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ducted very slowly from the upper part downwards, until the
whole mass is transformed into a cylinder of ice. The
tendency of the ice to form at the surface of the mercury, which
•on account of its superior conductivity is cooler than the
glass, must be prevented by keeping the nether part of the
instrument isolated with cotton. 1 The operation is best
performed on a cold winter day, by placing the instrument in
a-snowdrift or exposing it to the free air. The freezing process
is introduced from above, by surrounding the capillary tube
with snow and a few crystals of CaCL. + 6aq. Ordinarily the
formation of the whole cylinder lasted 6 to 8 hours. This
cylinder was without a single bubble of air and entirely
transparent, if the instrument was filled with pure water - and all
operations correctly performed. The ice from salt water is
somewhat opaque and shows a radial structure.
Afterwards the dilatometer is either dipped for a moment
in water of + 10’ or + 15° C or warmed by the hand, until
the outer layer of ice, nearest to the glass, is melted, then the
stopcock d is opened. The pressure of the mercury makes a
thin film of metal ascend between the glass and the compact
ice cylinder in the reservoir, which will thus be surrounded
by an envelope of mercury. The melted ice leaves the
instrument by the capillary tube. If this operation is conducted
with address, the ice will be almost completely enveloped with
a metallic rind and only be visible from without as a narrow
stripe, where the ice cylinder touches the glass, (see plate 20).
Finally the stopcock d is shut and the dilatometer placed
in an iron vessel, filled with mercury. An iron hook is fixed
in the bottom, which keeps the reservoir and a part of the
stem and the capillary tube of the dilatometer immersed in
the fluid, which is constantly stirred by an iron paddle. A
thermometer is placed beside the reservoir, and a calibrated
scale-tube is joined to the dilatometer at c. Outside the vessel
a cooling mixture of snow and NaCl is applied, which makes
the temperature of the mercury sink very slowly.
When an observation is to be made, the regular sinking
of the temperature in the mercury vessel is compensated by
1 This part of the experiment requires the constant watchfulness of
the experimentalist.
2 In this case the clanger of bursting is infinitely greater than
other–wise, on account of the extreme hardness of absolute!}’ pure ice. Ice from
«alt water is much weaker and seldom causes the bursting of the glass
reservoir.
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