STUDIES IN NATURE 



when they are carried out and laid down out- 

 of-doors. 



All this shows us that, besides the water, 

 there must be at least two other sorts of gases 

 or vapours, by which name we call these sub- 

 stances we cannot see or handle, in the air 

 around us. There is the good air which makes 

 our rooms pleasant and healthy to live in, and 

 there is the bad air that makes us feel faint and 

 sickly. This good air or gas is called oxygen, 

 while the bad air contains unpleasant vapours 

 given off by our bodies, one of which is known 

 as carbonic acid gas. We know now the 

 names and some of the properties of this mix- 

 ture of gases we call air ; there is also a fourth 

 substance, nitrogen, to be found in it, and it is 

 present in much larger quantities than any of 

 the other three. It does not take such an active 

 part in our daily life, and seems merely to dilute 

 or weaken the oxygen, as water may be used to 

 dilute whisky or wine, which would be too strong 

 for most of us alone. 



When we open the windows we let into our 

 rooms a fresh supply of air, and amongst the 

 air comes in oxygen, without which we cannot 

 live, while the used-up air goes out, taking the 

 carbonic acid gas in it. 



When we think this over we may well won- 

 der how it is that there is any pure oxygen at 

 all left in this world for us to breathe, seeing that 

 the animals and plants also require it. Surely it 



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