Because gases are widely dispersed under ordinary pressures, it is not unusual for several gases to occupy the same volume. However, as each gas is introduced into a fixed volume, the pressure must rise since more gas particles means more collisions with the walls of the container. It is logical to assume (and confirmed by experiment) that the number of gas particles is directly proportional to the part of the total pressure exerted by that gas. The fact that all of the "partial pressures" add up to the total pressure is known as Dalton's Law of Partial Pressures.

It would be nice if we had a law which would allow us to get information about a gas through a variety of changes and combinations of changes. Such a law does exist and is usually written as:

PV = nRT P is the pressure, V is the volume, n is moles, T is the temperature in K, and R is a number known as the gas constant, which makes the units work out. There are different values of R for different combinations of units.