Page:The fairy tales of science.djvu/128

 knowledge we have gained of the chemical powers of the sunbeam. The old alchemists, indeed, were acquainted with the isolated fact that a white substance called horn silver was blackened by exposure to the sun's rays, but it never struck them to investigate the cause of this curious phenomenon. It was reserved for a philosopher of modern times to prove that no substance can be exposed to the sun's rays without undergoing a chemical change.

The blackening of horn silver is but a single instance of a vast number of effects produced by that mysterious agent which is associated with light and heat in the sunbeam. All bodies are influenced by actinism, and undergo a chemical or molecular disturbance. The rock and the mountain, as well as the animal and the plant, are destructively acted upon during the hours of sunshine, and would soon perish under the delicate touch of the actinic rays, were it not for the counteracting influence of darkness. At night, the chemical disturbances are undone, and inorganic bodies as well as organized beings may be said to sleep!

The influence of actinism upon germination is very remarkable, as seeds will not germinate in light from which this principle is separated. But, after the leaves are formed, a larger amount of light than of actinism is necessary to enable the plant to separate carbon from the atmosphere and form wood. Again, the flowering and fruiting of a plant is more closely connected with the heat of the sunbeam than