THE OCEAN 177 



Under these circumstances the pressure, called 

 osmotic pressure by van't Hoff, may be very 

 great. 



According to the theories of van't Hoff 

 and Arrhenius this pressure is, in the case of 

 dilute solutions, proportional to the total 

 number of particles (molecules plus ions) 

 which are present in solution. In its magni- 

 tude and the laws governing its variation such 

 pressure corresponds exactly to gaseous pres- 

 sure. In fact the theory of solution consists 

 primarily in the extension of the laws of Boyle 

 and Gay-Lussac, of the hypothesis of Avo- 

 gadro, and of the manifold theoretical develop- 

 ments which have been based upon them, to 

 solutions. The great force of osmotic pres- 

 sure always comes into action when solutions 

 are in contact with permeable or semiper- 

 meable membranes. It must therefore al- 

 ways be reckoned with in physiology. The 

 biological importance of the constancy of the 

 osmotic pressure of sea water is strikingly 

 exemplified by the precision with which every 

 higher vertebrate preserves constant the os- 

 motic pressure of its own body fluids, all at 

 about seven or eight atmospheres. 



It may readily be shown that the osmotic 

 pressure of a solution is proportional to the 

 depression of its freezing point, and accord- 



