" The fish around her crowded, as they do To the false light which treacherous fishes show." As there can be no doubt, from the various preceding remarks, that the light of all these animals' is under the control of the will, it is a proper object of inquiry how this matter is managed. Here, however, we are fully as much at a loss as in the case of the luminous terrestrial insects. The whole question of its cause, its seat, and its chemical nature, is equally obscure ; and till these are explained, it is almost useless to think of the manner in which it is ei ther exerted or controlled.
Professor Smith, in Captain Tuckey's voyage, thinks he ascertained that, in a certain crab which he examined, the seat of the light was in the brain. That is possible; but we are much more inclined to imagine that there is a special organ provided for this purpose, wherever it is con fined to a limited space, as it is in many of the worms, and among others in the Medusa: and the Beroes, as well as in the shrimps and other insects. We have always observ ed that in these it occupies a single spot, often very small in proportion to the size of the animal, but we have in vain endeavoured to discover what that was. In some of the larger Beroes and Medusa?, we could distinctly ascertain at night, by the feel, that the luminous point was at the anterior or rounded end. Yct, on examining the same place by the light, we could discover no peculiar organization where it could have resided, as the whole substance was alike transparent. Yet we still think, from some observa tions, that it resides in the stomach or intestine ; an opin ion farther confirmed by observing that in the shrimps al so it lies within the thorax, and probably equally in that organ. We are persuaded that Professor Smith mistook this for the brain. But we must leave this point for fu ture investigations. In the larger fishes, however, it oc cupies the whole surface of the skin ; and hence it is that they show a general diffused and faintish light, while the various invertebrate animals exhibit distinct and defined sparks.
This much respecting the seat of the light, which, how ever, gives us no assistance towards explaining how it is controlled by the will. But we must, for the present, be content with our ignorance; nor have we any great reason to be surprised at it, if all the naturalists that have labour ed this subject, with an abundance of specimens, and the most ample means of investigation, arising from the vi tality of the animal, and the facility with which it can be preserved and examined alive in any manner, have as yet been unable to determine this point for the glow-worm.
We have last to inquire respecting the phosphorescent substance itself, if substance it be, and its chemical na ture. Here it appears to us that we are in a state of ab solute ignorance; no less so than in the phosphorescent terrestrial insects. Those who have published their ex periments on this subject, have, in the first place, assum ed that the phosphorescent matter of the dead animal was the only thing to be examined ; a conclusion which we think by no means warranted. It may possibly be so, but that is not necessary. As far as our own trials go, we have been able as yet to devise no method of getting at the phosphorescent matter in the living animal. The expe riments of Dr. Hulme on the luminous substance of the dead fishes are worthless, and lead to no information. Those of N. Dessaignes have an air of more accurate re search, and there is in them a principle of proceeding. We shall barely record them, without pretending to de cide on their accuracy ; because, even if they are correct, we consider that we are as far as ever from the cause of phosphorescence in living fishes.
A piece of phosphorescent fish was put into a saline so lution favourable to the production or maintenance of this property, but which had previously been deprived of its air by ebullition. After two hours the substance became dull, or lost its light. But on introducing a bubble of air, the phosphorescence was restored in a few minutes, and this effect was continued as long as fresh bubbles were in troduced, and till the water was perfectly saturated with air, when it remained permanently bright. Ho considers it, therefore, as a true combustion ; and, as in the phos phorescence of rotten wood, he thought that carbonic acid was produced, so phosphoric acid must have been the re sult in this case.
We shall conclude this part of our subject with one re mark, not less important as a question in natural history and in animal physiology generally, than it is on the sub ject of the final cause of the phosphorescence of marine animals, as we have already stated it. Like all that has preceded, we are indebted fot it to Dr. Alacculloch.