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SCIENCE 



[N. S. Vol. XXXII. No. 817 



peratures, an osmotic current will obviously be 

 set up in order to equalize the temperature. 

 If the fluids have the same temperature but 

 are of different chemical composition, the 

 osmotic current, if any, will be the resultant 

 of forces flowing from higher to lower levels 

 of chemical potency or potential energy. Or 

 as Van Laar and other followers of Gibbs have 

 interpreted it, the substances having the 

 higher chemical potentials will move towards 

 those having the lower. " Even when the 

 diaphragm is permeable to all components 

 without restriction," Gibbs insists, " equality 

 of pressure is not always necessary for osmotic 

 equilibrium." These conditions are, mathe- 

 matically, that t' = if' and fx = fj!' . . . where 

 t' , t" and ii!, ix" are the temperatures and 

 chemical potentials of the substances that can 

 pass through the semi-permeable diaphragm. 

 Now the Gibbsian potential ^u, was interpreted 

 by Clerk Maxwell as the intensity with which 

 a given component substance tends to expel 

 itself from the compound containing it, and is 

 equal mathematically to the surface energy 

 (marginal available energy) of the component 

 per unit mass at fixed temperature." Pro- 

 fessor Traube's " attraction-pressure " would 

 appear to be just the logical opposite of this 

 concept, viz., the tendency of the given com- 

 ponent to " stay put." But for chemical 

 equilibrium these " intensities " must neces- 

 sarily balance each other; in other words, the 

 " Haf tdruck " is conceivably the Gibbs poten- 

 tial /A with reversed sign ( — fn). The chem- 

 ical potential of Gibbs is identical with 

 Lewis's more recent concept of " fugacity " or 

 " escaping tendency," which the latter de- 

 fines " as " the tendency of every molecular 

 species to escape from the phase in which it 

 is " ; the " attraction-pressure " of Traube is 



"See Gibbs, Tr. Connect. Acad.. III., 150: "In 

 the case of a body of invariable composition, tlie 

 potential for the single component is equal to the 

 value of i (available energy at constant atmos- 

 pheric pressure) for one unit of the body, etc." 



^"Proc. Am. Acad. Arts and Sc, 1901-2, 

 XXXVII., 54. Lewis introduces " fugacity " as a 

 sort of generic variable to include all such con- 

 cepts as thermodynamic potentials, vapor-pres- 

 sure, solubility, etc. 



apparently a static expression of the " chem- 

 ical affinity " or " chemical attraction " of 

 other writers. These differences in funda- 

 mental conceptions may serve to illumLnate 

 some obscure features of the gigantic con- 

 troversy which has been waged of late years in 

 regard to osmosis and the theory of solution. 

 Up to 1887, as Professor Louis Kahlenberg 

 has recently called to mind," all theories of 

 osmosis and solution were purely chemical. 

 After that date, under the sway of the van't 

 Hoff-Arrhenius school they became purely 

 physical. But the van't Hoff theory depends 

 for its physical proof upon the assumption 

 that molecules, and ions exist as such, while 

 its mathematical proof is bound up with the 

 notion that liquid substances act like gases," 

 although a moment's common-sense reflection 

 will convince any one that they do not. This 

 theory which Lothar Meyer, Lord Kelvin and 

 Fitzgerald combated upon its first appearance, 

 and which chemists like Kahlenberg, Arm- 

 strong, van Laar, Mendelejeff and Eaoult have 

 latterly opposed with such striking ability, 

 owes most of its acceptance to the personal 

 influence and brilliant partizanship of Pro- 

 fessors Ostwald'" and Arrhenius; but, as Pro- 



" Science, 1910, No. 785, 41-52. 



" What van't Hoff set out to prove was that 

 the kinetic energy of a molecule in the dissolved 

 state is equal to that of the same molecule in a 

 gas occupying the same volume as the given liquid 

 solution, and he maintains that Iiis thermody- 

 namic demonstration is true whatever the mech- 

 anism of osmosis and whether the rOle of the 

 semi-permeable membrane be " selective " or other- 

 wise. Since van't Hoff took this stand in 1SS7, it 

 has become, as Kahlenberg insists, " a favorite 

 dodge of the therraodynamicists to claim that they 

 are not concerned with the mechanism of osmosis," 

 thus evading the crucial point at issue in favor of 

 computations tending to prove a theorem based 

 upon assumptions about- molecules and admittedly 

 true only of ideally diluted solutions (see Kahlen- 

 berg, J. Phys. Chem., 1909, XIII., 97). 



" To show how far such propagandism may be 

 carried, recent reviews of Ostwald's book on the 

 evolution of chemistry ( " Der Werdegang einer 

 Wissenschaft ") comment upon the fact that he 

 has suppressed all reference to Graham, to whom 

 chemists are indebted for many of their funda- 



