COMBUSTION AND OXIDATION 77 



the gas is a lighted candle. One should not enter caves, wells, 

 or cisterns in which a candle will not burn. Most fire extin- 

 guishers are made of substances which liberate a large amount 

 of carbon dioxide when necessary. A common form contains 

 a quantity of baking soda dissolved in water with some means 

 of adding acid when the extinguisher is to be used. The acid 

 combining with the baking soda and water generates so much 

 gas that the pressure forces both gas and water out upon the 

 fire. 



FIG. 26. Carbon dioxide generated in test- FIG. 27. Bunsen burner, 

 tube with limestone and dilute hydrochloric 

 acid. 



74. The Bunsen Burner. One of the most useful pieces of 

 apparatus in the chemical and physical laboratory is the bun- 

 sen burner, which is simply a device for securing a hot flame, 

 and is based on the fact that the more thoroughly oxygen is 

 mixed with a combustible substance the more rapidly it will 

 burn. A common form consists of an upright tube connected 

 with the gas supply and having an opening in the base for the 

 admission of air. When the gas is turned on, its passage 

 through the tube draws air in at the base and this, mixing 



