68 A TEXTBOOK OF OCEANOGRAPHY 



The mean quantity is approximately 35'976 grams in i,ooo 

 grams of sea-water — that is, 3J per cent. Sea-water has the 

 properties of a dilute solution of salts — that is to say, as com- 

 parée! with fresh water it has a lower freezing-point and vapour 

 tension, a higher osmotic pressure, electric conductivity, 

 viscosity, and surface tension. 



Sea-water is also an important médium for the development 

 of organisms, both animais and plants ; the latter can in the 

 présence of sunlight build up organic from inorganic matter, 

 thus playing an important part in the circulation of matter in 

 the sea. 



The Salts in Sea-Water. 



Of the 80 éléments known to modem chemistry, 32 hâve 

 been identifîed in sea-water. Probably ail are présent, the 

 majority in such small quantities as to élude identification. Of 

 the 32, the following 7 are the most important : Chlorine, 

 bromine, sulphur, potassium, sodium, calcium, and mag- 

 nésium. 



Analyses of sea-water give the following results. Those 

 of Thorpe were from the Irish Sea, where the salinity is less 

 than in the open océan. 



SALTS IN 1,000 GRAMS SEA-WATER. 



Dittmar's and Forchhammer's analyses were made, the 

 former from Challenger material collected from varions parts 

 of the world, the latter from water for the most part from mid- 



