tions
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.