text: how come a semipermeable membrane can't be used to harness osmotic pressure to form a perpetual motion machine how come a semipermeable membrane can't be used to harness osmotic pressure to form a perpetual motion machine: i.e., you separate two fluids, of different salinity, causing water to flow through the semipermeable layer, you then harness the pressure differential to create extremely small but continuing amounts of usable energy? hound has it.  a pm machine must work in a closed system.  the membrane would soon become swamped by its own waste products, just the way "developed" countries are doing.