192 



ATLANTIC OCEAN. 



Dec. 1833. 



also in my notes^ that having kept a Medusa of the genus 

 Diansea, 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 Crus- 

 tacea. 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 con - 

 siderable 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 undulation of the surface. The ship, which drew 

 thirteen feet 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 ex- 

 pected from a large fish moving rapidly through aluminous 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. With respect to any general obser- 

 vations, I have already stated that the display is very much 

 more common in warm than in cold countries. 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 gelatinous particles is in an impure state, and 

 that the luminous appearance in all common cases is pro- 

 duced by the agitation of the fluid in contact with the atmo- 

 sphere, I have always been inclined to consider that the 

 phosphorescence was the result of the decomposition of 

 the organic particles, by which process (one is tempted 



