EXPERIMENTS AND PRACTICAL WORK 



81 



over, and notice that the candle goes out as the oxygen 

 is used up. 



Expt. 193. Burning Substances add Carbon Dioxide 

 to the Air. After observing that the candle goes out 

 in the experiment above, add lime-water and shake. A 

 milky precipitate will be noticed. 



Expt. 194. Where the Water Vapour in the Air 

 conies from. 



(a) Plants. Tie a wide-mouthed bottle on to a branch 

 of a guango tree, and observe that water condenses in- 

 side. The mouth of the bottle should be closed. 



(b) Animals. Breathe against a piece of glass, and 

 observe the moisture deposited. 



(c) Water Surfaces. Observe how water exposed in a 

 saucer evaporates into the air. In a similar way water 

 in the sea, rivers, ponds, &c, is always passing ofl' into 

 the air in the form of vapour. 



WHY THE WIND BLOWS 



(See Tropical Readers, Book II, pp. 169-171.) 



Expt. 195. Air expands when 

 Heated. Fit up a flask as shown in 

 fig. 32; heat gently, and observe that 

 the hot air expands as shown by the 

 rising of the drop of water. 



Expt. 196. Warm Air rises. Put 

 light bits of paper or small feathers 

 above a fire, in a chimney for example, 

 and observe how they rise with the 

 warm air. 



Expt. 197. How a Balloon may be 

 made. Make a balloon about 2 ft. in 



(C282) 



Fig. 32. Expansion 

 of Air when heated 



6 



