§ 13. Light, which is now known to modify many inorganic compounds--light, which works those chemical changes utilized in photography, causes the combinations of certain gases, alters the molecular arrangements of many crystals, and leaves traces of its action even on substances that are extremely stable,--may be expected to produce marked effects on substances so complex and unstable as those which make up organic bodies. It does produce such effects; and some of them are among the most important that organic matter undergoes. The molecular changes wrought by light in animals are of but secondary moment. There is the darkening of the skin that follows exposure to the Sun's rays. There are those alterations in the retina which cause in us sensations of colours. And on certain eyeless creatures that are semi-transparent, the light permeating their substance works some effects evinced by movements. But speaking generally, the opacity of animals limits the action of light to their surfaces; and so renders its direct physiological influence but small.[7] On plants, however, the solar rays that produce in us the impression of yellow, are the immediate agents of those molecular changes through which are hourly accumulated the materials for further growth. Experiments have shown that when the Sun shines on living leaves, they begin to exhale oxygen and to accumulate carbon and hydrogen--results which are traced to the decomposition, by the solar rays, of the carbonic acid and water absorbed. It is now an accepted conclusion that, by the help of certain classes of the ethereal undulations penetrating their leaves, plants are enabled to separate from the associated oxygen those two elements of which their tissues are chiefly built up. This transformation of ethereal undulations into certain molecular re-arrangements of an unstable kind, on the overthrow of which the stored-up forces are liberated in new forms, is a process that underlies all organic phenomena. It will therefore be well if we pause a moment to consider whether any proximate interpretation of it is possible. Researches in molecular physics give us some clue to its nature.