8 



INTRODUCTION 



Nitrogen 



Sulphur 

 Phosphorus 

 Calcium 

 etc 



Biology. Before going farther it is well to get a clear idea 

 of what the common chemical terms mean. 



1. Oxygen is a gas which makes up a large part of the air. 

 It is the element in the air which sustains life in animals 

 and plants. Without it they cannot live. When given 



an undue amount of it, 

 they develop at an ab- 

 normal rate. It forms 

 about seventy per cent 

 of the bodies of plants 

 and animals. 



The most striking 

 property of oxygen is 

 the ease with which it 

 unites with other sub- 

 stances. Practically all 

 cases of burning are 

 caused by oxygen unit- 

 ing with paper, wood, 

 coal, or some other material. If a piece of glowing 

 charcoal is placed in a jar of oxygen, it bursts into flame. 

 This is the test for oxygen. 



2. Carbon is, next to oxygen, one of the most important 

 elements in biolog}^. It is usually black and solid and is 

 best seen as the charred remains of any material that has 

 been overheated but not burned up, as when toast or meat 

 is " burned." Carbon forms about fourteen per cent of the 

 body of plants and animals. 



3. Hydrogen gas is the lightest of all substances. For 

 this reason it is used in balloons and Zeppelins. It forms 

 a little less than ten per cent of the body of plants and 

 animals. 



4. Nitrogen is a gas which — unlike oxygen and hydro- 

 gen — does not burn. It dilutes the oxygen of the air and 



Figure 5. — Diagram. 



Showing proportion of chemical ele- 

 ments in living things. 



