18 ENVIRONMENT OF LIVING THINGS 



space occupied by the air in the jar. When the water reaches 

 this height, it goes no higher, and, no matter how many times 

 the experiment is repeated, the phosphorus invariably goes out 

 when the water displaces one fifth of the air in the jar. 



Evidently, the burning of the phosphorus uses up some gas within 

 the jar, which supports the flame, and the gas which remains in the 

 jar, occupying about four fifths of the space, does not have the power 

 of maintaining the flame. The former gas is called oxygen; the 

 latter, nitrogen. These two gases form the principal constituents 

 of the air in the proportion seen in the experiment. 



Chemical Elements. All the materials of this universe, both liv- 

 ing and lifeless, are classified by chemists as either chemical elements 

 or chemical compounds. A chemical element is a substance which 

 has never been decomposed into anything simpler in composition. 

 Examples of such elements are oxygen, making up about one fifth 

 of the atmosphere; nitrogen, composing nearly all the remainder of 

 pure air; carbon, an element that enters into the composition of all 

 organic matter; and over seventy others of more or less importance 

 to us in the study of biology. 



Nitrogen. The physical properties (those which we determine 

 through our senses) of nitrogen are its lack of color, taste, and odor. 

 Its chief chemical characteristics are its inability to support com- 

 bustion and its slight tendency to combine with other substances. 

 We shall later find that nitrogen is one of the most important chem- 

 ical elements found in living matter. In spite of this, animals and 

 most plants are absolutely unable to take any nitrogen from the 

 air, no matter how much they may need it. 



The other element in the air, oxygen, is taken out by the plants 

 and animals. We shall be able to see how, after studying the prop- 

 erties of oxygen. 



Preparation of Oxygen. Elements and Compounds. Oxygen 

 may be prepared by heating half a teaspoonful of chlorate of 

 potash with a little less than its bulk of black oxide of man- 

 ganese in a test tube over a Bunsen flame or a spirit lamp. 

 After a moment a glowing match inserted in the mouth of a test 

 tube bursts into a bright flame. Evidently the match burns more 

 brightly because of the presence of a gas which has been loosed 

 from the materials in the test tube. These materials are chemical 



