On the Physical Properties of Soil, 



179 



I. Weight of the soil. — In the determination of the weight of 

 the soil^ a particular distinction is to be made between the peculiar 

 specific gravity of the several portions of earth and the absolute 

 weight of a determinate volume^ as of a cubic inch or foot of the 

 several soils. 



The specific gra\dty of an earth is not found by the mere 

 weighing of a determinate volume;, as, for example^ of a cubic 

 inch ; and comparing such weight with that of an equal volume 

 of water, for we should in that case always obtain too small 

 a weight, as the interstices of every cubic inch of the earth, even 

 when closely compressed, contain much air. The real specific 

 gravity is obtained by the following process : — A glass bottle^ 

 with an accurately-fitted stopper, holding some 300 or 400 grains 

 of water, is completely filled with that liquid, and the weight 

 of the whole ascertainecl ; now empty the bottle of half the water, 

 and introduce into the half-filled vessel the soil to be investi- 

 gated, and which had been previously weighed in its dry state ; 

 again fill up the bottle with water, and close it with the stopper as 

 soon as it ceases after a few times shaking to give out air-bubbles 

 from the interstices of the soil, and determine now the weight of 

 the vessel thus filled with soil and water : the specific gravity is 

 found from the quantity of water excluded by the soil from the 

 bottle, by a simple calculation ; and we obtain the quantity of such 

 excluded water by subtracting the sum of the weights of the dry 

 soil and the vessel from the weight of the vessel filled with water. 

 An example will best elucidate the process : — 



The dry soil to be investigated weighs . . 240 grains. 

 The vessel filled merely with water weighs 600 „ 



Therefore the sum of both is .... 840 „ 

 The vessel filled with the soil and water 



together weighs 744 „ 



96 „ 



Therefore the soil has excluded 96 grains of water from the 

 bottle, or, in other words, 240 grains of soil require as great a 

 space as 96 grains of water, and the weight of the water bears 

 therefore to the weight of the soil the proportion of 96 : 240, or 



the specific gravity of this soil is ^ = 2.50, when we assume the 



weight of the water = 1 . 



The actual vreight of a determinate volume of soil, which is also 

 called its absolute weight, is obtained simply by weighing a cubic 

 inch or a cubic foot of the soil moderately compressed in the 

 vessel. As the weight of the soil is always very different according 

 to its different states of moistness or dryness, it is desirable to 



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