Since oxyluciferin can be reduced by palladium and sodium hypophosphite (Harvey, 1918), it is probable that we can write the equation for reduction of oxyluciferin and oxidation of luciferin in a similar manner to that of methylene blue: Luciferin + O ⇆ Oxyluciferin + H_{2}O Briefly--LH_{2} + O ⇆ L + H_{2}O. Just as in the case of methylene blue the reaction proceeds in the right hand direction spontaneously if the pressure of O is sufficiently high. If luciferase is also present we have luminescence. LH_{2} + O + luciferase ⇆ L + H_{2}O + luciferase (luminescence) The reaction proceeds in the left hand direction under low oxygen pressure, in the presence of nascent hydrogen or with some catalyzer which is able to split water, transferring the H_{2} to oxyluciferin and the O to an acceptor (A). NaH_{2}PO_{2} plays the part of the acceptor. L + H_{2}O + A + Pd = LH_{2} + AO + Pd. This appears to be the way in which the reducing enzymes or perhydridases (comparable to the Pd) of living tissues act (Bach, 1911-13) and the action of yeast cells, bacteria, muscle suspensions, etc., in reducing oxyluciferin must occur in the same manner. If we assume that the LH_{2} (luciferin) compound is dissociated to even the slightest extent into L and hydrogen, the hydrogen ion will shift the equilibrium toward the formation of that substance which involves the taking up of hydrogen. Consequently we may obtain a partial formation of luciferin by adding an acid to oxyluciferin. Reduction of the H-ion concentration tends to shift the equilibrium in the opposite direction. Consequently, addition of alkali favors the oxidation of luciferin, and it is quite generally true that biological oxidations are favored by an alkaline reaction. In addition oxygen in alkaline medium has a higher oxidation potential than in neutral or acid media. I believe this is the explanation of the action of acid in formation of luciferin from oxyluciferin. Addition of acid is not the only means of favoring the formation of luciferin from oxyluciferin. Any reaction which proceeds in one direction with evolution of light should, theoretically, proceed in the opposite direction under the influence of light. So far as I know the case of a reaction, photogenic in one direction and photochemical in the other direction, has never been described, unless we are to accept the cases of phosphorescence, for instance, the absorption of light by CaS and its emission in the dark. However, the reaction which occurs during phosphorescence cannot be stated.