TRANSITION POINT OF SODIUM SULPHATE. 197
water at 32°, filter, and divide the solution equally among five
small, clean flasks which have been freshly rinsed with distilled
water. In filling the flasks take care not to wet the necks with
the solution. Immediately stopper each flask with a loose plug
of cotton. Allow two of the flasks to stand in the ice-chest over
night, or longer, until the heptahydrate has crystallized out in
large, closely packed crystals which fill about one-third the vol-
ume of the liquid. Allow the three other flasks to cool to room
temperature; a thick, oily solution is obtained from which,
although it is supersaturated, no crystals separate even when
the liquid is gently rotated. Inoculate the first flask with a
minute fragment of sodium sulphate, whereupon crystals at once
begin to form at the point inoculated and grow rapidly until the
whole contents of the flask have solidified. Place a trace of
sodium sulphate upon a piece of filter paper, then brush it
off as completely as possible. Tear off a piece of this filter
paper and use it to inoculate the contents of the second flask.
Open the third flask, and close it with the thumb, which has
first been well rinsed with distilled water. If the thumb is per-
fectly clean and free from crystals of the salt and particles of
dust, the solution will bear shaking without the appearance of
crystals.
Introduce a trace of decahydrate into each of the two flasks
which contain the deposited heptahydrate. The supernatant
solution then crystallizes into the decahydrate so that the two
hydrates are obtained together in the flask one above the other.
After standing a long time, the more soluble heptahydrate goesover into the less soluble decahydrate.
- Transition Point of Sodium Sulphate,
Na 2 SO 4 .10H 2 O<^Na 2 SO 4 + 10 H 2 O.
32.383°.
If a mass of crystals of sodium sulphate decahydrate is heated slowly, the
temperature rises steadily until 32.383° is reached, at which point it remains
constant for a considerable time, because the heat then received from the
exterior is used entirely in dehydrating the salt. If the mixture of water
and sodium sulphate is heated to a higher temperature and then allowed to
cool slowly, the temperature again remains constant at the same point as
long as the heat of hydration is sufficient to compensate the loss of heat to
the surroundings. Compare the corresponding relations in the change in