Chapter III, in considering the efficiency of the firefly as a source of

light.

A body which emits light because of its (high) temperature is said to be incandescent and we speak of temperature radiation. We know, however, of many cases where substances give off light at temperatures much below 525 deg. C. They do not follow the Stefan-Boltzmann law. The light emission is stimulated by some other means than heat. Such bodies we speak of as _luminescent_, and in this category belong all luminous animals. The distinction between light and luminescence was first pointed out by Wiedemann (1888). It is usual to classify luminescences, according to the means of exciting the light, into the following groups:

Thermoluminescence Phosphorescence and Fluorescence Photoluminescence Cathodoluminescence Anodoluminescence Radioluminescence Triboluminescence and Piezoluminescence Crystalloluminescence Chemiluminescence

The luminescence which appears in a vacuum tube when an electric current is passed through it is sometimes spoken of as _electroluminescence_. As electroluminescence and also thermoluminescence are really special cases of phosphorescence or fluorescence and tribo-and crystalloluminescence are closely allied, the classification has only the merit of emphasizing the means of producing light. Let us examine each kind in turn in order that we may place the light of animals, _organoluminescence_ or _bioluminescence_ (or _biophotogenesis_), in one of these classes. All are examples of "cold light," light produced at temperature far below those observed in incandescent solids. In this category should be placed also the light from salts in the bunsen flame, for flame spectra and line spectra in general, while only obtained at relatively high temperatures, are not to be confused with the purely temperature radiation from the incandescent particles of carbon in a gas or candle light. The sodium or lithium flame, etc., is not a simple function of temperature and has been spoken of as a luminescence, _pyroluminescence_. As the luminescence of organisms could in no manner be regarded as a pyroluminescence, occurring at temperatures far above those compatible with life, a consideration of this form of luminescence will be omitted. Some other low temperature flames are known, such as that of CS_{2} in air, rich in ultra-violet rays, despite its relatively low temperature. While these are of interest to the physicist and chemist, they can have no direct bearing on the luminescence of animals and their consideration will also be omitted. (See Bancroft and Weiser, 1914-1915.)

THERMOLUMINESCENCE.--Some substances begin to emit light of shorter wave-length than red, well below 525 deg.. This is thermoluminescence. Diamond, marble, and fluorite are examples. Only certain varieties of fluorite show the phenomenon well. A crystal of one of these varieties heated in the bunsen flame on an iron spoon will give off a white light long before any trace of redness appears in the iron. Other crystals may luminesce in hot water. In all, this luminescence is dependent on a previous illumination or radiation of the crystal. If kept in the dark for a long time no trace of light appears when fluorite is placed at a temperature of 100 deg., but after a short exposure to the light of an incandescent bulb, although no light can be observed in the fluorite at room temperature, quite a bright glow appears at 100 deg.. Calcium, barium, strontium, magnesium and other sulphates containing traces of manganese sulphate, show a similar phenomenon after exposure to cathode rays (Wiedemann and Schmidt, 1895 b). They emit light during bombardment, but this soon ceases when the rays are cut off. If the sulphates are now heated they give off light, red in the case of MgSO_{4} + MnSO_{4}, green in the case of CaSO_{4} + MnSO_{4}. The power to emit light on heating may be retained for months after the exposure to cathode rays. The emission of light by bodies after previous illumination or radiation is called _phosphorescence_ and will be considered below. It would seem that the cases of thermoluminescence with which we are acquainted are really cases of phosphorescence intensified by rise of temperature. The spectrum of thermoluminescent bodies, also, is similar to that of phosphorescent ones. (See Fig. 3.) However, not all phosphorescent materials are also thermoluminescent. The production of light by animals is quite another phenomenon from thermoluminescence.

