USE OF THE HUMUS. 1 \7 



ture. For if humus possessed even a smaller de- 

 gree of solubility, than that ascribed to the sub- 

 stance called humic acid, it must be dissolved by 

 rain-water. Thus, the yearly irrigation of mea- 

 dows (see note at page 105), which lasts for several 

 weeks, would remove a great part of it from the 

 ground, and a heavy and continued rain would 

 impoverish a soil. But it is soluble only when 

 combined with oxygen ; it can be taken up by 

 water, therefore, only as carbonic acid. 



When kept in a dry place, humus may be pre- 

 served for centuries, but when moistened with 

 water, it converts the surrounding oxygen into 

 carbonic acid. As soon as the action of the air 

 ceases, that is, as soon as it is deprived of oxygen, 

 the humus suffers no further change. Its decay 

 proceeds only when plants grow in the soil con- 

 taining it ; for they absorb by their roots the car- 

 bonic acid as it is formed. The soil receives again 

 from living plants the carbonaceous matter it thus 

 loses, so that the proportion of humus in it does 

 not decrease. 



The stalactitic caverns in Franconia, and those in 

 the vicinity of Baireuth, and Streitberg, lie beneath 

 a fertile arable soil ; the abundant decaying vege- 

 tables or humus in this soil, being acted on by 

 moisture and air, constantly evolve carbonic acid, 

 which is dissolved by the rain. The rain-water 

 thus impregnated permeates the porous limestone, 

 which forms the walls and roofs of the caverns, and 



