70 THE FAIRY-LAND OF SCIENCE. 
interesting ; but if, when you read about the trade and 
other winds, you will always picture to yourselves 
warm air made light by heat rising up into space and 
cold air expanding and rushing in to fill its place, I 
can promise you that you will not find the study of 
aerial currents so dry as many people imagine it 
to be. 
We are now able to form some picture of our aerial 
ocean. We can imagine the active atoms of oxygen 
floating in the sluggish nitrogen, and being used up in 
every candle-flame, gas-jet and fire, and in the breath 
of all living beings ; and coming out again tied fast to 
atoms of carbon and making carbonic acid. Then we 
can turn to trees and plants, and see them tearing these 
two apart again, holding the carbon fast and sending 
the invisible atoms of oxygen bounding back again 
into the air, ready to recommence work. We can 
picture all these air-atoms, whether of oxygen or 
nitrogen, packed close together on the surface of the 
earth, and lying gradually further and further apart, as 
they have less weight above them, till they become so 
scattered that we can only detect them as they rub 
against the flying meteors which flash into light. We 
can feel this great weight of air pressing the limpet on 
to the rock ; and we can see it pressing up the mercury 
in the barometer and so enabling us to measure its 
weight. Lastly, every breath of wind that blows past 
us tells us how this aerial ocean is always moving to 
and fro on the face of the earth ; and if we think for a 
moment how much bad air and bad matter it must 
carry away, as it goes from crowded cities to be 
