Chloride of gold with nitrate of silver gives a precipitate of a yellow brown color. Paper impregnated with the acetate of lead, when washed with perfectly neutral chloride of gold, acquires a brownish-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's rays, and papers prepared with it afford very interesting photographs.-- Future experiments will undoubtedly more fully develope the photogenic properties of flowers, and practically apply them.
Certain precautions are necessary in extracting the coloring matter of flowers. The petals of fresh flowers, carefully selected, are crushed to a pulp in a 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. If alcohol be not added, it must be applied immediately, as the air changes or destroys the color instantly.
Most flowers give out their coloring matter to alcohol or water--but the former is found to weaken, and in some cases to discharge altogether these colors; but they are in most cases restored in drying. Paper tinged with vegetable colors must be kept perfectly dry and in darkness.
To secure an eveness of tint on paper it should be first moistened on the back by sponging, and blotting off with bibulous paper. It should then be pinned on a board, the moist side downwards, so that two of its edges--the right and lower ones--project a little over those of the board. Incline the board twenty or thirty degrees to the horizon, and apply the tincture with a brush in strokes from right to left, taking care not to go over the edges which rests on the board, but to pass clearly over those that project; and also observing to carry the tint from below upwards by quick sweeping strokes, leaving no dry spaces between them. Cross these with other strokes from above downwards, leaving no floating liquid on the paper. Dry as quickly as possible, avoiding, however, such heat as may injure the tint.