Pioneer Labourers 19 



work are oxygen and carbon dioxide, formerly called 

 carbonic acid. Rocks containing much iron are espe- 

 cially open to the attacks of the one, and those con- 

 taining lime, potash, soda, to the attacks of the other. 

 We are all familiar with the fact that iron and steel 

 become covered with rust if left exposed to the air. 

 Keys rust if left in their locks, and even polished 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 vapour 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 abovf the sur- 

 rounding 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. 



Now it would take a much harder blow to break a 

 piece of iron than it would to shatter a flint ; but if the 

 two were left exposed to the air, the metal would waste 

 away long before any impression was made on the stone. 



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 compact within, for nature's labourers do not 



