In 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

original 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.