bacteria) and the mucous secretion of Scolopendra electrica or the luminous matter of the glowworm to be this material, noctilucin, which, "in moist condition, takes up oxygen and gives off CO2 and when dry appears like mucin." Phipson says that it forms an oily layer over the seas in summer (he probably refers to masses of dinoflagellates), is liquid at ordinary temperatures and less dense than water, smells a little like caprylic acid, is insoluble in water but miscible with it, insoluble in alcohol and ether, dissolves with decomposition in mineral acids and alkalies and contains no phosphorus. We can see from this description that the word "noctilucin" does not indicate a chemical individual, but it is the earliest attempt to definitely designate the luminous substance.
The idea of a definite substance oxidizing and causing the light has been upheld by a number of investigators, and many years later Molisch called this substance the photogen. He contrasts the "photogen theory" with certain other views of light production, which may be spoken of as "vital theories," notably those of Pflüger (1875), who looked upon luminescence as a sign of intense respiration, and of Beijerinck (1915), who regarded the light as an accompaniment of the formation of living matter from peptone.
Fortunately biological science has advanced beyond the stage where a living process can be explained by calling it a vital process, and we must fall back upon the idea of a photogen oxidizing with light production. Indeed, it is now possible to go much further than this and describe the properties of the photogen, but we must not lose sight of the fact that it was recognized very early in the history of Bioluminescence, that water,
oxygen, and a photogenic substance were necessary for light production.
A very great advance in our knowledge of the chemistry of the problem was made by Dubois in 1885. He showed that if one dips the luminous organ of Pyrophorus in hot water, the light disappears and will not return again. Also if one grinds up a luminous organ the mass will glow for some time but the light soon disappears. If one brings the previously heated organ in contact with the unheated triturated organ it will again give off light. Later, Dubois showed that the same experiment could be performed with the luminous tissues of Pholas dactylus. A hot-water extract of the luminous tissue, and a cold-water extract of the luminous tissue, allowed to stand until the light disappears, will again produce light if mixed together. Dubois (1887 b) advanced the theory that in the hot-water extract there is a substance, luciferin, not destroyed by heating, which oxidizes with light production in the presence of an enzyme, luciferase, which is destroyed on heating. The luciferase is present together with luciferin in the cold-water extract, but the luciferin is soon oxidized and luciferase alone remains. Mixing a solution of luciferin and luciferase always results in light production until the luciferin is again oxidized. Similar substances have been found by me in the American fireflies, Photinus and Photuris, the Japanese firefly, Luciola, and in the ostracod crustacean, Cypridina hilgendorfii. Crozier[6] reports that they exist also in Ptychodera, a balanoglossid. I have been unable to demonstrate their existence in luminous bacteria; in the annelid, Chætopterus; the pennatulids, Cavernularia and Pennatula; the
squid, Watasenia; and the fish, Monocentris japonica. E. B. Harvey (1917) could not demonstrate them in Noctiluca. There are several reasons why the existence of such bodies might be difficult to demonstrate, but these reasons cannot be considered here. We thus see that the photogen is in reality of dual nature, that two substances are necessary for light production and that they may be very readily separated because of difference in resistance to heating. In this respect Bioluminescence is similar to some other biological processes, notably to certain immune reactions and to certain enzyme actions.
[6] Private communication.
Thus, for the hæmolysis of foreign red blood corpuscles, a specific immune body (amboceptor or substance sensibilatrice) not destroyed by moderate heating, and a thermolabile complement (alexin) are necessary.
For the alcoholic fermentation of glucose by the zymase of yeast juice two substances are also necessary. The zymase is made up of a heat resistant, dialyzing component, the co-enzyme, and a non-dialyzing substance, destroyed on boiling, the enzyme proper. Both must be present for alcoholic fermentation of glucose to proceed and the two may be separated by dialysis or by their difference in resistance to heating. Several other characteristics of living cells are known to depend on the joint action of two substances, one thermolabile, the other thermostable. The reducing action of tissues, according to Bach, requires a reducing enzyme proper or perhydridase and some easily oxidizable substance, such as an aldehyde. The aldehyde has been spoken of as the co-enzyme.
Because of the necessity of thermostable and thermolabile substances for light production in luminous animals and because I was unable to oxidize the thermostable