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 ot 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 ? 



FroblemSS: 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 dehvery 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 wiU Uberate 

 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. 



