64 PLANT GROWTH AND NUTRITION 



Materials. Bit of phosphorus, dish, float, bell jar. 



Method. Place a bit of phosphorus on cork and float it in a 

 pan of water. Ignite the phosphorus and quickly invert a bell 

 jar over it. 



Observations. What happens to the phosphorus ? What 

 happens to the water in the pan ? 



NOTE. Air is composed principally of two elements, nitrogen (about 79 per 

 cent) and oxygen (about 20 per cent). When the phosphorus burns, it unites with 

 one of the elements and forms a substance which dissolves in water. (See p. 20, 

 Civic Biology.) 



Conclusion. Judged by the amount of air which is dis- 

 placed by water, which of the two gases of the air was used up ? 



Problem 38: To test for oxygen. (Demonstration.) 



NOTE. Certain tests may be made by which the presence of some of the gases 

 which compose the air may be isolated and studied. Pure oxygen, a colorless and 

 odorless gas, is known by its ability to support combustion. 



Materials. Oxone, potassium chlorate, black oxide of man- 

 ganese, Bunsen flame, test tube with cork and delivery tube, 

 wide-mouth bottle, large dish. 



Method. Heat a little potassium chlorate in a test tube with 

 about the same amount of black oxide of manganese. Chemical 

 action takes place which results in the evolution of oxygen. This 

 may be collected by a delivery tube or used in the test tube. 

 Instead of this method, a patented substance known as oxone 

 may be used. A small piece of oxone placed in water will liberate 

 enough oxygen for several tests. The gas may be collected with 

 the aid of a delivery tube by displacing water from test tubes or 

 bottles. 



Observations. In a test tube containing oxygen plunge the 

 glowing end of a match. What happens to the glowing match? 

 What difference is there between the burning of the match in 

 air and in oxygen? 



Place a piece of red-hot iron wire in oxygen ; a piece of heated 

 magnesium wire. What happens in each case? 



NOTE. When oxygen combines with any substance, the process is called 

 oxidation. The substance with which the oxygen unites is said to be oxidized, and 

 heat is released as a result of the process. 



