THE PROPERTIES OF SEA-WATER 77 



The fact that light penetrated to an appreciable extent down to 

 550 fathoms was not previously suspected. 



Regnard has devisefl an apparatus for determining the 

 length of the day at different depths, a photographic film on 

 a cylinder being passed by a clock-work arrangement before 

 an aperture in the ** camera." Experiments made at Madeira 

 showed that at n fathoms the day lasted u hours, at \(>\ 

 fathoms 5 hours, but at 22 fathoms, although the sun was 

 shining brightly, the film only exhibited a slight influence of 

 light for a quarter of an hour about 2 p.m. 



As a general rule pure water is less transparent the higher 

 the temperature. In spite of this, the tropical seas are more 

 transparent than the temperate. Salinity does not appear to 

 be of much importance in this connection. The Baltic and 

 North Seas are of like transparency but very different salinity, 

 and the same is true of the Red and Sargasso Seas. 



The colour of the ocean has been the subject of frequent 

 misunderstandings and misdescription. Reflection from the 

 surface causes the various differences described by different 

 writers and painted by different artists. 



Omitting reflection from consideration, the colour of local 

 seas is green, that of the tropical seas, on the other hand, blue. 

 Occasionally the colour of the sea takes on a whitish, yellowish, 

 reddish, or olive-coloured tint, but these appearances are 

 invariably due to the presence of suspended matter or of 

 organisms. The normal colour of the sea is consequently 

 either green or blue. 



For the exact determination of the colour of the sea various 

 instruments and methods have been applied, of which one 

 only need be mentioned here. Forel invented a scale or 

 xanthometer which indicates all colour changes from pure 

 yellow through green to deep blue. Two solutions were made, 

 the first of i gram of copper sulphate with 9 grams of ammonia 

 and 190 grams of water; the second of i gram of neutral 

 potassium chromate in 199 grams of water. The first solution 

 was taken as the index and was numbered o. Then mixtures 



