A similar experiment can be performed with luciferase and oxyluciferin solution by addition of NH_{4}SH. This will serve also as another example of the reduction of oxyluciferin in an alkaline medium. Whenever we shake a tube of luciferase, oxyluciferin and NH_{4}SH, light will appear. When the tube is at rest it becomes dark. Even the merest touch is sufficient to agitate the tube contents, cause solution of oxygen and appearance of light. It is just as if we stimulate the tube to produce light and I believe the phenomenon has a deeper significance and a more fundamental similarity to the phenomena of stimulation than may at first appear. What more simple means of controlling a process can we think of than by admission or withdrawal of oxygen? The firefly turns on its light by stimulation through nerves of the luminous organ. _Noctiluca_ flashes on stimulation of any kind, even the slightest agitation causing a brilliant emission of light. If the stimulation process means merely the admission of oxygen to the photogenic cells we have a mechanism in the cell itself for automatically producing the light. The admission of oxygen results in aërobic conditions and luciferin in presence of luciferase can then oxidize to oxyluciferin with luminescence. When the oxygen is used up, the light ceases, anaërobic conditions prevail, and the oxyluciferin is reduced to luciferin again. Thus, luciferin is reformed during the rest period of _Noctiluca_ or between the flashes of the firefly. What more efficient type of light than this is to be desired? Again, methylene blue offers an interesting parallel to oxyluciferin. A little NH_{4}SH added to methylene blue solution will reduce (decolorize) it to the leuco-base. If the tube is now shaken the blue color returns. On standing reduction again occurs. The process can be repeated a number of times, the reaction going in one or the other direction, depending on the oxygen content of the mixture. As methylene blue contains no oxygen, its reduction consists in the addition of two atoms of hydrogen. When leuco-methylene blue oxidizes, water is formed by the union of these two atoms of hydrogen with oxygen, thus: C_{16}H_{20}N_{3}SCl + O ⇆ C_{16}H_{18}N_{3}SCl + H_{2}O (leuco-methylene blue) (methylene blue) Briefly--MH_{2} + O ⇆ M + H_{2}O To reduce methylene blue we can add the two hydrogen atoms directly from nascent hydrogen formed in the solution or we can split up water by a catalyzer in the presence of some substance, which will take up the oxygen of water, thus: NaH_{2}PO_{2} + H_{2}O + Pd = NaH_{2}PO_{3} + H_{2} + Pd (Sodium hypophosphite) (Sodium phosphite) This reaction occurs in presence of finely divided palladium. Methylene blue can be reduced by the H_{2} and the hypophosphite oxidized.