22O METHODS IN TEACHING 



the^fcottle is a little more than half full. The cork for 



the bottle should be bored for the delivery tube and for a 



" thistle," or funnel tube. The delivery tube 



Dioxide should have about two feet of rubber tubing 



attached to the outer end, and its inner 



end should extend only just through the cork. Pour small 



quantities of either sulphuric or hydrochloric acid into the 



bottle by means of the funnel tube, which should reach 



almost to the bottom of the bottle. 



The gas that is given off is carbon dioxide ; it may either 

 be collected by displacement or, by reason of its being con- 

 siderably heavier than air, it may be allowed to flow into 

 any deep vessel in a room where the air is quiet. When 

 sufficient amount is secured, test it by a burning paper. 

 Does it support burning? Would it support animal life? 

 If the jar is large enough, drop a small, light soap-bubble 

 into it, and notice that it floats in the gas much as a rubber 

 balloon would float in water. It may be dipped and poured, 

 run through troughs and pipes, or it may be siphoned like 

 heavier liquids. Make a deep trough of pasteboard and 

 place short, burning candles in the bottom. Pour the heavy 

 gas into the trough, after elevating one end of it, and note 

 that the lights are extinguished in order as it flows down 

 over them. 



Call attention of the pupils to other forms of oxidation 

 besides burning and breathing ; such as decay of animal and 

 vegetable tissues, fermentation, and the oxidation of met- 

 als, iron, zinc, lead, copper. How is the oxygen thus used 

 up replaced in the air? In connection with the experiments 

 with carbon dioxide, discuss with the pupils " choke damp " 

 of mines, and the " damps " that are sometimes found in 



