By the term Water-vapour, or Aqueous vapour, is meant the invisible steam always present in the air. Every one is familiar with the fact, that when water is heated it passes into an invisible vapour or gas, which becomes dissolved in the air. A vessel of water, for instance, may be placed on a table in the middle of a room, heated by means of a spirit lamp till it boils, and kept boiling till the water is entirely driven off into vapour or evaporated. The air in the room shows no visible change, though it has had all this water-vapour added to it. But it may be easily made to yield back some of the vapour. Let an ice-cold piece of glass, metal, or other solid substance be brought into the room. Though perfectly dry before, its surface instantly grows dim and damp. And if it is large and thick enough to require some minutes to get as warm as the air in the room, the dimness or mist on its surface will pass into trickling drops of water. The air is chilled by the cold glass, and gives up some of its moisture. Cold air cannot retain so much diffused vapour as warm air, so that the capacity of the air for vapour is regulated by its temperature. The passage in bold is an example of comparison.
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