M. Berthelot recently called the attention of the Academy (Paris) to
the memoirs of Carey Lea on the allotropic states of silver, and
exhibited specimens of the color of gold and others of a purple color
sent him by the author. He explained the importance of these results,
which remind us of the work of the ancient alchemists, but he reserved
the question whether these substances are really isomeric states of
silver or complex and condensed compounds, sharing the properties of
the element which constituted the principal mass (97-98 per cent.),
conformably to the facts known in the history of the various carbons,
of the derivatives of red phosphorus, and especially of the varieties
of iron and steel. Between these condensed compounds and the pure
elements the continuous transition of the physical and chemical
properties is often effected by insensible degrees, by a mixture of
definite compounds.
The following letter appears in a recent number of the _Chemical
News_.
_Sir_: In a recently published lecture, Mr. Meldola seems to call in
question the existence of allotropic silver. This opinion does not
appear, however, to be based on any adequate study of the subject, but
to be somewhat conjectural in its nature. No experimental support of
any sort is given, and the only argument offered (if such it can be
called) is that this altered form of silver is analogous to that of
metals whose properties have been greatly changed by being _alloyed_
with small quantities of other metals. Does, then, Mr. Meldola suppose
that a silver alloy can be formed by precipitating silver in the
presence of another metal from an aqueous solution, or that one can
argue from alloys, which are solutions, to molecular compounds or
lakes? Moreover, he has overlooked the fact that allotropic silver can
be obtained in the absence of any metal with which silver is capable
of combining, as in the case of its formation by the action of soda
and dextrine. Silver cannot be alloyed with sodium.
Mr. Meldola cites Prange as having shown that allotropic silver
obtained with the aid of ferrous citrate contains traces of iron, a
fact which was published by me several years earlier, with an
analytical determination of the amount of iron found. Mr. Prange
repeated and confirmed this fact of the presence of iron (in this
particular case), and my other observations generally, and was fully
convinced of the existence of both soluble and insoluble allotropic
silver. Mr. Meldola's quotation of Mr. Prange would not convey this
impression to the reader.
Of the many forms of allotropic silver, two of the best marked are the
blue and the yellow.
Blue allotropic silver is formed in many reactions with the aid of
many wholly different reagents. To suppose that each of these many
substances is capable of uniting in minute quantity with silver to
produce in all cases an identical result, the same product with
identical color and properties, would be an absurdity.
Gold-colored allotropic silver in thin films is converted by the
slightest pressure to normal silver. A glass rod drawn over it with a
gentle pressure leaves a gray line behind it of ordinary silver. If
the film is then plunged into solution of potassium ferricyanide it
becomes red or blue, while the lines traced show by their different
reaction that they consist of ordinary silver. Heat, electricity, and
contact with strong acids produce a similar change to ordinary gray
silver.
These reactions afford the clearest proof that the silver is in an
allotropic form. To account for them on suppositions like Mr.
Meldola's would involve an exceedingly forced interpretation, such as
no one who carefully repeated my work could possibly entertain.