PHOSPHORESCENCE AND FLUORESCENCE.--Although the word phosphorescence has been used in a very loose way to indicate all kinds of luminescence, and particularly that of phosphorus or of luminous animals, to the physicist it has a very definite meaning, namely, the absorption of radiant energy by substances which afterwards give this off as light. Phosphorescence does not strictly apply to the light of white phosphorus. If the radiant energy is light (visible or ultra-violet) we speak of _photoluminescence_, if cathode rays we have _cathodoluminescence_, if anode rays, _anodoluminescence_, and if X-rays (Roentgen rays) we have _radioluminescence_. Inasmuch as the [alpha], [beta], and [gamma] rays of radium correspond to the anode, cathode, and X-rays, respectively, radium radiation also produces luminescence in many kinds of material. If the material gives off the light only during the time it is radiated we speak of fluorescence; if the light persists we speak of phosphorescence. The distinction is perhaps a purely arbitrary one, as there are a great many substances which give off light for only a fraction of a second (1/5000 sec. in some cases) after being illuminated (_photoluminescence_). Some substances also, which fluoresce at ordinary temperatures, will phosphoresce at low temperatures. Phosphorescence is exhibited chiefly by solids, fluorescence also by liquids and vapors.

Special means must be used to observe a phosphorescence of short duration. E. Becquerel has devised an apparatus for doing this, a _phosphoroscope_. It consists of revolving disks with holes in them between which the object to be examined is placed. The holes are so arranged that the object is first illuminated and then completely cut off from light. The observer looking at it through another hole sees it at the moment it is not illuminated and can thus tell if it is phosphorescing. By determining the rate of revolution of the disks it is easy to calculate how long the phosphorescence persists.

While relatively few solids phosphoresce after exposure to light at ordinary temperature a large number of these acquire the property at the temperature of liquid air. Included in the list are such biological products as urea, salicylic acid, starch, glue and egg shells. The temperature also affects the wave-length and hence the color of the light given off. Usually the higher the temperature the shorter the wave-length, but in the case of some bodies (SrS) the wave-lengths become longer at the higher temperature.

The best known cases of phosphorescence which occur at room temperature and the group to which the word phosphorescence is commonly applied, are those of the alkaline earth sulphides (BaS, CaS, SrS) and ZnS. An Italian, Vicenzo Cascariolo, is said to have discovered the Bologna stone (BaSO_{4}) which, by calcination with charcoal, gave an impure phosphorescent BaS or _lapis solaris_. Canton's phosphorus (CaS) was later prepared "by heating a mixture of three parts of sifted calcined oyster shells with one part of sulphur to an intense heat for one hour." Hulme spoke of it as the "light magnet of Canton," because of its power of attracting and absorbing light. The pure sulphides do not show this property. Only if small amounts of some other metal such as Cu, Pb, Ag, Zn, Sb, Ni, Bi, or Mn are present, will the sulphide phosphoresce. One part of impurity in a million is often sufficient. Such mixtures, together with a flux of Na_{2}SO_{4}, Li_{3}(PO_{4})_{2} or some other fusible salt constitute a "phosphor." A "phosphor" is in reality an example of a solid solution and is the basis of some kinds of luminous paints.

The intensity and duration of a phosphorescent light depend chiefly on the nature of the exciting rays, the color chiefly on the impurity present but the alkaline earth metal also exerts an influence. Rise in temperature increases the intensity but diminishes the duration, so that the total amount of light emitted is about constant at different temperatures.

The spectrum of most phosphorescent substances is made up of one or more continuous bands having _maxima_ at different wave-lengths. In the light incident on a phosphorescent substance are also bands of light rays which are absorbed and whose wave-lengths are more efficient than others in stimulating phosphorescence. These bands in the phosphorescent light are usually of longer wave-length than those in the light which excites the phosphorescence. This fact is known as Stokes' Law, but it has been found not to be universally true. Curiously enough, red and infra-red rays have the power of annulling phosphorescence after a momentary increase in brightness and phosphorescing materials have been used to determine if infra-red rays are given off in the light of the firefly. Ives (1910) showed that infra-red radiation had no power of quenching the light of the firefly as it does the phosphorescent light of Sidot blende (ZnS), one fact tending to show that the firefly's light is not due to phosphorescence. Fig. 3 is a reproduction of a photograph of the phosphorescence spectrum of ZnS.

