54 THE OCEAN 



and becomes heavier when cooled, increasing in 

 weight down to freezing-point (which varies 

 with the salinity). Therefore, if the specific 

 gravity is always determined at the same tem- 

 perature, or if the determinations are reduced 

 to one standard temperature, differences of 

 specific gravity are due to differences of 

 salinity alone. It is usual nowadays to express 

 salinity in figures indicating the amount of 

 salts in 1000 parts of sea- water, but at the 

 time of the " Challenger " Expedition it was 

 expressed, and still is by some observers, as 

 specific gravity at 60° F. The following table 

 gives the figures of specific gravity at 60° F. 

 corresponding to various degrees of salinity: — 



Salinity per thousand... 000 100 200 300 

 Specific gravity ... 1-0000 1-0058 1-0138 1-0220 



Salinity per thousand... 32-5 350 37-5 400 

 Specific gravity ... 1-0240 10260 1-0280 1-0300 



The most recent method of determining 

 salinity is based on the fact that the ratio of 

 the salts in solution to each other, and to the 

 total salinity, is practically constant every- 

 where. It follows, therefore, that if we can 

 determine the amount of one salt, we can 

 calculate the total weight of all the salts 

 present. The amount of chlorine in sea- 

 water samples is easily and quickly deter- 

 mined by the method of chlorine titration. 



