58 THE FAIRY-LAND OF SCIENCE. 
lights would burn furiously. In fact, a flame once 
lighted would spread so rapidly that no power on earth 
could stop it, and everything would be destroyed. So 
the lazy nitrogen is very useful in keeping the oxygen- 
atoms apart; and we have time, even when a fire is 
very large and powerful, to put it out before it has 
drawn in more and more oxygen from the surround- 
ing air. Often, if you can shut a fire into a closed 
space, as in a closely-shut room or the hold of a ship, 
it will go out, because it has used up all the oxygen in 
the air. 
So, you see, we shall be right in picturing this in- 
visible air all around us as a mixture of two gases. 
But when we examine ordinary air very carefully, we 
find small quantities of other gases in it, besides oxy- 
gen and nitrogen. First, there is carbonic-acid gas. 
This is the bad gas which we give out of our mouths 
after we have burnt up the oxygen with the carbon 
of our bodies inside our lungs ; and this carbonic acid 
is also given out from everything that burns. If only 
animals lived in the world, this gas would soon poison 
the air ; but plants get hold of it, and in the sunshine 
they break it up again, as we shall see in Lecture VII, 
and use up the carbon, throwing the oxygen back 
into the air for us to use. Secondly, there are very 
small quantities in the air of ammonia, or the gas which 
almost chokes you in smelling-salts, and which, when 
liquid, is commonly called " spirits of hartshorn." 
This ammonia is useful to plants, as we shall see by 
and by. Again, there is a great deal of water in the 
air, floating about as invisible vapour or water-dust, 
and this we shall speak of in the next lecture. Lastly, 
