Boyle's law simply states that the volume of a gas kept at a constant temperature varies inversely with the pressure exerted upon it. In other words as the pressure of a gas increases the volume decreases and vice versa as long as the temperature stays the same. The equation thatrepresents this is:

P[1] * V[1] = P[2] * V[2] = constant

Where P is the pressure of the gas and V is the volume. The equation also states that the pressure times the volume is a constant. This means that if the pressure (or volume) changes the volume (or pressure) will also change to equal the product of the original pressure and volume. If, for example, the pressure of a gas is at 13.2 atm (atmospheric pressure) and it occupies a volume of 7.5 liters then the constant for it would be 99 atm*L. But if the volume increased to 9 liters then the pressure of the gas would be lowered to 11 atm in order to maintain the constant.

To determine what the equivalent was of one atmospheric pressure Boyle poured mercury into a long glass tube (eudiometer) and closed the open end. He then placed the sealed end into a large bowl filled with mercury and removed the seal. Some of the mercury flowed out of the tube into the bowl but 760 mm of mercury remained. Boyle was therefore able to assume that one atmospheric pressure was equal to the pressure that 760 mm of mercury exerts.

Boyle repeated this experiment with many other types of liquids and eventually he was able to accurately predict the height of the liquids inside the his manometer. Later he began using a J-shaped manometer to determine the relationship of pressure and volume. He would put some gas in the closed end of the manometer and begin filling it with mercury to increase the pressure. He recorded the volumes that the gases would be at different pressures. He eventually created the equation

V = C * P^a

where V is the volume of the gas, C is a constant that varies with the temperature and mass of the gas, P is the pressure that is exerted on the gas, and a was an exponent that was equal to the slope of a line when the pressure and volume were plotted on a graph.

Boyle's law has since been combined with Charle's law and Gay-Lussac's law in the General Gas law which states that the pressure of a gas varies inversely with the volume and in direct proportion to the temperature of the gas.

(P[1] * V[1])/T[1] = (P[2] * V[2])/T[2] = constant