Word: vapor
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Dates: during 1950-1959
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...standard Wilson cloud chamber for making atomic particles visible is a rather complicated apparatus. It contains moist air and a movable diaphragm or piston to rarefy it suddenly. This action cools the air by expansion and makes it "supersaturated" with water vapor which will condense into water droplets if given proper nuclei to condense upon. Fast-moving atomic particles provide the nuclei by ionizing (electrifying) the normally neutral atoms of the air. So particles (e.g., cosmic rays from outer space) that pass through the cloud chamber become visible as thin white trails of water droplets...
...chamber in operation, the felt under its cover is saturated with methyl (wood) alcohol. The alcohol vapor diffuses downward, becoming colder as it approaches the dry ice at the bottom. At some point in its downward motion it makes the air supersaturated. In this sensitive layer, cosmic rays or other fast-moving atomic particles leave trails that show up in a flashlight beam as brilliant white streaks...