HOW NITROGEN IS PREPARED . 53 



4. Why do we use, first, paper, then wood, and then coal in starting 

 a coal fire? 



5. Why are oxides used as fire-proofing materials? 



6. Compare the composition of the air that enters at the bottom of 

 a stove with that which passes out into the flue. 



7. Why does a kerosene lamp need a wick? Why a chimney? 



8. Why does a blanket or rug put out a fire? Why does water? 



9. When a gas is collected over water, why does the gas force the 

 water out of the .bottle? 



54. How Nitrogen is Prepared. One method of pre- 

 paring nitrogen is to remove the oxygen from air. We 

 may remove the oxygen, as Lavoisier did, by means of 

 heated mercury; but an easier 

 way is to burn phosphorus in a 

 bottle or jar of air (Fig. 47). 



A small heap of red phosphorus is 

 placed in the middle of a thin slice cut 

 from a cork about three fourths of an 

 inch in diameter. A wire pushed into 

 the cork supports ik To hold the wire 

 upright, we stick it into a rubber stop- 

 per, placed, large end downward, on the bottom of a pan of water. 

 The water is about two inches deep. The phosphorus is lighted, and 

 the jar of air (a fruit jar does very well) is immediately placed over 

 the burning phosphorus, and pressed tightly against the bottom of 

 the pan. After the phosphorus has burned for a moment, the hand 

 may be removed. Water rises into the jar to take the place of the 

 oxygen used up. 



The white smoke is phosphorus oxide. The jar is left until the 

 smoke has entirely disappeared (it dissolves. in the water); then a piece 

 of glass or cardboard is slipped under the jar, and the jar is set upright 

 on the table. A burning splinter "goes out" when put into the jar. 

 If too many bubbles did not escape when the jar was put over the 



