I think you are misunderstanding what is meant by "fire burns up air". Fire does use oxygen when it burns. But it also produces carbon dioxide. An oxygen molecule has two oxygen atoms in it. Fires use this to produce carbon dioxide by adding a single carbon atom from the fireís fuel (wood, for instance). So there is just as much carbon dioxide produced as there is oxygen used. It does not create a vacuum at all.
The jam jar experiment I think you are describing is actually a demonstration of how gases expand when they are heated. As long as the jar remains open, the pressure inside the jar is the same as outside (one atmosphere). If you heat the air in the jam jar with a candle burning inside, the air expands and becomes less dense, and some air molecules leave the jar. There are fewer air molecules in the jar when it is hot than when it is cold, but the pressure remains the same. Then the lid to the jar is closed, the candle burns out (runs out of oxygen), and the gases cool off. Only when it cools off does the pressure decrease and you have a partial vacuum. But all of the atoms in the original air are still somewhere (in this case some are outside of the jar). And others are shuffled around to make carbon dioxide and water vapor. If we consider the whole earth, we will not lose atoms because gravity keeps them all in.
So if there was a very large fire on the earth, it would convert a lot of oxygen to carbon dioxide. This carbon dioxide could then be used by plants and converted back to oxygen - unless, of course, the fire was big enough to burn all the plants up too. In that case, there probably wouldn't be enough animals left around to notice the difference.
Additional interesting links:
You donít need a fire to do the jam jar experiment. Hereís one experiment done by a professor at the University of Indiana with a big barrel and some boiling water (donít try this one at home): http://needmore.physics.indiana.edu/~rickv/barrel.html
Hereís a fun example of a railroad tank car which was crushed under atmospheric pressure after it was steam cleaned and the lid screwed back on (donít try this one at home either): http://www.delta.edu/slime/cancrush.html
Again, no fire problem, just a matter of heating up the gases to expel them and putting the lid back on while cooling it all off.
-Tamara (and Tom)
(republished on 07/24/06)