To find what fraction of air is used by magnesium in burning.
Invert the bell jar over water in a glass trough and mark off five equal divisions on the side of the bell jar above the water, using paper strips as before. Tie the magnesium ribbon to the iron wire and light it. When it starts to burn, quickly place it in the bell jar and replace the cork.
You will notice that the magnesium burns. You will also notice that a new substance is formed which is different from the original magnesium. The level of the water also rises.
In burning, magnesium makes use of about one-fifth of the air leaving about four-fifths of inactive air, and also forms a new substance which looks like white ash. This experiment confirms the conclusion that air consists of one-fifth by volume which is active and four-fifths by volume which is inactive.