A substance, fluorescent in solution, has been found in a few luminous animals, notably in several species of fireflies and also in a non-luminous beetle. It is called _pyrophorine_ or _luciferesceine_. Dubois (1886) has ascribed to pyrophorine the power of absorbing invisible rays and transforming them into visible ones, thus increasing the animal's light. That this is not the case has been shown by the work of Coblentz (1909). He photographed the spectrum of the firefly's light and the fluorescent spectrum of luciferesceine. The latter is almost complementary to the former (see Fig. 4) and no trace of the fluorescent spectrum appears in the spectrum of the light of the firefly. McDermott (1911 _a_) has studied the properties of luciferesceine and regards it merely as an incidental material found in many animals of the _Lampyridæ_ (in some non-luminous forms) and having no connection with the light production. A trace of alkali usually increases and acid inhibits the fluorescence of solutions. [Illustration: 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 CaCl_{2}, 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.