Now, the conclusions that we would derive from these statements, are the following : We think there is no rea son to doubt, that, in certain situations, and under cer tain circumstances, the water of the sea may be phospho rescent, in consequence of dead phosphorescent animal tt ild to determine whether that is to proceed solely from dead fishes, or occasionally also from living ones. But as we have shown that the property is very transitory in mat ter so detached, as the animal matter, from its mere defi ciency in quantity, cannot be diffused very far from its source, and as the sea cannot be supposed so to abound in dead fishes, or even in living ones, as to be rendered ex tensively luminous from this cause, we think that very lit tle need be allowed to it in inquiring respecting the origin of the phosphorescence of sea-water.
We must here also distinguish, what we shall have oc casion to do more fully immediately, between the several kinds of light which the sea affords. Sometimes it seems generally luminous, shining with a very pale and feeble whitish light ; at others it exhibits wide flashes, like a particular sort of lightning; and at others again it is filled with brilliant sparks of different sizes and colours. Now it is only the first of these, or the faint diffused light, which is caused by diffused dead matter, so that we have still the other two sorts left unaccounted for, and to be re ferred to other causes. But here also we do not admit that all the faint diffused lights are produced by dead animal matter. On the contrary, we have ascertained by nume rous observations, that this kind of light is produced by living animals, though of a very minute size ; by animal culx, in short, to use the common term, or microscopic creatures ; and in this we are supported by the remarks of many voyagers who have described the sea as appearing at nignt to be like A bCa of milk, or a plain of snow, while these phenomena have at the same time been discovered to be produced by minute animals. In the green water also, when no turbidness at all could be discovered, we have often found the diffused luminous appearance, and have in these cases ascertained that animalcule: were pre sent, but either too minute or too transparent in them selves to affect the clearness of the water. Hence our final conclusion is, that though sea-water may at times be rendered phosphorescent by detached and dead animal matter, that circumstance is so rare that it need scarcely enter into our calculations respecting the causes of the luminous appearance. With this slight exception, there fore, we shall now proceed to inquire in the causes which are most efficient and common, and describe the several appearances that occur, adding a description or enumera tion of the animals, as far as we have yet ascertained those by which it is produced.
It has been so positively asserted by so many observers, that they had seen, not only the general diffused light, but the sparkling appearance in sea-water when no animals were present, that we must first show that this assertion is founded in erroneous observation. We say, on the con
trary, and without hesitation, that this never happens, and that the mistake has arisen from negligence or ignorance of the subject where observations have been made, and from preconceived opinions where they have not. The sources of error require to be pointed out, as well as the method, not only of ascertaining the fact generally, but the very animals in which the luminous property resides in these more obscure cases.
In the first place, a very few bright lights dispersed through the sea are sufficient to produce a very brilliant effect, and to make observers imagine that the water is universally luminous. The phenomenon is in itself daz zling, particularly in a dark night ; and the general impres sion is also not a little augmented by a slight feeling to wards the marvellous, so apt to exaggerate every thing that is new or uncommon. Now, it is quite easy to take up from such a sea, and that repeatedly, as we have often • experienced, a bucket full of water without a single ani mal in it, while at another time two or three of the very medusa, orother creatures yielding the light, are obtained. There is another reason why they so often escape, and that is, their slippery nature; so that, floating on the sur face, as they generally do, they are washed out by the wave that takes place on hoisting the bucket on board. Hence one great source of the errors of careless observers with respect to the presence of these animals, and the consequent cause of the sparkling appearance of the sea.
In the next place, a great many animals that yield a very bright light, even in the form of distinct and brilliant sparks, are so transparent, although of visible dimensions, that it is quite impossible to see them, either in a ship's bucket, particularly at night, or in a vessel of earthen ware. Nay more, there are many, even among the me dusa, reaching to the tenth or eighth of an inch in diame ter, that cannot easily be seen, although in a glass, and by candle light. If there is even half an inch of water be tween them and the eye, or sometimes less, they are quite invisible, because their refractive density is exactly equal to that of the water. We have often been employed for half an hour or more in searching for such an animal in a common tumbler, where we had previously ascertained its presence, and it had merely escaped from the reach of the eye. Hence it is not surprising, if an impatient observer determines that nothing is present, merely because it is not visible at the very moment he expects. Had the na ture of these animals been better understood, such errors could never have taken place. In all investigations, of whatever nature, it is useful to recollect that we cannot always succeed, unless we are provided with more than one kind of information.