| Term | Definition |
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| Humidity (Absolute) | The absolute humidity is the amount of vapor actually present in the air and is expressed either in its expansive force, or in its weight per given volume.
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| Humidity (Relative) | Relative humidity is the ratio of the quantity of vapor actually present, to the greatest amount possible at the given condition. Complete saturation of the air by a vapor is designated as Humidity 100.
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| Hydration | A special case of solvation, where water is the solvent.
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| Hydrometer | A floating instrument for determining specific gravities; especially of liquids and solutions. It is usually a hollow glass or metal instrument, weighted at one end so as to float upright. The stem of the instrument is graduated so as to indicate the gravity of the liquid. Many instruments, for use with specific solutions, have arbitrary scales and are usually known by the names of the inventors, such as Balling and Plato.
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| Hygrometer | An apparatus for measuring the degree of moisture of the atmosphere.
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| Hygroscopic | Readily absorbing, becoming coated with, and retaining moisture, but not enough to make a liquid.
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