Later, or more accurate naturalists and seamen, also have, however, observed that some marine worms and insects were luminous ; and thus it was admitted that some at least of the luminous appearances of the sea might be produced by these. Still this was held to be independent of the general luminous property so widely extended through the ocean ; while many naturalists, who had as cett2ined the phosphorescent power of some species of marine animals, denied it to all the rest, and persisted in affirming that this quality depended on something inherent in the ocean itself. At one time, and that not very long ago, the Nereis noctiluca was supposed to be the only lu minous creature in the sea, and thus it was thought that wherever the light depended on an animal, it was produc ed by this worm. By degrees, however, others were add ed to the list by voyagers and naturalists; yet even when it had been considerably increased, the popular belief was not shaken. Such a persistence in wilful error might have excited surprise, had it not also happened, that not only was the general subject overlooked, but that even the very existence of the myriads of minute animals that crowd the waters of the ocean to a degree that appears almost miraculous vas unsuspected. We shall immediately show, not only that there are many luminous animals in the sea that were not suspected to possess this property, but that probably it belongs to every marine animal liv ing as well, as dead, and that uncounted and unaccounta ble myriads of minute creatures reside in the ocean, which, with scarcely any exceptions, have been overlooked by all naturalists.
We are indebted to Dr. Alacculloch fOr having first brought the whole of this question into one clear point of view, in his work on the Western Islands of Scotland, and for so great an extension of the luminous property to the marine species, as to have erected this into a general law ; and we shall therefore borrow from his writings, the following facts relating to this curious subject, as well as the conclusions which he has drawn from them.
It must, in the first place, be remarked respecting sea water, that it is far less frequently pure than is imagined. Within the vicinity of land it is so very rarely in that state, that it will not often fall to the lot of an observer to find it so, although this is generally believed to be the case, and that it appears sufficiently bright and clear on a superficial view, not only to a common observer, but even to one long practised in optical researches, and alive to minute distinctions of this nature. We have observed on several occasions, that not only it may contain animal matter in a state of solution without any sensible diminu tion of its transparency, but that it may abound with ani malcule without showing, from this cause, the slightest degree of turbidness or and consequently without exciting the smallest suspicion respecting its purity. It is probable, however, from our own observations, as well as from those of Mr. Scoresby, that there is an optical effect produced by the presence of animalcule, which has hitherto been a source of difficulty to philosophers. It is
known that the colour of sea-water varies absolutely, and independently of any effects arising from the state of the atmosphere. On our own coasts, and generally in the vi cinity of land, it is green, supposing that it is clear or transparent, and free from mud or any visible matter me chanically suspended in it. To judge of that colour truly, we must however view it in a bottom of white sand, or receive white light through it; and no where is it seen so bright as in the breaking of a wave in the sunshine, when the white foam is intermixed with the thin edge of the surf. Now in the ocean, on the contrary, or far from land, the green colour disappears, and it becomes blue; and that also independent of colour derived from the sky. 'Phis distinction appears to depend on the presence or ab sence of minute animals, which are to be found in the green water, but are wanting in the blue.
\\licit the water is green, however, it is also not un common to find that it has undergone a loss of transparen cy. Sometimes this is so slight that it cannot be disco vered without care; at others it is very sensible, and the effect resembles that which would follow from introducing into it a small quantity of milk. Now, if we attend to these appearances, when examining the luminous proper ty of the sea, we shall find that it always yields most light in proportion as its transparency is least perfect; and we shall also discover in such cases, that it contains the greatest number of animals. Further, when the opacity or milkiness is considerable, the microscope detects, be sides living animals, a quantity of delicate fibrous matter diffused through it. In a few instances, this is so abun dant as to he rendered highly sensible by re-agents, and to be even discovered by evaporation, and heating the re siduum to burning. In making these experiments on sea shores, we must however be Careful nut to confound with this kind of opaque matter, the fine mud which is brought down from the land by rivers, or forced up from the bot tom of shallow seas by gales of wind.
When the sea-water is, on the contrary, blue instead of green, it is neither opaque, nor does it contain ani mals. In the same cases it gives out no light ; but as er rors may arise in the examination of this property, we must here make some distinctions. When we say that it gives out no light, we only mean that this is the case as long as it contains no visible animals. It does not present that diffused and faint phosphorescence so general in green water; brit the presence or passage through it of any marine animal may still be attended by luminous ap pearances. It also happens that even in the wide ocean, and in the midst of a blue sea, tracts of light will occur at night when all the surrounding water is dark ; but, in such instances, it will always be found that in these places the sea is discoloured or turbid, and the microscope will then also show us that it abounds both in living animals and in diffused animal matter.