240 Mr Larmor, On the theory of Osmotic Pressure. [Jan. 25, 



the biloculine plan throughout, and when it does not, the bilocu- 

 line arrangement is sooner reached, that is after the formation of 

 a smaller number of chambers, than in the case of the micro- 

 spheric form. The exception does however appear to detract in 

 some measure from the sharpness of the contrast in the mode of 

 growth of the two forms, and, though I think without altogether 

 annulling it, from the force of the explanation which I have 

 offered. 



(3) On the theory of Osmotic Pressure. By J. Larmor, M.A., 

 F.R.S, St John's College. 



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 relation to general molecular theory, both in 

 the thermal and the electrical aspects. I fail to recognize 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 expounded — 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 

 paradoxical than the hypothesis of ionic dissociation ; anything 

 that throws light on the real nature of the one must also illuminate 

 the other. 



In his recent note on this subject*, Lord Kelvin appears to 

 allow, within certain limits, the cogency of the argument which 

 bases the law of osmotic pressure on Henry's empirical law of 

 solubility for gases, an argument which has recently been carefully 

 restated by Lord Rayleigh, having previously been employed, as 

 he remarks, in forms more or less explicit, by van't Hoff, Nernst 

 and other investigators. The connexion thus established however 

 hardly amounts to a physical demonstration, because it only 

 deduces one empirical relation from another. Yet it seems desirable 

 to draw attention to the fact, which I have not seen anywhere 

 remarked, that this method had been employed by von Helmholtz 

 in 1883, two years before van't Hoff announced his theory of the 

 correlation between osmotic and gaseous pressures ; and that the 

 principles given by him on an investigation of the work-equivalent 

 of gaseous solution, made in connexion with the theory of galvanic 



* Proc. E. S. Ediii., Jan. 1897 ; Nature, Jan. 21, 1897, p. 272. 



