i68 CAUSE OF PHOSPHORESCENCE, [chap. vm. 



northern hemisphere, to be the 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 Diancea 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 water, passed over, with- 

 out 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 atmo- 

 sphere was most favourable to its production. Certainly 

 I think the sea is most luminous after a few davs of more. 



