Pioneer Laborers 17 



with rust if left exposed to the air. Keys rust if left in 

 their locks, and even pohshed fire-irons often rust in the 

 summer, unless they are oiled or greased, and so protected 

 from the air. What happens in these cases is that the 

 oxygen, always present both in the air itself and in the 

 watery vapor floating in the air, lays hold of the metal 

 and combines with it to form a compound substance — an 

 oxide — which is looser and softer, and takes up more 

 room than the metal alone. At first the rust is a mere 

 reddish brown stain; but as the oxygen eats deeper and 

 deeper, and more and more oxide is formed, it swells up 

 unevenly above the surrounding surface, and feels rough 

 to the touch. It is so soft that it may be partly rubbed 

 off by the finger, and when the rust is cleaned away there 

 will be scars and indentations left, showing how much of 

 the metal has been removed. 



Very many rocks contain iron, as, for instance, the 

 slates, sandstones, granites, and basalts, some more, some 

 less, but hardly ever in a pure state. The basalt of the 

 Giant's Causeway contains so much iron that, on those 

 sides which are most exposed to the weather, it not only 

 looks rusty, but is also softer on the surface and less com- 

 pact within, for nature's laborers do not generally work 

 singly and alone, but in union one with the other, and the 

 great ally of oxygen is moisture. 



Let us take basalt as an example, and see how this 

 rock is crumbled into soil. In perfectly dry air, at the 

 ordinary temperature, oxygen is powerless to do even so 

 much as tarnish iron in the mass, though it would have 

 no difficulty in reducing it all to oxide — that is, rust — if 

 the same mass of iron were exposed to its action in the 

 form of powder. Fire-irons do not rust in winter, or 



