CHAPTER VI. 



THE ATMOSPHERE. 



Every one is familiar with the common phenomenon of 

 a piece of metal being eaten away by rust. A plate of 

 polished iron or steel, for example, exposed to a moist 

 atmosphere, soon loses its brilliancy, gradually becoming 

 coated with a dull reddish-brown rust ; and this process of 

 rusting, once set up, may go on until every particle of the 

 original metal has disappeared. But let the same piece of 

 bright metal be preserved in a vessel of pure water so as to 

 avoid contact with air, and it may retain its lustre unimpaired 

 for many years ; thus suggesting that the air must be directly 

 or indirectly connected with the phenomenon of rusting. 

 It is easy to show, indeed, that many metals rapidly rust or 

 tarnish when exposed to even the driest air. Cut a piece of 

 lead or of zinc, and observe the lustre of its fresh surface ; it 

 is, in fact, almost as brilliant as a piece of polished silver, but 

 this brilliancy is rapidly lost and the surface soon bedimmed 

 on exposure to the atmosphere. On the other hand, there are 

 many metals, such as gold, which never exhibit rust or 

 tarnish, however long they may be exposed. Other metals, 

 again, although they do not rust at ordinary temperatures, 

 may be caused to rust more or less rapidly when exposed to 



