THE PROPERTIES OF SEA-WATER 79 



and the Atlantic. Most abyssal animais — e.g., fish (except 

 those which live embedded in the mud of the floor of the 

 océan) — possess eyes, and consequently can see either by 

 means of the phosphorescent light which is présent at thèse 

 depths, or by such extremely small quantities of violet light 

 which penetrate to depths of thousands of fathoms. Thèse 

 indigo-blue rays would not be perceptible to the human 

 eye. 



The bluest of ail seas, the Sargasso, is also the poorest in 

 •floating organisms (plankton), so much so that " deep blue is 

 the désert colour of the deep sea." Not only does the plankton 

 minimise the transparency of the sea and so make it greener, 

 but in many cases the colour of the large masses of particular 

 organisms which are sometimes présent imparts a decided tint 

 to the sea. The chromatophores of the plankton plants, such 

 as diatoms and Peridineœ, are yellow to brown, and when 

 pelagic diatoms are présent in large quantities the whole sea 

 appears cloudy. In the main océans changes of colour due to 

 plankton are not frequently met with, though occasionally a 

 greenish tint is noticed at certain periods at places which are 

 usually deep blue in colour. Hudson (1807) noticed in northern 

 waters that the colour was olive-green, this being due to 

 the présence of, diatoms, and a similar phenomenon due 

 to the same cause was noted by James Ross in Antarctic 

 waters. 



The red colour frequently observed in the Red Sea is due 

 to masses of an alga {Trichodesmium erythrœuvi). A yellowish 

 colour due to a yellow-brown alga has also been observed in 

 Arafura Sea, not far from the mouth of the River La Plata 

 (South America). The sea is frequently tinted blood-red ; in 

 this case the colour is due to the présence of minute marine 

 Crustacea (Copepoda). A certain species of thèse Crustacea 

 (Calamis finmarchicus) abundant in northern waters produces 

 a similar coloration north of the Bank of Newfoundland. In 

 the blue Gulf Stream water between the Azores and the Bank 

 of Newfoundland green layers hâve been observed to be due 



