70 CARBONIC ACID. 



unfortunately enter the places thus filled with car- 

 bonic acid. Its presence in such places in the air, in 

 any quantity, may always be easily ascertained, by 

 letting down a lighted candle into the well or cellar: 

 if the candle continues to burn, we know that there 

 is enough oxygen present to support the life of an 

 animal ; whilst if the candle is extinguished, we are 

 certain that the place is full of carbonic acid gas, and 

 therefore that it must not be entered until the heavy 

 gas has been dispersed by proper ventilation. 



121. Carbonic acid is also evolved in large quan- 

 tities during the process of fermentation, and occa- 

 sions the pricking taste and effervescence of cider, 

 bottled ale, and other liquors. Common ale is allowed 

 to ferment in open vessels, so that nearly all the car- 

 bonic acid formed during that process (424) is dis- 

 persed ; but bottled ale being confined in close vessels 

 before its fermentation is completed, much of the 

 carbonic acid evolved subsequently is pent up in the 

 liquor, and escapes from it in innumerable small 

 bubbles, when the cork of the bottle is removed. 



122. Carbonic acid, then, is constantly being form- 

 ed in several different ways ; it is produced during 

 the combustion of all substances containing carbon, 

 during the respiration of animals, during the decay 

 of almost all vegetable and animal substances, and 

 during the process of fermentation. It is likewise, 

 in many situations, naturally given out by the earth 

 in large quantities. 



123. To counterbalance all these sources of in- 



