VIII.] COMPOSITION OF NATURAL WATERS. 119 



two grains in a gallon of water. But, when water is charged 

 with carbonic acid gas, the carbonate of lime is dissolved 

 with facility ; and, since most spring-water contains this gas, 

 it is easy to understand how it can act with great effect upon 

 limestone rocks. It has been seen that carbonic acid gas 

 is dissolved out of the atmosphere by rain-water ; and, in 

 like manner, every piece of water exposed to the air must 

 absorb it. Hence, all natural waters can dissolve carbonate 

 of lime, with more or less ease, and thus erode the lime- 

 stone rocks through which they drain. 



When such calcareous waters are used for domestic pur- 

 poses, they are found to curdle soap, and are consequently 

 termed /^^z/v/ waters. A portion of the soap is wasted, inas- 

 much as its fatty acids form insoluble salts with the lime. 

 As long therefore as this curdling continues, the soap is being 

 wasted, and a lather cannot be produced. It is clearly con- 

 venient to have some means of comparing the relative hard- 

 ness of different waters. To this end, Dr. T. Clarke, many 

 years ago, proposed a scale in which each degree corresponds 

 to one grain of carbonate of lime in a gallon of water. 

 According to this scale, the water of the Thames Head 

 Well has a hardness of 23 degrees ; that is to say, it 

 contains salts of lime equivalent to 23 grains of carbonate 

 to the gallon.' But it is possible to improve the condition 

 of such hard water by an easy process of softening, intro- 

 duced likewise by Dr. Clarke. This consists in simply 

 adding lime-water to the water the hardness of which is to 

 be corrected ; the lime combines with the excess of carbonic 



' The imperial gallon contains 70,(Xio grains. In many official reports 

 the proportion of carbonate of lime is given, not as grains per gallon, 

 but as so many grains in 100,000 grains of water. The conversion 

 of one form of result into the other is of course a mere question of 

 proportion. 



