LESSONS IN CHEMISTRY. 181 



that air that has been breathed once is not safe for breathing a 

 second time. But lesser quantities of the dioxide are harmful, 

 and a more delicate test of its presence is often desirable. Put a 

 small piece of quick lime into a tumbler or jar of water, after a 

 few minutes stir it well, and when the sediment settles pour off 

 the clear water for use. Fill a test-tube about two-thirds full of 

 lime water and force air from the lungs through the water by 

 means of a tube, and quickly the water becomes cloudy and 

 milky. This makes a delicate test for the dioxide. The lime of 

 the water combines with dioxide forming calcium carbonate 

 which is insoluble in water. If lime water is exposed to the air 

 in an open dish, the dioxide of the air uniting with the lime 

 soon forms a thin coating of calcium carbonate on the surface of 

 the water. 



The dioxide of carbon accumulates so rapidly as the pro- 

 duct of combustion and respiration that the air of a room 

 occupied by a number of people soon becomes loaded with 

 this gas unless special arrangements have been made for free 

 ventilation. Tests of the* air of almost any school or assembly 

 room, especially when fires or lights are in use, will show dan- 

 gerous quantities of a gas which cannot support life, and which 

 by limiting the supply of oxygen becomes detrimental to health. 



Under a pressure of 36 atmospheres (about 450 Ibs.) carbon 

 dioxide becomes a liquid; when the pressure is removed it begins 

 to evaporate rapidly, the evaporating portion absorbing so 

 much heat from the other that it freezes into a snow-like solid. 



Carbon dioxide is absorbed freely by water and other liquids, 

 the sparkling appearance of wines and mineral waters, the foam 

 of beer and " soda water " are due to the presence of this gas. It 

 is also used in the manufacture of fire extinguishers. An English 

 coal mine that had been smouldering for years was extinguished 

 completely by the use of carbon dioxide. This gas combined 

 with water forms carbonic acid, formula H 2 C0 3 , and it is prob- 

 able that in the tests with lime water we find the carbonic acid 

 not the dioxide. 



When coal is burned without a free supply of air, a gas is 



