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Air is a gas.
Careful, scientific observation has determined that these variables are related to one another, and the values of these properties determine the state of the gas. In the mid 1600's, Robert Boyle studied the relationship between the pressure and the volume of a confined gas held at a constant temperature. Boyle observed that the product of the pressure and volume are observed to be nearly constant. This relationship between pressure and volume is called Boyle's Law in his honor. For example, suppose we have a theoretical gas confined in a jar with a piston at the top.