The exposed portion retains its full metallic
brilliancy, giving an additional proof that the color depends upon
molecular arrangement, and this with the allotropic forms of silver is
subject to change from almost any influence.
_Stability._--These substances vary greatly in stability under
influences difficult to appreciate. I have two specimens of the gold
yellow substance, C, both made in December, 1886, with the same
proportions, under the same conditions. One has passed to dazzling
white, normal silver, without falling to powder, or undergoing
disaggregation of any sort; the fragments have retained their shape,
simply changing to a pure frosted white, remaining apparently as solid
as before; the other is unchanged, and still shows its deep yellow
color and golden luster. Another specimen made within a few months and
supposed to be permanent has changed to brown. Complete exclusion of
air and light is certainly favorable to permanence.
_Physical Condition._--The brittleness of the substances B and C, the
facility with which they can be reduced to the finest powder, makes a
striking point of difference between allotropic and normal silver. It
is probable that normal silver, precipitated in fine powder and set
aside moist to dry gradually, may cohere into brittle lumps, but these
would be mere aggregations of discontinuous material.
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