90 NATURE AND PROPERTIES OF SOILS 



The only marked variation here observed is in the clay 

 separates of tlie first eolmim. This may be due to the concen- 

 tration of the iron-bearing silicates in this grade and would 

 thus be an apparent rather than a real variation. 



Only one condition may vary the specific gravity of any 

 soil. This is the quantity of organic matter present. As the 

 specific gravity of organic matter usually ranges from 1.2 to 

 1.7, the more that is present the lower will be the figure for 



any given soil. A purely organic soil, such 



4 as muck, presents a variable specific grav- 

 ity ranging from 1.5 to 2.0, according to 

 the amount of inorganic wash it has re- 

 ceived from external sources. Some highly 

 organic minei-al soils may drop as low as 

 2.^. Nevertheless, for general calculations, 

 the average arable soil may be considered 

 to have a specific gravity of about 2.65. 

 The specific gravity of a soil is generally 

 Fig. 19,— Drawing determined by means of a picnometer, a 

 f pienometer bottle fitted with a perforated ground-glass 



generaBy used in stopper and accurately calibrated (Fig. 

 determining the ^^v -p. . ,, • i . <? , i , j. i 



specific gravity of 1^)- ^J comparing the weight of the total 



soil. The ground- water held by the bottle, usually 50 cubic 

 perforated. ^ centimeters, with the weight of the water 



when any given amount of dry soil, say 5 

 grams, is present in the bottle, the weight of the water dis- 

 placed by the soil can be determined and the specific gravity 

 calculated therefrom.^ 



^ Below will be found a sample calculation : 



Weight of picnometer 23.257 grs. 



Volume of picnometer 50 ce. 



Wt. of picnometer -f- 5 gis. soil + X grs. water. . . .76.347 grs. 



Wt. of picnometer + 5 grs. soil 28.257 grs. 



Wt. of X grs. water 48.090 grs. 



Water displaced (50 — 48.09) 1.910 grs. 



Specific gravity = -^ r— ==2.61 -4- 



