428 Dr. F. Tinker on Osmotic Pressure. 



in the crystalline state have a structure corresponding to 

 a formula MxC^M^C^, which we might call the constitution 

 formula of the solid state ; but this constitution formula may 

 be quite different from the chemical constitution formula 

 of the substances, or from the constitution of the liquid, 

 gaseous, or ionic form. Thus the chemical constitution of 

 xenotime may be that of a phosphate, and zircon may have 

 the chemical constitution of a silicate ; but we must also be 

 aware of the possibility that identity of crystal structure 

 does not involve identity or similarity of chemical con- 

 stitution. Thus, if we regard xenotime as a phosphate, we 

 need not regard zircon as a silicate or rutile as a titanate. 



In conclusion, I want to express my indebtedness to 

 Mr. H. Schjelderup for his valuable assistance during these 

 researches. My thanks are also due to Miss Liv Grleditsch 

 for making the models which are reproduced in this paper. 



University of Ohristiania, 

 January 1, 1917. 



XLI. Osmotic Pressure : its Relation to the Membrane, the 

 Solvent, and the Solute. By Frank Tinker, D.Sc* 



IT is a noteworthy fact that whereas most of the kinetic 

 explanations of the mechanism oE osmotic pressure have 

 focussed an almost exclusive attention on the solute, the 

 thermod} namic cycle by which the osmotic laws are derived 

 is carried out with the solvent, and with no reference to the 

 solute as distinct from the solution f. For this reason it has 

 come to be generally recognized, at least by those who 

 during recent years have treated the subject thermo- 

 dynamically, that the osmotic mechanism is to be explained 

 ultimately in terms of the solvent rather than the solute ; 

 osmotic flow, for instance, takes place because the potential 

 of the pure solvent is greater than the potential of the solvent 

 inside the solution. Experiment also lends strong support 

 to this view. With the exception that moisture travels 

 across a semipermeable membrane in a much more condensed 

 condition than vapour proper, the phenomenon of osmotic 



* Communicated by Sir Oliver Lodge, F.R.S., being part of a Thesis 

 approved for the Degree of Doctor of Science in the University of 

 London. 



f I refer to the process in which the osmotic pressure is first obtained 



in terms of the vapour pressure by the formula P= -^log^ ; and then 



afterwards in terms of the concentration by combining this formula 



, , P N+m 

 with Raoult's law - = -^— • 



