ATMOSPIJERE 25 



gen, neither of which resembles water. All compounds 

 can be divided into two or more elements. 



23. Mixtures. — Two or more elements, or compounds, 

 may be mixed and still retain all their individual prop- 

 erties. When this takes place the result is a mixture. 

 Salt is mixed with flour to make bread, and in the mix- 

 ture we cannot see any salt, but we can taste it and 

 know from its taste that the salt is unchanged. 



r, air is a mixture of several elements and com- 

 pounds. Let us see what these various substances are 

 that go to make up air. 



24. Oxygen. — This element is a gas, and makes up 

 nearly 23 per cent by weight of the air. It is the most 

 abundant substance on earth, making up eight-ninths 

 of the weight of water and about 50 per cent of the 

 weight of Band. About one-third of the weight of soil 

 and rocks is made up of oxygen, and all plants and ani- 

 mals contain large quantities of the gas. Oxygen n a 



re gas, always ready to combine with other 

 rabstances, which, when the union takes place, are said 

 come ottdtctd. When substances combine with 

 Hon produi ee beat Thus the carbon in 

 coal when it bums onitea with oxygen to form a gas 

 called carbon dioxide, and gives oiT much heat Com 

 billed with another ana called hydrogen* oxygen forms 

 a liquid which we know as water. Combined with iron, 

 it forms a solid called iron ore. In plants and animals 

 it occurs in combination as water, and also combined 

 with some other substances to form solids. We breathe 

 oxygen in the air, we drink oxygen in water, and we eat 



