On the Physical Properties of Soil. 



183 



a simple calculation^, determine tlie quantity of water absorbed, 

 and power per cent, which the earth exhibits of containing water : — 



Let the weight of the dry earth be . . . 400 grains. 

 The weight of the wet filter . . . .110 



Sum of the two . . . 510 „ 



The weight of the earth saturated with water, 



and the fiUer 706 „ 



Therefore the amount of water absorbed is . 196 „ 

 As 400 grains of this earth absorbed 196 grains of water^ 100 

 grains of the same would retain 49, and the power of this earth 

 to contain water would therefore be expressed by 49. 



Should the earth on the filter absorb the moisture with difficulty, 

 and receive it unequally into its interstices, it would be better to 

 mix it in its dry and previously-weighed state with water in a glass 

 vessel, and then pass it by degrees from this vessel to the filter. 



When an earth contains much humus and salts of humic acid, 

 it may be best to omit drying it before it is placed on the filter, 

 as the humic acid has the property of taking up less water after 

 it has been once thoroughly dried. In such a case the drying 

 may be made the last stage of the process. But, in earths which 

 contain only a small per centage of humus, as is the case with 

 most arable soils, the power of containing water can be only very 

 slightly affected by that circumstance ; and by drying them in the 

 first instance, we in fact obtain a far more decisive result, since it 

 is thus only that we can be sure we have taken them in equal 

 quantities. Clayey soils, too, absorb a different quantity of 

 water, according as they have been submitted in their half-mois- 

 tened state to a different pressure and different treatment^differ- 

 ences which can only be obviated by previous drying and pul- 

 verization. 



In an agricultural point of view, it is also of importance to know 

 how much water a given bulk as well as weight of soil can 

 take up, in order to be enabled to form a more correct judgment 

 of the quantity which any given space of ground can absorb. 

 This question is in every case easily answered when we know 

 (by the method already explained) the determinate power of 

 containing water by weight, and the weight itself also of a given 

 bulk of soil in its wet state. If we have found the power of con- 

 taining water of siliceous sand equal to 25 per cent, and the weight 



* It might appear, that this determination could be made by the mere 

 comparison of the weights of a cubic inch of dry and wet soil, or from the 

 absolute weight of a volume of the dry soil, and its power of containing 

 water ; we should, however, in this way obtain no correct result, because 

 many soils, especially those containing clay and humus abundantly, contract 

 considerably in drying, a cubic inch of such dry soils generally occupying 

 a greater space in their wet state. 



