3. The next Difference to be mentioned is, that a live Coal, being put into a small close Glass, will not continue to burn for very many Minutes; but a Piece of shining Wood will continue to shine for some whole Days....
4. A fourth Difference may be this: that whereas a Coal, as it burns, sends forth Store of Smoke or Exhalations, luminous Wood does not so.
5. A fifth, flowing from the former, is, that whereas a Coal in shining wastes itself at a great Rate, shining Wood does not....
6. The last Difference I shall take notice of betwixt the bodies hitherto compared is, that a quick Coal is actually and vehemently hot; whereas I have not observed shining Wood to be so much as sensibly lukewarm."
It should be clearly borne in mind that if we place luminous organisms, say bacteria or fungi, in an atmosphere devoid of oxygen and find that no light is produced, this may merely mean that certain functions of the cell are interfered with, including light production, but does not necessarily indicate that oxygen is actually used up in the photogenic process. If we find, however, that extracts of luminous cells or luminous secretions devoid of cells cease to light when the oxygen is removed and again luminesce when it is returned, we may be quite certain that the photogenic process itself requires free oxygen. As luminous extracts of fireflies, pennatulids, ostracods, Pholas and others give off no light when the oxygen is removed, we may safely conclude that for these luminescences, oxygen is necessary. Bacteria, fungi, and Noctiluca, whose light also disappears in absence of oxygen, although they are whole cells, we may by analogy also assume to require oxygen in the photogenic process.
Some of the earlier workers on fireflies and Noctiluca obtained light even after placing these organisms in absence of oxygen, but they did not realize how low is the amount of oxygen necessary to produce light. It is difficult to remove traces of oxygen from the water, traces which are nevertheless sufficient to cause luminescence. If the organisms are numerous, as in an emulsion of luminous bacteria, they will themselves use up all the oxygen and the liquid soon ceases to glow except at the surface in contact with air. We may gain an idea of the amount of oxygen necessary for luminescence from an experiment of Beijerinck (1902). He mixed luminous bacteria with an emulsion of clover leaves containing chloroplasts and kept the two in the dark until all the
oxygen was used up and the bacteria ceased to glow. If now a match was struck for a fraction of a second, sufficient oxygen was formed by photosynthesis to cause the bacteria to luminesce for a short time.
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.O2). 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.