referred: what is known as the general or diffused phosphorescence of
the sea. From this mode of describing it the reader must not infer
that the surface of the ocean is ever to be seen all aglow in one
sheet of continuous light. So far, at least, as was ever observed by
M. de Quatrefages, who studied this phenomenon carefully and during
long periods on the coasts of Brittany and elsewhere, no light was
visible when the surface of the sea was perfectly still. On the other
hand, when the sea exhibits in a high degree the phenomenon of
diffused phosphorescence no disturbance can be too slight to cause the
water to shine with that peculiar characteristic gleam. Drop but a
grain of sand upon its surface, and you will see a point of light
marking the spot where it falls, and from that point as a centre a
number of increasing wavelets, each clearly defined by a line of
light, will spread out in circles all around.
The cause of this diffused phosphorescence was long the subject of
curiosity, and was long unknown, but more than a hundred years ago (in
1764) the light was stated by M. Kigaut to proceed from a minute and
very lowly organism, now known as _Noctiluca miliaris_; and subsequent
researches have confirmed this opinion. This Noctiluca is a spherical
form of not more than one-fiftieth of an inch in size, with a slight
depression or indentation at one point, marking the position of a
mouth leading to a short digestive cavity, and having close beside it
a filament, by means of which it probably moves about. The sphere is
filled with protoplasm, in which there is a nucleus and one or more
gaps, or "vacuoles." Such is nearly all the structure that can be
discerned with the aid of the microscope in this simple organism.
Nevertheless, this lowly form is the chief cause of that diffused
phosphorescence which is sometimes seen over a wide extent of the
ocean. How innumerable the individuals belonging to this species must
therefore be, may be left to the imagination. Probably the Noctiluca
is not rivalled in this respect even by miscroscopic unicellular algæ
which compose the "red snow."
By filtering sea-water containing Noctilucæ its light can be
concentrated, and it has been found that a few teaspoonfuls will then
yield light enough to enable one to read holding a book at the
ordinary distance from the eyes--about ten inches.
A singular and highly remarkable case of diffused marine
phosphorescence was observed by Nordenskiöld during his voyage to
Greenland in 1883. One dark night, when the weather was calm and the
sea smooth, his vessel was steaming across a narrow inlet called the
Igaliko Fjord, when the sea was suddenly observed to be illumined in
the rear of the vessel by a broad but sharply-defined band of light,
which had a uniform, somewhat golden sheen, quite unlike the ordinary
bluish-green phosphorescence of the sea. The latter kind of light was
distinctly visible at the same time in the wake of the vessel. Though
the steamer was going at the rate of from five to six miles an hour,
the remarkable sheet of light got nearer and nearer. When quite close,
it appeared as if the vessel were sailing in a sea of fire or molten
metal. In the course of an hour the light passed on ahead, and
ultimately it disappeared in the remote horizon. The nature of this
phenomenon Nordenskiöld is unable to explain; and unfortunately he had
not the opportunity of examining it with the spectroscope.
If we come now to consider the more partial phosphorescence of the
sea, we find that it is due to animals belonging to almost every group
of marine forms--to Echinoderms, or creatures of the sea-urchin and
star-fish type, to Annelid worm, to Medusidæ, or jelly-fish, as they
are popularly called, including the "great domes" and the "silvery
disks" of the passage above quoted from Professor Martin Duncan, to
Tunicates, among which is the Pyrosoma, to Mollusks, Crustaceans, and
in very many cases to Actinozoa, or forms belonging to the type of the
sea anemone and the coral polyp.
Of these we will single out only a few for more special notice.
Many of the Medusidæ, or jelly-fish, possess the character of which we
are speaking. In some cases the phosphorescence is spontaneous among
them, but in others it is not so; the creature requires to be
irritated or stimulated in some way before it will emit the light. It
is spontaneous, for example, in the _Pelagia phosphorea_, but not in
the allied _Pelagia noctiluca_, a very common form in the
Mediterranean.
