Nature 55, 545-546 (08 April 1897) | doi:10.1038/055545a0

The Theory of Osmotic Pressure


AS osmotic theory is now attracting general attention in this country, it seems desirable that all the positions that are maintained in regard to it should be clearly set forth. The excuse for offering the following remarks is that for some time I have paid attention to the subject in its relations to general molecular theory, both in the thermal and the electrical aspects. I fail to recognise how the validity of the thermodynamic basis of the law of osmotic pressure can be shaken; and though the idea of ionic dissociation in solutions is an additional hypothesis which must be judged separately by the extent of its agreement with the facts, it appears to me that in some form—possibly not at all in the chemical imagery with which it is at present often associated—it holds the field. It is difficult, in fact, to see how the hypothesis that the same chemical element can have different valencies in different series of compounds, which is now usually accepted, is fundamentally any whit less parodoxical than the hypothesis of ionic dissociation; anything that throws light on the one must also illuminate the other.



  1. Proc. R.S. Edin., January; NATURE, January 21, p. 272.
  2. H.von Helmholtz, "Zur Thermodynamik Chemischer Vorgänge, iii." in Collected Papers, vol. iii. pp. 105–114, especially his equation (4) and the theory of diffusion at the end. [The theory had already been given explicitly in 1876 by Willard Gibbs, loc. cit. infra, p. 227.]
  3. Trans. Connecticut Academy, November 1875, p. 138: " Effect of a Diaphragm Equilibrium of Osmotic Forces)."