Fig. 5.—Dubois's figures showing transformation of photogenic granules to crystals (after Dubois).
The light of luminous organisms is quite generally associated with granules. In one of the centipedes (Orya barbarica), which produces a luminous secretion, Dubois (1893) has described the transformation of these granules into crystals and at one time he supposed the light to be a crystalloluminescence. He later reversed this opinion and, certainly, examination of his drawings which are reproduced in [Fig. 5] does not convince one of the actuality of crystal formation.
The phenomenon of lyoluminescence, described by
Wiedemann and Schmidt (1895) as a light accompanying the solution of colored (from exposure to cathode rays) crystals of Li, Na, or K chlorides, is probably due to a triboluminescence from stirring of the crystals during solution.
Chemiluminescence.—As the name implies, chemiluminescence is the production of light during a chemical reaction at low temperatures. This does not mean that the other types of luminescence are not connected with chemical reactions—using the word reaction in a broad sense—for we have reason to believe that in some cases spectra are not characteristic of the element as such but are rather characteristic of a particular reaction in which the element takes part (dissociation into ions, changes from monovalent to bivalent condition, etc.) and that this is the reason one element may show various spectra under different conditions (Bancroft, 1913). The chemiluminescences are rather oxidation reactions involving the absorption of gaseous or dissolved oxygen and may be very easily distinguished from all the previously mentioned luminescences by this criterion. They should, perhaps, more properly be called oxyluminescences.
The glow of phosphorus is the best known case, recognized since phosphorus was first prepared by Brandt in 1669. It is interesting to note that when first prepared phosphorus was regarded as a peculiarly persistent type of phosphor, i.e., a material akin to the impure alkaline earth sulphides.
Fresh cut surfaces of Na and K metal will glow in the dark for some time, especially if warmed to 60°-70° (Linnemann, 1858). A film of oxide is formed over the surface, showing definitely that oxidation has occurred.
Ozone oxidizes organic matter with an accompanying glow (Fahrig, 1890; Otto, 1896). The light from ozone acting on pyrogallol solution is especially bright under certain conditions.