192 



On the Physical Properties of Soil. 



General remarks on this pjroperty, tvith further experiments 

 on the same subject. Hence we obtain the following de- 

 ductions: — 



1 . The terms of a hot or cold, a dry or wet soil, rest chiefly on 

 this property of earths : sand, gypsum, and slaty marl, of all the 

 earths, are the quickest in becoming dry again ; on that account 

 they form what are called the hot soils. 



2. The carbonate of lime exhibits great differences in this re- 

 spect, according to the different form in which it occurs in soils. 

 Calcareous sand dries up very quickly, while fine carbonate of lime 

 yields the moisture it contains far more slowly to the air ; the 

 latter has, however, independently of its chemical action on hu- 

 mus, the important advantage over clay of loosening the soil after 

 it is dried. 



3. This property of the earths to require a longer or shorter 

 time to become dry, might seem to stand in the same relation as 

 their power of containing water ; and with thin layers this is 

 nearly ahvays the case : but with layers of some inches in depth, 

 the proportion deviates considerably, the deeper layers in this case 

 drying more slowly, according to their degree of consistency, and 

 to their greater or less contraction on drying : clay soils with a 

 large proportion of clay exhibit this variation in an especially 

 striking manner. 



In order to convince myself more accurately, by positive expe- 

 riments, of this slower process of evaporation in the case of deeper 

 soils, I placed ten earths of very different power of containing 

 water in round tin vessels, equal in size, 1 inch in depth and 1 J 

 in diameter — allowing them, after previous saturation with water, 

 to become gradually dry, in a closed room whose temperature 

 varied from 65|° to 72^° F. ; I determined their weight at the 

 commencement of the experiment, after thirty-six hours, and at the 

 end of four days. They gave off" their moisture to the air, at first, 

 according to that relation of their pov^er of retaining water which 

 had already been shown by the experiments with shallow layers ; 

 as soon, however, as their upper surface had become in some 

 measure dry, and they were contracted into a more or less dimi- 

 nished space, this result varied in the following different degrees ; 

 for easier comparison, the power of containing water pos- 

 sessed by the earths employed in these experiments is here also 

 annexed : 



