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.

I am, etc.,

M. CAREY LEA.
Philadelphia, October 22, 1891.

* * * * *


THE SCIENTIFIC AMERICAN

Architects and Builders Edition.

$2.50 a Year. Single Copies, 25 cts.