Aitken, John (1839-1919), a Scottish meteorologist, discovered that water vapor in the atmosphere will not condense to form clouds unless dust is present. He counted dust particles in saturated air by slightly expanding the air. Water vapor condensed on the dust particles, and the waterdrops that formed could be seen and counted. The Scottish physicist Charles T. R. Wilson modified Aitken’s apparatus to form what he called a cloud chamber. In the cloud chamber, the paths of moving charged particles are made visible. Aitken was born on Sept. 18, 1839, in Falkirk. He died on Nov. 14, 1919.
