All phosphorescent substances require friction to produce their
full volume of light; this is exemplified at sea during a calm
tropical night, when the ocean sleeps in utter darkness and
quietude and not a ripple disturbs the broad surface of the
water. Then the prow of the advancing steamer cuts through the
dreary waste of darkness and awakens into fiery life the spray
which dashes from her sides. A broad stream of light illumines
the sea in her wake, and she appears to plough up fire in her
rush through the darkened water.
The simple friction of the moving mass agitates the millions of
luminous animalcules contained in the water; in the same manner a
fish darting through the sea is distinctly seen by the fiery
course which is created by his own velocity.
All luminous insects are provided with a certain amount of
phosphorescent fluid, which can be set in action at pleasure by
the agitation of a number of nerves and muscles situated in the
region of the fluid and especially adapted to that purpose. It
is a common belief that the light of the glow-worm is used as a
lamp of love to assist in nocturnal meetings, but there can be
little doubt that the insect makes use of its natural brilliancy
without any specific intention.
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