Vapor
2 of the 6 encyclopedias on this shelf carry an entry for Vapor. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
in physics, an aëriform fluid into which some volatile substance is changed by the action of heat. Vapor is essentially the same as gas, but the word vapor is conventionally limited to the gaseous state of a body which is liquid or solid at ordinary temperatures, while the term gas is applied to aëriform bodies which are in that rarefied state at ordinary temperatures. Thus we speak of hydrogen gas, but of watery vapors. Vapors, like gases, have a certain elastic force, by which they exert a pressure on every part of any vessel in which they are inclosed. Vapors are formed instantly in a vacuum; in the atmosphere they are generated more slowly. When not saturated they exactly resemble gases in their action; when saturated and in contact with the liquid by which they were generated, they can neither be compressed nor expanded, but remain constant, both in their elastic force and in their density. Vapors of different composition vary in density. Thus if atmospheric air be taken as unity, the vapor of water 0.6235, that of alcohol 1.6138, that of sulphur 6.6542, and that of mercury 6.9760.
The Encyclopedia of Founding (1892)
Heat converts liquids into vapors, and the process is called vaporization. Heat applied to a solid first expands it, then melts it, and finally turns it into vapor. When vapor is formed sensible heat is absorbed and cold is produced. Hence when the skin is moistened with a volatile liquid (one that passes readily into vapor), like alcohol, a sensation of cold is produced; the heat has been consumed.
What Has Changed in the Last Century
The physics here still holds: Collier's 1921 figures for vapor density relative to air (water 0.6235, mercury 6.9760) are close to values still in use, and saturated vapor in contact with its liquid does hold a fixed pressure at a given temperature, as any steam table shows. What has firmed up is the vapor/gas distinction that Collier's called "conventional": since Thomas Andrews's work on carbon dioxide in 1869 and van der Waals's equation of state in 1873, the line has been drawn at the critical temperature, above which no amount of pressure will liquefy a substance, so a "vapor" is simply a gas below its critical point. The Founding volume's remark that "sensible heat is absorbed" during vaporization is now stated as latent heat of vaporization, and explained molecularly as the energy needed for fast-moving molecules to break free of the liquid surface. Chemists no longer speak of "watery vapors" in the plural, but the entries would still pass a physics class.
Written for this edition, 2026.