To find out if iron filings make use of the air in rusting and what fraction of the air is used.
Wet the inside of a gas jar, place a few iron filings inside and rotate the jar so that the iron filings stick to the sides. Invert the jar in a trough of water and mark off five equal divisions of the gas jar above the water with strips of paper. Leave the experiment for a few days until the water shows no further rise in level.
You will observe a new substance is formed which is different from the original iron filings. Also note the rise in level of the water in the jar. It rises to the first mark. The the new substance is iron rust. Since water rises in the tube, it shows that rusting makes use of a portion of the air. It makes use of active air.
Iron has used up about one-fifth of the given volume of air in rusting, forming a new substance. This is the same fraction used up by phosphorus and magnesium in burning to form new substances. We can therefore conclude that rusting appears to be another form of combustion. In fact, it is often called slow combustion. Since rusting plays such an important part both in industry and in our daily life, we would like to know more about it. For instance, in our homes, knives, forks, and spoons rusts after some time, our buckets also rust, and so do the hinges of our doors. In industry, metals generally, and iron in particular, are used in building houses, bridges, ships and cars; much damage is done if they are allowed to rust.