220 



UYGROMETRIC POWER OF SOILS. 



Gypsum, silicions, and calcareous siiid do not appeal in this table, 

 because they do not shrink in drying The humus appears to have 

 shrunk the most ; and dry humus is liable to swell in the same pro- 

 portion when it is moistened. This property explains the obvious 

 elevation of certain turfy or mossy soils at the period of the rains. 



Hygromelric property of soils. Agriculturists allow, that those 

 soils which have the property of attracting moisture from the atmo- 

 sphere are generally among the most fertile. This hygiometric 

 property must not be confounded with that in virtue of which moist- 

 ure is retained. It appears to depend especially on the porousness 

 of a soil, and, probably, also, in soi le degree, on the deliquescent 

 salts which it contains, even in veiy small quantity. Davy was 

 disposed to regard the hygrometric property of soils as a certain 

 index of their good quality ; and the experiments of M. Schiibler 

 upon the point, all tend to confirm the accuracy of this view. In 

 M. Schiibler's experiments, the increase of weight of dry soils was 

 ascertained by exposing them for a certain time in an atmosphere 

 kept at the point of saturation with moisture, and at the same tem- 

 perature, between 60° and 65° Fahr. 



From the results comprised in the preceding table, we may con- 

 clude, first, that the faculty of absorbing lessens as soils acquire 

 moisture; second, that humus is the most hygrometric of all the 

 substances examined ; third, that the clays which. absorb the largest 

 quantity of moisture are those which contain the smallest proportion 

 of .sand ; and fourth, that silicious sand and gypsum do not absorb 

 moisture in any appreciable quantity. 



Ahsorptum of oxygen gas hy arable soils. Humboldt had already 

 observed, before the year 1793, that argillaceous soils, the lydian 

 stone, certain schists, and humus, deprived the air of its oxygen. 

 He had also observed that the sides of the large cavities dug in the 

 salt mines of Saltzburg, absorbed this gas, and thus rendered the 

 stagnant atmosphere of the virorkings irrespirable and incapable of 

 supporting combustion. Finally, this illustrious observer had satis 



