ring themselves to their original
condition in the dark. This is more remarkably displayed in the iodide
of platinum, which readily recieves a photogenic image by darkening
over the exposed surfaces, but speedily loses it by bleaching in the
dark. The ioduret of Daguerre's plate, and some other iodides, exhibit
the same peculiarity--This leads us to the striking fact, that bodies
which have undergone a change of estate under the influence of
day-light have some latent power by which they can renovate themselves.
Possibly the hours of night are as necessary to inanimate nature as
they are to the animate. During the day, an excitement which we do not
heed, unless in a state of disease, is maintained by the influence of
light and the hours of repose, during which the equilibrium is
restored, are absolutely necessary to the continuance of health.

Instead of a few chemical compounds of gold and silver, which at first
were alone supposed to be photographic, we are now aware that copper,
platinum, lead, nickel, and indeed, probably all the elements, are
equally liably to change under the sun's influence. This fact may be
of benefit to engravers, for if steel can be made to take photographic
impressions, the more laborious process of etching may be dispensed
with. In fact, in the latter part of this work, a process is described
for etching and taking printed impressions from Daguerreotype plates.
As yet this process has produced no decided beneficial results--but
future experiments may accomplish some practical discovery of intrinsic
value to the art of engraving.

A very simple experiment will prove how essential light is to the
coloring of the various species comprising the vegetable and animal
kingdoms. If we transplant any shrub from the light of day into a dark
cellar, we will soon see it lose its bright green color, and become
perfectly white.

Another effect of light is that it appears to impart to bodies some
power by which they more readily enter into chemical combination with
others. We have already said that chlorine and hydrogen, if kept in
the dark, will remain unaltered; but if the chlorine alone be
previously exposed to the sun, the chlorine thus solarised will unite
with the hydrogen in the dark. Sulphate of iron will throw down gold
or silver from their solutions slowly in the dark; but if either
solution be first exposed to sunshine, and the mixture be then made, in
the dark, the precipitation take