Pressure is usually measured in atmospheres or millimeters of mercury; 1 atm = 760 mm of Hg.

As you can see on the previous page, throughout the experiment the volume is inversely proportional to pressure; or to express matters another way, the product of pressure and volume is unchanged. This can be written as

PV = k - Boyle's law

in which k is a constant that can be evaluated for a particular temperature from one particular set of pressurevolume conditions. In the table at the top of the page, this constant k is equal to 20 liter atmospheres.

If we want to compare two sets of experimental conditions at constant temperature, designated by subscripts 1 and 2, then Boyle's law can be written
PV = P V

Either form of Boyle's law can be used in an actual problem.