Since 1 gram glucose liberates 4000 calories on complete oxidation to CO_{2} and H_{2}O, it will be seen that the oxidation of luciferin is a very different type of reaction from the oxidation of glucose. As we shall see, it is probably similar to the oxidation of reduced hæmoglobin or the oxidation of leuco methylene-blue to methylene blue. According to Barcroft and Hill (1910), 1.85 calories are produced per gram of hæmoglobin oxidized. I have been unable to find figures for the heat exchange during oxidation of leuco-dyes, but it is no doubt also small. Since luciferin evolves no measurable amount of heat on oxidation, we have very good evidence in support of that obtained by electrometric measurements of H-ion concentration, that no carbon dioxide is produced during luminescence of luminous animals. In most animal cells it is perfectly clear that luminescence does not accompany respiration, since respiration is a continuous process, whereas light is only produced on stimulation. It is true that on stimulation respiration is accelerated, and we might suppose that luminescence is an accompaniment of accelerated respiratory oxidations; but this is not the case, for in luminous animals a rise in temperature of ten degrees centigrade will accelerate the respiratory oxidations 250 per cent. without necessarily causing the production of light. In fungi and bacteria, on the other hand, which continually emit light, it is quite natural to suppose that the light is an accompaniment of respiration, just as we know the heat of these forms to be. This view was accepted by such of the earlier workers as Fabre in 1855, who found that luminous portions of a mushroom, _Agaricus olearius_, gave off more CO_{2} (4.41 c.c. CO_{2} per gram in 36 hours at 12° C.) than non-luminous portions (2.88 c.c. CO_{2} per gram in 36 hours at 12° C.). This experiment has never been repeated and there are many reasons besides luminescence why one piece of fungus might have a more rapid respiratory rate than another piece. It is not true that rapidly respiring plant tissues, such as germinating seeds or the spadix of _Araceæ_, are luminous, although they produce considerable heat.