192 HYDRATES.
The method employed is the same in principle as that followed
in determining freezing-points with the well-known Beckmann
apparatus, but it is very much more simple because the freezing-
points here measured are so far apart that an accuracy of between
one and two degrees is sufficient. As container for the sub-
stance, a test-tube about 19 cm. long and 2 to 2.5 cm. wide suffices;
as thermometer, one graduated in whole degrees and reading
to 120° is suitable; an air-mantle is superfluous; as stirrer, a thin
glass rod or a piece of iron wire bent into a suitable shape may be
employed, although it is preferable to use a heavy platinum wire
as in the Beckmann apparatus. As heating and cooling baths,
two beakers of from 300 to 400 c.c. capacity, filled with paraffin
oil and provided with heavy iron wire stirrers, are used. Weigh-
ings need be accurate only to two, or at the most three, significant
figures.
Place about 12 g. of magnesium nitrate hexahydrate in the test
tube and close the mouth with a stopper through which the
thermometer and stirrer are inserted. Dip the tube well into the
heating-bath, heat rapidly to 70°, and from that point on, more
slowly. The melting point of the substance is shown, in the
first place, by the disappearance of crystals, and, second, by the
fact that the temperature remains for some time constant (89°).
Next heat the bath to 110°-120°, transfer the test-tube to the
cooling-bath, which has been brought to about 90°, and observe
the point at which crystals begin to appear and the temperature
ceases to fall during the crystallization; by repeating the experi-
ment several times it is possible to establish very sharply the point
at which the crystals appear or disappear. Now, from a weighed
pipette, or from a small weighed wash-bottle, add one or two
grams of water, and determine the amount added by a second
weighing. Mix the mass well with the water, melt it completely,
and determine as before the point where crystals begin to sepa-
rate; it lies considerably lower. Follow the curve in this way by
three or four similar experiments until the laboratory temperatureis reached. The heating-bath should be, in every case, about 10°
warmer and the cooling-bath 10° colder than the temperature
which is to be measured, the latter being first determined by a
preliminary rough experiment. As the concentration of the
water increases, the points of constant temperature become less