PURE AIR 99 



and less than one-hundredth is watery vapor and car- 

 bonic acid gas. Any one who has studied chemistry 

 will remember that water is formed of one part 

 oxygen and two parts of another gas called hydro- 

 gen. Nitrogen, oxygen, and carbonic acid gas do not 

 unite to form air, as do hydrogen and oxygen to form 

 water. They simply mix or diffuse, each keeping its 

 own characteristics. As a rule, in the free air out-of- 

 doors these components will be found mixed in the 

 proportion given above, about four-fifths nitrogen, 

 about one-fifth oxygen, and less than one-hundredth 

 vapor and carbonic acid gas. 



We found it useful to learn about the components 

 of food. Let us see whether a knowledge of these 

 mysterious air components is not equally valuable. 

 As oxygen is the most useful of all, we shall get ac- 

 quainted with it first. Some boys and girls may 

 think oxygen a very difficult acquaintance to make, 

 and may imagine a trip in a balloon or a long journey 

 in a flying machine really necessary to know it. But 

 what a mistaken idea ! This remarkable friend is not 

 distant or hard to reach. On the contrary, it is in, 

 about, and all around us. About five-sevenths of 

 our bodies, a little less than one-half of the earth's 

 crust, eight-ninths of the weight of the earth's water, 

 and, as we have already learned, nearly one-fifth of 

 its air are made up of oxygen. While there are many 

 interesting things about this useful element in the air, 

 the most important to know, at present, is that it 



