I20 PHYSIOGRAPHY. [chap. 



acid to form the almost insoluble carbonate of lime ; which 

 is then precipitated, in company with the original carbonate 

 of lime, thus rendered insoluble by removal of the carbonic 

 acid that held it in solution. In this way, the hardness of 

 the water of Thames Head Well may be reduced from 23° 

 to 5°. Such a process of softening is carried on upon a 

 large scale at several water-works, as at Caterham and at 

 Canterbury. 



The hardness which is thus capable of correction is 

 termed temporary hardness, to distinguish it from that which 

 cannot be removed by treatment with lime, and which is 

 consequently termed permanent hardness. Such permanent 

 hardness is due to the presence of sulphate of lime. It may 

 therefore be said that the water of Thames Head has a hard- 

 ness of 23°, of which 1 8° represent temporary hardness and 

 5° permanent hardness. The upland waters of the West of 

 England usually contain much more sulphate than carbonate 

 of lime. Sulphate of lime occurs crystallized in nature, 

 and is known to the mineralogist under the rather fanciful 

 name of Selenite^ whence waters containing much sulphate 

 of lime are termed selenitic waters. If a water be described 

 simply as calcareous, it is generally assumed that the parti- 

 cular salt of lime which it holds in solution is the carbonate. 



Waters flowing through limestone districts are generally 

 charged with this salt ; in many cases to so great an extent 

 that, if the water be exposed to the air, the carbonate of 

 lime is spontaneously thrown down in a solid form. Such 

 springs are vulgarly caWtd petrifying springs. To "petrify," 

 however, means literally to turn into stone ; it should there- 

 fore be distinctly understood that all such springs are able 

 to do is to simply cover the objects which receive the water 

 with a crust of carbonate of lime, and not actually to con- 

 ^ Selenite, from aiKT\vT\, selene, the moon. 



