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denote the average coefficients of expansion of mercury and
glass.
In the case, that q should be experimentally determined
with exactness for temperatures below zero, all the elements
are at hand for a recalculation of the following results.
B. Measurements of volume.
These are made by means of the dilatometer represented
by fig. 2 on plate 20 in the following way. The reservoir B is
filled to ’/;3 with mercury and cautiously heated, until the air
adhering to the surface of the glass, in contact with the
mercury, has escaped. When the last traces of air are removed,
and a real ebullition of mercury seems to begin, the stopcock
d is closed and the flame allowed to sweep the upper part of
B. The pressure of the heated air and the mercury vapour
makes the hot mercury rise in the capillary stem of the
dilatometer and All the side tubes b and c and the upper
reservoir A. The volume of the mercury, which remains in B,
must not exceed !/io of the space of the reservoir. Then the
stopcocks a, b and c are shut and the flame removed from
B to C, the water of which is heated to boiling. The stopcock
d is opened, and a jet of hot water passes from C to B, on
account of the rarefied air in the reservoir. When B is half
filled with water, a separate flame is applied to it, and the
wyater in B and C is assiduously boiled, until every trace of air
is removed. If B is allowed to cool, it is rapidly filled with
air-free water from C, which is still kept boiling. When the
temperature of the water in B has sunk to about 30° or 35° C,
the capillary tube of the dilatometer is made to dip in the
mercury, which covers the bottom of C, and then, on farther
cooling, a tiny column of mercury enters the capillary tube
in the manner shown in fig. 2. The stopcock d is shut, a is
opened, and the operation of filling the instrument with water
is completed.
If the dilatometer in this condition is combined with a
calibrated scale-tube, the wdiole dilatation of water, from its
boiling to far belowr its freezing point, can be easily
determined, but if also the change of volume by freezing and the
dilatation of the frozen water must be ascertained, which was
the case in my experiments, another preparatory operation,
which requires the greatest caution, is necessary.
The freezing of the water in the dilatometer must be con-
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