If the volume of a mixture of air and water vapor increases, small water drops form. If the air contains electrically charged particles - ions - then droplet formation occurs around these especially. C.T.R. Wilson exploited this phenomenon when he constructed his cloud chamber in 1911 - a glass container with air and water vapor and ingenious devices that allow traces left by ionizing radiation and particles that pass through the chamber to become visible and be photographed. The cloud chamber became an important tool in physics.