BREATHING AND PURE AIR 3 



Now this ocean of air is made up of several kinds of 

 gases mixed together. Each is different from all the 

 others, though not one of them is the gas that lights our 

 houses. Indeed, the gas we use for lighting is different 

 from the gases of the air in three ways : 



1. It will kill us if we breathe much of it. 



2. It has a smell. 



3. It will burn. 



Still we find that air gases are like this deadly gas 

 in two ways: 



1. We cannot see them. 



2. We can feel them when they blow against 

 us, or when we run through them. 



If you blow on your hand, you feel the air even though 

 you cannot see it. So, too, when the wind blows, you do 

 not see it, but you feel it rushing past. 



There are men who can take a bottle full of air, sepa- 

 rate it into its different gases, and study each gas by 

 itself. Just now, however, you and I need to pay attention 

 to only two of them, oxygen and carbon dioxid. 



If I should put a mouse into a jar with a good deal 

 of oxygen in it, he would act so merry that you would 

 think he had never before in his life felt so happy. If 

 I should then fill another jar with carbon dioxid and 

 put the same small mouse into it, he would surely die 

 in a minute unless I pulled him out again as promptly 

 as possible. 



