ORIGIN OF SOILS 57 



of all sizes of particles together. It is said that the Rhone, which 

 is fed chiefly from the glaciers of the Alps, carries such a volume 

 of rock dust that its muddy waters may be traced six or seven 

 miles after they have entered the Mediterranean. The action of 

 glaciers is mechanical ; the rock is ground up, but not decomposed. 



Chemical Action of Water and Air. Water acts chemically 

 upon rock-minerals by solution and by hydration. Rain, in pass- 

 ing through the air, dissolves oxygen, carbon dioxide, and other 

 substances. In the soil the water absorbs acids formed by the 

 decay of vegetable matter. These substances aid in its weather- 

 ing action. 



Hydration is a chemical change in which the mineral combines 

 with water. Some minerals take up water, increase in bulk, and 

 fall to a powder. Hydrated silicates are formed from various 

 silicates. 



Solution. There are very few minerals which do not give up a 

 portion of their constituents to water, though the amount of 

 material which goes into solution is usually very small. If 

 pulverized felspar, amphibole, etc., are moistened with pure water, 

 the latter at once dissolves a trace of alkali from the mineral, as 

 shown by its turning red litmus blue. This solvent action is slight 

 on a smooth mass of the material, being limited by the extent of 

 surface. Pulverization, which increases the surface, increases 

 the solvent effect considerably. 



This solution involves a chemical change, new bodies with new 

 properties being formed. Carbon dioxide and oxygen aid the 

 action greatly. For example, potash felspar is decomposed by 

 water with the formation of potassium silicate and aluminium 

 silicate. In the presence of carbon dioxide, potassium carbonate 

 is produced and hydrated silica set free, the quantity depending 

 upon the amount of carbon dioxide present. A lime felspar is 

 decomposed in the same way, and the calcium carbonate dissolved 

 by aid of the carbon dioxide the water contains. Silicates of iron 

 are decomposed with the production of hydrated oxides of iron 

 and silicic acid. If the silicate is a ferrous silicate, the iron is 

 oxidized by the oxygen in the water. 

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