Pioneer Laborers 19 



already combined with some amount of oxygen, but it 

 does not acquire the reddish brown color of what we 

 familiarly call *'rust" until it has absorbed as much oxygen 

 as it can hold. In this condition it is, of course, heavier, 

 and as we have seen, softer than before, and is therefore 

 more easily washed or blown away from the surface. But 

 it is also more bulky, and takes up more space than it did 

 before, so that if it be formed inside the rock where it 

 has not room to expand, the rock is cracked by it. This, 

 of course, opens the way for more water and more oxygen 

 to enter, and so the work proceeds, and the decay goes 

 deeper and deeper. 



We have chosen iron-rust as a sample of the way in 

 which oxygen works because it is one of which we all 

 know something, but it must not be forgotten that this is 

 only one of many oxides formed in the rocks; and that 

 whenever oxygen combines with any other substance in a 

 rock to form an oxide, it makes that substance take up 

 more room than before, and so the rock is cracked and 

 crum.bled. The other gas, carbon dioxide, works in a 

 different way, though it also helps the oxygen to rust iron 

 faster than it could do alone. But when it works on its 

 own account it is by combining with such substances as 

 Hme, potash, soda, and magnesia, which it makes much 

 more soluble than they were before. 



Some rocks are said to be impervious, or ''water- 

 proof, ' ' but this only means that they allow water to enter 

 so very slowly that unless they are actually soaking in it 

 for some time hardly any is taken up. Water has some 

 effect upon every known mineral, unless it be perhaps 

 upon gold and platinum. But water in nature is never 

 perfectly pure; how can it be, since it dissolves some, 



