ABOUT COMMON WATER. 837 



approach towards each other; from the little spear, 

 beginning like a small icicle in the roof, and the little 

 mound of stalagmite on the floor, exactly underneath, 

 up to the actual contact of both. The pillars and spears, 

 the arches and corridors, the fantastic stone drapery, 

 the fretted figures on the walls all contribute to pro- 

 duce an effect of extraordinary magnificence. 



What is the cause of the wonderful architecture and 

 decoration of St. Michael's Cave ? Probably some of 

 my clever readers will have anticipated both this ques- 

 tion and its answer. The rain, charged with its modi- 

 cum of carbonic acid by the air, falls upon the limestone 

 rock overhead, percolates through it, dissolves it, and, 

 thus laden, reaches the roof of the cave. Here it is 

 exposed to evaporation. The dissolved solid is, in part, 

 deposited, and the base of the stalactite is planted 

 against the roof. The charged water continues to drip, 

 and the stalactite to lengthen. Escaping from the 

 point of the stalactite, the drop falls upon the floor, 

 where evaporation continues, and mineral matter is de- 

 posited. The stalagmite rises ; the mound becomes a 

 pillar, towards which the spear overhead accurately 

 points, until, in course of time, they unite to form a 

 column. 



A similar process goes on over the fretted walls. They 

 shine with the water passing over them. Each water- 

 film deposits its infinitesimal load, the quantity de- 

 posited here and there depending on the inequalities of 

 the surface, which cause the water to linger longer, and 

 to deposit more at some places than at others. 



The substance most concerned in the production 

 of all this beauty is called by chemists carbonate of 

 lime. It is formed by the union of carbonic acid and 

 lime. What lime is, of course, you already know ; its 

 companion, carbonic acid, is, at ordinary temperatures, 

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