TO PREPARE OXYGEN 



49 



while others, like charcoal, merely glow. The explanation 

 is simple. A flame is a burning gas. Substances that do 

 not give off gases when burning do not 

 have flames. In fact, coke and charcoal 

 are the material that is left when the 

 gaseous part of coal and wood, respec- 

 tively, is driven off by heat (cf. 117 

 and 124). 



FIG. 41. 



Burning in a Coal 

 Stove. 



When a candle burns (Fig. 42), a little of the 



wick is consumed in melting the wax; then the 



melted wax is drawn, by capillary action (cf. 32), 



up the wick into the flame. The heat of the burn- 

 ing wick changes the liquid wax into a gas, or vapor, and 

 this burns with a flame, producing heat and light. The 

 flame exists only in the region in which oxidation is 

 taking place. The materials burning in the flame are 

 constantly changing, but they are constantly renewed; 

 hence the flame has a somewhat definite shape and size. 



FIG. 42. 



Air currents 



move toward a 



flame. 



50. To Prepare Oxygen. Since oxygen is 

 the active element of the air, we cannot pre- 

 pare it by simply removing the nitrogen. We 

 can, however, capture the oxygen by means 

 of some such substance as heated mercury (cf. 47). 

 By heating the mercury oxide that is formed to a higher 

 temperature we can get the oxygen by itself. 



Mercury and oxygen (at about 350 C.) give mercury oxide. 

 Mercury oxide (at about 380 C.) gives mercury and oxygen. 



This is what Lavoisier actually did. 

 In the laboratory, oxygen is generally made by heating 

 potassium chlorate. This is a white solid which melts 



