AIR AND LIVING THINGS 



23 



at the bottom, about one inch in diameter. These should be 

 closed with rubber stoppers. Any air leaks in the box may 

 be closed with putty. Place four candles in the box, close the 

 door, and stopper all the end openings, which may now be 

 thought of as closed windows. By varying the openings, find 

 out the best conditions for ventilation. 



FIG. 43 



In your notebook 1 record the notes of this experiment and 

 complete the statement given below. 



I find the best conditions for ventilation to be as in- 

 dicated by this experiment. 



OTHER INVESTIGATIONS WHICH YOU CAN MAKE 



Will things burn without air? Devise a method to test 

 out this question. 



READINGS WHICH WILL HELP ANSWER THE 

 PROBLEM QUESTIONS 



What are the differences between physical and 

 chemical changes? When two or more simple sub- 

 stances are combined in such a way that each has lost 

 the things, such as color, odor, and taste, by which it 

 was recognized, and a new substance is formed, they 

 are said to have been chemically united. The change 

 which has taken place is called a chemical change. The 

 rusting of a can, the cooking of meat, the burning of 

 wood, coal, or paper, the baking of a cake all are ex- 

 amples of this type of change. If simple substances 

 are put together in such a way that they may still be 

 taken apart and recognized by their color, odor, or 

 taste, they are said to have been mixed, and the 

 change is called a physical change. Some examples are 

 wood changed to sawdust, water changed to ice or 

 steam, iron changed to iron filings. Can you think of 

 others? 



Of what is air composed? The blanket of air which 

 surrounds our earth is a mixture of several gases, all 

 of which are colorless and odorless and have no taste. 

 These conditions explain why we are so little aware 

 of the presence of an atmosphere. The table below 

 shows the composition of the air and gives the 

 amounts of the gases present in the greatest quantity. 



Nitrogen 



Oxygen 



Argon 



78.02% 



21.00% 



.93% 



Carbon dioxide 

 Water vapor 

 Helium 1 



[.Rare gases present only in small traces 



'See accompanying workbook, p. 11. 



.04% 

 Variable 



Krypton 



Nitrogen, which makes up most of the air, is one 

 of the necessary substances for all living things, but 

 strange to say, they do not obtain it directly from the 

 atmosphere. It must first be put into the soil in a 

 form which can dissolve and then be absorbed into 

 plants whch are later eaten by animals and man. 

 Great amounts of money are spent each year for fer- 

 tilizers that contain nitrogen. During the recent war 

 our government spent several millions of dollars to 

 build a chemical plant at Muscle ^Shoals, Alabama, 

 which could take the nitrogen from the air and com- 

 bine it with other substances to make high explosives 

 in the time of war or fertilizers in peace times. 



Oxygen, which makes up about one fifth of the air 

 by volume, is necessary for all life. Without it this 

 earth would be a barren place, for no animal could 

 live and no fires could burn. Oxygen is very active, 

 that is, it uniies with a great many other things to form 

 chemical compounds. Many common substances in 

 our everyday lives contain oxygen chemically united 

 with one or more other elements. Iron rust is made 

 up of iron and oxygen ; water is hydrogen and oxygen ; 

 starch and sugar are made of carbon, hydrogen, and 

 oxygen ; sand is made of a substance called silicon 

 and oxygen in fact, oxygen is so active that it is 

 seldom found free except in the air, where the other 

 gases are so inactive that they will not unite with it 

 except under special conditions. 



Argon is a gas that resembles nitrogen in many 

 ways. It does not combine easily with other sub- 

 stances, and because of this it is sometimes used in 

 gas-filled electric lamps to keep the tungsten filament 

 from evaporating when it gets very hot. 



Carbon dioxide is a gas that is given off by all 

 living things. It is also formed whenever anything 

 containing carbon burns. Only a small percentage of 

 air is carbon dioxide. In 10,COO gallons pf air you 

 would find only three or four gallons of carbon dioxide. 

 This gas is important because it is again taken out of 

 the air by green plants as they build up food, material 

 in their leaves. 



There is always some water vapor present in the 

 air, but it varies greatly from day to day. The mois- 

 ture in the air, which is an important factor in weather, 

 climate, and health, will be studied in detail in the 

 next unit, "The Weather and Climate About Us." 



The other gases found in the atmosphere, helium. 

 neon, krypton, and xenon, are called rare because only 

 traces of them are found. Two of these gases have 



