Vapor

—1. Moisture in the air, any light cloudy substance in the air, as, smoke or fumes. 2. In physics, the semi-gaseous state of a substance which is liquid at ordinary temperatures; as, water in the state of saturated steam. A vapor that is not near the saturation point, behaves like a gas under changes of temperature and pre ure, but if it be sufficiently compre ed. or cooled, it reaches a point where it begins to condense. It then no longer obeys the same laws as a gas, but its pre ure cannot be increased by diminishing the size of the ve el containing it, but remains constant, except when the temperature is changed. The only gas that can prevent a liquid evaporating seems to be its own vapor.