sh-yellow
hue. The first impression of light seems rather to whiten than darken
the paper, by discharging the original color, and substituting for it a
pale greyish tint, which by slow degrees increases to a dark slate
color; but if arrested, while yet, not more than a moderate ash grey,
and held in a current of steam, the color of the parts acted upon by
light--and of that only--darkens immediately to a deep purple.

Here I must leave the subject of the action of light upon metalic
compounds--referring to Mr. Hunts work for any further information the
student may desire on the other metals--as I find myself going beyond
my limits. I cannot, however, entirely dismiss the subject without
giving a few examples of the action of light on the juices of plants,
some of which produce very good photographic effect.

CORCHORUS JAPONICA--The juice of the flowers of this plant impart a
fine yellow color to paper, and, so far as ascertained, is the most
sensitive of any vegetable preparation; but owing to its continuing to
change color even in the dark, photographic images taken on paper
prepared with it soon fade out.

WALL FLOWER.--This flower yields a juice, when expressed with alcohol,
from which subsides, on standing, a bright yellow finely divided
faecula, leaving a greenish-yellow transparent liquid, only slightly
colored supernatant. The faecula spreads well on paper, and is very
sensitive to light, but appears at the same time to undergo a sort of
chromatic analysis, and to comport itself as if composed of two very
distinct coloring principles, very differently affected. The one on
which the intensity and sub-orange tint of the color depends, is
speedily destroyed, but the paper is not thereby fully whitened. A
paler yellow remains as a residual tint, and this on continued exposure
to the light, slowly darkens to brown. Exposed to the spectrum, the
paper is first reduced nearly to whiteness in the region of the blue
and violet rays. More slowly, an insulated solar image is whitened in
the less refrangible portion of the red. Continue the exposure, and a
brown impression begins to be percieved in the midst of the white
streak, which darkens slowly over the region between the lower blue and
extreme violet rays.

THE RED POPPY yields a very beautiful red color, which is entirely
destroyed by light. When perfectly dried on paper the color becomes
blue. This blue color is speedily discharged by exposure to the sun