Since an increase of pressure reduces the volume of a gas, it is important to know whether there is a definite relation between the pressure exerted upon a gas and the resulting volume. This was experimentally determined independently by two physicists, Boyle and Mariotte. The results obtained were formulated in what is called Boyle's Law, which may be stated as follows: The temperature remaining the same, the volume of a given mass of gas varies inversely as the pressure acting upon it. This may be expressed by the proportion V: V' = P': P, from which we get
PV = P'V'
i.e., PV = a constant quantity.
The mass of the air remaining the same, it is evident that the density must increase as the volume diminishes; hence.
At a constant temperature the density of a gas is directly proportional to the pressure acting upon it.
Very careful measurements show that gases do not obey Boyle's Law exactly, and that different gases behave differently in this, respect. But for practical purposes the law may be considered to hold true, except at temperatures so low that the gas is about to liquefy.