Other facts show that the light of luminous animals is in no sense a phosphorescence and is quite independent of previous illumination of the animal. Luminous bacteria will continue to luminesce although they are grown in the dark for many weeks. Indeed strong light has a bactericidal action on these forms similar to that with ordinary bacteria. With some marine forms light has an inhibiting effect. They lose their power of luminescence during the day and only regain it at dusk or when kept in the dark for some time. Indeed, ordinary light never has the effect of causing luminescence in the same sense as it causes phosphorescence of CaS.

Fluorescence is most efficiently excited by the cathode rays of a vacuum tube. They not only cause the residual gas in the tube to glow (_electroluminescence_) by which their path may be followed with the eye, but also a vivid fluorescence of the glass walls of the tube, yellow green with sodium glass, blue green with lead and lithium glass. LiCl_{2} in the path of cathode rays gives off a blue light; in the path of anode rays a red light; NaCl a blue cathodoluminescence and a yellow anodoluminescence. The spectrum of the latter is a line spectrum of Li or Na, showing the characteristic red or yellow lines similar to those observed where Li or Na is held in the bunsen flame. The spectrum of the salts under excitation of cathode rays is a short continuous one in the blue region. Fluorescent spectra in general are of this nature, made up of short bands of light in one or more regions.

Diamonds, rubies and many minerals fluoresce brilliantly in the path of cathode rays. Some specimens of fluorite (CaF_{2}) show the phenomenon especially well, whence the name _fluorescence_. Fluorescent screens of barium platinocyanide, willemite (Zn_{2}SiO_{4}), Sidot blend (ZnS) or Scheelite (Ca tungstate) are frequently employed to render visible X-rays. The luminous paint most used at the present time is ZnS containing a trace of radium salt. The rays of the radium continually emitted cause a steady fluorescence of the ZnS. Indeed, if one examines the paint on the hands of a watch with a lens the flash of light from the impact of alpha particles on the ZnS can be distinctly seen, as in the _spinthariscope_.

Some animal tissues and fluids, especially the lens of the eye, will luminesce in the path of radium rays, as shown by the experiments of Exner (1903), but there is no evidence that luminous animals are especially active in this respect. Ultra-violet rays have the same action.

The luminous material of practically all luminous forms, if dessicated sufficiently rapidly, can be obtained in the form of a dry powder which will give off light when moistened with water. Coblentz (1912) has exposed this dry material to light, to the ultra-violet spark, and to X-rays and in no case has a phosphorescence or fluorescence ever been observed. I have examined the action of radium upon _Cypridina_ light. There was no intensifying or diminishing effect of twenty milligrams of radium (probably the bromide) on a luminous solution of _Cypridina_ material, nor was phosphorescence or fluorescence excited in a non-luminous extract of the animal. We must conclude that animal light is not a fluorescence of any substance due to radiation produced by the animals themselves.

Many solutions show fluorescence in strong lights. This is especially marked in quinine sulphate, mineral oils, eosin, fluorescein, esculin, rhodamin, chlorophyll, etc. The fluorescence of eosin in 10^{-8} grams per cubic centimetre is visible in daylight and 10^{-15} grams per cubic centimetre in the beam from an arc lamp. It is difficult to realize that the bluish fluorescence of quinine sulphate is really an emission rather than a reflection of light. But a test tube of quinine sulphate solution held in the ultra-violet region of a spectrum will glow with a pale blue light, although it is not illuminated with any rays that are visible to our eyes. Concerning this, Stokes, to whom the word fluorescence and much of our knowledge of the subject is due, says, "It was certainly a curious sight to see the tube" (containing quinine sulphate solution) "instantaneously lighted up when plunged into the invisible rays; it was literally 'darkness visible.'" Quinine sulphate absorbs the ultra-violet converting these rays into visible blue ones. Its spectrum is a short continuous one. Most fluorescent substances convert short into longer wave-lengths (Stokes' Law), but some may cause the reverse change.

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 _Lampyridae_ (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.

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.

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 crystalloluminescence and triboluminescence of AsCl_{3} and of K_{2}SO_{4}. 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 AsCl_{3}, 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^- = AsCl_{3}, 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