common cause of this phenomenon. The particles were so minute as
easily to pass through fine gauze; yet many were distinctly visible by
the naked eye. The water when placed in a tumbler and agitated, gave
out sparks, but a small portion in a watch-glass scarcely ever was
luminous. Ehrenberg states that these particles all retain a certain
degree of irritability. My observations, some of which were made
directly after taking up the water, gave a different result. I may
also mention, that having used the net during one night, I allowed it
to become partially dry, and having occasion twelve hours afterwards to
employ it again, I found the whole surface sparkled as brightly as when
first taken out of the water. It does not appear probable in this case,
that the particles could have remained so long alive. On one occasion
having kept a jelly-fish of the genus Dianaea till it was dead, the
water in which it was placed became luminous. When the waves
scintillate with bright green sparks, I believe it is generally owing
to minute crustacea. But there can be no doubt that very many other
pelagic animals, when alive, are phosphorescent.
On two occasions I have observed the sea luminous at considerable
depths beneath the surface. Near the mouth of the Plata some circular
and oval patches, from two to four yards in diameter, and with defined
outlines, shone with a steady but pale light; while the surrounding
water only gave out a few sparks. The appearance resembled the
reflection of the moon, or some luminous body; for the edges were
sinuous from the undulations of the surface. The ship, which drew
thirteen feet of water, passed over, without disturbing these patches.
Therefore we must suppose that some animals were congregated together
at a greater depth than the bottom of the vessel.
Near Fernando Noronha the sea gave out light in flashes. The appearance
was very similar to that which might be expected from a large fish
moving rapidly through a luminous fluid. To this cause the sailors
attributed it; at the time, however, I entertained some doubts, on
account of the frequency and rapidity of the flashes. I have already
remarked that the phenomenon is very much more common in warm than in
cold countries; and I have sometimes imagined that a disturbed
electrical condition of the atmosphere was most favourable to its
production. Certainly I think the sea is most luminous after a few
days of more calm weather than ordinary, during which time it has
swarmed with various animals. Observing that the water charged with
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