Thepercentageofcarbondioxide varies for a very different reason. Can you
guess why the air inindustrialcitiescontainsmorecarbondioxidethanthe air
in the countryside? One of theproducts ofcombustion (burning)is carbon
dioxide and thequantityoffuelsburnedin factories, homes, cars, trucks,and
buses gives off muchcarbon dioxide. The absenceofvegetationin cities is
anotherreason for the highpercentageofcarbondioxide in city air. Plants
remove a great deal of carbon dioxide from the air duringthe process of
photosynthesis.This is the process by which plants usesunlighttomanufacture
plantfood outofwater and minerals.
HOW YOU CAN MEASURE THE PROPORTION OF OXYGEN INAIR
Gather these materials: A largecork;a smallcandle; a tall thin glass; a
rubberband;a penknife;and a basincontainingI inchofwater.
Follow this procedure:Slice the cork to get a pieceabouttof an inch thick,
using your penknife.Rotatethe cork as you cut it, sothatyou will not have to
press very hard. In this wayboththe cork andyourfingers will be safe.Estimate
the centerofthe circleofcorkandattachthe candle tothatspotwith a few
dropsofcandle wax. Now, put it on the surfaceofthe water and see if it will
float. To do so, it must beproperlybalanced.Ifit isn't,remove thecandle
andtry again. When you have succeeded inmakingit float, light the candle.
Carefullyinvert the glass over the floating, lighted candle. Do this in such a
waythatonly the slightest bitofthe rimofthe glass isunderwater. Donot
push it down to thebottomofthe basin.Watchwhathappensinside the glass.
Whennothingelse seems to behappening,slip therubberbandaroundthe glass
and let itmarkthe surfaceofthe water on the inside.
Results:The candle burns for aminuteor two and then goes out. As itburns,
the water rises in the glass and when it goesout,the water stops rising.
Fires need oxygen inordertoburn.Theburningcandleused up the oxygen
containedin the air in the glassandthen went out. Becauseofthe missing
oxygen, the air inside the glass became lighterthanthe airoutsidethe glass.