According to Dubois, _Pholas_ luciferase is rather readily destroyed by chloroform and my own observations indicate that this is true also of firefly luciferase, so that a certain amount of variation exists in the group of luciferases. None of the luminescent animals which I have studied are at all affected by cyanides. The luminescence continues in extracts of _Cypridina_, firefly, and _Cavernularia_, or in _Noctiluca_ and luminous bacteria after addition of small or high (_m_/40) concentrations of KCN. In this respect the luciferases are very different from many types of oxidizing enzymes which are inhibited by exceedingly weak concentrations of cyanide. It should be borne in mind, however, that while KCN inhibits catalase and the catalytic decomposition of H_{2}O_{2} by Pt or Ag, it does not affect the catalytic decomposition of H_{2}O_{2} by thallium. OXYLUCIFERIN.--When luciferin is oxidized it must be converted into some substance or substances and I believe this change involves no fundamental destruction of the luciferin molecule as it is a reversible process. I shall speak of the principal (if not the only) product formed as _oxyluciferin_. If we assume that the oxidation of luciferin changes the molecule but slightly, we at once think of comparing the change luciferin ⇆ oxyluciferin with the change reduced hæmoglobin ⇆ oxyhæmoglobin. The condition is, however, not so simple as this, for oxyhæmoglobin will again give up its oxygen providing the partial pressure of oxygen is made sufficiently low, whereas oxyluciferin will not do this, at least in the dark. We can not reduce oxyluciferin solution by exhausting the oxygen with an air-pump. There is another oxidation-reduction system which can also be easily reversed, but not by merely removing the oxygen from the solution--that is, the reduction of a dye such as methylene blue to its leuco-base. I believe the change which occurs when luciferin is oxidized is similar to that which occurs when the leuco-base of methylene blue or sodium indigo-sulphonate is oxidized to the blue dye. Oxidation of leuco-dye bases occurs spontaneously in presence of oxygen and appears to consist in the removal of hydrogen from the leuco-base with formation of water. Reduction of these dyes may be effected in the same ways that oxyluciferin can be reduced. In the case of methylene blue, reduction consists in the addition of two hydrogen atoms. Whether a similar change occurs when oxyluciferin is reduced or whether oxygen is actually added as in formation of hæmoglobin cannot be definitely stated at present. We may write equations representing these possibilities as follows: C_{16}H_{20}N_{3}SCl + O ⇆ C_{16}H_{18}N_{3}SCl + H_{2}O (leuco-methylene blue) (methylene blue) Hæmoglobin + O ⇆ oxyhæmoglobin. Let us now turn to the methods which may be used in reduction of oxyluciferin. We may then endeavor to write an equation which will represent the fundamental changes in the luminescence reaction.