44 THE FOOD OF PLANTS CHAP. 



The burning candle has so altered the air that 

 substances can no longer burn in it. We may 

 call this air spoiled, or vitiated air. Having tested 

 the effect of the candle, we can try the effect of 

 spirits of wine, burning paper, burning oil, burning 

 hay, wood, or dried leaves. In each case the result 

 is the same. The air is so altered that it no longer 

 supports combustion. 



We find then by these experiments that sub- 

 stances when burnt alter the air in such a way that 

 it will no longer support combustion, that is to say, 

 that nothing will burn in it. 



Let us see now if we can obtain vitiated air in 

 any other way. 



The Air is Changed by the Breathing of 

 Animals 



Experiment 27 



Take a wide -mouthed stoppered bottle, remove 

 the stopper, fill it with water, and turn it upside 

 down in a basin of water. Then by means of a 

 piece of tubing blow air coming from the lungs 

 into the bottle. When the bottle is full of air 

 breathed out from the lungs, put in the stopper 

 and lift the bottle out of the basin. Light a 

 match, open the bottle, and plunge in the match. 

 The match goes out at once, showing that the 



