Prairies, Flats, and Barrens in Southern Illinois, 387 
humidity of other substances; those rich in humus, clay, or 
peroxyd of iron, rank highest in this respect. Tile drains in 
underdrained land of this kind frequently discharge water when 
the air is moist, but before any rain has fallen. This water has 
evidently been absorbed from the air by the soil, and this phe- 
nomenon proves conclusively the great importance of this 
absorbing power. 
Let us now enquire what becomes of all the water which the 
prairie soil takes up from the various sources. A portion of it 
sinks deeper into the substrata and finds an outlet in springs and 
creeks, but a large portion is retained by the formations nearest 
the surface, which become saturated with it, so that water can 
be obtained nearly all year round in shallow wells. Even where 
the main water level lies deeper, the underclay is generally of 
such a kind that it allows the water to percolate but slowly. 
The consequence is, that during that portion of the year when 
__ the evaporation is less powerful, in winter and spring, the soil is 
_. perfectly soaked with moisture, and the subsoil remains in that 
state till late in the season. This excess of moisture which must 
and then dissolved by carbonic and vegetable acids. It then 
its combina- 
the humidity disar pears , the soil is opened to the air, and the 
consequences of the wet spring are overbalanced by the natural 
Tichness of the soil. Later in the season these same prairies 
_ Am. Jour. Sc1.—Seconp Series, Vou. XXXVI, No. 108.—Nov., 1863. 
rs 
