228 CHEMISTRY. 



disappearance of a part of the precipitate. If now you let 

 the liquid stand for a time exposed to the air, it becomes 

 turbid again, because the carbonic anhydride escapes, 

 which takes from the water its power of keeping the car- 

 bonate in solution, and this salt is therefore again pre- 

 cipitated. And here we have a key to the explanation 

 of some very interesting phenomena. Water, as it makes 

 its way among the particles of the soil, finds carbonic acid 

 as one of the results of decay, and dissolves it ; and there- 

 fore, as it issues from the earth in springs, it contains not 

 only carbonic acid, but also carbonate of lime, which it 

 has found in the soil and dissolved by the aid of the acid. 

 But as soon as the water is fairly exposed to the air the 

 carbonic anhydride begins to escape from it, and ac- 

 cordingly the carbonate of lime begins to be deposited. 

 Hence comes the grand difference between the hard water 

 of springs and wells, and the soft water that runs in brooks 

 and rivers. The water as it runs along exposed to the air 

 has discharged much of its carbonic acid upward, and 

 therefore precipitated much of its carbonate of lime down- 

 ward. Water can be more thoroughly freed of its car- 

 bonic acid, and therefore of its carbonate of lime, by boil- 

 ing it than by 'mere exposure to the air, which explains 

 the considerable deposition of this salt in large steam-boil- 

 ers when hard water is used, collecting gradually as a hard 

 crust. Such incrustations are of course particularly apt 

 to occur in limestone districts. 



317. Stalactites and Stalagmites. The roofs of caverns 

 in limestone regions often have stalactites of carbonate of 

 lime suspended from them like icicles in shape. The rea- 

 son is obvious. The water, as it percolates through the 

 soil above the cavern, becomes charged with carbonic acid 

 from decaying vegetable matter, and therefore dissolves 

 some of the limestone ; and then., as it is exposed to the air 



