LXV
pans). Evaporation takes place only at the surface of a liquid
by pressure; as the pressure of the atmosphere is a hind
rance to the formation of vapours, the more this pressure is
diminished the more evaporation takes place.
§ 163. By evaporation heat is rendered latent or it is a ab
sorbed from the surrounding air: But in evaporation all the
heat is not rendered latent (as in boiling), else it would not be
possible to warm a liquid in an open vessel. We feel cool after
rain, 317; fire extinguished by water, 318; wet fire-wood, 319;
wine kept cool by wet clothes. 320. Filtration (see Porosity),
321; evaporation taking place on our skin, 322; we feel cold
coming out of a bath. 323. Moist linen is cold and injurious.
324.
§ 164. We have seen that temperature and evaporation are
in close connection, so it is of great importance to know the
actual quantity of vapour present in the air; this quantity is
very variable with the seasons, the climates, the temperature and
other causes. The degree of moisture or the distance of the
air from its point of saturation may be known by the Psychro
meter, 325 (also called "Wet-bulb Hygrometer").
§ 165. Also a great many Meteorological Phenomena de
pend upon heat and evaporation. Smoke does not rise before
rain, 328; a great number of animal and vegetable substances
(hair, strings etc.) elongate as the air becomes moist, and con
tract as it becomes dry; thus they indicate the quantity of va
pour contained in air. Instruments serving the purpose of
measuring the amount of moisture in the atmosphere are called
Hygrometers (or Hygroscopes). 329. Clothes get damp by
the moisture in the air. 330.
§ 166. Liquefaction or the condensation of vapours may
be due to 3 causes: a) chemical affinity (salt absorbs the
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