The expressed juice, alcoholic, or watery infusion of flowers, or
vegetable substances, may be made the media of photogenic action. This
fact was first discovered by Sir John Herschel. We have already given
a few examples of this in the third chapter.
Certain precautions are necessary in extracting the coloring matter of
flowers. The petals of fresh flowers are carefully selected, and
crushed to a pulp in a marble mortar, either alone or with the addition
of a little alcohol, and the juice expressed by squeezing the pulp in a
clean linen or cotton cloth. It is then to be spread upon paper with a
flat brush, and dried in the air without artificial heat. If alcohol
be not added, the application on paper must be performed immediately,
as the air (even in a few minutes), irrecoverably changes or destroys
their color. If alcohol be present this change is much retarded, and
in some cases is entirely prevented.
Most flowers give out their coloring matter to alcohol or water. Some,
however, refuse to do so, and require the addition of alkalies, others
of acid, &c. Alcohol has, however, been found to enfeeble, and in many
cases to discharge altogether these colors; but they are, in most
cases, restored upon drying, when spread over paper. Papers tinged
with vegetable colors must always be kept in the dark, and perfectly
dry.
The color of a flower is by no means always, or usually, that which its
expressed juice imparts to white paper. Sir John Herschel attributes
these changes to the escape of carbonic acid in some cases; to a
chemical alteration, depending upon the absorption of oxygen, in
others; and again in others, especially where the expressed juice
coagulates on standing, to a loss of vitality, or disorganization of
the molecules. To secure an eveness of tint on paper, the following
manipulation is recommended:--The paper should be moistened on the back
by sponging and blotting off. It should then be pinned on a board, the
moist side downwards, so that two of its edges (suppose the right-hand
and lower ones) shall project a little beyond those of the board. The
board then being inclined twenty or thirty degrees to the horizon, the
alcoholic tincture (mixed with a very little water, if the petals
themselves be not very juicy) is to be applied with a brush in strokes
from left to right, taking care not to go over the edges which rest on
the board; but to pass clearly over those that project; and observing
also to carry the tint from below upwards by quick sweeping strokes,
leaving no dry spaces between them, but keeping up a continuity of wet
spaces. When all is wet, cross them by another set of strokes from
above downwards, so managing the brush as to leave no floating liquid
on the paper. It must then be dried as quickly as possible over a
stove, or in a warm current of air, avoiding, however, such heat as may
injure the tint.
In addition to the flowers already mentioned in my third chapter, the
following are among those experimented upon and found to give tolerable
good photographic sensitives. I can only enumerate them, referring the
student, for any further information he may desire on the subject, to
Mr. Hunt's work; although what I have said above is sufficient for all
practical purposes; and any one, with the ambition, can readily
experiment upon them, without further research, on any other flower he
may choose.
Viola Odorata--or sweet sented violet, yields to alcohol a rich blue
color, which it imparts in high perfection to paper
Senecio Splendens--or double purple groundsel, yields a beautiful color
to paper.
The leaves of the laurel, common cabbage, and the grasses, are found
sufficiently sensitive.
Common Merrigold yields an invaluable faecula, which appears identical
with that produced by the Wall-flower, and Cochorus japonica mentioned
before, and is very sensitive, but photographs procured upon it cannot
be preserved, the color is so fugitive.
From an examination of the researches of Sir John Herschel on the
coloring matter of plants, it will be seen that the action of the sun's
rays is to destroy the color, effecting a sort of chromatic analysis,
in which two distinct elements of color are separated, by destroying
the one and leaving the other outstanding. The action is confined
within the visible spectrum, and thus a broad distinction is exhibited
between the action of the sun's rays on vegetable juices and on
argentine compounds, the latter being most sensibly affected by the
invisible rays beyond the violet.
It may also be observed, that the rays effective in destroying a given
tint, are in a great many cases, those whose union produces a color
complementary to the tint destroyed, or, at least, one belonging to
that class of colors to which such complementary tint may be preferred.
For instance, yellows tending towards orange are destroyed with more
energy by the blue rays; blues by the red, orange and yellow rays;
purples and pinks by yellow and green rays.