The decomposition of carbonic acid is directly dependent upon luminous agency; from the impact of the earliest morning ray to the period when the sun reaches the zenith, the excitation of that vegetable vitality by which the chemical change is effected regularly increases. As the solar orb sinks toward the horizon the chemical activity diminishes—the sun sets—the action is reduced to its minimum—the plant, in the repose of darkness, passes to that state of rest which is as necessary to the vegetating races as sleep is to the wearied animal.
These are two well-marked stages in the life of a plant, germination and vegetation are exerted under different conditions; the time of flowering arrives, and another change occurs, the processes of forming the alkaline and acid juices, of producing the oil, wax, and resin, and of secreting those nitrogenous compounds which are found in the seed, are in full activity. Carbonic acid is now evolved and oxygen is retained; hydrogen and nitrogen are also forced, as it were, into combination with the oxygen and carbon, and altogether new and more complicated operations are in activity.
Such are the phenomena of vegetable life which the researches of our philosophers have developed. This curious order—this regular progression—showing itself at well-marked epochs, is now known to be dependent upon solar influences; the
“Bright effluence of bright essence increate”
works its mysterious wonders on every organic form. Much is still involved in mystery; but to the call of science some strange truths have been made manifest to man, and of some of these the phenomena must now be explained.
_Germination_ is a chemical change which takes place most readily in darkness; _vegetable growth_ is due to the secretion of carbon under the agency of light; and the processes of _floriation_ are shown to involve some new and compound operations: these three states must be distinctly appreciated.
The sunbeam comes to us as a flood of pellucid light, usually colorless; if we disturb this white beam, as by compelling it to pass through a triangular piece of glass, we break it up into colored bands, which we call the _spectrum_, in which we have such an order of chromatic rays as are seen in the rainbow of a summer shower. These colored rays are now known to be the sources of all the tints by which nature adorns the surface of the earth, or art imitates, in its desire to create the beautiful. These colored bands have not the same illuminating power, nor do they possess the same heat-giving property. The yellow rays give the most LIGHT; the red rays have the function of HEAT in the highest degree. Beyond these properties the sunbeam possesses another, which is the power of producing CHEMICAL CHANGE—of effecting those magical results which we witness in the photographic processes, by which the beams illuminating any object are made to delineate it upon the prepared tablet of the artist.