No WATER QUITE PURE. 33 



air or on the earth. It is almost pure, indeed, when 

 drawn up by the sun from the ocean, but, the moment 

 it falls to the earth again it begins to gather up various 

 impurities, according to the nature of the soil over which 

 it passes; and even before it touches the ground, during 

 its passage through the air, it gathers up, not only dust, 

 but gases, of which the most important to us just now is 

 carbonic acid, or carbon dioxide, as it is called by 

 modern chemists. 



The quantity of carbonic acid in the air is relatively 

 small; but there are from twenty to a hundred times as 

 much in the soil ; it is poured forth in large quantities 

 from cracks in the earth in ' volcanic districts, and is 

 almost universal at great depths, so that, from one source 

 and another, all rain, spring, and river water is more or 

 less impregnated with it, and its solvent powers are there- 

 by much increased.* 



Iron oxide and various other oxides, which are called 

 insoluble because they may be kept a long time in pure 

 water without any perceptible alteration, are readily dis- 

 solved by water containing carbonic acid; but the gas 

 is also quite capable of working on its own account, 

 without the assistance of oxygen. It is the especial 

 enemy of all rocks containing felspar, as this mineral is 

 composed partly of potash or soda, together with lime 

 and magnesia, and with all these carbonic acid readily 

 unites to form carbonates, which are then easily dissolved 

 and washed away by the rain. 



* Ordinary springs and rivers contain less than one per cent. ; but water 

 from the Saratoga springs contains 231 cubic inches to the gallon. 

 D 