In both of the jelly-fishes just mentioned the phosphorescence, when
displayed at all, is on the surface of the swimming disk, and this is
most commonly the case with the whole group. Sometimes, however, the
phosphorescence is specially localized. In some forms, as in
_Thaumantius pilosella_ and other members of the same genus, it is
seen in buds at the base of tentacles given off from the margin of the
swimming-bell. In other cases it is situated in certain internal
organs, as in the canals which radiate from the centre to the margin
of the bell, or in the ovaries. It is from this latter seat that the
phosphorescence proceeds in _Oceania pilata_, the form which gives out
such a light that Ehrenberg compared it to a lamp-globe lighted by a
flame.
The property of emitting a phosphorescent light, sometimes
spontaneously and sometimes on being stimulated, is likewise
exemplified in the Ctenophora, a group resembling the Medusidæ in
the jelly-like character of their bodies, but more closely allied in
structure to the Actinozoa. But we will pass over these cases in order
to dwell more particularly on the remarkable tunicate known as
Pyrosoma, a name indicative of its phosphorescent property, being
derived from two Greek words signifying fire-body. As shown in the
illustration Pyrosoma is not a single creature, but is composed of a
whole colony of individuals, each of which is represented by one of
the projections on the surface of the tube, closed at one end, which
they all combine to form. The free end on the exterior contains the
mouth, while there is another opening in each individual toward the
interior of the tube. Such colonies, which swim about by the alternate
contraction and dilatation of the individuals composing them, are
pretty common in the Mediterranean, where they may attain the length
of perhaps fourteen inches, with a breadth of about three inches. In
the ocean they may reach a much greater size. Mr. Moseley, in his
"Notes of a Naturalist on the Challenger," mentions a giant specimen
which he once caught in the deep-sea trawl, a specimen four feet in
length and ten inches in diameter, with "walls of jelly about an inch
in thickness."
[Illustration: A. PYROSOMA. B. PONITON. (Magnified.)]
The same naturalist states that the light emitted by this compound
form is the most beautiful of all kinds of phosphorescence. When
stimulated by a touch, or shake, or swirl of the water, it "gives out
a globe of bluish light, which lasts for several seconds, as the
animal drifts past several feet beneath the surface, and then suddenly
goes out." He adds that on the giant specimen just referred to be
wrote his name with his finger as it lay on the deck in a tub at
night, and in a few seconds he had the gratification of seeing his
name come out in "letters of fire."
Among mollusks, the best known instance of phosphorescence is in the
rock-boring Pholas, the luminosity of which after death is mentioned
by Pliny. But it is not merely after death that Pholas becomes
luminous--a phenomenon perfectly familiar even in the case of many
fish, especially the herring and mackerel. It was long before the
luminosity of the living animal was known, but this is now a
well-ascertained fact; and Panceri, an Italian naturalist, recently
dead, has been able to discover in this, as in several other marine
phosphorescent forms, the precise seat of the light-giving bodies,
which he has dissected out again and again for the sake of making
experiments in connection with this subject.
A more beautiful example of a phosphorescent mollusk is presented by a
sea-slug called _Phyllirhoë bucephala_. This is a creature of from one
and a half to two inches in length, without a shell in the adult
stage, and without even gills. It breathes only by the general surface
of the body. It is common enough in the Mediterranean, but is not easy
to see, as it is almost perfectly transparent, so that it cannot be
distinguished without difficulty, by day at least, from the medium in
which it swims. By night, however, it is more easily discerned, in
consequence of its property of emitting light. When disturbed or
stimulated in any way, it exhibits a number of luminous spots of
different sizes irregularly distributed all over it, but most thickly
aggregated on the upper and under parts. These phosphorescent spots,
it is found, are not on the surface, but for the most part represent
so many large cells which form the terminations of nerves, and are
situated underneath the transparent cuticle. The spots shine with
exceptional brilliancy when the animal is withdrawn from the water and
stimulated by a drop of ammonia.
