light.
[Illustration: 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 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.
[Illustration: FIG. 3. Spectrum of zinc sulphide phosphorescence (_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 phosphorescent light does not shift its
position during decay of phosphorescence.]
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.
[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.
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