Exact figures on the minimal concentration of oxygen for luminescence cannot be given. The luminescent secretion of _Cypridina hilgendorfii_ will still give off much light if hydrogen containing only 0.4 per cent. of oxygen is bubbled through it, _i.e._, a partial oxygen pressure of 1/250 atmosphere (3.04 mm.Hg). However, addition of a fresh emulsion of yeast cells to a glowing _Cypridina_ secretion is sufficient to rapidly extinguish the light, because the yeast is capable of utilizing the last trace of oxygen in the mixture. Light only appears when, by agitation, we cause more air to dissolve. The minimal concentration of oxygen for luminescence of _Cypridina_ lies somewhere between 3.04 mm. and the amount which living yeast fails to extract from solution, a concentration approaching zero. It is probably nearer the latter figure. As the oxygen pressure is increased from 0 to about 7 mm., the intensity of the _Cypridina_ luminescence increases and at the latter figure the light is just as bright as if the solution were saturated with air (152 mm.O_{2}). Thus, the luminescence requires only a low pressure of oxygen and the similarity to the saturation of hæmoglobin with oxygen is obvious. Just as hæmoglobin is nearly saturated with oxygen at low pressures and becomes bright red in color, so the luminous material becomes saturated with oxygen at low pressures and glows intensely. Boyle also made many experiments to show that air was necessary for the life of animals and the germination of seeds and showed that repeatedly respired air was unfit for further breathing. About the same time R. Hooke discovered the true meaning of respiratory movements and by forcing a blast of air continuously through the lungs with bellows, was able to keep animals alive. He concludes "that as the bare Motion of the _Lungs_, without fresh air, contributes nothing to the life of the Animal, he being found to survive as well as when they were not moved as when they were; so it was not the Subsiding or Movelessness of the _Lungs_ that was the immediate cause of death, or the stopping of the circulation of the Blood through the _Lungs_, but the Want of a sufficient Supply of fresh Air." The cause of death on collapse of the lungs could not be better stated to-day. Thus combustion, respiration and luminescence of flesh or wood were early recognized as related phenomena.