30 THE WORLD'S LUMBER ROOM. 



a knife to see how the surface has been roughened by 

 the eating away of the steel. 



Those who have possessed specimens of a mineral 

 popularly known as "fool's gold," which looks, as to 

 colour, something like tarnished silver, may have been 

 puzzled by finding the cabinet drawer strewn with an 

 ash-like powder, which constantly re-appeared at intervals 

 of only a few weeks. And when at last they have traced 

 it to the " fool's gold " and found that this was gradually 

 crumbling away, perhaps they were hardly less puzzled 

 than before. 



In spite of its name, it has nothing to do with gold, 

 being in fact a variety of iron pyrites, called marcasite, 

 which is a compound of iron and sulphur, both of which 

 attract oxygen. 



The oxygen first attacks the sulphur, with which, and 

 the moisture of the air, it forms sulphuric acid. This, in 

 , its turn, seizes on the iron, and as the water evaporates, 

 minute needle-like crystals of sulphate of iron are formed. 

 Some of the pyrites is converted into this " salt," as it is 

 chemically called, and more is broken up by the crystals 

 as they force their way to the surface. 



Zinc is able to resist the attacks of oxygen at all 

 ordinary temperatures, and it is therefore used in thin 

 sheets as a covering for iron, which, when thus protected, 

 is said to be galvanised. As long as the zinc remains 

 entire, the iron is quite safe, but the smallest hole is 

 sufficient to admit the air, and then, as the iron rusts, the 

 zinc is forced up, because, as was said just now, oxide of 

 iron takes up more space than the iron alone. 



