Some luminous animals, e.g., Cypridina (Crustacea), contain luciferin which oxidizes to oxyluciferin in aqueous solution containing O. During attempts to excite luciferase to luminescence by energy of recombination of H atoms, the authors observed that dry oxyluciferin can be reduced to luciferin, confirming results obtained by reducing oxyluciferin in aqueous solution, and rendering it quite certain that the luciferin-oxyluciferin change is a dehydrogenation-hydro-genation reaction. When dry luciferase and luciferin are exposed to low concentration atomic H there may be observed at times a faint bluish glow which breaks into incandescence on raising the atomic H concentration. The faint glow of Cypridina may be a luminescence or a low temp. incandescence. Willemite, etc., show undoubted luminescence in atomic H.