The most remarkable peculiarity of _Cypridina_ luciferin is its stability. In my first paper on _Cypridina_ I stated that luciferin was not destroyed by momentary boiling but would be destroyed if boiled four or five minutes; also, that it was unstable at room temperatures and would disappear from solution in the course of a day or so. The reason for this is that luciferin oxidizes even in absence of luciferase and will then no longer give light with luciferase. This spontaneous oxidation, which occurs without light production, can be prevented by keeping the luciferin in a hydrogen atmosphere or by the addition of acid. Under these conditions the luciferin can be boiled without destruction or preserved for months without deterioration. The rapid disappearance of luciferin from neutral or alkaline solution on boiling in the air is entirely due to the more rapid oxidation at the boiling point. As the oxidation product, oxyluciferin, can be readily reconverted into luciferin again, we can not consider luciferin unstable in the sense that its molecule is actually destroyed as is the case when luciferase is boiled. Not only is luciferin stable on boiling but it will actually withstand boiling for 10 hours with 20 per cent. HCl (by weight, sp. gr. = 1.1) or with 4 per cent. H_{2}SO_{4}. After one day of boiling with 20 per cent. HCl the luciferin was completely destroyed and with 4 per cent. H_{2}SO_{4} destruction was almost complete. In these cases there was no question of a mere oxidation to oxyluciferin, as no oxyluciferin could be demonstrated after boiling with such strong acids. An actual destruction, probably an hydrolysis of the luciferin molecule, occurred. We shall have occasion to refer to this again in considering the protein nature of luciferin. It must be borne in mind that many proteins require four or five days' boiling with 20 per cent. HCl for complete hydrolysis to amino-acids. Luciferin forms a solution in water, probably colloidal, although the luciferin will dialyze through parchment or collodion membranes. It is rather readily adsorbed by various finely divided materials such as bone black, Fe(OH)_{3}, kaolin, talc and CaCo_{3}. It is not destroyed by any of the enzyme solutions which I have tried. These include such as are widely divergent in action: pepsin HCl, trypsin, erepsin, salivary and malt diastase, yeast invertase, urease, rennin and the enzymes of dried spleen, kidney and liver substances. By extracting the dried Cypridinas ground to a powder, the solubility of luciferin in non-aqueous solvents could be easily studied, and by adding such reagents as dilute acids, alkalies, neutral salts and the alkaloidal reagents to an aqueous solution of luciferin the general biochemical behavior of luciferin can be quite accurately stated. For convenience the results of this study are given in Table 8. TABLE 8 _Properties of Photogenic Substances from Cypridina_