EFFECTS OF OXYGEN. 29 



But another of Nature's most active dust-makers is the 

 air, which, though we breathe it with impunity, is yet a 

 deadly enemy to many of the rocks. 



This air is a mixture, not a compound, consisting chiefly 

 of oxygen and nitrogen, in the proportion of about twenty- 

 one parts of the former to seventy-nine of the latter, with a 

 very small quantity (-g-gVo) of carbonic acid, and a varying 

 amount of water-gas, or aqueous vapour. 



Minute quantities of everything capable of assuming the 

 gaseous form are also to be found in the atmosphere, but 

 these four together constitute its main bulk. The oxygen 

 and nitrogen are simple substances, and cannot be split up 

 into anything else, whereas each molecule of water-gas is a 

 compound, consisting of two atoms of hydrogen and one of 

 oxygen ; and each molecule of carbonic acid gas consists of 

 two atoms of oxygen and one of carbon. 



Now, oxygen will combine with every known elementary 

 substance but one, and foremost among those by which it 

 is particularly attracted stands iron, with which it unites to 

 form oxide of iron, or what we commonly call "rust." Not 

 an atom of the iron is lost, but the oxygen it has absorbed 

 has changed its colour, added to its weight and bulk, and 

 made it less compact than before. 



In perfectly dry air a mass of iron will remain untar- 

 nished, at the ordinary temperature, though if reduced 

 to powder it is so vigorously seized upon by the oxygen 

 as to take fire spontaneously and burn away to oxide. 

 But natural air is never free from moisture, and this fact 

 enables the oxygen to attack both iron and steel wherever 

 it finds them. We have only to rub away the rust from 



