38 CHANGES OF CONDITION.THE PASSIVE CONDITION.
The change of some metals, particularly chromium and iron, into the
passive condition is a phenomenon distinct from allotropy, and one of which
the significance is not yet fully understood. The surface of metals when
passive is more "noble" than when they exist in the ordinary state, as is
shown by the lesser tendency to react chemically, and by the electrochemical
behavior; in other respects there is no distinction between the ordinary and
the passive conditions.
Metals can be obtained in the passive state by treatment with nitric acid,
or other oxidizing agents, or by anodic oxidation.- Passive Iron.
Fill a small beaker with concentrated nitric acid, a large one
with water, and another small one with copper sulphate solution.
Suspend a piece of iron 4 to 5 cm. long (a thick iron screw will
answer) bv a platinum wire and submerge it completely in the
nitric acid, then lift it cautiously from the acid, rinse it in the
water, and finally dip it into the copper sulphate solution, using
care in each operation not to allow the iron to come in contact
with any solid object, nor to suffer any jar. When the iron is
taken from the copper sulphate solution it shows a gray color;
it has not reacted with the copper salt. The reaction takes place
immediately, however, when the passive condition is destroyed;
for example, by striking a sharp blow with a glass rod. Starting
from the point hit, a coating of copper, which precipitates from
the adhering film of copper salt solution, spreads over the sur-
face of the iron.
The experiment can be repeated immediately, for the coating
of copper is quickly dissolved in the nitric acid. When carried
out on a larger scale this experiment is well adapted for lecture
demonstration.AMORPHOUS STATE.As already stated, the phenomenon of supercooling is very generally observed
when melted substances solidify. This can often be obviated by means of
"seeding" with small fragments of the crystallized substance. In other
cases the supercooling persists far below the true temperature of crystalli-
zation, and a gradual changing into the crystalline form takes place only very
slowly, or perhaps not even to a detectable extent. Such strongly super-
cooled fluids are designated as amorphous substances when they, like glass