Page:Popular Science Monthly Volume 74.djvu/564

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1896, that "chemical forces are of a far more complex nature than electrolysis."[1] Accepting the contention of the van't Hoff school that the gas equation and the Arrhenius theory are only true for infinite dilution, Kahlenberg has turned a clever flank movement upon them by insisting that if liquids act like gases we should expect a solution of increased concentration to behave at least qualitatively as gases do on increase of pressure. As a matter of fact, although practically all gases act alike, different solutions do not, as a rule, and solutions of solids in liquids, or liquids in liquids, do not behave like solutions of gases in liquids or gases in gases. Furthermore, the Arrhenius theory does not agree with many facts about aqueous solutions, while it falls completely to the ground for solvents other than water. This does not mean that Kahlenberg opposes electrolysis or electrolytic dissociation as such, or that he would have us abandon hypotheses of such value before we have found better ones, but he insists that "the question why certain solutions, molten salts, etc., conduct electricity and others do not will probably not be answered until we can tell why a stick of silver conducts electricity and a stick of sulphur does not.[2] Morse and others have shown that the van't Hoff equation and the Arrhenius theory are true for very small dilutions, that is for solutions so mathematically ideal that they are practically independent of the nature of the solvent and the solute, but the experiences of Kahlenberg have shown that they are not always true for actual solutions of reasonable concentration. Moreover, the fact that the solute in tenth-normal solutions acts like a gas by no means explains all the phenomena of solution. Kahlenberg's experiments with semi-permeable membranes[3] show that such membranes, while passive for gases, are active or selective for different liquids, so that the initial movement and actual direction of the osmotic current are determined by the specific nature of the membrane itself and of the liquids bathing it. Semi-permeable membranes, therefore, exist as such, and although none are strictly ideal in Gibbs's sense, their true "semi-permeable" or selective character is indicated by Kahlenberg's discovery that in some cases true measurements of osmotic pressure can not be obtained unless the solution is stirred to increase chemical action. The semi-permeable membrane shows that osmotic

  1. "That other than purely electrical forces are operative in solution is indicated by Helmholtz's investigations of electrical diffusion through fine tubes." Fitzgerald, Helmholtz lecture, Nature, 1895-6, LIII., 297.
  2. Kahlenberg, Phil. Mag., 1905, 6. s., IX., 229.
  3. Kahlenberg, J. Phys. Chem., 1896, X., 141-209. Recently Tammann has advanced the view that in ideally diluted solutions the solute acts like a gas, while in concentrated solutions there is a chemical interaction between the solvent and the solute, and such solutions behave more like the solvent under higher pressure. (Tammann, "Ueber die Beziehungen zwischen den inneren Kräften und Eigenschaften der Lösungen," Leipzig, 1903.)