198 HYDRATES.
state of aggregation (No. 6, p. 15). Practical advantage may be taken of
this behavior of hydrated sodium sulphate for producing and maintaining
very accurately the temperature of 32.383°, as, for example, for a fixed point
in thermometry. By placing a mixture of the anhydrous salt and the
hydrate in surroundings of approximately 32 to 33°, the temperature of the
mass adjusts itself sharply to 32.383°, and remains constant a very long
time at this point, for if the external temperature is somewhat higher, the
heat conducted into the mixture is absorbed in dehydrating the salt, whereas
if the surroundings are cooler the temperature of the mixture is maintained
by the heat liberated in the hydration of the salt. A bath composed of such
a mixture, therefore, can be used as a very delicate thermostat.^1
The transformation MnCl 2 .4 H 2 O <z± MnCl 2 .2 H 2 O + 2 H 3 O takes place at
58.089° ± 0.005°. Richards and Wrede, Z. phys. Chem. 61, 313 (1907).
Dissolve 100 g. of crystallized sodium sulphate in 50 c.c. of
water at about 33°, filter the solution, and cause the salt to
recrystallize by shaking the solution and cooling it under the
water tap. Drain the crystalline meal on the suction filter, wash
the salt once with a little cold water, and use it moist in the follow-
ing experiment. Place about 12 g. of the crystals in a test-tube
and insert a thermometer which is graduated in tenths, or fifths,
of a degree. Place this test-tube inside a slightly larger one so
that an air space of about 2 mm. separates their walls, and clamp
the two tubes thus arranged so that they dip into a beaker con-
taining about 600 c.c. of water. Keep the temperature of the
water at 35° to 36°.
To prepare the equilibrium mixture in the inner test-tube,
remove this from its air mantle, and dip it directly into the warm
water in the beaker. When its contents are partly melted, wipe
the tube and replace it in the larger tube. The transition tem-
perature is quickly established, especially if the mixture is stirred,
and remains constant for more than half an hour. By removing
the inner tube and allowing it to cool somewhat, the mixture
again assumes the transition temperature. Before every read-
ing of the thermometer, the mass should be stirred.
The anhydride present in the mixture is recognized as a fine
turbid powder in the presence of relatively large crystals of the
decahydrate.
1
Cf. Richards, Z. phys. Chem. 26, 690 (1898); Richards and Mark, ibid, 43,
465 (1903).