The incredulity of naturalists on the subject, or their unwillingness to believe that the light was produced by animals, has been confirmed by two other circumstances. For this reason it is necessary to take notice of these ; besides which, they form an important part of the history of the phosphorescence of the sea. These things are, the apparently perfect diffusion or the abundance of the light, in some cases, through every part of the water, both in sparks and otherwise, and the distinction between the sparkling lights, which are commonly very bright, and the more faint light which seems inherent, and, as it were, incorporated with the whole fluid. We hope to show that, for the most part at least, these different appearances are owing to the same class of causes—to the phospho rescent powers of animals.
If naturalists had been aware how numerous or abundant these creatures were, it would never have appeared ex traordinary to them that the water should be so generally or extensively luminous ; no more than that, when their minuteness is considered, they should have escaped ordi nary observation. When even only one species is present, the numbers are sometimes such as even to confound the imagination ; when there are many, it has sometimes ap peared as if there was as much space occupied by the animals as by the water. On one occasion, a single spe cies was observed extending all the way from the Mull of Cantyre to Shetland, and occupying the breadth of sea which our vessel was obliged to traverse in beating against the wind ; a space probably of four or five miles broad.
How much wider than that it might have been we had no means of knowing, but it is probable that this animal was far from being limited to that narrow space. To what ever depth the eye or the ship's bucket could reach, the water appeared equally crowded with this animal ; which was an unknown species of what Muller has thought proper to include under the genus vibrio. In the sunshine the water appeared almost opake, and as if filled with mi nute scales of mica ; and, at night, there was no difficulty in ascertaining that this creature was the cause of the general luminous appearance. It would be a very mode rate computation indeed, to say, that an hundred of these were contained in every cubic inch of water ; were we to say a thousand, we should probably be nearer to the truth. To attempt to form a conception of their numbers, only for the space of a few yards, would be fruitless ; and the endless myriads of them existing throughout the extent of sea thus navigated baffles all the powers of imagination.
In the same seas, and nearly at all times, the water was found crowded with many different species of other ge nera ; many of them not visible without the lens, or truly microscopic. This very transparency of the sea was af fected by these, so numerous were they ; besides which, ten or twenty animals of various kinds, visible to the eye, were found in the same water, and in the space of a com mon drinking glass. Such facts as these, which are, as far as our experience goes, of common occurrence, are sufficient to account for all the light of the sea, without the necessity of seeking for any other causes.
In all these cases, where the animals were very minute, or microsopic, the sea was universally and diffusedly lu minous; and when they were more visible, it abounded in bright sparks. That the sparks were produced by these latter, admitted of no doubt, because they could be taken out of the water by means of a feather, in the very act of shining, and transferred to other water for examina tion. At the same time, the general luminous appearance was destroyed when the animals died, either from keep ing the water too long, or by warming it, or by the ad dition of spirits. The facility, indeed, by which the phos phorescence of water is extinguished by all those means which kill its inhabitants, is in itself a sufficient proof that this property resides in these.
1Vith some attention, it is even easy, to a certain ex tent, to distinguish the different sparks of light yielded by different animals ; as far, at least, as they differ in di mensions. In the larger kinds, the bright spot is quite distinct, and very often varies in colour, being white, or yellowish, or bluish, or reddish. In the smallest, agita tion produces a general luminous appearance ; the light of each separate individual being so small, or so faint, as not to be separately distinguishable. Thus, wherever sparks are observed, we may expect to discover visible animals. These sparks are frequent in the vicinity of sea-weed. They arc also found adhering to these plants, as well as to oysters and crabs, and to the larger fishes ; and by examining these by a lens, the animal itself is easily discovered. Small monoculi, podura, cyclopes, nereides, scolopendra, squilla, and other animals of con siderable dimensions, will thus be frequently detected, as well as minute hydra, and other creatures, which it is un necessary to enumerate ; as, in fact, we have never ob served one that was not phosphorescent.