SALINITY, GASES 53 



usually expressed as the amount of dissolved 

 salts contained in 1000 parts of water. In 

 average sea-water the dissolved solids amount 

 to about 35 per thousand or 3-5 per cent. 

 The term " salt " is applied not only to com- 

 mon salt (sodium chloride) but also to other 

 similar compounds, like sodium sulphate, 

 magnesium chloride, etc., which are present 

 in sea-water, though common salt forms the 

 principal constituent. 



Methods of determining Salinity. — For deter- 

 mining the salinity of a given sample of sea- 

 water various methods may be employed. The 

 most direct method is to evaporate a known 

 volume of water, drying and weighing the salts 

 left in the residue, but in the drying process 

 some of the chlorine is likely to be driven 

 off as hydrochloric acid. Another method 

 is to determine by means of the hydrometer 

 or specific gravity bottle or pycnometer the 

 relative density or specific gravity, that is the 

 weight of a certain volume of sea-water at a 

 given temperature compared with that of an 

 equal volume of pure water at its temperature 

 of maximum density, 39° -2 F. (a cubic centi- 

 metre of pure water at that temperature weighs 

 one gram). Specific gravity varies with 

 the amount of salts in solution, and depends 

 also on the temperature, for sea- water expands 

 and becomes lighter when heated and contracts 



