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.