To find out what happens during slow combustion of phosphorus
Invert a graduated tube over a trough of water with a piece of phosphorus attached to the end of a long piece of thin wire going to the top of the tube. Note the level of water in the tube at the beginning of the experiment and record it. Leave the apparatus for a few days to allow the smouldering phosphorus to use up all the active air in the tube. Read the level of the water in the tube at the end of the experiment. In reading this level you will have to take the following precaution: cover the mouth of the tube in water with your thumb and transfer it to a deep jar of water; remove your thumb. Push down the tube until the water in the tube is at the same level as the water in the jar. The volume of the remaining gas in the tube is now taken at atmospheric pressure as at the beginning of the experiment.
Observe that the smouldering ceases after a few days and the level of the water rises to about one-fifth as before.
In burning, phosphorus makes use of about one-fifth of the air leaving about four-fifths of inactive air. This experiment confirms the conclusion that air consists of one-fifth by volume which is active and four-fifths by volume which is inactive.