186 Mr. H. J. Cai-ter on the Colouring Matter of the Red Sea, 



was once observed. I am therefore inclined to infer that it is 

 chiefly confined to the sea some distance off shore. Yet Ehren- 

 berg, in 1823, saw the Bay of Tor covered with it, even up to 

 the sands. 



Lastly, I would advert, but, as before stated, with much diffi- 

 dence, to that part of the generic characters of Trichodesmium 

 Ehrenbergii in which we find the expression "prime rubro-san- 

 guinece, tandem virides," first used by Montagne (/. c. p. 346), 

 and then repeated by Kiitzing in his ' Species Algarum,' because 

 the facts connected with the accounts given of those who have 

 seen the scum formed by Trichodesmium, together with my own 

 experience of Algae generally, lead me to the opposite conclu- 

 sion, viz. that Trichodesmium is at first green, and subsequently 

 becomes red. 



It is true that its chief colour in the Bay of Tor, when seen 

 by Ehrenberg, was red ; it was red, like " red sawdust," when 

 seen by M. E. Dupont in the Red Sea {ap. Montagne, /. c.) : 

 but, on the other hand, what I saw in the Gulf of Aden and in 

 the Red Sea, together with what Mr. Latimer Clark saw in the 

 Sea of Oman, and Dr. Haines, as above stated, in the Indian 

 Ocean, was nearly all of a yellow oily colour ; and this is the 

 appearance that I have heard generally assigned to it by those 

 who have been in the habit of traversing the seas mentioned. 



Next to the yellow colour, red is the most prevalent, and 

 green least of all. Some of that seen by Ehrenberg was in- 

 tensely green ; this was the case also with the green portion 

 that I saw with the red above noticed ; while Ehrenberg saw 

 other portions of a less green colour. So much for what has 

 been stated respecting the colours under which Trichodesmium 

 has appeared. 



We come now to the usual course presented by other Algae 

 in arriving at a red colour. If we take the Peridinium which 

 colours the sea red on the shores of the island of Bombay, we 

 shall find, as above stated, that it is at first green, then yellowish, 

 and lastly red. In the green stage, the contents of the cell are 

 so thin and watery that they easily allow the light to traverse 

 them, and thus the Peridinium passes unobserved ; but as they 

 become inspissated, oil-globules generated, and the chlorophyll 

 changed first to yellow and lastly to red, these contents become 

 more opake ; and thus the Peridinium, by reflecting much more 

 light than it did at first, comes rapidly into notice, and by its 

 numbers gives a general red colour to that part of the sea in 

 which it may be present. The same is frequently, indeed com- 

 monly, the course with Euglena in freshwater ponds. The little 

 Protococcus which colours the salt red in the salt-pans of the 

 Island of Bombay, is green in the active period of its existence, 



