128 THE ART OF CULTURE. 



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-w^ater thus 

 impregnated permeates the porous limestone, which 

 forms the walls and roofs of the caverns, and dis- 

 solves in its passage as much carbonate of lime as 

 corresponds to the quantity of carbonic acid con- 

 tained 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 pro- 

 duction 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 sta- 

 lactites formed contain no trace of vegetable matter, 

 and no humic acid ; they are of a glistening white 

 or yellowish color, and in part transparent, like cal- 

 careous spar, and may be heated to redness without 

 becoming black. 



The subterranean vaults in the old castles near 

 the Rhine, the "Bergstrass," and Wetherau, are 

 constructed of sandstone, 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 experiments which 

 have been continued for a hundred or a thousand 

 years. Now, if water possessed the power of dis- 



