WATER IN THE AIR 



45 



limewater, we find that the liquid becomes milky in ap- 

 pearance. If we shake a little limewater in the jar after 

 burning wood or a candle in it, we find that the lime- 

 water turns milky. Evidently we might use this test, 

 known as the limewater test, to prove the presence in 

 the air of a gas formed when 

 certain things are burned. Do 

 you see why ? The chemist tells 

 us that this gas which is pro- 

 duced by mixing soda and vine- 

 gar is carbon dioxide. He uses 

 limewater to test for the pres- 

 ence of carbon dioxide. Carbon 

 dioxide is formed, as we shall find 

 later, by the union of oxygen 

 and a substance called carbon 

 when anything containing car- 

 bon is burned. Carbon dioxide 

 is always present in the air, al- 

 though in very small amounts, 

 about .04 per cent in out-of-door 

 air. It is constantly being given 

 off into the air by decaying vegetable matter, by burning 

 fuels, and by breathing animals. * 



Water in the air. Air is really a very complex sub- 

 stance, and contains besides the gases mentioned, more 

 or less water. You can easily prove this by mixing a 

 little salt with ice in a test tube, and observing the little 

 drops of moisture which form on the outside of the tube. 

 Where does it come from ? In addition to water, several 

 gases, among them argon, are found in small quantities in 

 the air. 



This shows that water is present 

 in the air. 



