1660, Robert Boyle published a book entitled "New Experiments Physico-Mechanical,
Touching [concerning] the Spring of the Air." In it he gave the
evidence for what is known today as Boyle's law. Air does have "spring."
If you compress it, it pushes back. Poke an airfilled plastic balloon
chair with your fingertip, and you will easily make a large dent,
which vanishes when you take your finger away. Yet if you sit down
on the chair, the air inside pushes back with enough force to hold
up your weight. If you compress an enclosed body of gas until it
is half its
volume, and keep the temperature constant, the pressure will be
doubled. If you continue to squeeze until the volume is a quarter
of the starting volume, the pressure will be four times as great.
Conversely, if you release the constraints on a gas and allow it
to expand to twice its initial volume, the pressure of the gas will
be halved, if the temperature is kept constant. This behavior is
illustrated in the table and PV plot above, which describe a hypothetical
experiment beginning with 20 liters of a gas at 1 atmosphere pressure.