CHAPTER III 



AIR 



THE air differs from many of the other things 

 we shall have to consider, in one striking way. 

 We cannot see it ; it is invisible. We can see 

 the effects of its movements when the branches 

 of the trees are swayed to and fro, or when the 

 draught in a house makes a door slam or a 

 chimney smoke. Now we are so much accus- 

 tomed to getting our information through our 

 eyes, that we are apt to forget that other senses 

 will sometimes serve us quite as well. 



On a still day out of doors, or in a quiet 

 room, it is perhaps difficult to believe that we 

 are surrounded with air ; there does not appear 

 to be anything. There is, as we said before, 

 nothing to see. Yet we are just as much 

 covered up by the air as a fish is by the water 

 it swims in. The air is always there, on every 

 side of us, whether in-doors or out-of-doors, 

 both by night and by day. Yet, however still 

 the air around us, there is one fact which 

 ought to tell us it exists. We all breathe. We 

 are all constantly taking something into our 

 chests and sending it out again, only to take in 



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