222 COMMON CHEMICAL PROCESSES 



a small portion of it. This part burns, giving off heat; 

 thus the parts right around it become heated until they 

 also burn; and in this way the whole mass is finally 

 heated and burned. 



We have seen that in order for any substance to burn 

 in air, it must be heated and constantly supplied with 

 oxygen. Clearly, then, a fire may be stopped by cooling 

 the burning mass or by cutting off the supply of oxygen 

 (or air). Water is commonly applied, and it serves both 

 purposes ; but water is not always the best thing to use. 

 Chemical fire extinguishers are of value when the fire is 



small ; they are usu- 

 ally devices for mak- 

 ing a large amount 

 of CO 2 on the spot 

 and CO 2 does not sup- 

 port combustion. The 

 FIG. 144 ,J[1L^ mos t effective way to 



stop a fire when first 

 started, is to cover it closely with rugs, clothing, earth, 

 flour, or any solid which does not easily burn; in this 

 way the air is kept away and the fire is " smothered." 



Experiment 138. Using a long or circular oil burner, turn the 

 wick up just above the metal and light it at one point. Note 

 the creeping of the flame along the wick as each part is heated 

 from the burning portions. 



Experiment 139. Try to set fire to small quantities each of 

 wood, alcohol, charcoal, sulphur, kerosene, phosphorus, hydrogen, 

 soft coal, etc. (To use alcohol and kerosene, pour a few drops on 

 a flat piece of wick.) Roughly compare the temperatures at 

 which these substances burn. Other things could be used. Be 

 careful with phosphorus, alcohol, kerosene, etc. 



