SOLUTION 43 



change to liquid again as the temperature fell. These 

 tiny bubbles are bubbles of air. 



If you have ever fished with minnows for bait, you know 

 that you must change the water frequently to keep the 

 minnows from dying. But if you take a pail of minnows 

 away from the stream, and cannot change the water fre- 

 quently, they will live much longer if you stir the water 

 frequently. Can you explain this? 



Air is not the only gas that is readily absorbed by water. 

 Some gases dissolve hi water to a remarkable extent. Thus 

 water can dissolve one thousand times its bulk of the gas 

 ammonia. A gas that is present in air, and is very impor- 

 tant in the lives of both plants and animals, is carbon 

 dioxide. This gas is given off from our lungs when we 

 exhale. It is also one of the products of burning. It is 

 used by plants in the manufacture of food. It is readily 

 absorbed by water, and all natural waters contain some 

 of it. Sea-water contains more carbon dioxide than does 

 the air above it. Soda-water is nothing but water charged 

 with carbon dioxide. The water is placed in a metal con- 

 tainer, and the gas forcea into it under high pressure. The 

 container is then firmly sealed. As soon as any of this 

 water is drawn off it begins to fizz and bubble the way 

 you have seen it in a soda-water glass. This is because 

 the pressure is removed, and the extra load of gas is rush- 

 ing to escape. 



We have been calling air a gas. It is time to explain 

 that it is a mixture of gases. Four-fifths of it is a gas 

 called nitrogen, and nearly all the rest of it is the gas called 

 oxygen. Less than half of one per cent of it is carbon 

 dioxide, yet that one-half of one per cent is absolutely 

 necessary to the making of our food. All these gases, as 