Among the Annelid worms a species of _Nereis_, or sea-centipedes, has
earned by its phosphorescent property the specific name of _noctiluca_
(night-shining), and the same property is very beautifully shown in
_Polynoë_, a near ally of the familiar sea-mouse. M. de Quatrefages
speaks with enthusiasm of the beauty of the spectacle presented by
this latter form when examined under a microscope magnifying to the
extent of a hundred diameters. He then found, as he did in the great
majority of cases which he studied, that the phosphorescence was
confined to the motor muscles, and was manifested solely when these
were in the act of contracting, manifested, too, not in continuous
lines along the course of the muscles, but in rows of brilliant
points.
More interesting than the Annelids, however, are the Alcyonarian
Actinozoa. The Actinozoa have already been described as formed on the
type of the sea-anemone and the coral polyp, that is, they are all
animals with a radiate structure, attached to one end, and having
their only opening at the other end, which is surrounded by tentacles.
In the Alcyonarian forms belonging to this great group these tentacles
are always eight in number, and fringed on both sides. Moreover, these
forms are almost without exception compound. Like the Pyrosoma, they
have a common life belonging to a whole stock or colony, as well as an
individual life.
Now, throughout this sub-division of the Actinozoa phosphorescence is
a very general phenomenon. Professor Moseley, already quoted as a
naturalist accompanying the Challenger expedition, informs us that
"all the Alcyonarians dredged by the Challenger in deep water were
found to be brilliantly phosphorescent when brought to the surface."
Among these Alcyonarians are the sea-pens mentioned in the quotation
above made from Professor Martin Duncan. Each sea-pen is a colony of
Alcyonarians, and the name is due to the singular arrangement of the
individuals upon the common stem. This stem is supported internally by
a coral rod, but its outer part is composed of fleshy matter belonging
to the whole colony. The lower portion of it is fixed in the muddy
bottom of the sea, but the upper portion is free, and gives off a
number of branches, on which the individual polyps are seated. The
whole colony thus has the appearance of a highly ornamental pen.
There is one British species, _Pennatula phosphorea_, which is found
in tolerably deep water, and is from two to four inches in length. The
specific name again indicates the phosphorescent quality belonging to
it. When irritated, it shines brilliantly, and the curious thing is
that the phosphorescence travels gradually on from polyp to polyp,
starting from the point at which the irritation is applied. If the
lower part of the stem is irritated, the phosphorescence passes
gradually upwards along each pair of branches in succession; but if
the top is irritated the phosphorescence will pass in the same way
downwards. When both top and bottom are irritated simultaneously two
luminous currents start at once, and, meeting in the middle, usually
become extinguished there; but on one occasion Panceri found that the
two crossed, and each completed its course independently of the other.
Those of our readers who have had opportunities of making or seeing
experiments with the sensitive plant (_Mimosa pudica_) will be
reminded of the way in which, when that plant is irritated, the
influence travels regularly on from pinnules to pinnules and pinnae to
pinnae.
In all the cases mentioned the phenomenon of phosphorescence is
exhibited by invertebrate animals; but though rare, it is not an
unknown phenomenon even in living vertebrates. In a genus of deep-sea
fishes called Stomias, Gunther mentions that a "series of
phosphorescent dots run along the lower side of the head, body, and
tail." Several other deep-sea fishes, locally phosphorescent, seem to
have been dredged up by the French ship Talisman in its exploring
cruise off the west coast of Northern Africa in 1883. During the same
expedition, a number of deep-sea phosphorescent crustaceans were
dredged up, the phosphorescence being in some cases diffused over the
whole body, in other cases localized to particular areas. In deep-sea
forms the phenomenon is, in fact, so common, as to have given rise to
the theory that in the depths of the ocean, where the light of the
sun cannot penetrate, the phosphorescence of various organisms diffuse
a light which limits the domain of absolute darkness.
So much by way of illustration regarding the phosphorescence exhibited
by animals, terrestrial and marine; but it ought to be noticed that
there are also a few cases in which the same phenomenon is to be
witnessed in plants. These are not so numerous as was at one time
supposed, the property having been mistakenly ascribed to some plants
not really luminous.
