330 



THE PUITCTION OP SALT IN SEA WATER, 



If the absorbing power of the air (by increase of temperature or other 

 cause) increase in strength, the effects of this increase become con- 

 trolled and rapidly neutralized by the stronger resisting power 

 imparted to the water by its increased degree of saltness. If, on 

 the other hand, the absorbent power of the air become, weakened 

 (by excess of rainfall, diminution of temperature, &c.), evaporation 

 becomes assisted by the weaker resistance of the water. These com- 

 pensating effects, it is contended, are due essentially to the presence 

 of saline matters in the waters of the sea. 



The original experiments published on this subject in 1855 — ' 

 although suj93.ciently exact to establish the striking difference which 

 the evaporation of fresh water exhibits as compared with that of sea 

 water — contained a source of error arising from the form of the 

 vessels employed, and partly from the evaporating surface of the 

 liquids not having been kept constantly at the same level. As the 

 surface of the fresh water soon occupied a lower level than th^t of 

 the other liquid, it necessarily became protected to a greater degree 

 from the action of the atmosphere : and thus the evaporation from 

 it (although always greater than the evaporation from the salt water) 

 was somewhat less than it should have been. Experiments of this 

 kind can scarcely be rendered absolutely faultless, but in after trials 

 these sources of error were remedied as far as possible. The annexed 

 results are from one of these trials extending over seven compara- 

 tively warm days : — 



The function of regulating evaporation, tkus attributed to the saline • 

 CDndition of sea water, may not seem at first thought to be a very 

 important one ; but, in its results, it is probably the most important 

 of all the natural phenomena of which the ocean is the stage. The 

 moisture of the earth, it is well known, comes essentially from the 

 sea. The salt-free vapour, taken up by evaporation from the surface 



