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. Si., V, 404, 1848) found that by continued digestion 
of pure water for a week, with powdered feldspar, horn- 
blende, chlorite, serpentine, and natrolite,+ these minerals 
yielded to the solvent from 0.4 to 1 per cent of their 
weight. 
In nature we never deal with pure water, but with 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, protoxide 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- 
cate 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, 
+ Mesotype. 
e 
