to
sunlight. With C the case is quite different, the color changes from
that of red gold to that of pure yellow gold. The experiment is an
interesting one. 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. With allotropic
silver the case is very different, the particles dry in optical
contact with each other, the surfaces are brilliant, and the material
evidently continuous. That this should be brittle indicates a totally
different state of molecular constitution from that of normal silver.

_Specific Gravities._--The allotropic forms of silver show a lower
specific gravity than that of normal silver.

In determining the specific gravities it was found essential to keep
the sp. gr. bottle after placing the material in it for some hours
under the bell of an air pump. Films of air attach themselves
obstinately to the surfaces, and escape but slowly even in vacuo.

Taken with this precaution, the blue substance, B, gave specific
gravity 9.58, and the yellow substance, C, specific gravity 8.51. The
specific gravity of normal silver, after melting, was found by G. Rose
to be 10.5. That of finely divided silver obtained by precipitation