On Thu, Mar 11, 2010 at 4:51 AM, Russell McMahon wrote: > The question is not HOW it works from a descriptive point of view or > what happens but how the existence of a particular fraction of a given > gas is "known" about by the same gas on the other side of the > membrane. I can calculate WHAT happens but need to know why. Isn't it as simple as the fact that they are all moving? If side A has 10 molecules, and B has 20 all at the same temperature, then the molecules are moving at the same speed. Since there are two times more molecules on B, then the statistical likelihood of a molecule hitting the membrane in a permeable spot due to its own random motion is two times more likely than a molecule in A hitting a permeable spot. Of course the molecules travel _both_ ways, and neither side's molecules know about the other side. But the random motion coupled with similar permeability, surface area, temperature, pressure, etc simply means that the higher concentration is going to move across the barrier more quickly than the lower concentration. But they'll both move across the barrier. Eventually the concentrations will be equal. -Adam -- http://chiphacker.com/ - EE Q&A site -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist