l8 SCIENCE OF HOME AND COMMUNITY 



candle. The gas left in the tumbler is nitrogen. What are the 

 proportions of oxygen and nitrogen ? 



2. To show the presence of carbon dioxid, pour some lime- 

 water into a dish and allow it to stand for an hour or two. The 

 white coating that forms on the surface of the water shows the 

 presence of carbon dioxid. 



3. To show the presence of water in the air. Bring into the 

 schoolroom a metal cup containing a mixture of ice and salt. 

 What is the source of the water that condenses on the outside 

 of the cup ? 



DEMONSTRATION 6 



Purpose. To learn how the air we breathe out differs from 

 the air we breathe in. 



Apparatus. Thermometer, limewater, straw, bicycle pump. 



Directions, i. Breathe on the bulb of a thermometer and 

 compare the temperature with that of the room. 



2. Pour some limewater into a test tube and blow through 

 it by means of a straw or glass tube. Pour limewater in another 

 tube and force air through by means of a bicycle pump. Which 

 solution gets the milkier ? What does this show ? 



3. Breathe on a window pane. What do the results show ? 



4. Remove the covers of two quart canning jars. Allow one 

 jar to stand outdoors. Have a pupil breathe into the other 

 by means of a glass tube. Screw the covers on both jars tightly 

 and let them stand in a warm place for a day or two. Just 

 after the children have come in from outdoors, have the pupil 

 who breathed into the bottle smell of the air in both bottles. 

 What difference is noted? What does this experiment show? 



5. What four changes have taken place in the air we breathe 

 out? 



Breathing. The air is made up of about one fifth oxygen, 

 four fifths nitrogen, a small fraction of one per cent of carbon 

 dioxid, besides minute quantities of other gases ; and it also 

 contains varying amounts of invisible water vapor. In 

 the process of breathing, the air is taken into the lungs, and 



