Colouring of the Waters of the Red Sea. 57 1 



his memoir, whether already known or entirely new and still unpub- 

 lished, are the following : — 



1. That the name of Erythrean Sea, given first to the sea of 

 Oman and to the Arabian Gulf by Herodotus, afterwards by the 

 later Greek authors to all the seas which bathe the coasts of Arabia, 

 probably owes its origin to the very remarkable phenomenon of the 

 colouring of its waters. 



2. That this phenomenon, observed for the first time in 1823 by 

 M. Ehrenberg in the bay of Tor only, then again seen twenty years 

 later by M. Dupont, but in truly gigantic dimensions, is owing to 

 the presence of a microscopic Alga sui generis, floating at the surface 

 of the sea, and even less remarkable for its beautiful red colour than 

 for its prodigious fecundity. 



3. That the reddening of the waters of the lake of Morat by an 

 Oscillatoria which DeCandolle has described, has the nearest re- 

 lation to that of the Arabian Gulf, although the two plants are gene- 

 rically very distinct. 



4. That as we may well suppose, according to the accounts of 

 navigators, who mention striking instances of the red colouring of 

 the sea, these curious phsenomena, though not observed till quite 

 recently, have nevertheless without doubt always existed. 



5. That this unusual colouring of seas is not exclusively caused, 

 as Peron and some others seem to think, perhaps as being chiefly 

 zoologists, by the presence of mollusca and microscopic animalcules, 

 but that it is often also due to the reproduction, perhaps periodical 

 and always very prolific, of some inferior algse, and in particular of 

 the species of the singular genus Trichodesmium. 



6. Lastly, that the phenomenon in question, although generally 

 confined between the tropics, is however not limited to the Red Sea, 

 nor indeed to the Gulf of Oman ; but that, being much more gene- 

 ral, it is found in other seas, for example in the Atlantic and Pacific 

 Oceans, as appears in the ' Journal of Researches' by Mr. Darwin, 

 and from the unpublished documents of Dr. Hinds, communicated 

 by Mr. Berkeley, and from which the following extract is given : — 



" Dr. Hinds, who sailed in the ship Sulphur, sent to explore the 

 western coasts of North America, first observed on the 11th of Feb- 

 ruary 1836, near the Abrolhos Islands, the same Alga doubtless 



