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means of a test tube filled with warm water, which is dipped
in the fluid. If the quantity of mercury is large [about 7 or
8 kilogrammes], and the stirrer is vigorously moved, the index
of the thermometer can be kept almost stationary by means
of this artifice,1 with exception of a few oscillations, which
only at very low temperatures amount to 0°.io C but
ordina-’rily do not exceed 0°.03 C. On account of the large capacity
of the dilatometer, the oscillations of the mercury index of its
scale-tube are not perceptible, the meniscus moves very
regularly over the usual number of millimetres (30 unto 50) for
every centigrade degree and then, two minutes after the
thermometer has indicated the state of equilibrium of temperature in
the mercury bath, attains a fixed position without any
oscillations. 2 The excellent conductivity of air-free ice, which is
surrounded by a metallic rind, is indeed surprising. In the
vicinity of the melting point, however, the behavior of the ice
is totally changed. In the next chapter it will be proved, that
at these temperatures a rapid change of the volume of the
ice-takes place. Immediately below the melting point the abnormal
change of volume increases enormously, and simultaneously
the index of the dilatometer becomes extremely sensitive to
slight oscillations of temperature. Happily, it is an easy task
to maintain the temperature absolutely constant for a long
time in this case, because we may dispense with the cooling
mixture, provided that the temperature of the air in the room
is kept a few degrees below zero. Then, by repeated
introduction of a warm body in the mercury bath, we may
compensate the slight loss of heat, suffered by conduction or
radiation, and approach the real melting point by regular
observations unto 0°.o2 or 0°.oi C.
The dilatation of the frozen water being ascertained by
one or two series of observations between —18° and the
melting point, it remains to determine the change of volume by
the melting of the ice and the expansion of the liquid water.
For this purpose b is made to dip in a cup containing a
weighed quantity of mercury, c is shut, b is opened and the
temperature of the mercury bath raised above the
melting-point of the ice. After several hours the instrument is allowed
1 The operation, like most of these experiments, requires the aid of an
assistant, who with a magnifying glass reads off the scale-tube of the
dilatometer, while the experimentor regulates the temperature and observes
the thermometer with a kathetometer.
2 The temperature was of course kept constant much longer.
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