LXIII
state), i.e. heat is disengaged or heat is set free. So e.g.
as long as snow falls the severity of cold is diminished; water
freezing near plants saves them from cold, 313; water in a
tube, from which air has been excluded can be cooled down to —
10°C without freezing, if it is not shaken; if shaken part of it
freezes at once and the temperature rises to 0° C.
§ 154. Whether some bodies cool quickly or slowly causes
a difference in their properties (cohesion); f.e. sulphur by a
sudden solidification is changed into a tough, tender mass; if
cooled down slowly we get a hard, brittle substance. By a quick
cooling glass and iron are made hard and brittle.
§ 155. The disengagement of heat can only be observed
if the processes of solidification and liquefaction take place
very quickly. So f.e. if vapour at 100° C, is led into a vessel
containing water at 0° C, it will be liquified and the whole mass
of water will have a temperature of 100° C. This can only be
the result of the vapour passing over from the gaseous into the
liquid state.
§ 156. Most chemical substances when passing from the
liquid or gaseous into the solid state assume some definite geo
metric form or are said to crystallize. Crystals are produced
when a substance, such as nitre, is dissolved in water and the
solution is allowed to evaporate gradually, or when a body, such
as sulphur, is melted and allowed to solidify by cooling. We
are ignorant of the mode, in which such crystallization takes
place; but the process, by which crystals are formed is always a
very slow one, and the more gradually a crystal has been pro
duced the larger and more perfect it is. As a rule any parti
cular body possesses a definite form, in which it always crystalli
zes and by which it can be distinguished. The many thousand
different forms are arrayed in 6 systems. There are crystals so
