INFLUENCE OF OXYGEN. 77 



position, as well as those which possess one, 

 are decomposed or dissolved by this means, with 

 comparative rapidity. 



Oxygen gas, forming a part of the air, also 

 acts in a powerful manner upon rocks of various 

 kinds. It does so chiefly when they contain 

 iron in their composition. Many times must 

 the reddish brown stains on the exposed surface 

 of various rocks have attracted notice. Such 

 stains generally indicate that the rocks contain 

 some compound of iron in their substance, and 

 show the influence of the air and water in de- 

 composing it, and so causing it to be removed 

 and washed away. This gas, uniting with the 

 iron, causes the particles of the rock to lose 

 their mutual cohesion, and consequently the 

 hard mass becomes cracked and softened, and 

 ultimately, after a sufficient lapse of time, is 

 actually reduced to a powdery condition. This 

 gas is, like carbonic acid, soluble in water, 

 although only to a very slight extent. We may 

 conceive, therefore, that in a dissolved state, as, 

 for example, in rain-water, it may have some 

 decomposing power over the rocks washed by the 

 winter shower, which, though trifling at a time, 

 may become important in its accumulated effects. 



Having thus alluded to the influence of the 

 most important agents employed in the che- 

 mistry of nature for the purpose of wearing down 



