194 HYDRATES.
chloride (weighing it to an accuracy of two significant figures),
and determine the freezing-point of the solution with a thermom-
eter which reads from — 20° to 100° and is graduated in whole
degrees. Repeat the measurements with successive additions of
1 g. of the salt. As cooling-bath a mixture of ice and salt can be
used at first. The solidification point is easily recognized by the
temperature remaining constant, but this point is usually pre-
ceded by a supercooling of about 0.5° before crystallization is
induced by vigorous stirring. When temperature equilibrium
has been established, withdraw the test-tube from the freezing
mixture in order to note the appearance of the separated solid.
Pure water usually shows a crust of ice and large needles; the
solutions show finely divided flakes of ice. If, in about the third
experiment, the neighborhood of the eutectic point is reached, a
freezing-point is at first observed as usual, but after the mixture
has stood in the freezing bath for some time the temperature
again falls until a second halting-point is registered at — 8.4°, and
at this temperature opaque, white masses of the cryohydrate, con-
sisting of ice and barium-chloride-dihydrate, separate. In the
succeeding experiments the solidifying point rises rapidly since
the region is reached in which the salt acts as the solvent for the
water. The deposited substance now consists of barium-chloride-
dihydrate which is easily distinguishable from ice. At the same
time the halting-point on the thermometer becomes less distinct
because the heat of solution of the salt is less than the heat of
fusion of ice. It is possible, however, to determine very sharply,
as in the magnesium nitrate series, the point at which crystalli-
zation begins; and this is especially true since the baths used in
these experiments are of ice water, water at the room temper-
ature, and finally liquid paraffin, instead of the opaque mixture of
ice and salt. Each measurement is repeated several times, leav-
ing a little of the barium salt undissolved each time to serve in
starting the next crystallization. It is sufficient to carry the
curve up to about 70° in three or four experiments. The tempera-
ture readings in the case of the first field (the ice curve) should be
accurate to about 0.1°, and in the second field (solubility curve)
to within one or two whole degrees.
The curve is plotted as before. Between 0% and 24% BaCl 2
the temperature of solidification falls from 0° to — 8.4°; between