184 



the solid condition at least double molecules occur. The difference 

 between «- and ^-cinnamic acid might then have its origin in the 

 manner in which the single molecules arc placed in the doul>Ie 

 molecule. 



The difference between these two assumptions consists in this that 

 the first admits of a difference in the solid condition onl}', whereas 

 the second renders possible a difference for the solution also. 



A further investigation will have to decide which representation 

 is in harmony with the facts. I hope to revert to this in detail, 

 shortly. 



Physics. — "Soiiw Remarks on the Osmotic Pressure". By Dr. 

 J. J. VAN Laar. (Communicated by Prof. H. A. Lorentz). 



(Communicated in the meeting of May 29, 1915). 



With much interest I read Prof. Ehrenfkst's paper [in the 

 Proceedings of this Academy (April 1915)] on the kinetic inter- 

 pretation of the osmotic pressure. 



However, I can concur neither with the deeper ground of iiis 

 interesting considerations, nor with the "Remarks" that are added 

 to them, which in some respect may be considered as resulting from 

 the foregoing considerations. 



Prof. Ehrenfkst knows that I feel a special interest in the osmotic 

 pressure and its correct interpretation, so that he will no doubt 

 excuse me if I once more return to it. 



I will therefore briefly summarize my objections, already set 

 forth in different papers'), in a number of Theses. 



Thesis I. The results of a kinetic theory must necessarily be in 

 accordance with the established results of Thermodynamics. 



If the results of the kinetic theory differ from those of Thermo- 

 dynamics, the kinetic theory in question is not valid. 



Thesis II. Through the equating of the molecular thermodynamic 

 potentials of the water in the solution and of the pure water outside 

 it [there exists namely only thermodynamic equilibrium between 

 the "mater" on either side of the membrane, as this is supposed to 

 be permeable only to water] the thermodynamic theory leads to') 



1) See particularly: Seciis Vorti-age (1906;, p. 17—36, and These Proc. of 

 June 1C06, p. 53 et seq. Also Zeitschr. f. pliysik. Ch. 64, p. 629 el seq. (190S). 



-) I gave this simple derivation already in 1894 (Zeitschr. f. physik. Ch. 15, 

 p, 463 ct seq ). 



