[Ill.u.s.tration: FIG. 2.--Distribution of energy throughout the spectrum of the sun, electric arc, and gas light (_after Nichols and Franklin_).

Ordinates show the relative intensities of different wave-lengths emitted. The notches in the curve represent absorption bands and the dotted line represents what the radiation from the sun would be if no selective absorption occurred. V=violet and R=red end of visible spectrum. (_Courtesy Macmillan Co._)]

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 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 cla.s.sify luminescences, according to the means of exciting the light, into the following groups:

Thermoluminescence Phosph.o.r.escence and Fluorescence Photoluminescence Cathodoluminescence Anodoluminescence Radioluminescence Triboluminescence and Piezoluminescence Crystalloluminescence Chemiluminescence

The luminescence which appears in a vacuum tube when an electric current is pa.s.sed through it is sometimes spoken of as _electroluminescence_. As electroluminescence and also thermoluminescence are really special cases of phosph.o.r.escence or fluorescence and tribo-and crystalloluminescence are closely allied, the cla.s.sification 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 cla.s.ses. 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. 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, 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. 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 _phosph.o.r.escence_ and will be considered below. It would seem that the cases of thermoluminescence with which we are acquainted are really cases of phosph.o.r.escence intensified by rise of temperature. The spectrum of thermoluminescent bodies, also, is similar to that of phosph.o.r.escent ones. (See Fig. 3.) However, not all phosph.o.r.escent materials are also thermoluminescent. The production of light by animals is quite another phenomenon from thermoluminescence.

PHOSPh.o.r.eSCENCE AND FLUORESCENCE.--Although the word phosph.o.r.escence 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. Phosph.o.r.escence 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 (Rontgen rays) we have _radioluminescence_. Inasmuch as the a, , and ? 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 phosph.o.r.escence. 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 phosph.o.r.esce at low temperatures. Phosph.o.r.escence is exhibited chiefly by solids, fluorescence also by liquids and vapors.

Special means must be used to observe a phosph.o.r.escence 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 phosph.o.r.escing. By determining the rate of revolution of the disks it is easy to calculate how long the phosph.o.r.escence persists.

While relatively few solids phosph.o.r.esce 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 sh.e.l.ls. 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 phosph.o.r.escence which occur at room temperature and the group to which the word phosph.o.r.escence 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 phosph.o.r.escent BaS or _lapis solaris_. Canton's phosphorus (CaS) was later prepared "by heating a mixture of three parts of sifted calcined oyster sh.e.l.ls 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 phosph.o.r.esce. 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 const.i.tute 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 phosph.o.r.escent 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 phosph.o.r.escent substances is made up of one or more continuous bands having _maxima_ at different wave-lengths. In the light incident on a phosph.o.r.escent substance are also bands of light rays which are absorbed and whose wave-lengths are more efficient than others in stimulating phosph.o.r.escence. These bands in the phosph.o.r.escent light are usually of longer wave-length than those in the light which excites the phosph.o.r.escence. 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 phosph.o.r.escence after a momentary increase in brightness and phosph.o.r.escing 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 phosph.o.r.escent light of Sidot blende (ZnS), one fact tending to show that the firefly's light is not due to phosph.o.r.escence. Fig. 3 is a reproduction of a photograph of the phosph.o.r.escence spectrum of ZnS.

[Ill.u.s.tration: FIG. 3. Spectrum of zinc sulphide phosph.o.r.escence (_after Ives and Luckiesh_). Photographs were taken by a special device one minute (middle) and fifteen minutes (bottom) after exposure to the light of the mercury arc and compared with a helium spectrum (top). In the middle photograph, the mercury exciting lines are visible. It will be noted that the narrow band of phosph.o.r.escent light does not shift its position during decay of phosph.o.r.escence.]

Other facts show that the light of luminous animals is in no sense a phosph.o.r.escence 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 phosph.o.r.escence 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 gla.s.s walls of the tube, yellow green with sodium gla.s.s, blue green with lead and lithium gla.s.s.

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 phosph.o.r.escence 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 phosph.o.r.escence 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.

[Ill.u.s.tration: 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.

CRYSTALLOLUMINESCENCE.--Crystalloluminescence is observed when solutions crystallize. It was described by Bandrowski (1894, 1895) in a.r.s.enious 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 ma.s.s 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 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.

[Ill.u.s.tration: FIG. 5.--Dubois's figures showing transformation of photogenic granules to crystals (_after Dubois_).]

The light of luminous organisms is quite generally a.s.sociated 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.

Radziszewski (1877, 1880) gives a long list of substances, chiefly essential oils, which luminesce if slowly oxidized in alcoholic solutions of alkalis. Formaldehyde, dioxymethylen, paraldehyde, metaldehyde, acrolen, disacryl, aldehydeammonia, acrylammonia, hydrobenzamid, lophin, hydroanisamid, anisidin, hydroc.u.minamid, hydrocinamid, besides waxes, and such biological substances as glucose, lecithin, cholesterin, cholic, taurocholic, and glycocholic acids, and cerebrin, all luminesce on oxidation. Radziszewski himself and many other authors have compared the light of organisms to this type of luminescence. Indeed the incorrect identification of granules found in the cells of practically all luminous tissues as oil droplets, is largely due to the influence of Radziszewski's work. Dubois (1901 _b_) has added esculin, and Trautz (1904-5) many aldehydes and phenol derivatives, including vanillin, papaverin, tannic and gallic acids, besides glycerol and mannite to the list of biological substances oxidizing with light production. Guinchant (1905) has described oxyluminescence of uric acid and asparagine, Weitlaner (1911) of substances in humus and McDermott (1913) of substances in urine and the anaerobic alkaline hydrolysis products of glue and Witte's peptone.

Pyrogallol is especially p.r.o.ne to luminesce, as was first noticed by Lenard and Wolf (1888) in developing a photographic plate with pyrogallol developer. Later the luminescence was studied in some detail by Trautz and Schloringin (1904-5) who developed the well-known luminescent mixture of pyrogallol, formaldehyde, K_{2}CO_{3} and H_{2}O_{2}. As I have shown, pyrogallol can be oxidized in a great many different ways, and some of these are of great interest, for they very closely imitate the mechanism for the production of light in organisms.

These are recorded in Table 3, which also includes various other types of oxyluminescence of general or biological interest.

TABLE 3

_Types of Oxyluminescent Reactions_

1. Oxidation in air spontaneously.

(_a_) At ordinary temperatures. [Phosphorus. Fresh-cut surfaces of Na or K. Thiophosgene and Thio-ethers (RCS.OR).]

(_b_) At melting or vaporizing points. (Fats, terpenes, sugars, resins, gums, ether, silk and others.)

2. Oxidation in aqueous or alcoholic alkalies. (Many organic substances.)

3. Oxidation in hypoiodites, hypobromites, or hypochlorites. (Many organic substances.)

4. Oxidation in peroxides (H_{2}O_{2} or Na_{2}O_{2}). (Many organic substances.)

5. Oxidation in ozone. (Many organic substances.)