THE ART OF CULTURE. 



carbonic acid as it is formed. But 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 vegetables 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 per- 

 meates the porous limestone, which forms the walls and roofs of 

 the caverns, and dissolves in its passage as much carbonate of 

 lime as corresponds to the quantity of carbonic acid contained 

 in it. Water and the excess of carbonic acid evaporate from 

 this solution when it has reached the interior of the caverns, and 

 the limestone is deposited on the walls and roofs in crystalline 

 crusts of various forms. There are few spots on the earth where 

 so many circumstances favorable to the production of humate of 

 lime are combined, if the humus actually existed in the soil in 

 the form of humic acid. Decaying vegetable matter, water, and 

 lime in solution, are brought together, but the stalactites formed 

 contain no humic acid ; they are of a glistening white or yellow- 

 ish color, in part transparent, like calcareous spar, and may be 

 heated to redness without becoming black. 



The subterranean vaults in the old castles near the Rhine, in 

 the " Bergstrass," and in the Wetterau, are constructed of sand- 

 stone, granite, or basalt, and present appearances similar to the 

 limestone caverns. The roofs of these vaults, or cellars, are 

 covered externally to the thickness of several feet with vegetable 

 mould, which has been formed by the decay of plants. The rain 

 falling upon them, sinks through the earth, and dissolves the 

 mortar by means of the carbonic acid derived from the mould ; 

 and this solution evaporating in the interior of the vaults, covers 

 them with small thin stalactites, which are quite free from humic 

 acid. 



In such a filtering apparatus, built by the hand of Nature, we 

 have placed before us the result of experiments which have been 

 continued for hundreds or thousands of years. Now, if water 

 possessed the power of dissolving a hundred-thousandth part of 

 its own weight of humic acid or humate of lime, and if humlo 



