56o THE POPULAR SCIENCE MONTHLY 



1896, that " chemical forces are of a far more complex nature than 

 electrolysis."®- 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 solu- 

 tions, 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."®^ 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. More- 

 over, 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®* show that such mem- 

 branes, 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 



°^ " 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. 



*^ Kahlenberg, Phil. Mag., 1905, 6. s., IX., 229. 



»* 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 

 Kraften und Eigenschaften der Losungen," Leipzig, 1903.) 



