JO SI AH WILLARD GIBBS 561 



pressure is not an initial force, but a secondary hydrostatic pressure due 

 " to the same affinity which produces adhesion, imbibition, absorption, 

 adsorption, solution and chemical action." But all these forces are 

 reducible in the simple reasoning of thermodynamics to the difference 

 in temperature (Carnot's principle) and differences of chemical po- 

 tentials which promote chemical change. As to molecular bombard- 

 ment, " osmotic pressure," said Fitzgerald, in 1896, " is more nearly 

 related to Laplace's internal pressure in a liquid which depends upon 

 intramolecular forces, than to a gaseous pressure which is practically in- 

 dependent of the forces acting between the molecules."^^ Van Laar 

 pictures a sugar solution of reasonable concentration as made up of 

 crooked movements of molecules, slowly crowding upon one another, 

 with no intervening spaces, totally different from the rapid billiard ball 

 movements with wide repulsions that are supposed to obtain in diluted 

 gases. Osmosis, in van Laar's theory depends not upon the molecules 

 of the dissolved substance, but upon the solvent itself, which, having 

 the higher chemical potential, moves toward the solute. To explain the 

 phenomena of osmosis by appealing to an initial osmotic pressure, says 

 van Laar,^^ is like saying that an angry man's loud talk and unseemly 

 gestures are due to his red face.^'^ Anger is the real cause of both. So 

 the movement in osmosis, which produces a difference in hydrostatic 

 pressure, depends initially upon differences of chemical and thermo- 

 dynamic potentials. Beyond this we know absolutely nothing of the 

 interaction between the solvent and the solute. Again Bancroft has 

 shown that the pressure for finite solutions in osmosis varies with the 

 heat of dilution, which again varies with the specific nature of the 

 solvent and the solute.®^ All this brings us back to Gibbs's fundamental 

 position that osmotic pressure " is a function of the temperature and 

 the n potentials.'"'^ From this point of view, Graham's original 

 doctrine, that osmosis is the conversion of chemical affinity into me- 

 chanical power,^'"' is at once true to the mathematical theory and the 

 laboratory facts. If now we agree with Whetham that " osmotic phe- 

 nomena are intrenched in the strongest part of the vast lines occupied 

 by the science of thermodynamics," it is clearly due to the early pioneer 

 work of Gibbs that this vantage ground was gained in the first instance, 

 while the molecular theory of osmosis remains in the debatable land of 

 controversy and a true theory of solutions is still far to seek. 



{To be continued) 



«= Fitzgerald, Nature, London, 1895-6, LIII., 297. 



"* Van Laar, " Sechs Vortrage iiber das thermodynamische Potential," 

 Braunschweig, 1906, 3. 

 "Ibid., 34. 



"^ Bancroft, J. Phys. Chem., 1906, X., 319-29. 

 »= Gibbs, loc. cit., 139. 

 "" Graham, Phil. Tr., 1854, 227. 



