Oldalképek
PDF
ePub

On the Animalcules and other Organized Bodies which give a Red Colour to the Sea. By M. CAMILLE DARESTE. (Translated from Annales des Sciences Naturelles.)

Mariners frequently observe in the ocean spaces of greater or less extent, the water of which presents colours passing through all the shades of yellow, blood-red, and brown. These coloured waters form bands, often of great extent, the edges of which are very clearly distinguished from the water which continues transparent. They have often been mistaken by seamen for shallows, though almost always observed in localities where the depth is considerable.

It has been long known that this colour does not belong to the water itself, but is produced by substances suspended in it, although their nature has generally been misunderstood. The prevalent opinion among seamen is, that it is chiefly caused by the spawn of fish, but naturalists, who have studied these phenomena, have thought that they were produced by organized beings; although, from not having had good microscopes at hand, they have frequently been unable to determine their exact nature.

Having had occasion to study one of these coloured spaces at the beginning of last year,* I felt desirous of becoming acquainted with facts of the same kind which have been mentioned by navigators and naturalists, and I have collected about fifty. The comparative examination of all these observations has enabled me to determine, in several cases with certainty, in others with a greater or lesser probability, the nature of the different organized beings which produce these colours. And it has also led to a result which appears to me as interesting to Physical Geography as to Natural History itself, that these colourings are, for the most part at least, permanent in certain localities, where they are generally reproduced at the same time of the year.

I have arranged these observations according to the certain or probable nature of the cause which produces them.

* See Memoire sur la Coloration de la Mer de la Chine.-Ann. des Scien. Nat., 4 scr., t. i.

§ I. Waters coloured by a small microscopic Alga, which Ehrenberg has described under the name of TRICHODESMIUM ERY

THRÆUM.

The first correct observation of a colour produced by this plant was made by Ehrenberg at Tor, on the Red Sea.

"On the 10th December 1823, I saw," says this observer, "the surprising phenomenon of the whole bay which forms the port of this town being of a blood-red colour. The offing outside the coral reef preserved its ordinary colour; but the gentle waves of a tranquil sea carried upon the shore, during the heat of the day, a mucilaginous matter of a blood-red colour, and deposited it upon the sand, so that in the space of half an hour the entire bay, at high water, was surrounded with a red band of several feet in width. I filled some glasses with the water, and brought them into a tent which I had near the sea. It was easy to see that this colour was owing to little tufts scarcely visible, often greenish, sometimes of an intense green, but for the most part of a deep red, although the water upon which they floated was entirely devoid of colour. As long as the sun shone above the horizon, I observed that these tufts kept on the surface of the water in the glasses which I had brought with me, and that during the night, and when I moved the vessel, they sunk to the bottom, rising again some time after to the surface." In examining these tufts with the microscope, M. Ehrenberg discovered an alga belonging to a new genus, and described it under the name of Trichodesmium erythræum. The description will be found in his Memoir, and in one by M. Montagne, who has reported and commented upon an analogous observation of M. Evenor Dupont.* M. Ehrenberg again observed this phenomenon on the 25th and 30th December 1823, and the 5th January 1824.

Nearly twenty years after the observation of M. Ehrenberg, an analogous fact was seen by M. Evenor Dupont. "On the 15th July 1843, we were crossing from the town of Cosseir. The sea was red all day; the next day, 16th, it was the same until mid-day, the hour at which we found our

* Memoire sur le Phénomène de la Coloration de la Mer Rouge.—Ann, des Scien. Nat., 3e ser., t. ii., P. 332.

selves opposite Tor. During this interval the packet making 8 knots an hour, as the sailors say, and had run over a space of 256 miles, or about 85 leagues."

M. Dupont saw that this water owed its colour to a particular colouring substance which he collected on a filter; and M. Montagne recognised in it the characters of the plant described by Ehrenberg. These observations, made with every possible care by eminent naturalists, cannot leave any doubt as to the results. We may, at the same time, presume that this phenomenon was observed at a much earlier period, because it gives us a satisfactory explanation of the term Red Sea, a name which that sea has borne since the time of Herodotus.

