Since Spallanzani's observation that the luminous material of medusæ could be dried, and upon moistening would again give light, many confirmatory observations have been made on other forms. _Pyrosoma_, _Pholas_, _Phyllirrhoë_, fireflies, _Pyrophorus_, copepods, ostracods, pennatulids, fungi, and bacteria can all be dessicated and the photogenic material preserved for a greater or less time. In a dessicator filled with CaCl_{2}, dried luminous bacteria lose, after a few months, their power to give light on being moistened. On the other hand, ostracods and copepods will still luminesce after years of dessication. The luminous material in the latter case appears capable of indefinite preservation, but it is possible that the quick loss of photogenic power with dried luminous bacteria is merely an indication that they contain very little photogenic substance and that the dried ostracods would also in time lose their power to luminesce. It is certainly a fact that the amount of luminous material in a single gland cell of an ostracod is vastly greater than that in the same mass of bacterial colony. When the dried powdered luminous material of an ostracod is sprinkled over the surface of water, it goes into solution and leaves luminous diffusion and convection trails plainly visible in the water. Many luminous marine forms give off a phosphorescent slime when they are handled, which adheres to the fingers. It is not surprising that this luminous matter should have early received a name. In 1872, Phipson called it _noctilucin_ and described some of its properties. He regarded the luminous matter which can be scraped from dead fish (luminous 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 CO_{2} 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.