When the outside temperature is the same as that of the air within a vessel, the product of the pressure and volume is constant. This is called Boyle's Law. To illustrate: If the volume of a gas is 2 cu. ft. at a pressure of 1 atmosphere (15 lbs.), then the volume would be decreased one-half as the pressure is increased twofold. Boyle's Law is sometimes expressed thus: At constant temperature volume of gas varies inversely as the pressure.
To calculate the volume of a gas at a given pressure, multiply the old volume by the old pressure and then divide by the new pressure.
P : P' - V : V P X V = P' X V' V' = P X V / P'
Where P and P' are the original and new pressures, and V and V' the original and new volumes.
The quotient will be the new volume. When a volume of gas is given it is understood to be at a pressure of 1 atmosphere unless some other pressure is expressed. One atmosphere is equal to 15 lbs. air pressure.
Note. - In order to convey to the mind the relationship between quantities, such as between volume and pressure, the expressions "varies directly" and "varies inversely" are used. The expression "varies directly" is used to convey to the mind the idea that one quantity grows larger in the same proportion as the other. When the relation between two quantities is such that one quantity grows larger in the same proportion as the other grows smaller, the first quantity is said to vary "inversely" as the other.