te a little longer exposure is required; and instead
of taking a picture in four or five seconds, two or three minutes are
necessary."

The durability of daguerreotypes has been, and is still, doubted by
many, but experiment has proved that they are more permanent than oil
paintings or engravings.

ETCHING DAGUERREOTYPES.--There are several methods of accomplishing
this object; discovered and applied by different individuals.

The first process was published at Vienna by Dr. Berres, and consisted
in covering the plate with the mucilage of gum arabic, and then
immersing the plate in nitric acid of different strengths.

Mr. Figeau, of whom I have already spoken, likewise discovered a
process for the engraving of Daguerreotypes; and founded on the belief
that the lights of a Daguerreotype plate consists of unaltered silver,
while the dark or shadows consists of mercury or an amalgam of mercury
with silver. He finds that a compound acid, consisting of a mixture of
nitric, nitrous, and muriatic acids, or of nitric mixed with nitrate of
potass and common salt, has the property of attaching the silver in
presence of the mercury without acting upon the latter. Bi-chloride of
copper answers the purpose also, but less completely.

"When the clean surface of a Daguerreotype plate is exposed to the
action of this menstruum, particularly if warm, the white parts, or
lights are not altered, but the dark parts are attacked, and chloride
of silver is formed, of which an insoluble coating is soon deposited,
and the action of the acid soon ceases. This coat of chloride of
silver is removed by a solution of ammonia, and then the acid applied
again, and so on, until the depth of biting in is sufficient. However,
it is not possible, by repeating this process, to get a sufficient
force of impression; a second operation is required, in order to obtain
such a depth as will hold the ink, to give a dark impression; for this
purpose the whole plate is covered with drying oil; this is cleared off
with the hand, exactly in the way a copper plate printer cleans his
plate. The oil is thus left in the sinkings, or dark bitten in parts
only. The whole plate is now placed in a suitable apparatus, and the
lights or prominent parts of the face are gilt by the electrotype
process. The whole surface is now touched with what the French
engravers call the "Resin Grain," (grain de resine), a species of
partial stopping out, and it is at once bitten