If we have thus succeeded in establishing these propo sitions, the water of the sea, when in a state of purity, yields no light, supposing also that it contains no visible animals. In proportion as it becomes green, which seems to be the result of some admixture of foreign matters, liv ing or dead, or both, its tendency to show light increases, and is greatest when it is milky, at which time it is easy to diccovci that it contains dead and living matters, both invisible to the naked eye. Hence it is a legitimate con clusion, that the phosphorescence of sea-water is a pro perty, not belonging to itself, but to substances accident ally contained in it.
hat we may attempt to ascertain what those sub stances are, we may first remark, that at a certain period after the death of fishes, they become luminous, even be fore putrefaction. In fact this property disappears before the proper putrefaction process has commenced. It is at the same time more remarkable in some fish than in others, although it occurs in the whole as far as our obser vations go. It is popularly remarked to be especially prevalent in the whiting: but we think that the reason of this is not only very intelligible, but that in assigning it we shall also explain why it has been overlooked in the other fishes in which it occurs. Among fishes, the whit ing is one of those which goes quickest into the putrefac tive state, while in others that process is far more tedious; in some indeed that is so much the case, that they may even be dried without experiencing it at all. This, in particular, is true of the salmon, the skate, and some others, while the turbot, the sole, the dory, and many more, can be preserved for days without undergoing any change. Now, as we have shown that the luminous pro cess occurs in a stage intermediate between the death of the animal, and the first occurrence of the putrefaction process, it is not difficult to see how that has been over looked in those fish which take it in slowly. In fact, these fishes are eaten long before it has commenced ; and as it is, when intended for this purpose, that observers arc chiefly familiar with them, it is not surprising if this phe nomena has been so often overlooked. It is only neces sary to keep the whole of them a sufficient length of time, to find that they will all become phosphorescent.
Now it happens, that during this luminous stage, as well as after it, there is a solution of some of the solid matter of the fish going on ; or, if not an actual sohoion, a disintegration of the cel:idar or muscular fibre. This is diffusible through the water, and may easily be examined by the microscope; at times indeed by the naked eye. For the luminous property, provided it was in a phosphores cent state before ; and it is in this way that a portion of water may be rendered luminous for a short time merely by agitating a luminous fish in it. This is the explanation of the experiments of Dr. Hulme, which led many per sons to imagine that the luminous appearance of sea-water was derived from putrescent animal matter. But, in the first place, there is no putrescence in this case, as that term is generally understood ; besides which, the light is very transitory. It is a mistake, moreover, to suppose
that the light resided in the mucus of the skin, and in agitating the fish in water this was the substance that carried the light with it. In succession, all the muscular and cellular matter of the animal becomes luminous, though the surface alone may appear so at first, because there the first change occurs. It is easy to see this by watching the progress in any fish, and it will be found that, at one period, the whole animal is luminous through out, and as if transparent.
It is now plain, that if all the fibrous matter thus sepa rated from fish at a certain stage after death, were perma nently luminous, we might thus account for the phospho rescence of turbid water, or of that in which the milki ness above-mentioned is discernible. But that is not the case; since, as we just remarked, the luminous property of this matter speedily disappears. We have also often succeeded in procuring water thus turbid and abounding in fibrous matter, which was utterly dark or incespc.bte of phosphorescence: and in these instances, as in others, we have also ascertained, as we shall hereafter fully show, that no phosphorescence was present unless the water con tained living animals, and that in this case it was invaria bly luminous.
We observed above, that it was supposed by some, that the luminous matter resided in the mucous secretions of the skin, and that thus being diffused through the water, even from the living animals, it was the cause of the lu minous property of the sea. We were at one time inclin ed to adopt this opinion ourselves, not indeed with regard to all the light which the sea exhibits, but to a particular feeble diffused phosphorescence which we shall presently describe. But we have since found reasons for withdraw ing this opinion, at least in a great measure, and for re curring to a different cause, even for this kind of light, which is intermixed with those more brilliant sparklings that the ocean exhibits. We do not profess indeed to be quite satisfied respecting the extent, either in the durabi lity or the intensity of the light which does reside in the surface of fishes, nor of the degree to which it is diffusi ble when removed from the animal. We have, however, ascertained that the whole surface of a living fish does be come luminous, if not always or permanently so, as in some cases it appears to be; and we therefore think it possible, that if the luminous matter is a secretion, it may be washed off together with the mucus, so as to be diffu sible through the water, as it is in the case of the dead animal. Even in this case, however, we should no more expect to find it permanent or durable, than when separat ed from a dead fish ; and we must add, that we even doubt whether such an event ever happens, as we have never succeeded in rendering water diffusedly luminous by means of a living fish, unless when we could ascertain that ani mals of a minute kind were also present.