Dandeno : Osmotic theories 293 



underlies osmotic pressure. The operation tends to show that 

 osmotic pressure is, however, regulated by the character of the 



i 



membrane, and this character depends upon two things ; the size 

 of the pores, and the actual relative attraction which the mem* 

 brane has for the solvent and for the substance in solution. Os- 

 motic processes are not different from diffusion. The membrane 

 simply shows this diffusion. 



Now, the actual amount of pressure developed by a substance 

 in solution will obviously depend upon the number of molecules 

 (or ions) in a given volume of the solution, hence any other con- 

 ditions, such as freezing point or boiling point, which have to do 

 with molecular activities, will not Hkely be very different from 

 osmotic phenomena. And in this connection, it is easy to realize 

 that such attraction as that postulated for osmotic pressure might 

 very probably be different in one substance from that in another. 

 From this it is easy to account for the high pressure of potassium 

 nitrate as compared with sugar, in equimolecular solutions, zvith- 

 oiit assuming the dissociation theory. In this case, there is no 

 necessity for assuming ionization. The cause of the high pres- 

 sure in one case as compared with that in another, is due to the 

 specific attraction of the molecule of the potassium nitrate, and 

 not to the assumption that there are two bodies or ions of K and 

 NO3, instead of one KNO3 body. If we assume the dissociation 

 theory to account for the relatively high pressure of such sub- 

 stances as are called electrolytes, we must ascribe as much attrac- 

 tive force to each ion as to the whole molecule, which is a more 

 unreasonable assumption than to ascribe to one molecule of a 

 different kind a greater attractive force than to that of another. 

 In the former case, we have to say, not that " The whole is greater 

 than its part," but that each part is equal to the whole, which, if 

 not unreasonable, is hovering near it. Jost "*" alludes to '* each 

 free ion having the same osmotic value as the entire molecule." 

 To prove this incorrect, is, of course, not easy, but the burden of 

 proof must rest on those who advance fhat theory. Tlie disso- 

 ciation theory itself is without much support, and in physiological 

 problems it gives no light. There are so many physiological 

 operations at variance with it that we need not assume the theory 



. ■ m ^ ^ "-^ 



* Plant physiology, i6. 1907 [Eng. trans.]. 



