It has been suspected that these three phenomena are not due to the same agency, but that, associated in the sunbeam, we have LIGHT, producing all the blessings of vision, and throwing the veil of color over all things—HEAT, maintaining that temperature over our globe which is necessary to the perfection of living organisms—and a third principle, ACTINISM, by which the chemical changes alluded to are effected. We possess the power, by the use of colored media, of separating these principles from each other, and of analyzing their effects. A yellow glass allows _light_ to pass through it most freely, but it obstructs _actinism_ almost entirely; a deep-blue glass, on the contrary, prevents the permeation of _light_, but it offers no interruption to the _actinic_, or chemical rays; a red glass, again, cuts off most of the rays, except those which have peculiarly a _calorific_, or heat-giving power.
With this knowledge we proceed in our experiments, and learn some of the mysteries of nature’s chemistry. If, above the soil in which the seed is placed, we fix a deep, pure yellow glass, the chemical change which marks germination is prevented; if, on the contrary, we employ a blue one, it is greatly accelerated; seeds, indeed, placed beneath the soil, covered with a cobalt blue finger-glass, will germinate many days sooner than such as may be exposed to the ordinary influences of sunshine:—this proves the necessity of the principle actinism to this first stage of vegetable life. Plants, however, made to grow under the influences of such blue media present much the same conditions as those which are reared in the dark; they are succulent instead of woody, and have yellow leaves and white stalks—indeed, the formation of leaves is prevented, and all the vital energy of the plant is exerted in the production of stalk. The chemical principle of the sun’s rays, alone, is not therefore sufficient; remove the plant to the influence of light, as separated from actinism, by the action of yellow media, and wood is formed abundantly—the plant grows most healthfully, and the leaves assume that dark green which belongs to tropical climes or to our most brilliant summers. Light is thus proved to be the exciting agent in effecting those chemical decompositions which have already been described; but under the influence of isolated light it is found that plants will not flower. When, however, the subject of our experiment is brought under the influence of a red glass, particularly of that variety in which a beautifully pure red is produced by oxide of gold, the whole process of floriation and the perfection of the seed is accomplished.