FUELS AND CARBON COMPOUNDS 



269 



cellars, where it is sometimes called " choke damp." 

 Before entering such places it will be well to test for the 

 presence of carbon dioxide by lowering a lighted candle 

 into the cavity. 



Under a pressure of 735 pounds to the square inch the 

 gas may be liquefied at ordinary temperatures. The 

 liquefied carbon dioxide is kept in strong cylinders from 

 which it is taken as needed for use, as 

 in soda fountains, by means of a valve. 



Fermentation. -- The fresh juice of 

 certain fruits is sweet on account of the 

 presence of sugar. If grape juice is 

 heated and sealed up while hot, it will 

 remain sweet, but when allowed to stand 

 exposed to the air it ferments. Bubbles 

 of gas escape and the sweet taste is 

 replaced by the mild alcoholic taste of J[' 2 ^ ~^ D t h e 

 wine. The fermentation is caused by a Cross Section of a Fire 



i n j mu Extinguisher. 



plant called yeast. The spores or yeast 

 seeds fall into the juice and sprout, producing innumerable 

 yeast plants. As these plants grow they change the sugar 

 to alcohol and carbon dioxide. The gas may be tested 

 by passing it over limewater. 



When yeast is put in a mixture of flour, water, and 

 sugar (dough) , the bubbles of carbon dioxide . cause the 

 bread to rise. The action of the yeast is arrested by the 

 intense heat of baking. 



For raising certain pastries, an artificial source of 

 carbon dioxide is often used. This may be ordinary 

 baking soda (sodium carbonate) used with sour milk, 

 or baking powder. When baking soda and sour milk 

 are used, the lactic acid of milk reacts with the baking 

 soda in the same way that acid reacts with the marble, 



