The French scientist, Jacques Charles, provided much of the foundation for the modern kinetic theory of gases. He found that all gases expand and contract in direct proportion to the change in the absolute tem

perature, provided the pressure is held constant. As a formula, this law is shown as follows:

- Volume 1 × Absolute Temperature 2 =
- Volume 2 × Absolute Temperature 1
- Or
- V1T2 = V2T1

Charles’ law also works if the volume is held constant, and pressure and temperature are the variables. In this case, the formula would be as follows:

- P1T2 = P2T1

For this second formula, pressure and temperature must be in the absolute.

*Example: A 15-ft ^{3} cylinder of oxygen is at a temperature of 70°F and a pressure of 750 psig. The cylinder is placed in the sun and the temperature of the oxygen increases to 140°F. What would be the new pressure in psig?*

- 70 degrees Fahrenheit = 530 degrees Rankine
- 140 degrees Fahrenheit = 600 degrees Rankine
- 750 psig + 14.7 = 764.7 psia
- P1T2 = P2T1
- 764.7 (600) = P2 (530)
- P2 = 764.7 (600) ÷ 530
- P2 = 865.7 psia
- P2 = 851 psig