ORIGIN AND FORMATION OF SOILS. 127 
Water acts chemically upon rocks, or rather upon their 
constituent minerals, in two ways, viz., by Combination 
and Solution. 
Hydration.—By chemically uniting itself to the mineral 
or to some ingredient of the mineral, there is formed in 
many instances a new compound, which, by being softer 
and more bulky than the original substance, is the first 
step towards further change. Mica, feldspar, amphibole, 
and pyroxene, are minerals which have been artificially 
produced in the slags or linings of smelting furnaces, and 
thus formed they have been found totally destitute of wa- 
ter, as might be expected from the high temperature in 
which they originated. Yet these minerals as occurring 
in nature, even when broken out of blocks of apparently 
unaltered rock, and especially when they have been di- 
rectly exposed to the weather, often, if not always, con- 
tain a small amount of water, in chemical combination 
(water of hydration). 
Solution.—As a solvent, water exercises the most im- 
portant influence in disintegrating minerals. Apatite, 
when containing much chlorine, is gradually decomposed 
by treatment with water, chloride of calcium, which is 
very soluble, being separated from the nearly insoluble 
phosphate of lime. The minerals which compose silicious 
rocks are all acted on perceptibly by pure water. This is 
‘readily observed when the minerals are employed in the 
state of fine powder. If pulverized feldspar, amphibole, 
ete., are simply moistened with pure water, the latter at 
once dissolves a trace of alkali, as shown by its turning 
red litmus-paper blue. This solvent action is so slight 
upon a smooth mass of the mineral as hardly to be per- 
ceptible, because the action is limited by the extent of 
surface. Pulverization, which increases the surface enor- 
mously, increases the solvent effect in a similar proportion. 
A glass vessel may have water boiled in it for hours with- 
out its luster being dimmed or its surface materially acted 
