PART I. LIVING THINGS IN RELATION TO 



THEIR ENVIRONMENT AND TO EACH 



OTHER 



UNIT I 



THE WORLD WE LIVE IN AND WHAT WE 

 TAKE FROM IT 



Preview. In our previous experience with science in the 

 elementary and junior high schools we have learned something 

 about our environment and what we get out of it. We know a 

 little about the air and how we use it, about water and how it 

 serves us, of fire, of the weather, and many other useful facts that 

 help us in our daily living. But now we are ready to learn some- 

 thing more about this environment from a different angle. 



Biologists realize more than ever before that living things are 

 dependent upon their environment and that they are composed 

 of many of the chemical substances that are found in that environ- 

 ment. Thus, our knowledge of biology depends upon an under- 

 standing of the chemists' and physicists' conception of the world 

 about us. This unit will help us to understand some of these 

 important facts about our surroundings. The physicist calls 

 anything that occupies space matter. The chemist in his turn 

 reduces all matter into over ninety simple substances called 

 chemical elements, substances that cannot be broken into more 

 simple substances. These elements are given symbols by the 

 chemist, such as O for oxygen, H for hydrogen, N for nitrogen, 

 and C for carbon. The soil and other things in nature are com- 

 posed largely of combinations of elements known as chemical 

 compounds. A few of these compounds, such as water, iron rust, 



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