Fig. 4.—Spectrum of fluorescent substance found in fireflies below (2) and of firefly luminescence above (2) compared with helium vacuum tube (1) (after Coblentz).
Triboluminescence and Piezoluminescence.—Under this head are grouped a number of light phenomena which at first sight may appear to be electrical in nature but in reality are not. The light is produced by shaking, rubbing, or crushing crystals, and only crystalline bodies appear to show triboluminescence or piezoluminescence. A striking case is that of uranium nitrate. Gentle agitation of the crystals is sufficient to give off sparks of light which much resemble the scintillations of dinoflagellates when sea-water containing these animals is agitated. If Romberg's phosphorus, which is fused CaCl2, is rubbed on the sleeve, it glows with a greenish light. Lumps of cane sugar rubbed together will glow. Saccharin crystals will also light if shaken and Pope (1899) found that the bluish light of saccharin was bright enough to be visible in a room in daytime. It only appeared from impure crystals and freshly crystallized specimens. Other crystals, also, have been found to lose their power of lighting after a time.
Among biological substances, cane sugar, milk sugar, mannite, hippuric acid, asparagin, r-tartaric acid, l-malic acid, vanillin, cocaine, atropin, benzoic acid, and many others show triboluminescence. A long list is given by Tschugaeff (1901), by Trautz (1905), and by Gernez (1905). The spectrum is a short continuous one, the waves emitted depending on the kind of crystal. Thus the color of the light varies among different santonin derivatives from yellow to green. In saccharin it is blue.
Although the light produced by some living organisms resembles triboluminescence in that it may be evoked by
rubbing or shaking the animals, it is in reality fundamentally different since it is dependent on the presence of oxygen whereas triboluminescence is not.
Crystalloluminescence.—Crystalloluminescence is observed when solutions crystallize. It was described by Bandrowski (1894, 1895) in arsenious oxide, in NaF, or if HCl or alcohol is added to hot saturated NaCl solution. A bluish light with sparkling points appeared. All well authenticated cases are exhibited by simple inorganic salts and these are also all triboluminescent. The reverse is not true, however; many triboluminescent substances are not crystalloluminescent. Crystalloluminescence is much less widespread than triboluminescence. Trautz (1905) has studied the matter in a number of compounds and comes to the conclusion that the light is really a special case of triboluminescence in which the growth of individual crystals causes them to rub together. The light becomes much brighter on stirring a mass of crystals which exhibit crystalloluminescence. While in some cases crystalloluminescence is unquestionably due to the triboluminescence of crystals rubbing against each other it is not in every case, as has been clearly shown by the work of Weiser (1918 b). He studied luminescence of saturated aqueous alkali halide solutions (NaCl, KCl, etc.,) upon addition of alcohol or of HCl. The salt crystallizes out under these conditions and Weiser found that the light is brightest when the conditions of concentration of alcohol or of HCl are such as to cause heaping up of Na and Cl ions. He believes that the bluish light which appears is due to the combination of ions in the reaction, Na+ + Cl- = NaCl. Only if this proceeds rapidly enough does luminescence occur. Weiser studied also the crystallo
luminescence and triboluminescence of AsCl3 and of K2SO4. By photographing the luminescence through color screens of different absorptive power (Weiser, 1918, a) a spectrum of the light could be obtained, and it was found to be identical in both the tribo- and crystalloluminescent light; in the case of AsCl3, a band in the green-blue, blue and violet. Weiser believes the light in this case also to come from recombination of the ions, As+++ + 3Cl- = AsCl3, and that crystalloluminescence in general is due to rapid reformation of molecules from ions broken up by electrolytic dissociation while triboluminescence is due to rapid reformation of molecules from ions broken up by violent disruption of the crystal. Of course in triboluminescent organic crystals which do not dissociate into ions, some other reaction must be responsible for the light. One thing seems certain, that the two types of luminescence are similar. As Bigelow[1] remarks, "It is altogether probable that the cause of this" (crystalloluminescence) "whatever it may be, is the same as the cause of triboluminescence, whatever that may be."
[1] Theoretical and Physical Chemistry, 1912, p. 516.
Crystals are not found in the luminous organs of animals with the exception of the fireflies. In these a layer of cells occurs (see Chapter IV) filled with minute crystals of one of the purine bodies (xanthin or uric acid). One might surmise that the light of the animal was a crystalloluminescence accompanying the formation of these crystals. It is easy to show, however, that the light comes not from the crystal layer but from another layer of cells containing large granules. It is also dependent on the presence of oxygen while crystalloluminescence takes place in the absence of oxygen. The crystal layer possibly
serves as a reflector. Its significance will be discussed in a later chapter.