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.