To find out if rusting will take place in the absence of air.
Place some new iron nails in two boiling tubes, one containing ordinary distilled water, the other containing distilled water which has been strongly boiled for ten minutes to drive off the air dissolved in it. Seal the latter tube from the air by pouring some oil on the surface of the water. Keep the tubes in a convenient place for two or three days.
Note that much rusting of the nails takes place in the tube containing unboiled water, and that the rust falls off as rapidly as it is formed, leaving a mass of reddish-brown deposit at the bottom of the tube. Little or no rusting has taken place in the other tube containing boiled water.
From this experiment it is observed that since boiling has driven out the air from the water in which rusting does not take place, we may conclude that rusting does not take place in the absence of air. This conclusion confirms that the process of rusting involves the use of some fraction of the air.