COMPOSITION OF SEA WATER 29 



tain the geographical distribution of the various 

 elements, and the changes which take place in them, 

 some general considerations may be stated before 

 proceeding to detailed description of the methods 

 employed. 



Composition of Sea Water. 



From comparison of all available analyses of sea 

 water, it may be assumed that the composition of sea 

 water in the open oceans is the same in all parts of 

 the world that is to say, ocean water contains 

 certain salts in solution, and the relative proportions of 

 these are constant (see Addenda, p. xvii). 



Their most important features are (a) the pre- 

 ponderance of sodium chloride ; (b) the small pro- 

 portion of calcium carbonate ; and (c) the " residuum " 

 oi 0-22 per cent., which contains traces of nearly all 

 the known elements, including gold. 



There is thus present in all oceans a " brine " of 

 particularly constant composition, and the variations 

 which are observed in the composition of sea water 

 consist merely of variations in the extent to which 

 this brine is diluted, just as variations in tea made 

 from the same leaves consist in variations of strength. 

 Hence one of the most important factors to be de- 

 termined in physical oceanography is the " strength " 

 or salinity of the water at different points and at 

 different depths. The salinity of a sample of water 

 starves as a label, which often makes it possible to trace 

 its origin and physical history and hence to draw con- 

 clusions repecting currents, and, along with its tem- 

 perature in place, enables us to calculate its specific 

 gravity. The ''salinity" is the total weight of salts 

 in bulk in solution in a given weight of water ; thus, 

 " Salinity 35 per mille" means 35 pounds of salts in 



