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SECOND BOOK. 
it can nourish the body; and more than that, it 
must itself become blood. 
10. “Some people call our blood ‘the life stream . 
Like a stream it is always flowing, and it carries to 
every part of the body the material that is needed 
for growth and repair, and brings away the waste. 
In short, the food is changed into blood, which in 
turn is changed into muscle, bone, and nerve.” 
THE AIR WE BREATHE. 
1. Without food we might live for a few days; 
but without air we should certainly not live many , 
minutes. It is well, then, for us to know what the 
air is, and what it contains that is so necessary to 
our lives. 
2. The air we breathe surrounds the earth to a 
height of many miles. It is a mixture of gases, 
of which the chief are nitrogen and oxygen. There 
is also argon , a gas which men who study such 
things have only lately found out; and there are 
small quantities of carbonic acid and other gases. 
Besides these there is water-gas, or vapour. 
3 . Oxygen is common everywhere. It forms 
about one-fifth of the air; with hydrogen gas it 
forms water; and it makes up about half the weight 
of the earth. 
4 . It is the oxygen of the air that causes fuel to 
