THE OCEAN FEEDING-GROUNDS 71 



of water at o C. ; that is, the specific weight as compared 

 with the weight of fresh water at +4 C. 1 " The water 

 of the ocean," says M. Thoulet, "is not everywhere of 

 the same chemical composition. We must not regard 

 sea-water as simply a solution, more or less concentrated, 

 of the various salts whose presence is revealed by 

 chemical analysis. This diversity of chemical composi- 

 tion is due to a host of causes, of which we will consider 

 a few. The freezing of the sea- water in the polar regions 

 causes a concentration of the sulphates in the ice thus 

 formed, and of chlorides in the water. The waters of 

 rivers reach the sea with small quantities of saline 

 matter in solution. Reactions take place even in the 

 interior of the upper layer of the ocean floor, where the 

 living organisms gather certain salts to fix them in their 

 tissues some silicon, others calcium, others lead, zinc, 

 bromine, or iodine salts ; in short, the reciprocal action 

 of these salts, when one of them is added to or subtracted 

 from the whole, gives rise to fresh combinations among 

 the remaining salts." The waters of the continental 

 plateau, the Channel, and the French Atlantic contain 34 

 to 35 grammes of salt per litre ; of the Mediterranean, 

 39 ; of the North Sea, 32 in the Skagerack, 34 and 34-8 in 



1 The temperature of distilled water at its maximum degree of 

 density. In practice salinity is often expressed in terms of density. 



Salinity. Density. 



13*1 grammes of salt per 1,000 grammes of sea-water = roi 



26'2 = 1-02 



30 > = 1*0226 



32-8 , = 1-025 



35 , = i'0253 



39*3 , ,, = 1*028 



4 , ,, = 1*0289 



45 > >, = 1*0326 



50 = 1*0365 



