Perrin and Langevin have made a successful attempt in this direction.

M.Perrin enunciates the following principle: _An isolated system never passes twice through the same state_.

In this form, the principle affirms that there exists a necessary order in the succession of two phenomena; that evolution takes place in a determined direction.

If you prefer it, it may be thus stated: _Of two converse transformations unaccompanied by any external effect, one only is possible_.

For instance, two gases may diffuse themselves one in the other in constant volume, but they could not conversely separate themselves spontaneously.
Starting from the principle thus put forward, we make the logical deduction that one cannot hope to construct an engine which should work for an indefinite time by heating a hot source and by cooling a cold one.