THE SUBSTANCES OF THE EARTH. 25 



Explain the difference between combustible and incombustible matter. 

 Mention some common substances which are partly combustible. 

 Mention some which are entirely combustible. Name some that 

 are entirely incombustible. 



How much oxygen is there in the earth? What is an oxide? Ex- 

 plain the process of burning. What js heat ? Why does a fire burn 

 better in a draft of air ? Why has decayed wood lost a part of its 

 value for fuel ? Why do we seal fruit to preserve it ? How does 

 breathing keep the body warm ? 



What are the peculiarities of hydrogen ? Why is water produced by 

 burning hydrogen ? Where is nitrogen to be found ? Why is it 

 so difficult to obtain it for practical purposes ? 



Name some objects that contain carbon. What proportion of the 

 earth is silicon ? Name some forms in which it is found. Name 

 some of the uses of sulphur. Describe phosphorus. Name some 

 useful forms of chlorine. 



Describe potassium. Name some of the salts of potash. Mention 

 some useful forms of sodium AVhere is calcium found? How 

 may ma.gnesium be distinguished ? Name some forms of aluminium. 

 Where does iron exist? 



