SCIENCE OF COMMON THINGS. 205 



Why a fire balloon rises. Where ventilation is perfect. Oxygen. 



be suspended, as is represented in Fig. 46, the upward 

 current of hot air will immediately put the paper in 

 motion, and make it revolve rapidly around the wire. 



1359 When a boy makes a fire-battoon, and sets fire to the cotton or 

 sponge (which has been steeped in spirits of wine), why is the balloon inflated f 



Because the air of the balloon is expanded by the 

 heat of the flame, and fills the balloon to its utmost 

 capacity. 



1360 Why does the balloon rise after it has been inflated by the expanded 

 air? 



Because the same quantity of air is expanded to three 

 or four times its orig^nal volume ; and made so much 

 lighter, that even when all the paper, wire, and cotton 

 are added, it is still lighter than common air. 



1361 In what situation is ventilation perfect ? 



In the open air, because the breath, as it leaves the 

 body, is warmer and lighter than the surrounding fresh 

 air, and ascending is immediately replaced by an in- 

 gress of fresh air ready to be received by the next 

 respiration. 



1363 Why is it desirable to avoid breathing the same air twice ? 



Air which has been once respired, is unwholesome, 

 and not suited to supply the wants of the animal system. 



1363 What are the elements of atmospheric air? 



Oxygen and nitrogen mixed together, in the propor- 

 tion of seventy-nine parts of nitrogen and twenty-one 

 of oxygen. 



1364 What is oxygen f 



A gas, colorless, tasteless, and odorless ; it is heavier 

 than atmospheric air, and is a non-conductor of elec- 

 tricity. 



1365 Is oxygen a substance existing in great abundance ? 



Oxygen is the most abundant of all known substances ; 

 it constitutes at least one third of the solid mass of the 

 globe, eight-ninths of all water, and nearly one-fourth 

 part of the atmosphere ; it also exists in most organic 

 substances. 



1366 Is oxygen ever found in a liquid or solid state f 



