LXVII
surface, from which moment water is transformed into vapour
or ebullition commences.
§ 169. For a given pressure boiling always begins at a cer
tain temperature, f.e. at 100°C for water; but this temperature
varies with different bodies. (About the latent heat of evapora
tion or ebullition, see I.) 339. 342. 343.
§ 170. The boiling point is influenced: a) by substances
in solution. If f.e. salt is mixed, the boiling point will be rais
ed, but sand or saw-dust put into the water makes no difference
whatever, why? b) by the nature of the vessel in which
water is boiled, 340; c) by pressure. Water or substances
heated in closed vessels, in which case boiling is impossible,
but temperature, the elastic force and density of the liquid in
crease. Papin's digester, 345 (a safety-valve necessary). The
greater the pressure of the atmosphere on the surface of a liquid
the higher its boiling point will be.
§ 171. A liquid cannot boil until heat has increased re
pulsion between the molecules of the liquid to such a degree as
to overcome the pressure of the atmosphere (see § 9) and of
the upper layers of the liquid; hence either the temperature must
be raised or the pressure diminished. Water boils under the
receiver of an air-pump. 346. Sulphuric ether boils even at
0°C under the receiver of an air-pump. 347. (Compare the
paradoxical experiment 42 in the Physics Primer, according
to which water in a glass-flask, which formerly boiled, begins
to boil again, when cooled by a stream of cold water). Water
boils at lower temperatures on high mountains than at the sea-
level (this is a great obstacle to the preparation of food), thus
heights may be measured by the boiling point. 348.
§ 172. 3) The elastic force of a aqueous vapour increases
at a high temperature. 349. In utilizing this force and consi
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