THE PROPERTIES OF SEA-WATER 73 



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

 2"4695 per cent, bas its maximum density at its freezing- 

 point, -1332° C. 



Knudsen bas pubHsbed tables giving tbe relationship 

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

 water of différent degrees of saHnity. Altbough tbe discussion 

 of tbese 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 fîsh 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 bas 

 at a température of 18° C, an osmotic pressure of 4"9 atmo- 

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

 30° C, bas osmotic pressure of 267 atmosphères. 



Tbe following table gives tbe boilin'g-point and vapour 

 tension for sea-water of différent degrees of salinity : 



Salinity, per cent. • ... 5 10 15 

 Boiling-point, increase 



°C 008 o"i6 0*23 



Vapour tension de- 



crease, mm. ... 2" 13 4-23 6'45 



From the above considérations it follows that sea-water, 

 under the same conditions, évaporâtes more slowly than fresh 

 water, and the higher the salinity the slower the evaporation. 

 The variations in tbe surface salinity of sea-water dépend on 

 tbe relation wbich evaporation bears to rainfall, and since the 

 sole source of atmospheric moisture is practically tbe sea, tbe 

 inter-relationsbips of salinity, evaporation, and atmospheric 

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

 fall, tbe végétation of the earth's surface, and incidentally the 

 distribution of population. The fact that evaporation from 

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



