THE BEARING OF OSMOTIC PRESSURE 



85 



We have seen that the osmotic pressure of solutions obeys the law 

 of Boyle for gases, and we now learn that diffusion obeys this 

 same law. This is, then, one analogy between the phenomena 

 of osmotic pressure and those of diffusion. 



We have already become familiar with the beautiful principle 

 of Soret, which shows that the temperature coefficient of diffusion* 

 is the same as that of a gas. In other words, temperature has 

 the same effect upon diffusion that it has upon gases, and the 

 law of Gay-Lussac applies to both phenomena. The law of Gay- 

 Lussac, as we now know, also applies to osmotic pressure, and 

 this is a second analogy between osmotic pressure and diffusion. 



By comparing the phenomena of diffusion in general with 

 those of osmotic pressure in the broadest possible way, we learn 

 that we are justified in concluding that the two sets of phenomena 

 obey throughout the same laws, and we conclude that osmotic 

 pressure is the cause of all diffusion. 



Osmotic pressure is then the force which drives the heavy 

 dissolved substance through the solvent even against the pull of 

 gravity until the whole system becomes homogeneous. It is 

 capable, as we have seen, of driving a very heavy substance up 

 through a light solvent to a very great height, and of establish- 

 ing and maintaining equilibrium of concentration in solution. 



Let us now think for a moment what diffusion really means 

 for general chemistry. If it were not for diffusion, therefore, 

 • if it w^ere not for osmotic pressure, it would be impossible to 



maintain the different parts of the solution at the same concen- 

 tration for any appreciable period of time. Indeed, this condi- 

 tion in solution could be secured only approximately by keeping 

 the solution in question in most violent agitation. If a solution 

 would not maintain itself homogeneous, it would be a most seri- 

 ous matter not only for quantitative chemistry, but for quantita- 

 tive work in any branch of the biological sciences in which solu- 

 tions are used, and most of them employ solutions in one way 

 or another. It would be especially serious for the recent develop- 

 ments in botany and zoology, which have come about by varying 

 the conditions to which the organism is subjected and observing 

 what changes are produced in it. Without the property which 