[Illustration: A PHOSPHORESCENT SEA.]
In some instances the mistake appears to have been due to a subjective
effect produced by brilliantly colored (red or orange) flowers, such
as the great Indian cress, the orange lily, the sunflower, and the
marigold. The fact that such flowers do give out in the dusk sudden
flashes of light has often been stated on the authority of a daughter
of Linnæus, subsequently backed by the assertions of various other
observers. But most careful observers seem to be agreed that the
supposed flashes of light are in reality nothing else than a certain
dazzling of the eyes.
In another case, in which a moss, _Schistostega osmundacea_, has been
stated to be phosphorescent, the effect is said to be really due to
the refraction and reflection of light by minute crystals scattered
over its highly cellular leaves, and not to be produced at all where
the darkness is complete.
Among plants, genuine phosphorescence is to be found chiefly in
certain fungi, the most remarkable of which is _Rhizomorpha
subterranea_, which is sometimes to be seen ramifying over the walls
of dark, damp mines, caverns, or decayed towers, and emitting at
numerous points a mild phosphorescent light, which is sometimes bright
enough to allow of surrounding objects being distinguished by it. The
name of "vegetable glow-worm" has sometimes been applied to this
curious growth.
Among other phosphorescent fungi are several species of Agaricus,
including the _A. olearius_ of Europe, _A. Gardneri_ of Brazil, and
_A. lampas_ of Australia, and besides the members of this genus,
_Thelaphora cærulea_, which is the cause of the phosphorescent light
sometimes to be seen on decaying wood--the "touchwood" which many boys
have kept in the hope of seeing this light displayed. The milky juice
of a South American Euphorbia (_E. phosphorea_) is stated by Martins
to be phosphorescent when gently heated. But phosphorescence is
evidently not so interesting and important a phenomenon in the
vegetable as it is in the animal kingdom.
The whole phenomenon is one that gives rise to a good many questions
which it is not easy to answer, and this is especially true in the
case of animal phosphorescence. What is the nature of the light? What
are the conditions under which it is manifested? What purpose does it
serve in the animal economy?
As to the nature of the light, the principal question is whether it is
a direct consequence of the vital activity of the organism in which it
is seen, of such a nature that no further explanation can be given of
it, any more than we can explain why a muscle is contracted under the
influence of a nerve-stimulus; or whether it is due to some chemical
process more or less analogous to the burning of a candle.
The fact of luminosity appearing to be in certain cases directly under
the control of the creature in which it is found, and the fact of its
being manifested in many forms, as M. de Quatrefages found, only when
muscular contraction was taking place, would seem to favor the former
view. On the other hand, it is against this view that the
phosphorescence is often found to persist after the animal is dead,
and even in the phosphorescent organs for a considerable time after
they have been extracted from the body of the animal. In the glow-worm
the light goes on shining for some time after the death of the insect,
and even when it has become completely extinguished it can be restored
for a time by the application of a little moisture. Further, both
Matteucci and Phipson found that when the luminous substance was
extracted from the insect it would keep on glowing for thirty or forty
minutes.
In Pholas the light is still more persistent, and it is found that
when the dead body of this mollusk is placed in honey, it will retain
for more than a year the power of emitting light when plunged in warm
water.
The investigations of recent years have rendered it more and more
probable that the light exhibited by phosphorescent organisms is due
to a chemical process somewhat analogous to that which goes on in the
burning of a candle. This latter process is one of rapid oxidation.
The particles of carbon supplied by the oily matter that feeds the
candle become so rapidly combined with oxygen derived from the air
that a considerable amount of light, along with heat, is produced
thereby. Now, the phenomenon of phosphorescence in organic forms,
whether living or dead, appears also to be due to a process of
oxidation, but one that goes on much more slowly than in the case of a
lighted candle. It is thus more closely analogous to what is observed
in the element phosphorus itself, which owes its name (meaning
"light-bearer") to the fact that when exposed to the air at ordinary
temperatures it glows in the dark, in consequence of its becoming
slowly combined with oxygen.
