Dandeno : Osmotic theories 297 



be called the membrane. We cannot follow him so far as this, for 

 in speaking of the membrane, he says, "The substance must be 

 soluble in the membrane/' In some of his experiments the mem- 

 brane was dissolved to some extent by the liquid in contact, but 

 so far as it was soluble, the condition was not ''typical osmosis.'* 

 Solubility of the membrane is not necessary. For instance, if we 

 use parchment paper (butter paper) and two liquids, fi) water, (2) 

 sugar solution, these arranged for an osmometer will give about 

 one third of an atmosphere pressure without extra support There 

 is no solubility whatever of the membrane in either liquid. 



On the other hand, we agree with Kahlenberg in the following 

 {loc, cit. 271) : ** To speak of osmotic pressure of any isolated solu- 

 tion . . .is nonsense." . . . And further, to assume that solutes are 

 polymerized or dissociated in dilute solutions because the osmotic 

 pressures developed by the latter in given cases happen to deviate 

 from values computed from the gas laws is evidently equally unjus- 

 tifiable practice/' We do not differ materially from him with re- 

 spect to the moving force causing osmotic pressure, excepting in the 

 name he employs; namely, chemical affinity. We think this term 

 not happily chosen because the affinity is purely a physical on^^ and 

 not chemical. There is no chemical change whatever. If there 

 be chemical action involved in the operation thus far, it is not 

 osmosis. From the above statement, it must not be assumed that 

 we undervalue the influence of the membrane as a part of the oper- 

 ation. The membrane is absolutely essential. It is a sine qua 

 non^ but it is merely a screen between the two liquids. 



The gas-pressure theory has also been attacked from another 

 point of view, quoting from Nature 72: 541: " Batelli and 

 Stefanini have brought forward, however, a number of facts which, 

 if subsequently verified, are likely to prove insuperable objections 

 to its [the van't Hoff law's] validity. ... It is contended that 

 osmotic pressure is essentially a capillary phenomenon." To what 

 extent this may hold good will depend upon how far the idea of 

 capillarity is carried, and if it is used in its broadest sense, the capil- 

 lary idea is not far removed from that offered in this discussion 

 that oi attraction. 



A summary of the salient points of this discussion, and of the 

 experiments involved, presents the following ; The chief argu- 



