ITS TRANSPAEEXCY AKD COLOUR. SI 



the sun's liglit, wliicli descends tlirougli the liquid depths to the 

 bed of the sea, and is reflected again to the surface ; or does it 

 result, as Cialdi thinks, from particles of mud that are floating in 

 the water ? * 



Another question, difficult to solve, is that of knowing what is the 

 natural colour of the sea- water. Not to mention local colouring, result- 

 ing like phosphorescence from numberless minute living organisms,t 

 the various parts of the ocean almost always present, whatever may be 

 the state of the atmosphere, a normal tint easy to be distinguished from 

 accidental shades. Thus, to cite one of the most striking contrasts, 

 the water of the Gulf of G ascony is of a sombre green, while in the 

 Gulf of Lyons the water of the Mediterranean is of a magnificent 

 azure, deeper than that of the sky. The wonderful blue colour which 

 rises from the depths of the water in the grotto of Capri, so frequently 

 visited by travellers, is a well-known example of the degree of inten- 

 sity to which the blue peculiar to the waters of the Mediterranean 

 can attain. In the tropical latitudes of the Atlantic and the South 

 Sea, the azure of the ocean is no less beautiful than that of the 

 Tyrrhenian Sea; while in the direction of the poles the water 

 gradually assumes a greenish tint. Naturalists have concluded 

 from this fact that the refraction of the rays of light, which are 

 much more vivid under tropical latitudes, play a principal part in 

 the blue colouring of the sea. Maury thinks that the saltness is 

 also one of the causes which contributes the most to give its azure 

 tint to the water; and observes that the Gulf-stream of the American 

 coasts, superior in salinity and in temperature to the water around it, 

 is also of a much deeper blue. In the same way the shallow water 

 let into the salt marshes of coasts, gains in intensity of colour in 

 proportion as the salt is concentrated there. Still it is very possible 

 that the colouring of the sea is due in great part, like the marvel- 

 lous tints of the Swiss lakes,:{: to innumerable corpuscules held in sus- 

 pension, upon which the light strikes. 



The law of the distribution of temperature, in the depth of the 

 ocean, is not as yet more determined than that of the colouring of 

 the water. At the surface of the sea it is as easy to make observations 

 as in the air, and it has been determined, without difficulty, that this 

 superficial sheet of water presents on an average, in all climates, 

 the same degree of heat as the superincumbent atmosphere. Thus, 



* Sul moto ondoso del mare, p. 287. 

 t See below, the section entitled, The Earth and its Fauna. 

 X See in Vol. I. the section entitled, Lakes. 



