4 2 ELEMENTARY SCIENCE 



In fresh milk the cream is distributed throughout in the 

 form of tiny droplets. So also droplets of oil are distrib- 

 uted in salad dressing. These are examples of one liquid 

 in suspension in another. 



In lemonade you have a solid (sugar) and a liquid 

 (lemon-juice) dissolved in a liquid (water). A solution may 

 also be made by dissolving a gas in a liquid, and this proc- 

 ess is also very important in life. It has something to 

 do with the way in which our blood absorbs the oxygen 

 that enters our lungs. 



A very important case of gas dissolved in a liquid is the 

 case of air dissolved in water. You have noticed how water 

 may enter the air as a gas. It does this when it evapo- 

 rates. Now it is important to understand that the reverse 

 of this also takes place. Air enters water. It is dissolved 

 by water. We can easily prove that water, as found in 

 nature, always has air in it. Fishes and other water 

 animals require air to breathe, and their source of air is 

 the air that is dissolved in water. 



If you gently heat water in a beaker, or in a porcelain- 

 lined vessel, the first change you will note will be the for- 

 mation of hundreds of tiny bubbles on the bottom and 

 sides of the vessel. These are very different from the 

 large bubbles of steam that form later, when the water 

 has reached the boiling-point. These tiny bubbles appear 

 long before steam arises from the surface. They appear 

 when the water is only a few degrees warmer than the 

 surrounding air. Then if you stop heating the water, and 

 set it aside without shaking, these bubbles remain for a 

 time, even after the water has cooled down to its ordinary 

 temperature. Of course they would not do this if they 

 were bubbles of steam. Steam bubbles would quickly 



