36 THE WORLD'S LUMBER ROOM. 



Most spring and river water contains some of this dis- 

 solved double carbonate ; when boiled, half the carbonic 

 acid is driven off, and the single, insoluble carbonate is 

 deposited in kettles or boilers, forming the crust which we 

 call "fur." 



Boiled water is "flat," however, from the loss of 

 the dissolved gases, and it may be as effectually softened 

 in another way. The Canterbury water, being derived 

 from the chalk, naturally contains a great deal of the 

 dissolved bi-carbonate, or double carbonate, and though 

 very clear is very hard. It is softened by the addition of 

 lime (calcium oxide), each atom of which takes away one 

 atom of carbonic acid from the dissolved bi-carbonate, 

 thus converting both the latter and itself into the insoluble 

 single carbonate, which makes the water milky at first, 

 but gradually sinks to the bottom and leaves it quite 

 clear. 



But enough has been said to show the strong attraction 

 existing between lime and carbonic acid j and there is 

 no difficulty in understanding how it is that the Portland 

 stone of which St. Paul's Cathedral was built not two 

 hundred years ago is already beginning to moulder, and 

 how tombstones of Italian marble lose their polish in a 

 year or two, and in the course of sixteen years are often 

 so roughened that the grains of carbonate of lime may 

 be rubbed off with the finger. 



From these examples we may also form some idea of 

 the way in which carbonic acid works away at the rocks ; 

 but on this head there will be more to say hereafter. 



