A COMMERCIAL USE OF CARBON DIOXIDE 55 



retain the carbon, which becomes a part of the plant structure. 

 Plants thus serve to keep the atmosphere free from an ex- 

 cess of carbon dioxide and, in addition, furnish oxygen to 

 the atmosphere. 



49. How to Obtain Carbon Dioxide. There are several 

 ways in which carbon dioxide can be produced commer- 

 cially, but for laboratory use the simplest is to mix in a 

 test tube powdered marble, or chalk, and hydrochloric acid, 

 and to collect the effervescing gas as shown in Figure 24. 

 The substance which re- 

 mains in the test tube 



after the gas has passed 

 off is a solution of a salt 

 and water. From a mix- 

 ture of hydrochloric acid 

 (HC1) and marble are ob- 

 tained a salt, water, and 

 carbon dioxide, the de- 

 sired gas. 



50. P^. Commercial Use FIG. 24. Making carbon dioxide from marble 



of Carbon Dioxide. If a 



lighted splinter is thrust into a test tube containing carbon di- 

 oxide, it is promptly extinguished, because carbon dioxide 

 cannot support combustion ; if a stream of carbon dioxide 

 and water falls upon a fire, it atts like a blanket, covering 

 the flames and extinguishing thern. The value of a fire 

 extinguisher depends upon the amount of carbon dioxide and 

 water which it can furnish. A fire extinguisher is a metal 

 case containing a solution of bicarbonate of soda, and a glass 

 vessel full of strong sulphuric acid. As long as the extin- 

 guisher is in an upright position, these substances are kept 

 separate, but when the extinguisher is inverted, the acid escapes 

 from the bottle, and mixes with the soda solution. The min- 



