A simple experiment uses Dalton's law of partial pressures, the sum of the partial pressures to measure the vapor pressure of a volatile liquid. A predetermined volume of air is injected into a closed-end tube filled with the liquid to be measured. The volume of liquid displaced will be greater than the volume of air injected because of the vapor pressure of the liquid. Using Dalton's law of partial pressures, the sum of the partial pressure of the liquid and of the air is directly related to the final volume, allowing the calculation of the vapor pressure of the liquid in the tube.