After many bibliographic researches, I happened to meet with a very curious passage in the Memoirs of Albuquerque, which shows that the phenomenon of the colour of the Red Sea has been observed under circumstances which render this observation very interesting on many accounts. "This name

of Mer Rouge or Mer Vermeillé suits it better than any other, and Albuquerque well knew why it had been thus named in old times: it was because the whole entrance of the Red Sea is filled with a great number of patches as red as blood. Alphonse Albuquerque having arrived with his fleet at the entrance of the strait, on the point of returning to India, saw, from the stern of his vessel, issuing from the strait, and expanding outside, a stream of very red water, which flowed towards Aden, and extended on that side of the strait as far as the eye could reach. Astonished at this fact, he asked the Moorish pilots the cause of this red colour which spread itself so widely over the sea; they told him that he need not be astonished, for the eddy made by the waters during the ebb and flow of the sea bristling with rocks and not very deep, was the cause of this red colour; and that it is principally during the reflux, because the force of the waters is greatest when they are going out, and there is no current inside the strait; also when the wind is strong, and from the west, the sea becomes more red. These reasons appeared satisfactory to Alphonse Albuquerque; he therefore gave his assent, and moreover

thought that the earthy matter from the bottom of the sea was the cause of the colour."

The exact date of this observation is unknown, but it is certain that the fleet of Albuquerque appeared before Aden on the 7th February 1513; and it would long since have determined the ideas of geographers upon the colour of the Red Sea, had it not been contradicted some years later by another Portuguese, whose name is equally celebrated in the history of the conquest of India, João de Castro. In the account of his voyage to the Red Sea, under the direction of Don Estebam de Gama, there is a curious dissertation (p. 257) upon the origin of the name Red Sea.

In this dissertation, João de Castro remarks that the Portuguese who preceded him on the Red Sea referred to the existence of the red patches: yet he does not mention the name of Albuquerque. "Portuguese who have navigated the Red Sea in times past, affirm that the sea is quite covered by patches of a deep red. The cause to which they attribute this phenomenon is the following: they say that the soil on the coast of Arabia is by nature very red; and that this coast is subject to very great hurricanes, during which clouds of dust are raised and carried away by the force of the wind, and falling into the sea, communicate their own colour to the water : this is why it has been called the Red Sea." João de Castro cites this opinion, in order to contradict it. He never observed either this red earth or the clouds of dust, and he never saw the water coloured red. He tells us that almost every day during the expedition he brought up some of the water, in order to observe it, and that he always found it perfectly clear and transparent; but we know, from the observations of M. Ehrenberg and M. Evenor Dupont, already referred to, that the colour of the Red Sea is neither a constant nor a general fact; and this explains how, during the course of his voyage, which lasted a little more than four months, João de Castro never observed this phenomenon, although he had devoted his special attention to it. He says, besides, with perfect truth, that he does not pretend to deny that the water of the Red Sea may sometimes present particular colours, but that for himself he had never met with them.

In order to explain the origin of the name Red Sea, there is also an imaginary hypothesis, which has become celebrated; and which attributes this name to the coral reefs carpeting the numerous shallows of this sea, and which, owing to the transparency of the water, are easily seen. This hypothesis cannot be maintained; for, even if it could be applied to some red and immovable patches, it certainly could not explain the movement of the great band of coloured water which Albuquerque saw issue from the Red Sea and flow into the Gulf of Arabia. It is true that Barros, who adopts João de Castro's explanation, supposes, in the latter case, that the colour of this band of water proceeded from the particles of coral carried away by the eddies. But, for many reasons, this is difficult to admit; and besides, we have a much more simple explanation in the phenomenon observed by MM. Ehrenberg and Evenor Dupont.

Nevertheless, this opinion of João de Castro, founded upon facts, collected with very great care, by an able observer for his time, and which, it is necessary to remember, had some very good arguments in its favour, was adopted at once by contemporaries. All the authors of that age who have spoken of the Red Sea, have, more or less, repeated the opinions and even the phrases of the "Roteiro;" and these ideas are circulated generally in treatises on physical geography even to our day, while the remarkable observation of Albuquerque has been entirely forgotten.

The Trichodesmium erythræum is likewise found in the Chinese Sea; I have proved its presence in the coloured water which was brought from it by M. Mollien; and have spoken of this fact in detail in a previous memoir. It will suffice to repeat here that the Chinese Sea presents many parts which are coloured red and yellow, the latter predominating to the north of the island of Formosa, and the former to the south; that the water which I examined was procured from a red patch, and was taken up in lat. 10° N., long. 106° E. On the same occasion, I mentioned the very curious fact of a shower of sand, observed at Shanghai, 4th March 1846, by Dr Bellott, surgeon in the English navy, and which, from the observations of Mr Piddington, appeared to be composed, for the most part, of a microscopic alga.

« ElőzőTovább »