68 A YEAR IN SCIENCE 



with the bottle upright; for, as the heavier gas enters, 

 the air is forced out. If a burning pine splinter is 

 inserted into a bottle of carbon dioxide it will be 

 extinguished immediately. It neither burns nor sup- 

 ports combustion. If lime water is poured into a bottle 

 containing carbon dioxide, and shaken, it at once turns 

 a milky color. This is the test for carbon dioxide and 

 is universally used. 



Balance of carbon dioxide maintained. From the 

 great supply of carbon dioxide constantly pouring into 

 the air it would seem that life on earth would soon be 

 endangered. However, carbon dioxide has its place 

 in nature as it provides some of the food for green 

 plants. In the process of food-making, green plants 

 take the carbon dioxide from the air and water from 

 the soil, and in the presence of sunlight convert them 

 into starch (or food for the plant). Oxygen is given 

 back to the air by the plant as a waste product from 

 this process. Thus the proper balance of carbon 

 dioxide in the air is maintained. 



Commercial uses of carbon dioxide. Carbon dioxide 

 is soluble in water, which at ordinary temperature 

 absorbs an amount about equal to its own volume. 

 Under pressure, however, it may be charged with many 

 times its own volume. This may be observed at any 

 ordinary soda fountain. Soda water is made by simply 

 charging water with carbon dioxide under pressure. 

 It is then kept in sealed jars away from the air. When 

 soda is drawn at the fountain, the effervescence is 



