RESPIRATION. 231 



and if allowed to stand, a fine powder falls to the 

 bottom. 



The air which you have blown into the water 

 has passed through your lungs, and during its pro- 

 gress has lost a part of its oxygen, and in its place 

 we find fixed air, or carbonic acid. It is this which 

 causes the lime water to become white, by uniting 

 with it. 



This fixed air is very unwholesome ; animals 

 soon die if confined in it, and this is the reason 

 why close and crowded rooms are so unhealthy, as 

 the air becomes very impure, and unfit for breath- 

 ing. We soon become oppressed and languid in 

 these situations, and whenever we feel these symp- 

 toms, we should have the room ventilated, or re- 

 move to more open places, 



Questions. 



What happens if our breathing is interrupted ? 



Of what do the lungs consist 1 



What is the cavity called in which they are placed ? 



Can you tell which is the most important muscle in breath- 

 ing? 



What are the divisions of the lungs called ? 



How is the windpipe or trachea kept open ? 



Through what is the air conveyed to the lungs 1 



Endeavour to describe the way in which the opening into 

 the windpipe is protected. 



How is the blood purified in the lungs, and what colour 

 does it become 1 



Is the air we have breathed changed 1 



What simple experiment shows the nature of this change ? 



What kind of air is formed in the lungs ? 



Is this injurious to life ? 



