By M. CAREY LEA.
In the first part of this paper were described certain forms of
silver; among them a lilac blue substance, very soluble in water, with
a deep red color. After undergoing purification, it was shown to be
nearly pure silver. During the purification by washing it seemed to
change somewhat, and, consequently, some uncertainty existed as to
whether or not the purified substance was essentially the same as the
first product; it seemed possible that the extreme solubility of the
product in its first condition might be due to a combination in some
way with citric acid, the acid separating during the washing. Many
attempts were made to get a decisive indication, and two series of
analyses, one a long one, to determine the ratio between the silver
and the citric acid present, without obtaining a wholly satisfactory
result, inasmuch as even these determinations of mere ratio involved a
certain degree of previous purification which might have caused a
separation.
This question has since been settled in an extremely simple way, and
the fact established that the soluble blue substance contains not a
trace of combined citric acid.
The precipitated lilac blue substance (obtained by reducing silver
citrate by ferrous citrate) was thrown on a filter and cleared of
mother water as far as possible with a filter pump. Pure water was
then poured on in successive portions until more than half the
substance was dissolved. The residue, evidently quite unchanged, was,
of course, tolerably free from mother water. It was found that by
evaporating it to dryness over a water bath, most of the silver
separated out as bright white normal silver; by adding water and
evaporating a second time, the separation was complete, and water
added dissolved no silver. _The solution thus obtained was neutral._
It must have been acid had any citric acid been combined originally
with the silver. This experiment, repeated with every precaution,
seems conclusive. The ferrous solution, used for reducing the silver
citrate, had been brought to exact neutrality with sodium hydroxide.
After the reduction had been effected, the mother water over the lilac
blue precipitate was neutral or faintly acid.
A corroborating indication is the following: The portions of the lilac
blue substance which were dissolved on the filter (see above) were
received into a dilute solution of magnesium sulphate, which throws
down insoluble allotropic silver of the form I have called B (see
previous paper). This form has already been shown to be nearly pure
silver. The magnesia solution, neutral before use, was also neutral
after it had effected the precipitation, indicating that no citric
acid had been set free in the precipitation of the silver.
It seems, therefore, clear that the lilac blue substance contains no
combined citric acid. Had the solubility of the silver been due to
combination with either acid or alkali, the liquid from which it was
separated by digestion at or below 100° C. must have been acid or
alkaline; it could not have been neutral.
We have, therefore, this alternative: In the lilac blue substance we
have either pure silver in a soluble form or else a compound of
silver, with a perfectly neutral substance generated from citric acid
in the reaction which leads to the formation of the lilac blue
substance. If this last should prove the true explanation, then we
have to do with a combination of silver of a quite different nature
from any silver compounds hitherto known. A neutral substance
generated from citric acid must have one or more atoms of hydrogen
replaced by silver. This possibility recalls the recent observations
of Ballo, who, by acting with a ferrous salt on tartaric acid,
obtained a neutral colloid substance having the constitution of
arabin, C6 H10 O6.
To appreciate the difficulty of arriving at a correct conclusion, it
must be remembered that the silver precipitate is obtained saturated
with strong solutions of ferric and ferrous citrate, sodium citrate,
sulphate, etc. These cannot be removed by washing with pure water, in
which the substance itself is very soluble, but must be got rid of by
washing with saline solutions, under the influence of which the
substance itself slowly but continually changes. Next, the saline
solution used for washing must be removed by alcohol. During this
treatment, the substance, at first very soluble, gradually loses its
solubility, and, when ready for analysis, has become wholly insoluble.
It is impossible at present to say whether it may not have undergone
other change; this is a matter as to which I hope to speak more
positively later. It is to be remarked, however, that these allotropic
forms of silver acquire and lose solubility from very slight causes,
as an instance of which may be mentioned the ease with which the
insoluble form B recovers its solubility under the influence of sodium
sulphate and borate, and other salts, as described in the previous
part of this paper.
The two insoluble forms of allotropic silver which I have described as
B and C--B, bluish green; C, rich golden color--show the following
curious reaction. A film of B, spread on glass and heated in a water
stove to 100° C. for a few minutes becomes superficially bright
yellow. A similar film of the gold colored substance, C, treated in
the same way, acquires a blue bloom. In both cases it is the surface
only that changes.
_Sensitiveness to Light._--All these forms of silver are acted upon by
light. A and B acquire a brownish tinge by some hours' exposure 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 is
stated to be 10.62.[1]
[Footnote 1: Watts' Dict., orig. ed., v. 277.]
I believe these determinations to be exact for the specimens employed.
But the condition of aggregation may not improbably vary somewhat in
different specimens. It seems, however, clear that these forms of
silver have a lower specific gravity than the normal, and this is what
would be expected.
Chestnut Hill, Philadelphia, May, 1889.
--_Amer. Jour. of Science._
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