Robert Boyle in the 1660's recorded his discoveries about air in the book "New Experiments Physio-Mechanicall, Touching the Spring of the Air and its Effects" (1660). Hooke had acted as Boyle's assistant and had designed an air pump that allowed Boyle to achieve a partial vacuum and make several discoveries.
In an appendix to the book, written in 1662, he mentioned that if the temperature does not change, a greater volume for a gas makes the pressure less, and increasing the pressure diminishes the volume. This became known as Boyle's law, sometimes called Mariotte's Law.
The law can be stated simply that the pressure times the volume equals some constant. This constant is different for different quantities of gas, but is always the same if the same quantity of gas is used. As you squeeze the gas into a smaller volume the pressure increases proportionally if the temperature is allowed to come back to equilibrium. The formula used is:
p1V1 = p2V2 
This relationship is really only true for what are called perfect gases or ideal gases. In real gases, the relationship breaks down when the gas starts to liquefy, or solidify.
- MacTutor Biography School of Mathematics and Statistics, University of St Andrews, Scotland. Accessed 09 June 2010.
- Halliday, David; Resnick, Robert; Walker, Jearl (2007). Fundamentals of Physics. 1 (8th ed.). Wiley. p. 592. ISBN 978-0-47004473-5.
- CRC Handbook of Chemistry and Physics, CRC Press, Inc.(Originally the Chemical Rubber Publishing Company), Boca Raton, Florida, 63rd Edition, 1982, page F-81