THE PROPERTIES OF SEA-WATER 73 



especially by Knudsen, who found that sea-water of salinity 

 2 '4^95 per cent, has its maximum density at its freezing- 

 point, - 1-332 C. 



Knudsen has published tables giving the relationship 

 between the freezing-point, density, and osmotic pressure for 

 water of different degrees of salinity. Although the discussion 

 of these relationships is beyond the scope of this work, there is 

 one point of practical importance to the biologist that must be 

 mentioned. If a frog is placed in sea-water, it loses at once 

 by osmosis, through its skin, large quantities of water, and 

 soon loses one-fifth of its original weight. On the contrary, a 

 true sea fish suddenly plunged into fresh water quickly absorbs 

 water, and, in fact, speedily dies from a kind of dropsy. 



The osmotic pressure of sea-water varies with the increase 

 of density. Water from the Baltic of 7-5 per 1,000 salinity has 

 at a temperature of 18 C. an osmotic pressure of 4/9 atmo- 

 spheres, while water of the Red Sea of 40 per 1,000 salinity at 

 30 C. has osmotic pressure of 267 atmospheres. 



The following table gives the boiling-point and vapour 

 tension for sea-water of different degrees of salinity : 



Salinity, per cent. ... 5 10 15 20 25 30 35 40 



Boiling-point,increase 

 C 008 0*16 o'23 0*31 0*39 o'47 0*56 0-64 



Vapour tension de- 

 crease, mm. ... 2-13 4-23 6-45 8-47 1073 12*97 15*23 17*55 



From the above considerations it follows that sea-water, 

 under the same conditions, evaporates more slowly than fresh 

 W 7 ater, and the higher the salinity the slower the evaporation. 

 The variations in the surface salinity of sea-water depend on 

 the relation w r hich evaporation bears to rainfall, and since the 

 sole source of atmospheric moisture is practically the sea, the 

 inter-relationships of salinity, evaporation, and atmospheric 

 moisture are of great importance. On the last depends rain- 

 fall, the vegetation of the earth's surface, and incidentally the 

 distribution of population. The fact that evaporation from 

 sea-water is much less than from fresh has long been known, 



