LXVI
vapour of the air), b) Pressure. If vapour is exposed to an
immense pressure, a point will be finally reached, where the
vapour present is just sufficient to saturate a given space. Any
pressure beyond this point will cause the vapour to pass into
the liquid state. c) Liquefaction by cooling. After vapours are
in a state of saturation the slightest lowering of temperature
will stop repulsion between the molecules and attraction will
preponderate, they agglomerate, form small drops, float in the air
and are deposited on the surrounding bodies. Thus windows
are covered with moisture. 331. Vapours passing over from the
gaseous to the liquid state may be seen over a vessel containing
hot water. In winter even the breath of men and animals will be
rendered visible. Dew depends not only on the degree of
temperature, but also on the quantity of vapour contained in
the air, for liquefaction begins only after the air is saturated.
332 (Hoar frost). No deposit of dew if there are clouds,
333; or if wind blows. 334. Fogs and mists are chiefly caused
by the moist soil being warmer than the air. 335. Clouds are
only a mist in higher regions and consist also of small hollow
vesicles of water suspended in the air ("small balloons"). They
may rise and disappear all of a sudden. 336. 337. Formation
of Rain, Snow, and Hail. 336.
§ 167. By pressure and cooling even some gases have
been forced to liquify and as some physicists (Cailletet, Raoul,
Pictet) have succeeded in the liquefaction of Oxygen and Hy
drogen even, there is no doubt, that all the gases are capable of
such a change, and taking this for granted there would be no
scientific difference between vapours and gases.
§ 168. Ebullition or boiling consists in the production of
elastic bubbles, which rise and in layers of lower temperature
are condensed again until large bubbles rise and burst on the
