COMMON ELEMENTS OF THE EARTH 157 



9. A toy balloon has a capacity of 10,000 cu. cm. What weight 

 of air will it displace? 



10. Could the hydrogen balloon be weighed in the ordinary way 

 with platform scales or a spring balance? Why? 



11. Select and write the symbols of all the elements that you know 

 to be gases. 



12. Write the symbols of all the elements that you know to be metals. 



