178 
LUMINOSITY OP THE SEA. 
seemed to be absolutely necessary to cause a certain 
action in it before the luminosity was emitted. How 
far this may depend on a peculiar stimulus or 
irritability incited in these diminutive animalcules, or 
on the presence of phosphorescent particles in such a 
chemical stage as to be easily acted on by friction ; it 
would be difficult to determine. One thing is very 
clear, that a certain movement is necessary, either by 
wind, tide, or mechanical means, to induce a luminous 
condition of the sea. In proof of this, we have only 
to instance the effect of a ship’s progress through 
phosphorescent water; its more luminous condition 
during fresh breezes in hot climates ; the greater 
quantity of light emitted by some of the larger aca- 
lephae in expanding and contracting their campanulate 
bodies. 
On the coast of Brazil, where the sea is so frequently 
luminous, we have often been enabled to determine 
the state of the tides in calm weather, by observing 
the effect on the phosphorescent mollusca ; their pre- 
sence being scarcely perceptible when the water was 
quiescent, but at once evidenced by innumerable scin- 
tillations, when the tide was actively ebbing or flow- 
ing ; or by putting a line overboard, when its downward 
course has been marked, by the luminosity, and on 
withdrawing it, we seldom failed to bring up some of 
the minute gelatinous mollusca. 
Between St. Catherine’s in Brazil, and the River 
Plate, we have frequently noticed a brownish disco- 
