4 ANIMAL BIOLOGY 



Experiment to show the Cause of Burning, or Oxidation. 



— Obtain a large glass bottle (a pickle jar), a short candle, 

 and some matches. Light the candle and put it on a table 

 near the edge, and cover it with the glass jar. The flame 

 slowly smothers and goes out. Why is this ? Is the air 

 now in the jar different from that which was in it before 

 the candle was lighted ? Some change must have taken 

 place or the candle would continue to burn. To try 

 whether the candle will burn again under the jar without 

 changing the air, slide the jar to the edge of the table and 

 let the candle drop out. Light the candle and slip it up 

 into the jar again, the jar being held with its mouth a little 

 over the edge of the table to receive the candle (Fig. 5). 

 The flame goes out at once. Evidently the air in the jar 

 is not the same as the air outside. Take up the jar and 

 wave it to and fro a few times, so as to remove the old air 

 and admit fresh air. The candle now burns in it with as 

 bright a flame as at first. So we conclude that the candle 

 will not continue to burn unless there is a constant supply 

 of fresh air. The gas formed by the burning is carbon 

 dioxid. It is the gas from which plants extract carbon. 

 (See Plant Biology, Chap. V.) One test for the presence 

 of this gas is that it forms a white, chalky cloud in lime 

 water ; another is that it smothers a fire. 



Experiment to show that Animals give off Carbon Dioxid. 



— Place a cardboard over the mouth of a bottle containing 

 pure air. Take a long straw, the hollow stem of a weed, 

 a glass tube, or a sheet of stiff paper rolled into a tube, 

 and pass the tube into the bottle through a hole in the 

 cardboard. Without drawing in a deep breath, send one 

 long breath into the bottle through the tube, emptying the 

 lungs by the breath as nearly as possible (Fig. 4). Next 

 invert the bottle on the table as in the former experiment, 



