THE EXCRETORY SYSTEM. 95 



it would be advisable to first lower into it a lighted 

 candle. If the candle continues to burn, it may 

 be concluded that the well is sufficiently free from 

 carbon dioxide to enter it with safety. 



That the lungs expel a gas which is 

 Test of the identical with that' which collects in 



i>reuin. 



wells may be illustrated by breathing 

 into a glass jar, after having held the breath in the 

 lungs for some time. If a lighted taper or wax 

 candle be lowered into the jar containing this 

 exhaled breath, it will go out, thus showing the 

 absence of pure air and the presence of carbonic 

 oxide. Again, to show that other substances, such 

 as particles of animal matter, are contained in air 

 once breathed, let the contents of the lungs after a 

 full inspiration be breathed into a bottle and corked 

 up. The effete matter excreted with the breath will 

 decompose and soon give off an offensive odor. Thus 

 we may understand the need of ventilation already 

 mentioned. We can understand how the excretions 

 from the lungs soon make the air in a closed roojn 

 unfit for breathing, causing drowsiness and headache. 

 The especial need of well- aired school-rooms is 

 therefore urged for the consideration of teachers 

 and pupils. Let pupils make the simple tests men- 

 tioned and see for themselves the importance of 

 proper ventilation. 



Besides what has already been men- 



tioned ' the lun s 8 take from the blood 



a watery vapor which is ordinarily not 



