To find what fraction of the air is used up when phosphorus burns, and hence what fraction of the air is active and used in combustion.
Place a porcelain evaporating dish on water in a glass trough. Hold a piece of phosphorus with a pair of tongs and cut a small piece. Place it in the dish and invert over it a bell jar. Using strips of paper andmark off five equal divisions on the bell jar surface above the water level. Warm the glass rod and quickly insert the cork at the mouth of the bell jar so that the rod touches the phosphorus.Observe what happens to the phosphorus and the changes in the level of the water in the jar.
You will notice that the phosphorus burns. It forms a new substance different from phosphorus. The level of the water in the bell jar rises.
From the experiment we see that in burning, phosphorus makes use of about one fifth of the air, leaving about four-fifths of inactive air, and forms a new substance which looks like ash. [phosphorus + active air ---> phosphorus ash + heat] This leads us to conclude further that air consists of one-fifth, by volume, which is active, i.e. which supports combustion (burning), and four-fifths, by volume, inactive air which does not.