wt. 

 W.=weight of water (i. e., weight of 10 cc. water). 



Questions: 1. What influence have stones upon the 

 apparent specific gravity? 2. What influence has plow- 



*In general when cubic centimeters and grams are used the 

 specific gravity of a body is found by the formula: 



Wt. body. 

 Sp. Gr. of Body= 



Vol. body. 



