ORIGIN' AND FOKSIATIOX OF SOILS. 127 



Water acts chemically upon rocks, or rather upon their 

 constituent minerals, in two ways, viz., hy 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 pyioxene, are minerals whicli have been artificially 

 produced in the slags or linings of smelting furnaces, and 

 thus formed they have been fotuid totally destitute of wa- 

 ter, as might be expected from the high temperature in 

 wliich they originated. Yet these minerals as occurrino- 

 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 Math water, chloride of calcium, Avhich is 

 very soluble, being separated from the nearly insoluble 

 phosphate of lime. The minerals Avhich compose silicious 

 rocks are all acted on j^erceptibly by pure water. This is 

 readily observed when the minerals are employed in the 

 state of fine powder. If pulverized feldspar, amphibole, 

 etc., 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 



