of the Electric and Luminiferous Medium. 



283 



Osmotic pressure is related to the total available energy, not 

 merely to the mechanical part of it : the limiting or maximum value 

 which it cannot exceed is equal to the change in available energy 

 produced per unit volume of transpiration across the partition. An 

 ultimate deduction of van't Hoff's law of analogy between osmotic 

 and gaseous pressure is offered, on the foundation of the principle of 

 available energy, which is independent of any assumption as to the 

 character of that pressure whether purely kinetic or otherwise : if 

 this be accepted, it will follow that no inference as to the physical 

 state of the dissolved substance, except as regards its degree of 

 effective dissociation, is deducible from the osmotic law. It appears 

 to have escaped general notice that what was virtually a prediction 

 of the Jaw for the cases of dissolved gases is involved in the equations 

 of von Helmholtz's discussion of the influence of dissolved gas on elec- 

 tromotive force, in which however the argument is based on Henry's 

 law of solution ; the law itself had indeed been formulated explicitly 

 on similar theoretical grounds by Willard Gibbs still earlier. The in- 

 fluence of an electric field on osmotic pressure between dielectric fluids 

 is estimated : this involves by cyclic processes the influence of an 

 electric field on the vapour pressure and on the freezing point of a 

 dielectric liqoid. Some considerations connected with the nature of 

 the process of ionisation are brought forward. The laws of chemical 

 equilibrium, as developed by Gruldberg and Waage and by van't Hoff, 

 are placed in relation to the principle of available energy. That 

 method is also applied in a discussion of the electromotive force of a 

 voltaic cell, and especially of the dissipative part which is established 

 by steady finite diffusion between solutions of different concen- 

 trations. 



8. A thermodynamic application which possesses interest, both 

 from the light it throws on the nature of magnetism and from the 

 circumstance that in it the heat supply is calculated indirectly from 

 the magnetic energy that runs down, is the relation between mag- 

 netic susceptibility and temperature in substances not in the very 

 susceptible or ferromagnetic condition. According to the Weberian 

 theory, which fits in with the present view, diamagnetic energy 

 which is not compensated mechanically goes to the induction of 

 Amperean currents in the molecules ; while paramagnetic energy 

 not thus compensated goes to orientating the molecules, and thus 

 into heat. It follows that the diamagnetic coefficient is independent 

 of temperature : on the other hand it is shown that the paramag- 

 netic coefficient should vary inversely as the absolute temperature. 

 These laws were discovered experimentally by Curie, who finds from 

 a very extensive investigation that they have the same order of 

 accuracy at sufficiently high temperatures as the ordinary gaseous 

 laws: at lower temperatures and in ferromagnetic substances the 