At one time it was believed that the presence of oxygen was not
necessary to the exhibition of phosphorescence in organic forms, but
it has now been placed beyond doubt that this is a mistake. Oxygen has
been proved to be indispensable, and hence we see a reason for the
luminous organs in the glow-worm being so intimately connected, as
above mentioned, with the air-tubes that ramify through the insect.
This fact of itself might be taken as a strong indication of the
chemical nature of the process to which phosphorescence is due. But
the problem has been made the subject of further investigations which
have thrown more light upon it. It was long known that there were
various inorganic bodies besides phosphorus which emitted a
phosphorescent light in the dark, at least after being exposed to the
rays of the sun; but it was not till quite recently that any organic
compound was known to phosphoresce at ordinary temperatures.
This discovery was made by a Polish chemist, named Bronislaus
Radziszewski, who followed it up with a long series of experiments on
the phosphorescence of organic compounds, by which he was able to
determine the conditions under which that phenomenon was exhibited. In
all the substances investigated by him in which phosphorescence was
introduced he found that three conditions were essential to its
production: (1) that oxygen should be present; (2) that there should
be an alkaline reaction in the phosphorescing mixture--that is, a
reaction such as is produced on acids and vegetable coloring matters
by potash, soda, and the other alkalies; and (3) that some kind of
chemical action should take place.
He found, moreover, that among the organic compounds that could be
made to phosphoresce under these conditions were nearly all the fixed
and ethereal oils. With reference to the phosphorescence of animals,
this observation is important, for it has been shown in a great many
cases that a fatty substance forms the main constituent in their
luminous organs. This has long been known to be the case in the
luminous insects belonging to the Lampyridæ and Elateridæ, as well as
in the luminous centipedes; and the researches of Panceri, already
referred to, on the luminous organs of many marine forms have shown
that it holds good with regard to these also.
We may, therefore, conclude that substances fitted to phosphoresce
under the conditions determined by the experiments of Radziszewski are
generally, and probably universally, present in the luminous organs of
phosphorescent animals. Now, what is to be said as to the occurrence
of these conditions? The access of oxygen is in all cases easy to
account for, but it must also be shown how the alkaline reaction is to
be produced. We need not expect to find in animal organisms potash,
soda, ammonia, and the other common alkalies; but it was established
by experiment that the alkaline organic compounds cholin and neurin,
which are present in animal tissues, would also serve to bring about
the phenomenon of phosphorescence in the substances on which the
experiments were made.
Accordingly, it seems fair to conclude that when all these conditions
for the production of phosphorescence in a chemical laboratory are
present in animal organisms, the phenomenon, when observed in these,
is exactly of the same nature as that which is produced artificially.
By that it is meant that animal phosphorescence is attended, like the
artificial phenomenon, by a slow chemical action, or in other words,
that the phosphorescent light is due to a gradual process of
oxidation.
One curious circumstance has been discovered which lends still further
probability to this explanation. It was mentioned above that among
phosphorescent plants there are several species of Agaricus. Now,
from one species of this genus, though not indeed one of the
phosphorescent species (from _A. muscarius_) there has been extracted
a principle called _amanitia_, which is found to be identical with
cholin. In the light of the results derived from the investigations
just referred to it is reasonable to draw the conclusion that, if
sought for, this principle would likewise be found in the
phosphorescent species in which the other conditions of
phosphorescence are also present.
On this theory of the production of the phenomenon now under
consideration, the effect of shaking or of vital action in giving rise
to or intensifying the exhibition of the light is accounted for by the
fact that by these means fresh supplies of oxygen are brought into
contact with the phosphorescent substance. The effect of ammonia on
the light emitted by the sea-slug _Phyllirhoë bucephala_, is also
fully explained, ammonia being one of those alkaline substances which
are so directly favorable to the exhibition of the phenomenon.