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This article describes a theoretical experiment, which was published 1974 in the journal "Sience"  by the chemist Octave Levenspiel. The experiment, known as Levenspiel's fountain ,
is discussed as a perpetual motion machine of second kind.

                                            Levenspielbild
The here described variation of "Levenspiel's fountain"  has a wheel , which is in constant rotation by falling water drops. In this device a tube with pure water immerse in a solution of salt. The lower end of the tube is closed by a semipermeable membrane. If the tube is short, the water level inside would be lower than outside, because of the osmotic pressure. But when the tube immerse deeper and deeper in the solution, this relation will change to a higher level inside, because the density of the salt solution is higher and so the hydrostatic pressure will grow faster. When the tube is long enough, falling water drops result a continual motion of the wheel.

Exist in a closed sytem a rotating wheel, which motion is not limited by the formation of a final equilibrium, this is in contradiction to the second law of thermodynamics. It is a perpetual motion machine of second kind.

The question here is, if it is possible-why ? The peculiarity, that the tubes have to be very long, because the osmotic pressure is even with low concentrations high, should be not important to a theoretical discussion.  Calculations are available in the link diffusionmachine .

At first we imagine, that there is no dropping out by  the added side tube. In this case the water level would rise to the equilibrium line. In this equilibrium state no further motion is possible. The connection between the solution and pure water exist only by the semipermeable membrane.   Single water molecules can move through the membrane in both directions and so it is a reversible connection between the systems.
If we add the side tube, the connection there is only in one direction, because a single water drop can only fall down. The irreversible step prevent  the genesis of a final equilibrium, which would stop the process.
So we can assume, that for a endless motion we need both processes, reversible and irreversible. An explanation for the importance of this priciple is available in the link
Irreversibility against thermodynamics .

The argumentation on this site is intentional short, because it should be only a starting point for thinking about things, which are absolute true for most scientists.

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