182 



On the Physical Properties of Soil 



This phenomenon is only to be explained by supposing a more 

 intimate approach in the interstices of the contiguous earthy par- 

 ticles ; something similar therefore, seems here to happen with 

 this mechanical commixture, to what takes place in a still higher 

 degree with natural mixtures of earthy and rocky materials, for 

 instance, with the dolomite sand and stony marls already men- 

 tioned, in which cases not only the absolute weight, but the real 

 specific gravity also is greater than in the separate earths. 



II. Power of soil to contain water. — We understand by the 

 power of the earths to hold or contain water, their property of 

 receiving and retaining more or less water within their interstices, 

 without allowing it again to flow away by dropping : it is of the 

 greater importance to vegetation, as on it depends the quantity 

 of the means of aqueous nourishment the soil is in a condition to 

 receive and supply to the roots of plants, and as the water itself is 

 likewise one of the most essential sources of nutriment to plants. 



The power of an earth to contain water may be found in the 

 following manner : we take 400 grains of the earth to be investi- 

 gated, and dry it at a temperature of about 144^° F., until it ceases 

 to lose weight ; in order to obtain results that may be compared 

 easily together, it is desirable that the experunent be made with 

 nearly equal quantities of each earth in its fine state, say, with 

 about 400 grains or one cubic inch, because, when a large quan- 

 tity is employed, the weight alone of the earth occasions the pres- 

 sure of a greater quantity of water out of it, and w e should in such 

 cases obtain different results for the same earth. We put this 

 dried earth on a round filter consisting of unsized paper, and which 

 has been previously weighed in its thoroughly moistened state, and 

 laid in a glass funnel, or on linen stretched over a frame (as in the 

 following figure). 



The latter mode is preferable, as the water poured on can more 

 easily flow off, and it also allows the wet filtering paper to be 

 raised up more easily and removed without tearing. We now 

 pour over the earth lying on the filter distilled or rain-water, until 

 it is fully moistened and saturated ; and when it has ceased drop- 

 ping, we bring it, while remaining hi this wet state, on the filter, 

 to the balance, to ascertain the weight of the whole ; and then, by 



