THE AERIAL OCEAN IN WHICH WE LIVE. 55 
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 surrounding 
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 
oxygen 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 car- 
bonic 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. Lastly, 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 
