PHOTOSYNTHESIS, the synthesis of chemical compounds by the action of light. The broader subject of photochemistry includes all chemical reactions that are induced or main tained by the action of light, whether they result in true synthesis or not; but photosynthesis, in the strict sense, is a constructive process, re salting in the production of substances that are higher in type than the materials from which they are formed, by reason of possessing a more complicated structure or of containing a greater quantity of energy per unit of weight. Photochemical processes occur in inorganic nature and also in the animal world (notably in the retina of the eye) ; but the most important case of photosynthesis consists in the formation of starch and other related compounds, from carbon dioxide and water, in the green leaves of plants.
Light cannot produce chemical changes un less it is absorbed. This rather evident fact was first distinctly recognized by Grotthus in 1819, and is usually known as "Grotthus' law," but it was also independently stated by Draper in 1841, and from this circumstance it is some times called *Draper's law." It does not follow, however, that chemical change will result merely because light is absorbed, and substances that show well-defined absorption bands in the spectrum may not be affected by light in the least degree so far as sensible changes in their chemical properties are concerned. Light-energy that is absorbed without producing chemical effects is doubtless transformed into simple heat-energy, which merely causes the absorbing substance to become warmer. In order to pro duce chemical action the light must apparently excite some special resonant mechanism in the molecule, and certain additional conditions at present unknown must doubtless be fulfilled also in connection with the chemical potentials of the absorbing substance and the other substances that might conceivably be produced from it. Light is an exceedingly rapid periodic disturb ance of the ether that fills space and pene trates between the ultimate particles of which matter is composed; and when a substance undergoes chemical modification in consequence of being subjected to the action of light, it is probable that some part of the molecular mech anism of the substance has a vibration period equal to that of the light that produces the change. When bodies or systems that are large
enough to be dealt with individually and which are capable of vibrating or oscillating in certain definite periods are subjected to a disturbance of external origin having this same periodicity, we know that their motions may undergo marked and important changes, and we can hardly doubt that the same thing is true of the molecular systems. A homely illustration of the absorption of vibratory energy in this way may be had by singing a loud note into a piano. Certain of the strings of the instrument— namely, those that are capable of vibrating with the same frequency as the air waves constituting the tone that is sung— are set in motion, and if the singing is abruptly stopped these par ticular strings give out a faint sound of the same pitch, thereby proving that they have absorbed vibratory energy from the air. When light penetrates a substance that is capable of having photochemical changes induced in it, energy is absorbed by the molecules or atoms of the substance in a presumably similar way, and it is this absorbed energy that brings about the change. This much appears fairly evident, though we do not know the precise mechanism by which the result is accomplished. In some cases in which an effect of this kind occurs the molecules may be merely shaken apart, with the consequent formation of new compounds of a simpler nature ; and when this occurs the disruption may be attended by the liberation of energy, so that the final products contain less energy, on the whole, than the parent sub stance from which they were produced. Other compounds, more stably constituted or more efficiently absorptive, may be able to store up the vibratory energy of the light until they become capable of entering into combinations of a higher type, and in a case of this kind we should have a true photosynthesis.