118 THE ART OF CULTURE. 



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 lime- 

 stone is deposited on the walls and roofs in crystal- 

 line crusts of various forms. There are few spots 

 on the earth where so many circumstances favour- 

 able to the production of humate of lime are com- 

 bined, 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 

 trace, of vegetable matter, and no humic acid ; they 

 are of a glistening white or yellowish colour, and 

 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, the " Bergstrasse " and Wetherau, are 

 constructed of sandstone, granite, or basalt, and 

 present appearances similar to the limestone ca- 

 verns. 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 inte- 

 rior of the vaults, covers them with small thin sta- 

 lactites, which are quite free from humic acid. 



