100 A YEAR IN SCIENCE 



air ships because of its lifting power. Hydrogen is 

 also important as a fuel. A number of mixtures of 

 combustible gases, consisting largely of carbon com- 

 pounds and hydrogen, are used extensively for the 

 production of light and heat. Those chiefly used are 

 coal gas, water gas, natural gas, and acetylene. 



Questions 



1. Is hydrogen found in the free state in nature? 



2. How may pure hydrogen be obtained? 



3. When generating hydrogen, why should the 

 first gas always be allowed to escape from the end of 

 the tube? 



4. What are the chief properties of hydrogen ? 



5. What use is made of the fact : (1) that hydrogen 

 is a very light gas, (2) that it unites energetically with 

 oxygen, giving out much heat, (3) that it is a con- 

 stituent of oil and natural gas, (4) that it is very hard 

 to liquefy it? 



