128 HOW CROPS FEED. 



upon, whereas the same glass * finely pulverized is attack- 

 ed by "water so readily as to give at once a solution alka- 

 line to the taste. Messrs. W. B. and R. E. Rogers {Am. 

 Jour. Sci.,Y, 4:0-1, 1848) found that by continued digestion 

 of pure water for a week, with powdered feldspar, horn- 

 blende, chlorite, serpentine, and natrolite,t these minerals 

 yielded to the solvent fiora 0.4 to 1 per cent of their 

 weiglit. 



In nature we never deal witli pure water, but witli wa- 

 ter holding in solution various matters, either derived 

 from the air or from the soil. These substances modify, 

 and in most cases enhance, the solvent power of water. 



Action of Carbonic Acid. — This gaseous substance is 

 absorbed by or dissolved in all natural waters to a greater 

 or less extent. At common temperatures and pressure 

 water is capable of taking up its own bulk of the gas. 

 At lower temperatures, and under increased pressure, the 

 quantity dissolved is much greater. Carbonated water, 

 as we may designate this solution, has a high solvent 

 power on the carbonates of lime, magnesia, pi'otoxide of 

 iron, and protoxide of manganese. The salts just named 

 are as good as insoluble in pure water, but they exist in 

 considerable quantities in most natural waters. The 

 spring and well waters of limestone regions are hard on 

 account of their content of carbonate of lime. Chalyb- 

 eate waters are those which hold carbonate of iron in 

 solution. When carbonated water comes in contact with 

 silicious minerals, these are decomposed much more rapidly 

 than by pure water. The lime, magnesia, and iron they 

 contain, are partially removed in the form of carbonates. 



Struve exposed powdered phonolite (a rock composed 

 of feldspar and zeolites) to water saturated with carbonic 



* Glass is a silicate of potash or soda, 

 t Musotj'pe. 



